Neoproterozoic I-type granites in the Central Tianshan Block (NW China):geochronology,geochemistry,and tectonic implications

The Central Tianshan Block is one of numerous microcontinental blocks within the Central Asian Orogenic Belt(CAOB)that overlies Precambrian basement rocks.Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB,and their place within the Rodinia supercontinent.However,to date,the timing and tectonic settings in which the basement rocks in the Central Tianshan Block formed are poorly constrained,with only sparse geochemical and geochronological data from granitic rocks within the northern segment of the block.Here,we present a systematic study combining U-Pb geochronology,whole-rock geochemistry,and the Sr-Nd isotopic compositions of newly-identified granitic gneisses from the Bingdaban area of Central Tianshan Block.The analyzed samples yield a weighted mean Neoproterozoic 206Pb/238U ages of 975-911 Ma.These weakly-peraluminous granitic rocks show a common geochemical I-type granite affinity.The granitic gneisses are calc-alkaline and enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs),but they are depleted in high field strength elements(HFSEs);these characteristics are similar to those of typical subduction-related magmatism.All samples show initial(^(87)Sr/^(86)Sr)(t)ratios between 0.705136 and 0.706745.Values forεNd(t)in the granitic gneisses are in the range from-5.7 to-1.2,which correspond to Nd model ages of 2.0-1.7 Ga,indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths.The documented geochemical features indicate that the protoliths for the gneisses have a similar petrogenesis and magmatic source,which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material.The Central Tianshan Block probably constitute part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the Early Neoproterozoic and underwent a transition from subduction to syn-collision compression at 975-911 Ma.

Zircon U–Pb and Hf isotopic study of Neoproterozoic granitic gneisses from the Alatage area, Xinjiang:constraints on the Precambrian crustal evolution in the Central Tianshan Block

The architecture and growth history of Precambrian crustal basements in the Central Tianshan Block play a key role in understanding the tectonic evolution of the Chinese Tianshan Orogenic Belt.In this study,we present precise LA-ICP-MS zircon U–Pb dating and LAMC-ICPMS zircon Hf isotopic data for two granitic gneisses from Alatage area in the Central Tianshan Block.The magmatic zircons from both samples yield similar protolith ages of 945±6 and 942±6 Ma,indicating that the early Neoproterozoic magmatism is prevailed in the Alatage area.These zircons have crustal Hf model ages of1.82–2.22 and 1.70–2.03 Ga,respectively,which are significantly older than their crystallization ages.It indicates that their parental magmas were derived from the reworking of ancient crust.However,we suggest that these Paleoproterozoic Hf model ages might result from mixing of continental materials with different ages in the Neoproterozoic crust.The inherited(detrital)zircon cores not only yield a wide age range of ca.989–1617 Ma,but also exhibit large Hf-isotope variations with Hf model ages of1.54–2.30 Ga.In particular,some 1.4–1.6 Ga zircons show high initial176Hf/177Hf ratios,consistent with those of depleted mantle,which indicates that the Mesoproterozoic event involved both reworking of older crust and generation of juvenile crust.The Central Tianshan Block has different Precambrian crustal growth history from the Tarim Craton.Therefore,it would not be a fragment of the Precambrian basement of the Tarim Craton.

Tectonic Framework of Late Paleozoic Intrusions in Xingxingxia： Implications for Final Closure of South Tianshan Ocean in East Tianshan

This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.

The Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China——Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton;the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.

