大兴安岭晚中生代火山岩成因与古老地块物质贡献:锆石U-Pb年龄及多元同位素制约

The Petrogenesis of Late Mesozoic Volcanic Rock and the Contributions from Ancient Micro-Continents:Constraints from the Zircon U-Pb Dating and Sr-Nd-Pb-Hf Isotopic Systematics

系统的元素及多元同位素地球化学研究表明,以西乌珠穆沁旗地区和塔河地区为代表的部分大兴安岭晚中生代火山岩地幔源区组分中存在明确的古老地块物质贡献.与大兴安岭其他地区晚中生代火山岩源区组分主要为新生地壳物质特征相比,塔河火山岩中的碎屑锆石记录了晚元古代和早古生代年龄;配套的微量元素和多元同位素特征则清晰地指示了上述两个地区火山岩的源区组分是在继承古老地块富集地幔的基础上叠加了古生代岛弧等新生地壳物质.上述研究首次为兴蒙造山带内存在前寒武古老地体以及该造山带组成特征提供了明确和系统的深部地球化学制约.晚中生代全球深部事件以及蒙古-鄂霍茨克洋闭合后的伸展作用,触发岩石圈地幔部分熔融,是本区晚中生代强烈火山活动成因可能的地球动力学背景.

Comprehensive geochemical investigations on the Late Mesozoic volcanic rocks from Tahe and Xiwu areas, Great Xing＇an range reveal that the mantle sources for these rocks are of attributes of old continent blocks, while the mantle sources for the coeval volcanic rocks from other area show juvenile features. The detrital zircons from Tahe volcanic rocks have records of Late Proterozoic and Early Paleozoic. Trace elements and Sr, Nd, Pb and Hf isotopic features imply that the mantle sources for these two volcanic fields were inherited from enriched lithospheric mantle beneath old blocks with overprint of subducted juvenile island arc materials. This study provides solid deep geochemical constraints on the existence of Precambrian blocks among the Paleozoic Xingmeng orogen. During the Late Mesozoic, with the closure of the Mongol-Okhotsk and Mesozoic global event, regional extensional geological setting developed in these regions, which trigged decompressional melting of the lithos- pheric mantle and caused the voluminous eruption of volcanic rocks.