西秦岭厂坝、黄渚关岩体的锆石U-Pb年龄、Hf同位素组成及其地质意义

摘要

Changba and Huangzhuguan plutons are located in the Qinling microplate between the Shang-Dan and theMian-Lue suture zones.The plutons consist of granodiorite and monzonitic granite.LA-ICP-MS zircon U-Pb dating suggests that the Changba and Huangzhuguan plutons formed in Early Mesozoic,and can be divided into two stages:Middle Triassic (229 Ma) and Late Triassic (215～209 Ma).Because of the cooling of the intrusion,the age of the middle part((229.2 ± 1.0) Ma) of the Huangzhuguan pluton is older than that of the marginal part ((215.8 ± 0.8) Ma),while the age of the middle part (218.3 ± 1.2) Ma)of the Changba pluton is older than that of the marginal part ((209.4 ± 0.8) Ma).The ε Hf(t) values of the Huangzhuguan pluton are -3.13～～-14.51,mainly between-10 and-6,whereas the εHf(t) values of the Changba pluton are-5.17 ～-14.51,mainly between-12 and-6.On the εHf(t) versus age diagram,εHf(t) is above the evolution curve of chondrite and below the evolution curve of depleted mantle.Moreover,the age of stage (TDM2) in Changba pluton is mainly between 1.8 and 2.1 Ga,whereas the age of stage (TDM2) in Huangzhuguan pluton is mainly between 2.0 and 2.2 Ga.Combined with the inherited zircon ages of 1932 Ma,2039 Ma,the dark en claves in Huangzhuguan pluton might suggest that the rock-forming materials were mainly derived from the Palaeoproterozoic crustal partial melting with the addition of mantle material.%厂坝及黄渚关岩体位于商丹缝合带和勉略缝合带之问的秦岭微板块,主要由花岗闪长岩及二长花岗岩组成.LA-MC-ICP-MS锆石U-Pb定年显示,黄渚关及厂坝岩体的演化可划分为中三叠世(229 Ma)和晚三叠世(215～209 Ma)2期.黄渚关中心部位的年龄 (229.2±1.0) Ma)老于边部岩体 (215.8±0.8) Ma),厂坝岩体中心部位年龄 (218.3±1.2) Ma),老于边部年龄 (209.4±0.8) Ma),结合岩体中心相和边缘相渐变的特征,说明2个岩体是由中心部位侵位,边部岩体的侵位冷却时间较晚造成的.黄渚关岩体的εHf(t)为-3.13～-14.51,集中于-10～-6;厂坝岩体的εHf(t)为-5.17～～-14.51,集中于-12～～-6.在εHf(t)-t图解中,ε Hf(t)均在球粒陨石之下和古元古地壳演化线之上;二阶段模式年龄黄渚关岩体主要集中在1.8～～2.1 Ga,厂坝岩体主要集中在2.0～～2.2Ga,结合继承锆石年龄1932Ma、2039Ma,表明古元古代地层可能为岩体的源区之一,在黄渚关岩体发现的暗色包体说明岩浆在上升的过程中发生了壳-幔岩浆的混合.因此,笔者认为黄渚关、厂坝岩体起源于古老地壳重熔,并在上升的过程中与幔源岩浆发生一定比例混合.