Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2>59%, enrichment in A12O3(15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g) and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg<0.35), weak Eu anomaly (Eu/Eu=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magma resulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and the petrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.

40Ar-39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang, Northern Tibet, Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China （84 30＇ E, 34024＇ N）, provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite- and glaucophane-bearing assemblages equilibrated at 375-400 C and 11 kbar. 4Ar-39Ar analysis on mineral separate from one blueschist sample yielded a well-defined plateau age of 242 Ma. Geochemical studies show the blueschist is metamorphosed within-plate basalts. The high pressure-low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in situ suture between eastern and western Qiangtang.

Reconstruction of the Triassic Tectonic Lithofacies Paleogeography in Qiangtang Region, Northern Qinghai-Tibet Plateau, China

The Triassic petrostratigraphic system and chronologic stratigraphic sketch have been updated and perfected in the Qiangtang area, Qinghai-Tibet Plateau based on the integrated 1：250000 regional geological survey and the latest research progeny. The first finished 1：3000000 Triassic tectonic lithofacies paleogeographic maps in the Qiangtang area shows that the Triassic tectonic unit in the Qiangtang area can been divided into three parts from north to south： northern Qiangtang block; Longmucuo-Shuanghu suture zone; and southern Qiangtang block. The early-middle Triassic tectonic paleogeography in the Qiangtang area is divides into three sub- units： northern Qiangtang passive continental marginal basin （NQPB）, Longmucuo- Shuanghu residual basin （LSRB） and southern Qiangtang residual basin （SQRB）. The NQPB can be subdivided into four paleogeography units： The Tanggula-Zangxiahe shallow and bathyal sea; The Wangquanhe- Yingshuiquan carbonate platform; The Rejuechaka-Jiangaidarina littoral- shallow sea; and Qiangtang central uplift. The above units of The NQPB possess EW trend, geomorphology high in the south and low in the north, the seawater depth northward. The basinal paleo-current direction is unidirectional, and basinal tectonic subsidence center is in accord with the depo-center, located in the Tanggula-Zangxiahe belt, north of the basin. The sedimentation and tectonic evolution of the NQPB are characterized with passive continental marginal basin. The Qiangtang central orogenic denuded area （ancient land） may be as a sedimentary materials source of the NQPB. SQRB can be divided into two units： Duoma carbonate platform and southern Qiangtang neritic-deep sea. The late Triassic tectonic paleogeography in the Qiangtang area is the framework of the ＂archipelagic-sea＂ as a whole, and it may be divided into three sub-units： northern Qiangtang back- arc foreland basin（NQFB）, Longmucuo-Shuanghu residual basin（LSRB） and southern Qiangtang marginal-sea basin（SQMB）. Thereinto, NQFB can be divided into five paleogeography units： the Zangxiahe-Mingjinghu bathyal basin characterized with the flysch; the Tanggula shallow-sea shelf with the fine-clastics; the Juhuashang platform with carbonates; the Tumenggela-Shuanghu coastal- delta with coal-bearing clastics and the Nadigangri- Geladandong arc with volcanics and tuffs. In transverse section, the NQFB fills is wedge-shaped, and the sediments characterized with thicker in north and thinner in south, and with double materials derived from the Ruolagangri orogenic belt in north and the Shuanghu central orogenic belt in south. The late Triassic depocenter of NQFB is located in the middle of the basin, the Yakecuo-Bandaohu-Quemocuo belt, but the subsidence center in the north, the Zangxiahe- Mingjinghu belt, and basinal tectonic subsidence center not concordant with the depo-center. Late Triassic, the SQMB may be divided into three sub-units： Xiaochaka shallow-sea; Riganpeicuo platform~ and South Qiangtang southern bathyal basin. In transverse section, the basement of the SQMB is characterized with low in the northern and southern, but high in the middle; forming wedge shaped sediments with thicker in the north and thinner in the south; the sedimentary materials derived from the Qiangtang central uplift and Nadigangri arcs in north. The late Triassic subsidence centre of the SQMB is located in the northern （Xiaochaka area）, but the depocenter in the southern （Qixiancuo Suobucha area）. The sedimentation and tectonic evolution of the SQMB are characterized with marginal sea.

