伊犁地块北缘早古生代构造属性:来自温泉地区闪长岩的证据

Early Paleozoic Tectonic Evolution of Northern Yili Block:Evidence from Diorite in Wenquan,Xinjiang

温泉地区位于伊犁地块与哈萨克斯坦板块结合部位,广泛发育早古生代侵入岩,侵入岩带对探究伊犁地块北缘早古生代构造背景及洋陆演化具有重要意义。在野外地质填图和岩相学观察基础上,对温泉地区奥陶纪闪长岩开展了系统的锆石U-Pb年代学、Hf同位素和地球化学研究。年代学表明,石英闪长岩3个样品锆石U-Pb加权平均年龄分别为(467.7±5.4)、(457.1±3.9)和(455.4±5.0)Ma,表明其结晶时间从中奥陶世一直持续到晚奥陶世早期。地球化学成分显示,闪长岩SiO2质量分数(50.19%~61.91%,平均56.57%)、TiO2质量分数(0.84%~1.13%,平均1.01%)较低,MgO质量分数(2.36%~8.17%,平均4.60%)、CaO质量分数(3.94%~9.45%,平均6.50%)和全碱质量分数(3.74%~7.06%,平均5.41%)相对较高,并具有高的Na2O/K2O值(1.42~4.90,平均2.76),以及低的A/CNK值(0.70~0.95,平均0.84),为准铝质岩石。微量元素地球化学显示闪长岩富集大离子亲石元素(K、Rb等),亏损高场强元素(Ta、Nb、Ti等);球粒陨石标准化稀土元素分布模式表现为'左陡右缓'的变化趋势,Eu显示弱的负异常(δEu=0.67~1.12,平均值0.90),重稀土元素质量分数较低,具有大陆边缘弧火成岩地球化学特征。Harker二元图解表明,岩浆演化过程中发生了分离结晶作用,存在于岩体中的地层捕虏体显示其经历了一定的地壳混染。两件石英闪长岩样品具有相近的Hf同位素组成:εHf(t)值为3.9~7.0,两阶段模式年龄(TDM2)为991~1191Ma(平均1 083Ma),远远大于锆石U-Pb年龄。以上特征表明亏损地幔在闪长岩源区占主导地位,并有早期大洋板片流体交代的富集岩石圈地幔组分的混入。综合地质资料和前人研究成果,推测区内闪长岩形成时的动力可能与准噶尔洋奥陶纪持续向南俯冲有关,洋盆最终于晚奥陶世—早志留世闭合。

Located at the junction of Yili block and Siberia plate,the northern Yili block develops a belt of Early Paleozoic intrusive rocks,which are the key to study the northern Yili block tectonic setting and evolution of the old Junggar Ocean.On the basis of a field geological survey and petrographic observation,we present the whole-rock data,U-Pb ages and Hf isotopic data for the Early Paleozoic diorite from Wenquan of northern edge of Yili block.The weighted mean 206 Pb/238 U ages of the zircons from quartz diorite are(467.7±5.4)Ma,(457.1±3.9)Ma and(455.4±5.0)Ma,indicating that these diorite bodies crystallized from the Early Ordovician to the Middle Ordovician.The petrological and chemical composition show that these diorites are characterized by low SiO2(50.19%-61.91%,average56.57%)and TiO2(0.84%-1.13%,average 1.01%),slightly higher MgO(2.36%-8.17%,average4.60%),CaO(3.94%-9.45%,average 6.50%),total alkali(Na2 O+K2 O)(3.74%-7.06%,average5.41%).The ratio of Na2 O/K2 O is high(1.42-4.90,average 2.76),while the ratio of A/CNKis low(0.70-0.95,average 0.84).The geochemical data suggest that these diorite bodies belong to quasialuminous series.The diorites are enriched in large ion lithophile elements(K,Rb),but depleted of high field strength elements(Ta,Nb,Ti).The REE pattern shows a trend of'left high and right low'with minor negative Eu anomalies(0.67-1.12,average 0.90),and heavy REE content is low,indicating the geochemical characteristics of arc igneous rocks in continental margin.Various binary diagrams reveal fractional crystallization of olivine,clinopyroxene,and hornblende.The existence of stratigraphic xenoliths and the La/Sm value(2.99-8.59,average 5.95)suggest the limited crustal contamination during the magma rise.The zircon Hf isotopic compositions of the quartz diorite are mainly characterized byεHf(t)values of 3.9-7.0,two-stage model ages of 991-1 191 Ma(average 1 083 Ma),which is older than U-Pb age.The petrological,geochemical data and zircon Hf isotopic compositions imply that the primary magma derived from a depleted mantle probably mixed with metasomatic enriched lithospheric mantle components and the early fluid metasomatism of oceanic plate.Based on the regional magmatism and the previous study,the authors infer that the old Junggar Ocean crust subducted southward beneath the northern edge of Yili block during 467.7 Ma-455.4 Ma,and ended in the Late Ordovician-Early Silurian.