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Various tectonic models have been proposed to account for the widely distributed igneous activities in the southeastern part of the South China Block (SCB) during the Triassic–Jurassic period. One of the major contending debates is on the timing of initiation of the palaeo-Pacific plate subduction under the SCB, due to lack of unequivocal evidence for arc magmatism during the period in this region.The 191 ± 10 Ma (N = 5, MSWD = 12) calc-alkalic high-K I-type Talun metagranite occurs in the southern Tailuko belt of the Tananao metamorphic complex, Taiwan. In terms of age, this metagranite belongs to the Early Yanshanian igneous activity in the southeastern part of the SCB. However, its geographic position does not accord with the well-known general oceanward younging trend of the Yansnanian igneous rocks. In view of the large age uncertainty reported, this metagranite is redated in this study. Some zircons of this metagranite are high in U content and are metamict. Zircons with low U contents are a...

Major and trace elements and whole rock Sr–Nd isotope data of Late Permian silicic plutonic and volcanic rocks from the Phan Si Pan–Tu Le region in NW Vietnam were collected in order to establish their petrogenetic relationship with the magmatic rocks of the Song Da zone (NW Vietnam) and the alkaline silicic rocks of the Panxi area of the Emeishan large igneous province (ELIP) in SW China. The granites and rhyolites have geochemical characteristics of anorogenic granites (e.g. high Fe# and high Ga/Al) and are further subdivided based on mineralogy. The Phu Sa Phin and Muong Hum granites contain sodic to sodic–calcic amphiboles and sodic pyroxene, while the Phan Si Pan granite does not. The Phu Sa Phin and Muong Hum granites occasionally show peralkaline to metaluminous compositions, while the Phan Si Pan granites and the Tu Le rhyolites have metaluminous to peraluminous compositions. The chondrite-normalized REE patterns [(La/Yb)N = 3.6–51.3, (Gd/Yb)N = 1.4–3.4] and the primitive mantle-normalized spidergrams (enrichment in high field strength elements) are similar to those of the Song Da silicic rocks and the Panzhihua and Taihe granites in the Panxi area. Rhyolite-MELTS modeling suggest that the silicic magmas were likely generated by fractionation of the Song Da high-Ti basalts. The eNd(t) values range from weakly negative to moderately positive values (−2.2 to +2.2), suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement. Permian magmatic rocks in the Phan Si Pan–Tu Le region are petrologically, geochemically and geochronologically comparable to those of the inner zone of ELIP. Although Permian lithospheric mantle and lower crustal structure of the Phan Si Pan–Tu Le region may be destroyed by a lithospheric removal in response to Cenozoic India–Eurasia collision, our results along with recent geophysical analyses in NW Vietnam suggest that the upper to middle crust of Phan Si Pan–Tu Le region represent a remnant upper to middle crust of inner zone of ELIP.

In-situ zircon U–Pb and Hf isotope analyses were carried out for alkaline silicic magmatic rocks from the Phan Si Pan-Tu Le region in northern Vietnam to constrain their possible sources and to determine their petrogenetic relationship with the Emeishan Large Igneous Province (ELIP), SW China. Nine granites and nine rhyolites yield zircon 206Pb/238U ages from 262 Ma to 249 Ma, coinciding with the timing of silicic magmatism in the Panxi area of the ELIP. The zircon eHf(t) values (+14 to +3) of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the Panxi area [i.e. eHf(t) = +14 to +4]. The zircon Hf isotope ratios show that the zircons probably record the original source characteristics whereas whole-rock Nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement. The synchroneity, coupled with petrological and geochemical similarities, indicate that silicic rocks from the Phan Si Pan-Tu Le region are cogenetic with the Panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes (i.e. fractionation of mafic magmas or partial melting of mafic rocks). Therefore, we propose that the Phan Si Pan-Tu Le region represents a displaced portion of the ELIP inner zone.