Geochronology and Genetic Model for Early Cretaceous Volcanic Rocks from the Southern Qiangtang Terrane,Northern Tibet,China: Constraints from U-Pb Zircon Dating, Whole-Rock Geochemical and Sr-Nd Isotopic Data

Post-collisional volcanic rocks of Mesozoic age occur in the regions adjacent to Gerze, part of the southern Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and wholerock Sr-Nd isotopic analyses were performed on the volcanic rocks to better characterize their emplacement age and models for their origin. Laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS) U-Pb zircon analyses yielded consistent ages ranging from 123.1±0.94 Ma to 124.5±0.89 Ma for six volcanic rocks from the study area. The intermediate volcanic rocks belong to the alkaline and sub-alkaline magma series in terms of K2 O+Na2 O contents(5.9%–9.0%), and to the shoshonitic and calc-alkaline series on the basis of their high K2 O contents(1.4%–3.3%). The Gerze volcanic rocks are characterized by the enrichment of light rare earth elements [(La/Yb)N=34.9–49.5] and large–ion lithophile elements(e.g., Rb, Ba, Th, U, K, Pb, and Sr), slightly negative Eu anomalies(Eu/Eu*=0.19–0.24), and negative anomalies in high field strength elements(e.g., Nb, Ta, Hf and Ti), relative to primitive mantle. The samples show slightly elevated(87 Sr/86 Sr)i values that range from 0.7049 to 0.7057, and low εNd(t) values from-0.89 to-2.89. These results suggest that the volcanic rocks studied derived from a compositionally heterogeneous mantle source and that their parent magmas were basaltic. The more mafic, parental magmas to the Gerze volcanic rocks likely underwent fractional crystallization of clinopyroxene, hornblende, biotite, and potassium feldspar, during ascent, with little to no crustal contamination, prior to their eruption/emplacement. While these volcanic rocks exhibit geochemical signatures typical of magmas formed in a destructive plate-margin setting, it is plausible that their mantle source might also have acquired such characteristics in an earlier episode of subduction.

DEFINITING AND ITS GEOLOGIC MEANING OF SOUTH-NORTH TREND FAULTED STRUCTURE BELT IN QIANGTANG BASIN, NORTH PART OF TIBET

There were more expounding to north—west (west) trend fault and north\|east trend fault within Qiangtang Basin, North Part of Tibet, in the past literature. With increasing of geophysical exploration data, nearly east\|west trend structure began to be taken note to. Since the year of 1995, by a synthetic study to geophysical and geological data, that south\|north trend faulted structures are well developed. These structures should be paid much more attention to, because they have important theoretical meaning and practical significance.1 Spreading of south\|north faulted structure belt According to different geological and geophysical data, the six larger scale nearly south\|north faulted structure belt could be distinguished within the scope of east longitude 84°～96° and near Qiangtang Basin. The actual location of the six belts are nearly located in the west of the six meridian of east longitude 85°,87°,89°,91°,93°,95° or located near these meridian. The six south\|north faulted structure belts spread in the same interval with near 2° longitude interval. The more clear and much more significance of south\|north trend faulted structure belts are the two S—N trend faulted structure belts of east longitude 87° and 89°. There are S—N trend faulted structure belts in the west of east longitude 83°,81°, or near the longitudes. The structure belts spreading features,manifestation,geological function and its importance, and inter texture and structure are not exactly so same. The structure belts all different degree caused different region of geological structure or gravity field and magnetic field. There is different scale near S—N trend faulted structure belt between the belts.

Evaluation of the oil and gas preservation conditions, source rocks, and hydrocarbongenerating potential of the Qiangtang Basin: New evidence from the scientific drilling project

The Qiangtang Basin of the Tibetan Plateau,located in the eastern Tethys tectonic domain,is the largest new marine petroliferous region for exploration in China.The scientific drilling project consisting primarily of well QK-1 and its supporting shallow boreholes for geological surveys(also referred to as the Project)completed in recent years contributes to a series of new discoveries and insights into the oil and gas preservation conditions and source rock evaluation of the Qiangtang Basin.These findings differ from previous views that the Qiangtang Basin has poor oil and gas preservation conditions and lacks high-quality source rocks.As revealed by well QK-1 and its supporting shallow boreholes in the Project,the Qiangtang Basin hosts two sets of high-quality regional seals,namely an anhydrite layer in the Quemo Co Formation and the gypsum-bearing mudstones in the Xiali Formation.Moreover,the Qiangtang Basin has favorable oil and gas preservation conditions,as verified by the comprehensive study of the sealing capacity of seals,basin structure,tectonic uplift,magmatic activity,and groundwater motion.Furthermore,the shallow boreholes have also revealed that the Qiangtang Basin has high-quality hydrocarbon source rocks in the Upper Triassic Bagong Formation,which are thick and widely distributed according to the geological and geophysical data.In addition,the petroleum geological conditions,such as the type,abundance,and thermal evolution of organic matter,indicate that the Qiangtang Basin has great hydrocarbon-generating potential.

