Abstract:The gneiss dome is an important tool to reveal the orogeny processes of subduction-exhumation and collision-exhumation. In the Tibetan Plateau, many gneiss dome swarms are distributed in the Paleotethyan and Neotethyan terranes, formed during the subduction-exhumation and/or collision-exhumation process. Superlarge spodumene deposit has been discovered in the pegmatite veins intruded the schist of the Triassic Jiajika gneiss dome in the Songpan-Ganze orogen, showing close spatio-temporal linkage between gneiss domal structure, syn-tectonic granite, ore-bearing pegmatite, and indosinian large decollement zone. The prospecting and exploration of relevant deposits in gneiss domes are a target in the future.

Abstract:Gneiss dome is an important structure resulting from magmatism, metamorphism and deforming during orogeny. A gneiss dome is first reported closely along the northern part of the Xianshuihe sinistral strike-slip fault zone in the Songpan-Ganzi block. It is composed of undeformed granite in the core and foliated granite in the edge, whose lithology close to the fault has been mylonitized. LA-ICP-MS U-Pb dating shows that the undeformed granite formed at 14.51±0.82 Ma and the ages of foliated granites are 14.39±0.23 Ma, 4.25±0.34 Ma and 18.2±1.3 Ma，which indicates the gneiss dome formed at 18~14 Ma. The U-Pb age of mylonite and granite vein in the Xianshuihe shear zone are estimated 14.57±0.34 Ma and 16.73±0.97 Ma, respectively. These ages show a close relationship between the formation of the gneiss dome and the shearing of the Xianshuihe fault zone that the former results from partial melting and bulging of the upper crustal rocks under the shearing of the latter. And in return, the melting promoted the southeastward spreading of the Xianshuihe fault zone, resulting in the formation of the Red River fault along its southeastern part at 11~10 Ma. Meanwhile, they accelerated the clockwise rotation of the Chuandian block, coordinated the eastward motion of the materials in the eastern Tibet and boosted the thickening of the upper crust and subsequent mountain building and river incision. These resulted in rapid cooling of the gneiss dome with a rate of ~61℃/Ma and ~146℃/Ma at 12~5.5 Ma and 4.4~3.1 Ma, respectively.

Abstract:The West Kongur Detachment (WKD) is located in the w estern margin of the Kongur gneiss dome in northeast Pamir. Based on the detailed field investigation, microstructure observation, quartz EBSD testing and zircon U-Pb dating, this study analyzed the geometry, kinematics and evolution time of the WKD. The presence of garnet, sillimanite and kynite in the mylonites from the WKD indicates that the shear zone moved from mid-lower crust to the surface. S/C fabric, rotating porphyroclasts and asymmetric folds in the WKD indicate the characteristic of W- or SW-dipping shearing movement of the hanging wall of the shearing zone. High-grade metamorphic mylonites and leucosomes records the zircon 206U-238Pb age of 20 Ma, suggesting that the WKD initially formed at early Miocene. Combined with previous studies, it can be concluded that the WKD should be the northernmost segment of Cenozoic Pamir mid-lower crust. Due to the fact that the Indian plate continued to northward thrusting after the Cenozoic Indian-Asian collision and this led to thickening of the Pamir crust from south to north, prograde metamorphism firstly occurred in south Pamir in the Eocene, and reached northern Pamir ~20 Ma later. The exhumation of Mid-Lower crust materials in northeastern Pamir started in middle Miocene. It was not until 6~4 Ma when the Kongur region started to uplift rapidly and the Cenozoic metamorphic rocks exhumed quickly.

Abstract:The Great Counter Thrust (GCT) in the northern margin of the Himalayan orogen belt is located in the boundary of India-Asia collision and is also parallel to the Yarlung Tsangpo suture zone (YTSZ). Northward thrusting of the GCT resulted in the superimposition of the Tethys-Himalaya over the Gandese magmatic belt. The GCT constrains the development of the Miocene Kailas-type molass basin (including Kailas, Qiuwu, Dazhuqu, Luobusha and Langxian), with molass deformation gradually increasing from west to east. This study carried out the geochrological analysis of the conglomerate from the east section of the GCT. The results show that sedimentary age of detrial zircon from the Luobusha-Langxian conglomerate is early Oligocene (28.7~29.1 Ma). The pseudotachylyte, metamorphosed and mylonitic conglomerates are extremely well-developed in the Luobusha-Langxian formation. The Luobusha-Langxian conglomerate is characterized by matrix mylonitization, pseudotachylyte and northward shearing, suggesting that the molass basin controlled by the GCT faulting has experienced three stages of tectonic evolution, (1) formed at shallow depths, (2) underthrusted into depths about 10~15 km and with the pseudotachylyte formation, (3) exhumed into surface.