Cordilleran-type batholiths are useful in understanding the duration, cyclicity and tectonic evolution of continental margins. The Dalat zone of southern Vietnam preserves evidence of Late Mesozoic convergent zone magmatism superimposed on Precambrian rocks of the Indochina Block. The Dinhquan, Deoca and Ankroet plutons and their enclaves indicate that the Dalat zone transitioned from an active continental margin producing Cordilleran-type batholiths to highly extended crust producing within-plate plutons. The Deoca and Dinhquan plutons are compositionally similar to Cordilleran I-type granitic rocks and yield mean zircon U/Pb ages between 118 ± 1.4 Ma and 115 ± 1.2 Ma. Their Sr–Nd whole rock isotopes (ISr = 0.7044 to 0.7062; eNd(T) = − 2.4 to + 0.2) and zircon Hf isotopes (eHf(T) = + 8.2 ± 1.2 and + 6.4 ± 0.9) indicate that they were derived by mixing between a mantle component and an enriched component (i.e. GLOSS). The Ankroet pluton is chemically similar to post-orogenic/within-plate granitic rocks and has a zircon U/Pb age of 87 ± 1.6 Ma. Geobarometric calculations indicate that amphibole within the Ankroet pluton crystallized at a depth of ~ 6 kbar which is consistent with the somewhat more depleted Sr–Nd isotope (ISr = 0.7017 to 0.7111; eNd(T) = − 2.8 to + 0.6) and variable eHf(T) compositions suggesting a stronger influence of crustal material in the parental magma. The compositional change of the Dalat zone granitic rocks during the middle to late Cretaceous indicates that the tectonic regime evolved from a continental arc environment to one of post-orogenic extension. The appearance of sporadic post-90 Ma magmatism in the Dalat zone and along the eastern margin of Eurasian indicates that there was no subsequent orogenic event and the region was likely one of highly extended crust that facilitated the opening of the South China Sea during the latter half of the Cenozoic.

To constrain the paleoposition of Indochina within Gondwana during the Early Paleozoic, we performed in-situ U–Pb and Hf isotope analyses on detrital zircons from three river sediment samples in the Truong Son Belt of the Indochina block. The age distributions yield dominant Neoarchean (~ 2.5 Ga), Mesoproterozoic (1.7–1.4 Ga), Grenvillian (~ 0.95 Ga), and Pan-African (0.65–0.5 Ga) age groups and minor Paleo- to Meso-archean zircons. Hf isotope compositions of zircons for each age group exhibit large ranges of eHf (T) , suggesting that the zircon host rocks have diverse sources. The oldest Hf model ages for zircons of Neoarchean, Grenvillian, and Pan-African age group yield ~ 3.7 Ga or older, while those of Mesoproterozoic age group show ~ 3.3 Ga. The remarkable similarity of age distribution and Hf isotope compositions among detrital zircons of Indochina and those of Tethyan Himalaya, western Cathaysia, and Qiangtang suggests that Indochina was located outboard of Qiangtang and south of South China in the Indian margin of Gondwana during the Early Paleozoic. Our results are consistent with the paleontological correlations of east Gondwana margin during the Early Paleozoic.

Taiwan formed during the Plio-Pleistocene collision of Eurasia with the outboard Luzon arc. Its pre-Tertiary basement, the Tananao metamorphic complex, consists of the western Tailuko belt and the eastern Yuli belt. These circum-Pacific belts have been correlated with the high-temperature/low-pressure (HT/LP) Ryoke belt and the high-pressure/low-temperature (HP/LT) Sanbagawa belt of Japan, respectively. To test this correlation and to reveal the architecture and plate-tectonic history of the Tananao metamorphic basement, detrital zircons were separated from 7 metasedimentary rock samples for U–Pb dating by LA-ICPMS techniques. Results of the present study, coupled with previous data, show that (1) the Tailuko belt consists of a Late Jurassic to earliest Cretaceous accretionary complex sutured against a Permian–Early Jurassic marble ± metabasaltic terrane, invaded in the north by scattered Late Cretaceous granitic plutons; the latter as well as minor Upper Cretaceous cover strata probably formed in a circum-Pacific forearc; (2) the Yuli belt is a mid- to Late Cretaceous accretionary complex containing HP thrust sheets that were emplaced attending the Late Cenozoic Eurasian plate–Luzon arc collision; (3) these two Late Mesozoic belts are not coeval, and in part were overprinted by low-grade metamorphism during the Plio-Pleistocene collision; (4) accreted clastic sediments of the Tailuko belt contain mainly Phanerozoic detrital zircons, indicating that terrigenous sediments were mainly sourced from western Cathaysia, whereas in contrast, clastic rocks of the Yuli accretionary complex contain a significant amount of Paleoproterozoic and distinctive Neoproterozoic zircons, probably derived from the North China craton and the Yangtze block ± eastern Cathaysia, as a result of continent uplift/exhumation after the Permo-Triassic South China–North China collision; and (5) the Late Jurassic–Late Cretaceous formation of the Tananao basement complex precludes the possibility that the early Yanshanian (Early Jurassic) granitoids in southern China represent a landward arc contemporaneous with the later, outboard Tananao accretionary event.

