Jian-Jun Fan

Petrology, geochemistry, and geochronology of the Zhonggang ocean island, northern Tibet: implications for the evolution of the Banggongco–Nujiang oceanic arm of the Neo-Tethys

This study presents new data relating to the tectonic evolution of the Zhonggang ocean island, within the Mesozoic Banggongco–Nujiang suture zone of northern Tibet, and discusses the implications of these data for the evolution of this region. Thirteen basalt and ten gabbro samples were collected from a sampling transect through this area; these samples have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and are enriched in highly incompatible elements, yielding primitive-mantle-normalized trace-element variation patterns that are similar to ocean island basalts (OIB). A gabbro dike intruded into basalt of the Zhonggang ocean island and was overlain by basaltic conglomerate, suggesting that this dike was formed after the basalt, but before the basaltic conglomerate. The gabbro dike yields an LA–ICP–MS zircon U–Pb age of 116.2 ± 4.1 Ma, indicating the timing of formation of the Zhonggang ocean island, and suggesting in turn that the Banggongco–Nujiang Neo-Tethys Ocean remained open at this time. These data, combined with the geological history of the region, indicates that the Banggongco–Nujiang Neo-Tethys Ocean opened between the late Permian and the Early Triassic, expanded rapidly between Late Triassic and Middle Jurassic time, and finally closed between the late Early and early Late Cretaceous.

Petrology, geochemistry, and geological significance of the Nadong ocean island, Banggongco–Nujiang suture, Tibetan plateau

We studied oceanic mafic igneous rocks of the Mesozoic Banggongco–Nujang suture zone in western Tibet to constrain the tectonic evolution of these rocks and the region as a whole. Two transects were accomplished. Seven basalt samples from the base of the Nadongshan transect (N1 basalts) have flat chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element variation diagrams that are similar to MORB. Two basalt samples from the base of the Nadongshan transect (N1 basalts), ten gabbro samples from the middle of Nadongshan transect (N2 gabbros), four basalt samples from the bottom of Tanjiuxiama transect (T1 basalts), and four basalt samples from the top of the Tanjiuxiama transect (T2 basalts) are alkali basalts and have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and have primitive-mantle-normalized trace element variation diagrams that are enriched in highly incompatible elements, similar to OIB. LREE concentrations increase from N1 basalts to the T1 and T2 basalts, which have (La/Yb)N up to 16 and have even higher (Ce/Sm)N. These data indicate that the Nadong ocean island is an Azores-type ocean island that formed during the mature stage of development of the Banggongco–Nujiang Ocean. The conformable nature of the Nadong ocean island with the Mugagangri Group flysch indicates that the Banggongco–Nujiang Ocean was never a large ocean.

Age and nature of the late Early Cretaceous Zhaga Formation, northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.

Petrology, geochemistry, and geochronology of boninitic dikes from the Kangqiong ophiolite: implications for the Early Cretaceous evolution of Bangong–Nujiang Neo-Tethys Ocean in Tibet

The Kangqiong ophiolite is exposed in the central–western part of the Bangong–Nujiang suture zone (BNSZ) of central Tibet. This study reports new data for boninitic dikes with the aim of reconstructing the geodynamic and petrogenetic evolution of the Kangqiong ophiolite. Ten samples of boninitic dikes that cross-cut the mafic cumulates have very low TiO2 (0.34–0.42%) contents and high MgO (6.65–8.25%) contents. LA-ICP-MS U–Pb analyses of zircon from the boninitic dikes yield an age of 115 Ma. They are characterized by positive εHf(t) values varying from +13.1 to +15.0. Taking into account the geochemical characteristics of the mantle section, the Kangqiong ophiolite should be generated in a fore-arc spreading setting resulting from intra-oceanic subduction. Based on our data and previous studies, we propose that the BNSZ represents the major suture and records the Early Cretaceous intra-oceanic subduction of the Bangong–Nujiang Neo-Tethys Ocean, and the Shiquan River–Yongzhu–Jiali ophiolitic mélange belt represents a back-arc basin. These two belts, together with the northern Lhasa subterrane should, represent an Early Cretaceous intra-oceanic subduction system and back-arc basin in central Tibet that is similar to present-day active intra-oceanic subduction systems in the western Pacific Ocean. The final closure of the Bangong–Nujiang Neo-Tethys Ocean might have taken place later than the Early Cretaceous.