THE LITHOSPHERIC EVOLUTION IN THE QIANGTANG BLOCK OF NORTHERN TIBET PLATEAU: EVIDENCE FROM CENOZOIC VOLCANISM

Introduction The widespread occurrence of Cenozoic magma of the Tibetan Plateau suggest that they are common to most destructive plate margins and orogenic belts and active faults. In northern Tibet, volcanic rocks are divided into two volcanics subzone, i.e., the Qiangtang subzone and Kunlun subzone.We found most shoshonitic lavas in Kunlun subzone had extruded for 0～17Ma..The sodic volcanics had erupted for 44～60Ma in western Qiangtang subzone and the most potassic lavas had extruded for 18～45Ma, in eastern Qiangtang subzone.The volcanism in Qiangtang starting 60Ma ago, most volcanic rocks dominantly formed between 45 and 18Ma (Fig.1). From 45Ma to 18Ma, there is a trend that the strength of volcanism gradually decreases. The peralkaline volcanism occurred in late Palaeogene and was re\|active at local area in Neogene.

STUDY ON THE BASIC IGNEOUS EVENT IN CENTRAL QIANGTANG, TIBET

It bas been considered that large\|scale basic igneous activities are related to results of the break\|up of paleo\|continent. Basic magmatism could record the dynamics information for orogenic process. Thus, studying it can help us to reconstruct the forming and evolution process of continental orogenic zone.The basic igneous zone in the central part of Qiangtang, Tibet, starts at the Sino\|Indian border in the west and ends at Shuanghu region in the east, being over 800km long and several to more than 100km wide. Based on published data, there are coeval basic magmatic activities with similar features in Sanjiang region, Western Yunnan and Thai\|Malaysian Peninsula. the basic dykes from Boyamalong to Qomu in central Qiangtang occur in swarms with E\|W trending and are more than one thousand in number. Single or composite igneous body is 1 to 7 km long and 0.3 to 3 km wide, being of the area of 0.3 to 20km\+2 . The total exposed area of the dykes is more than 400km\+2 . The country rocks of the intrusives are the upper Carboniferous Mushirbuka group, which consists of rift\|type deposits containing glaciomarine petromictic conglomerates.No basic dyke has been found in the strata since late Permian (Mokou Period)

THE DECOLLEMENT IN THE QIANGTANG BASIN, TIBET

Qiangtang Basin in northern part of Tibet is significant on geological research, and it is also a prospect area for petroleum and gas exploration. Qiangtang Basin mainly consists of Triassic\|Jurassic carbonate strata, extending E—W. The basement of the Qiangtang Basin composed of Lower\|Middle Proterozoic exposed in the central part, and is called Central Upright Zone.The decollement and thrust structure occurred both in Qiangtang Basin and the Central Upright Zone, which have resulted in important influence for petroleum and gas exploration.(1) Tectonic style:① The suprastructure of Qiangtang Basin is dominated by parallel folds (Ramsay’s classification Ib\|Ic) and brittle faults.② Most of the folds are open folds with interlimb angles 80～120°and lack of axial cleavage.③ The 3\|D shape of fold is cylindrical, without or little change on area and volume.④ The folds association is ejective folds (i.e. with the characteristics of the Jura\|type fold).⑤ The plastic bed flowing with the higher zone of the folds formed diapir structure, which is the important evidence indicating decol lement.