中天山地块沙垄铁建造矿石矿物原位微量元素特征及其意义

位于中天山地块的沙垄铁建造是新近识别出的新元古界铁建造之一。前人已经对该铁建造的矿床特征、形成时代和构造背景进行了研究,并通过矿石的全岩元素地球化学和铁同位素分析,探讨了其矿床成因和沉积环境。但是,目前缺乏针对沙垄铁建造矿石矿物的矿物学和微区地球化学分析等的研究,制约了对其铁矿物形成过程和形成环境的深入认识。本文以沙垄铁建造中代表性铁矿石的磁铁矿和赤铁矿为主要研究对象,在详细的矿相学研究基础上,利用飞秒激光剥蚀电感耦合等离子体质谱(fs-LA-ICP-MS)开展磁铁矿和赤铁矿的原位地球化学分析,进一步揭示磁铁矿和赤铁矿形成的过程和环境。沙垄铁建造的赤铁矿主要呈半自形微细粒板状,并且顺层定向分布,磁铁矿主要呈中粗粒状变斑晶产出。原位地球化学分析结果表明:磁铁矿总体具有较高质量分数的Si、Ca、Mn和较低质量分数的Ni、Cu、Zn,不同样品的磁铁矿V质量分数相对一致,Ti质量分数变化较大;赤铁矿相对于磁铁矿具有较高的Si、Ca、Ti、Cr质量分数和较低的Mn、Ni、Zn质量分数。磁铁矿的稀土总量较低(w(ΣREY)=1.49×10^(-6)~51.16×10^(-6),平均值为12.15×10^(-6)),具有轻稀土相对重稀土亏损、Eu异常不明显和正La异常的特征,不同样品的磁铁矿和赤铁矿具有与全岩的稀土元素一致的配分模式,表明了铁建造中磁铁矿和赤铁矿是稀土元素的主要载体,磁铁矿和赤铁矿微量元素特征可以为揭示铁建造的成矿物质来源和形成时的氧化还原条件提供新的依据。结合前人研究,本文认为沙垄铁建造中的磁铁矿变斑晶是赤铁矿重结晶作用的产物,磁铁矿的稀土元素特征指示沙垄铁建造具有低温热液和海水混合的成矿物质来源,而Ce异常的缺乏和质量分数相对一致的V、Cr,说明铁建造形成于相对缺氧的环境,并且在形成过程中氧逸度的变化较小。

Nature of the Precambrian metamorphic blocks in the eastern segment of Central Tianshan:Constraint from geochronology and Nd isotopic geochemistry

Granitoid gneisses are widespread in Precambrian metamorphic blocks of eastern segment of the Central Tianshan Tectonic Zone, and they have intrusive contact relationships with their metamorphic sedimentary country rocks of Proterozoic Xingxingxia and Kawabulag groups. Zircon U-Pb ages from a granodioritic gneiss (IW11-1) and a parametamorphic schist (W05-9) are determined at the Weiya area. Euhedral prismatic zircons from the granodioritic gneiss (IW11-1) provide U-Pb isotopic discordia intercept ages of 1218±17 Ma and 426±26 Ma, respectively, and euhedral prismatic zircons from the parametamorphic schist (W05-9) display U-Pb isotopic discordia intercept ages of 1216±74 Ma and 290±15 Ma, respectively. A whole-rock Sm-Nd isotopic isochron is determined in augen granitoid gneiss samples at the Gang- gou-Kumishi area and we obtain the isochron age of 1142±120 Ma, and its ε Nd (t) = -4.3. These geochronological data suggest that these Precambrian metamorphic basement blocks within eastern segment of the Central Tianshan Tectonic Zone can be produced during 1140—1220 Ma, and occur a nearly homochronous metamorphism. Integrated to these geochronological data, Nd depleted mantle model ages (T DM ) and epsilon Nd(t) values of these granitoid gneiss samples indicate that they can derive from mixing in various scales both magmas from mantle and crust sources at a late Mesoproterozoic active continental margin tectonic environment. Similarity in geochronology, Sm-Nd isotopic geochemistry between Weiya-Xingxingxia, Pargangtag and Ganggou-Kumishi areas suggests that they could be a bigger uniform metamorphic basement block, which could be formed by the assembly of the supercontinent Rodinia and be separated by late geological processes.

