Granite provinces and basement terranes in the Lachlan Fold Belt,southeastern Australia

Many granites have compositional features that directly reflect the composition of their source rocks. Since most granites come from the deeper parts of the Earth's crust, their study provides information about the nature of parts of that deep crust. Granites and related volcanic rocks are abundant and widely distributed in the Palaeozoic Lachlan Fold Belt of southeastern Australia. These granites show patterns of regional variation in which sharp discontinuities occur between provinces which internally are of a rather constant character. Such a discontinuity has long been recognized at the I‐S line and the extent of that line can now be defined more fully. Breaks of this type are thought to correspond to sharp changes in the composition of the deep crust that correspond to unexposed or basement terranes. Nine such basement terranes can be recognized in the Lachlan Fold Belt. The character of these basement terranes appears to be different from that of the terranes recognized in the Mesozoic‐Cainozoic Cordilleran fold belt, in which the plates accreted during the period of tectonism reflected in the exposed surface rocks. In the Lachlan Fold Belt, it is postulated that fragments of continental crust, or microplates, were assembled in the Late Proterozoic or Early Palaeozoic to form the substrate of the presently exposed Palaeozoic sedimentary rocks; the compositional features of these fragments were later redistributed vertically by magmatic processes. The identification of basement terranes of this type shows that models which involve the lateral growth of the Lachlan Fold Belt during the Palaeozoic, in a manner analogous to the accretion of younger belts, are untenable. These basement terranes have implications for mineral exploration because the content of heavy metals can vary from one to another and this would ultimately affect the probability of concentrating these metals to form a mineral deposit.     

准噶尔盆地东部的前晚奥陶世陆壳基底——来自盆地东北缘老君庙变质岩的证据

出露在准噶尔盆地东北缘的老君庙变质岩 , 主要由绿片岩相的石英片岩构成 , 其上残存含前泥盆纪床板珊瑚和海百合茎化石的大理岩。对石英片岩中的白云母进行的4 0 Ar/ 39Ar定年 , 获得了 (4 6 1.5± 0 .2 ) Ma的坪年龄和 (4 6 2 .0± 4 .1) Ma等时线年龄 , 表明该变质岩的变质时代不晚于中奥陶世晚期 , 准噶尔盆地东部具有前晚奥陶世的陆壳基底。结合区域地质资料 , 推测准噶尔盆地的基底主体是古生代期间的陆块。     

内蒙古石哈河地区黑云母二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

通过对乌拉特中旗石哈河地区黑云母二长花岗岩的锆石U-Pb年代学、岩石地球化学的系统研究,探讨了其形成时代及构造环境。LA-ICP-MS锆石U-Pb年龄加权平均值为(275.0±0.7)Ma,代表了其成岩年龄。岩石的A/CNK值为0.93~1.10,属于偏铝质-弱过铝质I型花岗岩,σ值为2.01~2.88,属于钙碱性系列。岩石ΣREE偏低,轻稀土富集而重稀土亏损,LREE/HREE为9.86~15.63,轻重稀土分异程度较高,Eu异常不明显或呈弱的负异常。原始地幔标准化蛛网图上富集大离子亲石元素Rb、K等,而亏损高场强元素Nb,蛛网图特征与正常弧花岗质岩石基本一致。根据石哈河地区黑云母二长花岗岩所处大地构造位置、侵位时代及岩石地球化学特征,推断其形成于活动大陆边缘构造环境,这也标志着研究区由洋壳俯冲阶段转入陆-陆碰撞构造演化阶段的时间上限应晚于早二叠世晚期。     

