伊犁北部博罗霍努岩体年代学和地球化学研究及其大地构造意义

Geochronological and geochemical studies on the Borohoro plutons,north of Yili,NW Tianshan and their Tectonic implication.

博罗霍努岩体是发育在新疆伊犁北部的一个大型海西期花岗岩体,总体沿近 SE—NW 向分布,出露面积逾2000km^2。该岩体主要包括三类花岗岩:灰黑色辉石闪长岩、浅色黑云母花岗岩和紫红色黑云母钾长花岗岩。锆石 U—Pb La-ICP-MS 定年表明,辉石闪长岩的年龄为301±7Ma,黑云母花岗岩的年龄范围为294±7～285±7Ma,而黑云母钾长花岗岩则形成于280±5～266±6Ma。岩石地球化学分析显示,黑云母花岗岩和钾长花岗岩以准铝或弱过铝Ⅰ型花岗岩为主,个别属于弱过铝 S 型花岗岩。在微量元素方面,这些花岗岩均富集轻稀土而亏损重稀土,但来自两个剖面的花岗岩具有不同的稀土元素配分模式,可能代表它们的岩浆源区有所不同,因此需要进一步对这些花岗岩进行同位素地质学研究。相对于洋脊花岗岩而言,博罗霍努岩体的花岗岩明显富集 K,Rb,Ba 和 Th,同时,显著亏损 Nb,Ta,Y 和 Yh。以上地球化学特征及微量元素判别图表明,这些花岗岩类形成于俯冲有关的火山岛弧环境。结合伊犁及邻区岩浆岩的特征及其时代,可以认为博罗霍努岩体的形成与天山北部洋壳向南的俯冲造山作用有关。西天山北部俯冲造山作用最终在中二叠世结束,并在中一晚二叠世进入陆内造山和伸展拉张阶段。

Abstract Hercynian Borohoro plutons extending SE-NW along the northern margin of Yili Block develop in a wide area of more than 2000km^2. It is predominantly composed of dark gabbroid diorite, light-coloured biotite granite and pink biotite K-granite. U-Pb La-ICP-MS dating on zircons yields 301±7 Ma for gabbroid diorite, 294±7Ma to 285±7 Ma for light-coloured biotite granite and 280± 5Ma to 266±6Ma for biotite K-granite, respectively. Geochemical analyses suggest that the Borohoro plutons are mainly subaluminous I-type granite with minor aluminous S-type granite. They have higher components of LREE than HREE, and dissimilar REE distribution patterns of granitoids from two sections indicate that they were probably derived from different magma sources, further isotopic study is in urgent need. These granites are rich in K, Rb, Th and Ba, and depleted in Nb, Ta, Zr and Hf with respect to Ocean Ridge Granite. Trace element discriminations suggest that these granitic rocks formed in a subduction-related volcanic arc setting. Comparing to the previous results, this study proposes that the Borohoro plutons were resulted from south-directed subductionary orogeny of an oceanic basin in North Tianshan, the convergent process was completed in Middle Permian, and was followed by intra-continental orogeny and subsequent extension in Late Permian.