Abstract:Generally ductile shear zone (DSZ) is a combination of simple and pure strain. The only difference is which factor (simple shear or pure shear) plays a significant role. With this theory, decomposing the strain and defining shearing types are very effective in quantitative study of a ductile shear zone. Two common methods of kinematic vorticity (Wk) measurements, the polar Mohr diagrams and quartz c-axis fabrics were applied for the first time in the Xaitongmoin-Quxu ductile shear zone, which show the similar results with uncertainties. By means of nine mylonitic samples analyzed with the polar Mohr diagram, the analyzed results suggest that kinematic vorticity ranges from 0.73 to 0.96, mean value Wk around 0.83. In addition, according to polar Mohr diagrams, we calculated the thinning ratios (S) of the shear zone, showing that the thinning ratio ranges from 0.09 to 0.35, with a mean value around 0.20. We also calculate the kinematic vorticity of the four samples through the quartz c-axis fabrics and strain ratio method, and the results show that Wk ranges from 0.85 to 0.93, with a mean value Wk around 0.88. Above integrated results of kinematic vorticity analyses indicate that the Xaitongmoin-Quxu ductile shear zone is general shear.The results show that the simple shear plays a dominate role in assessing formation mechanism of the Xaitongmoin-Quxu shear zone, which should be related to the tectonic transition of the Lhasa terrain from tectonic compression to extension during Miocene. The structural deformational characteristics and kinematic vorticity determines the tectonic evolution process of the Gangdese region and further advances geologic studies of the middle-south Tibetan plateau.

Abstract:The Himalayan Cenozoic leucogranites, a typical example of S-type leucogranites worldwide, are distributed mainly within two parallel W-S-trending structure belts, i.e. Tethyan Himalayan belt and the Higher Himalayan belt. Experimental results and theoretical calculations indicate that these leucogranites were derived from different partial melting of middle-lower crustal material, closely coupled with structural deformation. (1) During the period of 46~35 Ma and under the condition of thickening of the crust, the partial melting occurred mainly in amphibolite, forming granites with high Sr/Y ratio. (2) During the period of 28~9 Ma, rapid exhumation of deep crustal rocks resulted in the dehydration melting of muscovite, giving a rise to granites with high Rb/Sr ratios. (3) During the period of 21~16 Ma, fluxed melting of muscovite in the metapelites related to E—W extension along the southern Tibet rift zone, producing the granites with low Rb/Sr ratios and high Sr and Ba contents. (4) During the period of 25~27 Ma, partial melting of metapelites occurred in some areas at the high pressure.

Abstract:This study systemically presents the geochemical features and zircon U-Pb and Lu-Hf isotopic compositions of Yinmabin plutons and mafic dykes in the central Mogok metamorphism belt, Myanmar, with an emphasis on their petrogenesis and sources and geological implications. The Yinmabin granodiorites are geochemically characterized by medium SiO2 (64.3%~65.5%) and high Na, with N2O/K2O =1.5~1.75 and A/CNK=0.94~0.97, enrichment in LREE (Sr and Ba >500×10-6) and high field strength elements (HFSE) such as Nb, Ta, Zr and Hf, nd depleted in HFSE. These geochemical features of the Yinmabin granodiorites are of attribute of active continental margin calc-alkaline I-type granite. The LA-ICP-MS analysis yielded an age of 131~135 Ma for the zircon from Yinmabin granodiorite and 120~118 Ma for diabases. The Yinmabin granodiorite are heterogeneous in zircon εHf(t) values (-15.66~-2.99) with a mean of -8.77, yielding younger zircon Hf crustal modal ages of 795~1569 Ma. While zircons from diabases dykes show εHf(t) values from -13.72 to +5.82 (averaging -2.72) and a corresponding mean tDM age of 570~1341 Ma. This suggests that the rocks mainly derived from partial melting of ancient crust with the addition of depleted mantle material. The similar Lu-Hf isotopic compositions to that of the Tengchong and North Lasha magmatism belt indicate that early Cretaceous magmatic belt in the Mogok-Maodanmian block formed in the similar geological background as the Bomi-Chayu-Gaoligong-Lasha magmatic belt. This suggests it be part of southward extension of the Tengchong magmatic rock belt. Combined with petrogenesis, source characteristics and the previous research results, it can be concluded that the formation of Early Cretaceous Yinmabin granodiorite and mafic dyke were closely related to the subduction of the Luxi-Mogok oceanic plate and that the ocean-continent transformation occurred at about 120Ma. The research has important geological significance in understanding the process of subduction and collision of the mid-Tethys ocean during the Late Mesozoic in the Mogok metamorphism belt.

