Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan

In this paper, cataclastic shear zones along the northern margin of the Mino Belt, central Japan are described, and the significance of the shearing in the tectonic evolution of SW Japan is examined. The Mino Belt in SW Japan is composed of accretionary complexes of Jurassic to Early Cretaceous age. Field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the Mino Belt. Closely spaced cleavage is developed in these shear zones. Lineation on the cleavage plunges at shallow to moderate angles. Deformation structures (e.g. composite planar fabric and asymmetric structure of clasts) in the sheared rocks clearly indicate a sinistral sense of shear. The shearing ceased by latest Cretaceous time, because the sheared rocks are overlain by unsheared Upper Cretaceous volcanic rocks. The sinistral shearing may be closely related to Cretaceous sinistral movement along the eastern margin of Asia. Sinistral shearing along the northern margin of the Mino Belt can be considered as a key for re-examining the tectonic development of SW Japan.     

Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt,China

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The ε_Hf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ¹⁸O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H₂O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H₂O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H₂O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.     

STRUCTURAL GEOLOGY

正20141283 Bai Daoyuan(Hunan Institute of Geological Survey,Changsha 410016,China);Zhong Xiang Nature,Origin and Tectonic Setting of Jinzhou Basin in the South Segment of Xuefeng Orogen(Geology in China,ISSN1000-3657,CN11-1167/P,40(4),2013,p.1079-1091,10 illus.,47 refs.)Key words:foreland basins,strike-slip faults,Hunan Province     

A long‐lived metamorphic history in the contact aureole of the Mooselookmeguntic pluton revealed by in situ dating of monazite grains preserved as inclusions in staurolite porphyroblasts

Emplacement of the Mooselookmeguntic pluton, located in the western Maine region of the northern Appalachians, was thought to have occurred towards the end of the Acadian deformation at around 370 Ma. Crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c. 20 Myr. Foliation inflection/intersection axes (FIAs) within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth. The orientation of FIAs in both garnet and staurolite porphyroblasts trend successively from ESE–WNW, NNW–SSE, E–W, ENE–WSW to NE–SW. Electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of FIA development reveals a succession of apparent ages from 410 Ma to 345 Ma. A similar spread of crystallization ages can be observed for plutons from Maine and adjacent regions. This succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the Acadian orogeny. The thermal structure of the orogen progressively evolved to enable pluton emplacement, and it continued to develop afterwards with magmatic fluids still forming at depth.     

湘西合仁坪金矿床硫、铅同位素地球化学

湘西柳林汊一带广泛分布钠长石_石英脉型金矿,合仁坪金矿床是其典型代表。文章对合仁坪金矿床的硫、铅同位素进行了研究,并与区域石英脉型金矿床进行对比,探讨了该矿的成矿物质来源,并初步确定了其矿床成因。研究表明,合仁坪金矿床硫化物的δ34S值范围较窄(-4.8‰~4.4‰),平均为-0.6‰,该矿床的硫为深源硫,由深部变质流体带入;铅的同位素组成较均一,并表现出明显的造山带铅的特点。结合区域成矿作用,进一步研究揭示,合仁坪金矿床为一典型的造山型金矿,其成矿可能与湘西雪峰山地区加里东期的造山作用有关。     

广西油麻坡钨钼矿床成岩成矿年代学研究及其地质意义

广西博白_岑溪断裂带位于钦杭成矿带南段,既是一条长期活动的地体边界断裂带,也是一条岩浆活动频繁的W_Mo_Sn_Pb_Zn_Sb_Au_Ag多金属成矿带。文章以该带中的广西博白县油麻坡矽卡岩型钨钼矿床为研究对象,在详细的岩性鉴别和划分的基础上,对矿区内花岗岩类进行了单颗粒锆石LA_ICP_MS锆石U_Pb测年,获得深灰色花岗斑岩的等时线年龄为(479.7±3.9)Ma,灰色黑云母花岗闪长岩为(109.7±1.1)Ma,浅灰色细粒白云母花岗岩为(103.3±1.2)Ma。辉钼矿Re_Os测年获得加权平均年龄为(97.4±1.9)Ma。由此可见,油麻坡岩体为一个复式岩体,由早奥陶世(加里东期)的花岗斑岩和早白垩世(燕山晚期)的黑云母花岗闪长岩和细粒白云母花岗岩组成,其中,黑云母花岗闪长岩构成油麻坡岩体的主体。钨钼矿形成于早白垩世晚期(即燕山晚期),与黑云母花岗闪长岩和白云母花岗岩密切相关。这些高精度测年数据的获得,进一步表明博白断裂带是一条长期活动的断裂,其在加里东期已经活动,并伴有岩浆岩的侵位。燕山晚期,该断裂重新复活,并导致大量的岩浆活动和一定强度的W_Mo_Sn_Pb_Zn_Sb_Au_Ag成矿作用,形成由多个大_中型矿床组成的多金属成矿带。博白_岑溪成矿带属于华南100~80 Ma大规模成矿的一部分,成矿背景与华南地区白垩纪的地壳伸展、钦杭成矿带的再次裂陷有关。     

