An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous: Evidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarze, Southern Tibet

Widely distributed in Gyangze-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase(gabbro)-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarze were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean 206Pb/238U ages were obtained in this paper, which are 134.9±1.8 Ma (MSWD= 0.65) and 135.5±2.1 Ma (MSWD=1.40), respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.     

Temporo-Spatial Distribution and Evolution of Ore Deposits in the West Sector of the Northern Qilian Mountains

The west sector of the northern Qilian Mountains is well-known for the Jingtieshan-type iron deposits. A new breakthrough has been made in prospecting for gold and copper in recent years. In this paper, the distribution characteristics of ore deposits in the study area are discussed from the viewpoint of tectonic evolution. It is suggested that there are 9 stages of mineralization from the Palaeoproterozoic to Indosinian. Four minerogenetic series and two minerogenetic subseries of ore deposits are recognized. Iron mineralization occurred in several stages, while most of the metals were accumulated in large amounts in the Caledonian. The enrichment and mineralization of gold is related to large-scale shear-strike-slip faults and the ascent and unloading of deep-seated fluids.     

~(207)Pb/~(206)Pb Zircon Dating of the Huangtuling Hypersthene-Garnet-Biotite Gneiss from the Dabie Mountains, Luotian County, Hubei Province, China: New Evidence for Early Precambrian Evolution

The Huangtuling hypersthene-garnet-biotite gneiss at Luotian County, Hubei Provine, is a typicalgranulite-facies rock of the Dabie Group Complex in the Dabie orogenic belt. Investigations on the morphology andoccurrence of zircons and their internal structures shown in the thin sections lead to the recognition of three types ofzircons, which are in good agreement with the types identified on the basis of morphology, colour and external fea-tures from the related zircon concentrates. The observation of zircons in the rock reveals that part of type 1 zirconsshow signs of a double-layered structure. The interval part existed in the protolith prior to the granulite-facies meta-morphism. Type 2, the prismatic zircons which mainly occur in garnet and hypersthene are metamorphic minerals ofthe granulite-facies metamorphism. Type 3, the round multifaceted zircons in felsic minerals and biotite, are proba-bly attributed to a later geological event related to migmatization. The ~(207)Pb/~(206)Pb zircon dating by direct evaporationon (thermal evaporation ion mass spectrometer) yields ages ranging from 2814 Ma to 1992 Ma. The age discrepancyamong these different zircon types is conspicuous. The yellow-brown(type 1) zircons give ages of 2814±29 Ma to2527±6 Ma, the prismatic euhedral zircons (type 2), 2456±7 Ma to 2254±4 Ma, and the round multifaceted zircons(type 3), 1992±10 Ma. The results are geologically interpreted in consideration of the complicated behaviours of zir-cons during Precambrian geological evolution of the Dabie area. (1) If the protolith of the gneiss is a sedimentaryrock, then type 1 zircons are clastic ones and the ages 2814±29 Ma and 2811±27 Ma may reflect the minimum age ofthe rocks of its source region. also the first geological event in the area. Sedimentation of the protolith occurred be-tween 2814 Ma and 2527 Ma, probably close to 2814 Ma. If the protolith is a volcanic rock, then the formation age ofthe supracrustal rocks of the Dabie Group Complex is around 2814 Ma. The age 2456±7 Ma reflects the time whenthe granulite-facies metamorphism took place. The later migmatization event is dated at aboat 1992±10 Ma, and isprobably the latest early Precambrian event in the area. The present work provides geochronological evidence for the existence of the Dabie Archaean craton, whichhad probably experienced 3 or 4 geological events during its early Precambrian evolution.     