Abstract:The Nyainqentanglha Group of the western part of the Lhasa terrane, distributed along the Shiquanh-Shenzha-Jiali ophiolite m lange and its vicinity, has been traditionally grouped as the Precambrian metamorphic basement. Detailed petrology, geochemistry, structural geology and zircon U-Pb dating were conducted in this study. The results show that characteristic metamorphic minerals, such as schist, gneiss and leptynite suite follows the granularity differentiation rules, suggesting the protolith of the rocks is likely turbidite deposited in active continental margin of Neoproterozoic arc-basin system in the north margin of Gondwana nucleus. Amphibolites are characterized by low Si, high Fe and Mg and Ca, indicating its island-arc type mafic volcanic rock in origin. Granite pegmatite (P37RZ1) of the Nyainqentanglha Group has a LA-ICPMS U-Pb age of 1150±13 Ma, and is of peraluminous affinity to S-type granite, representing the basement rock of the Nyainqentanglha Group started to deposit due to partial melting before the mesoproterozoic (1150±13 Ma). The formation of the granite pegmatite is considered to be associated with the tectonothermal event during the Grenville orogeny. In contrast, the plagioclase gneiss (P38RZ2) has two types of zircons, namely the recrystallization one and the altered one by hydrothermal melt. The first group zircons yield a weighted mean age of 701±15 Ma, indicating a HP-MT amphibolite facies metamorphism during the disperse of the Rodinia supercontinent and/or the assemblage of the Gondwana landmass. The zircons in the second group yield a weighted mean age of 301±8.4 Ma, which is likely related to with the magmatic hydrothermal process derived from the Paleo-Tethys evolution along the northern margin of the Gondwana landmass.

Abstract:The Jiesha mafic-ultramafic body is distributed mainly in the eastern Yarlung-Zangbo suture zone of Tibet. The massif consists of harzburgite, dunite, and podiform chromitite. Chromitite orebodies with a length of 20~40 m and a width of 1~3 m trend from east to west and dip northwest. The surrounding rocks are mainly harzburgites and dunites. Microscopic characteristics and electron microprobe analysis show that the chromian spinel in the Jiesha chromite has Cr# of 679~885 and Mg# of 646~682, with TiO2 content of 006%~018% and Al2O3 content of 131%~165%, suggesting that the Jiesha chromite is high-Cr type chromite Mineral chemistry of olivine, orthopyroxene and clinopyroxene in harzburgite shows that the Jiesha harzburgite are similar to abyssal peridotite and the fore-arc peridotite Based on the chemical composition of spinel-olivine/clinopyroxene, it can be identified that the Jiesha mafic-ultramafic body had experienced early partial melting (degree about 20%～30%) and late rock/melt reaction (degree about >35%) Therefore, the Jiesha peridotites and chromite, like other rock bodies in the Yarlung Zangbo suture zone, might experience multi-stage superimposition processes such mid-ocean ridge （MOR） and late suprasubduction zone （SSZ）

Abstract:Study of mineral inclusions in chromitites is of great significance in recognizing the origin of chromitites. In this paper, a large number of mineral inclusions including CPXs, OPXs, olivines, aspidolites, Na-Cr pargasites, CPX exsolution lamellaes were discovered in Sartohay high-Al chromitites, which were trapped in Cr-spinels as single phase and multiple phase when the Cr-spinels crystallized. Hydrous minerals, aspidolites and Na-Cr pargasites, imply hydrous condition, which probably indicates that parental magmas of chromitites were hydrous in back arc spreading ridge influenced by subducted fluid/melt. Taking account into the UHP diamonds discovered in chromitites, the formation of Sartohay chromitites probably experienced the deep mantle and shallow oceanic lithosphere setting.

