SHRIMP dating of the Bangong Lake SSZ-type ophiolite: Constraints on the closure time of ocean in the Bangong Lake-Nujiang River,northwestern Tibet

The Bangong Lake ophiolite is located in the westernmost part of the Bangong Lake-Nujiang River suture zone. It is a tectonic mélange consisting of numerous individual blocks of peridotite, pillowed and massive lavas and mafic dykes with SSZ-type ophiolitic geochemical affinity formed at the end of a Wilson circle. The SHRIMP U-Pb ages of the co-magmatic zircon domains from one gabbroic dyke (Sample 01Y-155) range from 162.5±8.6 Ma to 177.1±1.4 Ma with an average of 167.0±1.4 Ma (n = 12, MSWD = 1.2), suggesting that the subduction of the Bangong Lake Neo-Tethyan Ocean started before the Middle Jurassic. It is inferred that the tectonic transform from spreading to subduction of the Neo-Tethyan Ocean began before the Middle Jurassic in the Bangong Lake area.

深反射地震成像揭示的班公湖-怒江缝合带中段Moho断阶及其大地构造意义

揭示班公湖-怒江(班怒)缝合带Moho(莫霍面)结构对于认识中特提斯洋壳俯冲和南羌塘坳陷成因具有重要地球动力学意义。基于横跨班怒缝合带的深反射地震数据(88°30′E),本文采用了中长波长静校正、噪声压制、优化叠加和叠前深度偏移(PSDM)等地震处理技术,获得了深度域地震反射偏移剖面、层速度场和高分辨率Moho结构。由深度域剖面显示,班怒缝合带Moho位于地表以下65~80 km,呈不连续北向抬升趋势,指示在拉萨地块与南羌塘地块之间存在岩石圈上地幔断阶,最大阶步可达15 km。综合分析缝合带两侧的Moho形态认为,这些断阶受南侧拉萨地体的岩石圈上地幔以19.5°北倾俯冲与北侧南羌塘地块的上地壳抬升驱动,可能与深部存在局部熔融相关。班怒缝合带下的Moho结构表明,随着晚侏罗世—早白垩世中特提斯洋闭合,南羌塘地体由边缘海沉积向前陆盆地转换,形成南羌塘坳陷。

西藏东巧—北拉地区班公湖—怒江洋俯冲闭合及对南羌塘盆地演化的制约

应用地层对比、砂岩岩相学和碎屑锆石U-Pb年代学的方法,重建东巧—北拉地区物源转换和班公湖—怒江洋多期次俯冲及微陆块的拼合过程。研究表明:东卡错微陆块南侧的中下侏罗统希湖群下段表现为上三叠统确哈拉群的再旋回沉积,而北侧上段则开始出现来自羌塘地区的物质。这标志着北侧早侏罗世俯冲的东巧分支洋盆消亡,东卡错微陆块在中侏罗世与羌塘地块拼合开始形成初始周缘前陆盆地。接奴群的物源完全来自南羌塘地区,表明周缘前陆盆地在微陆块南侧北拉洋俯冲挤压下持续发育。晚侏罗世—早白垩世(147~141 Ma)拉萨地块和羌塘地块东西向全面碰撞,至早白垩世晚期(约120 Ma)南侧的分支洋盆北拉洋消亡代表碰撞结束。南羌塘地区受班公湖—怒江洋俯冲作用控制在早侏罗世发育由弧前—岩浆弧—弧后盆地组成的“一隆两坳”古地貌,并沉积了曲色组页岩和布曲组石灰岩。微陆块碰撞导致南羌塘盆地的隆起和海平面的下降,形成夏里组含膏质泥岩的潮坪相沉积。随着拉萨地块和羌塘地块的全面碰撞,南羌塘盆地从弧相关盆地卷入前陆盆地褶皱冲断带中,发生差异埋藏和隆升剥蚀。晚侏罗世—早白垩世,南羌塘盆地曲色组烃源岩和布曲组石灰岩在构造挤压作用下发生快速埋藏,进入生油和白云石化阶段,成为南羌塘盆地最重要的成藏期。