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However, its thermochronological history has not been fully examined yet. Here we applied U–Pb and 40 Ar/ 39 Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of 40 Ar/ 39 Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.

Zircons from one jadeitite sample, collected from serpentinite melange north of the Motagua fault, Guatemala, were separated for SHRIMP-RG U–Pb dating and trace element analyses. The sub- to anhedral crystal form, lack of typical magmatic oscillatory zoning, the presence of fluid and albite/quartz/jadeite inclusions, and the low Th/U ratios ( Both the whole-rock jadeitite and its zircons have very low REE contents, ∼ 1 ppm and 0.5–42 ppm, respectively. The whole-rock jadeitite shows a flat and slightly concave REE pattern and a positive Eu anomaly (Eu/Eu* = 1.24). Zircons are enriched in HREE and their REE patterns can be divided into two groups: one with negative Eu anomaly and one with positive Eu anomaly. The latter tends to have smaller positive Ce anomalies. The fluid from which jadeitite formed probably evolved over time, becoming more reducing and more dominated by plagioclase decomposition reactions. Alternatively, trace element compositions of zircons simply demonstrate complicated variations of fluid chemistry during jadeitite formation. A reducing fluid with high pH values capable of mobilizing Al, Na, Zr and Hf is inferred to be the media during jadeite/zircon formation. If jadeitite formation was related to serpentinization fluid, the ultramafics must contain olivine as the major primary phase subjected to serpentinization, yielding reducing fluids with high pH values. Two additional requirements are (1) the presence of feldspar/mica-bearing protolithic blocks survived from rodingitization in peridotites to provide Al and Na, and (2) the presence of fluid channels as well as pulses of in/out-flux fluid to trigger metasomatic reactions. These preconditions in subduction zones may contribute to the rare occurrences of jadeitite in global high-P metamorphic belts.

To constrain the timing of collisional event in the Indochina block, SHRIMP U-Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc Complex, central Vietnam. Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1), and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2). The change in mineral assemblages indicates a clockwise P-T path composed of a high- or medium-P + low-T stage (M1) and a subsequent low- P + high-T stage (M2). The U-Pb concordia ages of zircon rims are 447 ± 6 Ma and 452 ± 6 Ma for the two samples, respectively. These results are distinctly different from the available Ar-Ar mineral ages of 254–225 Ma. Following the clockwise P-T path and phase equilibrium analyses of the Complex, we suggest that the zircon rims were formed near peak temperatures during the decompression. The ∼450 Ma zircon rim thus gives the minimum age constraint for a possible crustal thickening event during Early Paleozoic, whereas the reported Permo-Triassic Ar-Ar ages would result from an Indosinian overprint. This Early Paleozoic event is most likely related to a collisional orogeny between the Indochina and South China blocks. Late Neoproterozoic to Neoarchean ages are recorded from detrital zircon cores of the Kham Duc Complex, the Kontum Massif and Truong Son Belt, suggesting that their protoliths might have derived from sediments at the Gondwana margin.

In-situ U-Pb geochronology and Hf isotopic studies of zircons from the Pre-Tertiary basement complex and Eocene rocks of the Central Range of Taiwan were carried out to elucidate the history of detrital zircons older than 2.3 Ga. Zircons from the eastern Backbone Range and Eastern Central Range show Paleoproterozoic to late Neoarchean ages (2.3−≥2.5 Ga), whereas those from the Hsuehshan Range yield Neoarchean to Mesoarchean ages (2.6–3.2Ga). The ɛHf(T) for the Paleoproterozoic to late Neoarchean zircons varies from −7.4 to +5.1, implying both juvenile crustal growth and reworking of old crusts. This dual origin could extend back to Mesoarchean-Paleoarchean (3.0–3.4 Ga) based on Hf isotopic model ages. Consistent but narrower ɛHf(T) values and Hf crustal model ages for zircons from Taiwan compared to Cathaysia suggest a possible common origin for these zircons, although Taiwan has a generally younger crustal evolution history than SE China.

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma...