伊犁地块北缘温泉地区托克赛岩组的构造变形与年代学研究及其地质意义

伊犁地块是中亚造山带西南缘的主要大陆块体之一,记录了新元古代‒古生代多期岩浆活动、复杂构造变形和高级变质作用,是研究天山乃至中亚造山带构造演化的理想场所。本文对位于伊犁地块北缘的温泉地区托克赛岩组中石榴角闪岩、花岗片麻岩、云母片岩、石英片岩和侵入其中的未变形花岗岩脉进行了详细的野外构造分析和岩相学观察,发现石榴角闪岩主要分布在一条近EW走向的韧性剪切带中,与其共生的花岗片麻岩、云母片岩和石英片岩均发生了强烈韧性变形,发育一致的陡立面理和近水平线理,运动学指示右旋走滑剪切变形。锆石U-Pb年代学研究显示,韧性变形的角闪岩中变质重结晶锆石具有453±4 Ma和414±4 Ma两个主要年龄值,其中前者与云母片岩中热液改造成因的锆石年龄(465±7 Ma)基本一致,代表这些岩石经历了高角闪岩相变质作用,该变质事件与温泉地区中奥陶世陆缘弧岩浆作用同期。角闪岩中较年轻的年龄峰值与石英片岩(408±3 Ma)和花岗片麻岩(399±7 Ma)中热液成因的锆石生长边,以及花岗岩脉(394±5 Ma)的年龄在误差范围内基本一致,代表了岩浆热液作用的时代,该事件与伊犁北缘泥盆纪岩浆弧花岗岩的侵入时代相当。同时,在角闪岩和花岗片麻岩中均获得了~900 Ma和~780 Ma两个年龄峰值,分别代表花岗片麻岩和角闪岩的原岩结晶时代。此外,石英片岩和云母片岩中锆石的核部还给出了~1135 Ma、~1286 Ma、~1517 Ma和~1725 Ma共4个中元古代的年龄峰值,代表其原岩物源区的岩浆事件。结合前人研究结果,笔者认为伊犁北缘温泉地区在463~443 Ma和423~395 Ma期间分别经历了两期俯冲相关的岩浆‒变质和岩浆‒热液事件,是认识准噶尔洋早古生代向伊犁地块俯冲作用的重要依据。

星星峡石英闪长质片麻岩的锆石年代学:对天山造山带构造演化及基底归属的意义

天山造山带是中亚造山带(CAOB)的主要组成部分,对于其前寒武纪古老基底的起源、古生代构造单元划分和造山作用过程的认识还存在很大分歧。本文对分布在星星峡镇西的石英闪长质片麻岩开展了系统地岩相学观察和锆石U-Pb年龄、Hf同位素及全岩地球化学分析。根据矿物组成推测它们的原岩为石英闪长岩,两个样品中的锆石具有基本一致的内部结构特征,均发育黑色、均一的边部和具震荡环带的核部,部分保留有更早的继承锆石核。分析结果表明,它们的原岩形成于～425Ma,变质作用年龄为约320～360Ma,继承锆石的年龄为1381～1743Ma。原岩结晶锆石具有正的且变化较大的εHf(t)值(0.9～17.8),继承锆石的tDM2模式年龄变化于1.54～2.44Ga。在全岩地球化学组成上,石英闪长质片麻岩具有明显富集Rb、Ba、Th、K等大离子亲石元素和Pb、U元素,亏损Nb、Ta、Ti等高场强元素的特点。结合区域上已有的前寒武纪基底、高级变质岩、蛇绿混杂岩、岩浆岩的研究资料,获得以下认识:中天山地块的前寒武纪基底的起源与塔里木板块没有明显的相关性,可能是中元古代时期,由东欧板块边缘的弧增生造山作用形成;中天山地块东部在早古生代为大陆边缘弧的构造环境,可能形成于南天山洋向中天山板块的俯冲作用;南天山洋在天山造山带的东部和西部可能具有一致的闭合时间。

中天山地块南北两缘的韧性剪切带

本文详细描述了天山中段沿乌鲁木齐—库尔勒公路后峡—乌瓦门段的构造变形特点。综合室内外观察研究结果 ,在中天山地块南、北缘 ,及内部的乌拉斯台地区鉴别出三条规模较大的韧性剪切带 :中天山北缘剪切带前人已有研究 ,出露宽达 10 km的右行斜冲剪切带 ,见大量糜棱岩、超糜棱岩 ;乌拉斯台剪切带中的糜棱岩宽超过 2 0 0 m,具有左行走滑运动特点 ;中天山地块南缘剪切带宽度达 5 km,内部发育大量闪长质糜棱岩、超糜棱岩 ,运动方式为左行斜冲。中天山块体南北两侧的脆性边界断层与糜棱岩带变形中心基本重合。这些剪切带具有相当大的走滑位移量 ,指示天山各地质单元间目前的空间关系可能是走滑拼接的结果。研究天山造山带内剪切带的展布、运动方式、总变形量 ,对于恢复各地质单元的初始位置。

