The 2.73 Ga I-type granites in the Lengshui Complex and implications for the Neoarchean tectonic evolution of the Yangtze Craton

ABSTRACT

We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 ± 13 Ma and 2738 ± 25 Ma, respectively. The more precise age of 2732 ± 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO₂ (73.11–74.01 wt%), K₂O (3.93–5.48 wt%), Na₂O (3.93–4.86 wt%) and low CaO (0.30–0.69 wt%), MgO (0.17–0.30 wt%), TiO₂ (0.14–0.17 wt%), P₂O₅ (0.01–0.06 wt%), Al₂O₃ (14.11–14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04–1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2–34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their ε_Hf(t) values (?2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive ε_Nd(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67–2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.     

北天山四棵树花岗岩体锆石U-Pb年代学及地质意义

四棵树岩体位于北天山缝合带中,是侵入依连哈比尔尕岛弧带中最大的一个花岗岩体,具有"钉合岩体"的构造意义。组成该岩体的岩石类型较多,大体上可分为3个侵入期次,依次为石英闪长岩和花岗闪长岩、二长花岗岩和钾长花岗岩、花岗斑岩;其中第二侵入期次构成了该岩体的主体。岩石SiO2含量为57.85%~74.28%,K2O含量为1.95%~5.25%,Na2O含量为3.00%~4.50%,整体表现为高K、Ca碱性特征。∑REE=113.91×10-6~169.47×10-6,(La/Sm)N为1.77~3.16,(Gd/Yb)N为1.33~2.12,(La/Yb)N为3.41~5.64,δEu为0.37~0.82,表现为轻稀土元素富集的右倾型特征;岩石具有Nb、Ta、Eu的亏损以及Th、Zr、Hf相对富集。通过形成环境判别,第一期石英闪长岩和花岗闪长岩大部投在火山弧环境,板块碰撞前—后碰撞隆起阶段;第二期二长花岗岩和钾长花岗岩大部投在后碰撞阶段;第三期花岗斑岩主要投在晚造山阶段。通过LA-ICP-MS对该岩体中石英闪长岩、二长花岗岩和钾长花岗岩进行较为系统的锆石U-Pb测年,得出其加权平均年龄分别为(324.1±4.3)Ma、(314.9±4.1)Ma、(311.5±3.9)Ma。结合前人得出的巴音沟蛇绿岩中辉长岩的(344±3.4)Ma年龄,可初步得出北天山缝合带的形成时代应该为344~311.5Ma,碰撞造山作用主要发生在324~311Ma,随后进入后碰撞伸展阶段,而四棵树岩体3期侵入岩可能分别代表了北天山造山带俯冲汇聚-碰撞-陆内造山各阶段构造-岩浆作用的产物。     

青藏高原羌塘中部果干加年山一带晚三叠世花岗岩的特征、锆石定年及其构造意义

藏北羌塘果干加年山的东部出露有一处花岗岩岩体,其主要岩性为黑云母二长花岗岩和花岗细晶岩岩脉,侵入晚三叠世望湖岭组和晚石炭世—早二叠世展金岩群之中。锆石LA-ICP-MS U-Pb年龄测定结果表明,该花岗岩的形成时代为晚三叠世（210.3Ma±2.6Ma）,略晚于区域上高压变质带的变质峰期年龄和望湖岭组底部流纹岩的年龄。岩石地球化学数据显示,该花岗岩岩体属高钾钙碱性过铝质花岗岩,形成于后碰撞环境,是岩石圈碰撞加厚之后减压过程中岩浆活动的产物,说明在210Ma左右果干加年山地区的构造环境开始由碰撞阶段向板内阶段转化,进入了后碰撞阶段。     

北祁连金佛寺岩体地球化学特征及构造意义

金佛寺岩体为北祁连华力西早期形成的一个较大的复式深成岩基,主要岩石类型为二长花岗岩、黑云二长花岗岩和正长花岗岩,为高钾钙碱性岩系,具高钾、高铝和低钛的特点;REE分布模式为弱Eu异常;在Peace的Rb-Y-Nb图解中位于后碰撞花岗岩类区。岩体的同位素锆石U-Pb年龄为345.0±75.0Ma,代表其形成时代为晚泥盆世...     

