扬子陆核古元古代A型花岗岩的年代学与地球化学研究及其构造意义
Geochronological and Geochemical Study for the Paleoproterozoic A-type Granite in the Nucleus of the Yangtze Craton and Its Tectonic Implication

摘要： 以侵入于宜昌崆岭杂岩中的圈椅埫花岗岩体为研究对象，系统研究了其年代学和地球化学特征，并据此对岩石成因和扬子陆核古元古代构造演化过程进行探讨。锆石LA-ICP-MS U-Pb定年结果表明，圈椅埫花岗岩形成年龄为(1 822±44) Ma，说明其为扬子陆核古元古代岩浆活动产物。地球化学研究表明，该花岗岩体富Si，贫Al、Mg，微量元素组成上富集Rb、Th，具有Eu、Ba、Sr和高场强元素的负异常。岩石具有高Ga/Al比值和(Zr+Nb+Ce+Y)含量，其锆石饱和温度计算值较高(＞862 ℃)，综合地质地球化学特征表明该岩体应属铝质A型花岗岩。岩体的εNd(t)值在-12.4~-10.3之间变化，对应两阶段Nd同位素模式年龄值为3.3~3.2 Ga，暗示岩体可能形成于扬子陆核深部古老的长英质地壳物质在后碰撞伸展构造背景低压、高温条件下部分熔融。结合前人已有的研究成果，认为其可能与区域上2.0~1.9 Ga板块碰撞造山后发生的由碰撞挤压向伸展作用的构造转换作用有关。扬子陆核古元古代构造岩浆事件与全球范围内2.1~1.8 Ga的与Columbia超大陆演化有关的碰撞造山-裂解事件时间吻合，表明扬子陆核可能是Columbia超大陆的重要组成部分之一。
Abstract: This study carries out systematical geochemical and geochronological investigations on the Quanyi-shang granitic pluton intruding into the Kongling Complex, and discusses the petrogenesis and Paleoproterozoic evolution of the nucleus of the Yangtze craton. The zircon LA-ICP-MS U-Pb dating shows that crystallization age of the granite is (1 822±44) Ma, hence a typical product of Paleoproterozoic magmatism of the Yangtze continental interior. The geochemical research illustrates a depletion of Al, Mg and enrichment of Rb, Th, as well as negative anomalies of high field strength elements (HFSE) and Eu, Ba and Sr. High Ga/Al ratio and Zr+Nb+Ce+Y content and relatively high calculated zircon saturation temperature (＞862 ℃), combined with other geochemical features, indicate that the Quanyishang granitic pluton is aluminous A-type granite. The εNd(t) value varies between -12.4 to -10.3 and the correspondent two-stage Nd isotopic modal ages range from 3.3 to 3.2 Ga. These characteristics imply that the granite may have formed by partial melting of ancient felsic crustal material in the nucleus of the Yangtze craton under a low-pressure, high-temperature condition at a post-collisional extensional tectonic setting. Combined with the documented work, it may have relationship with the regional tectonic transition from collisional extrusion to extention in a post-collisional setting during the period from 2.0 to 1.9 Ga. The Paleoproterozoic tectonic magmatic events of the Yangtze craton is consistent with the global 2.1-1.8 Ga collisional orogeny to breakup events related to the evolution of Columbia supercontinent, indicating that the nucleus of the Yangtze craton may have been an important part of Columbia supercontinent