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The Youjiang Basin is usually regarded as an important foreland basin in the southern part of the South China Block that is related to the convergence of the South China and Indochina blocks during the Permian-Triassic. However, the nature of the basin remains controversial due to questions about the subduction polarity and suture boundary between these two blocks. Permian-Triassic felsic volcanics across the Dian-Qiong and Song Ma suture zones could offer new insights into the convergent processes of the South China and Indochina blocks. This study presents detailed petrological, zircon U-Pb dating, and Hf-O isotope and whole-rock geochemical analyses for the Triassic felsic volcanics of the Youjiang Basin (northeast of the Dian-Qiong). The dacites and rhyolites from the Beisi and Baifeng Formations were dated at ca. 240–245 Ma. All of the felsic volcanics are characterized by high SiO2 (69.40–73.15 wt%), FeOt/MgO, 10000*Ga/Al, and TZr, δ18O (9.7–11.8‰) and negative εNd(t) (from −9.6 to −12.3) and zircon εHf(t) (from −6.2 to −14.5) with A-type granitoid geochemical affinities, suggesting the reworking of crustal rocks in an extensional setting. Permian-Triassic felsic igneous rocks display similar geochemical signatures across the Dian-Qiong suture zone, whereas they show distinctive Sr-Nd and zircon Hf-O isotopes across the Song Ma suture zone. The felsic igneous rocks to the northeast of the Song Ma suture zone have much lower εNd(t) and higher δ18O with negative zircon εHf(t) than those to the southwest, which have positive zircon εHf(t). Combined with other geological and geophysical features, it is inferred that the Song Ma suture zone was probably the suture boundary between the South China and Indochina blocks, and the Youjiang Basin was likely a peripheral foreland basin in response to the southwestward convergence of the South China Block toward the Indochina Block.

In order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464–485 Ma, 455–471 Ma, and 427–446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96–1.99 and K2O > Na2O, which are marked by enrichment in large-ion lithophile elements and depletion in high field strength elements with remarkable Nb-Ta, Sr-P, and Ti negative anomalies. Their initial 87Sr/86Sr ratios range from 0.70510 to 0.71559, εNd(t) from −9.5 to −3.0, (206Pb/204Pb)i from 18.65 to 19.72, (207Pb/204Pb)i from 15.66 to 15.80, and (208Pb/ 204Pb)i from 38.84 to 39.79. The corresponding zircon ɛHf(t) and δ18O values are in the range of −10.6 to +1.0 and 6.88‰ to 8.94‰, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined “single-ancient” Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.

The Cretaceous igneous rocks in the South China Block (SCB) were associated with the slab subduction and roll-back of the Pacific Plate. Thus, they provide excellent opportunities to examine the spatial–temporal geochemical migration of magmatism in the retreating subduction margins. The Cretaceous mafic–intermediate igneous rocks from the southeastern SCB were aged between 142 and 71 Ma, and can geochemically be subdivided into three groups: Group A (126–129 Ma and 83–93 Ma), Group B (126–142 Ma and 71–108 Ma) and Group C (116–142 Ma and 70–110 Ma). Group A and B were mainly distributed in the SCB interior and derived from asthenosphere and asthenosphere–lithosphere interaction sources, respectively. Group C occurred to the east of the Ganjiang Fault and originated from slab–lithosphere interaction. From the coastal provinces to the interior, these mafic–intermediate igneous rocks show increasing incompatible element ratios and Nd isotopic compositions, reflective of a westerly decreasing involvement of slab-derived components. They show two similar age-pulses at c. 125 Ma and c. 90 Ma as well as the Cretaceous A-type granites, indicating two episodes of subduction retreat of the Pacific slab during the Cretaceous. This spatial–temporal pattern of the Cretaceous mafic–intermediate igneous rocks suggests that the Cretaceous slab metasomatism of Pacific subduction retreat was limited to the east of the Ganjiang Fault. Supplementary material: Tables of geochemical data and additional figures are available at https://doi.org/10.6084/m9.figshare.c.4938576

The South China Block (SCB) consists of the Yangtze and Cathaysia Blocks which, in the northeast, are separated from each other by the Jiangnan Orogen. The southward continuation of the Jiangnan Orogen into the Youjiang Basin remains much debated. Here, we present detailed petrological, chemical and isotope evidence from the Late Jurassic (~159 Ma) Yangtun and Liuliang high-Mg andesites (HMA), eastern Youjiang Basin, to place important constraints on an ancient subduction event. These HMA are enriched in large ion lithophile elements and depleted in Nb-Ta-Ti, with negative whole-rock eNd(t) and zircon eHf(t) values, similar to Neoproterozoic high-Mg rocks that occur in the eastern Yangtze Block. Considering the distance of our study areas to penecontemporaneous Pacific or Tethyan trenches, we infer that these HMA derived from a metasomatised lithospheric mantle preserved beneath the eastern Yangtze Block, which was associated with a Neoproterozoic subduction event. The Late Jurassic HMA in the eastern Youjiang Basin provide potential clues for the southward continuation of the Jiangnan Orogen into the basin.

The volcanic rocks in the Chanthaburi zone are rarely reported, but are important in investigating the tectonic evolution of the Palaeotethyan Ocean in SE Thailand. Four rhyolitic samples from Ko Chang Island yield zircon ages of 258–254 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K2O contents of 4.92–7.10 wt% and A/CNK values of 1.10–1.69. They are enriched in the light rare earth elements, Rb, Th, U, Zr and Y and show negative anomalies of Ba, Sr, Nb, Ta and Ti, with clear Eu anomalies. Their whole-rock eNd (t) values range from −1.7 to −3.1. Zircon in situ eHf (t) and δ18O values range from 0.0 to +5.6 and from 8.2 to 9.6‰, respectively. They belong to the group of peraluminous, ultrapotassic A-type rhyolites and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of a subducted Palaeotethyan oceanic slab beneath the Indochina Block. Combining these results with previous geological observations, we propose that there were some sporadically distributed continental rift basins along the Eastern Palaeotethyan domain during the Permian. Supplementary material: Major oxides, trace element analytical and Nd isotopic data, LA–ICP–MS and SIMS zircon U–Pb ages and Zircon in-situ Hf–O isotopic compositions are available at https://doi.org/10.6084/m9.figshare.c.5635390

The eastern Tibetan Plateau is a key part of the eastern India–Asia collisional zone, a region that records multiple overprinting tectonic and magmatic events. This study presents new geochronological, geochemical, and Sr–Nd–Hf–O isotopic data for Cenozoic potassic granitoids in eastern Tibet, southwestern China, which recorded the tectonic evolution of the eastern Tibetan Plateau. These potassic granitoids are formed between 37.6 and 32.9 Ma and are geochemically subdivided into the following: Group 1, adakite-like granites; Group 2, syenites; and Group 3, low-εNdt granitoids. The Group 1 samples are similar to high-silica adakites in that they have variable SiO2 contents (63.31–73.62 wt.%) and high Sr/Y and La/Yb ratios. These samples have εNdt and εHft values that range from −5.8 to −0.6 and from −4.3 to +5.2, respectively, with δ18O values of 6.78‰–7.36‰. The Group 2 samples are syenites, contain 56.36–63.86 wt.% SiO2 and high concentrations of Y and Yb, and have εNdt values from −8.4 to −2.4, εHft values from −6.1 to +1.1, and δ18O values of 6.37‰–6.89‰. The Group 3 samples have a narrow range of SiO2 concentrations (62.27–64.59 wt.%), high Sr/Y and La/Yb ratios, δ18O values of 6.31‰–6.82‰, and low εNdt and εHft values (−12.6 to −10.9 and −11.4 to −6.6, respectively) that are similar to the values obtained for the contemporaneous Yao’an lamprophyres. These data indicate that the Group 1 samples are formed from magmas sourced from a heterogeneous and thickened region of the lower crust containing an enriched mantle component. Group 2 magmas were most likely derived from contemporaneous mafic melts sourced from an ancient region of the lithospheric mantle previously modified by the incorporation of recycled components. The Group 3 samples have distinct Sr–Nd–Hf isotopic compositions that are indicative of derivation from magmas generated by the fractional crystallization of lower crustal melts sourced from ancient enriched mantle of the Yangtze Block. Combining these new data with the results of previous research suggests that the Cenozoic potassic igneous rocks of eastern Tibet were formed as a result of the thinning of the lithospheric mantle and an associated crustal collapse event, potentially representing a regional late Eocene to early Oligocene transition from compression to transtension in the eastern Tibetan Plateau. These potassic igneous rocks are contemporaneous with or are younger than igneous rocks in the Qiangtang Block, suggesting that the magmatic response to the India–Asia collisional event was initiated in the central Tibetan Plateau before propagating towards the eastern margin of this region.

To better reveal the tectonic process of the Jiangnan Orogen in creating the united South China Block (SCB), a set of new geochronological, geochemical, and Sr-Nd-Hf-O isotopic data is herein presented for the coeval early Neoproterozoic mafic to acid rocks in the Lengjiaxi Group and its equivalents. Our data show that these rocks belong to subalkaline basalt, basaltic andesite, andesite, and dacite in composition, and can be geochemically classified into N-MORB- and arc-like mafic (Group 1 and Group 2, respectively), along with high-Mg andesitic-dacitic (Group 3) rocks. They yield zircon U-Pb ages of 837 ± 4 Ma, 832 ± 11 Ma, and 835 ± 7 Ma, respectively, suggesting their early Neoproterozoic (ca. 835 Ma) origin. Group 1 has SiO2 = 47.21–52.45 wt%, MgO = 7.36–10.07 wt% with Mg-number of 56–68, 87Sr/86Sr(t) = 0.7062–0.7136 and εNd(t) = +1.3–+9.4, along with N-MORB-like REE- (rare earth element) and PM (primitive mantle)-normalized patterns, suggesting its derivation from a refractory MORB-like source newly introduced by slab-derived fluid. The Group 2 samples are classified as tholeiitic basalt and basaltic andesite with SiO2 = 48.78–55.75 wt%, MgO = 7.70–12.74 wt%, Mg-number = 63–77, 87Sr/86Sr(t) = 0.7046–0.7098 and εNd(t) = –1.6 to –0.4. They exhibit similar “spiky” REE- and PM-normalized patterns to typical arc basalt, with pronounced enrichment in (large ion lithophile elements) and sharp depletion in (high field strength elements), probably inheriting from a sub-arc source modified by fluids and/or melts released from recycled sedimentary component. The Group 3 rocks show SiO2 = 57.74–66.30 wt%, FeOt = 6.22–8.59 wt%, Mg-number = 43–66, K2O+Na2O = 3.18–6.17 wt% with K2O/Na2O of 0.59–3.28, classified as high-Mg andesites and dacites. Diagnostic elemental and isotopic ratios of Group 3 include (La/Yb)cn = 7.7–11.4, (Gd/Yb)cn = 1.44–1.87, Eu/Eu* = 0.49–0.80, Nb/La = 0.27–0.42 and εNd(t) = –7.3 to –5.4, resembling to those of other early Neoproterozoic high-Mg intermediate rocks in the SCB, originating from a mantle wedge source with proportional addition of the recycled SCB sediment-derived components. In combination with the available data and geological observations, it is concluded that the N-MORB- and arc-like mafic, along with high-Mg andesitic rocks are coeval in the Lengjiaxi Group and its equivalents along the central Jiangnan Orogen and formed at ca. 847–832 Ma of the early Neoproterozoic period. The synthesis of these observations suggests the development of an early Neoproterozoic continental arc-basin setting. The arc-basin system westerly subducted and finally closed at ca. 830 Ma and the assembly of the Yangtze and Cathaysia blocks terminated at ca. 810 Ma along the central Jiangnan Orogen.