东准噶尔北缘两类钙碱性花岗岩特征及其构造-成矿意义

依据前人对东准噶尔地区花岗岩类年代学和地球化学研究,将东准噶尔北缘钙碱性花岗岩划分为两类：一类为俯冲钙碱性花岗岩;另一类为后碰撞高钾钙碱性花岗岩。岩石地球化学对比研究发现,两类钙碱性花岗岩存在系统差异,兼具有相似性。岩石地球化学特点差异表现为俯冲钙碱性花岗岩,显示亲岛弧岩浆性,后碰撞高钾钙碱性花岗岩具有陆内后碰撞深成岩浆的特点。两者之间的相似性表明,造山带内花岗岩类由俯冲钙碱性→后碰撞高钾钙碱性系列演化是由于俯冲造山向碰撞造山快速转化、空间上位于同一造山带内,源区岩浆有双重性的特点。钙碱性花岗岩产出的构造环境具有多样性,成岩过程受造山带不同演化阶段相应构造背景制约,洋陆俯冲的岛弧环境和陆内造山的后碰撞环境分别是俯冲钙碱性和后碰撞高钾钙碱性花岗岩形成的构造背景。埃达克质岩浆亲和性显示俯冲钙碱性花岗岩与部分后碰撞高钾钙碱性花岗岩均有形成不同背景（岛弧和后碰撞）斑岩型矿床的潜力。后碰撞高钾钙碱性花岗岩与IOCG型矿床的空间联系,表明造山带伸展背景的后碰撞环境,也可能是IOCG型矿床产出的构造背景。两类钙碱性花岗岩构造背景的差异可能初步印证了具有岛弧斑岩型矿床的哈腊苏-卡拉先格尔和IOCG型矿床的老山口南东一带成矿环境的差异。     

东准噶尔北缘两类花岗质岩石及其地质意义

对东准噶尔北缘造山带内花岗质岩石以岩石地球化学特征为主的对比研究发现,该区主要存在两类花岗质岩石：造山带钙碱性花岗岩类和富碱花岗岩类。部分钙碱性花岗岩与新疆北部的埃迭克质岩具有相似的岩浆亲和性,而富碱花岗岩更多的表现出碱性花岗岩的岩石地球化学特点？两类花岗质岩石分别产于大陆碰撞造山带造山演化的俯冲环境和陆内后碰撞环境,...     

内蒙古镶黄旗乌兰哈达中二叠世碱长花岗岩 LA-ICP-MS锆石U-Pb年龄和地球化学特征

内蒙古镶黄旗乌兰哈达岩体位于华北克拉通北缘中段,岩性以中细—中粗粒碱长花岗岩为主。LA-ICP-MS锆石U-Pb定年结果表明,该花岗岩的侵位年龄为265.1Ma±1.5Ma（MSWD=1.5）,形成于中二叠世。该花岗岩富硅(SiO2为73.63%~75.47%),富碱(Na2O+K2O介于9.25%~9.49%)之间。在SiO2-K2O图中,样品多数落入高钾钙碱性系列。A/CNK值在1.00~1.03之间,属弱过铝质花岗岩。稀土元素含量高(∑REE=94×10-6~383×10-6),轻重稀土元素分异明显,(La/Yb)N=6.59~13.20,具有明显的负Eu异常（δEu=0.22~0.38）。相对亏损Ba、Sr、Nb、P、Ti元素,而高场强元素含量高（Zr+Nb+Ce+Y>350×10-6）,且具有较高的10000×Ga/Al比值(2.36~2.79)。地球化学特征暗示了乌兰哈达碱长花岗岩具有典型的A型花岗岩特征。相对低的Nb的含量进一步暗示了该岩体具有A2型属性。构造环境R1-R2判别图和构造环境（Y+Nb）-Rb判别图解显示,该岩体可能形成于后碰撞的拉张环境。结合乌兰哈达岩体的大地构造位置、构造环境及侵位时代,初步推断研究区内华北板块北缘与西伯利亚板块南缘碰撞缝合时代应早于265.1Ma。     

