Upper mantle seismic anisotropy beneath a convergent boundary: SKS waveform modeling in central Tibet

This study used SKS waveforms from the International Deep Profiling of Tibet and the Himalayas (INDEPTH) III dataset and a new 2D method for modeling seismic waves in anisotropic media to construct an image of anisotropic structures beneath central Tibet. A preferred model revealed three-segment anisotropic structures in the upper mantle beneath the study region. Waveform modeling demonstrated that the anisotropy was mainly generated by the lithosphere but not the asthenosphere, and that an anisotropic model with a flatter axis of symmetry provides a more consistent interpretation of the observations than models having steeply dipping symmetry axes. A relatively low velocity zone may underlie or intermingle with the anisotropic structures in the northern portion of the region. Synthetic tests also indicate that variations in the elastic constants and depth extent of the anisotropy assumed by the calculations do not affect the general conclusions, although trade-offs exist among certain model parameters. The modeling results suggest that the complex seismic structures in central Tibet were associated with underthrusting of the Indian lithosphere beneath the Asian lithosphere; the inferred flat symmetry axis of the anisotropy was likely generated during this collision process. If this were not the case, the inherited anisotropy would exhibit a steeply dipping axis of symmetry, parallel to the direction of underthrusting.     

西藏冈底斯中段雄村铜金矿床成矿流体特征与成因探讨

雄村铜金矿是在西藏冈底斯中段新发现的一个矿床,流体包裹体组成分析揭示该矿床成矿流体为富含CO2、N2和有机气体的Na _K _Ca2 _Cl-_SO2-4_HS-_CO2-3体系,Na/K、Na/Ca和Cl-/SO2-4摩尔比值分别为1.5～7.1、0.2～2.1和1.0～29.3。氢、氧同位素分析显示,成矿流体δ18O值范围为-4.7‰～ 2.2‰,δD值范围为-104‰～-82‰,表明成矿流体以大气水为主,但岩浆流体可能也对矿床的形成起了重要作用。硫化物硫同位素组成也显示硫主要来自深源岩浆,雄村矿床高盐度流体的形成可能与岩浆流体的低压不混溶密切相关。高盐度流体是雄村贱金属元素活化、迁移的重要介质;地下水中高的有机质还原SO2是诱发贱金属沉淀成矿的重要机制。有机质还原SO2也为Au的活化、迁移提供了介质和有利的还原环境。雄村矿床的形成与岩浆流体和循环地下水共同作用有关,浅部稳定的热体制和高的有机质还原SO2机制是形成雄村矿床特殊蚀变矿化样式的重要原因。     

Ar/Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes

Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite ⁴⁰Ar/³⁹Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at  14.6 Ma. The Kampa Granite yields the youngest biotite ⁴⁰Ar/³⁹Ar ages of  13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.     

西藏申扎地区奥陶纪鹦鹉螺化石

申扎地区塔尔玛桥东－扎扛一带的奥陶系是藏北地区有可靠古生物化石依据的第二个出露区。地层发育好,含化石门类多,数量大,时代依据可靠,界线清楚,是藏北地区目前已知研究奥陶系（纪）的最理想地区。经鉴定,该区鹦鹉螺化石有29种,10未定种,分别隶属3目8科15属,其中3新属9新种。这为该区奥陶纪地层划分、对比及研究古地理格局,探讨藏北地质构造演化提供了重要资料。文中仅描述了1新属6新种,老属种描述从略。     

西藏班公湖岛弧带含硫化镍超基性岩的源区性质与基底背景

青藏高原班公湖一怒江成矿带上的超基性岩型硫化镍矿化是近年来在西藏地区发现的矿化新类型.文章根据成矿带西段班公湖地区含镍超基性岩体的岩石地球化学和Sr、Pb同位素分析结果,论述了含镍超基性岩浆的源区性质及生成条件,并根据锆石U-Pb LA-ICP-MS年龄测定结果,探讨了藏西北地区的基底背景.研究发现,班公湖地区的含镍超基性岩体以富集大离子不相容元素Rb、Th、U、Sr、Pb,亏损Ba、K为特点,高场强元素亏损Nb、Ti,富集Ta;稀士元素相对球粒陨石亏损强烈,但轻稀土元素相对富集.这些特点一致反映出含矿岩浆产生于受俯冲沉积物熔体交代的富集型岩石圈地幔源区.岩浆的生成深度较浅,为尖晶石二辉橄榄岩相,源区部分熔融程度较低,大体在10%左右.岩石中集中出现一批24.79亿年的残留锆石年龄,标志着当时班公湖中特斯洋盆沉积物的物源区较为单一,主要来自太古代末-元古代初的古老基底,并且推测太古代与元古代之交(25亿年)有可能是藏西北的一个古陆壳快速生长期.     

