华北陆块南缘中生代合峪花岗岩的地球化学特征与成因

Geochemistry and petrogenesis of the Heyu batholith in the southern margin of the North China block.

合峪花岗岩基位于华北陆块南缘外方山地区,为豫西地区燕山期最大的岩基,出露总面积达784km2,有多期侵入的特点,可划分为6个单元。主要岩石类型为黑云母二长花岗岩。LA-ICP-MS锆石U-Pb年龄资料表明,合峪花岗岩基侵位时间为148.2~135.3Ma。合峪花岗岩基的SiO2=67.16%~75.43%,Al2O3=13.29%~15.92%,MgO=0.26%~1.12%,K2O+Na2O>8%,Na2O/K2O=0.88~1.43,属于高钾钙碱性系列,ACNK=0.94~1.09,为准铝质-弱过铝质花岗岩。岩体轻稀土富集、重稀土亏损((La/Yb)N=14.5×10-6~49.9×10-6,平均值27.2×10-6),Sr含量变化较大(102×10-6~848×10-6,平均290×10-6),Y、Yb含量低(Y=3.21×10-6~17.3×10-6;Yb=0.43×10-6~2.16×10-6),Eu弱亏损(δEu=0.57~0.89),反映熔体发生过长石分离结晶作用,源岩部分熔融生成熔体时残留相组合中没有或很少有长石的存在。合峪花岗岩基ISr=0.7071~0.7090,εSr(t)=40.8~65.9,εNd(t)=-16.4^-11.2,其tDM2为1.85~2.27Ga;锆石的εHf(t)主要集中于-25.39^-5.25之间,tDM2年龄介于1301~2846Ma。同位素数据显示合峪花岗岩基的源岩主要为南秦岭及扬子地块结晶基底,还混有少量熊耳群及太华群的物质。综合区域地质演化,认为合峪花岗岩基形成于秦岭造山带中生代陆内俯冲,岩石圈增厚,幔源岩浆上涌底侵导致下地壳部分熔融形成。

Heyu batholith in Waifang Mountain,Henan Province,located in the southern margin of the North China block.It is a multi-stage intrusion with a total area of outcrop up to 784km2.Heyu batholith mainly consists of biotitic monzogranite and can be subdivided into 6 units.LA-ICP-MS zircon U-Pb dating of the Heyu batholith porphyry gives concordant ages of 148.2~135.3Ma.SiO2,Al2O3 and MgO contents are 67.16%~75.43%,13.29%~15.92% and 0.26%~1.12%,respectively.Heyu batholith is high K calc-alkaline and metaluminous-peraluminous,with K2O+Na2O>8.0%,Na2O/K2O=0.82~1.43,ACNK=0.94~1.09.The rocks are relatively enriched in LREE and depleted in HFSE((La/Yb)N=14.5×10-6~49.9×10-6,average 27.2×10-6).Variable Sr content comparing with typical adakite(102×10-6~848×10-6,average 290×10-6) and low Y and Yb contents(Y=3.21×10-6~17.3×10-6;Yb=0.43×10-6~2.16×10-6),and slightly negative Eu anomaly(δEu=0.57~0.89) suggest that the fractional crystallization of the magma is obvious and a feldspar residual in the source is unlikely.The initial ISr ratio range from 0.7071 to 0.7090,and the εSr(t) value is 40.8~65.9.Negative εNd(t) value(-16.4^-11.2) with old tDM2 ages of 1.85~2.27Ga and negative εHf(t) values(-25.39^-5.25) with old tDM2 age of 1301~2846Ma suggest that the batholith was probably formed by partial melting of the south Qinling and Yangtze crystalline basement with the participation of Xiong'er Group and Taihua Group.In combination with the regional tectonic evolution,we suggested that the crust in the study area was thickened in early stage of intra-continental subduction of the Qinling orogenic belt intra-continent collision during Mesozoic,and the mantle-derived magma upwelling,and thereby the Heyu batholith formed in the southern margin of the North China Block.