新疆南天山缝合带的形成时限 ——来自阔克萨彦岭花岗岩体的锆石年龄新证据

南天山缝合带位于塔里木克拉通与中天山-伊犁-哈萨克斯坦地块之间,其形成时限对于研究古亚洲洋西南缘的南天山洋最终闭合具有重要的意义.利用LA-ICP-MS锆石U-Pb法对南天山阔克萨彦岭地区的花岗岩体进行测试,结果表明花岗岩体的形成时代为早二叠世(273～283Ma),而且大量早二叠世侵入体同时出现在中天山-伊犁-哈萨克斯坦地块和塔里木克拉通.其中,巴雷公蛇绿岩也被早二叠世侵入体所侵入,限定了南天山缝合带的形成时限应早于早二叠世.研究结果进一步支持南天山洋的最终闭合应发生在300Ma之前.     

Post-Orogenic Granites in Pingwu Region, Northwest Sichuan: Evidence for North China Block and Yangtze Block Collision during Triassic

The Nanyili (南一里), Laohegou (老河沟), and Shaiziyan (筛子岩) granitic intrusions are located in the southern margin of the Bikou (碧口) block in Pingwu (平武) area, Northwest Sichuan (四川). The petrography and geochemical characteristics of the granitic intrusions as well as their source and tectonic settings are reported and discussed in this article. The Laohegou and Shaiziyan granites are with high SiO2 (69.89 wt.%-73.05 wt.%) and Al2O3 contents, and A/CNK=1.04-1.12. They are typi-cal strongly peraluminous granites, with supersaturation in Al and Si. The abundance of ∑REE varies in the range of (33.13-89.12)×106. The rocks show an LREE enrichment pattern and obvious Eu nega-tive anomaly. The trace element geochemistry is characterized evidently by a negative anomaly of Ta, Nb, Ti, etc. and a positive anomaly of Rb, Ba, Sr, etc.. Zircons of the Nanyili granite have higher Th/U ratios, and their CL images have internal oscillatory zoning, suggesting that the zircons of the samples are igneous in origin. The LA ICP-MS zircon U-Pb isotopic concordia diagram yields an age of 223.1±2.6 Ma (MSWD=1.4), which indicates that the granodiorite intrusions formed in the early Late Triassic. The Nanyili, Laohegou, and Shaiziyan granites have the characteristics of post-eollisional granites and are regarded as post-orogenic granites. Thus, the granite intrusions are interpreted as syn-collisional granites that resulted from the crustal thickening caused by the collisions between the North China plate and the Yangtze plate during the Indosinian. The granitic intrusions formed in a transitional environment from syn- (compres-slonal environment) to post-collision (extensionalenvironment).     

西藏安多地区粗面岩的特征及其锆石SHRIMP U-Pb定年

安多粗面岩位于班公湖-怒江缝合带中段,紧邻安多蛇绿岩带的北侧,出露面积约30km2,堆积厚度约1500m,属陆相火山岩。火山机构面貌保存较完整。岩石类型包括安粗岩、石英安粗岩、粗面岩及少量火山碎屑岩,以粗面岩为主,属钾玄岩系列。岩石地球化学和同位素示踪特征表明岩石属相对高压型的粗面岩,是与大陆碰撞后造山有关的钾质火山岩。岩浆主要来源于加厚的陆壳下部,但也有幔源组分的贡献。粗面岩中锆石SHRIMP U-Pb年龄为79.9Ma±1.9Ma,是粗面质岩浆上侵结晶的年龄,表明安多粗面岩的喷溢时代为晚白垩世, 也暗示班公湖-怒江洋盆安多段早白垩世晚期已经闭合,晚白垩世时期羌塘地块与拉萨地块已成为统一的陆块,并具有加厚的陆壳。