川西新元古代灯杆坪花岗岩成因与岩浆演化:来自斜长石和黑云母矿物学依据

新元古代灯杆坪花岗岩体位于龙门山造山带中段, 处于青藏高原东缘与扬子地块西缘的过渡地带,是研究扬子西缘构造-岩浆事件的典型地区。通过对新元古代灯杆坪岩体开展野外地质调查、岩相学分析以及电子探针测试工作,重点对花岗岩中斜长石结构成因特征、黑云母成分特征、岩浆演化进行了分析讨论。结果表明,灯杆坪二长花岗岩中斜长石具有简单“反环带”结构,主要为更长石(An_12.51Ab_47.43Or_40.06),正长花岗岩中“熔蚀结构”显著,碱性长石居多,多为透长石(An_7.46Ab_42.39Or_50.16); 花岗岩体中的黑云母具有富铝、富铁、低镁、低钾的特征,属铁质黑云母。同时I_Fe值为0.47~0.50,表明该岩体中的黑云母未遭受后期流体的改造。黑云母地球化学特征分析结果表明,灯杆坪岩体属于过铝质花岗岩,其形成与壳源岩浆有关。结晶压力和深度分别为1.81×10 ⁵~2.23×10 ⁵ Pa、6.58~8.11 km,黑云母的结晶温度为750~786 ℃,并且有较高的氧逸度,属于中深成相。这一结果为扬子地块西缘新元古代800~740 Ma期间的强烈伸展-岩浆事件提供了新的证据。

Origin and Magmatic Evolution of the Neoproterozoic Dengganping Granite in Western Sichuan: Mineralogical Evidence from Plagioclase and Biotite

The Neoproterozoic Dengganping granite is located in the middle part of the Longmenshan orogenic belt, which is in the transition zone between the eastern margin of the Qinghai-Tibet Plateau and the western margin of the Yangtze block. It is a typical area for studying the tectonic-magmatic evolution of the western margin of the Yangtze block. In this paper, field geological, petrographic and electron microprobe analyses were carried out on the Neoproterozoic Dengganping intrusion. We discuss the genetic features of plagioclase structure, compositional characteristics of biotite, and the magmatic evolution of the granite. The results show that the plagioclase in the Dengganping monzonitic granite has a simple “anti-ring” texture, mainly for the oligoclase (An_12.51Ab_47.43Or_40.06), In the syenogranite, the corrosive texture is clear, and alkaline feldspar predominates, most of which is sanidine (An_7.46Ab_42.39Or_50.16). Biotite in the granite is characterized by being Al-rich, Fe-rich, low-Mg and low-K, and belongs to iron biotite. Meanwhile, the I_Fe value is 0.47-0.50, indicating that the biotite was subjected to late fluid modification. Geochemical features of the biotite show that the Dengganping intrusion belongs to the peraluminous granite, whose formation was related to a crustal source. The crystallization pressure and depth are 1.81×10 ⁵-2.23×10 ⁵ Pa, 6.58-8.11 km, and the biotite crystallization temperature is 750-786 ℃. The granitic magma may have had high oxygen fugacity, belonging to the medium-deep formation phase. The results provide new evidence for the 800-740 Ma strongly-extensional magmatism in the Neoproterozoic western margin of the Yangtze block.