Abstract:Besides the widespread granitoids formed during Late Cretaceous, the late Triassic to Jurassic granites are also known to occur in the Gangdese magmatic belt. However, their petrogenesis remains a controversial issue. In this study, we conducted petrological, geochemical, zircon U-Pb dating and Hf isotopic studies for the Xiangmucun tonalites from the eastern parts of southern Gangdese magmatic belt. The tonalites consist mainly of plagioclase, quartz, biotite with minor amphibole. LA-ICP-MS zircon U-Pb dating yielded a crystallization age of ca. 184±4 Ma for the tonalites. They have positive εHf(t) values ranging from 7.90 to 10.26. The geochemical data further indicate that the Xiangmucun tonalites are middle K calc-alkaline and metaluminous, and generally show characteristics of adakitic rocks, i.e., the high SiO2, Al2O3, Nae2O/K2O (>1) and Sr/Y (35.5～105.6), low Y (4.46×10-6～12.9×10-6) and Yb (0.51×10-6～1.39×10-6), and enrichment in large ion lithophile elements (LILE) of Rb, Ba and K, and depletion in high field strength elements (HFSE) of Nb, Ta, Ti and P. They exhibit concave-upward LREE-riched and MREE-depleted patterns with negligible Eu anomalies. We suggest that the Xiangmucun tonalites were derived from the partial melting of mafic lower crust, with amphibole and garnet as the main residual phases and free of plagioclase. Combined with previous data, the zircon Hf isotopes of the Gangdese Late Triassic-Jurassic granites show a gradual enrichment trend from west to east and from south to north, which indicates that the contribution of ancient crustal components gradually increased and the ancient crust basement may exist in the eastern parts of the southern Gangdese belt.

Abstract:The Baishan tectonic belt is located in the conjunction region between northern and southern tectonic units in the Beishan orogenic collage. Various plutons, widely distributed in the Baishan tectonic belt, play important roles in recognizing the late Paleozoic teconic evolutions of the northern Beishan. In this study, we report new zircon U–Pb ages, bulk–rock major and trace element, and zircon Hf isotopic data for the Shuanjingzi pluton in the Baishan belt. Shuangjingzi granodiorite and K-feldspar granite have zircon LA-ICP MS U-Pb ages of 284.9±1.5 Ma and 327.6±1.6 Ma, respectively. Geochemical data show that they both are I-type granites with weak peraluminous, calc- alkaline characteristics, and display enrichments in large ion lithophile elements and depletions in high field strength elements. In addition, Shuangjingzi plutons display similar zircon Hf isotopic compositions. They both have relatively high εHf(t) values (7.92—12.24 and 5.87—10.28, respectively) and relatively juvenile TDM2 model ages ( 510—796 Ma and 679—961 Ma, respectively). Zircon Hf isotopic compositions indicate strong crustal growths in the northern Beishan during Carboniferous—Middle Permian. Our study and previous studies on Hongshishan ophiolitic mélange and volcanic rocks in the Baishan group all support a Paleo-ocean subduction setting in the Baishan tectonic belt during Carboniferous—Middle Permian. The Shuangjingzi pluton may derive from southward subduction processes of the Paleo-ocean represented by the Hongshishan ophiolitic mélange.