The Mesoproterozoic Baoban Complex, southwest Hainan Island, has played a crucial role in deciphering Nuna (Columbia) supercontinent reconstructions. Metasedimentary rocks in the Baoban Complex are intruded by Mesoproterozoic gneissic granites and mafic rocks. Our new geochemical and geochronological data show that these gneissic granites are monzogranites and granodiorites and crystallized at ∼1430 Ma. Geochemically, they are subdivided into two groups with I- and A-type affinities. The groups show broadly similar eNd (t) values ranging from −2.4 to +0.9, zircon eHf (t) from −5.4 to +7.8 and δ18O values from +5.0 to +8.6‰, suggesting derivation from the same source involving juvenile mafic melt and ‘‘ancient” continental crust components. Metasedimentary rocks have high La/Sc (1.4–5.8) but low Sc/Th (0.6–1.6) and Co/Th (0.3–1.1) ratios, as well as negative eNd (t) of −4.5 to −4.1, indicative of an “ancient” felsic igneous source with a recycled sedimentary component. Detrital zircon grains from the metasedimentary rocks yield three primary age-peaks of ∼1780 Ma, ∼1610 Ma and ∼1440 Ma, with the depositional age synchronous with those of the gneissic granites and metabasites (∼1430 Ma). Our geochemical and geochronological data, along with regional geological constraints, suggest a rift setting. The age populations and hafnium compositions of detrital zircons from the metasedimentary rocks in the Baoban Complex, southwest Hainan are comparable to those igneous zircons from East Antarctica, pointing out a probable affinity between southwest Hainan and East Antarctica in the Nuna supercontinent.

The Early Paleozoic orogeny in the South China Block was accompanied by crustal anatexis, as indicated by the voluminous granites and migmatites. Here we report on the migmatite-granite suite within the Neoproterozoic biotite gneiss of the Baiyunshan Domain, which has directly recorded the process of anatectic melt formation, segregation and extraction. The crystallization of leucosome in migmatite was dated at ca. 438 Ma by U–Pb analysis of zircon, and the granitic vein that intruded the migmatite was dated at ca. 433 Ma. Petrological and geochemical data suggest that anatexis resulted from fluid-present melting of the biotite gneiss at low temperature and pressure conditions (~650–700 °C and ~4–6 kbar). The leucosome mainly consists of plagioclase + K-feldspar + quartz and has positive Eu anomaly (Eu/Eu* = 1.70–2.89), which is attributed to feldspar accumulation. This means that some melt has been extracted from the migmatite, and the granitic veins are likely to represent these extracted melts since they have similar trace element compositions and zircon Lu Hf isotopes to the leucosomes. The degree of partial melting and chemical differentiation to generate the migmatite and the granitic veins are revealed by mass balance calculations. Calculations suggest that the primary melt could be a result of ~40 wt% melting of the Baiyunshan biotite gneiss, and fractionated melt (~24–26 wt%) was extracted from the deeper sections to form the intrusive veins. Combined with regional dating results, it is suggested that the crustal anatexis in the Wuyi-Yunkai orogen started no later than 465 Ma with a duration of >55 Myr. Generation of the granitoids and migmatites are a result of water-fluxed melting and linked to extra heat from mantle magmas provided to the middle-lower crust.

The NE–SW-trending Qin-Hang Belt and east–west-trending Nanling Range are two critical magmatic and mineralization zones in the southeastern South China Block (SCB). However, the formation timing and petrogenesis of the magmatism in these zones, along with the relationship with the Pacific subduction, remain hotly debated. Herein, this study presents a set of zircon U–Pb geochronological, Lu–Hf isotopic and whole-rock geochemical data for the Late Jurassic Mashan and Nandu shoshonitic intrusions in the Qin-Hang Belt. All the samples are enriched in large ion lithophile elements and light rare earth elements. They have high eNd(t) (from −1.4 to +4.4) and zircon eHf(t) values (from −1.0 to +3.4) and low initial 87Sr/86Sr (0.70426–0.70696), distinct from those of the Triassic shoshonitic rocks but comparable with those of the Jurassic shoshonitic rocks in the region. Such signatures suggest that the Mashan and Nandu igneous rocks mainly derived from a Jurassic metasomatized lithospheric mantle in response to asthenospheric upwelling, without significant contribution from subducted slab-derived components. Our new data, together with other geological observations, reveal a spatial change of magmatic activities, structural patterns, sedimentary basins and palaeocurrent directions during the Middle–Late Jurassic, probably associated with the far-field effects of the Pacific subduction in the interior SCB.

Felsic igneous rocks in northern Laos are key elements of the Truong Son and Song Ma tectonic zones and are important for constraining the Paleotethyan evolution in southeastern Asia. This paper presents new geochronological, geochemical and Sr–Nd–Hf–O isotopic data from granitoids and rhyolites in northern Laos. Geochronological data show that these samples have zircon U-Pb ages ranging from ca. 281 Ma to 221 Ma. These samples can be subdivided into four groups based on their lithology and geochemical composition. Group 1 samples are granodiorite and granite with I-type geochemical characteristics. They have lower 87Sr/86Sr (i) ratios (0.7036–0.7059) and higher eNd (t) values (+0.7 − +0.8) than Group 2 biotite granite samples (87Sr/86Sr (i) = 0.7087–0.7129 and eNd (t) = −5.3 to −9.2) and Group 3 rhyolite samples (87Sr/86Sr (i) = 0.7092–0.7103 and eNd (t) = −5.9 to −8.9). Zircon grains from Group 1 samples yield positive eHf (t) values of +5.1 − +13.8, TDM ages of 0.37–0.71 Ga and δ18O values of 5.6–6.3‰. In contrast, zircons from Group 2 and Group 3 samples have mainly negative eHf (t) values (−9.6 to +1.6 and −4.7 to −14.7), older TDM ages (1.08–1.70 Ga and 1.42–1.97 Ga), and higher δ18O values (5.7–10.3‰ and 6.7–8.0‰), respectively. Our geochemical data suggest that the Group 1 granite samples are the products of the partial melting of newly underplated mafic rocks, whereas the Group 2 and 3 samples originated from a mixed source of “ancient” metaigneous and metagreywacke components. Group 4 samples include Late Triassic I-type granodiorite and granite and have eNd (t) = −1.9 to −3.9, 87Sr/86Sr (i) = 0.7079–0.7084, zircon eHf (t) = 0.0 − +7.5, and δ18O = 5.1–6.9‰, indicating a derivation from a metagreywacke source with a component of juvenile mafic crust. Our data suggest that the Permian felsic igneous rocks in northern Laos formed in an arc setting related to the Song Ma Paleotethyan branch/BAB subduction, whereas the Late Triassic I-type granitoids formed in a post-collisional setting in response to the gravitational collapse of thickened crust during the collision of the South China and Indochina blocks. The temporal-spatial pattern of igneous rocks along the Truong Son zone is related to the Paleotethyan evolution and resembles that along the Ailaoshan-Song Ma zone and its eastward extension in northern Hainan.

It is well established that the Late Jurassic magmatism in the southeastern area of the South China Block (SCB) was associated with an extensional event. However, the triggering mechanism for this event remains unclear, with models commonly invoking either back-arc extension or intra-continental extension in response to slab roll-back of the Pacific plate or asthenospheric upwelling. The key issue about these models revolves around whether the subducted Pacific slab contributed to the mantle source beneath the SCB during the Jurassic, or not. Basalts erupted during the Late Jurassic can provide significant clues for probing the nature of the lithospheric mantle and underlying convective mantle. In this study, we present detailed zircon U-Pb geochronological, whole-rock element and Sr-Nd isotope data for Late Jurassic trachybasalt and trachyandesite from the Mashan Complex, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA-ICP-MS zircon U-Pb dating suggests that these volcanic rocks erupted in the Late Jurassic (~158 Ma). All volcanic samples have shoshonitic geochemical affinities with high K2O (2.03–4.89 wt%) and K2O/Na2O (0.55–2.50). They are strongly enriched in LILE and LREE with positive K anomalies, positive eNd(t) values (from +0.8 to +3.0), moderate Dy/Yb (1.98–2.36) and low Ba/La (11.1–30.7), Th/Yb (2.29–5.77), U/Th (0.18–0.35), Th/Nb (0.16–0.27) and Th/Ce (0.08–0.15) ratios. Such geochemical signatures suggest that the Mashan volcanic rocks were derived from low-degree (1–5%) partial melting of a metasomatised lithospheric mantle with little evidence supporting the involvement of the Pacific slab in the genesis of the magmas. The synthesis of the available data shows that the Jurassic shoshonitic and mafic rocks have higher eNd(t) values than the Triassic shoshonitic and mafic rocks (eNd(t)

The northern margin of the Yili Block, NW China, contains widespread Permian volcanic rocks that provide insight into the late Paleozoic evolution of the North Tianshan Orogenic Belt in the southwestern Central Asian Orogenic Belt. This paper presents geochronological, geochemical and Sr–Nd isotopic data for Permian volcanic rocks of the Hamisite and Wulang formations in the Nileke area of the western Awulale Mountains, and provides new constraints on their ages, petrogenesis and tectonic settings. The whole-rock 40 Ar/ 39 Ar plateau age of 281.6 ± 1.0 Ma for basalt in the Hamisite Formation suggests that these volcanic rocks were erupted in the early Permian. The studied samples comprise mainly basalt and basaltic andesite, with positive eNd(t) values, enriched light rare earth and large-ion lithophile elements, and depleted high-field-strength elements, indicating an origin from a subcontinental lithospheric mantle wedge that was probably metasomatised by slab-derived fluid and sediment-derived melt beneath the Yili Block. Based on geochemistry and the regional geological records, we suggest that the Permian Nileke volcanic rocks of the Yili Block were formed in a post-collisional setting, following slab break-off after closure of the North Tianshan Ocean. However, the Hamisite volcanic rocks were derived from a deeper mantle source, probably through thickening of the lithospheric mantle and crust of the Yili Block.

