西天山前寒武纪天窗花岗片麻岩的锆石U-Pb年龄及Nd-Sr同位素特征

Zircon U-Pb age and Nd-Sr isotopic composition of granitic gneiss and its geological implications from Precambrian window of western Tianshan, NW China

采用颗粒级锆石U－Pb定年方法，精确测定了西天山前寒武纪天窗花岗片麻岩的结晶锆石年龄为798Ma，它代表了花岗质岩浆的结晶时代。花岗片麻岩具有较高的IAS。值（1.17～1．55）和LILE、LREE含量，明显的Eu负异常（δEu＝024～0.38）和Ba、Sr、P、Ti、Nb、Ta的亏损，表明源岩为成熟度很高的陆壳物质。其负的εNd（t）值（-4.4～-6.4）和较高的初始87Sr／86Sr比值（0．7196～0．7207），均表明熔融母岩具有很长的地壳存留时间，1．93～2．01Ga的Nd模式年龄值可能代表了源岩的平均地壳存留年龄。

Granitic gneiss at Precambrian window in western Tianshan has been dated by the U-Pbzircon method. The results give a concordant age of 798 Ma, and it was interpreted as thecrystallization age of the protolith. Granitic gneisses have high IAS value (1.17～1. 55), highLILE and LREE contents, significantly negative Eu depletion (δEu = 0. 24～0. 38), and dis-tinctly negative Ba, Sr, P, Ti and Nb anomalies, indicating a S-type parentage, which isconsistent with high initial 87Sr/86Sr ratios value (0. 7196 ～0. 7207 ) and very negative εNd(t)value (- 4. 4～- 6. 4). The protolith magma is intemreted as a product of partial melting ofthe basement rocks, and the Nd model age probably represents mean crust-residence age ofsource rocks.