内蒙古霍各乞海西期侵入岩形成时代、成因及其与铜矿体的关系

文章通过对狼山霍各乞矿区侵入岩中锆石U-Pb年代学和铪同位素组成、岩石化学及空间上毗邻铜矿体地球化学的研究,探讨了其岩石成因及与铜矿体的关系.结果表明,岩体岩性为辉长-闪长岩,岩体中锆石阴极发光图像具明显的振荡环带结构,Th/U比值均大于0.1(0.20～0.77),指示岩浆成因特征;岩体中锆石加权平均206Pb/328U年龄为(273.9±1.2)Ma,表明岩体形成于海西晚期;锆石的εHf(t)值变化范围为-4.1～-10.0;Hf模式年龄分布于1.56～1.92 Ga之间,其峰值与Nd两阶段亏损地幔模式年龄一致,这说明成岩物质主要来自古老地壳物质的熔融,并非来自亏损地幔源区;铜矿体钕模式年龄变化比较大,变化范围为1.78～2.37 Ga,反映了成矿物质多来源的特征;野外地质特征和爆裂测温数据表明铜矿体受后期改造,与海西期侵入岩有关.认为海西期辉长-闪长岩体应由太古代结晶基底部分熔融并混入少量幔源组分形成,该岩体对霍各乞铜矿床没有提供大量成矿物质,但岩体的形成对矿体局部有明显的富集改造作用.     

塔里木盆地西北缘早二叠世后碰撞花岗质岩浆活动及其构造意义

塔里木地块西北缘的阔什布拉克钾长花岗岩富碱(Na_2O+K_2O平均8.36%8%),富钾(K_2O/Na_2O=1.27～1.47),准铝质(A/CNK=0.82～0.88),属于高钾钙碱性系列岩浆岩。岩石的稀土含量较高(∑REE=263.90×10~(-6)～445.75×10~(-6)),富集Th、U、Ta、Nb、Hf和Y等高场强元素和大离子亲石元素Rb,具有强的负Eu异常(δEu=0.003～0.019),富集高不相容元素(Zr+Nb+Ce+Y=368×10~(-6)～531×10~(-6)350×10~(-6)),高Ga(Ga/Al×10 000=4.17～4.722.6),显示出A型花岗岩的地球化学特征。岩石Th/U比值(平均为3.86)、Nb/Ta比值(平均为12.75)和Rb-Th富集、Ti亏损指示其壳源成因。对花岗岩进行的LA-ICP-MS微区原位锆石U-Pb定年结果表明,花岗岩的结晶侵位年龄为275.4±2.8 Ma。综合西南天山与塔里木盆地早二叠世花岗质岩浆活动的特点,认为早二叠世西南天山的后碰撞岩浆活动不仅在西南天山内部引起了强烈的花岗质岩浆活动,而且对塔里木地块西北边缘的花岗质岩浆活动也有显著的影响。阔什布拉克A型花岗岩也说明西南天山地区的碰撞造山作用在275.4±2.8 Ma之前已经趋于结束,以南天山洋盆为代表的古亚洲洋已基本结束了其演化历史。     

江西省早古生代晚期花岗岩的地球化学特征及其地质意义

上犹、陡水、龙回和乐安岩体是江西省早古生代晚期花岗岩的重要组成部分。它们具有较高的S iO2含量(平均73.80%)、K2O/Na2O比值(平均为1.33)和ACNK值(平均为1.05);大离子元素明显富集,Ba、Sr、P、Ti负异常明显;轻稀土元素富集(LREE/HREE比值平均为6.78),Eu亏损相对明显(δEu值平均为0.39)。同时,它们具有稍高的εNd(t)值(平均为-5.7)和相对年轻的Nd模式年龄(平均为1617M a)。它们在ACF和(La/Yb)N-δEu图解中,投影于S型或壳源型花岗岩范围内;在εNd(t)-t图解中,位于华南元古代地壳演化域内;在Rb/Sr-Rb/Ba和A/MF-C/MF图解中,它们的源区物质是泥质岩和砂质岩而不是幔源基性岩;此外,在研究区域内,迄今未发现早古生代幔源岩浆活动。这些特征表明,研究区内早古生代晚期花岗岩应归属于S型或壳源型花岗岩。其形成机制与早古生代晚期的板内作用有关,即早先受区域挤压应力场影响,武夷、南岭、赣中等块体彼此拼贴,进而增生到扬子地块之上;稍后,由于应力场的转化,进入后碰撞伸展环境。并在减压、导水等因素的综合影响下,位于中、上地壳层位的低成熟度泥砂质碎屑岩发生部分熔融,形成S型或壳源型花岗岩。     

