青藏高原冈底斯带东南部新生代多期岩浆作用及其构造意义

本文对拉萨地体东南缘东冈底斯带的花岗岩进行了详细的野外地质观察,基于侵入关系划分出了5期花岗岩.前4期花岗岩经历了不同程度的变形,呈片麻状构造,而最后1期花岗岩未经历变形,呈块状构造.锆石U-Pb定年揭示,5期花岗岩的形成年龄分别为63Ma、51Ma、50Ma、29Ma和26Ma.岩石学和岩石化学分析表明,前两期为花岗闪长岩,具有高的MgO(1.28％～1.84％)和CaO(3.16％～4.18％)含量,高的Mg#值(平均为43),中等K2O含量(1.5％～2.53％),低的K2O/Na2O(0.37～0.68),为准铝质Ⅰ型花岗岩.而后三期为花岗岩,其中第四期和第五期分别为浅色花岗岩脉和伟晶岩脉,具有明显低MgO(0.03％～0.27％)、Mg#值(2～15),低CaO(1.04％～1.6％),较高K2O(3.75％～6.93％)、K2O/Na2O(0.94～2.04),弱过铝质,显示S型花岗岩特征.研究表明,前三期花岗岩在冈底斯带其它地区也有广泛分布,而后两期花岗岩仅在研究区出露,这很可能说明拉萨地体东南缘从渐新世以来经历了与冈底斯带其它地区不同的构造演化历史.     

A preliminary study on the origin and metallogenetic characteristics of Mesozoic granites in Guangxi,China

Granites are widespread in Guangxi Province and distributed in four tecto-magmatic zones. Geochemical characteristics of these Mesozoic granites can be outlined as: (1) petrochemically rich in silica, alkali and volatiles; (2) low oxidation ratio and relatively high Al index and K₂O/Na₂O ratio. As indicated by their geochemical, petrological and mineralogical features, many granites in Guangxi belong to S-type, frequently associated with mineralizations of rare-earth, tin, tungsten niobium and tantalum.     

西藏当惹雍错吉松环斑花岗岩的发现及地质意义

在1∶25万区域地质调查工作中,于当惹雍错中部的吉松一带发现环斑花岗岩,获同位素K-Ar法年龄45.3Ma,时代为始新世。吉松环斑花岗具特殊的卵状环斑结构,K2O Na2O含量较高,K2O>Na2O;稀土元素含量高,具明显的铕亏损,为造山型环斑花岗岩,形成于青藏高原陆内碰撞造山晚期的后碰撞环境。     

Early Proterozoic postcollisional granitoids of the Biryusa block of the Siberian craton

Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberian craton basement. U-Pb zircon dating of the granites yielded an age of 1874 ± 14 Ma. The rocks of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-type granites. The low CaO/Na₂O ratios (< 0.3), K₂O - 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7-1.9) and Rb/Sr (3.9-6.8) ratios indicate that the two-mica granites resulted from the melting of a metapelitic source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in £_Nd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at 740-800 °C (zircon saturation temperature). The age of the S-type two-mica granites agrees with the estimated ages of I- and A-type granitoids present in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik-Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik-Iya terrane, and Archean Tunguska superterrane into the Siberian craton.     

北秦岭马河钼矿区花岗岩类的锆石U-Pb年龄、地球化学特征及其地质意义

本文对北秦岭马河钼矿区桃官坪中-细粒二长花岗岩和西沟斑状二长花岗岩进行了LA-ICP-MS锆石U-Pb年龄测定,分别获得了157±1Ma(NSWD=13)和153±1Ma(NSWD=14)的谐和年龄,表明其为晚侏罗世侵入体。马河钼矿的形成与这两个侵入体密切相关,据此限定该钼矿床成矿时代略晚于157~153Ma,为侏罗世晚期成矿。桃官坪和西沟花岗岩具有高硅(68.12%~76.33%),富碱(Na₂O+K₂O=6.35%~10.38%)的特征,K₂O/Na₂O变化较大(0.91~1.89),A/CNK在0.85~1.14之间,为准铝质-弱过铝质高钾钙碱性系列。微量元素上富集U、Pb、Hf等高场强元素,亏损Rb、Ba、K、Sr、P等大离子亲石元素。稀土元素配分曲线为轻稀土相对富集的右倾型,多数具有弱的负铕异常(δEu=0.5~0.9),少数样品具有弱正铕异常(δEu=1.05~1.08)。与华北地块南缘与钼矿有关的花岗岩相比,二者均具有高硅、高钾、富碱的特征。但马河钼矿区成矿花岗岩比华北地块南缘的稍基性,这可能是导致马河钼矿的矿床规模比华北地块南缘较小的原因之一。     