The Habo Au polymetallic deposit is located in the south of the Ailaoshan-Red River tectonic zone. Mineralization is controlled by a strike-slip fault, which was intruded by aplite. Ore minerals are dominated by pyrite and arsenopyrite. Gold is invisibly and ionically bound in pyrite and arsenopyrite. Three hydrothermal alteration-mineralization stages can be identified: quartz-pyrite (py1) ± arsenopyrite vein associated with sericite alteration (I), quartz-pyrite (py2)-polysulfide (II) and quartz-calcite (III). The aplite yielded zircon U−Pb ages of ca. 35.4 Ma while the sericite in pyritized siltstone yielded an Ar-Ar age of ca. 37.5 Ma, constraining the mineralizing age. In situ S–Pb isotopic compositions for sulfides were measured by fs-LA-MC-ICP-MS. Sulfides from the deposit show uniform δ34S values ranging from –5.1‰ to +2.5‰, a typical characteristic of magmatic-derived S. Pb isotopic compositions of galena are similar to those of the granite in the Habo granite complex, suggesting that the metal is probably sourced from the Habo granite complex. In-situ LA-ICP-MS spot analysis indicates that py1 has high Ni, Co, As and Se concentrations while py2 contains higher concentrations of Au, As, Cu, Pb and Zn. These characters reveal that the ore-forming fluids are As-rich and largely unsaturated with respect to Au. Based on the ore deposit geology, dating and isotopic data, the Habo Au polymetallic deposit mineralization is similar to typical intrusion-related vein-type deposit, formed by fluids originated from magma differentiations of the Habo granite complex.

Granites that have high eNd(t) and zircon eHf(t) (here phrased as ‘HNHGs’) can offer significant insights into the processes of crustal growth and reworking. This study investigates Early Jurassic HNHGs in the Southeastern (SE) South China Block (SCB) to determine whether they reflect crustal growth or reworking processes, which has implication for understanding the tectonic setting of the SE SCB. We presented new zircon geochronological and Hf-O isotopic, and geochemical analyses on these HNHGs to assess the fundamental crustal processes. Zircon U-Pb geochronological analyses yield the crystallization ages of 192.9 ± 0.8 Ma, 186.7 ± 2.5 Ma and 188.6 ± 1.3 Ma for the Wengong, Gaokeng and Chabei granites, respectively. These HNHGs display high SiO2 (69.75–77.50 wt%), low MgO (0.07–0.83 wt%) and Rb/Sr (0.74–14.3), belonging to unfractionated granites. They have low (87Sr/86Sr)i and zircon δ18O, high eNd(t) and zircon eHf(t), originating from partial melting of a juvenile crust. The HNHGs show a spatial distribution that overlaps with Early Paleozoic lithospheric mantle-derived mafic igneous rocks in the SE SCB. Isotope modeling results imply that the juvenile crust was likely a consequence of mixing among depleted mantle, subordinate enriched lithospheric mantle and SCB basement rocks during Early Paleozoic. These HNHGs record crustal reworking, rather than crustal growth during Early Jurassic, and formed in a continental extension setting.

The early Paleozoic Kwangsian event, the first compressive orogeny after the Rodinia assemblage in the South China Block (SCB), is characterized by the angular unconformity between the Devonian and pre-Devonian sequences and widespread granitoids and small-volume mafic rocks. However, the mechanism triggering this event is highly debated and the associated mantle source is poorly known. Our recent investigations identified the occurrence of early Silurian high-Mg basalt–andesite–dacite in the Dashuang area along the northwestern Yunkai Domain in the SCB. This study presents a set of new zircon U–Pb geochronological, whole-rock geochemical and Sr–Nd isotopic data for these volcanic rocks. They yield weighted mean 206 Pb/ 238 U ages of c. 430–443 Ma, similar to the age-peaks of early–middle Paleozoic magmatism and metamorphism in the SCB. These volcanic rocks have high MgO, Cr and Ni contents and Mg-numbers. They also exhibit arc-like geochemical affinities with crustal-like Sr–Nd isotopic compositions, similar to the Neoproterozoic high-Mg basalt–andesite in the eastern SCB. Such signatures, combined with other geological observations, suggest that the Dashuang high-Mg rocks were derived from an ancient (probably early Neoproterozoic) mantle wedge source. The early Paleozoic intra-continental reactivation in response to late-orogenic extension triggered the melting of this palaeosubduction-modified mantle, which led to the formation of these volcanic rocks.

It has been generally accepted that the Neoproterozoic amalgamation of the Yangtze and Cathaysia Blocks along the Jiangnan orogenic zone created the united South China Block (SCB). However, its final oceanic consumption time and subsequent assemblage process remain controversial. This paper presents a set of new geochronological, geochemical and Sr-Nd-Hf-O isotopic data for the early Neoproterozoic mafic to felsic igneous rocks in the previously-mapped Chencai Group and the associated equivalents. Our data show that the compositions of these rocks plot into the fields of the subalkaline and alkaline series, and they can be geochemically subdivided into five groups. Group 1 has SiO2 = 56.68–63.97 wt.% and MgO = 2.31–8.90 wt.% with Mg-number of 42–69, resembling the high-Mg intermediate rocks. Their 87Sr/86Sr(t) ratios range from 0.7022 to 0.7043, eNd(t) from +3.0 to +5.5, eHf(t) from +4.9 to +7.7 and δ18O from 4.78‰ to 5.57‰, most likely showing their petrogenetic relevance to the interaction of young slab-derived fluids with the overlying refractory mantle. Group 2 with SiO2 of 63.68–74.89 wt.% shows high Sr, Ba, Sr/Y and (La/Yb)cn, but low Y and Yb, resembling high-silica adakites. Its arc-like elemental signatures and highly positive eNd(t) -eHf(t) values, along with δ18O vaules of 4.95–5.39‰, suggest its derivation from the recently-ponded mafic lower crust with a mantle wedge source similar to Group 1. Group 3 is characterized by the arc-like basaltic and andesitic rocks with SiO2 = 45.06–63.53 wt.%, MgO = 3.38–12.99 wt.%, 87Sr/86Sr(t) = 0.7032–0.7047, eNd(t) = +2.0 ∼ + 5.9, eHf(t) = +3.2∼+7.8, originating from a MORB-like source newly modified by slab-derived fluids. Group 4 has SiO2 = 44.55–52.46 wt.%, MgO = 3.34–8.07 wt.%, Nb = 18.2–55.0 ppm and Nb/La = 1.01–1.76. These rocks can be classified as the high-Nb OIB-like alkaline mafic rocks, and interpreted as the product of the enriched component-modified asthenosphere. Group 5 has similar elemental signatures to the Nb-enriched basalt, with SiO2 = 44.52–48.80 wt.%, Nb/La = 0.38–0.81 and Nb = 8.2–17.4 ppm, and 87Sr/86Sr(t) = 0.7032–0.7044 and eNd(t) = +2.7∼+5.5, suggestive of an origin from a mantle wedge source recently modified by both slab-derived and OIB-like components. Our zircon U-Pb geochronological results for the representative samples of five groups indicate that all the five groups formed at 830–859 Ma with a cluster age of ∼830–845 Ma in the early Neoproterozoic, identical to the deposited time of the Lengjiaxi Group and its equivalents. The coexistence of the 830–859 Ma high-Mg intermediate (Group 1) and high-Si adakitic (Group 2) rocks, arc-like basaltic and andesitic (Group 3) rocks, and high-Nb (Group 4) and Nb-enriched (Group 5) mafic rocks in the Chencai Complex suggests an early Neoproterozoic arc-trench setting and the ongoing of subduction until ∼830 Ma. The initial assemblage of the Yangtze with Cathaysia Blocks along the whole Jiangnan Orogen might be roughly synchronous (∼820–830 Ma). An early Neoproterozoic slab window model at subduction termination (∼830 Ma) is proposed, representing an improved understanding of the petrogenesis of the early Neoproterozoic Chencai igneous rocks.

Mafic-intermediate igneous rocks in the Bangxi-Chenxing tectonic zone of Hainan Island, South China, can be used to constrain the Paleotethyan tectonic evolution in Southeast Asia. This paper presents petrologic, geochronological, elemental and Sr–Nd–Hf–O isotopic data for Carboniferous basalts, Permian andesites and Triassic gabbro-dolerites in this zone. Zircon grains from these rocks have U-Pb ages of ∼330 Ma, ∼257 Ma and ∼240 Ma, respectively. The ∼330 Ma basalts contain 1400 Ma captured zircon grains, and display MORB-like geochemical characteristics with positive eNd (t) values of +5.86–+9.85, reflective of a MORB source with the input of a slab-derived component in a continental back-arc basin setting. The ∼257 Ma andesites contain older captured zircon grains, and have arc-like geochemical signature with weakly positive eNd (t) values of +0.26–+0.38, zircon eHf (t) values of −0.5–+1.4 and δ18O values of 6.35–7.16‰, which were derived from a metasomatized mantle wedge modified by the sediment-derived component in a continental arc setting. The ∼240 Ma gabbro-dolerites exhibit enrichment in LILEs, depletion in HFSEs, negative eNd (t)-eHf (t) values (−8.45 to −1.05 and −5.9 to −2.7, respectively) and crustal-like δ18O values (7.26–8.70‰), indicative of a post-collisional environment in response to the asthenosphere upwelling shortly after the closure of the Bangxi-Chenxing back-arc basin. The Carboniferous-Triassic mafic-intermediate igneous rocks thus record the Carboniferous-Permian arc-basin system and its Triassic closure. The Bangxi–Chenxing tectonic zone separated South Hainan from North Hainan and westerly extended to the Song Ma–Ailaoshan suture zone.

New U-Pb geochronological, elemental and Nd-Hf isotopic data for the metamorphic mafic rocks from the Baoban Complex in southwest Hainan provide new constraints on their petrogenesis and geodynamic setting. Magmatic zircons from the representative samples yielded weighted mean 207Pb/206Pb ages of 1441–1424 Ma, representing their crystallization ages. These samples show subalkaline to alkaline characteristics and can be geochemically subdivided into three groups in spite of similar formation time. Group 1 has TiO2 of 2.85–3.18 wt.%, Nb of 20.5–21.6 ppm, Nb/U of 45–49, Nb/La of 1.22–1.37, eNd (t) of +6.8–+6.9 and zircon eHf (t) of +9.3–+12.7, suggestive of the derivation from OIB-like source. Group 2 shows Nb contents ranging from 1.0 ppm to 12.6 ppm, eNd (t) values from +5.5 to +5.9 and zircon eHf (t) values from +10.3–+12.4 with right-sloping REE-normalized pattern, indicative of an E-MORB source. Group 3 is characterized by negative Nb-Ta anomalies with Nb/La ratios of 0.51–0.88 and eNd (t) value of ∼+5.1, indicative of the derivation from a previously-metasomatized mantle source. The coeval mafic and felsic rocks in the Baoban Complex are geochronologically and geochemically comparable to the Mesoproterozoic intra-plate magma in southwest Laurentia. The Mesoproterozoic Baoban igneous rocks are generated in a syn-rift setting in response to the Columbia breakup. Southwest Hainan might connected with west Laurentia and Australia at ∼1.43 Ga.

