点苍山-哀牢山变质杂岩带中、北段多期花岗质岩浆事件及其构造意义

点苍山-哀牢山变质杂岩带位于青藏高原东南缘扬子板块与印支板块的结合部位。杂岩带经历了多期构造-岩浆-变质事件的叠加,带内物质组成极其复杂,是研究古特提斯演化、印度-欧亚板块俯冲-碰撞-隆升以及渐新世印支地体沿红河-哀牢山左行走滑侧向挤出的关键地区。本文对点苍山-哀牢山变质杂岩带中、北段戛洒地区11件花岗质岩石进行了LA-ICPMS锆石年代学和岩石地球化学分析,研究揭示出新元古代、中三叠世、始新世和渐新世四期不同类型的花岗质岩浆活动。其中4件花岗质岩石中锆石206Pb/238U加权平均年龄为780.0±6.3Ma~743.6±6.7Ma,部分岩石内发育有渐新世深熔锆石;1件花岗质片麻岩内27颗锆石的206Pb/238U加权平均年龄为234.3±2.0Ma;3件花岗斑岩锆石U-Pb测年获得36.67±0.54Ma~33.01±0.39Ma的加权平均年龄;3件花岗质片麻岩/糜棱岩中获得渐新世(28.58±0.57Ma~24.53±0.29Ma)侵位年龄。岩石地球化学研究揭示新元古代花岗质岩石具有较高的Cu、V、Zn、TiO_2含量及较高铝指数,A/CNK变化范围为1.09~1.13,TiO_2含量为0.44~0.60;中三叠世花岗岩具有钙碱性特征,A/CNK1.0,R1-R2图解中落入深熔花岗岩范围内;始新世花岗岩具有高Sr、Ba、K_2O含量,高Sr/Y、Sr/Nd、Ba/Th比值,低Nd、Na)2O含量的特点,岩石中全碱含量为9.2%~10.4%,Sr/Y值为48.8~94.5,Sr/Nd值为30.4~65.3,Ba/Th值为151~358;渐新世花岗岩全碱含量较低变化范围为3.25~9.03,A/CNK范围为1.09~1.14,属富铝质S型花岗岩。花岗质岩石野外产状、锆石年代学与岩石地球化学综合研究揭示新元古代花岗质岩石可能属扬子板块西缘原特提斯洋俯冲形成的陆缘岩浆弧,印支期造山作用过程中卷入到杂岩带内,新生代区域性走滑剪切使岩石进一步发生变形和深熔;中三叠世花岗岩为古特提斯洋闭合后扬子板块与印支板块碰撞后伸展构造热事件的响应;始新世花岗质岩石为印度-亚欧板块碰撞后伸展背景下地幔上涌交代下地壳进而产生的富钾花岗质岩石,且侵位年代由北向南逐渐变新记录了印度-亚欧板块汇聚角度的调整过程;渐新世花岗质岩石为红河-哀牢山剪切-走滑抬升过程中变沉积岩、新元古代和晚二叠世-三叠纪花岗质岩石的部分熔融而成。

Multiple granitic magma events in north-middle segment of Diancang Shan-Ailao Shan complex zone:Implications for tectonic evolution

The Diancang Shan (DCS)-Ailao Shan (ALS) metamorphic complex belt is located on the southeastern margin of Tibetan Plateau, which separated the Yangtze Block to the east and the Indochina Block to the west. The DCS-ALS complex belt consists mainly of various metamorphic rocks, and was overprinted on multiple deformation and metamorphism events, where is a key region for revealing the complicated evolution of the Paleo-Tethys, subduction-collision-exhumation between the Indian-Eurasian plate and the Indochina Block extrusion alone the Red River (RR)-Ailao Shan sinistral strike-slip shearing zone in Tertiary. In this study, eleven granitoid rocks are collected from the Gasa area within the north-middle segment of DCS-ALS metamorphic complex. Neoproterozoic, Mid-Triassic, Eocene and Oligocene four periods of granitic magmatism have been constrained by LA-ICP-MS dating. Thereinto, four granitic rocks yield the weighted mean ²⁰⁶Pb/²³⁸U ages ranging from 780.0±6.3Ma to 743.6±6.7Ma, indicating a Neoproterozoic magmatic event occurring in the southwestern margin of Yangtze Block. In contrast, 27 zircons collected form one granitic gneiss yield the weighted mean ²⁰⁶Pb/²³⁸U ages of 234.3±2.0Ma. Whereas three granitic rocks show the weighted mean ²⁰⁶Pb/²³⁸U age ranging from 36.67±0.54Ma to 33.01±0.39Ma. Three granitic mylonite yield the youngest Oligocene ages of 28.58±0.57Ma~24.53±0.29Ma. Geochemical analysis of various granites have revealed that the Neoproterozoic granitic rocks are rich in Cu, V, Zn and TiO₂ content with a higher Al index. The value of A/CNK and the TiO₂ content change from 1.09 to 1.13 and 0.44 to 0.60, respectively. The Mid-Triassic granitic gneiss with A/CNK rations <1.0 belong to calc-alkaline series in SiO₂-K₂O diagram, and plot into the region of anatectic granitoids in R1-R2 diagram. The Eocene granitic/granodioritic rocks contain the total alkali contents ranging from 9.18 to 10.43 show higher Ba, Sr, K₂O contents, and lower Na₂O content with higher Sr/Y (48.8~94.5), Sr/Nd (30.4~65.3) and Ba/Th (151~358) ratios. The Oligocene granitic rocks have lower total alkali contents (3.25~9.03) and higher A/CNK ratios 1.09~1.14, which belong to typical S-type granite. Combine studies of the field occurrence, geochronology and geochemical features with previous published data have revealed the petrogenesis and tectonic evolution for the various granitic rocks within the Diancang Shan-Ailao Shan complex belt. The Neoproterozoic granitic rocks are similar to the continental arc granites which were genetically related to the Proto-Tethys ocean subduction to the southwestern margin of Yangtze Block, and subsequently involved in the DCS-ALS metamorphic complex during late collision orogeny. The Mid-Triassic granitic gneiss shows closly relationship with the post-collisional extension between Yangtze Block and Indochina Block. Post-collisional extension between Indian plate and Eurasia plate in Eocene triggered the potassic porphyry intruded in/around DCS-ALS area. The emplacement ages decrease from north to south reflect the the adjustment of convergence angel. The Oligocene granitic rocks are genetic connect to the RR-ALS regional strike-slip. The preexisted rocks in deep level within the complex zone were uplifted to shallow crust during strike movement and induce partial melting eventually formed the syn-tectonic granitic rocks.