Temporal-Spatial Structure of Intraplate Uplift in the Qinghai-Tibet Plateau

The intraplate uplift of the Qinghai-Tibet Plateau took place on the basis of breakup and assembly of the Precambrian supercontinent, and southward ocean-continent transition of the Proto-, Paleo-, Meso-and Neo-Tethys during the Caledonian, Indosinian, Yanshanian and Early Himalayan movements. The intraplate tectonic evolution of the Qinghai-Tibet Plateau underwent the early stage of intraplate orogeny characterized by migrational tectonic uplift, horizontal movement and geological processes during 180-7 Ma, and the late stage of isostatic mountain building characterized by pulsative rapid uplift, vertical movement and geographical processes since 3.6 Ma. The spatial-temporal evolution of the intraplate orogeny within the Qinghai-Tibet Plateau shows a regular transition from the northern part through the central part to the southern part during 180-120 Ma, 65-35 Ma, and 25-7 Ma respectively, with extensive intraplate faulting, folding, block movement, magmatism and metallogenesis. Simultaneous intraplate orogeny and basin formation resulted from crustal rheological stratification and basin-orogen coupling that was induced by lateral viscous flow in the lower crust. This continental dynamic process was controlled by lateral flow of hot and soft materials within the lower crust because of slab dehydration and melted mantle upwelling above the subducted plates during the southward Tethyan ocean-continent transition processes or asthenosphere diapirism. Intraplate orogeny and basin formation were irrelevant to plate collision. The Qinghai-Tibet Plateau as a whole was actually formed by the isostatic mountain building processes since 3.6 Ma that were characterized by crust-scale vertical movement, and integral rapid uplift of the plateau, accompanied by isostatic subsidence of peripheral basins and depressions, and great changes in topography and environment. A series of pulsative mountain building events, associated with gravity equilibrium and isostatic adjustment of crustal materials, at 3.6 Ma, 2.5 Ma, 1.8-1.2 Ma, 0.9-0.8 Ma and 0.15-0.12 Ma led to the formation of a composite orogenic belt by unifying the originally relatively independent Himalayas, Gangdisê, Tanghla, Longmenshan, Kunlun, Altyn Tagh, and Qilian mountains, and the formation of the complete Qinghai-Tibet Plateau with a unified mountain root after Miocene uplift of the plateau as a whole.     

Late Cretaceous-Cenozoic Exhumation History of the Lüliang Mountains, North China Craton: Constraint from Fission-track Thermochronology

The Lüliang Mountains, located in the North China Craton, is a relatively stable block, but it has experienced uplift and denudation since the late Mesozoic. We hence aim to explore its time and rate of the exhumation by the fission-track method. The results show that, no matter what type rocks are, the pooled ages of zircon and apatite fission-track range from 60.0 to 93.7 Ma and 28.6 to 43.3 Ma, respectively; all of the apatite fission-track length distributions are unimodal and yield a mean length of ~13?μm; and the thermal history modeling results based on apatite fission-track data indicate that the time-temperature paths exhibit similar patterns and the cooling has been accelerated for each sample since the Pliocene (c.5 Ma). Therefore, we can conclude that a successive cooling, probably involving two slow (during c.75-35 Ma and 35-5 Ma) and one rapid (during c.5 Ma-0 Ma) cooling, has occurred through the exhumation of the Lüliang Mountains since the late Cretaceous. The maximum exhumation is more than 5 km under a steady-state geothermal gradient of 35°C/km. Combined with the tectonic setting, this exhumation may be the resultant effect from the surrounding plate interactions, and it has been accelerated since c.5 Ma predominantly due to the India-Eurasia collision.     

