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The late Paleozoic tectonic evolution of the Lhasa terrane and subduction of Sumdo Paleo-Tethys ocean remains controversial due to limited data and variable Permian magmatism in central and southern Lhasa. We report new zircon U–Pb ages and Hf-isotope ratios as well as whole-rock major element, trace element, and Sr–Nd isotope data from middle Permian (266 ~ 257 Ma) arc-related magmatism, including high-Mg quartz diorite and trondhjemite in the Tangjia-Sumdo area. The quartz diorite has high SiO2 (53.92–57.90 wt.%), MgO (3.69–8.03 wt.%) and low TiO2 (0.60–1.01 wt.%) contents. It is enriched in the large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and depleted in the high field strength elements (HFSEs, e.g. Nb, Ta, Ti). We interpreted these chemical features, in combination with uniform whole-rock (87Sr/86Sr)i (0.70375–0.70452), εNd(t) (+4.48 – +5.55), and zircon εHf(t) (+11.2 – +13.7) values, as indicative of dioritic melt derived by the interaction of fluids expelled from the subducting slab with the overlying mantle wedge in an island arc setting. The trondhjemite has high SiO2 (71.16–73.52 wt.%), Al2O3 (14.92–16.24 wt.%) and K2O + Na2O (7.34–8.27 wt.%) contents, is enriched in the light rare-earth elements (LREEs) and is depleted in Th and heavy rare-earth elements (HREEs). These rocks are characterized by positive εNd(t) (+5.91 – +6.68) and εHf(t) (+12.0 – +16.1) values and may have been derived from partial melting of subducted oceanic crust. Based on the elemental and isotopic composition of the quartz diorite and trondhjemite, we suggest that middle Permian magmatism occurred in an intra-oceanic subduction setting in the Sumdo Paleo-Tethys ocean. The Sumdo Paleo-Tethys ocean was probably a small ocean basin within the eastern Lhasa terrane and not a vast ocean basin that completely separated central Lhasa from the southern Lhasa subterrane. [ABSTRACT FROM AUTHOR]