哈密宏源铅锌矿地质地球化学特征及找矿标志

宏源铅锌矿位于塔里木板块北缘中天山地块东段,于2003年检查1∶50000水系沉积物测量铅锌银综合异常时发现,目前规模为中型铅锌矿。在前人工作基础上,对矿床地质地球化学特征和找矿标志进行总结,为该区寻找此类型矿床提供依据及找矿标志。

中天山喀拉塔格一带麻粒岩包体的发现及其地质意义

为探讨中天山地块前寒武纪结晶基底组成及时代,进而制约天山造山早期地壳演化及构造归属,在野外调查基础上,结合岩石学、矿物学、矿物化学等手段,对新疆东部喀拉塔格地区(中天山地块东段)新发现的下地壳代表性岩石-麻粒岩进行了系统研究。结果表明,麻粒岩主要矿物组合为紫苏辉石+堇青石+黑云母+石英+斜长石;麻粒岩相变质发生于1899±32Ma(MSWD=2.4,error bars=2δ),原岩应形成于2354±23Ma(MSWD=1.2,error bars=2δ)。该麻粒岩为目前中天山地块有确切年龄依据的最老地质体,证实了中天山地块存在古元古代结晶基底并经历了古元古代晚期高温变质,为中天山地块基底组成与时代、与周缘地块对比研究提供了重要信息。

新疆哈密黑山金矿点地质特征及找矿方向

黑山金矿处于阿其克库都克-沙泉子断裂以南的卡瓦布拉克-星星峡地块,由天湖岩群变质岩和晚古生代中酸性火山岩组成矿床发现于2016年,2017年-2018年通过钻探工程,对金矿(化)带进行了验证,本文介绍了矿点地质特征、总结了找矿标志,初步认为其成因为浅成低温热液型,该区工作程度低,结合综合研究成果,指出了矿化带的西南、东北方向以及深部为下一步的找矿方向。

南天山中央地块的厘定与古气候事件

古气候变化与地质构造演化有着一定联系 ,构造活动的结果将引起地球表面海陆分布、陆地古纬度、火山活动、山脉形成以及地形地貌的巨大变化 ,是直接影响古气候的重要因素。根据志留—泥盆纪构造古地理再造、深部层次变形、基底时代、深部构造特征、构造混杂带研究 ,在确定南天山中央地块构造演化的基础上 。

中天山地块南缘两类混合岩的成因及其地质意义

中天山地块是位于中亚造山带西南缘的西天山造山带的重要组成块体,其基底演化和构造亲缘性对恢复西天山的增生造山方式和大地构造格局具有重要意义。混合岩在中天山地块的高级变质地体中广泛分布,是揭示中天山地块基底演化和构造属性的窗口。本文通过开展锆石U-Pb年代学和Hf同位素及岩石地球化学研究,确定了中天山地块南缘乌瓦门杂岩的两类条带状混合岩的原岩性质和形成时代以及混合岩化作用时代和成因机制。第一类条带状混合岩的原岩为中基性岩屑砂岩,混合岩化时代为~1. 8Ga,是在同期角闪岩相变质过程中通过变质分异形成的。第二类条带状混合岩的古成体包括黑云角闪斜长片麻岩和黑云斜长角闪片麻岩,原岩均形成于~2. 5Ga,并叠加~1. 8Ga角闪岩相变质作用,是洋陆俯冲背景下由俯冲洋壳或岩石圈地幔部分熔融形成。侵入古成体的变基性岩墙形成于~1. 72Ga,具有Fe-Ti玄武岩的地球化学特征,起源于后碰撞伸展背景下的软流圈地幔。该类混合岩的浅色体同时穿插古成体和变基性岩墙,呈现突变的野外接触关系,与区域内约787~785Ma混合岩化同期,即混合岩化作用是外来岩浆注入的结果,可能是造山带垮塌引发地壳深熔作用的产物。乌瓦门杂岩记录的~2. 5Ga岩浆活动、~1. 8Ga变质作用和~790Ma混合岩化作用可以和塔里木北缘进行对比,暗示中天山地块是一个具有确切新太古代-古元古代结晶基底的微陆块,并且和塔里木克拉通存在构造亲缘性。