羌塘地块中西部布木错走滑断裂系的第四纪晚期地表变形特征与构造意义

班公-怒江缝合带(班怒带)是青藏高原内部羌塘地块与拉萨地块之间的重要边界,研究该边界带上共轭走滑断裂第四纪晚期的几何结构与变形特性对于理解高原内部在印度-欧亚板块碰撞作用下形成的空间差异响应和构造模型具有重要意义。位于班怒带西段的布木错断裂系包括北东向布木错断裂和北西向纳屋错断裂,通过遥感解译和野外地质调查,明确了这两条断裂在第四纪晚期的构造特征和最新的地表变形特征。结果显示,两条断裂自第四纪晚期以来的活动特征明显,并且近期都经历过一次大地震,产生了地表破裂。据此推测班怒带西段北西、北东两组断裂的最新活动强度接近,羌塘地块南部边界现今变形可能受控于两组断裂的共同影响,并已延伸至块体内部。以上发现进一步证明,青藏高原内部物质受中—下地壳流的驱动作用,通过走滑断层和正断层持续向北扩展。

青藏高原西北部班公湖流域夏季主要水体H-O同位素组成及其影响因素

作为高寒内陆区的青藏高原西北部班公湖流域水文数据资料较为匮乏,导致对区域水循环过程和气候环境的认识不够深入,氢氧稳定同位素示踪技术可为解决该问题提供理想途径。本文系统分析班公湖流域夏季含湖水、河水、冰川融水和地下水4种不同类型的水体中氢氧同位素的组成,阐释水体氢氧同位素、氘盈余参数沿程变化的特征及其与水体矿化度、高程、经纬度和全球大气降雨线之间的关系,并进一步剖析同位素组成变化的控制因素。结果显示,水体中δ^(2)H和δ^(18)O的波动范围分别为-112.37‰~-24.90‰和-14.84‰~2.01‰,平均值及标准差分别为-76.73‰±26.49‰和-8.43‰±5.24‰。湖水δ^(2)H和δ^(18)O相对其他水体富集,测定结果更偏正值,冰川融水δ^(2)H和δ^(18)O相对其他水体更为贫化。流域水体氢氧同位素的大陆效应总体不明显,部分水体有一定高程效应体现。水体氢氧同位素演化趋势表明,冰川融水与全球大气水线接近,易受大气降雨影响。同时其他水体线的较小截距和斜率值,表明大气降雨不是这些水体的直接补给源。河流沿程的水体δ^(2)H和δ^(18)O呈递增趋势,越远离冰川融水补给端的河水和湖水值越偏正,反映强蒸发的特点。水体氘盈余参数及其与矿化度之间的关系也指示蒸发作用是影响该区域水体氢氧同位素变化的主要因素。

藏北盐湖地区晚白垩世辉长岩:对班公湖‒怒江缝合带碰撞后拆沉作用的指示

青藏高原中部班公湖-怒江缝合带的两侧在晚白垩世经历了大规模的岩浆活动,但其深部动力学机制至今仍存有较大争议。本文对藏北盐湖地区新识别出的辉长岩进行了系统的锆石U-Pb定年、原位Hf同位素以及全岩主微量元素分析,旨在为该缝合带的构造演化过程提供新的约束。分析结果显示,辉长岩形成于~100 Ma,其地球化学特征与洋岛玄武岩(OIB)相似,具有明显的Nb-Ta元素亏损,εHf(t)值介于6.60~8.33之间。这些特征表明,辉长岩具有板内基性岩的地球化学属性,是软流圈地幔部分熔融熔体混染地壳物质的产物。结合区域上近同期基性岩脉和大规模高Mg埃达克质岩浆活动,本文认为盐湖辉长岩的形成于碰撞后伸展环境,是拉萨-羌塘地块碰撞后增厚榴辉岩相下地壳拆沉过程中,深部软流圈地幔上涌减压熔融的产物。盐湖辉长岩的发现为青藏高原中部晚白垩世碰撞后增厚下地壳拆沉模型提供了岩石学上的证据。