中国阿尔泰造山带后造山喇嘛昭花岗岩体锆石SHRIMP年龄、成因及陆壳垂向生长意义

喇嘛昭岩体是阿尔泰造山带典型的后构造岩体,其时代问题一直没有解决。本次锆石SHRIMP定年给出 206Pb／238U年龄276±9Ma(MSWD=1．1),代表其形成年龄。这为阿尔泰后造山花岗岩和后造山作用提供了一个可靠的年代学证据。主量、微量元素地球化学特征显示该岩体具有I-A过渡的高钾钙碱性或高分异钙碱性花岗岩的特点。依据岩体构造特征、年代学、地球化学和地质背景综合分析,该岩体应为后造山岩体。该岩体Sr初始值变化于0．704-0．71659之间。εNd(T) 值为正值( 1．3- 2．8),模式年龄TDM-2为0．93-0．81Ga,与中亚造山带典型的高(正)εNd(T)值花岗岩相同。而且,其εNd (T)值略高于该地区同造山花岗岩。这说明,在具有陆壳基底的阿尔泰造山带后造山阶段,除了可能的俯冲下埋的年轻洋壳或岛弧物质外,可能有新的幔源物质参与了花岗岩的形成。这为中亚造山带后造山阶段陆壳垂向生长提供了一个证据。     

依连哈比尔尕山石炭纪侵入岩锆石SHRIMP U-Pb测年及其地质意义

博罗科努石炭纪侵入岩为一巨大的岩基,主体岩性为闪长岩,边部发育石英闪长岩、花岗闪长岩、二长花岗岩、花岗岩、斑状花岗岩等,属富含角闪石强—中等钙碱性系列。该岩体的形成主要与板块的俯冲作用有关。在闪长岩中获得锆石SHRIMPU-Pb年龄为308.2Ma±5.4Ma。岩体侵入最新地层为下石炭统大哈拉军山组,为石炭纪活动陆缘火山岩的深成组分,它们共同组成该区的陆缘型岩浆弧,从而说明该区在晚古生代存在古板块的俯冲作用。     

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835-1280 m), DFb (2010-2280 m) and DFc (2920-3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. ⁴⁰Ar/³⁹Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223-202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m}<a> and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N-S-striking foliation and a subhorizontal N-S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001)<a> is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.     

新疆阿尔泰南缘萨尔布拉克铁矿区花岗岩年代学及地球化学研究

本文对新疆阿尔泰南缘萨尔布拉克铁矿区花岗岩体进行了锆石SHRIMP U-Pb年龄测定,获得岩体形成时代为410±4Ma(MSDW=0.88),是晚志留世—早泥盆世岩浆活动产物。岩体的年龄限定它侵入的地层时代大于410Ma,为前泥盆系而不是前人认为的中泥盆统;同时限定萨尔布拉克矽卡岩型铁矿的形成时代略晚于410Ma,为晚志留世末期—早泥盆世早期成矿。萨尔布拉克花岗岩具有富硅、偏碱性,LREE富集且分馏明显,HREE亏损且分馏不明显,中等负铕异常(δEu=0.35～0.47),微量元素Th、U、La、Ce、Nd、Zr、Hf相对富集,Nb、Ta、Sr、P、Ti相对亏损,具明显负异常的特点。Na2O/K2O集中于1附近,属于钙碱性—高钾钙碱性系列。铝过饱和指数变化于1.07～1.14,为过铝质。其微量元素地球化学特征与蒙库岩体(400～404Ma)具有很好的相似性,表明萨尔布拉克花岗岩形成于与板块俯冲有关的活动大陆边缘的陆缘弧环境中,可能是洋壳铁镁质岩石在板块俯冲过程中熔融形成,且加入了陆壳物质。     

南秦岭随枣地块耀岭河组双峰式火山岩: 对扬子克拉通北缘大陆裂解过程的约束

长期以来,扬子克拉通北缘新元古代地质构造演化一直是地学界研究的热点。在南秦岭随州地区耀岭河组中新识别出一套双峰式变火山岩,对其开展锆石U-Pb年代学研究结果表明,变玄武岩中锆石较少,且多为捕获锆石,年龄组成特征与下伏武当岩群顶部岩石中碎屑锆石基本一致;白云母千枚岩中锆石年龄集中,获得的U-Pb年龄为623±3 Ma,代表了南秦岭随枣地区耀岭河组的形成时代,该结果与侵入武当岩群中的基性岩墙年龄650~600 Ma一致,两者应为同时期不同产出相的岩浆作用的产物。结合区域研究成果,扬子克拉通北缘裂解可能经历了更长期且复杂的地质构造过程,西部裂解较早,东部（现今方向）较晚,总体呈"拉链式"裂解模式。     