Owing to the impact of the Paleo-, Meso-, and Neo-Tethys Oceans, the Triassic evolution of the Lhasa Terrane is complex and remains debated. The little-known subduction of the oceanic lithosphere of the Sumdo Paleo-Tethys Ocean (SPTO) in the Triassic has especially constrained our understanding of the evolution of the Lhasa Terrane. We investigated the newly identified Triassic granite and metabasalt in the Sumdo area to detail new constraints regarding the Triassic subduction of the SPTO oceanic lithosphere and evolution of the Lhasa Terrane. Zircon LA-ICP-MS U Pb dating for granite and metabasalt yielded crystallization ages of 238.4–236.1 Ma. The granites were strongly peraluminous I-type granites with weakly fractional crystallization. High Nb contents (on average 51.91 ppm), low ε Hf (t) values (−19.7 to +2.8), and ε Nd (t) values (−2.86 to −2.2) suggest that the granites were likely generated by mixing of mantle melts and their induced melts of ancient crustal materials. Metabasalts were analogous to island arc tholeiite with negative Th, Nb, Ti, Zr, and Hf anomalies and positive ε Nd (t) values (+6.1 to +7.1), demonstrating that the magma was likely derived from the partial melting of the depleted mantle metasomatized by subduction fluids. Combined with Triassic magmatism in the Lhasa Terrane, we consider that a divergent double subduction of the SPTO oceanic lithosphere produced the Early Triassic to early Late Triassic magmatism (248–221 Ma) concentrating on both sides of the SPTO suture zone. The Middle Triassic granites and metabasalts in this study were formed in the continental and intra-oceanic arcs, respectively, of the SPTO. Subsequently, closure of the SPTO and collision between the central- and southern Lhasa Terrane might have triggered the subduction of the Indus–Yarlung Tsangbo oceanic lithosphere broadly generating the late Late Triassic magmatism (217–201 Ma) in the Lhasa Terrane. • Middle Triassic I-type granite and tholeiite occurred in the Central Lhasa Terrane. • Mixing of the mantle- and ancient crustal melts produced the I-type granite. • Partial melting of the depleted mantle metasomatized by fluids formed the tholeiite. • They were resulted in the subduction of the Sumdo Paleo-Tethys Oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Previous studies of the Southern Qiangtang (SQ) Terrane in Tibet have shown significant growth of continental crust during the late Mesozoic (ca. 170–120 Ma) through underplating of mantle-derived melts beneath ancient lower crust. However, the mechanism of crustal growth remains defective during the Late Jurassic. In this contribution, we report new zircon U–Pb ages, whole-rock major and trace element concentrations, and zircon Hf isotopic compositions of Late Jurassic rhyolites from the Duobuza area in the SQ Terrane. Zircon LA–ICP–MS U–Pb dating yields concordant ages and weighted mean 206Pb/238U ages of 151–149 Ma, indicating that the rhyolites were erupted during the Late Jurassic. The rhyolite samples yield high SiO2 (69.1–73.4 wt.%) and Na2O (3.82–6.49 wt.%) concentrations, high values of the differentiation index (95.4–98.5), moderate Al2O3 (13.1–15.5 wt.%) and K2O (2.55–3.26 wt.%) concentrations, and relatively low TiO2 (0.34–0.44 wt.%), MgO (0.14–0.67 wt.%), and MnO (0.01–0.10 wt.%) concentrations and Mg# values (8.9–33.5). They are enriched in Rb, Th, Ba, and Pb, depleted in Nb, Ta, Sr, Ti, and P, and yield negative Eu anomalies. These characteristics suggest that the Duobuza rhyolites represent medium- to high-K calc-alkaline, highly fractionated I-type granitoids. Variations in major and trace element concentrations indicate that the rhyolites underwent significant fractionation of Fe–Ti oxides, plagioclase, K-feldspar, hornblende, biotite, and apatite. The samples yield positive zircon εHf(t) values (+0.08 to +10.9) and young TCDM model ages (816–386 Ma), indicating that they were generated through partial melting of juvenile lower crust of the SQ Terrane. These data are distinct from those of Late Jurassic intermediate–felsic plutons in the SQ Terrane that generally yield negative εHf(t) values and ancient TCDM model ages. We suggest that the occurrence of the Duobuza rhyolites is the significant witness. The presence of juvenile lower crust beneath the SQ Terrane during the Late Jurassic (ca. 150 Ma) rather than during the Early Cretaceous (ca. 120Ma). The transition from ancient to juvenile lower crust has already began in the SQ Terrane during the Late Jurassic. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Early Carboniferous clastic sediments in the North Lhasa Terrane was mainly from the Indian continent and partly from the Australian continent. • The North Lhasa Terrane may be located at the northern margin of the Indian- and Australian continent during the Early Carboniferous. • The reduction of Grenvillian detrital zircons in the Yongzhu Formation might be associated with the Early Carboniferous rifting process in the eastern Lhasa Block. The Lhasa Block was located at the northern margin of East Gondwana before it drifted away, but its exact paleogeographic location in the Late Paleozoic remains controversial. The Late Paleozoic strata in the Lhasa Block, especially the Lower Carboniferous, for which the rock assemblage and sedimentary age have been described but the in-depth research on provenance and tectonic environment are still lacking, may provide vital information about its paleogeographic location. This study presents a systematic field geological survey of the Lower Carboniferous Yongzhu Formation, along with U-Pb dating and Hf isotope analysis of detrital zircons. The detrital zircons U-Pb data revealed major age peaks at 530 and 980 Ma as well as several subordinate age peaks at 784, 890, 1160, 1514, 1690, 2480, and 2600 Ma. The Hf isotopes of the detrital zircons showed similarity with zircons from the Indian continent on the age peaks at 980 and 890 Ma and with those from the Australian continent on the age peaks at 1160 and 2600 Ma, which indicates mixed provenance for the Yongzhu Formation. From this study, we infer that the Lhasa Block might have been located at the northern margin of the Indian and Australian continents during the Early Carboniferous. The weaker age peak at 1160 Ma of the Lower Carboniferous compared to the Pre-Carboniferous and Upper Carboniferous-Lower Permian might be related to the opening of the Sumdo Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

