川西稻城岩体北部花岗岩年代学、地球化学和成岩构造环境

Zircon U-Pb Chronology,Geochemistry,and Their Tectonic Setting of Granitic Rocks from Northern Daocheng Batholith in West Sichuan

川西稻城岩体北部花岗岩成因和成岩构造环境争议较大.本次工作选择该岩基北部花岗岩为研究对象,进行了岩石学、锆石U-Pb年代学、地球化学和Hf同位素研究.LA-ICP-MS锆石U-Pb测年结果显示,岩体北部花岗岩形成时代213~212 Ma,略晚于南部.岩性以花岗闪长岩和二长花岗岩为主,前者含较多暗色微粒包体(mafic microgranular enclaves,MME).代表性样品地球化学上具有富钠、偏铝-弱过铝质、富大离子亲石元素的特征.ε_(Hf)(t)值非常分散,介于-16.6~-2.4(均值-5.0),对应的t_(2DM)为2.04~1.25 Ga(均值1.4).结合区域地质证据分析,稻城岩体为I型花岗岩,其源区以中元古代康定群变质玄武岩为主,酸性端元含较多变质杂砂岩.花岗岩的形成与后造山环境下俯冲板片断离导致的软流圈上涌、中下地壳受烘烤部分熔融关系密切.

The genesis and tectonic setting of granitoids in the northern part of Daocheng batholith in West Sichuan are still debated.In this paper,a compilation of petrology,zircon U-Pb chronology,geochemistry and Hf isotopes was conducted on the granitoid to solve this uncertainty.LA-ICP-MS zircon U-Pb dating result shows that granitoids in the northern part of the batholith were formed at 213-212 Ma,slightly later than those in the southern part.Granitoids researched are mainly composed of granodiorite and monzogranite,the former of which contains many MME(mafic microgranular enclaves).Accessory minerals contain apatite,epidote,zircon and opaque minerals.Enrichment in sodium,and large ion lithophile elements,metaluminous to weak peraluminous are typical geochemical characteristics of the representative granitic samples.Theirε_(Hf)(t)values are very scattered,ranging from-16.6 to 2.4(average-5.0),reflecting average crustal residence age(t_(2DM))of 2.04-1.25 Ga(average 1.4)of the granitic source.Combined with regional geological evidence,the granitoids are I-type garanites derived from partial melting of basement metabasalt from the Mesoproterozoic Kangding complex,part of metagreywacks maybe involved in the acid member.Upwelling of asthenosphere and partial melting of the middle and lower crusts triggered by slab detachment under postorogenic extension environment are responsible for the formation of granitoids.