PALAEOMAGNETIC STRATIGRAPHIC STUDIES OF THE JURASSIC AND LOWER CRETACEOUS ROCKS FROM THE QIANGTANG BASIN, NORTHERN TIBET

The Qiangtang Basin located in northern Tibet is a Jurassic foreland basin, whereas the sedimentation for the arc\|basin system during the Late Triassic. Paleomagnetic sampling sites and sections include the Lower Jurassic section in Juhuashan, Shuanhu district, the Middle and Upper Jurassic section in Nadigangri Mountain and the Lower Cretaceous section in Abushan, Shuanhu district. The Lower Jurassic Nadigangri Fm. is composed of tuffaceous volcanic rocks and turbidite (lower) and purple clastic rocks. The Middle Jurassic consists of Quemocuo Fm. purple clastics , Buqu Fm. carbonates and Xiali Fm. gypsum\|bearing varicolored clastics.The Upper Jurassic includes Suowa Fm. carbonates and Xueshan Fm. purple clastics. The Lower Cretaceous Abushan Fm. is lacustrine clastics.1723 oriented paleomagnetic samples of Jurassic and Cretaceous strata were collected in 1997.The sampling sections is located in Shuanhu district of northern Tibet. Although it is unlikely that the sections studied formed by constant and continuous deposition, field evidence indicates no major breaks in the Jurassic sedimentation, except early Bajocian stage. Based on sections of actual survey, all sampling was done using a portable gas\|powered core drill, and cores were oriented with magnetic compass and inclinometer. Samples were obtained at common stratigraphic spacing of 0 5～5m, partly 5cm at some important geological boundary\|surfaces and beds/members. 25mm diameter and 20～50mm length paleomagnetic core samples were drilled in cropping field. The measuring of most samples was completed at the China Academy of Geosciences Paleomagnetic Laboratory using a type DSM\|2 digital rotational magnetometer (its sensitivity of reaching 10 -5 A/m) made by SCHONSTEDT Company of U.S.A., and SCHONSTEDT TSD\|1 for thermal demagnetization. The apparatus used for AF demagnetization was a commercial (SCHONSTEDT GSD\|5)instrument, capable of reaching 100mT peak field. 10% of total measuring samples were completed at the Beijing Geological Institute Paleomagnetic Laboratory of China Academy of Sciences using a type 2G\|755R magnetometer made by Superconducting Technology for weak magnetized samples (sensitivity of reaching 10 -8 A/m). Most samples were stepwise thermally demagnetized, at 50℃ intervals from 100 to 700℃.

BASIN-RANGE TRANSITION AND GENETIC TYPES OF SEQUENCE BOUNDARY OF THE QIANGTANG BASIN IN NORTHERN TIBET

The surface of sequence boundary is a negative record. Its recognition largely depends on the physics of the sediments below and above the boundaries, or on the different sedimentary structures are synthetic marks for the sedimentation and tectonic movements in the sedimentary basin. The Qiangtang Basin that is in 5000m above the sea level is located in Northern Tibet. The Lazhulung—Jinshajiang suture zone now bound it to the north and the Bangong—Nujiang suture zone to the south. Three second\|order tectonic units have been distinguished, i.e. North Qiangtang depression, Central rise and South Qiangtang depression from north to south.The Upper Permian Riejuichaka Formation is built up of mudstone and mud\|limestone, which is represented by sediments in seamarsh. The Lower Triassic Kuanglu Formation, which exhibits the structure unconformable contact with the overlying Upper Permian strata, is characterized by terrigenous clastic rocks in the lower area and is carbonate rocks in the upwarding area and the Middle Triassic Kuangnan Formation. The Upper Triassic Xiachaka Formation consisting of terrigenous clastic rocks, carbonates rocks and mixed sediments, is confined to the uplift zones. The lower Jurassic volcanic rocks are deposited in continental rift. The middle and Upper Jurassic Yangshiping Group are conformable contact and assembled by the gypsum\|bearing terrigenous clastic rock formations and carbonate rock formation. The Middle Cretaceous and the Paleocene strata is built up of the terrigenous clastic rock formations.

TECTONIC CHARACTERISTICS AND EVOLUTION OF THE QIANGTANG BASIN IN NORTHERN TIBET PLATEAU

The Qiangtang basin is located in the north of Qinghai—Tibet plateau and sandwiched by Nianqingtangula continental block and Kekexili\|Bayuankal continental block. Its southern boundary is the Bangongfu—Nujiang suture zone and its northern boundary is the Xijinwulan\|Jinshajiang suture zone.The basement of Qiangtang basin is composed f metamorphic rock of Proterozoic age, which can be divided into two parts. The competent lower part with isotope age of 2056～2310Ma experienced multi\|stage deformation and the soft upper part is dated 1111～1205Ma. Within the basin, it groups into Northern Qiangtang Depression, Central Rise and Southern Depressions and are complicated by a number of subdepressions and subuplifts.The strata of Middle Devonian\|Tertiary are overlain on the basement and composed of marine carbonate rocks, clastic rocks and terrestrial sandstone and conglomerate. Several sets of faults and folds have developed in the cover sequence and the deformation is very strong, characterized by orientation, zonation and equidistance in space and by diversity and disharmony in the profile. The major deformation occurred in Yanshan\|Himalayan period.