Abstract The Gangdese magmatic belt is oriented E–W and located between the Indus–Yarlung Tsangpo and Bangong–Nujiang suture zones in Tibet. It records the subduction of the Neo-Tethyan oceanic crust, Indian–Eurasian collision, and uplift of the Tibetan Plateau. Here we present new zircon U–Pb geochronology, Hf isotopic compositions, and whole-rock geochemistry of the granodiorites in the Sumdo area of the Gangdese Belt. Our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early-stage evolution of the Neo-Tethyan Ocean during the Late Triassic to Early Jurassic. The granodiorites yield zircon U–Pb ages of 193–201 Ma, indicating an Early Jurassic magmatic event. They are metaluminous, with medium- to high-K calc-alkaline geochemistry, and show characteristics of I-type granitoids. They show enrichment in light rare earth elements (LREEs) and depletion in high-field-strength elements (HFSEs) in primitive-mantle-normalized spider diagrams, indicating that their initial magma was generated in a subduction-related active continental margin. They have negative ε Hf (t) values (−11.3 to −4.1) and crustal model ages (TC DM) of 1.50–1.95 Ga. Hf isotopes and whole-rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower-crust material mixed with mantle material. Our data, in combination with published geochronological and geochemical data, show that the Late Triassic–Early Jurassic arc magmatism in the Gangdese magmatic belt was formed by the northward subduction of the Neo-Tethyan oceanic crust beneath the Lhasa Terrane. Highlights • Ca. 201–193 Ma granitoids occur in the Sumdo area of the Gangdese belt, Tibet • The rocks were derived from crustal melting and subsequent crust-mantle magma mixing. • Coeval magmatism in the Gangdese Belt is related to the Neo-Tethyan Ocean. [ABSTRACT FROM AUTHOR]

In this paper, we present new major and trace element chemical data for the basalts and phonolites of the Nare ocean island fragment (NaOI), as well as zircon U-Pb age data and Hf isotope compositions for the NaOI phonolites in the middle segment of the Bangong-Nujiang Suture Zone, northern Tibet. Our aim is to assess the genesis of these rocks and to reconstruct the Middle Triassic evolution of the Bangong-Nujiang Tethyan Ocean (BNTO). The NaOI retains an ocean island-type double-layered structure comprising a basaltic basement and an oceanic sedimentary cover sequence (conglomerate and limestone, the latter accompanied by layers of erupted phonolite near the top of the sequence). The basalts in the NaOI are enriched in light rare earth elements and high field strength elements (Nb, Ta, Zr, Hf, and Ti), and they exhibit chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns similar to those of ocean island basalts. Taking into consideration their high Dy/Yb, Sm/Yb, and La/Sm ratios, we conclude that the NaOI basalts were derived from the partial melting of garnet peridotite in the mantle. The NaOI phonolites have LREE-enriched chondrite-normalized REE patterns with negative Eu anomalies (Eu/Eu* = 0.41-0.43) and primitive mantle-normalized trace element patterns with enrichments in Nb, Ta, Zr, and Hf, and depletions in Ba, U, Sr, P, and Ti. Given the high contents of Nb (172-256 ppm), Ta (11.8-16.0 ppm), Zr (927-1117 ppm), and Hf (20.8-26.9 ppm), and the very low contents of MgO (0.11-0.25 wt%), the very low Mg# values (5-10), and the near-zero contents of Cr (1.27-7.59 ppm), Ni (0.43-7.19 ppm), and Co (0.11-0.38 ppm), and the small and homogeneously positive εHf(t) values (+ 4.9 to + 9.5), we infer that the NaOI phonolites were formed by the fractional crystallization of an OIB-derived mafic parent magma. The phonolites of the NaOI contain zircons that yielded U-Pb ages of 239 and 242 Ma, indicating that the NaOI formed during the Middle Triassic. These data, combined with data from modern ocean islands (e.g., Canary Islands, Cape Verde, Fernando de Noronha, Tristan da Cunha, and Gough in the Atlantic Ocean, and Society and Austral-Cook in the Pacific Ocean), lead us to infer that the BNTO was open for a long time before the Middle Triassic, and that the ocean had already developed into a mature ocean with a thick oceanic lithosphere by at least the Middle Triassic. [ABSTRACT FROM AUTHOR]