The NNE-SSW-trending Dien Bien Phu fault, which dextrally displaces the NW-SE-trending Song Ma suture, in northwestern Vietnam, is widely considered one of the most seismically active faults in Indochina. In order to better understand the fault’s activity, this study reports new 40Ar/39Ar geochronological data for mica schists and mylonites from several areas along the Dien Bien Phu fault, showing 40Ar/39Ar ages of 194–212 Ma for mica schists and 158–198 Ma for mylonites. These ages suggest that after the collision of the South China and Indochina blocks, dextral shearing deformation initiated along the Dien Bien Phu fault in the Jurassic, significantly earlier than previously thought. In light of the relevant tectonic events in Indochina, the successive suturing/collision of the Indochina, Simao, and Sibumasu blocks may have been responsible for the initiation of dextral shearing along the Dien Bien Phu fault, which in turn resulted in offset of the Song Ma suture.

The Day Nui Con Voi (DNCV) metamorphic complex in North Vietnam is the southernmost high-grade metamorphic zone along the NW–SE trending Red River Shear Zone (RRSZ) in Indochina. The RRSZ was considered as a classical large-scale continental strike-slip fault that had played a significant role in the continental extrusion of Southeast Asia since the collision of India and Eurasia. Earlier ideas determined the RRSZ as a steep shear zone that penetrated the entire lithosphere. Both metamorphism and structures within rocks along the DNCV metamorphic complex have been previously thought to be formed syn-tectonically by left-lateral shearing of the RRSZ during the Oligocene–Miocene continental escape tectonics. However, our meso- and microstructural re-examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike-slip shearing. High angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the Triassic to the Tertiary. D1 is preserved as NW–SE striking upright folds under garnet grade regional metamorphism during the Triassic Indosinian orogeny as South China block amalgamated with the Indochina block. The large-scale horizontal D2 folds with a dominant top to N–NW bottom to S–SW sense of shear, and sub-horizontal fold axial planes suggest that the DNCV metamorphic complex remained at midcrustal depths since the Indosinian orogeny. The youngest ductile deformation event, D3, refolded D2 recumbent folds into a dome, and uplifted the DNCV as lower-temperature fabrics, S3, indicated. Steep mylonite zones with left-lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep Song Hong and Song Chay faults during left-lateral movement of the RRSZ. Our new spatial, temporal and kinematic correlations of metamorphic fabrics within the DNCV metamorphic complex support the suggestion that the RRSZ developed after regional metamorphism and remained purely a crustal fault. The complicated deformation history recorded within the DNCV metamorphic complex provides an alternative interpretation and suggests that crustal scale strike-slip faults (such as the RRSZ) need not root from the mantle.

Permo-Triassic intermediate–felsic magmatism is developed along the Truong Son fold belt, located in the eastern margin of the Indochina Block. It comprises a succession of the active continental margin associations: calc-alkaline volcano-plutonic associations (272–248 Ma), peraluminous granites (259–245 Ma), and subalkaline felsic volcano-plutonic associations (younger than 245 Ma). Detailed study of geochemical characteristics such as trace elements (LILE, REE, HFSE) and isotopes (Sr, Nd, Pb) indicates that they are homogeneous and that they are products of the Palaeotethys subduction process in relation to Indochina (IC)/North Vietnam–South China (NV–SC) amalgamation (S.L. Chung et al., Abstr., GEOSEA 98, Malaysia, 1998, pp. 17–19). The Indosinian characteristics are represented by mantle–crust interaction in magma generation, controlled by their emplacement localities in relation to the Kontum Uplift. The spatial and temporal evolution of Permo-Triassic magmatism allows reconstructing the geodynamic history of the Indosinian orogeny. It confirms that this event ended in Early to Middle Triassic (246–240 Ma, after C. Lepvrier et al., Tectonophysics 393 (2004) 87–118).

Late Yanshanian magmatism in SE China includes three stages of thermal event induced by the interaction between the continental margin of Eurasia and the paleo-Pacific plate during the Cretaceous period. Products of syn-orogenic magmatism (130–110 Ma) include high-Al gabbros (HAG), and gneissic tonalite, trondhjemite and granodiorite (TTG), which intruded into the deep basement (18–24 km). Rocks of the post- and an-orogenic magmatism are shallow-level (6–8 km) I-type granitoids (110–99 Ma), and miarolitic A-type granites plus rhyolite-dominate bimodal volcanics (94–81 Ma), respectively. Geochemically, HAG and TTG belong to the medium-K calc-alkaline affinity with high Sr/Y, whereas other granitoids are mainly high-K calc-alkaline to shoshonitic rocks with low Sr/Y. Sr and Nd isotope compositions suggest different sources of HAG and TTG from other rocks. Progressive depletions of Ba, Sr, Eu and P from I- to A-type granites reflect partial melting of felsic granulites from hydrous to dry conditions, whereas high Sr/Y in HAG and TTG are compatible with dehydration melting of amphibolites. Tectonic models which accommodate HAG and TTG may involve thickening of the lithosphere to convert the pre-existing lower-crust basic rocks into amphibolites. It was followed by basaltic underplating which is attributed to delamination of the thickened lithosphere and led to triggering of crust melting under exten