Cambrian granitic gneiss within the central Qiangtang terrane, Tibetan Plateau: implications for the early Palaeozoic tectonic evolution of the Gondwanan margin

ABSTRACT The Tibetan Plateau is located in the eastern Himalayan–Alpine orogen, an area where previous research has focused on ophiolites and a high-pressure metamorphic belt, whereas comparatively little research has been undertaken on the Tibetan basement. Cambrian granitic gneiss crops out in the Duguer area of the South Qiangtang terrane in northern Tibet and yields zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages of 502–492 Ma, providing insight into the possible existence of basement rocks within the South Qiangtang terrane. The granitic gneisses are geochemically similar to high-K, calc-alkaline S-type granites, and Hf isotopic analysis of zircons within the gneisses yields negative εHf(t) values (–7.4 to – 1.1) and old zircon Hf model ages (TDMC = 1757–1406 Ma). These granitic gneisses were generated by partial melting of ancient pelitic rocks, and the resulting melts were contaminated by a small amount of mantle-derived material. Combining our new data with previous research, we conclude that these Cambrian granitic gneisses developed in a post-collisional tectonic setting after Pan-African tectonism. This suggests that the South Qiangtang terrane might have the same early Palaeozoic crystalline basement as the Lhasa, Himalaya, Baoshan, Gongshan, and Tengchong terranes.

Depositional environment and provenance of the upper Permian–Lower Triassic Tianquanshan Formation, northern Tibet: implications for the Palaeozoic evolution of the Southern Qiangtang, Lhasa, and Himalayan terranes in the Tibetan Plateau

Abstract This article reports the depositional environment and provenance for the Tianquanshan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone, and uses these to better understand the tectonic evolution of this region. Zircons in the andesite of the Tianquanshan Formation yielded concordia ages of 246, 247, and 254 Ma, indicating that the Tianquanshan Formation formed during the late Permian–Early Triassic. The Tianquanshan Formation consists of flysch and ocean island rock assemblages, indicating that the Longmuco–Shuanghu–Lancangjiang Palaeo-Tethys Ocean continued to exist as a mature ocean in the late Permian–Early Triassic. The detrital zircons in the greywackes of the Tianquanshan Formation yielded peak ages of 470–620, 710–830, 910–1080, 1450–1660, and 2400–2650 Ma, indicating the provenance of the Tianquanshan Formation was either Indian Gondwana or terranes that have an affinity with Indian Gondwana in the Tibetan Plateau (i.e. the Southern Qiangtang, Lhasa, and Himalayan terranes). The Ordovician quartzites, Carboniferous sandstones, Carboniferous–Permian diamictites, and the Upper Permian–Lower Triassic greywackes in the Southern Qiangtang, Lhasa, and Himalayan terranes all contain detrital zircons with youngest ages of ca. 470 Ma, indicating their source areas have been in a stable tectonic environment since the Ordovician, and this inference is supported by the continuous deposition in a littoral–neritic passive margin in these regions from the Ordovician to the lower Permian. Combining the present results with regional geological data, we infer that the Southern Qiangtang, Lhasa, and Himalayan terranes were all in a stable passive continental margin along the northern part of Indian Gondwana during the long period from the Ordovician to the early Permian. At early Permian, because of the opening of the Neo-Tethys Ocean, the tectonic framework of this region underwent a marked change to a rifting and active environment.

Reconstructing in space and time the closure of the middle and western segments of the Bangong–Nujiang Tethyan Ocean in the Tibetan Plateau