Geochronology and Geochemistry of Early Cretaceous Granitic Plutons in the Xing’an Massif, Great Xing’an Range, NE China: Petrogenesis and Tectonic Implications

In this study, we present zircon U-Pb ages, whole-rock geochemical data and Hf isotopic compositions for the Meiguifeng and Arxan plutons in Xing'an Massif, Great Xing'an Range, which can provide important information in deciphering both Mesozoic magmatism and tectonic evolution of NE China. The zircon U-Pb dating results indicate that alkali feldspar granite from Meiguifeng pluton was emplaced at ～145 to 137 Ma, and granite porphyry of Arxan pluton was formed at ～129 Ma. The Meiguifeng and Arxan plutons have similar geochemical features, which are characterized by high silica, total alkalis, differentiation index, with low P_2O_5, CaO, MgO, TFe_2O_3 contents. They belong to high-K calc-alkaline series, and show weakly peraluminous characteristics. The Meiguifeng and Arxan plutons are both enriched in LREEs and LILEs(e.g., Rb, Th, U and K), and depleted in HREEs and HFSEs(e.g., Nb, Ta and Ti). Combined with the petrological and geochemical features, the Meiguifeng and Arxan plutons show highly fractionated I-type granite affinity. Moreover, the Meiguifeng and Arxan plutons may share a common or similar magma source, and they were probably generated by partial melting of Neoproterozoic high-K basaltic crust. Meanwhile, plagioclase, K-feldspar, biotite, apatite, monazite, allanite and Ti-bearing phases fractionated from the magma during formation of Meiguifeng and Arxan plutons. Combined with spatial distribution and temporal evolution, we assume that the generation of Early Cretaceous Meiguifeng and Arxan plutons in Great Xing'an Range was closely related to the break-off of Mudanjiang oceanic plate. Furthermore, the Mudanjiang Ocean was probably a branch of Paleo-Pacific Ocean.     

Subduction and Collision of the Jinsha River Paleo-Tethys: Constraints from Zircon U-Pb Dating and Geochemistry of the Ludian Batholith in the Jiangda–Deqen–Weixi Continental Margin Arc

The Jiangda–Deqen–Weixi continental margin arc(DWCA) developed along the base of the Changdu–Simao Block and was formed as a result of the subduction of the Jinsha River Ocean Slab and the subsequent collision. The Ludian batholith is located in the southern part of the DWCA and is the largest batholith in northwest Yunnan. Granite samples from the Ludian batholith yield an early Middle Permian age of 271.0 ± 2.8 Ma. The geochemical data of the early Middle Permian granitoids show high Si2 O, low P2 O5 and MgO contents that belong to calc-alkaline series and peraluminous I-type rocks. Their εHf(t) values range from-5.01 to +0.58, indicating that they were formed by hybrid magmas related to the subduction of the Jinsha River Tethys Ocean. The monzonite and monzogranite samples yield Late Permian ages of 250.6 ± 1.8 Ma and 252.1 ± 1.3 Ma, respectively. The Late Permian granitoids are high-K calc alkaline and shoshonite series metaluminous I-type rocks. Their εHf(t) values range from-4.12 to-1.68 and from-7.88 to-6.64, respectively. The mixing of crustal and mantle melts formed the parental magma of the Late Permian granitoids. This study, combined with previous work, demonstrates the process from subduction to collision of the Jinsha River Paleo-Tethys Ocean.     

Geochemistry of K‐feldspar and Muscovite in Rare‐element Pegmatites and Granites from the Totoral Pegmatite Field,San Luis,Argentina

The geochemistry of K‐feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S‐type granites and LCT (Li, Cs, Ta) rare‐element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb‐Cs, K/Cs‐Rb, K/Rb‐Rb/Sr, K/Rb‐Ba in K‐feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites → barren‐ to transitional pegmatites → beryl type, beryl‐columbite‐phosphate pegmatites → complex type of spodumene subtype pegmatites → albite‐spodumene type → albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta‐Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare‐element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites     