丝绸之路经济带甘肃黄金段找矿勘查新进展

丝绸之路经济带已成为国家战略,赋于了上合组织新的内涵,也给地质工作者提出了新的要求。甘肃是古丝绸之路的锁匙之地,是丝绸之路经济带上的“黄金路段”。2014年甘肃省提出利用拥有古丝绸之路贯穿境内1600 km的战略通道优势,打造丝绸之路经济带的黄金段,使其成为向西开放的战略平台、经贸物流的区域中心、产业合作的示范基地、人才交流的桥梁纽带。本文就甘肃省矿产资源优势、找矿勘查新进展及向西走出去提出建议。     

内蒙古二连—东乌旗铜钼多金属成矿带中西段 找矿进展

二连—东乌旗成矿带位于西伯利亚板块和华北板块之间的兴蒙造山带东段,属西伯利亚板块东南南大陆边缘早古生代火山岛弧带、晚古生代二连－贺根山板块对接带。自中生代开始,区内逐渐向滨太平洋构造域发展过渡,并在中生代晚期经历了明显的由挤压构造体系向伸展构造体系的转换过程,在一系列构造演化及其相应的岩浆作用、火山作用和变质作用过程中,分别伴随着金、钼、铜、铅、锌等多金属富集和矿化,该地区具有优越的成矿条件。     

苏鲁超高压变质带超镁铁岩深部碳酸盐交代作用的证据

中国大陆科学钻探工程第三先导孔(PP3钻孔),位于秦岭-大别-苏鲁超高压造山带的东部,赣榆岗上超镁铁岩体中。钻孔中超镁铁岩的岩性包括纯橄岩和石榴超镁铁岩等,该超镁铁岩的SiO_2含量均值在43.68%,变化范围为41.9%~47.11%;MgO均值在44.71%,变化范围为47.12%~48.62%;Cr_2O_3均值在0.39%,变化范围为0.30%~0.47%;CaO均值0.12%(变化范围为0.006%~0.34%);Na2O均值在0.05%(变化范围为0.005%~0.32%)。超镁铁质岩的Mg#(Mg/(Mg+Fe)×100)稳定在91.9~93.0之间,Cr#(Cr/(Cr+Fe)×100)均值在38,较高;变化在30~45之间,变化小。其中闪石化金云母超镁铁岩具有最低的值(Cr#=19)。Ni含量在2100×10~(-6)~2500×10~(-6),CaO含量均值为0.13%,Al_2O_3含量均值为0.41%。岩体成分均一,表现为高镁,低钙和低铝的特征。超镁铁岩的不相容元素和稀土元素总量很低,稀土元素总量均值在0.60×10~(-6)。(La/Yb)N比值在6.9~51.2,均值在16.1,重稀土元素严重亏损,是中国东部最亏损的地幔岩之一。超镁铁岩中橄榄石成分(Fo在88.7~93.1之间,Fa在6.8~11.1之间),从早期到晚期,岩石Fo值从93→91~92.4→88.7~89.1。铬尖晶石Cr#值从51到89变化,TiO_2和MnO_2值分别低于0.26%和0.46%,晚期铬尖晶石Cr#值增大,Ti含量减小。单斜辉石由透辉石(Wo_(45.9)8En_(47.89)Fs_(2.73)Ac_(3.39))和顽透辉石(Wo_(27.61)En_(68.78)Fs_(2.27)Ac_(1.34))两种,透辉石(Cpx_Ⅰ)在顽透辉石(Cpx_Ⅱ)中呈被交代的残余粒状。角闪石和金云母呈明显的条带状和脉状,局部发育。岩石具有弱的Na和K交代作用,较高的Sr和Ba等元素,亏损高场强元素(HSFE),以及特征的稀土元素和微量元素配分曲线;岩石中主要组成矿物橄榄石从早期到晚期,矿物Mg#指数下降(Mg#从93→88);单斜辉石中可看到透辉石被顽透辉石交代现象;次生角闪石和金云母的形成等特征都显示超镁铁岩经历了碳酸盐交代作用。在超镁铁岩矿物橄榄石中发现细小白云石和菱镁矿等碳酸盐矿物更有力证明了苏鲁超高压变质带超镁铁岩经历了深部碳酸盐交代作用。PP3钻孔超镁铁岩属于强烈亏损地幔岩区域,单斜辉石和角闪石等含水矿物在超镁铁岩中含量低,交代作用的范围和规模有限,且交代作用在动力学上是快速的和不平衡的过程。