桐柏-大别造山带南缘中侏罗世基性岩的发现及其构造意义——来自锆石U-Pb年代学和Hf同位素证据

摘要

桐柏-大别造山带南缘大洪山地区出露一套变质辉长辉绿岩脉,该岩脉沿襄樊-广济断裂带侵入到大洪山造山带增生杂岩之中,本文采用锆石U-Pb LA-ICP-MS定年方法,获得了该变质辉长辉绿岩脉的岩浆锆石年龄为163±3 Ma,代表岩脉的侵位结晶年龄,是目前桐柏-大别造山带已知最早的造山后晚中生代岩浆活动记录之一,表明桐柏-大别造山带南缘晚中生代岩浆活动开始于中侏罗世晚期,与北大别地区初始深融作用时间相当,比早白垩世大规模岩浆活动的峰期(～130 Ma)早约30 Ma.锆石Hf同位素分析结果显示εHf(t)值为+5.8～+10.8(平均值为+9.7±0.7),对应的单阶段Hf模式年龄tDM1为387～595 Ma(平均值为434±29 Ma),明显大于岩脉成岩年龄,表明岩浆源区主要来自于亏损地幔,亏损地幔为早古生代伸展过程的产物.结合区域研究资料,163±3Ma是桐柏-大别碰撞造山带南缘中生代陆-陆碰撞后最古老的岩浆岩年龄之一,与大别造山带山根软化的时代一致,可能代表桐柏-大别造山带由挤压环境向伸展环境的初始转换时间,据此认为桐柏-大别造山带从山根软化到大规模伸展垮塌间隔～30 Ma,地幔经历了亏损-富集的转化过程.%Dahongshan area,located on the southern margin of the Tongbai-Dabie orogenic belt,has extensively exposed metamorphic gabbro-diabase.This dike intruded into the accretionary complex of the Dahongshan orogenic belt.In this paper,zircon U-Pb LA-ICP-MS dating method shows that the emplacement age of rock vein is 163 ± 3 Ma.The age of this mafic dike is one of the earliest known records of Late Mesozoic magmatic activities after the collision in the Tongbai-Dabie orogenic belt,suggesting that the Late Mesozoic magmatic activities in the Tongbai-Dabie orogenic belt started in late period of Middle Jurassic,which is similar to the time of initial anatexis in northern Dabie orogenic belt,and is about 30 Ma earlier than the peak period of the Early Cretaceous magmatism (～ 130 Ma).The analytical results of Hf isotope from zircon show that εHf(t) is + 5.8～ + 10.8(with an average + 9.7 ± 0.7),and the corresponding single stage Hf model age tDM1 is 387～595 Ma (mean value being 434 ± 29 Ma),obviously larger than the age of diagenesis.These data show that the source of magma was derived from depleted mantle,resulting from the remelting of the Early Paleozoic partially melted depleted mantle basaltic rocks.All the data show that 163 ± 3 Ma is one of the oldest magmatic rock records after the continent-continent collision of the Mesozoic Tongbai-Dabie orogenic belt,and this age is in agreement with that of root softening in Dabie orogenic belt.It possibly represents the conversion time of Dabie orogenic belt from compressional environment to extensional environment.It is therefore considered that the interval from root softening to large scale extensional collapse in Tongbai-Dabie orogenic belt is ～30 Ma.