PALEOGENE(?) DEPOSYSTEMS AND BASIN EVOLUTION IN THE EASTERN TIBETAN PLATEAU: NANGQIAN AND XIALAXIU BASINS

Paleogene sedimentary basins exposed across much of the central and eastern Tibetan Plateau may record the early history of plateau uplift related to the Indo\|Asian collision. We conducted sedimentological and stratigraphic investigations in the northeastern Qiangtang terrane, eastern Tibetan Plateau. Our results indicate development of several nonmarine basins during Paleogene(?) time, probably synchronous with northeast\|southwest contractional deformation. The Nangqian and Xialaxiu basins (96°～97°E, 32°～33°N) are composed of 500～ 20000m thick successions of primarily clastic sediment indicative of lacustrine and alluvial\|fan depositional processes. Paleocurrent measurements and sediment compositional data indicate local sediment source areas composed of Carboniferous\|Triassic carbonate and sandstone and minor Tertiary volcanic rocks. The large variability of provenance and facies types suggest that each basin evolved independently, as opposed to regional development of a single integrated basin which was partitioned by later deformation.

Permian Fusuline Fauna from the Lower Part of the Lugu Formation in the Central Qiangtang Block and its Geological Implications

A Kubergandian （Kungurian） fusuline fauna from the lower part of the Lugu Formation in the Cuozheqiangma area,central Qiangtang Block is described.This fusuline fauna belongs to the Southern Transitional Zone in palaeobiogeography,and is characterised by the presence of the distinctive bi-temperate genus Monodiexodina and many genera common in lower latitude Tethyan areas such as Parafusulina and Pseudodoliolina.The occurrence of Monodiexodina in the fauna confirms that the seamount-type carbonates of the Lugu Formation did not originate from the Palaeotethys Ocean,but rather from a branch of the Neotethys Ocean after the rifting of the Qiangtang Block from the Tethys Himalaya area in the Artinskian.

The Cretaceous tectonic event in the Qiangtang Basin and its implications for hydrocarbon accumulation

The tectonic event during Cretaceous and its relationship with hydrocarbon accumulation in the Qiangtang Basin is discussed based on zircon U-Pb dating and the study of deformation, thermochronology and hydrocarbon formation. LA-ICPMS zircon U-Pb dating indicates that the tectonic event took place during the Early-Late Cretaceous （125-75Ma）. The event not only established the framework and the styles of structural traps in the basin, but also led to the cessation of the first hydrocarbon formation and the destruction of previous oil pools. The light crude oil in the basin was formed during the second hydrocarbon formation stage in the Cenozoic, and ancient structural traps formed during the Cretaceous event are promising targets for oil and gas exploration.

TWO PHASES OF CENOZOIC DEFORMATION IN NORTHEASTERN TIBET: THRUSTING FOLLOWED BY STRIKE-SLIP FAULTING

In order to establish deformation history for the Cenozoic development of the Tibetan Plateau, we conducted geologic mapping along a 120km traverse between Nangqian and Yushu in the northeastern Qiangtang terrane. This work reveals a complex interaction among Tertiary thrusting, strike\|slip faulting, sedimentation, and igneous activity. Two phases of deformation are recognized. The older northeast—southwest shortening, expressed by thrusting and folding, is followed by left\|slip faulting along northwest\|trending faults. Tertiary thrusts, predominantly southwest\|dipping, are distributed throughout the traverse, and typically juxtapose Mesozoic strata over Paleogene strata. The latter were deposited in several separated basins during folding and thrusting, as indicated by well\|developed growth strata. A preliminary construction of balanced cross\|sections suggests a minimum estimate of 45km of crustal shortening along the traverse. Numerous hypabyssal intrusions were mapped in the southern part of the traverse near Nangqian. They were emplaced into the Paleogene sediments and are dated between 36 and 33Ma by 40 Ar/ 39 Ar and U\|Pb methods. Paleogene sediments are also interbedded with volcanics in both the southern and northern parts of the study area. In the northernmost part of the traverse, a volcanic unit overlies a Tertiary thrust. This unit itself is broadly folded. This relationship suggests that Tertiary igneous activity was coeval with contractional deformation in the region, implying strongly the causal relationship between the two processes. The youngest event in the area is the development of northwest\|trending left\|slip faults. They cut Tertiary thrusts, folds, and about 35Ma igneous intrusions. In contrast to widely distributed Tertiary folds and thrusts, strike\|slip faulting is restricted only to the southern portion of our mapped area near Nangqian. The strike\|slip faults apparently control the distribution of modern drainage systems, suggesting that they may have been active recently. As the younger strike\|slip faults are subparallel to the older folds and thrusts, we have not been able to determine the magnitude of left\|slip on these faults. We interpret the termination of contractional deformation and the subsequent replacement by strike\|slip faulting as a result of both clockwise rotation of the region and westward propagation of strike\|slip deformation in eastern Tibet.