西藏改则县扎多嘎尔穷勒铜金矿地质特征及找矿地质意义

以西藏康托地区矿产远景调查成果为基础,论述了改则县扎多嘎尔穷勒铜金矿的地质特征,并对其成因、找矿意义等进行了初步讨论。成矿岩体是一个以花岗闪长斑岩为主的杂岩体,主要岩性为花岗闪长斑岩、闪长玢岩、辉绿玢岩。成矿围岩主要为岩屑砂岩、白云质灰岩。控矿因素主要有地层、构造、岩浆岩。铜金矿成因属岩浆热液型。含钙质蚀变碎屑岩是形成本矿床的围岩,而花岗闪长岩为成矿母岩,断裂构造活动是含矿气水溶液运移、沉淀的场所,岩浆-热液活动是成矿的必要条件。该铜金矿的发现为在班公湖-怒江结合带两侧寻找铜金矿提供了新的线索。

^40Ar/^39Ar and Rb-Sr Ages of the Tiegelongnan Porphyry Cu-(Au)Deposit in the Bangong Co-Nujiang Metallogenic Belt of Tibet,China:Implication for Generation of Super-Large Deposit

The Tiegelongnan deposit is a newly discovered super-large porphyry-epithermal Cu-（Au） deposit in the western part of the Bangong Co-Nujiang metallogenic belt, Tibet（China）. Field geology and geochronology indicate that the porphyry mineralization was closely related to the Early Cretaceous intermediate-felsic intrusions（ca. 123–120 Ma）. Various epithermal ore and gangue mineral types were discovered in the middle-shallow part of the orebody, indicating the presence of epithermal mineralization at Tiegelongnan. Potassic, propylitic, phyllic and advanced argillic alteration zones were identified. 40Ar/39Ar dating of hydrothermal biotite（potassic zone）, sericite（phyllic zone）, and alunite（advanced argillic zone） in/around the ore-bearing granodiorite porphyry yielded 121.1±0.6 Ma（1σ）, 120.8±0.7 Ma（1σ） and 117.9±1.6 Ma（1σ）, respectively. Five hydrothermal mineralization stages were identified, of which the Stage IV pyrite was Rb-Sr dated to be 117.5±1.8 Ma（2σ）, representing the end of epithermal mineralization. Field geology and geochronology suggest that both the epithermal and porphyry mineralization belong to the same magmatic-hydrothermal system. The Tiegelongnan super-large Cu-（Au） deposit may have undergone a prolonged magmatichydrothermal evolution, with the major mineralization event occurring at ca.120–117Ma.

Geological Features of the Eastern Sector of the Bangong Co-Nujiang River Suture Zone:Tethyan Evolution

According to an analysis of the geological features in the eastern sector of the Bangong Co-Nujiang River suture zone, the Tethyan evolution can be divided into three stages. (1) The Embryo-Tethyan stage (Pz1): An immature volcanic arc developed in Taniantaweng (Tanen Taunggyi) Range, indicating the existence of an Embryo-Tethyan ocean. (2) The Palaeo-Tethyan stage (C-T2: During the Carboniferous the northern side of the Taniantaweng Range was the main domain of the Palaeo-Tethyan ocean, in which developed flysch sediments intercalated with bimodal volcanic rocks and oceanic tholeiite, and Pemian-Early Triassic are granites were superimposed on the Taniantaweng magmatic are; on the southern side the Dêngqên-Nujiang zone started secondary extension during the Carboniferous, in which the Nujiang ophiolite developed, and the Palaeo-Tethyan ocean closed before the Middle Triassic. (3) The Neo-Tethyan stage (T3-E): During the Late Triassic the Dêngqên zone developed into a relatively matural ocean basin, in which the Dêngqên ophiolite was formed. By the end of the Triassic intraocean subduction occurred, and the ocean domain was reduced gradually, and collided and closed by the end of the Early Jurassic, forming the Yazong mélange; then the Tethyan ocean was completely closed.

