The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis,melt productivity,melt loss and the preservation of cordierite

Cordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C^?1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.     

藏南特提斯喜马拉雅带中段二叠纪-白垩纪的火山活动(Ⅰ):分布特点及其意义

在野外实地考察和追索的基础上,详细厘定了特提斯喜马拉雅带中段晚古生代以来火山岩的分布特点和迁移规律。结果表明,在特提斯喜马拉雅带中段晚古生代以来的地层系统中,从二叠纪→三叠纪→侏罗纪→白垩纪,共有11个层位含规模不等的火山岩,它们以透镜体、薄夹层或以块状玄武岩、玄武质安山岩等形式产出于不同地层系统中;从二叠纪→早中三叠世→晚三叠世→侏罗纪和白垩纪,具有由西向东、从南→北→南→北的迁移规律。这些火山活动的发现和厘定,对填补特提斯喜马拉雅带火山岩研究的空白,了解陆下岩石圈地幔和软流圈地幔之间的相互作用和新特提斯洋盆的形成演化都具有一定的指示意义。     

喜马拉雅造山带东、西构造结的地质特征与对比

喜马拉雅造山带的东、西两端分别有一个构造急剧转向的地区——构造结, 这里是探讨喜马拉雅造山带构造演化的重要场所.区域地质调查资料对比显示这2个构造结有: (1)相似的地貌景观; (2)相似的地质特征和演化历史, 即都缺失喜马拉雅沉积岩(寒武纪—第三纪); (3)结晶岩系中都有高压变质岩, 且在10 Ma以来均发生过深熔与混合岩化作用; (4)25 Ma以来, 特别是10 Ma以来两地都经历了快速剥露和隆升作用; (5)印度板块-欧亚板块的碰撞时间接近, 分别为75 Ma和65 Ma, 均早于喜马拉雅造山带的其他地区.这些相似性表明: 伸展拆离和以河流作用为主的地表过程是喜马拉雅造山带的东、西构造结快速剥露的主导因素; 因强烈剥露减压所致的地壳部分熔融作用形成的岩浆向地表减压处的流动在构造结的演化过程中起着重要作用.      

Paleo-glacial reconstruction of the Thajwas glacier in the Kashmir Himalaya using 10Be cosmogenic radionuclide dating

Quantitative glacial chronologies of past glaciations are sparse in the Himalaya, and mostly absent in the Kashmir Himalaya. We used cosmogenic ¹⁰Be exposure dating, and geomorphological mapping to reconstruct glacial advances of the Thajwas Glacier (TG) in the Great Himalayan Range of the Kashmir Himalaya. From ¹⁰Be exposure dating of ten moraine boulders, four glacial stages with ages ～20.77 ± 2.28 ka, ～11.46 ± 1.69 ka, ～9.12 ± 1.39 ka and ～4.19 ± 0.78 ka, were identified. The reconstructed cosmogenic radionuclide ages confirmed the global Last Glacial Maximum (gLGM), Younger Dryas, Early Holocene, and Neoglaciation episodes. As per area and volume change analyses, the TG has lost 51.1 km² of its area and a volume of 2.64 km³ during the last 20.77 ± 2.28 ka. Overall, the results suggested that the TG has lost 64% of area and 73% of volume from the Last glacial maximum to Neoglaciation and about 85.74% and 87.67% of area and volume, respectively, from Neoglaciation to the present day. The equilibrium line altitude of the TG fluctuated from 4238 m a.s.l present to 3365 m a.s.l during the gLGM (20.77 ± 2.28 ka). The significant cooling induced by a drop in mean ambient temperature resulted in a positive mass balance of the TG during the gLGM. Subsequently the melting accelerated due to the continuing rise of the global ambient temperature. Paleo-glacial history reconstruction of the Kashmir Himalaya, with its specific geomorphic and climatic setting, would help close the information gap about the chronology of past regional glacial episodes.     

隐伏在青藏高原中部的东西走向断裂的航磁异常场特征及其意义

1998～2000年完成的青藏高原中西部航磁数据的综合分析结果显示,在青藏高原中部存在一个重要的隐伏断裂带（大致沿革吉北-改则南-错勤北-申扎北）。结合藏东位场分析结果,革吉北-改则南-错勤北-申扎北隐伏断裂可与嘉黎走滑断裂相连。沿该隐伏断裂发生的地震震源机制解显示,该断裂性质与嘉黎走滑断裂相同。因此,该隐伏断裂与位于藏东的嘉黎走滑断裂一起构成了位于高原中部的东西走向的巨型走滑断裂。这种推测得到了相关地质、地球物理证据和数值模拟结果的证实。此外,该巨型走滑断裂可能调节着青藏高原内部的应力分布,从而控制着高原内部的变形特征。     