We present our new investigation into the depositional environment and provenance of the Yingshuiquan Formation in the central Qiangtang region of northern Tibet, in order to further our understanding of the environment of the Longmu Co-Shuanghu Palaeo-Tethys during the Early Triassic. The Yingshuiquan Formation is composed of oolitic limestone, calcareous sandstone, calcarenite, thin-bedded ribbon limestone, bioclastic limestone, and coarse oolite limestone that were deposited in a shallow-marine basin and contain abundant Lower Triassic conodont fossils (e.g. Hadrodontina anceps, Pachycladina sp., gen. et sp, Pachyclaina oblique, Hibbardelloides sp). We selected detrital zircons from four calcareous sandstone samples for U-Pb dating, yielding minimum age peaks of 263, 269, 275, and 280 Ma, respectively, and a minimum age of 249 Ma, based on several zircons around the same age. Analysis of the conodont biofacies and zircon LA-ICP-MS dating of calcareous sandstone indicates that the data is consistent with deposition in the Early Triassic. The Yingshuiquan Formation records Early Triassic shallow-water sediment in the Longmu Co-Shuanghu Palaeo-Tethys, and has a Southern Qiangtang and Northern Qiangtang terranes provenance. During the Early Triassic, the carbonate sediments of the Yingshuiquan Formation were deposited in an active environment around the Longmu Co-Shuanghu Palaeo-Tethys, which has became a residual sea basin. [ABSTRACT FROM AUTHOR]

Zircon U-Pb dating of two samples of metagabbro from the Riwanchaka ophiolite yielded early Carboniferous ages of 354.4 ± 2.3 Ma and 356.7 ± 1.9 Ma. Their positive zircon εHf(t) values (+7.9 to +9.9) indicate that these rocks were derived from a relatively depleted mantle. The metagabbros can be considered as two types: R1 and R2. Both types are tholeiitic, with depletion of highfield- strength elements (HFSE) and enrichment of large-ion lithophile elements (LILE) similar to those of typical back-arc basin basalts (BABB), such as Mariana BABB and East Scotia Ridge BABB. Geochemical and isotopic characteristics indicate that the R1 metagabbro originated from a backarc basin spreading ridge with addition of slab-derived fluids, whereas the R2 metagabbro was derived from a back-arc basin mantle source, with involvement of melts and fluids from subducted ocean crust. The Riwanchaka ophiolite exhibits both mid-ocean ridge basalts- and arc-like geochemical affinities, consistent with coeval ophiolites from central Qiangtang. Observations indicate that the Qiangtang ophiolites developed during the Late Devonian-early Carboniferous (D3-C1) in a back-arc spreading ridge above an intra-oceanic subduction zone. Based on our data and previous studies, we propose that an oceanic back-arc basin system existed in the Longmuco-Shuanghu-Lancang Palaeo-Tethys Ocean during the D3-C1 period. [ABSTRACT FROM AUTHOR]

New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon εHf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area. [ABSTRACT FROM AUTHOR]

