秦岭蟒岭高Sr花岗岩的锆石Lu-Hf同位素特征及其成因

蟒岭花岗岩体位于商丹构造带北侧的北秦岭构造带上,为一呈近东西走向的中生代花岗岩基。蟒岭花岗岩属高钾钙碱性—钾玄岩系列、过铝质I型花岗岩,其w(SiO2)=67.3%～73.7%,w(Al2O3)=14.0%～16.3%,w(Na2O)=3.17%～3.93%、w(K2O)=3.9%～6.3%,具高Sr、Ba、LREE、Sr/Y、La/Y,低HREE、Y、Mg#(50)、Rb/Sr,亏损Nb、Ta、Ti和P,无明显负Eu异常的特征,与中国东部的高Sr、Ba低HREE花岗岩的地球化学特征相似。锆石εHf(t)值为-9.4～-3.1,二阶段模式年龄(tDM2)集中于1.4～1.8 Ga,暗示蟒岭高Sr花岗岩的原岩主要为中新元古代地壳物质,并混入少量幔源物质;源区残留石榴石,而角闪石、斜长石为主要熔融相。蟒岭高Sr花岗岩形成于陆内造山阶段,由增厚的下地壳物质发生减压部分熔融形成,而底侵的镁铁质岩浆可能为部分熔融作用提供了热量。

Zircon Lu-Hf Isotopic Compositions and Petrogenesis of Strontium-Rich Granite from Mangling,Qinling Orogen Belt

Mangling granite,a Mosozoic granitic batholith with nearly east-west trending,is located in the north Qinling tectonic belt adjacent to the Shangdan tectonic belt in north.Mangling granites belong chemically to peraluminous and high-K calc-ackaline-shoshonitic series,similar to I-type granite.The contents of SiO2 are from 67.3% to 73.7%,Al2O3 from 14.0% to 16.3%,Na2O from 3.17% to 3.93%,and K2O from 3.9% to 6.3%.Additionally,they are enriched in Sr,Ba,LREE,and depleted in Nb,Ta,Ti,Y and P,and are high Sr/Y and La/Y ratios,and low HREE,Mg#(<50),Rb/Sr ratios as well as no significant negative Eu anomalies.The mangling granites are geochemically similar to high-Sr-Ba granitoids in Eastern China.The εHf(t) values of zircons from Mangling granite range from-9.4 to-3.1,and their tDM2 are from 1.4 to1.8 Ga,suggest that the protolith of strontium-rich granites from Mangling is mainly the Neoproterozoic crust with small amount of mantle-derived component.Garnet is a main residual phase in source,whereas hornblende and plagioclase are melted.Mangling strontium-rich granites formed in the intracontinental orogenic stage and were probably produced by partial melting of the thickened lower crust due to the underplating of mafic magmas.