Abstract:The acidic volcanic rocks are widespread in the Coqen basin, central Lhasa subterrane. Although numerous studies have been conducted on theses volcanic rocks, many uncertainties and controversies still remain as to their petrogenesis and geodynamic setting. In this paper, we take the volcanic rocks outcropped in both south and north sides of the Tanuochuo-Coqen fault as an example, and conducted an integrated study including zircon U-Pb dating and Hf isotopes, and whole-rock elemental geochemistry, aiming to obtain the precise ages, and to discuss magma source and geodynamic mechanism for their formation. Previously regional survey suggests that the volcanic rocks in the north side of the fault belong to the Lower Cretaceous Zenong Group and those in the southern side belong to the Paleocene Dianzhong Formation. However, our new zircon LA-ICP-MS U-Pb dating results demonstrate that volcanic rocks from both sides of the fault share similar ages of 110~120 Ma, indicating that they all belong to the Lower Cretaceous Zenong Group. Lithologically, volcanic rocks in the studied region are mainly a set of dacitic and rhyolitic rock association. They are chemically belong to high-K calc-alkaline series, and are enriched in LREEs and LILEs (e. g., Rb, Ba, etc.), depleted in HFSEs (e. g., Nb, Ta, Ti, etc.), showing typical signatures of arc-related igneous rocks under a subduction setting. All samples have enriched zircon Hf isotope compositions, with εHf(t) values of -12.9~-1.6 and tDM2 values of 1.28~1.99 Ga, indicating that these volcanic rocks were derived mainly from anatexis of the Paleoproterozoic to Mesoproterozoic basement of central Lhasa subterrane, with contributions of depleted mantle components. Based on a synthesis of geology, geochemistry and tectonics, we suggest that these volcanic rocks were likely generated under a continental marginal arc setting with a relatively thick crust, and the slab break-off of the southward subducted Bangong-Nujiang oceanic lithosphere occurred during the middle-late stage of Early Cretaceous is the most possible mechanism that triggered the magmatism in this area.

Abstract:The volcanic rocks in the Triassic Manghuai Formation (MHF) and Xiaodingxi Formation (XDF) are an important component of igneous rock belt in the central-southern Lancangjiang orogeny, and the product of the tectonic evolution of the Lancangjiang Paleotethyan suture. This study reports the petrology, geochemistry and geochronology of the andesitic and rhyolitic interlayers in the thick XDF basalts. Zircon LA-ICP-MS U-Pb dating shows that the XDF volcanic rocks erupted at 230 Ma. Andesite is characterized by high K calc-alkaline, strongly negative Nb, Sr and Ti anomalies, slightly negative Eu anomalies, low Mg#, (La/Yb)N ratios, Cr and Ni contents, which are significantly similar to that of the XDF high Al basalts, indicating the magma of the andesite are the same as that of the basalt and derived from partial melting of metasomatized mantle. The rhyolite in the XDF is strongly peraluminous igneous with negative Ba, Nb, Ta, Sr, P, Eu and Ti anomalies, which are similar to that of the Lincang granite. The Nb/Ta, Th/Ce and Th/U ratios of rhyolites are consistent with that of the continental crust, with εHf(t) values range from -11.6 to +9.6 (the peak εHf(t) values of -6.0~+2.0), which corresponds to crustal Hf model ages (tDM2) of 658~1990 Ma. Most of the ages range from 1100 to 1600 Ma, showing the XDF rhyolitic magma originated mainly from the partial melting of middle/upper crustal rocks, and mingled with a minor volume of mantle-derived melt. Andesites and rhyolites in the thick XDF basalt have similar geochemical characteristics, such as the enrichment in LILE and LREE, strongly negative Nb, Ta and Ti anomalies, and all the samples falling within the active continental-margin arc or syn-collision magma field in tectonic discrimination diagrams. In combination with the lack of the early Triassic strata, unconformable contact between late Triassic molasse formation with the underlying strata, and occurrence of the high-pressure metamorphic belt in the west segment of the Lancangjiang igneous belt, it can be reasonably concluded that the Mesozoic volcanic rocks in the central-southern Lancangjiang orogeny, similar to Lincang granitic batholith, was generated in syn-collision tectonic setting related with the collision between Baoshan and Indochina blocks. The mafic magma derived from metasomatized mantle wedge migrated upward to the crust, and triggered the partial melting of crustal material to form granitic magma. The basaltic magma from lower mafic magmatic chamber and the granitic magma from upper felsic magma chamber erupted intermittently during middle Triassic to form the Mesozoic volcanic belt consisting of the MHF rhyolite with thin layers of andesite and basalt in the low and the XDF basalt with thin layers of andesite and rhyolite on the upper.

