Early Jurassic metamorphism of the eastern segment of the Lhasa terrane in South Tibet and its tectonic significance

The metamorphic belt in the Dongjiu area is located in the eastern segment of the Lhasa terrane in South Tibet. The Dongjiu metamorphic rocks are primarily composed of schist and gneiss, with minor amounts of marble, and the protoliths are sedimentary rocks with Precambrian and early Palaeozoic zircons probably deposited during the Palaeozoic or late Neoproterozoic. On the basis of petrology and phase equilibria modelling, this study shows that the Dongjiu metamorphic belt has experienced a kyanite-grade metamorphism, which is characterized by a decompressional vector with slight cooling from a peak of 9.6 kbar and 745°C to medium-pressure amphibolite-facies metamorphic overprinting at 5–6 kbar and 600–630°C. This P–T path was well recorded and recovered by garnet zoning profiles. Laser ablation inductively coupled plasma mass spectrometry in situ U–Pb analyses on metamorphic zircons and zircon rims yielded concordant ²⁰⁶Pb/²³⁸U ages of c. 194–192 Ma, suggesting that the Dongjiu metamorphic rocks were formed during the Early Jurassic. Therefore, the Dongjiu metamorphic belt, together with the western Nyainqentanglha, Basongco, and Zhala metamorphic belts, constitutes a nearly continuous tectonic unit with an E–W extension of at least 500 km between the northern and southern Lhasa terranes. The metamorphic ages of these belts, ranging from 230 to 192 Ma, show a younger trend from west to east, indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the Late Triassic to Early Jurassic.     

上扬子东南缘民乐-古丈地区锰矿地质特征及找矿方向

湖南省民乐-古丈地区锰矿位于上扬子地块东南缘之武陵台陷的半局限浅海盆地中,为典型的海相沉积型锰矿床。区内主要有民乐和古丈2个锰矿产地,位于松桃成锰盆地的中东部,与贵州道坨锰矿、杨立掌锰矿处于同一断陷槽中,成矿地质条件较好,找矿潜力较大。通过对区内锰矿床（点）成矿地质特征、古构造、岩相古地理等综合分析,总结区域锰矿成矿规律和找矿方向,为上扬子地块东南缘南华系锰矿找矿突破提供依据。     

西藏班公湖-怒江成矿带主碰撞期成矿作用:荣嘎钼矿岩石地球化学及同位素年龄的证据

荣嘎钼矿是新近在班公湖-怒江成矿带发现并证实的首例斑岩型钼矿床。为了查明班公湖-怒江成矿带主碰撞期的成岩成矿作用,探讨荣嘎钼矿的富Mo机制,对该矿床进行岩石地球化学以及同位素年代学的研究。本次研究获得荣嘎含矿二长花岗斑岩的锆石SHRIMP206Pb/238U加权平均年龄为99. 8±1. 9Ma (MSWD=2. 0),属晚白垩世早期。7件辉钼矿样品的模式年龄较为一致,介于98. 0±1. 6Ma～101. 8±1. 7Ma之间,加权平均年龄为99. 7±1. 1Ma。荣嘎含矿二长花岗斑岩样品主量元素表现为富硅(SiO_2=72. 83%～75. 49%)、富碱(Na_2O+K_2O=8. 12%～8. 55%)、过铝质(A/CNK=1. 08～1. 13)的高钾钙碱性系列岩石。样品明显富集Rb、K、Th、U等元素,中等亏损Nb、Ta,强烈亏损Ba、Sr、P、Ti等元素,具中等程度的负Eu异常(δEu=0. 53～0. 63),稀土元素配分曲线呈轻稀土相对较陡,重稀土较为平坦的右倾型。荣嘎含矿二长花岗斑岩样品的(~(87)Sr/~(86)Sr)i值为0. 7042～0. 7055,ε_(Nd)(t)值介于0. 03～0. 68之间,锆石ε_(Hf)(t)值介于+12. 4～+17. 9之间,两阶段模式年龄t_(DM2)较为年轻(31～314Ma)。基于以上同位素以及岩石地球化学特征研究,荣嘎含矿二长花岗斑岩很可能是幔源岩浆诱发了富Mo洋底沉积物的熔融并与壳源熔体混合形成母岩浆,再经历结晶分异作用而形成。结合本文以及区域上的研究资料,荣嘎钼矿可能形成于拉萨地块与南羌塘地块碰撞的大地构造背景,富Mo大洋沉积物熔体的加入及随后的结晶分异是荣嘎含矿岩浆中金属Mo富集的主要原因。班公湖-怒江成矿带南缘侏罗-白垩纪海相地层分布的地区,可能为勘查钼矿床的有利靶区,应重点评价晚白垩世早期高分异中酸性岩体的含矿性。     

