内蒙古巴林右旗新开坝地区大石寨组火山岩形成时代及构造背景

Age and tectonic setting of volcanic rocks of the Dashizhai Formation from Xinkaiba,Balinyouqi area,Inner Mongolia

对内蒙古巴林右旗新开坝地区大石寨组玄武安山岩进行了锆石U-Pb年代学、锆石原位Hf同位素分析及地球化学研究,定年结果表明玄武安山岩结晶年龄为280Ma,形成于早二叠世。地球化学特征显示高场强元素Nb、Ta强烈亏损,Ti轻微亏损,大离子亲石元素Sr、Ba、Rb、K富集,表现出岛弧火山岩特征。地球化学结果和Hf同位素的不同来源(亏损地幔及古老地壳的再循环),表明岩浆起源于俯冲沉积物熔体和流体交代地幔楔橄榄岩。与大石寨地区玄武安山岩进行了对比研究,表明巴林右旗大石寨组玄武安山岩在俯冲的构造背景,形成于岛弧-活动大陆边缘弧的环境,古亚洲洋板块由南向北进行俯冲,说明早二叠世古亚洲洋未闭合。

Zircon U-Pb chronological,geochemical and in situ Hf isotope research have been undertaken on the basaltic andesites of the Dashizhai Formation in Xinkaiba,Balinyouqi area,southeast part of the Central Asian Orogenic Belt in Inner Mongolia,China.The basaltic andesites,which yield age of - 280Ma（ the Early Permian）,display an enrichment in large ion lithophile elements（ Sr,Ba,Rb and K） and depletion in high field-strength elements（ Nb,Ta and Ti）,exhibited the transitionary geochemical features of island arc volcanics. The results of geochemistry and different sources of Hf isotopes（ depleted mantle and ancient crust）,indicate the magma originated from the subduction sediment melt and fluid metasomatism peridotite which from the mantle wedge. A comparative study on the basaltic andesites of the Dashizhai Formation,in Dashizhai area,indicates that basaltic andesites from Balinyouqi area are formed under the island arc-active continental margin environment in a subduction tectonic setting. It ＇s demonstrated that the ocean crust was subducting from south to north and Paleo-Asian Ocean wasn＇t closed in Early Permian.