Abstract:SHRIMP U-Pb dating was performed on the dunite and harzburgite samples collected from the Kangjinla No.11 chromite in the Luobusha ophiolite, Tibet. Age results of the two samples are not coupled. Dunite sample has an age range from from 125.6±2.6 Ma to 1791±23 Ma (from Archean to Early Cretaceous), with a minimum weighted average value of 130.0±2.8 Ma (2.1%) for four zircon samples. Harzburgite has an age range from 208.9±3.7 Ma to 2770±44 Ma (from Archean to Late Triassic) with a weighted average value of 209.7±5.2 Ma (2.5%) for two zircon samples. The complex characteristics of zircon grains imply the multi-stage geological evolution. Laser Raman Spectroscopy analysis shows that zircons in dunite and harzburgite contain inclusions of crustal material consisting of apatite, feldspar and quartz, indicating its crustal genesis and recording a process of crustal contamination of the mantle. In combination with a large amount of unusual mantle minerals discovered in the Kangjinla chromite and mantle peridotite, there might be a cycle process that crustal material returned surface from oceanic ridge after subducting to mantle, which also proves a multi-stage formation model of the podiform chromitite.

Abstract:Yidun Arc, located between Songpan-Garzê Fold Belt and Qiangtang Block, southwestern China was the result of Late Triassic large-scale subduction orogenic process. Daocheng granite and Haizishan granite are the two outcropped granites in the Yidun Arc during the Late Triassic and Cretaceous, respectively. On the basis of field investigation, this study carried out the analysis of petrology and zircon U-Pb ages for these two plutons. The results show rock-forming minerals of the two granitic plutons are mainly composed of plagioclase, K-feldspar, quartz and biotite, and the accessory minerals include zircon, sphene, magnetite and apatite. Sodic plagioclase grains and occasionally present K-feldspar and quartz indicate that the two granitic plutons belong to subsolvus granite and the formation of two plutons was related to orogenic event under water-bearing condition. The Daocheng granite has emplacement age of 217.4 Ma (Late Triassic), with εHf（t）ranging from -7.1 to -0.1, and two-stage model age（tDM2）of 1.26~1.7 Ga, indicating that the Yidun Arc and Yangtze Block may experience the same evolutionary history of continental crust during the Mesoproterozoic time. The Haizishan granite has an emplacement age of 98.3 Ma (Cretaceous) , with εHf（t）ranging from -12.1 to +2.5, and a tDM2 age of 1.0~1.93 Ga. In combination with previous studies about magmatism in the Yidun Arc, it can be concluded that the source rocks of Daocheng granite was probably derived from partial melting of the Mesoproterozoic lower crust material related to Yangtze Craton with addition of minor depleted mantle-derived magma, which was triggered by the underplating of large-scale mantle-derived magma under the syn-collisional tectonic setting following the westward subduction and closure of the Garzê-Litang paleo-ocean. The Haizishan granite was an A2-type granitic pluton resulting from post-collisional extensional tectonic setting related to arc-continent collision during Early Cretaceous. Its source rocks were also derived from Mesoproterozoic crustal material with addition of minor mantle-derived magma, but emplaced at shallow crust and experienced rapid cooling.

Abstract:The metamorphism and deformation process of Fenzishan Group and Jingshan Group in the Jiaobei massif are important for understanding the evolution of North China Craton. Previous geochronological study in this area mainly focused on zircons. The metamorphism and deformation age occurred around Paleoproterozoic because most geochronology data published are around 1.8 ~ 1.9 Ga. Monazite and rutile separated from 2 garnet mica schist and 2 mica schist samples, which were collected from Fenzishan Group and Jingshan Group, were analyzed for U-Pb isotopic age in this study. The results reveal that Fenzishan and Jingshan groups not only experienced the Proterozoic (1869±16~1864±14 Ma) metamorphism, but also overprinted by Triassic deformation and metamorphism (215.1±4.2~217.8±6.3 Ma). Using Zr thermometry in rutile yielded three U-Pb ages of 215.1±4.2 Ma, 217.7±2.7 Ma and 217.8±6.3 Ma, which correspond to 658℃, 667℃ and 680℃. Our study can conclude that the Jiaobei massif involved in the Triassic Sulu-Dabie HP-UHP subduction-collision orogenic process, superimposed by Triassic amphibolite facies metamorphism and deformation.

