内蒙古林西早中生代O型高镁埃达克质安山岩的发现及其意义

对内蒙古林西地区火山岩样品的地球化学研究表明,其具有大洋型(O型)高镁埃达克质安山岩的地球化学特征w(SiO2)=57.89%～63.97%、w(Al2O3)=15.39%～16.17%、w(Na2O)/w(K2O)=1.5～2.2(除一个为0.9)、A/CNK=0.83～0.97、Mg#=49～64、w(Cr)=91～209μg/g、w(Ni)=52.2～111.4μg/g、w(Sr)=551～1018μg/g、w(Yb)=1.18～1.71μg/g、w(Y)=12.6～17.7μg/g、δ(Eu)=0.86～0.94]。利用LA-ICP-MS对其锆石进行了U-Pb定年,得到206Pb/238U加权平均年龄为(244±2)Ma(MSWD=0.36)。此时,区域内古亚洲洋已闭合,综合地球化学特征和区域地质环境说明,林西高镁埃达克质安山岩是由残余洋壳部分熔融并随后与地幔橄榄岩相互作用形成的。内蒙古林西早中生代高镁埃达克质安山岩的产出表明,洋壳来源的埃达克岩(O型埃达克岩)不仅可以随俯冲同时产生,而且也可以在非俯冲环境下由保存在古俯冲带地幔中的残余洋壳部分熔融产生。

Early Mesozoic O-type High-Mg Adakitic Andesites from Linxi Area, Inner Mongolia and Its Implication

This paper studies geochemical compositions of volcanic samples from Linxi area, Inner Mongolia. The results show that these samples have similar geochemical characteristics of ocean-type (O-type) high-Mg adakitic andesitesSiO~2 = 57.89%～63.97%,A1_2O_3= 15.39%～16.17%,Na_2O/K_2O=l.5～2.2 (except one of 0.9), A/CNK =0.83～0.97, Mg~#=49～64、Cr=91～209 μg/g, Ni = 52.2～111.4 μg/g, Sr=551～1 018 μg/g, Yb=l.18～1.71 μg/g, Y=12.6～17.7 μg/g, and δ(Eu)=0.86～0.94]. LA-ICP-MS zircon U-Pb dating results for the Linxi high-Mg adakitic andesites yield a weighted mean ~(206)Pb/~(238)U age of (244±2)Ma (MSWD=0.36), when the Paleo-Asian ocean was closed. The combination of geolog-ical setting and geochemical characteristics of the Linxi high-Mg adakitic andesites suggests that the Linxi high-Mg adakitic rocks were most probably derived from partial melting of the remnant of Paleo-Asian oce-anic crust and subsequently hybridized by mantle peridotite. Generation of the Linxi early Mesozoic high-Mg adakitic andesites indicates that Otype high-Mg adakitic rocks could occur in a variety of tectonic set-ting, where oceanic lithosphere subduction had occurred and subsequent p-T conditions permit melting of the remnants of the oceanic crust.