The Kuching zone, the most important tectonic boundary in Borneo, separates the Sibu zone in Central Sarawak from the Schwaner Mountains granitoids in SW Borneo. It potentially provides a key window for probing the paleo-Pacific subduction process. This paper presents new geochronological, elemental and Sr Nd isotopic data for the Pakong-Serabang and Serian mafic rocks, along with detrital zircon U Pb age-data for the associated greywacke in the Sarawak Kuching zone. These mafic rocks are dated at ~77–98 Ma and show distinctive geochemical signatures. The Pakong- Serabang mafic rocks are subalkaline basalt with SiO2 = 46.22–51.75 wt%, TiO2 = 0.49–2.42 wt% and MgO = 6.30–10.54 wt%. They exhibit MORB-like PM-normalized patterns with depletion in LILEs and HFSEs. Their 87Sr/86Sr(t) and eNd(t) range from 0.70285 to 0.70728 and + 7.4 to +9.4, respectively, originating from a slab-derived fluid-modified MORB-like source. The late Cretaceous Serian Volcanics are marked by SiO2 of 51.52–57.90 wt%, MgO of 3.50–5.06 wt% with mg-number = 36–46 and show arc-like geochemical signatures with (Nb/La)n = 0.37–0.47. They have 87Sr/86Sr(t) = 0.71188–0.71417 and eNd(t) = −11.1 ~ −7.3, and were derived from a mantle wedge with the input of recycled sedimentary components. The detrital zircon grains from the associated sandstones give similar age-spectra to those in East Peninsular Malaysia and SE Vietnam. Our data revealed a Cretaceous Kuching trench-arc system. In combination with other geological observations, it is concluded that NW Borneo to the southwest of the Sarawak Kuching zone was a part of the Indochina/East Peninsular Malaysia fragment in SE Asia prior to the Jurassic and re-activated as an active continental margin in the Cretaceous. Widespread accretionary orogenesis along the Kuching zone likely initiated at or before the early Cretaceous and ended no earlier than the latest Cretaceous in response to the westward subduction of the paleo-Pacific slab.

The Chang’an Au deposit is a large Au deposit located in the south of the Ailaoshan-Red River tectonic zone, southwestern margin of the South China block. The deposit exhibit characters similar to Carlin-type Au deposits in Nevada and Dian-Qian-Gui “Golden Triangle”, including tectonic evolution (deformed and rifted cratonic margins), orebody controlled by lithology and structure and invisible Au ionically bound in arsenian pyrite and pyrite. Mineralization is controlled by a strike-slip fault and occurs as barren milky quartz veins, disseminated gold-bearing pyrite and arsenopyrite in the Early Paleozoic sandstone, siltstone and limestone. Gold is invisibly and ionically bound in arsenopyrite and pyrite or locally occurs as isolated native gold crystals (micron scale). Wall rock alteration includes sulfide, silicic, sericite and decarbonized alterations. Three hydrothermal mineralizing stages (I, II, III) can be identified: milky quartz-pyrite (I), quartz-sericite-pyrite-arsenopyrite (II) and dolomite-calcite-quartz (III). Different from those deposit in Dian-Qian-Gui, numerous plutons intruded in and around the mine. The syenite dikes are pyritized, thus, formed prior to the mineralization while lamprophyre dikes are unaltered and crosscut by the syenite dikes. The syenite dikes yielded zircon U − Pb ages of ca. 35.8 Ma and the lamprophyre yielded similar Ar-Ar age of ca. 36.6 Ma, largely constraining the age of Au mineralization. In situ S–Pb isotopic compositions for ore pyrite and arsenopyrite were measured by fs-LA-MC-ICP-MS. Sulfides from Chang’an show a uniform range from –6.4‰ to + 4.6‰, a typical characteristic of magmatic-derived S. Such sulfur isotopic compositions are similar to those of sulfides from Carlin-type Au deposits, but different from those of sulfides from orogenic-type deposit, which commonly exhibit a wide range of δ34S values. Pb isotopic ratios of pyrite and arsenopyrite are similar to those of the intrusive rocks in the deposit with more radiogenic Pb, suggesting that the metal is probably sourced from the coeval intrusive rocks and sedimentary wall rocks. Combined with the low Au concentrations of the sedimentary wall rocks, Au in the Chang’an deposit are probably sourced from the intrusive rocks. Differences between the Chang’an deposit and typical orogenic-type Au deposit include: (1) Orebodies in the Chang’an deposit exhibit transitional contact relationship with the host rocks and boundaries between them are defined by cut-off grade while these boundaries in typical orogenic-type deposits are clearly defined. (2) Sulfidized and decarbonized alterations are quite significant in the Chang’an deposit but commonly absent in typical orogenic-type deposits. (3) The Chang’an deposit have lower CO2 contents of ore-forming fluids and more shallow ore-forming depth relative to typical orogenic-type deposits. Based on the ore deposit geology, dating and isotopic data, the Chang’an mineralization is similar to typical Carlin-type gold deposit, formed by ore fluids originated from a deep magmatic S source in an post-collisional intracontinental setting at ca. 36 Ma.

The consumption of the East Paleotethyan Ocean was the final step in creating the united SE Asia continent. However, uncertainties remain as to the significance of Paleotethyan-associated granitoids and when ocean closure finally occurred in west Indonesia and Malaysia. This paper presents new U-Pb geochronological, petrologic, elemental and Sr-Nd-Pb-Hf isotopic results for granitoids from west Kalimantan, West Java, Bangka-Belitung and Peninsular Malaysia. These granitoids are geochemically subdivided into the Eastern and Main Range granites. The Eastern granites include the granitoids from west Kalimantan, West Java and East Peninsular Malaysia and are dated at ~256–207 Ma, ~208–207 Ma and ~289–217 Ma, respectively. They have eNd(t) = +0.2 ~ −9.1, with two-peaks of +0.1 and −5.6, (206Pb/204Pb)i = 18.64–19.49, (207Pb/204Pb)i = 15.61–15.78 and (208Pb/204Pb)i = 38.56–39.47, and originated from a mixed source involving juvenile crust and Indochina basement. The Main Range granites occur in Bangka-Belitung (West Indonesia) and West Peninsular Malaysia, and have crystallization ages of ~226–198 Ma. They share the similar Sr-Nd-Pb-Hf isotopic compositions with those in the South China and Inthanon Triassic granites, with 87Sr/86Sr(t) = 0.70725–0.72652, eNd(t) = −5.6 ~ −10.8, ∆8/4 = 28.1–109 and ∆7/4 = 13.7–23.6, and were derived from a meta-greywacke source. Our data reveals the development of the Paleotethyan-associated Permo-Triassic granitoids in West Java, Belitung and west Kalimantan. The Bentong-Raub suture forms the East Paleotethyan suture boundary in Peninsular Malaysia, and extends southwards into the easternmost Bangka-Belitung Islands (and even West Java). To the north, it links to the Inthanon-Changning-Menglian suture in NW Thailand and SW China. It is concluded that the temporal pattern of the East Paleotethyan Ocean in west Indonesia and Malaysia correlates with that in NW Thailand and SW China. Its evolution switched from subduction to assembly of the Indochina-East Malaysia and Sibumasu at ~230 Ma. The Eastern granites might reflect active margin processes in response to the eastward subduction of the Paleotethyan Ocean, and the Main Range granites represent the post-collisional igneous rocks.

Early Mesozoic volcanic rocks in northwestern Laos are rarely reported and important for investigating the Paleotethyan tectonic evolution and regional correlation in SE Asia. In this study, New zircon ages and whole-rock geochemical data are presented for the newly identified volcanic rocks in northwestern Laos. Twelve volcanic rocks yield zircon U–Pb ages of 228–234 Ma. The volcanic rocks in northwestern Laos contain basalt, basaltic andesite, andesite, and rhyolite and can be geochemically subdivided into three groups. Both Group 1 and 2 samples belong to the calc-alkaline mafic-intermediate volcanic rocks. Group 1 has eNd (t) values of −0.52 to +1.20, eHf (t) values of +0.9 to +3.6 and δ18O values of 6.3–7.0‰. In contrast, Group 2 has lower eNd (t) values of −2.82 to −5.89 and eHf (t) values of −4.0 to +1.4 and higher δ18O values of 6.6–8.1‰ than Group 1. Group 3 samples are high-K calc-alkaline rhyolites with high eNd (t) (−0.08 to +0.74) and positive eHf (t) (+4.6 to +13.2) values. Geochemical characteristics suggest that the Group 1 and 2 samples were derived from a metasomatized lithospheric mantle modified by sediment-derived components, and Group 3 rhyolites were derived from partial melting of juvenile mafic rocks with a crustal component. Our results, along with the geological observations and reported ages suggest that the Late Triassic volcanic rocks in northwestern Laos were formed in a post-collisional setting, and provide the key studies for the connection between the Lancangjiang and Chiang Khong-Lampang-Tak zones.

Volcanic rocks preserved in the Lampang–Den Chai area in NW Thailand are important components of the giant Paleotethyan igneous belt. Constraining their age and petrogenesis is critical for better understanding their temporal-spatial relationship with the Lancangjiang igneous zone and the Paleotethyan tectonic evolution in SE Asia. The volcanic suite is constituted by intermediate to acid rocks with zircon U–Pb ages of 240.4 ± 1.7 Ma and 240.6 ± 1.9 Ma for the representative andesitic and rhyolitic samples, respectively. Volcanic sequence is dominated by calc-alkaline andesites, dacites and rhyolites. The andesitic and dacitic samples are characterized by high Mg# (37–57) and TiO2 (0.91–1.59 wt%), and can be classified as high-Mg series. They are enriched in LILEs and LREEs and depleted in HFSEs. Representative andesitic samples have 87Sr/86Sr (i) ratios of 0.70398–0.70567, eNd (t) values of +3.6–+3.9, zircon eHf (t) values of +2.8–+8.0 and δ18O values of 7.01–8.11‰, respectively. The rhyolitic samples are characterized by high Mg# (38–70) and low TiO2 (0.25–0.61 wt%). They are enriched in LILEs and LREEs, along with 87Sr/86Sr (i) = 0.70468–0.70645, eNd (t) = +2.0–+4.3 and zircon eHf (t) = +5.7–+13.6. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a newly modified mantle source by slab-derived fluids and recycled sediments, and rhyolitic samples were derived from juvenile mafic crust. It is proposed that the Middle Triassic high-Mg volcanic rocks in the Lampang–Den Chai area formed in response to slab roll-back during transition of tectonic regime from subduction to continental collision between the Sibumasu and Indochina blocks. These rocks constitute part of the Chiang Khong–Lampang–Tak igneous zone, and can northerly link with the Lancangjiang igneous zone and southerly extend to the Chanthaburi, Malaysia and Singapore areas.