The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean and was located on the Qinghai–Tibet Plateau. Doubts exist as to the internal dynamics and evolution of the Sumdo Paleo-Tethys Ocean region during the Permian, owing to insufficient knowledge regarding the remnants of oceanic crust. Here, we report results for petrology, laser ablation–inductively coupled plasma–mass spectrometry zircon U Pb dating, whole-rock geochemistry, and zircon Hf and whole-rock Sr Nd isotopes of mafic rocks in the Sumdo area. Geochemical characteristics of these mafic rocks allow them to be divided into two groups, A and B. Group A rocks are characterized by high SiO 2 and TiO 2 contents, low Al 2 O 3 contents, and Na 2 O > K 2 O. Their chondrite-normalized rare-earth-element (REE) and primitive-mantle-normalized trace-element patterns are similar to those of N-MORB. The samples exhibit enrichment in U and depletion in Th, depletion in light REEs, and high 143Nd/144Nd values. Group B rocks have high SiO 2 , TiO 2 , and Al 2 O 3 contents. Their chondrite-normalized REE patterns and primitive-mantle-normalized trace-element patterns are similar to those of E -MORB. Group B rocks are enriched in Nb and Ta, depleted in Th and U, and show high 87Sr/86Sr and low 143Nd/144Nd values. Zircon U Pb dating of five meta-gabbro rock samples yielded ages of 268.9 ± 1.6 Ma (STN66), 273.8 ± 2.2 Ma (STN79), 265.1 ± 4.1 Ma (STE26), 258.7 ± 4.4 Ma (STE89), and 261.0 ± 3.1 and 263.7 ± 5.4 Ma (STE41). STN79 shows N-MORB geochemical characteristics and has ε Hf (t) values of +12.4 to +14.6, whereas STE41 displays E -MORB geochemical characteristics with ε Hf (t) values of −2.6 to +0.4. The Sumdo mafic rocks were not contaminated by subducted sediments and/or slab-derived fluids, and formed in a mid-ocean ridge setting. Combining results of previous studies with our new geochemical data for mafic rocks in the Sumdo area, we infer the occurrence of plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean during the middle–late Permian. Mixing of mantle plume/hotspot melt and depleted asthenospheric mantle explains the origin of basaltic magma that formed rocks with the observed geochemical and isotopic characteristics in the Sumdo area. • The Sumdo mafic rocks are formed in a MOR setting during the middle–late Permian. • The mafic rocks in Sumdo show a continuous evolution from N-MORB to E-MORB to OIB. • The genesis of the mafic rocks was related to plume/hotspot-ridge interaction. [ABSTRACT FROM AUTHOR]

• The newly discovered Middle Permian granite dikes are adakitic in composition. • The adakitic magma was generated by partial melting of subducted oceanic slab. • The granite dikes were related to the subduction of the Sumdo Paleo-Tethys Ocean. The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean, located on the Qinghai–Tibet Plateau. However, the evolution of the Sumdo Paleo-Tethys Ocean and the origin of eclogites in the Sumdo area of Tibet are still matters of debate. Here we report newly discovered adakitic granite dikes from the Sumdo area of the central Lhasa terrane. The rocks have high SiO 2 , Al 2 O 3 , and Sr contents, high Sr/Y and La/Yb ratios, enrichment in light rare-earth elements (REEs), and depletion in Y, Yb, Th, and heavy REEs. They are characterized by low 87Sr/86Sr (0.703678–0.704427) and high 143Nd/144Nd (0.512825–0.512904) ratios, and positive ε Nd (t) (+6.41 to +7.38) and ε Hf (t) (+13.8 to +15.7) values. Zircon U–Pb dating yielded ages of 269.1 ± 4.1 and 266.4 ± 2.0 Ma for the rocks. Our data indicate that the Sumdo adakitic granite dikes were derived from partial melting of subducted oceanic slab, with the magma being influenced by slab-derived fluids and slight interaction with peridotite in the mantle wedge during ascent. By combining our new geochemical data with results of previous studies, we infer a subduction event involving the Sumdo Paleo-Tethys Ocean during the middle Permian, which led to the formation of the Sumdo adakitic granite dikes and the Sumdo, Jilang, and Xindaduo eclogites. [ABSTRACT FROM AUTHOR]