中天山地块尾亚地区早古生代构造格架——来自野外地质和锆石U-Pb年代学的证据

野外地质调查发现尾亚地区分布的主体为岛弧杂岩,被更晚的造山后花岗岩如钾长花岗岩等侵入改造。尾亚地区的岛弧杂岩受造山作用的改造,局部发生了一定程度的变质变形作用。这套变质岩分布局限,变质程度较低且主要为绿片岩相,劈理化强。2个样品YM01、YM02分别为黑云斜长变粒岩、角闪斜长变粒岩,其原岩分别为中酸性火山碎屑岩及闪长岩。2个样品的锆石年龄记录了中天山地块北侧的岛弧岩浆事件,表明古亚洲洋(北天山洋)从奥陶纪已经开始,一直持续到泥盆纪由北向南持续俯冲。俯冲过程在晚寒武世开始,暗示中天山地块北侧在当时已经发生了构造转换,作为一个微陆块由被动大陆边缘转变为主动大陆边缘。另外,岛弧杂岩体中没有前寒武纪年龄记录的特征,表明区域内的前寒武纪基底物质分布有限,而变质变形强烈的部分多为造山过程的产物,是在造山带后期改造形成的。

新疆哈密黑山金矿流体包裹体及Pb同位素特征

黑山金矿是在中天山地块中发现的第一个颇具规模的金矿。该矿床产于受沙泉子断裂的次级断裂控制的晚石炭世-早二叠世中酸性火山中。通过蚀变岩特征和流体包裹体实验确认黑山金矿的成矿流体为中低温(163~216℃)、低盐度(3.05%~5.09%NaCleq)、H2O-NaCl流体。黄铁矿中Pb同位素特征(206Pb/204Pb值为18.5365~18.7465,207Pb/204Pb值为15.6033~15.6294,208Pb/204Pb值为38.4035~38.6851)表明黑山金矿的成矿元素来源于俯冲带的岩浆作用。综合黑山金矿床地质与地球化学特征,认为黑山金矿是晚石炭世-早二叠世形成于造山带环境中与中酸性岩有关的浅成低温热液型金矿。

中亚造山带内微陆块的起源——以中国天山造山带研究为例

中亚造山带内分布有众多的具有前寒武纪基底的微陆块,其构造起源一直是中亚造山带地质演化研究的核心问题之一。中国天山造山带呈东西向展布于中亚造山带的南部,发育伊犁和中天山两个古老微陆块。已有成果认为,伊犁地块和中天山地块可能存在古元古代地壳物质,其古老陆核主要形成于中元古代(ca.1.4~1.3Ga),并在新元古代早期(0.97~0.85Ga)发生强烈的地壳改造作用而固结形成统一的结晶基底。笔者最新的碎屑锆石研究表明,伊犁地块在1.8~1.7Ga和1.6~1.3Ga期间发生同时期的地壳生长和改造作用,此后主要以地壳改造为主(1.0~0.78Ga)。基于最近的岩浆研究认为中天山地块在中元古代(1.6~1.3Ga)经历了一期地壳生长事件,随后在新元古代早期开始以地壳的改造作用为主。在中-新元古代时期,伊犁和中天山地块可能都处于大陆地壳伸展环境(0.76~0.71Ga)。与塔里木克拉通及周缘其他主要古老块体的综合对比研究揭示,伊犁地块和中天山地块在前寒武纪时期具有相似的地壳演化过程,但不同于塔里木克拉通及其他古老块体。由此认为,伊犁地块和中天山地块与塔里木克拉通不具有亲缘性,其构造起源于相邻的哈萨克斯坦地块。