The tectonic transition from the palaeo-Tethyan to palaeo-Pacific dynamic domains in the South China Block (SCB) is still a matter of debate. The A-type granites collected from the southeastern SCB offered an opportunity to illustrate this tectonic transition. This article records a set of petrographic, geochronological, and geochemical data for the Wengong granitic pluton from the eastern Nanling Range. LA-ICP-MS zircon U–Pb dating shows a crystallization age of 196.9 ± 4.4 Ma with eHf(t) values ranging from +2.1 to +7.7. The samples have high SiO2, Zr+Nb+Ce+Y, FeOt/MgO, Ga/Al, and Y/Nb and are depleted in Nb–Ta, Zr–Hf, Ba, Sr, Ti, and Eu, similar to those of the A2-type granite. Their initial 87Sr/86Sr ratios range from 0.70885 to 0.70983 and the eNd(t) values range from −2.9 to −1.1, close to those of the Early Palaeozoic mafic rocks in the southeastern SCB. The Wengong A2-type granite was derived from partial melting of the mafic rocks underplated into the lower crust during the Early Palaeozo...

The Indochina block is one of a number of continental fragments that lay along or outboard of the East Gondwana northern margin. Its early Paleozoic tectonic nature and the relationship with East Gondwana remain poorly-known. This paper shows the general geology of northeastern Indochina and presents a set of new geochronological and geochemical data with respect to its early Paleozoic tectonics in East Laos. These results reveal early Paleozoic (~485-410 Ma) igneous activity in the northeastern Indochina, which are divisible into four geochemical groups. The ~450-434 Ma Group 1 and ~485-434 Ma Group 2 mafic-intermediate igneous rocks were derived from MORB-like and supra-subduction zone wedge source modified by the recycled-sedimentary components, respectively. The Group 3 rhyolites at ~424 Ma represent the melting products of juvenile mafic crust. The Group 4 granitoids and equivalent volcanics formed at ~485-407 Ma and were derived from crustal materials with variable input of juvenile crust. The detrital-zircon age-spectra are marked by peaks at ~560 Ma, ~960 Ma, ~1600 Ma, ~1850 Ma and ~2450 Ma for the pre-Silurian paragneiss in the NE Indochina, reflective its geographical proximity to the India-Antarctica segment of East Gondwana until the earliest Paleozoic. The age-peaks of ~447 Ma and ~780 Ma is additionally given for the detrital-zircon from the Silurian sandstones in the Truong Son zone. An accretionary assemblage model is proposed for the early Paleozoic tectonic pattern of the SW Indochina with Yangtze fragments. Such an orogenesis formed the part of the North Indo-Australie Orogen related to subduction of the Prototethyan Ocean and the subsequent accretion of Asian micro-continental fragments along the Peripheral East Gondwana until earliest Devonian.

The Neoproterozoic tectonic pattern of the South China Block (SCB) is considered to be a key element in the Rodinia reconstruction and subsequent Gondwana assembly. Numerous studies have been done on the Neoproterozoic igneous rocks along the SE and NW Yangtze Block of the SCB, but the contemporaneous igneous rocks in the SW Yangtze Block has poorly been known so far. This paper documents a comprehensive set of U–Pb geochronological, elemental and Sr–Nd–Hf isotopic data for the granitic gneiss and migmatite from the Ailaoshan tectonic zone of the SW Yangtze Block on the basis of field investigations. These granitic rocks are mainly granodiorite, monzogranite and quartz diorite and spatially associated with the synchronous mafic and dioritic rocks. On the basis of the A/CNK values and CIPW-normative corundum contents, these samples are subdivided into Group 1 with A/CNK > 1.1 and corundum of 1.5–6.5 vol.% and Group 2 with A/CNK = 0.87–1.09 and corundum = 0–1.5 vol.%, generally similar to the S- and I-type granitic rocks, respectively. The representative samples from Group 1 and 2 give the weighted mean ages of 810–785 Ma and 790–764 Ma, respectively, interpreted as the crystallization ages. They show strong Nb–Ta and P–Ti negative anomalies and variable Eu/Eu∗ (0.10–0.92). The eNd(t) and eHf(t) values are in the ranges of −6.5 ∼ −4.2 and −3.8 ∼ +3.1 for Group 1 and of −6.2 ∼ +1.8 and −6.3 ∼ +5.7 for Group 2, respectively. These Neoproterozoic granitic rocks might originate from a hybridization source of metagreywacke with proportionally “ancient” mafic component for Group 1 but with abundant juvenile mafic-derived rocks for Group 2. Our data, along with the available geological observations, demonstrate the extensive occurrence of the Neoproterozoic (761–833 Ma) igneous rocks in the Ailaoshan tectonic zone. The previously-mapped Ailaoshan and Yaoshan groups might be the Complexes including the Neoproterozoic, Triassic and Cenozoic igneous rocks. A persistent and long-lived Neoproterozoic (∼830–760 Ma) subduction zone along the southwestern and northwestern margins of the Yangtze Block from Ailaoshan to Panxi and then to Hannan is proposed. The Yangtze Block might locate at the periphery of Rodinia during the early Neoproterozoic.

Volcanic rocks are widely exposed within the Chiang Khong–Lampang–Tak igneous zone in NW Thailand. A representative rhyolite sample from the Chiang Khong area yielded a zircon U–Pb age of 230.7 ± 1.1 Ma (n = 20, MSWD = 0.98). The Chiang Khong rhyolites are characterized by low TiO2 (0.29–0.62 wt%) and MgO (0.04–0.82 wt%) with A/CNK values of 0.95–1.06 (one outlier at 1.47), and can be classified as highly fractionated I-type rhyolites. They are enriched in LILEs and LREEs, and depleted in HFSEs. Two representative samples have 87Sr/86Sr (i) ratios of 0.70497 and 0.70527, and the eNd (t) values fall at +1.1 and +1.3, respectively. eHf (t) and δ18O in zircon are between +4.7 to +12.0 and 5.3 to 5.9 ‰, respectively. Our geochemical data suggest that the Chiang Khong rhyolites formed by partial melting of juvenile mafic lower crust in a post-collisional setting. Deep crustal anatexis was probably induced by upwelling asthenospheric mantle, shortly after slab detachment subsequent to closure of the Paleo-Tethys.

The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand (the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone). Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high FeOt (20.96 wt.%–25.56 wt.%) and TiO2 (6.07 wt.%–6.34 wt.%) and low SiO2 (38.54 wt.%–43.46 wt.%) and MgO (1.61 wt.%–2.40 wt.%) contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the “normal” Bowen trend. Their chondritenormalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial 87Sr/86Sr ratios and eNd(t) values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB (oceanic-island basalt). These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.

The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ∼259 Ma, and four subordinary age-peaks at ∼1 859, ∼941, ∼788, and ∼447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.

The volcanic rocks exposed within the Chiang Khong–Lampang–Tak igneous zone in NW Thailand provide important constraints on the tectonic evolution of the eastern Paleotethys ocean. An andesite sample from the Chiang Khong area yields a zircon U–Pb age of 229 ± 4 Ma, significantly younger than the continental-arc and syn-collisional volcanic rocks (ca. 238–241 Ma). The Chiang Khong volcanic rocks are characterized by low MgO (1.71–6.72 wt.%) and high Al 2 O 3 (15.03–17.76 wt.%). They are enriched in LILEs and LREEs and depleted in HFSEs, and have 87 Sr/ 86 Sr (i) ratios of 0.7050–0.7065, e Nd (t) of − 0.32 to − 1.92, zircon e Hf (t) and δ 18 O values of 3.5 to − 11.7 and 4.30–9.80 ‰, respectively. The geochemical data for the volcanic rocks are consistent with an origin from the enriched lithospheric mantle that had been modified by slab-derived fluid and recycled sediments. Based on available geochronological and geochemical evidences, we propose that the Late Triassic Chiang Khong volcanic rocks are equivalent to the contemporaneous volcanic rocks in the Lancangjiang igneous zone in SW China. The formation of these volcanic rocks was possibly related to the upwelling of the asthenospheric mantle during the Late Triassic, shortly after slab detachment, which induced the melting of the metasomatized mantle wedge.

The Jiangnan Orogen is traditionally considered the suture zone, along which the Yangtze and Cathaysia blocks collided to form the united South China Block. There are still major controversies surrounding the pre-collisional tectonic pattern and amalgamation process along the Orogen and the relationship of the united South China Block (SCB) with the Rodinia. Our recent field investigations identified the presence of the volcano-sedimentary unit of the Lengjiaxi Group at Jianxichong, with the thickness of over 1700 m. A set of new zircon U–Pb geochronological, whole-rock elemental and Sr–Nd isotopic data for the metabasites have been obtained and presented in this paper. The layered diabase and tuffaceous samples from the Jianxichong volcano-sedimentary unit yielded the weighted mean ages of 837 ± 5 Ma and 835 ± 6 Ma, respectively. The U–Pb ages of detrital zircons from the phyllite interlayer form a main age-cluster at the peak age of 870 Ma with the youngest apparent age of 845 ± 23 Ma. This indicates that the metabasites in the Jianxichong volcano-sedimentary unit formed at ∼845–835 Ma rather than in the previously-thought Neoarchean in the Central Jiangnan Orogen. The metabasitic samples have SiO2 ranging from 48.78 wt.% to 53.72 wt.% and MgO from 7.71 wt.% to 12.74 wt.% with mg-number of 63–71, and hence can be classified as tholeiitic basalt and basaltic andesite. They demonstrate a sub-parallel and right-sloping chondrite-normalized REE pattern with slightly negative Eu anomalies and sub-parallel spiky PM-normalized patterns with enrichment in LILEs, depletion in HFSEs and P–Ti negative anomalies, similar to those of arc volcanic rocks. Their ɛNd(t) values range from −1.64 to −0.37 and initial 87Sr/86Sr values from 0.7046 to 0.7081, respectively. Such crust-like geochemical characteristics might have been inherited from a sub-arc source modified by fluid/melt released from the subduction sedimentary component. In conjunction with available data and geological observations, we herein propose the development of the early Neoproterozoic (∼840 Ma) continental arc-basin system along the Central Jiangnan Orogen in response to an arc-continent accretionary assembly along the Rodinia exterior.