Abstract:Mesozoic marine Qiangtang Basin, lying on the Qiangtang Terrane, generally has the features of median mass basin. It can be divided into five first-order tectonic units: northern Qingtang depression, western uplift, middle lower uplift, southern Qingtang depression and northeastern slope zone. Qiangtang Basin has experienced three evolution stages which are Triassic marine basin, Jurassic—early Early Cretaceous marine basin and late Late Triassic tectonic transition during the above two stages. The Triassic basin with its basement higher in south and east, controlled by the opening and closing of Lazhulong-Xijinwulan tectonic belt on its north side, is characterized by volcanic, clastic, carbonate, gypsum-salt rocks and coal-bearing clastic rocks. At the end of Late Triassic, responding to the closing of Lazhulong-Xijinwulan ocean and collaging of Qiangtang Terrane and Kekexili Terrane, this marine basin uplifted as a whole and there followed a sedimentary hiatus with the formation of karst weathering crust, after which this region underwent regional rifting and volcanic activity, indicating the formation of Bangonghu-Dingqing intercontinental rift. During Jurassic, Qiangtang Basin once again subsided and received marine sediments, approximately synchronous with the development of Bangonghu-Dingqing intercontinental rift on its south side, indicative of the marine transgression from south to north. The Jurassic marine basin developed multiple NWW-trending depo-subsidence centers, with higher relief in southwest and northeast and relative subsidence centers in east part of southern Qiangtang depression and central-west part of northern Qiangtang depression. Jurassic basin was characterized by clastic, carbonate and gypsum-salt rocks. In early Cretaceous, the convergence between Gangdise (Lhasa) Terrane and Qiangtang Terrane and the closure of Bangonghu-Dingqing intercontinental ocean put an end to the marine sedimentary history of Qiangtang Terrane and since then Qiangtang Terrane stepped into continental sedimentation of several small basins. Late Early Cretaceous—Late Cretaceous basin inversion and Cenozoic modification largely reformed the original appearances of the Mesozoic Qiangtang Basin and its north and south suture zones, and disappearance of the northwestern part of the basin. The superposed evolution and modification of Qiangtang Basin creates favorable conditions for Mesozoic marine successions to contain multi-set source rocks, have high oil-gas exploration prospect and form multi-layer and multi-type petroleum accumulation. The discoveries of several different kinds of oil-gas shows and large paleoreservoirs indicate and prove the ever-existing processes of hydrocarbon formation and accumulation in Qiangtang Basin. In conclusion, late modification is the main factor influencing the hydrocarbon accumulation and distribution, resource quantities and prospect, and exploration effectiveness of Qiangtang Basin.

Abstract:The available paleomagnetic data from Duila limestones provide important constraints on paleogeography of the northern margin of Greater India but controversy still remains. Hence we carried out a further study on the late Cretaceous marine sediments in this area. On the basis of systematic demagnetization and statistical analysis on samples collected from 10 sites, characteristic remnant magnetizations (ChRMs) were successfully isolated with tilt-corrected direction of D=168.2°，I=-7.3°，α95=7.5°, which is insignificantly different from the one obtained from the middle and upper part of the Zongshan Formation in the area of Duila and have passed a positive fold test suggesting a pre-folding in nature. The uniform reversed polarity of the ChRMs for the Zongshan and Gamba CunKou formations is, however, incompatible with the geomagnetic polarity time scale during the time interval from the Turonian to Maastrichtian suggesting that the primary remanence of the rocks had most probably been completely overprinted and hence could not be used for paleogeographic reconstruction of Greater India. Excluding the data obtained from the Zongshan Formation in the Duila area, the remaining 8 sites obtained from the area of Gamba suggest that the Tethyan Himalayan Terrane was located at ~9°S, and the amount of the northern extension of Greater India is recalculated to be ~1500 km, which is compatible with the one obtained from the Zongpu Formation by former studies.