新疆东部卡瓦布拉克地区古生代岩浆活动对中天山地块构造归属的指示意义

卡瓦布拉克地区位于中天山地块南缘。该区古生代侵入岩类广泛发育,是探究中天山构造演化及地壳生长历史的重要窗口。本文获得该区两件二长花岗岩样品的LA-ICP-MS锆石U-Pb定年结果分别为447.4±4.9Ma、422.6±5Ma。LA-MC-ICP-MS锆石Hf同位素原位分析结果显示,其εHf(t)值分别为7.1~11.9、5.8~11.8,对应的二阶段模式年龄为674~963 Ma、514~1043 Ma。侵入于晚古生代花岗岩中的中-基性岩脉携带多期捕获锆石,其εHf(t)介于-7.4~4.8,对应的模式年龄变化范围为1.47~2.54Ga。卡瓦布拉克地区古生代花岗岩及中-基性岩脉中捕获锆石所反映的基底信息表明,中天山与塔里木在中元古代(~1.4Ga)、新元古代(~800Ma)以及古生代(~400Ma)期间岩浆活动具有截然不同的源区特征。结合前人研究成果,认为中天山与塔里木不具有构造亲缘性。

新疆中天山地块星星峡群形成的构造背景及峰期变质条件

中天山地块是天山造山带的重要组成部分,其前寒武纪基底的年龄自西向东逐渐变老。星星峡群位于中天山地块最东端,是天山造山带最古老的前寒武纪基底。目前为止,有关星星峡群原岩形成的构造背景与后期发生的变形变质作用尚无详细研究。文章重点对星星峡群的变质片岩和片麻岩进行了野外观察和取样,室内开展了薄片观察、矿物电子探针分析以及全岩地球化学测试。全岩主量元素地球化学分析表明,星星峡群变质片岩和片麻岩的原岩主要为泥质岩和砂岩;微量与稀土元素结果显示,星星峡群沉积形成于大陆弧或活动大陆边缘环境。依据矿物电子探针成分分析数据结合全岩主量元素测试结果计算获得,星星峡群所经历的峰期变质温压条件大致为610~730℃,6.8~8.5 kbar。此外,通过最新的基底岩石变质年龄限定星星峡群发生峰期变质的时间大概在380 Ma。

西天山伊犁地块早古生代拼贴:来自碎屑锆石U-Pb年代学和Hf同位素的制约

西天山伊犁地块记录着中亚造山带构造演化方面的关键信息,前人多认为伊犁地块是一个单独块体,但其西延境外部分对应着多个块体。这些块体在奥陶纪末期完成拼贴,它们之间的缝合带在西天山的延伸部分被晚古生代‒中新生代地层覆盖,但通过境内外地质对比,认为其很可能沿着伊犁地块中部向东延伸,并将伊犁地块划分为南、北伊犁两个块体。目前,对于南、北伊犁地块的拼贴时限缺少精确限定,其是否类似于境外部分于奥陶纪末期完成拼贴仍不清楚。为此,本项研究选取北伊犁地块志留系库茹尔组为研究对象,通过开展碎屑锆石U-Pb年代学和Hf同位素分析,约束南、北伊犁两个块体的拼贴时限。结果显示,库茹尔组碎屑锆石U-Pb年龄主要集中在早古生代(422~519 Ma),其最年轻峰值432 Ma约束了库茹尔组的最大沉积时代。在对比南北伊犁地块岩浆活动的基础上,发现库茹尔组沉积地层同时接收到了南北伊犁地块的物源供给,表明南北伊犁地块已在库茹尔组沉积前完成拼贴。通过综合解析碎屑锆石及区域岩浆岩锆石Hf同位素数据,发现其在奥陶纪末期同时向富集和亏损的方向演化,与显生宙以来东亚多块体拼贴过程中的Hf同位素演化趋势相似,因而我们将这种ε_(Hf)(t)值扇形演化样式归因于南北伊犁地块的碰撞拼贴,进而约束伊犁地块的拼贴时限在奥陶纪末期。