新疆温泉地区晚石炭世古老地壳重熔:花岗斑岩脉侵位年龄及其Sr、Nd同位素证据

Ancient Crust Remelting in Wenquan Area,Xinjiang:Evidence from the Age and Sr-Nd Isotopes of the Late Carboniferous Granite Porphyry Dykes

新疆阿拉套山南缘分布有由众多岩性复杂的脉岩所组成的岩墙群,部分岩墙与成矿作用密切相关,是了解该地区地壳生长过程的重要窗口。其西段出露大量中酸性脉岩,脉岩侵入到上泥盆统托斯库尔他乌组地层,以闪长岩脉为主,含少量花岗斑岩脉和霏细岩脉。为了探讨花岗斑岩的形成时代、岩浆源区和构造背景,对其进行了锆石U-Pb年龄测定和Sr、Nd同位素研究。分析结果显示,花岗斑岩锆石U-Pb(加权平均)年龄为310 Ma,说明花岗斑岩脉形成于晚石炭世早期,该年龄与北天山沙湾流纹岩(形成于陆板内拉张环境)的喷发时代(310 Ma)以及区内察哈乌苏岩基的形成时代(313 Ma)十分接近,是阿拉套山西段最晚一期岩浆活动的产物。Sr、Nd同位素分析结果表明,该岩脉具有中等I_(Sr)值(0.709328~0.710018)、负的ε_(Nd)(t)值(-3.92~-2.33)和较大的Nd同位素模式年龄(1294~1502 Ma)特征,明显不同于同时期西天山乃至新疆北部众多的花岗岩类,后者普遍拥有低I_(Sr)值、正的高ε_(Nd)(t)值和较年轻的Nd同位素模式年龄,是幔源玄武质岩浆底侵、发生岩浆的同化分离结晶作用或岩浆混合作用的结果,说明该花岗斑岩脉不太可能是幔源岩浆底侵演化的产物,其岩浆源区可能来自成熟度比较高的前寒武纪基底岩石。310 Ma以后,阿拉套山地区逐渐进入后碰撞伸展阶段,处于陆壳垂向生长、区域构造应力场显示拉张的构造环境,花岗斑岩脉可能形成于同碰撞挤压向后碰撞伸展背景转换的过渡阶段。

Dyke swarms with complex lithology are distributed on the southern margin of the Alataw Mountains(some closely ore-related),providing a good window to understand the crustal growth process.Numerous intermediate-felsic dykes are exposed in the western section,which intruded the Upper Devonian Tuskuertau Formation.These dykes are dominated by diorite with minor granite porphyry and felsite.We carried out zircon U-Pb age and Sr-Nd dating on the granite porphyry dyke to reveal its formation age,magma source and tectonic setting.The zircon U-Pb dating yielded weighted average age of 310 Ma,indicating that the granite porphyry dykes were formed in the early Late Carboniferous.This age is very close to the eruption age(310 Ma)of the Shawan intraplate rhyolite formed under an extensional environment and the formation age of the Chahawusu batholith(313 Ma),which is the product of the latest magmatism in the western Alataw Mountains.The Sr-Nd isotopic compositions indicate that the dykes are featured by medium I_(Sr)(0.709,328-0.710,018)and negativeε_(Nd)(t)(-3.92 to-2.33)values and older Nd model age(1,294-1,502 Ma),which are obviously different from those of coeval granitoids in the western Tianshan and northern Xinjiang.The latter has generally low I_(Sr) and highly positiveε_(Nd)(t)values and younger Nd isotope model ages,and were likely derived from the mantle,and underwent magma underplating and ACF or magma mixing processes.After 310 Ma,the Alataw Mountains gradually entered a post-collision extension stage,which was in a vertical growth of continental crust.We conclude that the granite porphyry dykes were unlikely to be formed by mantle-derived magma underplating and evolution,and the magma source is the Precambrian basement rock with relatively high maturity.The dykes may have formed in a post-collision extensional setting.