藏南冈底斯岩基东段石炭纪岩浆作用记录

藏南冈底斯岩基记录了大量中生代和新生代以来的岩浆作用信息,虽然晚古生代的岩浆岩报道较少,但对限定拉萨地块在新特提斯洋俯冲之前的构造作用具有重要意义。本文通过综合已有资料,进一步对加查县和朗县晚古生代花岗岩开展了锆石U-Pb地质年代学及全岩元素地球化学组成的研究。加查县和郎县花岗片麻岩的原岩年龄分别为~344.8Ma和344.0~362.0Ma,表明加查和朗县岩体都形成于石炭纪。这些岩石具有较高的SiO₂含量（67.46%~75.33%）,Al₂O₃含量较低（12.66%~15.82%）,CaO含量为0.79%~4.32%,FeO和MgO的含量分别为0.48%~3.00%和0.28%~1.64%。依据它们的K₂O/Na₂O比值,这些岩石可分为富钾和富钠两个演化系列。这些岩石富集轻稀土元素,亏损Nb、Ta和Ti,但Zr和Hf无明显异常。地球化学特征和年代学信息表明：（1）冈底斯岩基东段石炭纪花岗岩形成于弧后伸展环境,可能与古特提斯洋向冈瓦纳大陆北缘的俯冲作用有关;（2）镁铁质岩浆演化形成富钠花岗岩,幔源岩浆和中下地壳岩熔体的混合形成富钾花岗岩;和（3）石炭纪岩浆作用持续时间至少~30Myr。

     

黑龙江河口林场锡多金属矿床地质特征及矿床成因

黑龙江河口林场锡多金属矿床位于我国东部大陆边缘完达山陆缘增生带内,锆石ICP-MS U-Pb测年结果显示该矿床的成矿母岩体花岗斑岩形成于121.4±0.5 Ma,属早白垩世晚期,成矿时间略晚于此。地球化学测试结果显示,河口林场花岗斑岩SiO_2含量较高,为61.21%~73.81%,K_2O含量较高,为3.74%~4.61%,Na_2O为0.06%~0.20%,属高钾钙碱性系列的A型花岗岩,形成于伸展背景下。(La/Yb)N-Yb、K/Rb-Rb等微量元素比值图解显示矿体的形成与壳-幔岩浆的混合作用有关。中生代晚期(130~120 Ma)我国东部进入地壳伸展减薄的高峰期,古太平洋板块的持续西向俯冲使区内岩石圈不断增厚发生拆沉,软流圈地幔直接加热下地壳使其部分熔融导致壳幔混合作用的发生,形成的岩浆携带大量成矿物质沿伸展作用形成的空间上侵,并于运移过程中不断萃取沿途围岩中的有用组分,随后就位于三叠纪地层内,岩浆演化晚期,Sn、W等金属元素随流体不断聚集在岩浆房顶部及构造薄弱处,随着温度、压力等物化条件的改变,最终沉淀于岩体与地层的内外接触带成矿。本次研究确定了该矿床为区内发现的首例斑岩型锡多金属矿,与东侧俄罗斯境内的锡霍特-阿林锡多金属成矿省具有良好的亲缘性,为该成矿省在我国境内的延伸部分,其发现为我国东北地区锡矿的探寻工作提供了新的思路。     

滇西澜沧江构造带南段沙乐花岗岩的锆石U-Pb年龄、地球化学特征及其地质意义

沙乐花岗岩位于滇西思茅地块南涧县南西沙乐乡一带,主要由黑云母二长花岗岩和花岗闪长岩组成,被少量正长斑岩侵入。LA-ICP-MS锆石U-Pb测年得出黑云母二长花岗岩的形成年龄为246.4±2.6 Ma,花岗闪长岩的形成年龄为245.7±3.6Ma,表明该花岗岩体的形成时代属于早三叠世。岩石SiO₂含量71.44%~76.39%,Al₂O₃含量12.72%~16.15%,K₂O/Na₂O=0.1~1.30,均属高钾钙碱性过铝-强过铝花岗岩。根据地球化学特征和微量元素构造判别图解,样品点主要落入“火山弧花岗岩”、“板内花岗岩”区域,少量落入“碰撞花岗岩”区域,表明其形成于俯冲-碰撞岩浆弧转换环境。在Sr-Yb花岗岩分类图解中,主要属常见的低Sr高Yb花岗岩,少数样品属低Sr低Yb花岗岩,其物质来源为含砂质的变质泥质岩。结合锆石定年结果及岩体产出的区域地质背景,认为沙乐花岗岩形成于缅泰马陆块与思茅地块大陆碰撞造山过程的初始阶段,同时表明昌宁-孟连洋碰撞最早时限为早三叠世。     

