内蒙古锡林浩特毛登牧场早石炭世花岗岩锆石U Pb年龄、地球化学特征及其地质意义

通过对内蒙古锡林浩特毛登牧场早石炭世花岗岩体进行野外观察、LA ICP MS锆石U Pb测年以及地球化学测试，讨论其构造环境，进一步为研究古亚洲洋闭合时限提供依据。测年结果表明：花岗闪长岩为(3306±18) Ma，二长花岗岩为(3277±26) Ma，成岩时代为早石炭世。岩石地球化学分析表明:花岗闪长岩为强过铝质、钙碱性系列岩石，具有活动大陆边缘的亲缘性特征。微量元素特征指示花岗闪长岩具有典型下地壳来源特征并伴有部分幔源岩浆混合作用，为弧岩浆岩。二长花岗岩为具高硅、富碱、相对低铝特征的高钾钙碱性系列岩石。两类差异明显的岩石稀土配分曲线表明二长花岗岩具有下地壳岩浆重熔的演化特征。微量元素特征指示样品为大陆弧环境下壳源重熔的成熟弧花岗岩。构造判别图显示花岗闪长岩为代表活动大陆边缘环境的I型花岗岩，而二长花岗岩则为指示活动大陆边缘弧后伸展环境的A2型花岗岩，二者构成I－A型复合岩体，说明研究区在早石炭世仍存在古亚洲洋向西伯利亚板块的俯冲作用，推测古亚洲洋此时尚未闭合。

Zircon U-Pb Dating and Geochemistry of Granites in Early Carboniferous in Maodeng of Xilin Hot,Inner Mongolia and Their Geological Implications

In this paper, a series of analyses including field observation, LA ICP MS zircon U Pb isotopic dating, major element and trace element were performed on the granites in Early Carboniferous in Maodeng of Xilin Hot, Inner Mongolia in order to make the discussion of tectonic environment of the study area. Two types of granites were identified which of granodiorite and monzonite granite are invaded into the former. The result of the LA ICP MS zircon U Pb isotopic dating are (3306±18) Ma for granodiorite and (3277±26) Ma for monzonite granite, which suggests that the granites formed in the Early Carboniferous. The geochemical feature of major elements in the granodiorite is strongly peraluminous (A/CNK=132)and of calcium alkali. The pattern of LREE enrichment type indicates the active continental margin environment with the feature of low ∑REE(6765×10-6), (La/Yb)N (837) and slight negative anomaly. The trace element geochemistry shows evident enrichment of LILE(Rb, Ba, Sr), heat producing element(Th, U, K) and HFSE(Zr, Hf) with strong deficit of Nb, P and Ti, which imply that the samples are arc magmatic suites coming from the lower crust with few mantle derived magma. The geochemical feature of major elements in the monzonite granites is of high SiO2(7585%) and K2O+Na2O(854%), low Al2O3 relatively, which belongs to weak peraluminous(A/CNK=101) series and high K calc alkali series. The feature of REE are divided into two types. One type is of high ∑REE(12028×10-6) with negative Eu anomalies(088), LREE/HREE (496), (La/Yb)N (1009), which shows the strong differentiation between the HREE and LREE. The patterns of the other type granites show the slightly differentiation like the “sea gull” with low ∑REE(5428×10-6), strong Eu negative anormalies (027), LREE/HREE (199), (La/Yb)N (150), which comes from the remelting of the low crust.The trace element geochemistry shows evident enrichment of LILE(Rb), heat producing element(Th, U) and HFSE(Zr,Hf) with strong deficit of Sr,Ba,P,Ti,which implies that the sample are mature arc granite coming from the remelting of the low crust magma in the continental marginal environment. Discrimination diagrams display that the granodiorites are I type representing active continental margin environment, but the monzonitic granites are A2 type representing the stretch environment behind the arc of the active continental margin which form the I A complex. These geochemical granites of different tectonic environments display that the subduction from Paleo Asian Ocean to the Siberian plate still existed during the Early Carboniferous in Maodeng of Xilin Hot, Inner Mongolia, which indicates that the Paleo Asian Ocean didnt close in Early Carboniferous.