西藏中拉萨地块西段左左乡晚侏罗世-早白垩世花岗岩年代学、地球化学与岩石成因

近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素,以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期(163Ma和160Ma)的年龄,属于晚侏罗世;南侧岩体花岗岩年龄为142～147Ma,属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩,富集大离子亲石元素和轻稀土元素,亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分(ε_(Hf)(t)=-16.8～-13.6),可能来源于古老下地壳的部分熔融;包体具有相似的Hf同位素成分(ε_(Hf)(t)=-15.7～-13.6),结合晚侏罗世寄主岩与包体较高的MgO(4.13%～6.90%)、Cr(146×10~(-6)～370×10~(-6))和Ni(31×10~(-6)～113×10~(-6))含量,说明包体代表的中基性岩浆可能源于古老富集地幔的熔融,并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的ε_(Hf)(t)值(-8.2～-4.8和-3.8～+0.2),指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入,并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合,经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶,最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。     

西藏南拉萨地块西部鸭洼地区侵入岩的锆石年代学和Hf同位素特征

拉萨地块南缘发育了广泛的中、新生代岩浆作用,然而与中、东部地区相比,南拉萨地块西部地区的研究程度还相对偏低,尤其是缺乏中生代以前岩浆活动的发现和研究。本文对南拉萨地块西部鸭洼地区新发现的中、新生代侵入岩开展了详细的锆石U-Pb定年和Hf同位素研究,发现鸭洼地区发育了晚三叠世(210～208Ma)辉长岩和闪长玢岩、晚侏罗世(约155Ma)似斑状二长花岗岩和始新世(约50Ma)花岗斑岩。晚三叠世辉长岩和闪长玢岩具有相对亏损的锆石Hf同位素组成,ε_(Hf)(t)值分别是+1.9～+6.2和-3.0～+7.4。晚侏罗世似斑状二长花岗岩和始新世花岗斑岩则具有明显富集的锆石Hf同位素组成,其ε_(Hf)(t)值分别是-10.0～-5.2和-7.9～-2.7,对应的地壳模式年龄分别是1534～1836Ma和1297～1624Ma,分别反映了古元古代和中元古代古老陆壳物质为主的再循环作用。结合区域文献数据,鸭洼-打加错地区中生代以来由于新特提斯洋的俯冲而发生了明显的地壳增生作用,新生地壳的生长从中生代到新生代早期逐渐进行,并且具有从陆内向海沟逐渐增加的趋势。     

西藏拉萨地块东部尼木—加查地区林子宗群火山岩年代学、地球化学特征及其构造意义

拉萨地块广泛分布中—新生代岩浆活动,研究其对于认识新特提斯洋的演化和理解整个青藏高原的形成过程有着重要意义。对拉萨地块东部尼木—加查地区比马组火山岩进行了年代学与元素地球化学研究,结果显示该比马组火山岩LA-ICP-MS锆石U-Pb年龄为55.2~57.6 Ma,明显晚于已发表的比马组火山岩年龄177~195 Ma,而与林子宗群火山岩年龄(30.4~70.0 Ma)相符。因此,该地区原来认为的比马组火山岩可能需要重新厘定为林子宗群年波组火山岩。研究区火山岩主体为玄武岩、玄武安山岩、安山岩和英安岩,属中、高钾钙碱性系列。地球化学特征显示火山岩整体上富集大离子亲石元素Rb、Th、U等,Nb、Ta、Zr、Ti等高场强元素则表现明显的负异常,具有典型的岛弧火山岩特征。稀土元素配分模式主要表现为轻稀土元素右倾型,Eu表现出无异常到中等的负异常。此外,研究区年波组火山岩年龄晚于拉萨地块中西部年波组形成时代,结合前人的研究成果,可能指示印度与亚洲大陆的碰撞具有西早东晚的特点。     

Microbial Geochemical Characteristics of the Coalbed Methane in the Shizhuangnan Block of Qinshui Basin,North China and their Geological Implications

Methanogens and sulfate reducing bacteria were detected by the 16SrRNA sequencing of coalbed methane(CBM)co-produced water in the south of the Qinshui Basin,which is indicative of the presence of secondary biological gas in the south of this basin,in contradiction to the previous understanding of thermogenic gas.This work systematically collected water samples from the CBM wells in the Shizhuangnan Block and analyzed the microbial geochemical characteristics from the aspects of water ions,hydrogen and oxygen isotopes,dissolved inorganic carbon and microbial diversity.It is shown that the Shizhuangnan Block has a nearly SN-trending monoclinic structure,and the elevation of coal seam decreases gradually from the east to west.Because of the water blocking effect of Sitou fault in the west,the precipitation flowed from the east to west,and gradually transited to stagnant flow area.The concentration variation of some ions such as Na~+,K~+,Ca~(2+),Mg~(2+),Cl~-,HCO~-_3 and total dissolved solids(TDS)suggest the variation of redox condition in the coal reservoir water.The 16SrDNA sequencing analysis of the collected water samples detected the presence of methanogens and sulfate reduction bacteria.The presence of methane production zone and sulfate methane transition zone(SMTZ)was identified.The effect of methanogens in the methane production zone leads to an increase in the methane concentration,resulting in a high gas content in the study area.In the SMTZ,most methane is consumed by anaerobic oxidation due to high sulfate concentrations.     