新疆富蕴县北部金格岩体斜长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

对新疆富蕴县北部金格岩体斜长花岗岩进行LA-ICP-MS锆石U-Pb年龄测定,获得了402.7±2.9Ma(MSWD=0.17)的岩浆结晶年龄,表明岩体形成时代为早泥盆世。金格岩体岩性组合为英云闪长岩-奥长花岗岩-花岗闪长岩,具有TTG组合特征,暗示与洋俯冲作用有关。岩石地球化学具有高硅(SiO_2=72.55%～77.41%),富钠(Na_2O=4.94%～6.63%),贫钾(K_2O=0.24%～0.95%)的特征,Al_2O_3含量为12.11%～15.30%,铝饱和指数在0.94～1.04之间,呈低钾拉斑玄武岩系列、准铝质-弱过铝质、I型花岗岩的特征。稀土元素显示轻稀土元素富集、重稀土元素相对亏损,中等-弱负Eu异常的右倾型,微量元素Rb、Th、U等大离子亲石元素富集,呈明显的"峰",Nb、Ta、Ti等高场强元素强烈亏损,呈明显"TNT"负异常,显示了与俯冲作用有关的岩浆岩特征。结合区域地质资料,金格岩体形成于与洋俯冲作用有关的活动大陆边缘的陆弧环境,金格—正格河地区在早泥盆世处于局部拉张环境。     

黑龙江东安金矿床赋矿岩浆岩锆石U-Pb年代学及岩石地球化学特征

东安金矿床是环太平洋成矿域的一处大型低硫型浅成低温热液金矿床,赋存于燕山期碱长花岗岩和中酸性火山岩中。本文通过LA-ICP-MS锆石U-Pb同位素定年,获得赋矿的碱长花岗岩和光华组流纹岩的加权平均年龄分别为183.2±1.3Ma和109.1±1.2Ma,表明碱长花岗岩的侵位年代为早侏罗世,光华组火山岩的喷出时代为早白垩世。在地球化学组成上,东安碱长花岗岩具高硅、高钾和低磷的特征,富集Rb、Th和K,亏损Nb、Ta、Sr、P和Ti,属于高分异的I型花岗岩,是太平洋板块俯冲作用的产物。光华组中酸性火山岩富集Rb、Th、U和K,亏损Nb、Ta、P和Ti,为太平洋板块俯冲方向发生改变后的岩石圈伸展减薄环境下,镁铁质下地壳部分熔融而形成的。东安金矿床成矿年龄(107~108Ma)与光华组火山岩的成岩年龄在误差范围内一致,表明成矿与成岩作用为同一地质事件,均形成于早白垩世太平洋板块俯冲背景下的拉张构造环境中。结合区内其他浅成低温热液型金矿床的赋矿围岩特征,认为早白垩世陆相火山岩是东北地区寻找浅成低温热液金矿床的有利场所。     

西藏八宿吉利地区新发现寒武纪变质花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

八宿吉利地区寒武纪变质花岗岩位于曲扎湖-提卡一带,主要由变质二长花岗岩和变质花岗闪长岩组成。这一新发现对于认识和恢复原特提斯构造历史演化具有重要意义。锆石CL图像显示变质花岗岩锆石为岩浆成因。锆石LA-ICP-MS测年得出片理化变质二长花岗岩年龄为503.7±4.7Ma、变质花岗闪长岩年龄为494.7±3.4Ma,表明该岩体形成时代属于寒武纪。通过岩石地球化学分析,变质二长花岗岩SiO2含量介于69.87%~79.89%之间;变质花岗闪长岩SiO2含量介于66.63%~70.15%之间。前者Al2O3含量变化于12.36%~14.82%,Na2O含量为2.54%~7.16%,K2O含量为0.15%~5.95%,K2O/Na2O=0.02~2.34;后者Al2O3含量变化于14.66%~15.41%,Na2O含量为3.60%~5.63%,K2O含量为0.77%~2.78%,K2O/Na2O=0.14~0.77,属于钙碱性-碱性过铝质花岗岩。在侵入岩构造环境Rb-（Y+Nb）判别图解、Rb-（Yb+Ta）判别图解中,样品均落入"火山弧花岗岩"区域中,表明其形成于大陆边缘火山弧环境。结合锆石测年结果及区域地质背景分析,认为吉利地区变质花岗岩形成于冈瓦纳大陆裂离卡穷微陆块阶段,同时表明原特提斯洋形成最早时限可追溯至寒武纪。     

