西秦岭勉县北部光头山二长花岗岩独居石电子探针U-Th-Pb化学法定年及其地质意义

EPMA monazite U-Th-Pb chemical dating for monzogranites from Guangtoushan intrusion in western Qinling Mountains,Central China and its geological significance

南秦岭勉略缝合带北侧光头山花岗岩体由早期的石英闪长质—英云闪长质岩石和晚期的S型二长花岗岩组成。其中早期侵入相明显地经历了韧性剪切带的改造,形成于勉略构造带闭合之前。晚期的S型二长花岗岩侵位于早期的石英闪长岩、英云闪长岩和韧性剪切带中,是与勉略洋盆闭合密切相关的同碰撞—后碰撞阶段侵位的花岗岩。所以光头山岩体的形成年龄可以较好地限定西秦岭勉略洋盆闭合、扬子板块与微秦岭板块碰撞的时代。2个二长花岗岩样品的独居石电子探针U-Th-Pb定年分析,分别得到3组年龄和4组年龄。结合前人的研究成果、独居石BSE图像和表观年龄分布,确定S型花岗岩的结晶年龄为209～196Ma,它限定了勉略构造带闭合隆升的时代为晚三叠世—早侏罗世;较老的240Ma±4Ma、318Ma±5Ma和249Ma±4Ma记录了早期与勉略缝合带闭合洋壳俯冲相关的残留独居石的年龄,年轻的184Ma±3Ma和154Ma±3Ma反映了后期的变形作用和流体活动等的改造作用。

The Guangtoushan granitoid pluton consists of early quartz diorites, tonalities and late intrusive monzogranites. Early lithological assemblages were deformed during the collision event between the South China Craton （SCC） and the Qirding-Dabie Microblock （QDM）, and some rocks of them have been partly changed into mylonites, which indicates they were formed before the clo- sure of Mianxian-Lueyang paleo-ocean basin. However, later stage S-type monzogranites emplaced into early quartz diorites and tonalities and the ductile shear zones as one main mark of the closure of Mianxian-Lueyang paleo-ocean basin, which reveals that the S-type monzogranites are correlated to the closure of the Mianxian-Lueyang suture zone. Therefore, the geochronological data of Guangtoushan granitic pluton can be used to constrain the timing of collision between the SCC and the QDM. Two representative samples of monzogrartites that formed at the later stage are dated using the electron microprobe monazite Th-U-Pb chemical dating method. The electron microprobe monazite Th-U-Pb data reveal three age groups and four age groups, respectively. Integrated distribution of the apparent ages on the BSE images, internal structures of analyzed monazite grains with isochron ages, the 209-196 Ma are interpreted as the main formation ages of the S-type monzogranites, which implies that the coUisional event between the SCC and the QDM, namely closure of the Mianxian-Lueyang paleo-ocean, occurred from Late Triassic to Early Jurassic. The older ages of 240±4 Ma, 318±5 Ma and 249±4 Ma could sign early geological events relating subduction of oceanic crust slab in the Mianxian-Lueyang suture zone, which are ages of inherited monazite remnants. And the younger ages of 184±3Ma and 154±3 Ma reflect overprint events of deformation and fluid alteration during post-collision to late stages.