Petrology, geochemistry and geochronology of the Zhongcang ophiolite, northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ^(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).

Hydrothermal Zircon Geochronology in the Shangxu Gold Deposit and its Implication for the Early Cretaceous Orogenic Gold Mineralization in the Middle Bangonghu–Nujiang Suture Zone

As a typical orogenic gold deposit in Tibet,Shangxu gold deposit is located at the Bangong Lake–Nujiang River Metallogenic Belt in the south of Qinghai–Tibet Plateau.In this paper,zircon U-Pb dating,trace elements and Hf isotopic analysis were performed on Au-bearing quartz veins in the Shangxu gold deposit.Zircons from Au-bearing quartz veins can be divided into three types:detrital,magmatic,and hydrothermal zircons.There are two age peaks in detrital zircons:ca.1700 Ma and ca.2400 Ma.There are two groups of concordant ages including 157±4 Ma(MSWD=0.69)and 120±1 Ma(MSWD=0.19)in magmatic zircons,in whichεH f(t)value of ca.120 Ma from the magmatic zircons range from+8.24 to+12.9.An age of 119±2 Ma(MSWD=0.42)was yielded from hydrothermal zircons,and theirεH f(t)values vary between+15.7 and+16.4.According to sericite Ar-Ar age,this paper suggests that an age of 119±2 Ma from hydrothermal zircons represent the formation age of the Shangxu gold Deposit,and its mineralization should be related to the collision between Lhasa Block and Qiangtang Block.The metallogenic age is basically the same as the diagenetic age of Mugagangri granite,andεH f(t)value of hydrothermal zircon is significantly higher than that of the contemporaneous magmatic zircon,which indicates that there is a genetic relationship between the gold mineralization and the deep crust-mantle magmatism.

The Characteristics and Significance of Peng Co Peridotites in the Middle Segment of Bangong Co-Nujiang Suture in Tibet

The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila-Yilashan sub-belt of the the middle Bangong Co-Nujiang ophiolitic belt. The Peng Co ophiolite is mainly composed of mantle peridotites, cumulates, diabase dikes. About 70 percent peridotites are harzburgites and 30 percent are lherzolites. Mineral chemistry of the Peng Co lherzolitesare characterized by low Fo contents(88.85–90.33) of olivine and high Al2O3 content(4.26%–7.25%) in pyroxenes. Compared to the primitive mantle, the Peng Co peridotites have relatively higher MgO contents, lower CaO, Al2O3 and TiO2 contents. The total rare-earth element(REE) contents of the lherzolites are 1.11–1.53 ppm, which are lower than those of the primitive mantle. The chondritenormalized REE patterns of the Peng Co peridotites display slight loss in LREE. In the primitive mantle-normalized spider diagram, the Peng Co peridotites exhibit negative Rb and Zr anomalies and intensively positive U, Ta, Sr anomalies. The PGE contents of Peng Co lherzolites are between 22.9–27 ppb. The chondrite-normalized PGE patterns of the Peng Co lherzolites are consistent with that of the primitive mantle. Mineral and whole-rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites, indicating that it may have formed in the mid-ocean ridge setting. Through quantitative modeling, we conclude that the Peng Co lherzolites formed after 5%–10% degree of partial melting of the spinelphase lherzolite mantle source. The sharp increase of Cr#(56.74–60.84)in Spinel of harzburgites and relatively high Pd/Ir and Rh/Ir ratios suggest that they have experienced melt-rock reaction. The crystallization sequence of Peng Co cumulate is olivine-clinopyroxene-plagioclase. The Mg# value of clinopyroxene in cumulate peridotite ranges from 86.92 to 89.93, and the mean value of Fo is 84.45, which is obviously higher than that of MOR-type ophiolite cumulates. The mineral composition, sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ-type ophiolite in the subduction zone. Therefore, we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%. In the later period, due to the intra-oceanic subduction, it experienced the rock-meltinteraction, and thus formed the SSZ-type cumulate and harzburgite of high Cr value.

