天山0.9Ga新元古代花岗岩SHRIMP锆石U-Pb年龄及其构造意义

Formation of the 0.9 Ga Neoproterozoic granitoids in the Tianshan Orogen,NW China:Constraints from the SHRIMP zircon age determination and its tectonic significance

天山造山带的中天山隆起带中广泛出露着新元古代花岗岩类岩石。获得了东天山星星峡和西天山温泉地区片麻状花岗岩中锆石的SHRIMPU-Pb年龄,分别为(942±7)Ma(2σ)和(919±6)Ma(2σ)。这些花岗岩以特有的粗粒、巨大的眼球状片麻结构为特征。主元素数据表明它们属于过铝质的二长花岗岩,具有高SiO2(≥70%)、高K2O+Na2O(7%～≥8%),且K2O>Na2O。这些新元古代花岗岩类岩石具不同的REE含量,但它们有近于平行的REE分布曲线和明显的Eu负异常,同时代的星星峡混合岩具有较高的REE含量,明显富集LREE。它们的微量元素蛛网图几乎完全相同,均有明显的Nb、Sr、Ti、P负异常。基于Nd同位素组成研究,其εNd(t)在-4至～0之间,Nd模式年龄tDM为2.0～1.6Ga。由于放射成因Sr同位素组成非常高,不可能得到精确有意义的Sr同位素初始值。上述特征均表明这些新元古代花岗岩形成于大陆边缘构造环境,由古元古代地壳重熔并经历了高度的结晶分异作用而形成。综合近年来发表的关于天山各地段,诸如温泉、赛里木湖以东、拉尔敦达坂、巴仑台和星星峡等地区较为可靠的锆石U-Pb年龄数据,我们认为天山新元古代花岗岩类岩石主要形成于960～910Ma。结合塔里木盆地周边古老地块年龄的研究结果,可以推断在早新元古代时期塔里木周边和天山的古老地块可能曾组成Rodinia超大陆的一部分。

Neoproterozoic granitoids occur widely in the central uplift zone of the Tianshan Orogen. Two SHRIMP zircon U-Pb ages of （942±7） and （919 ± 6） Ma （2σ） have been obtained for granitic gneisses of the Xingxingxia area in East Tianshan and the Wenquan area in West Tianshan, respectively. These granitoids are characterized by a very coarse-grained and huge augen gneiss texture. Major element data indicate that they belong to peraluminous monzogranite, with SiO2 （≥70% ）, K2O ＋ Na2O （7% to ≥ 8% ）, and K2O 〉 Na2O. These rocks show nearly parallel REE patterns with different abundances and distinct negative Eu anomalies. Contemporaneous migmatites from Xingxingxia have high REE abundances and show more pronounced LREE enrichment. All the granitoids from West Tianshan and East Tianshan disply similar spidergrams with obvious negative anomalies in Ba, Nb, Sr, P, and Ti. In temls of Nd isotopic compositions, these rocks have end（t） from -4 to near zero, and tDM ages from 2.0 to 1.6 Ga. Due to the highly radiogenic nature of the Sr isotopic compositions, precise or meaningful initial ratios cannot be obtained. Based on the chemical characteristics and Nd isotopic compositions, the Neoproterozoic granitoids were likely generated in a continental margin tectonic setting and by remelting of Paleoproterozoic rocks, followed by advanced fractional crystallization. With consideration of the available zircon U-Pb ages from several areas （Wenquan, eastern Sayram Lake, Laerdundaban, Balguntay and Xingxingxia） in the Tianshan, we conclude that the formation of the Neoproterozoic granitoids in the Tianshan Orogen took place mainly in the period of 960 to 910 Ma. Furthermore, with the age information on the ancient terranes of Tarim basin, we suppose that the Tarim-Tianshan ancient blocks probably formed a part of Rodina during early Neoproterozoic period.