粤北白沙地区晚侏罗世高分异Ⅰ型细粒花岗岩年代学、地球化学特征及其地质意义

Geochemistry and geochronology of highly fractionated Ⅰ-type fine grain granite in Late Jurassic from Baisha area of northern Guangdong Province and its significance

受东西向佛冈-丰良断裂控制的佛冈复式岩体形成于多期次和不同来源的岩浆侵入作用。白沙地区细粒黑云母二长花岗岩体位于佛冈岩体北缘,是佛冈岩体的一部分,成因目前还不明确。文章以该岩体细粒黑云母二长花岗岩为研究对象,进行了岩石学、锆石U-Pb年代学、地球化学和Hf同位素组成研究。研究结果显示,白沙地区细粒黑云母二长花岗岩LA-ICP-MS锆石U-Pb加权平均年龄为155.6~157.1 Ma,表明其侵位于晚侏罗世。岩体具高硅(w(SiO2)=70.74%~77.47%)、富碱(w(Na2O+K2O)=5.78%~8.62%)、低磷(w(P2O5)=0~0.10)等特征,富集Rb、Th、U、K、Pb、Nd、Zr和Hf等元素,亏损Ba、Nb、La、Ce、Sr、P、Eu和Ti等元素。岩石总体上明显富集轻稀土元素((La/Yb)N=0.94~54.69,1个为0.94),具明显的负Eu异常(δEu=0.07~0.57,n=6),个别正Eu异常(1.00~1.16,n=3)。岩体地球化学特征指示,细粒黑云母二长花岗岩为高分异I型花岗岩。岩体形成于古太平洋板块(库拉板块)与欧亚板块俯冲作用和菲律宾地块与南中国-印支地块的岩石圈消减作用下,岩石圈伸展-减薄,导致地幔物质上涌,在高温条件热流作用下幔源物质诱导下地壳中-元古代物质部分熔融形成。

The Fogang complex controlled by East-West Fogang-Fengliang Fault formed from Multi-stages and different source magmatic intrusion.The petrogenesis of the fine-grained monzogranite in the Baisha area at the northern margin of the Fogang complex is still unknown.To solve this question,a compilation of petrology,zircon U-Pb chronology,geochemistry,and Hf isotopes were carried out on these granites.Our results show that the weighted average of the LA-ICP-MS zircon U-Pb ages of fine-grained biotite monzogranite in the Baisha area is 155.6~157.1 Ma,indicating that these granite formed in the late Jurassic.Our results show high content value of w(SiO2)(70.74%~77.47%),w(Na2O+K2O)(5.78%~8.62%),low content value of w(P2O5)(0~0.10%),the enrichment in Rb,Th,U,K,Pb,Nd,Zr and Hf and depletion of Ba,Nb,Ta,La,Ce,Sr,P and Ti.These samples are characterized by obvious enrichment of REE[(La/Yb)N=0.94~54.69]and obvious depletion of Eu[δEu=0.07~0.57(n=6)]with a little enrichment of Eu[1.00~1.16(n=3)],indicating that these granites belong to highly fractionated I-type granite.The fine-grained granite from the Baisha area in the northern Guangdong likely formed in the post-collision stage of the subduction of the Paleo-Pacific plate and Philippine Blocks to the Eurasian plate.The resulting lithosphere extension and thinning may lead to the upwelling of mantle material,revealing that the Late Jurassic fine particle biotite adamellite mainly originated from the Palaeoproterozoic rock of the crust in the high-temperature condition,accompanied by a small amount of the mantle-derived material.