西藏荣玛地区上三叠统日干配错组沉积环境及其构造意义

西藏荣玛地区上三叠统日干配错组发育厚度较大的碳酸盐岩夹少量碎屑岩沉积,在野外剖面实测的基础上,对这套碳酸盐岩进行了岩相与微相分析。通过对碳酸盐岩样品显微薄片观察分析,共识别出13个微相。根据岩石特征及其组合类型划分出6个沉积相:陆源碎屑滨岸相、局限台地相、开阔台地相、台地边缘浅滩相、台地边缘礁相和斜坡相。分析结果表明,晚三叠世荣玛地区主要为陆源碎屑滨岸-浅海碳酸盐台地环境,夹有较深水的斜坡相沉积,共发生了4次海侵-海退旋回,并间隔有若干次小规模的海平面升降变化。区域上,班公湖—怒江洋北缘晚三叠世的沉积环境大体一致,且广泛发育晚二叠世—晚三叠世之间的区域不整合,故认为班公湖—怒江洋的开启时间为晚三叠世之前,且东西段呈准同时开启模式打开。     

藏东吉塘地区早古生代花岗岩锆石U-Pb年龄、Hf同位素及其对原特提斯洋演化的启示

以前对藏东吉塘地区的早古生代岩浆事件鲜有报道,在吉塘地区的浪拉山新发现了中—晚奥陶世花岗岩。采用LA-ICP-MS锆石U-Pb测年,获得该花岗岩的206Pb/238U年龄加权平均值为455±2.0 Ma（WSWD=1.15,n=26）,属于中—晚奥陶世。岩石地球化学特征表明,该花岗岩具有高SiO2（78.72%~79.48%）、富钾（K2O/Na2O=5.55~10.20）,低MgO（0.10%~0.25%）的特征,铝饱和指数A/CNK为1.08~1.26,岩石属于高钾钙碱性强过铝S型花岗岩。岩石稀土元素总体表现为轻稀土元素富集、重稀土元素亏损的特征,具强烈负Eu异常,δEu值为0.35~0.39;在原始地幔标准化微量元素蛛网图上,明显富集Rb、Th、Nd、Pb等元素,亏损Ba、Nb、Sr、P、Ti等元素。结合区域地质资料分析,该岩体是由原特提斯洋俯冲消减引起古老地壳物质部分熔融形成的,是原特提斯洋俯冲消减的岩浆事件响应;说明在中—晚奥陶世吉塘地区存在原特提斯洋的俯冲消减作用。     

西藏北部雄梅-曲松波地区早白垩世多尼组及沉积环境探讨

在西藏北部区域地质调查过程中,发现多尼组岩性自西向东有一定的变化。西区(雄梅地区)下段为深灰、灰、浅灰色微、薄层状粉砂质泥岩、粉砂岩、细砂岩韵律层;中段为成熟度略高的细、中、粗粒石英砂岩韵律层;上段为新建立的地层单位达过火山岩,为中性火山岩。东区(曲松波地区)下段以黄绿、灰绿、土黄色中细砂岩为主夹少量粉砂岩及含砾粗砂岩;中段为灰绿、黄绿、紫、灰紫色粉砂岩、细砂岩韵律层;上段为新建立的地层单位弄巴砾岩,为紫色粗砂岩、砂砾岩、砾岩。多尼组呈带状分布于班公错-怒江缝合带与永珠蛇绿岩带之间。在研究区内多尼组以海岸砂堤及海滩沉积环境为主;研究区以西以浅海相沉积环境为主;研究区以东以潮坪泻湖及沼泽沉积环境为主。中特提斯海东部海岸在多尼组早期可能在研究区以东,中期可能曾移至西区,晚期可能移至东区。这一结论确定了多尼组沉积时期中特提斯海海进、海退过程,它为研究中特提斯海的演化历史、古地理轮廊的确定及青藏高原的形成演变历史提供了新的资料。     

TECTONIC FEATURES AND HYDROCARBON POTENTIAL OF THE EASTERN QAIDAM BASIN

TECTONIC FEATURES AND HYDROCARBON POTENTIAL OF THE EASTERN QAIDAM BASIN     

西藏康金拉铬铁矿床刚玉中的包裹体研究

西藏康金拉铬铁矿石的矿物学研究中,发现大量的微粒金刚石和碳硅石等超高压异常地幔矿物,表明它们产在一个强还原的高压环境.本研究在铬铁矿石中还发现了刚玉及其中大量的矿物包裹体.电子探针等方法研究表明,包裹体的种类包括简单氧化物,如金红石;自然钛;Ti-N、Ti-Si、Ti-C、Ti-Si-P、Ti-B等合金类;含稀土元素的硅酸盐矿物,以及一些未知矿物.结合对铬铁矿石中其他矿物的研究成果,认为康金拉铬铁矿石中的刚玉及其中的强还原环境形成的矿物组合形成于深部地幔.因此,康金拉铬铁矿石中的刚玉可以认为是一种新的带有高压环境信息的标志性矿物.