呼伦湖早白垩世碱性流纹岩的地球化学特征及其意义

大兴安岭呼伦湖一带的上库力组第3段流纹岩具有高硅(SiO₂=75.41%～77.32%)、富碱(Na₂O+K₂O=7.98%～8.74%)、富Y、Nb、Zr及低Al、Mg、Ca、Ni、Cr、Ti和高Ga/Al比值等特点,类似于A型花岗岩,而与高度分异的I型和S型花岗岩有明显差别。该流纹岩的地球化学特征类似于A₁型花岗岩,且与该区A型花岗岩同时,很可能是A型花岗岩浆喷出相的产物。流纹岩的全岩Rb-Sr等时线年龄127±5Ma,与伊列克得组玄武岩(125±2Ma)时代一致,两者构成了双峰式火山岩组合,形成于早白垩世岩石圈拉张环境。     

内蒙古苏尼特左旗东苏A型花岗岩的锆石U-Pb年龄、地球化学特征及地质意义

位于苏尼特左旗东部的东苏二长花岗岩是索伦缝合带重要组成部分,具有高硅(SiO_2=69.76%~75.58%)、高碱且相对富钾(K2O+Na2O=6.99%~9.16%,K2O/Na2O=0.70~1.69)、相对富铝(铝饱和指数A/CNK=1.04~1.13)、高TFeO/MgO值(3.43~11.26)、贫镁(MgO=0.08%~0.50%)的特征;微量元素地球化学性质表现出相对富集轻稀土,Ba、Sr、Nb、Eu强烈负异常,Rb、Th、Ta等不相容元素富集的特征。主量和微量元素均表现出铝质A型花岗岩的特征。采用锆石LA-ICP-MS U-Pb法获得206Pb/238U值的加权平均年龄为221.5±0.81 Ma(MSDN值为0.57),表明该岩体为晚三叠世岩浆活动的产物。结合区域地质资料及地球化学信息可以判断该东苏花岗岩为后造山伸展阶段长英质地壳物质(变杂砂岩)部分熔融的产物。它的出现,标志着古亚洲洋演化的结束,索伦缝合带在晚三叠世的碰撞已达末期。     

内蒙古赤峰地区早二叠世花岗岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

以赤峰地区铭山复式岩体、尖山子岩体为研究对象,通过锆石U-Pb年龄、地球化学研究,确定了其形成时代,探讨了岩石成因和岩浆源区性质及其形成的构造背景。LA-ICP-MS锆石~(206)Pb/~(238)U测年结果表明,铭山复式岩体中灰白色斑状黑云二长花岗岩、尖山子岩体中浅肉红色细粒正长花岗岩分别形成于284.4±7.9Ma、294.7±8.5Ma,为早二叠世。根据地质体间接触关系可知,铭山复式岩体中的二长花岗岩岩体和尖山子岩体均为复式岩体,有待进一步解体。斑状黑云二长花岗岩SiO_2含量较高,K_2O+Na_2O含量较高,A/CNK1,属于高钾钙碱性I型花岗岩。细粒正长花岗岩Al_2O_3含量为14.32%~15.14%,Na_2O/K_2O=0.71~0.99,A/CNK=1.17~1.20,在标准矿物中出现刚玉分子。二者微量元素特征相似,富集大离子亲石元素,亏损高场强元素Nb、Ta,具Sr、P、Ti的负异常,表现出岛弧岩浆岩类特征。结合岩石地球化学、区域地质特征,认为赤峰地区早二叠世岩体形成于岛弧/活动大陆边缘构造背景,其形成与古亚洲洋向南俯冲有关。     

Transition from I‐type to A‐type magmatism in the Sanandaj–Sirjan Zone,NW Iran: an extensional intra‐continental arc

The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO₂ content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A‐type affinity with high Na₂O + K₂O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO₂ levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na₂O + K₂O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO₂ content (48.5%–55.9 wt.%) with metaluminous and calc‐alkaline compositions, relatively lower Na₂O + K₂O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geochemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic subduction of the Neo‐Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley & Sons, Ltd.     