Variation of Moho Depth across Bangong-Nujiang Suture in Central Tibet—Results from Deep Seismic Reflection Data

There is a long-term dispute at Moho depth across the Bangong-Nujiang suture (BNS). Due to the complicated and changeable seismic geological condition, it is not easy to acquire images of the reflective Moho in central Tibet. In the support of the SinoProbe project, a series of deep seismic reflection profiles were conducted to image Moho structure across the BNS and the Qiangtang terrane. These profiles extend from the northern Lhasa terrane to the Qiangtang terrane crossing the BNS. Both shot gathers and migration data show clear Moho images beneath the BNS. The Moho depth varies from 75.1 km (~24 s TWT) beneath the northmost Lhasa terrane to 68.9 km (~22 s TWT) beneath southmost Qiangtang terrane, and rises smoothly to 62.6 km (~20 s TWT ) at ~28 km north of the BNS beneath the Qiangtang terrane. We speculate that the Moho appears a 6.2 km sharp offset across the BNS and becomes ~12.5 km shallower from the northmost Lhasa terrane to the south Qiangtang terrane at ~28 km north of the BNS. The viewpoint of Moho depth across the BNS based on deep seismic reflection data is inconsistent with the previous 20 km offset.

西藏格玛铅多金属矿床的发现、地质特征、年代学与成矿地质背景

格玛铅多金属矿床是2010年由笔者团队发现的矿床,也是目前在班公湖—怒江成矿带东段发现的唯一一处中型富铅锌银多金属矿床。该矿床大地构造位置处于昂龙岗日—班戈—腾冲岩浆弧带的东段,矿体均呈脉状产于早白垩世花岗闪长岩体外接触带围岩中,矿体产出的形态、位置严格受近东西向和北西向两组断裂构造控制,赋矿围岩为中侏罗统马里组(角岩化)砂岩、粉砂岩。本文通过对格玛铅多金属矿床地质特征的阐述和对矿床岩矿石地球化学、花岗闪长岩LA-ICP-MS锆石U-Pb年代学的研究,分析了矿床的成矿物质来源、成矿时代与成矿地质背景,探讨了矿床的成矿作用,并初步建立了班公湖—怒江成矿带东段铅锌银多金属矿床的成矿地质模型。结果表明:区内花岗闪长岩具有低硅、高铝、相对富钾主量元素特征,为板块碰撞前俯冲阶段岩浆作用的产物;花岗闪长岩LA-ICP-MS锆石U-Pb定年结果为(119.86±0.45)Ma,表示其成岩时代为早白垩世;花岗闪长岩与矿石、围岩的稀土元素特征均表现为轻稀土元素富集、重稀土元素亏损、Eu负异常,暗示三者在成因上具有一定联系;与中国陆壳丰度值相比,花岗闪长岩中Pb、Ag分别属较富集和强富集元素,Zn属亏损元素,马里组变质粉砂岩中Pb、Ag属极强富集元素,Zn属强富集元素。该矿床成矿物质不仅来源于花岗闪长岩,同时也有围岩马里组中成矿物质大量的加入,成矿物质来源具有明显的“双源性”特征,矿床形成于班公湖—怒江洋盆闭合前的岛弧环境,成矿时代为早白垩世,矿床成因类型为岩浆期后热液型矿床。

The Cretaceous of the Eastern Bangong-Nujiang Suture Zone (Tibet):Tectono-Sedimentation

The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. The Cretaceous of the area includes Chuanba Formation (K 1c ), Duoba Formation (K 1d ), Langshan Formation (K 1l ) and Jiangba Formation (K 2j ). The K 1c is composed of black shale, sandy pelite, siltstone, sandstone, coal beds and volcanic rocks, of shallow marine facies. The K 1 d consists of terrestrial siliciclastics intercalated with some calcareous sandstone beds bearing Orbitolina sp. indicating marine influence. The K 1j is carbonate platform deposits of shallow marine and lagoon. The K 2j is characterized by terrestrial thick massive red conglomerate. An active margin related to B-subduction zone is considered to be the geological setting of the Cretaceous sedimentation.

