相山高分异A型花岗岩的识别及地质意义

传统地球化学判别图解能够较好地区分I-S-M-A花岗岩岩石成因类型,但对于高分异花岗岩的判别并不可靠。相山火山侵入杂岩岩石成因一直存在S型和A型的争议,高度分离结晶作用可能是导致这一争议的原因所在。系统收集了相山火山侵入杂岩相关地球化学数据,进行了高分异花岗岩的地球化学判别及岩石成因类型的判别。结果表明,相山火山侵入杂岩属于高分异花岗岩范畴(Zr/Hf<38,Nb/Ta<17)。地球化学判别图解表明,相山火山侵入杂岩具有A型花岗岩演化趋势和A₂型花岗岩特征,为高分异A₂型花岗岩。相山火山侵入杂岩A₂型花岗岩可能的成因模式如下:中新元古代上中地壳变质基底,在燕山晚期强烈的拉张作用下,受上涌地幔物质加热而发生部分熔融,形成A₂型花岗质岩浆,该岩浆经历了较强的结晶分异作用,最终形成高分异的A₂型花岗岩。晚期,壳幔混合作用明显,形成了与流纹英安岩、碎斑熔岩、花岗斑岩完全不同的石英二长斑岩。

Identification of highly differentiated A-type granite in Xiangshan and its geological significance

Traditional geochemical discrimination diagrams can better distinguish the genetic types of I-S-M-A granites, but they are not reliable for discriminating highly differentiated granitic rocks. The genesis of Xiangshan volcanic-intrusive complex has always been disputed between S-type and A-type which may be due to highly differentiated crystallization. In this paper, 116 groups of related geochemical data of Xiangshan volcanic-intrusive complex have been systematically collected, and the geochemical and genetic types of highly differentiated granites have been identified. 92 samples of Xiangshan volcanic-intrusive complex belong to highly differentiated granite (Zr/Hf<38, Nb/Ta<17). The geochemical discriminant diagram shows that the Xiangshan volcanic-intrusive complex has the evolution trend of A-type granite and the characteristics of A2 type granite, belonging to highly differentiated A2 type granite. Possible model for Xiangshan highly differentiated A2 type granite is as follows: the upper or middle crust of the Neoproterozoic in metamorphic basement was partial melted due to upwelling mantle material heating under the influence of late Yanshanian strong tension, and finally formed the highly differentiated A2 type granitic magma. In the late stage, quartz-monzonite-porphyry was formed differently from other types of rock in Xiangshan because of obvious crust-mantle mixing.