喜马拉雅东段库曲岩体锂、铍和铌钽稀有金属矿物研究及指示意义

稀有金属矿物记录了花岗伟晶岩成岩成矿的重要信息。喜马拉雅是全球著名的淡色花岗岩带,库曲岩体位于喜马拉雅东段的特提斯喜马拉雅岩系中。本文调查了库曲岩体的二云母花岗岩、白云母花岗岩、电气石花岗岩和花岗伟晶岩,其中,花岗伟晶岩涉及花岗岩的伟晶岩相和独立伟晶岩脉。库曲岩体产出的稀有金属矿物包括锂辉石、锂绿泥石、绿柱石、铌铁矿-钽铁矿、钇铀钽烧绿石和细晶石,它们主要赋存于似文象伟晶岩、石英-钠长石-白云母伟晶岩、块体长石-钠质细晶岩、块体长石-电气石钠质细晶岩、锂辉石-块体长石-细晶岩、白云母花岗岩的伟晶岩相以及电气石花岗岩内。显微镜观察、电子探针和LA-ICP-MS测试结果显示锂辉石具有四种产状,包括粗粒锂辉石自形-半自形晶、细粒锂辉石-石英镶嵌晶、中细粒锂辉石-钾长石-钠长石-云母镶嵌晶以及发育锂绿泥石的粗粒锂辉石,揭示了其形成时复杂的熔流体动荡结晶环境。绿柱石背散射电子图像（BSE）下呈均一结构和不均一结构（蚀变边、不规则分带和补丁分带）,元素替代机制包括通道-八面体替代、通道-四面体替代以及通道中碱金属阳离子间的置换。铌铁矿族矿物包括原生、蚀变边和不规则分带结构,部分被钇铀钽烧绿石和细晶石交代。与原生铌铁矿相比,蚀变边和不规则分带铌铁矿族矿物总体上富钽贫锰,显示了结晶分异、过冷却引起的过饱和以及流体作用。根据稀有金属矿物揭示的成因信息,独立伟晶岩脉（似文象伟晶岩）、白云母花岗岩的伟晶岩相和电气石花岗岩在岩浆分异程度、经历的演化过程、以及流体活动方面存在差异,很可能是不同期次岩浆活动的产物。库曲岩体绿柱石的Rb和Zn含量、以及铌铁矿族矿物的Sc₂O₃、SiO₂和PbO含量,与已有指示标志存在相关性,作为潜在指示标志仍需开展更多的研究工作。综合含锂辉石伟晶岩的产出、岩浆分异演化程度、多期花岗质岩浆活动、复杂的流体作用以及所属锂丰度高值区等因素,库曲岩体是喜马拉雅东段找锂的有利地段。

     

赣中周潭群变质脉体的微量元素及氧同位素地球化学研究

笔者等对周潭群沉积变质岩及其中脉体的微量元素、氧同位素地球化学进行了研究,结果表明,变质石英脉继承了其围岩的稀土元素及氧同位素地球化学特征,表明形成石英脉的流体来源于围岩;脉体的微量元素对Zr-Hf、U-Th、Nb-Ta和Y-Ho与变质岩有一定的差异,且有一定的分异,指示脉体是流体作用的产物。变质流体通过侧向扩散作用形成了周潭群变沉积岩中的变质脉体。     

西昆仑山东段库牙克断裂与康西瓦断裂、阿尔金断裂关系的地球化学证据

新疆昆仑山中部库牙克地区,处于不同大地构造单元的结合部位,一些重要构造线均交会于此,确定它们之间的关系,关键在于库牙克断裂。1：20万区域化探最新成果显示,库牙克地区绝大多数元素两分性特征突出、南北差异明显,北部整体富集MgO、K2O、Na2O、Al2O3、CaO、Cr、Ni、Co、V、Ti、Au、La、Be,而贫Hg、As、Sb、B、Li,南部正好相反。两者之间的分界线从东向西依次对应于阿尔金断裂西段、库牙克断裂、阿什库勒断裂和康西瓦断裂东端。因此,从地球化学角度对库牙克断裂的延伸进行了推断,提出库牙克断裂东连阿尔金、西经阿什库勒与康西瓦断裂相接的新认识。     

惠民凹陷中央隆起带沙四上亚段滩坝与风暴岩组合沉积

惠民凹陷沙四上亚段沉积时期,盆地边缘发育三角洲和扇三角洲沉积,受波浪和沿岸流的影响,在中央隆起带滨浅湖区形成了与岸线走向平行或斜交的滩坝砂体和风暴沉积。滩坝沉积成熟度较高,泥质含量少,岩性以中细砂岩和粉砂岩为主,也有少量砾石,砂岩中常见含双壳类化石、生物碎屑和炭屑。滩坝沉积可分为砂坝、砂滩和滩坝间微相,平面上该区滩坝砂体可分为近岸砂滩、近岸砂坝、坝间砂滩、远岸砂坝、远岸砂滩、水下隆起处砂坝和水下隆起处砂滩。此外,受风暴浪的影响,在浪基面和风暴浪基面之间的地带形成风暴沉积,主要由块状层理段、递变层理段、丘状交错层理段、平行层理段和块状泥岩段构成。该组合沉积体系的形成主要受古地形、物源性质、湖平面变化等方面的影响。     

中亚陆间区南带上古生界构造和油气远景（代序）

安加拉-阿尔丹和华北-塔里木两大克拉通之间地壳相对活动性的地区,地质界多称为中亚陆间区,其南带大部分在中国境内。近年来,中国开始把其上古生界作为油气新区、新领域。多数人认为,其在晚古生代已形成过渡型地壳区,其中有许多地块已逐步拼合,在其上发育裂谷型沉积地质体,未受到区域变质,若干实体变形微弱并被上覆层掩盖而得到保存。源于上古生界的油气既可赋存在其内,也可运聚于中生界或基岩内,在其西段已发现了大中型油气田群。但其整体地质研究程度偏低,仍有许多重大区域地质、油气地质问题存在歧见。针对其近年来的进展、存在的问题和油气远景评价作了概括性的评论。