The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event in Southwest China, but the event of the subduction preceding the final collision is still not well-constrained. The mafic blueschists exposed in the Lancang accretionary complex provide crucial records of the Paleotethyan subduction process. In this paper, we present a set of new petrologic, geochronological and geochemical data for the Suyi mafic blueschists in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25–30%), albite (15–20%), epidote (25–30%), phengite (5–10%), chlorite (~ 5–10%), and minor amounts of actinolite, apatite, sphene, zircon, ilmenite, quartz and secondary limonite. This suggests a prograde metamorphism from ~ 0.5 to ~ 0.9 GPa and retrograde metamorphic overprinting (back to ~ 0.6 GPa) within the temperature range of 300–450 °C. The Suyi blueschists give a zircon U–Pb age of 260 ± 4 Ma and glaucophane minerals formed during prograde metamorphism yield a 40Ar/39Ar plateau age of 242 ± 5 Ma (MSWD = 0.77; P = 0.54). The blueschists have geochemical compositions of subalkaline basalt and show typical OIB-type REE and multi-elemental patterns and eNd(t) values ranging from + 3.35 to + 4.85. Based on available data, it is inferred that the protolith formed at 260 Ma and originated from a basaltic seamount. The basaltic rocks subducted down to 30–35 km depths beneath the Lincang arc to form the epidote blueschists at ~ 242 Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the subducted slab and the continent–continent collision in the middle–late Triassic. These results provide a systematic constraint on the tectonic evolution and temporal framework of the southeastern Paleotethyan belt in Southwest China.

The early Paleozoic tectonic nature and subsequent orogenic process remain poorly known in the Indochina interior. This study introduces the general geology of South Laos and presents a set of new zircon U Pb and Hf O isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic data for the mafic-intermediate igneous rocks in South Laos. The representative samples from the previously-mapped Ordo-Silurian strata in the southern Truong Son zone give the formation ages of 434–438 Ma. Five samples from the Tam Ky-Phuoc Son tectonic zone yield zircon U Pb ages of 447–485 Ma. The corresponding zircon eHf(t), δ18O and TDM values are in range of +1.6~ −6.5, 6.1–8.6‰, and 1.02–1.79 Ga, respectively. Forty-five mafic-intermediate igneous rocks in South Laos with mg-number = 42–69 have SiO2 = 53.04–61.66 wt%, Al2O3 = 12.13–20.92 wt% and MgO = 2.73–9.55 wt%, and are characterized by enrichment in LILEs and depletion in HFSEs with Nb Ta negative anomalies. Their (87Sr/86Sr)i ratios range from 0.7044 to 0.7109 and eNd(t) values from −6.2 to −2.6, with (206Pb/204Pb)i = 18.08–18.68, (207Pb/204Pb)i = 15.50–15.77, and (208Pb/204Pb)i = 38.04–38.97. Ten gabbroic samples, with mg-number = 77–79, show similar incompatible elemental ratios and Sr-Nd-Pb-Hf-O isotopic compositions with the other mafic-intermediate igneous rocks, but have lower SiO2 and higher MgO, Cr, Ni and V contents, reflective of a primary mafic magma. Such characteristics suggest a newly-modified wedge source by recycled sediment-derived components. In combination with other geological observations in Central Vietnam, we propose a northerly-younging early Paleozoic igneous event in response to the on-growing accretionary orogenesis in the Indochina interior. The southward consumption of the early Paleozoic Tam Ky-Phuoc Son Ocean in the Indochina interior continued until at least ~430 Ma and the assemblage of the Northern Indochina with the Kontum terrane likely terminated at the early Devonian.

Southeast Asia was mainly created by the late Paleozoic assemblage of the Eastern Cimmeria with other continental fragments. However, the tectonic nature and geodynamic setting of the key tectonic zones in the Indochina block are still in dispute, hindering the understanding of the Eastern Paleotethyan evolution of Southeast Asia. This study presents a set of new geochronological, elemental and Sr-Nd isotopic data for the late Paleozoic mafic rocks in the Nan and Luang Prabang tectonic zones. The mafic rocks from the Nan and Luang Prabang zones give zircon U-Pb ages of 279–294 Ma and 265–294 Ma, respectively, with xenocryst ages of 302–2524 Ma. They have SiO2 = 45.99–57.47 wt%, Al2O3 = 12.74–21.06 wt%, FeOt = 8.34–15.03 wt% and MgO = 2.79–14.74 wt% with mg-number of 34–74, and fall into the fields of basalt/basaltic andesite and its equivalents. These rocks are characterized by enrichment in LILEs and LREEs, depletion in HFSEs, low initial 87Sr/86Sr ratios (0.7032–0.7057) and high eNd(t) values (+4.8 to +7.4). They might be derived from a mantle wedge source modified by slab-derived components, and formed in a continental back-arc basin setting. Our analysis indicates the development of the Carboniferous-Permian mafic igneous event in the Nan and Luang Prabang tectonic zones, and that the Nan tectonic zone linked northerly with the Luang Prabang zone during the late Paleozoic, tectonically comparable to the Banpo-Nanlinshan back-arc basin in SW Yunnan, SW China. The back-arc basin finally closed in the early-middle Triassic and the subsequent assemblage continued till the latest Triassic in response to the eastward subduction of the Paleotethyan slab.

Geochemical and isotopic data for metabasic rocks in NW Yunnan constrain the nature of the mantle source beneath the East Himalayan Syntaxis during the Neotethyan subduction and the timing of initial collision between India and Asia. Our results indicate that these mafic rocks crystallized at 50–55 Ma and were metamorphosed at ~ 39 Ma, contemporaneous with magmatic flare-ups and high-grade metamorphism in South Tibet. The mafic rocks in NW Yunnan are divisible into three groups. Group 1, from Nabang, shows geochemical affinity to a MORB-like source modified by the recycled component with e Nd (t) of + 5.1 to + 9.5 and e Hf (t) of + 5.1 to + 11.6, and resembles back-arc basin basalt. Group 2 is mainly from Jinzhouzhai and Tongbiguan to the east of Group 1, and is marked by high Al 2 O 3 , enrichment in LILEs and depletion in HFSEs, along with negative e Nd (t) (− 1.1 to − 5.6) and e Hf (t) (− 4.6 to − 0.9) values. Group 2 is interpreted to have originated from the lithospheric mantle modified by input from a slab-derived component in a continental arc setting. Group 3, from Nanjingli, is located a considerable distance east from Group 1, and exhibits negative e Nd (t) (− 6.7 to − 7.6) and e Hf (t) (− 7.1 to − 2.2), and is inferred to be derived from the Tengchong lithospheric mantle but with limited involvement of a slab-derived component. Overall Nd–Hf isotopic compositions show a sharp decrease from Group 1 to Group 2 and then a more subdued decrease to Group 3, with the three groups corresponding to an overall spatial distribution from west to east. Our results suggest that early Eocene magmatism in NW Yunnan represents the eastward continuation of the Gangdese magmatic belt and that the Neotethyan subduction continued until ~ 50 Ma followed by the India–Asia collision. It is proposed that at least two eastward-dipping subduction zones are involved in the Neotethyan suprasubduction system prior to 55 Ma. The sudden decrease in convergence rate in the early Eocene (55–50 Ma), together with the inferred positive buoyancy of the Tengchong lithosphere, stimulated rollback of down-going slab and induced melting of heterogeneous mantle sources to result in a flare-up in magmatic activity.

Financial support from National Science Foundation of China (41190073, 41372198 and 40490613), National Basic Research Program of China (2014CB440901 and 2016YFC0600303) and “the Fundamental Research Funds for the Central Universities to SYSU” are gratefully acknowledged. The basaltic rocks in NW Thailand belong to part of giant Southeast Asian igneous zone that delineates the extension of the Paleotethyan Ocean from SW China into NW Thailand. The Chiang Mai basaltic samples from the Chiang Dao, Fang, Lamphun and Ban Sahakorn sections are divisible into two groups of high-iron basalt. Group 1 has SiO2 of 38.30–49.18 wt.%, FeOt of 13.09–25.37 wt.%, MgO of 8.38–1.60 wt.%, TiO2 of 3.92–6.30 wt.%, which is rarely observed in nature. Group 2 shows SiO2 = 44.71–49.21 wt.%, FeOt = 10.88–14.34 wt.%, MgO = 5.24–16.11 wt.%, TiO2 = 2.22–3.07 wt.% and mg# = 44–70. Olivine and pyroxene are responsible for the fractionation of the Group 2 magma whereas low oxygen fugacity during the late-stage differentiation of the Group 1 magma prolonged fractionation of ilmenite and magnetite. The onset of ilmenite and magnetite fractionations controls the distinct differentiation commencing at MgO = ~ 7 wt.%. Both groups show similar REE and primitive mantle-normalized patterns with insignificant Eu, Nb-Ta and Zr-Hf anomalies. They have similar Nd isotopic compositions with εNd(t) values ranging from + 2.8 to + 3.7 and similar Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y and Zr/Y, resembling those of OIB-like rocks. The representative basaltic sample yields the argon plateau age of 282.3 ± 1.4 Ma, suggestive of early Permian origin. Our data argue for Group 1 and Group 2 are coeval in the intra-oceanic seamount setting within the Paleotethyan Ocean, which at least continued till 283 Ma. These data, along with other observations, suggest that the Inthanon zone defines the main Paleotethyan suture zone, which northerly links with the Changning-Menglian zone in SW China. Postprint

The South China Block (SCB) is composed of the Yangtze Block in the northwest and the Cathysia Block in the southeast. The earliest Neoproterozoic granitic rocks in the SCB are important for our understanding of the tectonic regime and location of this ancient continent in relation to the Rodinia supercontinent. However, such rocks have rarely been reported for the Cathaysia Block so far. In this study, six granitic gneisses from the Wuyi-Yunkai domain gave zircon U–Pb ages of 985–913 Ma, indicating the presence of the early Neoproterozoic granitic magmatism in the Cathaysia interior. These granitic gneisses are peraluminous with high FeOt, MgO, TiO 2 and CaO/Na 2 O but low SiO 2 , Al 2 O 3 /TiO 2 , Al 2 O 3 /(MgO + FeOt) and Rb/Sr ratios. They show steep REE chondrite-normalized patterns with δEu values of 0.35–0.70. These samples have strongly negative Ba, Sr, Nb-Ta, P and Ti anomalies in primitive mantle-normalized diagrams, and ɛNd( t ) values of −9.54 to −3.81, similar to those of the Kwangsian granitic rocks and Precambrian volcanosedimentary package in the Cathysia Block. The zircon grains with the earliest Neoproterozoic age gave ɛHf(t) values of −9.64 to +2.78 with the peak values at −2.5 and −7.5, and Hf model ages of 1.35–2.00 Ga with the two-peaks of 1.57 Ga and 1.75 Ga, respectively. These data suggest that the granitic gneisses originated dominantly from a binary mixing of metapelitic and metaigneous components, with insignificant input of juvenile materials. We consider that these rocks are closely related to the closure of the earliest Neoproterozoic (∼980 Ma) Wuyi-Yunkai back-arc basin in an accretionary continental margin setting. In combination with other data, it is proposed that the previously-defined Cathaysia block might consist of various sub-blocks separated by the arc-back-arc basin prior to the early Neoproterozoic period. The Wuyi-Yunkai arc-back-arc system was finally closed at ∼920 Ma, significantly younger than the classical Grenvillian Orogenic event, and thus the SCB was located on the margin of the Rodinia supercontinent.