兴蒙造山带东南缘黄松群中压型变质作用及其构造意义:岩石学、矿物学和年代学证据

本文报道了兴蒙造山带东南缘黄松群代表性岩石的岩相学、矿物化学成分和黑云母40Ar/39Ar定年结果,以确定它们的变质作用类型、变质温压条件、变质时代及其构造意义。岩相学研究表明黄松群主要由黑云斜长片麻岩、二云斜长片岩、斜长角闪片岩等代表性岩石以及不同类型的糜棱岩所组成。前者往往呈规模较小的面状分布,反映了主期区域动热变质作用,后者主要呈条带状分布,反映了后期低温动力变质作用的叠加与改造。此外,在与侵入体接触处产出的红柱石角岩反映了局部接触变质作用的存在。对黄松群代表性岩石的温压条件计算结果表明,该群主期变质作用的温压范围分别为525~597℃和5.8~7.5kb,地热梯度集中在21~27℃/km,揭示出黄松群中压型(绿帘角闪岩相-低角闪岩相)变质作用的存在。对含石榴二云斜长片岩中黑云母40Ar/39Ar定年结果为239.22±3.02Ma,而黑云母糜棱岩中黑云母的40Ar/39Ar定年结果为193.91±2.16Ma,结合黑云母的封闭温度和研究区已有的年代学资料,认为前者反映黄松群中压型变质作用的时代为晚二叠世-早三叠世,而后者反映黄松群经历的后期低温动力变质作用发生在早侏罗世。结合区域上同时代变质作用、岩浆作用以及沉积建造特征和区域构造演化历史,认为黄松群晚二叠世-早三叠世中压型变质作用与华北板块与西伯利亚板块碰撞拼合的地球动力学背景有关,该期变质作用标志着古亚洲洋东段的最终闭合已经完成。     

Metamorphic patterns and SHRIMP zircon ages of medium‐to‐high grade rocks from the Tongbai orogen,central China: implications for multiple accretion/collision processes prior to terminal continental collision

The Qinling‐Tongbai‐Dabie‐Sulu orogenic belt comprises a Palaeozoic accretion‐dominated system in the north and a Mesozoic collision‐dominated system in the south. A combined petrological and geochronological study of the medium‐to‐high grade metamorphic rocks from the diverse Palaeozoic tectonic units in the Tongbai orogen was undertaken to help elucidate the origins of Triassic ultrahigh‐pressure metamorphism and collision dynamics between the Sino‐Korean and Yangtze cratons. Peak metamorphic conditions are 570–610 °C and 9.3–11.2 kbar for the lower unit of the Kuanping Group, 630–650 °C and 6.6–8.9 kbar for the upper unit of the Kuanping Group, 550–600 °C and 6.3–7.7 kbar for the Erlangping Group, 770–830 °C and 6.9–8.5 kbar for the Qinling Group and 660–720 °C and 9.1–11.5 kbar for the Guishan complex. Reaction textures and garnet compositions indicate clockwise P–T paths for the amphibolite facies rocks of the Kuanping Group and Guishan complex, and an anticlockwise P–T path for the granulite facies rocks of the Qinling Group. Sensitive high‐resolution ion microprobe U–Pb zircon dating on metamorphic rocks and deformed granite/pegmatites revealed two major Palaeozoic tectonometamorphic events. (i) During the Silurian‐Devonian (c. 440–400 Ma), the Qinling continental arc and Erlangping intra‐oceanic arc collided with the Sino‐Korean craton. The emplacement of the Huanggang diorite complex resulted in an inverted thermal gradient in the underlying Kuanping Group and subsequent thermal relaxation during the exhumation. Meanwhile, the oceanic subduction beneath the Qinling continental arc produced magmatic underplating and intrusion, leading to granulite facies metamorphism followed by a near‐isobaric cooling path. (ii) During the Carboniferous (c. 340–310 Ma), the northward subduction of the Palaeo‐Tethyan ocean generated a medium P/T Guishan complex in the hangingwall and a high P/T Xiongdian eclogite belt in the footwall. The Guishan complex and Xiongdian eclogite belt are therefore considered to be paired metamorphic belts. Subsequent separation of the paired belts is inferred to be related to the juxtaposition of the Carboniferous eclogites with the Triassic HP metamorphic complex during continental subduction and exhumation.