Taiwan is an active mountain belt formed by oblique collision between the Luzon are and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio-Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at similar to0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO approximate to 15 wt.%) and potassic (K2O approximate to 5 wt.%, K2O/Na2O approximate to 1.6-3.0). Whereas these basic rocks (SiO2 approximate to 48 wt.%) have relatively low Al2O3 approximate to 12 wt.%, total Fe2O3 approximate to 7.5 wt.% and CaO approximate to 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, > 1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high Field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * approximate to 0.90 - 0.84), and Sr and Nd isotope ratios are uniform (Sr-87/Sr-86 approximate to 0.70540-0.70551; Nd-143/(144) Nd approximate to 0.51268-0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 +/- 1.8; 1 sigma deviation), which are in agreement with the whole rock Mg-values (83 - 80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94-0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet, We therefore suggest that the Tsaolingshan lavas resulted from a phlogopite-bearing harzburgitic source in the lithospheric mantle that underwent a recent metasomatism by the nearby Ryukyu subduction zone processes. The lavas exhibit unique incompatible trace element ratios, with Rb/Cs approximate to 8, Ba/Rb approximate to 1, Ce/Pb approximate to 2, Th/U approximate to 1 and Nb/ U approximate to 0.8, which are significantly lower than the continental crust values and those of most mantle-derived magmas. Nonmagmatic enrichment in the mantle source is therefore required. Based on published experimental data, two subduction-related metasomatic components, i.e., slab-released hydrous fluid and subducted sediment, are proposed, and the former is considered to be more pervasive for causing the extraordinary trace element ratios observed. Our observations lend support to the notion that dehydration from subducting slabs at convergent margins, as a continuing process through geologic time, can account for the fractionation of these elemental pairs between the Earth's crust and mantle. (C) 2001 Elsevier Science B.V. All rights reserved.

Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [eNd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [eNd(T)=−4.7 to −9.7; TDM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate eNd(T) values (+0.6 to −2.8) and younger TDM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.

Left-lateral motion along the Ailao Shan‐Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-floor spreading in the South China Sea. Our new 40 Ar/ 39 Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began 027 Ma and lasted until 022 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y ˇ1 . Such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea. # 2000 Elsevier Science Ltd. All rights reserved.

We report Rb−Sr and Sm−Nd isotope data to better constrain the origin of the Seoul granitic batholith in Uijongbu-Dongduchon-Pocheon area. The dominant rocks of the Seoul granitic batholith intruding the Precambrian Gyonggi massif are biotite granite and garnet biotite granite. Rb−Sr whole rock data define three subparallel lines in an isochron diagram, which is related to geography rather than to lithology. Biotite granite and garnet biotite granite from the northeastern part of the study area give an isochron age of 170.0±5.2 Ma with an initial Sr isotopic ratio of 0.71669±0.00043, while those from the central part give an isochron age of 167±19 Ma with an initial ratio of 0.71280±0.00075. On the other hand, three biotite granites from the northern part, having low Rb/Sr ratio, plot below the isochron of the central samples with a similar slope (∼168 Ma) and have calculated initial Sr isotopic ratios of 0.7102–0.7109. Whole rock Sm−Nd data do not show isochron relationship, nor do they show distinct initial isotopic ratios for different areas as defined by initial Sr isotope ratio. Calculated eNd (T=170 Ma) values range from −15.6 to −19.9 and are generally higher than those of basement gneisses (an average of −23.4) around the batholith. These isotope data indicate that three distinct magmas with different Sr but similar Nd isotope ratios intruded during middle Jurassic time to form the Seoul batholith in the study area. The Sr and Nd isotope data from the basement rocks of the Gyeonggi massif suggest that the initial Sr and Nd isotope data of the Seoul batholith can be explained by mixing of three components: the mantle component and two crust components with high and low87Sr/86Sr, which rules out the possibility of its origin from direct partial melting of basement rocks, but requires significant mantle input. The isotope data also suggest that geochemical differences (MnO, Rb, Sc, Nb, Y, and Yb contents) between biotite granite and garnet biotite granite in our previous major and trace element study might be related to some sort of fractionation in each magma.