This study was supported by China Natural Science Foundation (41190073 and 41372198), National Basic Research Program of China (2014CB440901) and Natural Environment Research Council (grant NE/J021822/1). Lenses of amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U-Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping amphibolite, has mg-number of 71-76 and (La/Yb)cn ratios of 7.2-7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3-18.4 ppm), resembling Nb-enriched basalts. The epsilon(Nd)(t) values for Group 1 range from -3.45 to -2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U-Pb zircon age of 803 7 Ma, whereas two samples for Group 2 give U-Pb zircon ages of 813 +/- 11 Ma and 814 +/- 12 Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these amphibolites are inferred to constitute part of an early Neoproterozoic (similar to 815-800 Ma) arc-back-arc basin system. The Neoproterozoic amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block. (C) 2014 Elsevier B.V. All rights reserved. Postprint

This study was jointly funded by the National Science Foundation of China (41190073), National Basic Research Program of China (2014CB440901 and 2016YFC0600303) and the Fundamental Research Funds for the Central Universities to SYSU. New U-Pb geochronological, petrologic, elemental and Sr-Nd-Hf-O isotopic data for the granites from the Inthanon and Sukhothai zones in NW Thailand in conjunction with correlations with SW China are presented to constrain the age and position of the Paleotethys Ocean in this region and the associated assembly of Southeast Asia. The geochronological data show that the granitic rocks in the Inthanon and Sukhothai zones, herein named Group 1 and Group 2 granites, respectively, yield similar crystallization ages of 230-200 Ma. Group 1 samples are characterized by monzogranite and granite with I- and S-type geochemical affinity and Group 2 samples by I-type monzogranite and granodiorite. They have generally similar chondrite-normalized REE and PM- normalized multi-element patterns but distinct Sr-Nd-Hf-O isotopic compositions. Group 1 samples have slightly higher initial 87Sr/86Sr ratios (0.7111- 0.7293) but lower εNd(t) values (-11.1 ~ -14.1) than those of Group 2 samples (87Sr/86Sr(i)=0.7073-0.7278 and εNd(t)=-8.3 ~ -11.0). Group 1 samples show the lower εHf(t) values (-5.4 ~ -18.2), older TDM (1.62-2.40 Ga) and higher δ18O values (+ 7.95-+9.94) than those of Group 2 samples (εHf(t) of -11.1-+4.80, TDM of 0.96-1.95 Ga and δ18O of + 4.95 ~+7.98) for the Triassic crystallization zircons. These geochemical signatures are similar to the Kwangsian and Indosinian granites in the South China and Indochina blocks but distinct from those of the Gangdese I-type granite and Sibumasu Paleozoic granite. Our data suggest that Group 1 samples mainly originated from the early Paleozoic supracrustal rocks containing metapelite and metavolcanic components, which had previously experienced the surface weathering. Group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component. Synthesis of our data with available regional observations indicate that the Inthanon zone represents the main suture zone of the eastern Paleotethyan Ocean in NW Thailand and links with the Changning-Menglian suture zone in SW Yunnan (SW China). In NW Thailand, a switch from the eastward subduction of the Paleotethyan oceanic plate to the collision of the Sibumasu with Indochina blocks occurred at ~ 237 Ma, and syn- and post-collisional time being at ~ 237-230 Ma and ~ 200-230 Ma, respectively. The late Triassic granites in the Inthanon and Sukhothai zones are representative of the post-collisional magmatic products. Postprint

The assembly time and process of the South China Block and its subsequent breakup remain controversial. This paper presents a set of new mineral chemical, zircon U-Pb geochronological, elemental and Sr-Nd-Hf-O isotopic data of the Group 1 Yiyang pillow basalt, Group 2 Aikou and Xinyu mafic–ultramafic rocks, and Group 3 Yuanling-Anjiang gabbro-diabase from the central Jiangnan Orogen. Group 1 is ∼830 Ma high-Mg subalkaline basalt with the mineral assemblage of clinopyroxene, epidote, chlorite and minor plagioclase. It shows arc-like geochemical signatures and derived from a wedge source recently infiltrated by recycled component. Group 2 is mainly characterized by peridotite, pyroxenite, gabbro and diabase and crystallized at ∼791–780 Ma. It has zircon eHf(t) and δ18O values of −0.4∼+5.6 and +4.99∼+5.90‰, respectively. The Group 2 rocks geochemically resemble OIB-like high-Nb/Nb-enriched basalt and originated from an OIB-like origin with the ultramafic rocks being the cumulates of the mafic magma. Group 3 contains normal zoned clinopyroxene and mainly formed at ∼780–786 Ma. It has 87Sr/86Sr(t) = 0.7066–0.7116, eNd(t) = −2.5∼−0.3, zircon eHf(t) = −6.6∼−1.0 and δ18O = +6.06∼+6.65‰, originated from the lithosphere-asthenosphere interaction. These data synthetically reflect the South China Block being created by an early Neoproterozoic accretionary orogen along the periphery of Rodinia and transformed into lithospheric extensional regime since ∼785 Ma in response to Rodinia breakup. Along the Jiangnan Orogen, the Paleo-Huanan Ocean was completely closed by ∼830 Ma. The Yangtze with Cathaysia blocks finally amalgamated at ∼810 Ma and subsequent rifting of the united SCB occurred at ∼785 Ma, respectively.

The Neoproterozoic mafic rocks of the Jiangnan Orogen in South China provide a critical geological record for unraveling regional tectonic history and testing different tectonic models. However, the geochronology, geochemistry and petrogenesis of >835 Ma mafic igneous rocks in the Central and Southwestern Jiangnan Orogen have not been well reported so far, in contrast to the extensive study of such rocks in the Eastern Jiangnan Orogen. An integrated study of geochronology and geochemistry has been carried out for the basaltic rocks from NE Hunan and NW Jiangxi Provinces of the Central Jiangnan Orogen. SHRIMP, SIMS and LA-ICP-MS zircon U–Pb dating of three mafic samples yielded zircon U–Pb ages of 860 ± 20 Ma, 838 ± 12 Ma and 847 ± 18 Ma, respectively. This provides evidence for the development of early Neoproterozoic (∼850 Ma) mafic lava along the Central Jiangnan Orogen. SiO2 ranges from 49.43 wt% to 52.07 wt% and MgO from 7.36 wt% to 9.04 wt% with mg-number of 68–57. Their chondrite-normalized REE patterns exhibit a left-sloping pattern with more depleted LREEs relative to HREEs. On the N-MORB-normalized multi-element patterns, they are characterized by enriched highly incompatible elements, depleted moderately incompatible elements relative to those in N-MORB, insignificant Nb–Ta depletion and remarkably positive Sr and weakly negative Ti anomalies, resembling those of modern Eastern Lau back-arc basin basalts. In addition, these rocks have positive ɛNd(t) values of 1.30–9.36 with initial 87Sr/86Sr ratios in the range of 0.70796–0.71126. The geochemical characteristics above indicate an origination of the MORB-like source modified by the flux of the slab-derived fluid and input of the recycled sediment-derived melt. This finding, together with other geological observations, suggests that the mafic rocks in the Central Jiangnan Orogen might be formed in a back-arc basin in the eastern SCB. In conjunction with other data, it is inferred that the closure of the back-arc basin along the Central Jiangnan Orogen is contemporaneous with that in the Fuchuan area.

In the eastern South China Block (SCB), the presence of middle Paleozoic volcanics and mafic rocks is poor in spite of the extensive occurrence of granites in response to the synchronous (Kwangsian) intracontinental orogen event. The nature of the lithosphere and its associated orogenic process are poorly known for such a major orogen. In this paper, a set of new U–Pb zircon geochronological, elemental and Sr–Nd isotopic data are presented for several gabbroic plutons, which were recently identified in the Cathaysia Block. The representative samples of the Longhugang, Xinchuan and Xinsi plutons yielded the weighted mean 206Pb/238U ages of 423 ± 8 Ma, 434 ± 6 Ma and 420 ± 3 Ma, respectively, and their gneissoid country-rocks are dated at 442–466 Ma. This evidences the presence of Silurian mafic rocks in the Cathaysia Block. These gabbroic rocks can be geochemically classified into three groups. Group 1 from the western Longhugang pluton exhibits flat REE patterns represented by (La/Yb)cn = 1.1–1.7, (Gd/Yb)cn = 0.99–1.15, and δEu = 1.29–1.51 with negative Nb–Ta and P and markedly positive Sr anomalies. It shows enrichment in LILEs and 87Sr/86Sr(t) ratios of 0.70412–0.70727 and eNd(t) values of − 2.6 to − 0.6. Group 2 from the Xinchuan, Zhouya and Yunlu plutons is characterized by high MgO (11.4–15.8 wt.%), mg-number (66–74) and Cr and Ni contents. It is marked by significantly negative Nb–Ta, Zr–Hf and P–Ti anomalies with high 87Sr/86Sr(t) (0.70920–0.71072) and low eNd(t) values (− 3.9 to − 7.9). Group 3 from the Xinsi and eastern Longhugang plutons has eNd(t) values from − 2.8 to − 5.7 and shows high Nb content of 7.05–9.89 ppm with pronounced enrichment in LILE and weak depletion in HFSE, resembling Nb-enriched arc basalt in HFSE composition. A synthesis of these geochemical data points to the following petrogenesis: Groups 1 and 2 derived from a spinel-bearing, plagioclase-rich source and a garnet-bearing, orthopyroxene-rich source, respectively, proportionally modified by slab- and sediment-derived melts plus fluid fluxing from the earliest Neoproterozoic subduction. The HFSE composition for Group 3 inherited from a metasomatized mantle wedge column with the input of large-proportional slab melts and the absence of residual rutile during partial melting. In conjunction with other available observations, it is proposed that there preserved a paleosubduction-modified wedge column beneath the Cathaysia Block, which might be undisturbed until the Kwangsian intracontinental orogen. During the Silurian post-orogenic collapse of the event, the elevated lithosphere thermal boundary facilitated the partial melting of the column to generate these Silurian gabbroic rocks.

