秦岭晚中生代花岗岩时空分布、成因演变及构造意义

Temporal-spatial variations,origin and their tectonic significance of the Late Mesozoic granites in the Qinling,Central China

秦岭晚中生代花岗岩主要发育于秦岭北部的华北地块南缘和北秦岭,南秦岭仅有零星出露。本文报道了3个晚中生代大岩体的年代学和其中2个岩体的地球化学和同位素数据,并系统收集了发表的相关资料,对秦岭晚中生代花岗岩进行了初步总结。华北地块南缘蓝田花岗岩的锆石LA-ICPMSU-Pb年龄为133±1Ma,εNd(t)=-11.8～-18.3,锆石εHf(t)=-37.7～-5.7。北秦岭构造带中牧护关花岗岩的锆石LA-ICPMSU-Pb年龄为150±1Ma,εNd(t)=-7.6～-11.4,锆石εHf(t)=-7.3～-17.4;蟒岭花岗岩的锆石SHRIMPU-Pb年龄为149±2Ma。依据收集到的26个锆石U-Pb和3个黑云母Ar-Ar年代学资料,秦岭晚中生代花岗岩浆的演化可分为2个阶段:第一阶段为160～130Ma(晚侏罗世-早白垩世),以I-型花岗岩为主,主要发育于华北地块南缘和北秦岭,在南秦岭仅零星分布;第二阶段为120～100Ma(早白垩世中晚期),以I-A过渡型和A-型花岗岩为主,主要分布在华北地块南缘的东部和北秦岭,出露面积比第一阶段小。第一阶段(160～130Ma)花岗岩主要形成于古老地壳物质的部分熔融,并有年轻幔源组分的参与,形成于挤压向伸展转换的构造环境。第二阶段(120～100Ma)花岗岩的形成除了古老地壳物质的部分熔融外,有更多的年轻幔源组分加入,发育于陆内伸展环境。花岗岩的同位素特征显示,从华北地块南缘到南秦岭,底基物质中年轻组分有增加的趋势,花岗岩的物源受深部地壳物质组成特征的控制。

Zircon U-Pb dating for three large granitoid batholiths,geochemical and Hf isotope analyses for two of them have been carried out in the different blocks of the Qinling.Zircon dating by LA-ICPMS for the Lantian granitoid in the southern margin of the North China Block gives an age of 133±1Ma.The granitoids show whole-rock εNd（t）=-11.8^-18.3,zircon εHf（t）=-37.7^-5.7.The Muhuguan and the Mangling granitoids in the North Qinling orogen yield zircon LA-ICPMS and SHRIMP age of 150±1Ma and 149±2Ma,respectively.The Muhuguan granitoids show whole-rock εNd（t）=-7.6^-11.4 and zircon εHf（t）=-7.3^-17.4.Combined with statistical analyses of 26 zircon ages and 3 Ar-Ar ages of granitoid plutons collected from the literature,the Late Mesozoic magmatism in the Qinling can be divided into two stages.The first-stage magmatism （160~130Ma） widely occurred in the southern margin of the North China Block,the North Qinling and occasionally in the South Qinling,and has features of I-type granitoids.The second-stage magmatism （120~100Ma） characterized by transition of I-to A-type and A-type granitoids took place in the North Qinling and the eastern part of the southern margin of the North China Block.The first-stage magmatism occurred in a transition from contractional to extensional settings,while the second-stage in the extension setting.The formation of the grantoids are interpreted as partial melting of old crust,mixed with juvenile mantle component.The second-stage granitoids contain more juvenile component than the first-stage.The isotope data of the granitoids show that juvenile compositions of the basement increas from the southern margin of the North China Block to the Shangdan suture.