When and how the Bangong–Nujiang Tethyan Ocean closed is a highly controversial subject. In this paper, we present a detailed study and review of the Cretaceous ophiolites, ocean islands, and flysch deposits in the middle and western segments of the Bangong–Nujiang suture zone (BNSZ), and the Cretaceous volcanic rocks, late Mesozoic sediments, and unconformities within the BNSZ and surrounding areas. Our aim was to reconstruct the spatial–temporal patterns of the closing of the middle and western segments of the Bangong–Nujiang Tethyan Ocean. Our conclusion is that the closure of the ocean started during the Late Jurassic and was mainly complete by the end of the Early Cretaceous. The closure of the ocean involved both “longitudinal diachronous closure” from north to south and “transverse diachronous closure” from east to west. The spatial–temporal patterns of the closure process can be summarized as follows: the development of the Bangong–Nujiang Tethyan oceanic lithosphere and its subduction started before the Late Jurassic; after the Late Jurassic, the ocean began to close because of the compressional regime surrounding the BNSZ; along the northern margin of the Bangong–Nujiang Tethyan Ocean, collisions involving the arcs, back-arc basins, and marginal basins of a multi-arc basin system first took place during the Late Jurassic–early Early Cretaceous, resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the Southern Qiangtang Terrane on the northern side of the ocean. However, the closure of the Bangong–Nujiang Tethyan Ocean cannot be attributed to these arc–arc and arc–continent collisions, because subduction and the development of the Bangong–Nujiang Tethyan oceanic lithosphere continued until the late Early Cretaceous. The gradual closure of the middle and western segments of Bangong–Nujiang Tethyan Ocean was diachronous from east to west, starting in the east in the middle Early Cretaceous, and being mainly complete by the end of the Early Cretaceous. The BNSZ and its surrounding areas underwent orogenic uplift during the Late Cretaceous.

Ordovician sedimentation and bimodal volcanism in the Southern Qiangtang terrane of northern Tibet: Implications for the evolution of the northern Gondwana margin

ABSTRACT This article reports our new interpretations of the depositional environment and provenance of the Dawashan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ), in the Southern Qiangtang terrane of northern Tibet, in order to gain a better understanding of the Ordovician tectonic evolution of the northern margin of Gondwana. The Dawashan Formation is dominated by greywacke and shale, with interlayered bimodal volcanic rocks that were deposited in a bathyal to abyssal marine basin. The detrital zircons in the greywacke of the Dawashan Formation have peak ages of 550, 988, 1640, and 2500 Ma, indicating a northern Gondwana margin provenance. The bimodal metavolcanic rocks from the Dawashan Formation are dominated by metarhyolite with subordinate metabasalt. The results of zircon LA-ICP-MS U–Pb dating indicate that the metarhyolite formed between 470 and 455 Ma. The metavolcanic samples are bimodal (SiO2 = 45.27–55.05 and 66.09–74.59 wt.%). In comparison, the metabasalt has a wide range of MgO concentrations and Mg# values, contains variable Cr and low Ni concentrations, is depleted in Rb, Ba, and Sr, and is enriched in TiO2, Th, U, Nb, and Ta. Geochemical diagrams show that the metabasalt erupted in an intra-plate environment. The metarhyolites have high SiO2, Th, and U concentrations, low concentrations of MgO, P2O5, Nb, Sr, and Ti, and negative Eu anomalies. The metarhyolites yield negative zircon εHf(t) values (–2.08 to – 4.50) and TCDM model ages of 1436–1567 Ma. The metarhyolites formed from magma derived from the partial melting of old continental crust. These data indicate that the Dawashan Formation records Middle–Upper Ordovician bathyal to abyssal turbidite deposition in a deep-water rift basin at the northern margin of Gondwana.

Geochronology of the Duguer range metamorphic rocks, Central Tibet: implications for the changing tectonic setting of the South Qiangtang subterrane

ABSTRACT The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected for in situ zircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments.

Geochronology and geochemistry of the Dabure basalts, central Qiangtang, Tibet: evidence for ~550 Ma rifting of Gondwana

Newly discovered basalts in the Dabure area (central Qiangtang block, northern Tibet) were subjected to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating, geochemical analyses, and zircon Hf isotope analyses. Dating of magmatic zircons from three basalt samples indicates that the Dabure basalts formed during the late Ediacaran (~550 Ma). Xenocrystic zircons yield ages of 700–1150 Ma, providing evidence of the Cryogenian crust in the Tibet block. The Dabure basalts are alkaline, rich in Ti and Fe, and are strongly enriched in light rare earth elements without Eu anomalies. The basalts are geochemically similar to within-plate basalts but are relatively depleted in Nb and Ta. Although the analysed zircons show differences in their Hf isotope compositions, the geochemical data suggest that the Dabure basalts were derived from enriched mantle and that the source magmas were contaminated by the continental crust. The basalts may have erupted during rifting at ~550 Ma (from the dating of magmatic zircons), and may have been a product of the initial breakup of Gondwanaland.