The tectonic nature of the Cathaysia Block during earliest Neoproterozoic period has been long disputed and attracted considerable attentions. One reason for this problem is rare exposure and few data for the igneous rocks of Grenvillian age in this Block. In this paper, a set of new U–Pb zircon geochronological, elemental and Nd isotopic data are presented for the metabasites in the Yunkai Group which shows WNW-trending extension with a length of more than 100 km along the northern Yunkai Domain of the Cathaysia Block. The plagioclase amphibolite and metabasite from Shiwo (Beiliu) and Tantu (Xinyi) from the northern Yunkai Domain, representative of Groups 1 and 2, yielded the 206Pb/238U weighted mean ages of 997 ± 21 Ma and 978 ± 19 Ma, respectively, evidencing the presence of the Grenvillian mafic rocks within the Cathaysia Block. Based on the Nb, Nb/La and Nb/U, these samples can be geochemically subdivided into two groups (Groups 1 and 2). Group 1 has Nb/La = 0.33–0.54 and Nb = 1.71–2.85 ppm, and exhibits similar geochemical signatures to those of Saunders island-arc. Group 2 gives Nb/La of 0.57–0.86 and Nb of 7.58–12.70 ppm, resembling to those of Phanerozoic Nb-enriched basalts, and shows the geochemical affinity to the Okinawa back-arc basin basalt. Both groups have positive ɛNd(t) values, ranging from +3.84 to +5.36 and +3.93 to +6.55, respectively. These geochemical and isotopic characteristics suggest that Group 1 might have been originated from a MORB-like source modified by subduction-derived fluids. The source of Group 2 Nb-enriched basalt might be metasomatized by melt released from a newly subducted slab. In conjunction with other geological observations, it is identified for the Grenville-aged (ca. 1.0 Ga) arc and back-arc basin along the northern Yunkai Domain of the Cathaysia Block, analog to the modern West Pacific. The South China Block probably located at the margin of Rodinia supercontinent between west Australia and east India.

The amalgamation process of the Yangtze with Cathaysia Blocks has been one of the disputed issues that have attracted considerable attention due to its significance in understanding the assembly and breakup of the supercontinent Rodinia. However, it has been a matter of debate as to the tectonic nature (plume vs. subduction/collision vs. plate-rift) and timing of the amalgamation (1000 Ma vs. 880 Ma vs. 800 Ma) along the Jiangnan domain. To evaluate the previously proposed models and to constrain the final amalgamation timing of the Yangtze with Cathaysia Blocks, this paper presents a set of integrated geochemical and geochronological data for the high-mg volcanic sequence that is bounded between the Lengjiaxi and Banxi Groups. Two andesitic samples from the volcanic sequence (Cangshuipu) were dated at 824 ± 7 Ma (CSP-35) and 822 ± 28 Ma (CSP-8) using the SIMS and SHRIMP techniques, respectively. These samples have mg-numbers ranging from 71 to 25 with SiO2 of 57.51–73.80 wt%, classified as high-mg andesite and dacite. They show significant enrichment in LILEs and depletion in HFSEs with marked negative Nb–Ta, P and Ti anomalies. Their ɛNd(t) values range from −1.7 to −4.7 and initial 87Sr/86Sr values from 0.7051 to 0.7123. The generation of the Cangshuipu high-mg rocks might have been attributed to an interaction of the subducted sediment-derived fluid with the overlying refractory mantle. The contemporaneous dacitic rocks should be the product of the highly fractional crystallization from the high-mg andesitic magma. Integrated with the regional geology and available geochronological data, we propose that the Cangshuipu volcanic sequence might have formed in a collisional setting related to the Neoproterozoic orogenic process along the Jiangnan domain. The final amalgamation of the Yangtze with Cathaysia Blocks along the Jiangnan domain probably occurred between 815 Ma and 822 Ma (probably 820 Ma) rather than 1000 Ma, ca. 880 Ma or 800 Ma as previously proposed. Taking into account the subsequent development of the Nanhua rifting (probably after 815 Ma), it is inferred that a plate-rift model might be a more reasonable mechanism for the Neoproterozoic tectonic evolution along the Jiangnan domain.

To constrain temporal–spatial patterns of the Kwangsian and Indosinian orogenic events and the Phanerozoic tectonic evolution of the eastern South China Block, this paper documents the zircon U–Pb geochronology of the high-grade metamorphic rocks for key outcrops in the eastern SCB and the P–T path for the Indosinian metamorphic amphibolites. Our data show that the representative amphibolites from the originally-defined Badou, Mayuan, Chencai, Zhoutan and Yunkai Groups and associated granulites yielded zircon U–Pb metamorphic ages of 428–468 Ma. The weighted mean ages of 221–246 Ma are also given by the metamorphic zircons from amphibolites and granulites in the Yunkai, Shiwandashan, northern Wuyi and southern Hunan regions. The two age groups are followed by the 40Ar/39Ar plateau ages of 406–438 Ma and 195–217 Ma for synkinematic minerals from major shear zones in the eastern South China Block, respectively. The age-spans of 406–468 Ma and 195–246 Ma roughly coincide with the formation time of the Kwangsian (396–462 Ma) and Indosinian (202–248 Ma) granites of the eastern South China Block, respectively. The strongly compressive deformation, metamorphism and granitic magmatism are roughly contemporaneous for the Kwangsian and Indosinian tectonothermal events in the eastern South China Block. They occurred over a similar spatial extent, only reaching westward to the Jiangnan–Xuefeng Domains across the Jiangshan–Shaoxing Fault. The amphibolites from Zhouyuan (SW Zhejiang Province) of the Cathaysia Block, which have the Indosinian zircon U–Pb metamorphic ages of 243–246 Ma, underwent three episodes of metamorphism (M1–M3) and yielded a near-isothermal decompression clockwise P–T path. In combination with other geological data, it is proposed that the ages of ca. 430 Ma and ca.230 Ma might be interpreted as the transformational time from the prograde- to retro-metamorphism for the Kwangsian and Indosinian events, respectively. The Kwangsian and Indosinian orogenic events occurred in a similar tectonic setting where the Cathaysia and Yangtze Blocks remained contiguous, and both involved initial crustal thickening followed by rapid exhumation and final cooling, respectively. Their occurrence can be attributed to intra-plate deformation and orogenesis without the Phanerozoic subduction of the Cathaysia Block underneath the Yangtze Block, in response to the far-field stress fields for the subduction/collision beyond the southern margin of the South China Block. Our geochronological data for the amphibolites and granulites also indicate the development of abundant Neoproterozoic (ca. 800 Ma) mafic magmatism in the Cathaysia interior.

To achieve a better understanding of the Kwangsian orogenic event of the eastern South China Block, this paper documents a set of new zircon U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the representative massive granite intrusions across the Jiangshan-Shaoxing fault. The studied samples are classified into two groups, representing the rocks from the Cathaysia Block to the east of the Jiangshan-Shaoxing Fault (Group 1) and those from the eastern Yangtze Block between the Anhua-Luocheng and Jiangshan-Shaoxing faults (Group 2). The Group 1 samples gave the zircon U–Pb ages of 405–454 Ma and e Hf (t) values of − 3.6 to − 15.2 with Hf model ages of 1.6–2.4 Ga. Group 2 yielded the zircon U–Pb ages of 400–432 Ma and e Hf (t) values of − 0.2 to − 12.7 with Hf model ages of 1.3–2.2 Ga. Geochemically, the Group 1 samples (A/CNK = 1.02–1.43) have relatively lower Al 2 O 3 , MgO, CaO, P 2 O 5 and e Nd (t) but higher K 2 O + Na 2 O than those of Group 2 (A/CNK = 0.93–1.44). Both groups show similar chondrite-normalized patterns of rare-earth elements with Eu/Eu* values of 0.15–0.92 and strongly negative Ba, Sr, Nb, P and Ti anomalies in primitive mantle-normalized spider diagrams. Their e Nd (t) values range from − 11.1 to − 8.0 for Group 1, and − 8.9 to − 5.0 for Group 2, generally similar to those of Precambrian paragneiss and contemporaneous gneissoid granites in the eastern South China Block. Our geochronological results indicate that the Kwangsian massive granites in the eastern South China Block were crystallized between 400 Ma and 454 Ma, synchronous to the Kwangsian gneissoid granites along the Wugong and Wuyi-Baiyun-Yunkai domains in the eastern South China Block. The synthesis of these whole-rock geochemical and in-situ zircon Hf isotopic data suggests that both the Group 1 and 2 granites across the Jiangshan-Shaoxing Fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the Precambrian basement. The input of juvenile mantle-derived magma is insignificant. Taking into account other available evidences, it is proposed that the Jiangshan-Shaoxing Fault might be a pre-existing boundary between the Yangtze and Cathaysia Blocks, which was reactivated during the middle Paleozoic (Kwangsian) orogenic event. The massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime.

Gneissoid granites were traditionally thought to be the components of the Precambrian basement in the eastern South China Block, but twenty-four gneissoid granite samples from the Wugong, northern Wuyi, southern Wuyi and Yunkai Domains gave zircon U–Pb ages of 424–456 Ma, 410–457 Ma, 426–438 Ma and 415–450 Ma, respectively. The existence of the abundant Kwangsian gneissoid granites, which have incorrectly been assigned to the Precambrian basement, indicates that the “so-called” Precambrian stratigraphic sequence of the Cathaysia Block should be termed “Complex” rather than “Group”. These gneissoid granites are peraluminous granites with relatively high Al 2 O 3 , MgO, TiO 2 , FeOt and CaO/Na 2 O but low CaO, Al 2 O 3 /(MgO + FeOt) and Rb/Sr ratios. They exhibit strongly negative Ba, Sr, Nb, P and Ti and positive Pb anomalies in primitive mantle-normalized diagrams, and have initial 87 Sr/ 86 Sr ratios ranging from 0.70924 to 0.72935 and negative eNd(t) values from − 6.4 to − 11.4, similar to those of paragneissic enclaves and Precambrian sedimentary rocks in the eastern South China Block. The zircons crystallized in the Kwangsian gneissoid granites gave eHf(t) values ranging from + 2.4 to − 19.4 with the peak at − 4 (almost all grains clustering at − 1 to − 12) and Hf model ages of 1.1–2.1 Ga. The synthesis of these geochemical and in-situ zircon Hf isotopic data indicates that the Kwangsian gneissoid granites dominantly originated from Proterozoic metapelitic and meta-igneous components with insignificant input of juvenile mantle-derived magmas. A model of two-stage crustal anatexis is proposed for the Kwangsian granitic magma in the eastern South China Block, with (1) the formation of 460–430 Ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust, and (2) the generation of the 430–400 Ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust. The crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far-field response to the assembly of the Australian–Indian plate with the Cathaysia Block during middle Paleozoic (Kwangsian) time.