Cai Li

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.

Cenozoic high Sr/Y volcanic rocks in the Qiangtang terrane, northern Tibet: geochemical and isotopic evidence for the origin of delaminated lower continental melts

Geochemical and Sr–Nd–Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO 2 contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29–58) and Sr/Y (43–92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low e Nd (t) values (−3.8 to −5.0) and highly radiogenic Sr (( 87 Sr/ 86 Sr) i = 0.706–0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers.

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.

Nature and evolution of the Neo-Tethys in central Tibet: synthesis of ophiolitic petrology, geochemistry, and geochronology

In this paper, we summarize results of studies on ophiolitic mélanges of the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Yongzhu–Jiali ophiolitic mélange belt (SYJMB) in central Tibet, and use these insights to constrain the nature and evolution of the Neo-Tethys oceanic basin in this region. The BNSZ is characterized by late Permian–Early Cretaceous ophiolitic fragments associated with thick sequences of Middle Triassic–Middle Jurassic flysch sediments. The BNSZ peridotites are similar to residual mantle related to mid-ocean-ridge basalts (MORBs) where the mantle was subsequently modified by interactions with the melt. The mafic rocks exhibit the mixing of various components, and the end-members range from MORB-types to island-arc tholeiites and ocean island basalts. The BNSZ ophiolites probably represent the main oceanic basin of the Neo-Tethys in central Tibet. The SYJMB ophiolitic sequences date from the Late Triassic to the Early Cretaceous, and they are dismembered and in fault contact with pre-Ordovician, Permian, and Jurassic–Early Cretaceous blocks. Geochemical and stratigraphic data are consistent with an origin in a short-lived intra-oceanic back-arc basin. The Neo-Tethys Ocean in central Tibet opened in the late Permian and widened during the Triassic. Southwards subduction started in the Late Triassic in the east and propagated westwards during the Jurassic. A short-lived back-arc basin developed in the middle and western parts of the oceanic basin from the Middle Jurassic to the Early Cretaceous. After the late Early Jurassic, the middle and western parts of the oceanic basin were subducted beneath the Southern Qiangtang terrane, separating the Nierong microcontinent from the Southern Qiangtang terrane. The closing of the Neo-Tethys Basin began in the east during the Early Jurassic and ended in the west during the early Late Cretaceous.

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.

Early Cretaceous (100–105 Ma) Adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: implications for the Bangong–Nujiang Ocean subduction and slab break-off

We performed U–Pb zircon dating and whole-rock major and trace element geochemical analyses of monzonites and granodiorites from the Dachagou area, northern Lhasa terrane, Tibet. Zircon U–Pb dating of two plutons yielded magmatic crystallization ages of 100 and 105 Ma, indicating that the intrusive rocks were emplaced in the Early Cretaceous. The monzonites and granodiorites are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and high Sr/Y ratios, and secondarily by low K2O contents, high Mg # values, and relatively high Cr and Ni abundances. Major and trace element geochemical analyses indicate that the rocks have adakitic signatures, suggesting generation by partial melting of an oceanic slab. However, there are some differences between the monzonites and granodiorites, which imply that the monzonitic magmas were contaminated by enriched mantle and crustal components and that the granodioritic magmas were contaminated by enriched mantle melts without a significant crustal contribution. Based on these data and the regional tectonic setting, we propose that the oceanic slab break-off model best explains adakitic magma generation in the Dachagou area. Identification of adakites in the area provides important evidence of slab break-off during the southward subduction of the Bangong–Nujiang Ocean plate.

Petrology, geochemistry and geochronology of gabbros from the Zhongcang ophiolitic mélange, central Tibet: Implications for an intra-oceanic subduction zone within the Neo-Tethys Ocean

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt, the gabbros from new discovered Zhongcang ophiolitic mélange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element (REE) patterns of most of these rocks show flat types with slightly light REE (LREE) depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements (HFSE) (Nb, Ta) and enrichment in large ion lithophile elements (LILE). These gabbros have island arc and mid-ocean ridge basalt affinities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive ɛNd(t) values show that these gabbros were derived from ∼30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Cretaceous, resembling modern active intra-oceanic subduction systems in the western Pacific.

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.

Geochronology, geochemistry, Hf isotopic compositions and formation mechanism of radial mafic dikes in northern Tibet

A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.