新疆东准噶尔地区奥陶纪斜长花岗岩的发现

Discovery of Ordovician Plagiogranite from East Junggar,Xinjiang

通过1:5万区域地质调查,在东准噶尔地区老君庙东北部发现了一套侵入前早奥陶世变质岩的斜长花岗岩。锆石SHRIMP U-Pb测年结果显示^(206)Pb/^(238)U表面年龄加权平均值为(468.1±5.5)Ma,代表斜长花岗岩的结晶年龄,这是首次报道东准噶尔地区的奥陶纪斜长花岗岩。斜长花岗岩SiO_2含量70.09%、高Na_2O(6.12%)、低K_2O(1%)、Mg含量较高(1.82%),标准矿物中刚玉分子含量达到1.96。稀土总量较低(72.97×10^(-6)),Eu负异常不明显,轻重稀土分馏明显[(La/Yb)_N=16.05],具低Sr、低Y、Yb,较高的K、Rb、Ba、Th,Nb、Ta负异常明显,正Ce异常。反映奥陶纪斜长花岗岩为幔源岩浆成因,并受到了地壳的混染,形成环境可能为大洋环境。奥陶纪斜长花岗岩的发现,表明东准噶尔地区奥陶纪时期存在一次岩浆热事件。

A set of Ordovician plagiogranites, of which the SHRIMP U- Pb zircon dating showed that the crystallization age was 468.1±5.5 Ma, was found intruding the Pre - paleoordovician metamorphic rocks in the northeastern Laojunmiao of East Junggar. The Ordovician plagiogranite age was first reported in the East Junggar. The content of SiO2 is 70.09%; MgO （1.82%） and Na20 （6.12 %） content are high while K2O （1%）content is low. The content of standard mineral corundum molecules reached 1.96. The total REE is low（72.97 × 10-6）, and negative Eu anomaly is not obvious, while the fractionation between LREE and HREE （（La/Yb）N = 16.05） is evi- dent. There is lower St, Y, and Yb content, but higher K, Rh, Ba, Th content; there are also Nb and Ta negative anomalies and Ce positive anomaly, reflecting that the magma of plagiogran- ites was mantle - derived probably formed in ocean environment, and contaminated by the crust. The discovery of the Ordovician granites confirmed the existence of an extensive magma - meta- morphic thermal event in the east Junggar through Ordovician period.