Formation and geodynamic implication of the Early Yanshanian granites associated with W–Sn mineralization in the Nanling range,South China: an overview

ABSTRACT

The Nanling range (Nanling) is characterized by intense and widespread Mesozoic magmatism related large-scale W–Sn mineralization. A summary of geochemistry, geochronology, and petrogenesis for the W–Sn-bearing granites has been carried out in this study. A series of rock- and ore-forming ages in Nanling indicate that the W–Sn mineralization is closely related to the Early Yanshanian granitic magmatism both in temporal and spatial dimensions (165–150 Ma). Geochemical features show that both of the W- and Sn-bearing granites, which mainly belong to highly fractionated I-type granites with a few A-type granites, are characterized by high contents of SiO₂, Al₂O₃, Na₂O, and K₂O; enrichment in Rb, Th, U, Zr, Hf, and REE; depletion in Sr, Ba, P, and Ti; and high ratios of A/CNK. Furthermore, the different Sr–Nd–Hf isotopic compositions indicate that they are mainly originated from the partial melting of the Precambrian basement rocks of the Cathaysia Block at low oxygen fugacity, and the estimated temperatures for the tungsten-bearing and tin-bearing granites are ca. 700°C and ca. 800°C, respectively. The model of the mantle–crust interaction exhibits that different percentages of mantle-derived magma were likely involved in the generation of the tin-bearing granites and tungsten-bearing granites. In combination with previous studies, we propose that these granites in Nanling were emplaced in an extensional setting, as a response to the break-off and roll-back of the subducted Palaeo-Pacific Plate during 175–150 Ma.     

佳木斯地块中部岩浆岩锆石U-Pb年代学、地球化学及其大地构造意义

佳木斯地块中部桦南隆起区广泛发育古生代-中生代岩浆岩,这些岩石的成因对深入探讨中亚造山带的形成演化具有重要的指示意义。LA-ICP-MS U-Pb定年和地球化学测试分析结果表明,研究区广泛分布的片麻状花岗闪长岩形成于中二叠世(267±2Ma),具有I型花岗岩的地球化学特征,表明其岩浆起源可能和俯冲板片产生的熔体有关,形成于岛弧构造环境。而正长花岗岩形成于中三叠世(244±2Ma),地球化学揭示其岩浆起源于下部陆壳物质的部分熔融,具有同碰撞花岗岩的地球化学特点。上述特征表明,研究区在中二叠世处于大陆边缘的构造背景,与古亚洲洋板块俯冲于佳木斯板块之下的构造作用相关,而中三叠世处于同碰撞的构造环境,俯冲此时已经消失,古亚洲洋已经最终闭合,因此三叠纪应该为中亚造山带重要的地质转折期。     

Discussion: Macrofaunal zonation of Middle Permian coal sequences in Sydney Basin

Three suites of alkaline granite can be recognised in the Narraburra Complex at the triple junction of the Tumut, Giralambone‐Goonumbla and Wagga Zones, central southern New South Wales. On the basis of K₂O/Na₂O ratios, biotite and hornblende‐biotite potassic I‐type granites have been assigned to the Gilmore Hill (K₂O/Na₂O 1.00) and Barmedman Suites (K₂O/Na₂O > 1.2). These are metaluminous to weakly peraluminous suites that crystallised from high‐temperature,reduced magmas with the least fractionated members of each suite having high Ba and low Rb abundances compared to other Lachlan Fold Belt granites. Fractionated members of these suites have high abundances of high‐field‐strength elements, similar to those observed in A‐type granites. Arfvedsonite and aegirine‐arfvedsonite granites have been assigned to the peralkaline Narraburra Suite. Granites from this suite have chemistry consistent with them being the intrusive equivalents of comendites and they are also similar in some respects to A‐type granites: they have, for example, particularly high abundances of Zr. The A‐type signature is, however, at least in part the result of strong fractionation. Total‐rock Rb–Sr isotopic analyses from both I‐type suites plot on the same isochron, giving an age of 365 ± 4 Ma (Sr_l = 0.70388 ± 53). A total‐rock isochron for the peralkaline Narraburra Suite gives a less well‐defined age of 358 ± 9 Ma (Sr_l = 0.7013 ± 80). The Late Devonian Rb–Sr ages may be emplacement ages or a result of resetting during fluid‐rock interaction. Although granites of the Narraburra Complex have geochemical affinities with alkaline granites formed late in orogenic cycles, they post‐date arc magmatism by at least 75 million years and they formed in a within‐plate setting. Magmatism was related to localised reactivation of major faults (Gilmore Fault and the Parkes Thrust) in the region, and to partial melting involving both enriched mantle and Ordovician shoshonitic crustal components. Emplacement of the Narraburra Complex was contemporaneous with magmatism in the Central Victorian Magmatic Province and A‐type magmatism in eastern New South Wales. Collectively, all these magmatic events were related to extension post‐dating amalgamation of the western and central/eastern subprovinces of the Lachlan Fold Belt.     