新疆北部晚石炭世至晚三叠世地壳热演化：——东准噶尔考克塞尔盖？…

在野外考察和室内研究的基础上,＾４０Ａｒ－＾３９Ａｒ定年揭示出新疆东准噶尔考克塞尔盖山前志留纪荒草坡群下组的主变质作用发生在石炭纪晚期,此后还受到了晚三叠世热事件的改造。结合区域地质资料,对新疆北部晚石岩世至三叠纪的地壳热演化进行了初步探讨。     

太平洋古陆的破碎消亡与中新生代华北克拉通板内构造岩浆活化

回顾了太平洋撞击成因假说的主要内容, 重点围绕太平洋古陆消亡的内外动力学机制进行讨论, 认为P/ T之交发生在太平洋地区的撞击事件撞裂了岩石圈板块, 导致岩石圈下的地幔对流方式发生重大转折.在这种全新的对流方式驱动下, 太平洋古陆板块伴随新生洋壳板块的俯冲和碰撞而逐渐消亡并拼接到环太平洋大陆边缘, 与此同时, 环太平洋构造域开始形成.这种以太平洋构造域为中心的深部构造热体制深刻地影响了环太平洋大陆板块在中新生代的构造岩浆活动.      

异常压力环境下流体活动及其油气运移主通道分析

沉积盆地的油气生成、运移和成藏过程与盆地流体作用密切相关, 而异常压力环境下流体活动有其特殊性, 因而与之相关的油气运移和成藏也有其特殊性.对于异常压力体系(包括高压和低压)而言, 存在2类流体系统, 即半开放型和封闭型流体系统.前者由于封闭层的间歇开启导致流体幕式释放, 后者以封闭层内热对流作用为主.按照封闭层开启的成因机制可划分为3类, 即水力破裂或流体压裂型、断裂型、断-压双控型.封闭层的间歇开启构成异常压力环境下油气运移的主通道.这些流体活动和油气运移在海底以及盆地的浅层和深层显示不同地球物理特征, 如气苗、麻坑、气烟囱和流体底辟带等.同样, 还可以利用岩石中残留的一些标记或异常现象示踪盆地流体活动, 如流体包裹体、地层水化学、有机地球化学异常、成岩作用异常、流体场动态模拟等.这些特征和标志为流体和油气运移主通道的识别, 同时也为异常压力环境下油气勘探提供重要的依据.      

宁强碳质球粒陨石的岩石、矿物特征研究

宁强碳质球粒陨石的结构复杂,有粒状、斑状和炉条状等结构。球粒分为玻璃质和晶质两种。聚合体含量丰富、形态各异,有钢性和塑性两种聚合体。目前已查明的矿物有20余种,主要是镁橄榄石、贵橄榄石、烦火辉石、普通辉石和黄长石类矿物等。在宁强陨石中出现罕见的方解石、白云石。不透明矿物有陨硫铁、铁纹石—镍纹石、镍铁矿、磁铁矿和非晶质碳质。Fe/SiO_2为0.71—0.72,SiO_2/MgO为1.37—1.40。根据化学及矿物特征,该陨石应属C_2群。     

含锡自然铜研究

本文对发现于河南毛堂金矿田氧化矿石中的含锡自然铜进行了矿物学研究。该矿物属等轴晶系,O_h~5-F_(m3m),a_0=3.655?,Z=4。化学成分中Cu 90.38(wt)%,Sn 9.38(wt)%,分子式Cu_(18)Sn。含锡自然铜相当于Cu_(18)Sn合金中的a相,在自然界属首次发现。     

江苏发现的硼锡钙石及其研究

硼锡钙石是一种罕见的岛状硼酸盐矿物,其化学式为CaSn[BO_3]_2。一轴晶,负光性,折光率No=1.774,Ne=1.661,比重为4.09,发育{0001}解理,乳白色。矿物产于苏州花岗岩与大理岩接触带矽卡岩中的一个萤石矿床内。它是一种富含硼、氟的高温气液流体作用于钙质围岩而发生交代的产物。它的发现,不仅对矿物产状提供了新的资料,而且对锡、硼的地球化学以及硼酸盐矿物的研究也具有一定的学术意义。