UPb zircon/monazite dating and Sr-isotope results for granitoids from Taiwan and Chinmen-Lieyu, SE China place important constraints for the tectonic evolution and the timing of felsic magmatism associated with mid-Cretaceous subduction process. The Chinmen gneiss (139.4 Ma) and granite (100.9 Ma) represent the oldest granitic units and belong to the previously recognized late Yanshanian phase of magmatism. A younger suite of granitoids occurs in the Tananao Metamorphic Complex of Taiwan and includes the Yuantoushan and Tachoshui granites (87.3 Ma) and the Kanagan gneiss (90.2 Ma). These mid-Cretaceous ages for the younger granitoid suite mitigate against the existence of the hypothetical late Paleozoic “Dongnanya” microcontinent in the study area as proposed by other investigators. UPb zircon inheritance ages, ranging between 1.67 and 2.09 Ga indicate the presence of a Precambrian basement beneath much of coastal SE China and Taiwan, however, there is no evidence of Archean crust as indicated by zircon inheritance patterns for rocks west of the present study area. On a 87 Sr 86 Sr vs. 87 Rb 86 Sr plot, the available data for the granitoids in Chinmen and coastal Fujian, SE China, however, define pseudoisochrons with apparent ages of 158–167 Ma. Although the pseudoisochron could be interpreted as a result of a two-end-member mixing process with subsequent fractional crystallization, the apparent age of the pseudoisochron, substantiated by the circumstantial evidence, is postulated as the approximate time of Sr-isotope homogenization of the crustal protoliths of the granitoids during the previous (early Yanshanian) tectonic event in this region. Such a Sr-isotope homogenization process must have occurred over a large area, possibly facilitated by a metamorphic fluid phase or by large-scale anatexis. The Cretaceous granitoids in this area were probably produced by partial melting of this crust in a more restricted scale. The Cretaceous calc-alkaline granitoids in coastal Fujian and Taiwan are of I-type or contaminated I-type. A subduction component is inferred during the magma genesis. Although the Taiwan granitoids are suggested to be subduction related, the possible tectonic environment for the genesis of coastal Fujian granitoids is yet to be solved.

Samples of minerals from the Manaslu leucogranite in the central Himalaya were microsampled and measured for RbSr mineral ages. The ages obtained are used in conjunction with models for RbSr closure temperatures, to reconstruct the thermochronological history of the Manaslu granite. The RbSr results indicate a rapid change in temperature between 21 Ma and 17 Ma of 60–70°C/million yr, which, in turn, indicates unroofing at 2 mm/yr. The temperature-time curve inferred from RbSr data is in excellent agreement with existing ArAr data on the Manaslu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly in the 20-17 Ma time period, and this erosion could account for the observed rapid increase in seawater 87 Sr 86 Sr . A tourmaline-whole rock age of 29.5 Ma was obtained on a sample from the Chokkang Arm, but may be unreliable because of post-crystallization alteration of feldspar. Microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 μm or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar39Ar approach.

Late Cretaceous granitic rocks constitute an essential part of the pre-Tertiary Tananao metamorphic basement complex of Taiwan. They are dominantly of granodiorite to quartz monzonite composition. Most granitic rocks are peraluminous (A/CNK > 1.0 and normative corundum > 1%) and display moderately fractionated LREE and relatively unfractionated HREE patterns with negative Eu anomalies. On a primitive mantle-normalized trace-element diagram, they show a significant Nb depletion which is typical of the calc-alkaline magmatism from the subduction-zone environment. They fall within the volcanic arc field on the discrimination diagram of Pearce, Harris and Tindle (1984). The lack of systematic inter-element relationships suggests that the role of fractional crystallization is not significant and that these granitic rocks were derived from heterogeneous protoliths. Geochemical data suggest Taiwan granitic rocks are contaminated I-type and I-type granites related to the subduction of the Paleo-Pacific plate beneath the eastern margin of the Eurasia plate during late Mesozoic time.