滇西北休瓦促Mo-W矿区印支晚期和燕山晚期岩浆活动与成矿作用:来自锆石U-Pb年代学和地球化学的证据

本文首次在格咱岛弧休瓦促Mo-W矿区识别出印支晚期似斑状黑云母花岗岩,并确定其结晶年龄为200.93±0.65Ma,同时获得燕山晚期二长花岗岩结晶年龄83.57±0.32Ma;即首次在休瓦促Mo-W矿区内厘定出印支晚期和燕山晚期两期花岗岩浆叠加活动,而Mo-W成矿作用与燕山晚期二长花岗岩具有成因关系。岩石地球化学显示燕山晚期二长花岗岩具有较高的SiO_2和全碱含量及较低的Fe、Mg、Ca和P含量,呈准铝质-弱过铝质;富集Rb、Th、U、Nb、Zr和轻稀土元素,亏损Ba、Sr、P、Eu,具有高分异I型花岗岩特征;其形成于与拉萨-羌塘板块碰撞相关的陆内伸展环境,主要来自中-基性下地壳物质的部分熔融,为Mo-W成矿作用提供了重要的物质基础。相对于二长花岗岩,印支晚期似斑状黑云母花岗岩具有较低的SiO_2、Na_2O+K_2O含量和A/CNK比值,较高的Mg、Ca和P含量;富集Th、U、Rb和轻稀土元素,强烈亏损Nb、Ta、Zr、Hf等高场强元素,为准铝质高钾钙碱性具有岛弧岩浆性质的花岗岩,可能形成于甘孜-理塘洋壳俯冲作用结束后,松潘-甘孜地块和义敦岛弧碰撞后伸展环境,为俯冲期改造后形成的下地壳部分熔融的产物。     

Age,origin, and tectonic implications of Palaeozoic rapakivi granites in the North Qaidam orogen,Northwest China

Palaeozoic rapakivi granites occur in the western segment of the China Central Orogenic System. Exhibiting typical rapakivi texture, these granites contain magmatic microgranular enclaves of intermediate compositions. SHRIMP zircon U–Pb ages for the granites and enclaves are 433 ± 5 Ma and 433 ± 3 Ma, respectively. The rapakivi granites are magnesian to ferroan, calc-alkalic to alkalic, and are characterized by high FeO_t/(FeO_t + MgO) (0.74–0.91) and Ga/Al ratios, and SiO₂, Na₂O + K₂O and rare earth element (apart from Eu) contents, but low CaO, Ba, and Sr contents. These are typical A-type granite geochemical features. The granites and enclaves exhibit a uniform decrease in TiO₂, CaO, Na₂O, K₂O, FeO, and MgO with increasing SiO₂, and both lithologies have similar trace element patterns. Whole-rock ?_Nd(t) values vary from??9.2 to??8.7 for the granites and from??9.0 to??8.4 for the enclaves, but zircon ?_Hf(t) values vary more widely from??5.8 to??0.2 and??4.6 to +5.1, respectively. Our data suggest that the granites and enclaves have crystallized from different magmas. The granites appear to have been derived from old continental crust, whereas the enclaves required a source having a juvenile component. The spherical shape and undeformed nature of the granites and their geochemical characteristics, coupled with the (ultra)-high pressure metamorphism and evolution of Palaeozoic granitoid magmatism in the North Qaidam orogen, indicate that the rapakivi granites were generated in a post-collisional setting. These rocks are therefore an example of Palaeozoic rapakivi granites emplaced in a post-collisional, extensional orogenic setting.