东天山咸水泉片麻状花岗岩特征、年龄及成因

Characteristics,age and origin of the Xianshuiquan gneissose granite in eastern Tianshan.

野外地质关系及岩相学、地球化学、同位素和年代学研究表明,成水泉片麻状花岗岩是典型的原地改造型花岗岩,其原岩(即围岩)为糜棱岩化花岗闪长岩。本次锆石 LA-ICP-MS U-Pb 定年获得其原岩糜棱岩化花岗闪长岩^(206)Pb/^(238)U 加权平均年龄为369.9±5.6Ma(MSWD=36),片麻状花岗岩锆石核部年龄为367.9±5.4Ma(MSWD=2.2),此年龄与其原岩年龄在误差范围内一致,为片麻状花岗岩原地改造成因提供了年代学证据。此外,还获得其锆石热液增生边不一致线下交点年龄254±10Ma(MSWD=1.4),该年龄代表了片麻状花岗岩的形成时间。片糜状花岗岩在地球化学上继承了许多原岩的特征,同时又受到了深源流体-熔体的显著影响。将原岩糜棱岩化花岗闪长岩改造成片麻状花岗岩的流体-熔体具有富硅富钾和正ε_(Nd)(t)值的特征,表明这种流体-熔体可能主要地来自幔源岩浆内侵体,以及在内侵岩浆热量作用下的初生地壳岩石的变质和熔融。原岩糜棱岩化花岗闪长岩及其派生的片麻状花岗岩ε_(Nd)(t)值分别为+3.87及+3.05～+6.09,表明糜棱岩化花岗闪长岩源自于年轻的陆壳,而片麻状花岗岩除继承了其原岩糜棱岩化花岗闪长岩的高ε_(Nd)(t)值这一特征外,还可能受到了深源流体-熔体中幔源组分的影响。咸水泉岩体的实例研究表明,原地改造型片麻状花岗岩不但可以形成于造山带的挤压-伸展转折阶段,还可以形成于碰撞后进一步伸展的时期,而经过强烈韧性剪切的岩石是转变为片麻状花岗岩的最有利原岩。东天山咸水泉、平顶山和天湖东等原地改造型片麻状花岗岩的形成均与内侵有关,因而片麻状花岗岩可被看作是内侵的重要产物。

Abstract Studies in field relations, petrography, geochemistry, isotopes and geochronology indicate that the Xianshuiquan gneissose granite is typical of the in situ transformation-type granite. It was formed through transformation of the parent granodiorite by fluids and melts. A zircon LA-ICP-MS weighted average ^206Pb/^238U age of 369.9 ± 5.6Ma （ MSWD =36 ） was obtained by this research for the mylonitised granediorite that is the parent of the gneissose granite. The cores of zircon from the gneissose granite gave a LA-ICP- MS weighted average ^206Pb/^238U age of 367.9 ± 5.4Ma （MSWD = 2.2 ）. This age agrees well with the age of the parent mylonitised granodiorite within errors, and thus supports the interpretation that the gneissose granite was formed as a result of in situ transformation of the mylonitised granediorite by deep-sourced fluids-melts. In addition, we obtained a lower-interception age of 254 ± 10Ma （ MSWD = 1.4 ） for the hydrothermal overgrowth rims of zircon from the gneissose granite. This age can be explained as formation age of the gneissose granite. The gneissose granite has inherited various geochemical features from its parent rock, but has been remarkably affected by deep-sourced fluids-melts. The fluids-melts that transform the parent mylonitised granodiorite are interpreted to have high silica and potassium contents and positive БNd（t） value, and hence could have been derived partly from intraplated mantle magma, and partly by metamorphism and melting of juvenile crustal rocks in response to the heat from the intraplated body. The parent mylonitised granodiorite and the derivative gneissose granite have БNd（t） values of ＋3. 87 and ＋3.05～＋6.09, respectively, indicating that the mylonitised granodiorite might have originated from juvenile continental crust, and the gneissose granite, apart from its originally high БNd （t） value inherited from the parent granodiorite, might have been geochemically affected by mantle components from deep-sourced fluids-melts. Case study of the Xianshuiquan granite shows that gneissose granite of the in situ transformation-type can be formed not only at the stage of compression-extension conversion, but also during post-collisional extension in orogenic belts. Apart from the Xianshuiquan granite, other gneissose granite of the in situ transformation-type reported from Pingdingshan and Tianhudong have genetical affiliation to mantle magma intraplating. Hence, such a type of granite can be regarded as an important product of intraplating.