To aid a better understanding of the crustal evolution of SE China, isotopic studies of mid-Cretaceous granitoids and associated late Paleozoic metapelites of the basement complex of Taiwan have been undertaken. Initial ϵNd-values and 87Sr86Sr ratios for the granitoids range from −12 to +1 and 0.7051 to 0.7108, respectively, suggesting a heterogeneous nature of their protoliths. Furthermore, these initial Nd and Sr isotopic ratios define a covariant array lying between those of nearby metabasites and metapelites. This indicates that the sources of granitoids could well be two-component mixtures of a depleted component of mantle origin (fresh to altered oceanic crust with ϵNd = + 11 and 87Sr86Sr = 0.703−0.708, represented bythe adjacent amphibolite) and an enriched continental component (ϵNd = −12 and 87Sr86Sr = 0.712, respresented by the metapelites), with the latter making up from 15% to 60%. Oxygen isotope data are also consistent with this mixing model. Nd model ages with respect to the depleted mantle reservoir (TDM) for Taiwan granitoids range from 0.9 to 1.5 Ga. These model ages are slighlty younger than those of the S-type granitoids of SE China and world-wide Phanerozoic sedimentary rocks, implying a significant contribution from juvenile material. Similar to the granitoids, Taiwan metapelites are also heterogeneous in Nd and Sr isotopic compositions with each body possessing its own characteristic ranges. Nd crustal residence ages (TDM) for Taiwan metapelites range from 1.1 to 1.6 Ga which are younger (1.5–2.3 Ga) than those of the Paleozoic sediments of South China. Thus, the oldest sediments in the Taiwan basement received not only recycled continental crustal material but also fresh mantle input. Therefore, Taiwan is a good witness of at least three episodes of fresh mantle inputs to the crust of the Eurasian continent: the ongoing accretion of the intra-oceanic Luzon Arc, the Mesozoic event recorded in granitoids and the Paleozoic event recorded in metapelites.

We present a new synthesis of the tectono-thermal events of Taiwan, excluding the Coastal Range, based on existing isotopic, geochemical and geochronological data for granitic, metamorphic, volcanic and sedimentary rocks. Nd model ages (TDM) and the inherited zircon ages consistently yielded Proterozoic ages, suggesting that the source rocks from the exposed rocks in Taiwan were formed in the Proterozoic, starting from about 2 Ga ago. The crustal evolution of Taiwan began in the Late Paleozoic (250 ± 20 Ma). Since then, five tectono-thermal events can be delineated: (I) an Early Jurassic event (200 175 Ma) registered in the marble and metapelites of the Tananao metamorphic basement complex of northern Taiwan and crystalline limestone of the basement rocks in western Taiwan; (II) a Late Jurassic event (~153 Ma) revealed by a meta-granite of the Tananao metamorphic basement complex of southern Taiwan; (III) a Late Mesozoic event (97 - 77 Ma) recorded in the rocks of the Tananao metamorphic basement complex and offshore of northern and western Taiwan; (IV) a Cenozoic of prePliocene event (episodic from 56 to 9 Ma) registered in the dikes in the Central Range and the intraplate basalts of mainland Taiwan and offshore of northern and western Taiwan; and (V) an ongoing Late Cenozoic event (since 5 Ma) shown in the recent volcanics of onshore and offshore northern Taiwan and offshore northeastern Taiwan. In comparison with the Cathysia foldbelts, Taiwan demonstrates similar Paleoproterozoic crustal residence ages, coeval Jurassic magmatism (Early Yanshanian) and Late Cretaceous (Late Yanshanian) orogeny. However, subduction-related magmatism was prevalent in Taiwan in comparison with both extension- and subduction-related magmatism in Cathaysia foldbelts during the Yanshanian orogeny. Rifting-related magmatism has continued in the Cathaysia foldbelts ever since Jurassic. Since Late Cretaceous, rifting-related magmatism migrated from the interior toward the coastal region in the Cathaysia foldbelts but it did not reach Taiwan until Paleogene. Since 5 Ma, subduction-related activity started in northern Taiwan. The volcanism in northern Taiwan was attributed to the Late Pliocene extensional collapse of the northern Taiwan mountain belt in response to the oblique collision of the northern Luzon arc with the Asian continent, but not directly related to the magmatism of the Ryukyu subduction zone. However, volcanism resulting from the spreading of the Okinawa trough manifests in the offshore region of northeastern Taiwan.

