新丰猴岩岩体LA-ICP-MS锆石U-Pb年龄及地质意义

Abstract

猴岩岩体位于佛冈复式岩体北东缘，总体呈南北向展布，出露面积约30 km2，岩性多为中细粒黑云母二长花岗岩、花岗闪长岩。本文对猴岩岩体花岗闪长岩样品进行LA-ICP-MS锆石U-Pb测年，获得的同位素年龄为(155.5 ± 4.2) Ma (MSWD = 2.0)，表明猴岩岩体侵位时代为晚侏罗世中期，是燕山第三幕岩浆活动之产物。通过对比，猴岩岩体与佛冈主体花岗岩的侵位时代相一致, 表明二者形成于同期构造–岩浆活动，猴岩岩体是佛冈主体花岗岩分异结晶之产物。猴岩岩体的花岗闪长岩是岩浆分异结晶时捕掳地层残留体，导致岩石成分产生差异而形成，为岩体的边缘相。佛冈复式岩体形成于古太平洋板块向欧亚大陆板块俯冲的构造背景下，其主体花岗岩的地球化学特征表明其物质主要来源于地壳，属陆壳改造型(即S型)花岗岩。 Located at the northeast of Fogang composite pluton, Houyan pluton, which is mainly composed of intermediate-fine grained biotite monzogranites and granodiorites, generally presents a north- south directional distribution and covers an area of 30 km2. In this paper, the LA-ICP-MS zircon U-Pb age of granodiorites of the Houyan pluton is (155.5 ± 4.2) Ma (MSWD = 2.0). The result shows that the emplacement age of Houyan pluton belonged to the middle stage of Late Jurassic, and it was the product of the third magma activity of Yanshan. As its age is the same as the main granite of Fogang composite pluton, they were in the same period of tectono-magmatic activity, and Houyan pluton was crystallized from magmas of main granite of Fogang composite pluton. As the crystallization differentiation of magmas captured the remained stratums, it caused differences in rock composition, and formed the marginal facies which are composed of granodiorites. Fogang composite pluton was formed in the process of subduction of the Pacific plate under Eurasian plate. The geochemical characteristics of its main granite indicate that its source mainly derived from the crust, which belongs to crustal transformation type (S-type) granite.