西藏班公湖蛇绿混杂岩中硅质岩的地质地球化学特征及其形成构造环境

蛇绿混杂岩是研究洋陆转换、板块构造向板内构造转换过程的关键证据。本文以西藏班公湖蛇绿混杂岩中的硅质岩为研究对象,通过岩石学以及主量、稀土元素地球化学特征研究,分析了硅质岩成因和沉积环境。研究区硅质岩呈夹层产出在沙木罗组或断片产出在玄武岩中,具有隐晶质结构、生物碎屑结构和块状构造,含有晚侏罗世—早白垩世放射虫。硅质岩w(SiO_(2))为73.80%~90.44%,w(Al_(2)O_(3))为3.67%~12.33%;MnO/TiO_(2),Fe_(2)O_(3)/SiO_(2),Fe_(2)O_(3)/TiO_(2)和Al_(2)O_(3)/(Al_(2)O_(3)+Fe_(2)O_(3))分别为0.14~0.91,31.44~141.88,1.83~11.80和0.66~0.96;稀土元素北美页岩标准化后显示平坦型稀土配分模式,δCe值为0.77~2.03,Ce整体呈负异常特征,δEu值为0.99~2.11,Eu整体呈弱的正异常特征;(La/Yb)_(N),(La/Ce)_(N),(La/Lu)_(N),(La/Sm)_(N),(Ce/Yb)_(N)和(Gd/Lu)N值分别为0.43~1.12,0.51~1.27,0.44~1.26,0.59~1.12,0.41~2.01和0.82~1.16。硅质岩的地球化学特征指示,班公湖蛇绿混杂岩中伴生的硅质岩为生物成因,形成明显受陆源物质的影响,其形成环境并非典型的大陆边缘环境,而是一个与大陆边缘有一定距离的大洋盆地到大陆边缘构造环境。

西藏那曲地区早白垩世A型花岗岩成因及其构造意义

班公湖-怒江缝合带及其两侧广泛分布早白垩世岩浆岩,它们是班公湖-怒江洋俯冲消减及拉萨地块与南羌塘地块碰撞过程的直接响应,为研究特提斯大洋演化、青藏高原早期陆块聚合提供了重要素材。本文报道了班公湖-怒江缝合带中段那曲地区黑云母二长花岗岩的锆石U-Pb定年、岩石地球化学和锆石Hf同位素分析结果。锆石LA-ICP-MS U-Pb定年结果表明,黑云母二长花岗岩形成于114~113 Ma(早白垩世晚期)。地球化学分析表明,岩石显示出高钾钙碱性—钾玄岩系列特征,同时具有高的Ga/Al×10000比值(介于2.02~3.15之间,平均为2.61)和Zr+Nb+Ce+Y含量(平均为524.87×10^(-6))。此外,锆饱和温度和锆石Ti温度计共同指示岩浆形成于高温的环境(>800℃),这些特征与典型的A型花岗岩相一致。黑云母二长花岗岩中锆石具有低的ε_(Hf)(t)值(10.1~6.4),对应的Hf同位素二阶段模式年龄(t_(DM2))为1852~1547 Ma,指示其岩浆可能源自安多微陆块中下地壳古老结晶基底的部分熔融。结合区域研究成果,认为那曲地区黑云母二长花岗岩可能形成于后碰撞伸展的构造背景,早白垩世晚期伴随着拉萨和南羌塘地块的拼合,班公湖-怒江中特提斯洋中段地区已经闭合。