Mantle xenoliths entrained in the Neogene basalts at central and southern Vietnam provide important constraints on the nature of the lithospheric mantle beneath the eastern Indochina block. The basalts contain mantle xenoliths (garnet lherzolite, spinel lherzolite, harzburgite, and eclogite), cumulate xenoliths (wehrlite, websterite, and pyroxenite) and megacrysts (olivine, Al-rich clinopyroxene, orthopyroxene, anorthoclase, Ti-amphibole, phlogopite, sapphire, and zircon). The mineral compositions of spinel lherzolite xenoliths studied are typical of mantle phases: Fo-rich olivine, En-rich orthopyroxene, Di-rich clinopyroxene and Cr-rich spinel. Concentrations of trace elements for clinopyroxene were determined in situ by laser ablation ICP-MS. Clinopyroxenes exhibit large compositional variations ranging from LREE-depleted (LaN/YbN = 0.5–0.7) to cryptically metasomatized LREE-enriched (LaN/YbN = 6.7–20.3). Slightly LREE-depleted REE patterns with YbN of 10 indicate these peridotites were originated from very low degree of partial melting. However, most REE patterns present much lower HREE concentrations with YbN of 2–6 suggesting these peridotites were products of either high degree of partial melting or the residual from the removal of garnet. Spidergrams of most clinopyroxenes show Ba, Nb–Ta, Hf–Zr and Ti depletion. Therefore, the hydrous silicate melts/fluids of subduction zone origin are likely candidates for the required metasomatic agent. The subcontinental lithospheric mantle beneath central and southern Vietnam was complicated and heterogeneous.

Southeast Asia, or, in a more strict sense, the Indochinese continent, was previously considered to be composed entirely of Proterozoic to Phanerozoic rocks, and reliable evidence for Archean crust was lacking. Gneisses from the Cavinh Complex, south of the Red River shear zone, northern Vietnam, however, show Archean Nd model ages of 3.4–3.1 Ga. Zircon separates from the rocks yielded U-Pb dates of 2.8–2.5 Ga, the first convincing evidence for the presence of a Late Archean complex in Southeast Asia. Given that the Red River shear zone was propagating in the South China block with a left-lateral offset of ∼600 km, the Cavinh Complex can be correlated with the Late Archean Kangding Complex in the western margin of the Yangtze craton, southwestern China. The Cavinh Complex therefore represents one of the oldest crustal nuclei of the South China block.

In order to explore the timing of strike-slip movement along the Ailao Shan–Red River shear zone, an 40Ar/39Ar thermochronological study of the Day Nui Con Voi metamorphic massif in northern Vietnam was undertaken. The massif, exposed in the southeastern segment of the Ailao Shan–Red River shear zone, reveals a rapid cooling in the early Miocene (25–21 Ma) after a very slow cooling ca. 34–25 Ma. The slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late Paleogene (40–30 Ma) extension in eastern Tibet and western Yunnan, China. The rapid cooling, consistent with evidence of a wider rapid cooling span from 27 to 17 Ma compiled from the entire Ailao Shan–Red River shear zone, constrains the duration of left-lateral shearing, which eventually offset Indochina from South China by about 600 km. The time constraints we have established reinforce the argument that the onset of the Ailao Shan–Red River shear zone postdates the opening of the South China Sea that began ca. 30 Ma. This result highlights the necessity for reevaluating the Cenozoic tectonic models of Southeast Asia.

We carried out a Sm-Nd isotopic reconnaissance study of South Korea. Eight river sediments gave crustal residence ages (T DM ) between 1.7 and 2.7 Ga, implying that, on average, the crust of South Korea was formed in Early to Middle Proterozoic time. Twenty-six Precambrian basement rocks yielded T DM between 1.4 and 3.8 Ga. More than half of them exceed 2.5 Ga, indicating that South Korea has a long history of crustal evolution. Four of the samples have T DM from 3.2 to 3.8 Ga suggesting the possibility that there may be ancient continental crust in Korea having ages comparable to those of the oldest cratons in the world. However, these samples also have preferentially higher Sm/Nd, so their old ages could be artifacts, thus requiring confirmation. The large variation in T DM of South Korea is reminiscent of that for the South China block. However, the existence of Early Archean ages better matches that of the North China block, because the basement of south China was thought to be mostly Proterozoic in age. The recent discovery of rare Archean basement rocks in the South China block may favor models that attempt to correlate South Korea with south China.