华北地块北缘泥盆纪岩浆活动及其构造背景

晚古生代—早中生代岩浆岩广泛分布在华北地块北缘,构成了沿华北地块北缘呈近东—西向分布的上千公里的岩浆岩带。泥盆纪岩浆岩作为其中的组成部分,虽然分布范围不及石炭—二叠纪岩浆岩广泛,但近十多年来随着研究工作的深入,越来越多的泥盆纪岩浆岩被发现,其构造背景及意义也逐渐引起重视。对华北地块北缘近年来发现的泥盆纪岩浆活动进行总结分析表明,泥盆纪岩浆活动时限在400~360 Ma左右,其中碱性侵入岩和基性—超基性侵入岩时代主要集中在400~380 Ma之间,沿华北地块北缘自东向西均有分布;火山岩则主要集中在400 Ma和360 Ma,主要分布在华北北缘东段的赤峰地区。泥盆纪侵入岩在岩石组合上以碱性杂岩及碱性花岗岩为主,其次为二长闪长岩、基性—超基性杂岩。泥盆纪火山岩则以变质流纹质火山岩—次火山岩—火山碎屑岩及变质玄武安山岩、玄武岩（斜长角闪岩）为主,总体上具有双峰式岩石组合特征。综合研究该地区构造演化历史及泥盆纪岩石组合、弱构造变形特征、岩石地球化学与同位素地球化学组成等,认为泥盆纪岩浆活动的形成与白乃庙岛弧和华北地块在晚志留世发生弧—陆碰撞后的伸展背景有关。华北北缘泥盆纪岩浆岩带的形成对于认识古生代期间地壳增生过程、方式及古亚洲洋最终闭合时间有重要科学意义。      

Volcanogenic Cu–Pb–Zn bodies in granulites of the central Arunta Block, central Australia

Abstract Small unexploited copper-lead-zinc deposits, characterized by a distinctive wall-rock association of cordierite quartzite, silica-undersaturated rocks, calc-silicate rocks and impure marbles, occur in quartzofeldspathic gneisses and mafic granulites of the Strangways Metamorphic Complex, central Arunta Block, central Australia. Available data support the hypothesis that these are metamorphosed volcanogenic ore bodies. The chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks, particularly the felsic igneous rocks emplaced in the North Australian Orogenic Province in the interval 1880–1800 Ma; and the mafic granulites are chemically similar to basalts (olivine-normative tholeiites). The wall-rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits. That the protolith of the cordierite quartzites may well have been leached tuff, similar to the illite-chlorite-quartz tuff found in volcanogenic ore deposits, is also shown by retrogression of the granulitefacies assemblage: cordierite-garnet-ortho-pyroxene-biotite-quartz in the cordierite quartzites to cordierite-anthophyllite-bearing assemblages and thence to chlorite-muscovite-quartz assemblages. Lenses of silica-undersaturated rocks with spinel and, less commonly, sapphirine are interpreted as the metamorphosed equivalents of chlorite-rich pods found within leached tuffs in volcanogenic ore deposits. The wall rocks form sheet-like bodies; this suggests that they were deposited in relatively shallow water, thus precluding the formation of massive sulphides.     

华北克拉通对前寒武纪超大陆旋回的基本制约

全球大陆克拉通在前寒武纪至少记录了3次超大陆聚合－裂解的构造旋回。不同大陆前寒武纪地质的研究证明，板块的构造模式可以前推至新太古代。超大陆的聚合表现为大规模造山带的穿时性发育，而裂解则表现为大陆裂谷系、非造山花岗岩及巨型基性岩浆岩省的同期快速发育。广泛的区域地质研究揭示华北克拉通前寒武纪地质构造演化具有明显的阶段性差异特征，克拉通主体形成于新太古代陆壳增生与碰撞造山过程。华北克拉通在太古宙末期首次经历强烈的裂解作用，在古元古代晚期涉及强烈的陆缘再造作用。在古元古代末期发生第二次大规模的裂解活动，随后以中元古代末期的造山带拼合为Rodinia超大陆的组成部分。详细的区域构造对比证明，华北克拉通长期以来与波罗的地质、东南极克拉通、印度南部克拉通、巴西克拉通等具有构造亲缘关系。