P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.

ANISOTROPIC FEATURE OF QIANGTANG MASSIF TEXTURE

Qiangtang Massif is located in the hinterland of Qinghai—Tibet plateau, which belong to the mid\|east section of Tethys Tectonic Domain.1 Features of the whole texture and structure of Qiangtang massif By synthetic analysis of gravity,magnetic field,MT,seismic surveying,etc. Geophysical data, the massif, lied in the tectonic setting and geodynamic setting mingled by the south,north tectonic belts, have the features of massif,basin and tectonic belt three forming an organic whole,multi\|degree coupling in plane and section with division of region in south\|north trend,division of block\|fault in east\|west trend,division of sphere\|layer in vertical direction. (1) Belting in south\|north trend: Qiangtang massif could be divided into four units from north to south, that is north edge doming zone, west part doming area,Qiangtang Basin and south edge doming zone. Qiangtang Basin also can be divided into four tectonic units—north Qiangtang down\|warping region, middle downing zone, south Qiangtang down\|warping and east part slope region. The near east\|west trend tectonic zones are well developed. There is aero\|magnetic anomaly distributed in belting with east\|west trend but also concentrated in section. Gravity anomaly is high in the south\|west part and low in the northeast part. Inter\|crust low resistance layer alternately distributed with high and low belting of sou th\|north trend in plane.

PROVENANCE OF LOWER TERTIARY REDBEDS IN HOH XIL BASIN AND UPLIFT OF NORTHERN TIBET PLATEAU

Eocene Fenghuoshan Group and lower Oligocene Yaxicuo Group are represented by mainly fluvial, lacustrine and fan\|deltaic redbeds cropping out in Hoh Xil basin, the largest redbed basin in northern Tibet plateau. Lithic sandstone, lithic quartzose sandstone, conglomerate, as well as siltstone, consist of the major rock composition of the redbeds. The petrography of sandstone and conglomerate reveals the relationship between regional tectonic uplift events and the provenance of lower Tertiary redbeds.The outcrop rock data show that the detritus were derived from sedimentary rocks, volcanic rocks, and metamorphic rocks. Phyllite and quartzite rock fragments are most common in the metamorphic rock fragments .The sedimentary rock fragments are composed of chert, limestone, siltstone, and mudstone fragments. But volcanic rock fragments are minor and occasionally occur. In Hantaishan area, the northwestern part of Hoh Xil basin, clastic composition of sandstones and conglomerates testify to mainly southward and northeastward provenances. The metamorphic rock fragments, which are more common than volcanic and sedimentary rock fragments, can be directly correlated with metamorphic rock of Triassic metasedimentary Bayankara Group around the basin. In Fenghuoshan area, the southwestern part of the basin, there are less metamorphic rock fragment composition and more limestone fragments than in Hantaishan area.The limestone fragments were obviously derived from Carboniferous—Permian or Jurassic limestone beds. These limestone strata can not be found in the Bayankara block mass at which Hoh Xil basin locates, thus it is deduced that the limestone detritus were derived from the south, that is, Qiangtang blockmass.

BIOLOGICAL MARKER OF MIDDLE JURASSIC OIL SHALE SEQUENCE FROM SHUANGHUI AREA,NORTHERN TIBETAN PLATEAU

The oil shale with marine origin was first reported in 1987 from Shuanghui of the Qiangtang region. Its depositional sequence consists of brown\|black oil shale interbedded massive to thin limestone. Eleven oil shale beds occur and aggregated thickness is up to 47 38m. It deposit age is confined in middle Jurassic by fossils identification. Nine samples selected from horizons with high\|organic contents have been examined by organic geochemistry approach. The oil\|shale range widely in organic carbon content (Toc), average in 8 34%, maximum values reaching 26.12%. Toc are markedly varied in vertical section. The Upper and lower members are slightly low and increase in the middle. The oil\|shale sediments are characterized by high concentration in chloroform bitumen“A”(608～18707)×10 -6 )and total hydrocarbon ((311～5272)×10 -6 ).The Rock\|Eval T \|max data (434～440℃) and vitrinite reflectance values (0.88%～1.26%) indicate that oil\|shale sequence are mature in all samples. The organic matter is predominantly made up of typeⅡ kerogen.