西藏谢穷地区晚白垩世淡色花岗岩体和岩脉年龄及地球化学特征:对班公湖-怒江带后碰撞背景的制约

藏东左贡地块的晚白垩世侵入岩鲜有报道。对谢穷地区侵入于新元古界酉西岩群中的花岗岩体和岩脉进行了岩石学、地球化学及锆石U-Pb测年研究,揭示其岩石成因及形成环境。岩体岩石类型为含电气石白云母二长花岗岩,岩脉岩石类型为细粒花岗质岩脉,二者的LA-ICP-MS锆石U-Pb年龄分别为88.5±0.7 Ma和85.8±2.3 Ma,均侵位于晚白垩世。岩体与岩脉含白云母,无角闪石,均具有高硅(SiO_(2)=74.17%~78.88%)、富碱(K_(2)O+Na_(2) O=5.92%~9.00%)和富铝(Al_(2)O_(3)=12.61%~15.13%)的特点,铝饱和指数(A/CNK=1.11~1.40)和里特曼指数(δ<3.3),指示二者均属于过铝质钙碱性系列;均富集Rb、U、K、P和Pb,亏损Nb、Ti和Ba,具强烈的负Eu异常(δEu=0.18~0.56),表明为一套强过铝质亚碱性S型花岗岩,具有电气石型淡色花岗岩的特征。其可能为地壳缩短增厚挤压环境向地壳伸展环境转换过程中,构造减压导致的上地壳变质泥岩中含水矿物(白云母)脱水部分熔融的产物,为班公湖-怒江缝合带在东部的后碰撞演化过程提供了新的制约。

班怒带西段达如错斜长角闪岩变质温压条件及地质意义

班公湖—怒江缝合带是中特提斯构造演化的重要构造带之一,目前班公湖—怒江洋盆的俯冲演化过程存在诸多争议,尤其是俯冲过程中的变质作用鲜有报道。本文通过对班公湖—怒江缝合带西段的达如错斜长角闪岩开展角闪石和斜长石矿物的显微岩相学分析和原位EPMA测试工作,查明岩石的矿物组合为辉石+角闪石+斜长石。利用相平衡模拟得出的斜长角闪岩变质温度660℃~750℃,压力2~6 kbar,对应角闪岩相的温压条件,与角闪石全铝压力计(3.7~5.4 kbar)及斜长角闪石温度计(615℃~881℃)计算结果一致。综合前人研究成果,本文认为达如错地区的斜长角闪岩原岩形成于侏罗纪晚期洋内俯冲引起的弧后盆地扩张环境,角闪岩相变质作用的发生与早白垩世班怒洋壳俯冲过程中的板片底板变质有关,班公湖—怒江洋盆西段达如错地区的闭合时间为103.7~100.1 Ma。

西藏班公湖—怒江缝合带东段多金属矿床地质特征与时空分布

班公湖—怒江缝合带东段矿产资源丰富,成矿地质条件优越,成矿潜力巨大,由于交通条件较差,地质找矿工作程度较低,理论研究薄弱。基于前人研究及出波松多、白库、热昌、扎罗卡矿区等典型矿床勘查成果,通过对矿床成因类型、控矿因素、成岩成矿时代及岩浆岩等研究,确认成矿时代为早—中侏罗世燕山早期和早白垩世燕山晚期两个期次。矿床地质特征受地层、构造及燕山期岩浆岩的约束,铅锌多金属矿床以中低温热液充填-交代型为主,矿体赋存于层间破碎带或断层带内,少数为MVT型铅锌矿床,矿体呈囊状—似层状产出于碳酸盐岩地层中;金多金属矿床主要为中低温热液-构造破碎带蚀变岩型,矿体或金异常分布于侵入岩体外接触带附近发育的断裂构造上;Cr-Ni-Co-Pt-Fe矿床矿体赋存于燕山早期超基性岩脉及附近断裂带中。通过对区域成矿规律的总结,划分出唐古拉前陆复合盆地Cu-Au-Pb-Zn、丁青Cr-Ni-Co-Pt-Fe和冈底斯—念青唐古拉弧盆系Au-Cu-Ag-Pb-Zn三个成矿带,可以为该地区的进一步找矿工作提供参考。