Late-Stage Reservoir Formation Effect and Its Dynamic Mechanisms in Complex Superimposed Basins

<正>Complex superimposed basins exhibit multi-stage tectonic events and multi-stage reservoir formation;hydrocarbon reservoirs formed in the early stage have generally late-stage genesis characteristics after undergoing adjustment,reconstruction and destruction of later-stage multiple tectonic events.In this paper,this phenomenon is called the late-stage reservoir formation effect.The late-stage reservoir formation effect is a basic feature of oil and gas-forming reservoirs in complex superimposed basins,revealing not only multi-stage character,relevance and complexity of oil and gas-forming reservoirs in superimposed basins but also the importance of late-stage reservoir formation. Late-stage reservoir formation is not a basic feature of oil and gas forming reservoir in superimposed basins.Multi-stage reservoir formation only characterizes one aspect of oil and gas-forming reservoir in superimposed basins and does not represent fully the complexity of oil and gas-forming reservoir in superimposed basins.We suggest using "late-stage reservoir formation effect" to replace the "late-stage reservoir formation" concept to guide the exploration of complex reservoirs in superimposed basins. Under current geologic conditions,the late-stage reservoir formation effect is represented mainly by four basic forms:phase transformation,scale reconstruction,component variation and trap adjustment.The late-stage reservoir formation effect is produced by two kinds of geologic processes: first,the oil and gas retention function of various geologic thresholds(hydrocarbon expulsion threshold,hydrocarbon migration threshold,and hydrocarbon accumulating threshold) causes the actual time of oil and gas reservoir formation to be later than the time of generation of large amounts of hydrocarbon in a conventional sense,producing the late-stage reservoir formation effect;second, multiple types of tectonic events(continuously strong reconstruction,early-stage strong reconstruction, middle-stage strong reconstruction,late-stage strong reconstruction and long-term stable sedimentation) after oil and gas reservoir formation lead to adjustment,reconstruction and destruction of reservoirs formed earlier,and form new secondary hydrocarbon reservoirs due to the late-stage reservoir formation effect.     

新生代板内造山作用研究--认识中国区域地质构造基本特征的关键

中国区域地质构造特征深受新生代初印度板块对欧亚板块陆陆碰撞的影响 ,原属显生宙不同时代构造单元拼贴的中国区域发生了普遍而强烈的构造叠加改造 ,导致板内造山作用。由作者新编的中国主要活动断裂构造图显示 ,印度板块的碰撞对中国地质构造演化的影响远比自中生代侏罗纪以来太平洋亚洲板块会聚造成的影响更为强烈。中国帕米尔山结东、西两侧的现今大陆动力环境差别巨大 ,其东侧的东移因遇太平洋板块向西俯冲而获得上冲的自由空间。阿尔金断裂带把中国西部分划为南、北断裂构造活动性质不同的两大构造域。有人否认板内造山作用是因为他们只承认经典的板块边缘造山带的存在。要改变这一观念深入研究板内造山作用及演化 ,综合分析造山带构造地貌、山体隆升扩展、与前陆盆地的耦合、陆相沉积建造序列、冲断推覆构造系及其匹配、壳源花岗岩侵位及深部地球物理场观测等是关键。中国是研究大陆拼贴地块区板内造山作用的理想区域 ,在这一地区从事研究可大大丰富大陆动力学的内涵 ,把地质构造综合成果及对演化规律的认识作为编制大地构造图新的重要要素 ,对理论研究与实际应用都有巨大意义     

Large-scale Migration of Fluids toward Foreland Basins during Collisional Orogeny: Evidence from Triassic Anhydrock Sequences and Regional Alteration in the Middle-Lower Yangtze Area

