东天山觉罗塔格构造带晚三叠世镁安山岩的厘定及其构造意义

新疆东天山地区印支期岩浆活动的岩石成因、源区性质及相关动力学背景一直是区域地质问题争论的焦点。对东天山构造带阿奇山安山岩开展岩石学、地球化学及锆石U-Pb-Hf系统研究,结果表明安山岩具斑状结构和气孔状构造,斑晶矿物主要为单斜辉石和斜长石,基质以针状斜长石微晶为主。锆石U-Pb定年结果表明镁安山岩喷发于235 Ma(晚三叠世),并含有石炭纪年龄的继承锆石。锆石Hf同位素分析显示岩浆期锆石具有低的ε_Hf(t)值(-31.7~-7.6)和古老的亏损地幔模式年龄(T_DM=2.2~0.7 Ga)。岩石地球化学特征显示样品为高MgO含量(3.7%~4.7%)和K₂O/Na₂O比值(0.6~1.1)及低TFeO/MgO比值(1.6~1.8)的高钾钙碱性镁安山岩。岩石地球化学特征结合区域构造-岩浆-成矿作用揭示阿奇山镁安山岩为板内伸展背景下含金云母的富集岩石圈地幔在较浅深度(<80 km)部分熔融的产物,且母岩浆混有古老地壳物质。晚三叠世阿奇山镁安山岩的厘定反映了该时期东天山地区处于岩石圈伸展-减薄阶段。

Late Triassic Mg andesite of Juluotage,eastern Tianshan: Identification and tectonic implications

The petrogenesis and sources of Indosinian magmatic rocks and the associated geodynamic settings in eastern Tianshan, Xinjiang have long been debated. In this study, the Aqishan andesites were investigated through integrated petrographic, geochemical and zircon U-Pb-Hf isotopic analyses. The andesites had a typical porphyritic texture and vesicular structure. The porphyritic texture was made of clinopyroxene and plagioclase phenocrysts in a matrix of acicular plagioclase. Zircon U-Pb dating results showed the Mg andesites erupted in ca. 235 Ma (Late Triassic) and contained Carboniferous inherited zircon grains. In-situ Hf isotopic analysis showed the magmatic zircon grains had negative ε_Hf(t) values (-31.7--7.6) and Hf model ages of 2.2-0.7 Ga. The samples were geochemically marked by high MgO contents (3.7%-4.7%) and K₂O/Na₂O ratios (0.6-1.1) and low TFeO/MgO ratios (1.6-1.8) resembling high-K calc-alkaline Mg andesites. Geochemical characteristics combined with regional tectono-magmatic activities revealed the Aqishan Mg andesites were derived from partial melting of a phlogopite-bearing lithospheric mantle in an intraplate extensional setting. Therefore, during the Late Triassic, the lithosphere in eastern Tianshan was in an extension-thinning stage.