柴北缘乌兰北部三叠纪辉长岩的年代学和地球化学特征

doi:10.13278/j.cnki.jjuese.20190224

摘要/Abstract

摘要： 柴北缘乌兰地区中生代岩浆岩分布广泛，主要出露辉长岩、闪长岩、花岗闪长岩和花岗岩。乌兰北部两件辉长岩样品的LA-ICP-MS锆石U-Pb测年显示其加权平均年龄分别为（241.9±0.9） Ma和（245.4±1.9） Ma，说明其形成于中三叠世早期。辉长岩的w（SiO₂）为47.94%和52.01%，全碱质量分数较低（ALK为1.25%和1.47%），里特曼指数为0.33和0.26，属钙碱性系列岩石；w（Cr）（1 661.00×10^-6和1 418.00×10^-6）、w（Ni）（394.00×10^-6和280.00×10^-6）和Mg^#值（81和79）极高，具幔源原生玄武岩浆特征。轻、重稀土元素分馏较弱，LREE/HREE为3.92和3.44，无明显负Eu异常，表明未发生明显的岩浆分异作用；富集K、Rb等大离子亲石元素，不同程度地亏损Nb、Ta、Ti等高场强元素，表现为与俯冲作用相关的弧岩浆岩特征。辉长岩的ε_Hf（t）值（-3.0~9.3）变化较大，暗示来自地幔的基性岩浆中有地壳组分的加入。结合区域地质演化特征研究成果，认为辉长岩形成于古特提斯洋向北俯冲背景下的大陆边缘弧环境，俯冲的洋壳板片脱水产生的流体导致上覆地幔楔部分熔融，形成玄武质岩浆，岩浆上升侵位过程中遭受地壳物质混染或与壳源岩浆发生混合作用。

关键词: 柴北缘, 辉长岩, 锆石U-Pb测年, 地球化学, 岩浆弧, 古特提斯洋

Abstract: The Mesozoic magmatic rocks are widespread in Wulan area of northern Qaidam, and consist largely of gabbro, diorite, granodiorite and granite. The LA-ICP-MS zircon U-Pb dating of two gabbro samples yield well-constrained ages of (241.9±0.9) Ma and (245.4±1.9) Ma, indicating their formation of the Middle Triassic. Their SiO₂ content of gabbro in northern Wulan area is 47.94% and 52.01%, the total alkali content is low (1.25% and 1.47%), and the Ritman index is 0.33 and 0.26 respectively, belonging to cala-alkaline series. They have high Mg^# (81 and 79), Cr (1 661.00×10^-6 and 1 418.00×10^-6), and Ni (394.00×10^-6 and 280.00×10^-6), similar to those predicted of the original basaltic melts. They have weak LREE/HREE fractionation (3.92 and 3.44) with nearly no Eu anomalies and no obvious magma differentiation. The primitive mantle normalized trace element spider diagrams show enrichment of large ion lithophile elements (e.g., K, Rb) and depletion of high field strength elements (e.g., Nb, Ta, Ti), characteristics considered typical of subduction-related arc magmatic rocks. The large variation of zircon ε_Hf(t) values (from -3.0 to 9.3) implies that the crustal components seem to have been incorporated in the mantle-derived mafic magmas. Combined with regional geological evolution, we suggest that the gabbro formed in the continental marginal arc environment related to the northward subduction of the Paleo-Tethys Oceanic plate. The fluids released from the subducting oceanic slab promote partial melting of the overlying mantle wedge to produce basaltic magma, which was contaminated by crustal materials or mixed with crust-derived magma during magma ascending.

Key words: northern margin of Qaidam, gabbro, zircon U-Pb dating, geochemistry, magma arc, Paleo-Tethys Ocean

中图分类号:

P588.12

岳悦, 孙德有, 侯可军, 彭银彪. 柴北缘乌兰北部三叠纪辉长岩的年代学和地球化学特征[J]. 吉林大学学报(地球科学版), 2021, 51(1): 154-168.

Yue Yue, Sun Deyou, Hou Kejun, Peng Yinbiao. Geochronology and Geochemistry of Triassic Gabbro in Northern Wulan, Northern Margin of Qaidam Basin[J]. Journal of Jilin University(Earth Science Edition), 2021, 51(1): 154-168.

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