The middle-lower Yangtze area underwent a series of complex tectonic evolution, such as Hercynian extensional rifting, Indosinian foreland basining, and Yanshanian transpression-transtension, resulting in a large distinctive Cu-Fe-Au metallogenic belt. In the tectonic evolution, large-scale migration and convergence of fluids toward foreland basins induced during the collisional orogeny of the Yangtze and North China continental blocks were of vital importance for the formation of the metallogenic belt. Through geological surveys of the middle-lower Yangtze area, three lines of evidence of large-scale fluid migration are proposed: (1) The extensive dolomitic and silicic alteration penetrating Cambrian-Triassic strata generally occurs in a region sandwiched between the metallogenic belt along the Yangtze River and the Dabie orogenic belt, and in the alteration domain alternately strong and weak alteration zones extend in a NW direction and are controlled by the fault system of the Dabie orogenic belt; it     

Granulite facies xenoliths from the Yuhuashan complex,central Jiangxi,South China: constraints on Late Palaeozoic orogeny and middle‐lower crust components

The Cretaceous Yuhuashan igneous complex contains abundant xenoliths of high‐grade metamorphic rocks, with the assemblage garnet ± hypersthene + biotite + plagioclase + K‐feldspar + quartz. The biotite in these samples has high TiO₂ (>3.5%), indicating high‐T metamorphism (623–778 °C). P–T calculations for two felsic granulites indicate that the peak metamorphism took place at 880–887 °C and 0.64–0.70 GPa, in the low pressure/high temperature (LP‐HT) granulite facies. Phase equilibrium modelling gives equilibrium conditions for the peak assemblage of a felsic granulite of >0.6 GPa and >840 °C, consistent with the P–T calculations, and identifies an anticlockwise P–T–t path. LA‐ICPMS U–Pb dating of metamorphic and detrital zircon from one xenolith reveals that the granulite facies metamorphism took place at 273.6 ± 2.2 Ma, and the protolith was a sedimentary rock deposited later than 683 Ma. This represents the first Late Palaeozoic (Variscan) granulite facies event identified in the South China Block (SCB). Coupled with other geological observations, the LP‐HT metamorphic conditions and anticlockwise P–T–t path suggest that Variscan metamorphism probably occurred in a post‐orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of mantle‐derived magma. Based on P–T estimates and the comparison of the protolith composition with mid‐ to low‐grade metamorphic rocks in the area, it is suggested that the mid‐lower crust under the Xiangshan–Yuhuashan area consists mainly of these felsic granulites and gneisses, whose protoliths were probably subducted to these depths during the Early Palaeozoic orogeny in the SCB, and underwent two episodes of metamorphism during Early Palaeozoic and Late Palaeozoic time.     

A reassessment of late glacial – Holocene diatom oxygen isotope record from Lake Baikal using a geochemical mass‐balance approach

We present a new palaeoenvironmental record of hydrological variability in Lake Baikal, based on re‐modelled δ¹⁸O_diatom values of diatom silica (δ¹⁸O_modelled), where the residual contaminants are identified and compensated for using electron optical imaging and whole‐sample geochemistry. δ¹⁸O_modelled interpretations are based on the balance between rivers with high δ¹⁸O values and rivers with low δ¹⁸O values. Isotopic variability is related to latitudinal differences in precipitation which feed these rivers. The δ¹⁸O_modelled record suggests that rather moist conditions prevailed in the Lake Baikal region during the latter stages of the Younger Dryas. Throughout the Holocene, episodes of low δ¹⁸O_modelled values are, in general, in good agreement with increases in percentage haematite‐stained grains in North Atlantic sediments (indicative of ice‐rafted debris events). Rivers with southerly catchments dominate fluvial input especially between c. 3.3 and 2 cal ka BP, concurrent with high precipitation in the Lake Baikal region. Copyright © 2011 John Wiley & Sons, Ltd.