• The Banduo Pb-Zn deposit formed at the end of the Late Triassic, which is the oldest deposit in the Gangdese Metallogenic Belt. • The metal-rich magma was a mixture of mantle and crustal melts that experienced fractional crystallization. • The Banduo Pb–Zn deposit formed as a result of the northward subduction of the Neo-Tethys oceanic crust in the Late Triassic. The newly discovered Banduo Pb–Zn deposit in the Somdo area of Tibet is hosted by a Late Triassic granodiorite. A combination of zircon U–Pb dating, Hf isotopic compositions, sulfide Rb–Sr dating, and whole-rock geochemistry was used to determine the age of the mineralization and characterize the metallogenesis of the deposit. The weighted-mean U–Pb ages of zircon from the granodiorite (203.8 ± 1.7 Ma, MSWD = 0.53; 206.1 ± 0.4 Ma, MSWD = 0.098) and the weighted-mean Rb–Sr isochron age of sulfides from the Pb–Zn mineralization (202.7 ± 2.3 Ma; MSWD = 1.3) suggest that the timing of mineralization was close to that of granodiorite crystallization during the Late Triassic. The average (87Sr/86Sr) i value of 0.71026 implies that the ore-forming metals were sourced from a mixture of mantle and crustal material. The weakly altered granodiorite of the deposit is characterized by SiO 2 = 66.78–67.85 wt%, K 2 O = 2.57–3.37 wt%, Al 2 O 3 = 13.53–15.15 wt%, and Mg# = 45.5–51.3. It is enriched in LREEs and LILEs and depleted in HREEs and HFSEs, and it exhibits a negative Eu anomaly. The granodiorite has positive and negative εHf(t) values (−2.5 to 1.7) and crustal model ages (TC DM) of 1.14–1.40 Ga. Hf isotopic analyses and whole-rock geochemistry indicate that the Late Triassic granodiorite was likely sourced from of a mixture of mantle and ancient lower-crust material that had undergone variable degrees of fractional crystallization. Based on regional data, the Banduo deposit is the oldest Pb–Zn deposit in the Gangdese belt and likely formed as a result of the northward subduction of the Neo-Tethys Ocean. The metallogenic epoch indicates that deposits related to the northward subduction of the Neo-Tethys oceanic crust exist in this area. The Late Triassic age of granitoid-hosted mineralization provides additional prospects for exploration in the Gangdese Metallogenic Belt. [ABSTRACT FROM AUTHOR]

A recent study suggests that the central Qiangtang is a key locality to investigate the evolution of the opening and closure of the Paleo-Tethys Ocean. Basalts are commonly associated with supercontinent fragmentation, and they have the potential to indicate the tectonic environment into which they were erupted. In this study, laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb zircon dating, whole-rock geochemistry, and zircon Hf isotope analysis were used to study newly discovered basalts in the southern Gangma Co area of central Qiangtang. Dating of magmatic zircons from three basalt samples indicates that the basalts erupted in the Late Devonian to early Carboniferous (early Tournaisian) at 360–350 Ma. The basalts are geochemically similar to within-plate basalts and formed from a depleted mantle source. We suggest that the basalts may have been a product of the breakup of the northern margin of Gondwana in the Late Devonian to early Carboniferous (360–350 Ma), which may be linked to the initial rifting and opening of the Paleo-Tethys Ocean in the Qiangtang area. [ABSTRACT FROM AUTHOR]

In this paper we present new major and trace element compositions of basaltic rocks in the Gufeng ocean island (GFOI) area in the western segment of the Bangong–Nujiang Suture Zone, northern Tibet. Our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the Bangong–Nujiang Tethyan Ocean. An ocean-island-type double-layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the GFOI provides direct evidence for the interpretation that the assemblage is a typical ocean island. The basalts in the GFOI can be divided into three types (named G1, G2 and G3 basalts), and these basalts range in composition from MORB to OIB types, which is typical of ocean islands. The G1 basalts have MORB-type affinities, possibly indicating the existence of MORB oceanic crust under the GFOI. The G2 basalts represent the early stage of formation of the GFOI, and are produced by the interaction of rising OIB-type basaltic magma and the existing MORB oceanic crust. The G3 basalts are typical OIB basalts and they are the products of the direct eruption of OIB-type basaltic magmas. The G3 basalts have high (La/Yb) N (12.3–14.4), (Ce/Yb) N (10.8–11.8), (La/Sm) N (2.39–2.76), and (Sm/Yb) N (4.89–5.23) ratios, indicating the presence of oceanic lithosphere below the GFOI with a thickness of 50–60 km. Geochemical analyses of the GFOI cherts show that they contain terrigenous material, indicating the GFOI formed close to a continental margin. Norian conodont fossils within the GFOI limestones indicate the GFOI formed during the Late Triassic. These data, combined with geological evidence and a half-space model of lithosphere cooling, where the thickness of the oceanic lithosphere is determined from the age of the lithosphere, indicate that the western segment of the Bangong–Nujiang Tethyan Ocean opened initially in the late Permian, expanded rapidly during the Early–Middle Triassic, and was a mature ocean during the Late Triassic. [ABSTRACT FROM AUTHOR]