松潘—甘孜造山带南缘二叠系变质玄武岩的成因与构造意义

摘要

A set of bedded metabasalts occurred in the Permian strata,in the southern margin of the Songpan-Garze orogenic belt.These rocks have apparent pillow and vesicular structures.In this paper,we performed geochemical analyses on these rocks and studied their petrogenesis and diagenetic tectonic settings.The metabasalts exhibit low SiO2 contents (47.56％-37.98％) and Nb/Zr ratios and relative high abundance of MgO (average 18.95％),Mg (Mg# average value 74.1) and Cr (average 1712× 10-6).These geochemical features indicate that the magma of their protolith was most likely derived from the depleted mantle and formed mainly by the partial melting (20％-25％) of garnet peridotite.Their MgO and Cr contents show a positive correlation with the extremely low La/Srn (2.86-1.02),(Th/Yb)pM (2.42-0.56),(Th/Ta)pM (1.21-0.43) and (La/Nb)PM (1.55-0.27) ratios,suggesting fractional crystallization of the Cr-bearing minerals and no contamination with crustal materials during the magma evolution processes.REE,trace element patterns and tectonic discriminations show that the chemical compositions of metabasalts are similar to those of E-MORB and N-MORB.In the field,the metabasalts are commonly sandwiched in the Permian marble,indicating a mature back-arc basin setting.Combined with the fact that some ultrabasic rocks were also developed in the Permian strata,we propose that the metabasalts are most likely a remnant of the Paleo-Tethys oceanic crust.When compared with the chemical compositions of Emeishan basalts,it is clear that the Permian metabasalts in the southern margin of the Songpan-Garze orogenic belt were not the products of the Late Permian Emeishan basaltic magma activities.%松潘-甘孜造山带南缘的二叠系之中发育一套顺层产出的变玄武岩,岩石具有明显的变余枕状构造及气孔构造.本文对其进行了主微量元素分析,并探讨其岩石成因与成岩构造背景.变玄武岩具有低的SiO2质量分数(47.56％～37.98％)与Nb/Zr值,其MgO(平均为18.95％)、Mg#值(平均为74.1)和Cr(平均为1 712×10-6)含量较高,表明原岩岩浆很可能起源于亏损地幔,主要由石榴石橄榄岩发生20％～25％的部分熔融形成.岩石MgO与Cr含量呈现良好的正相关,具有极低的La/Sm(2.86～1.02)、(Th/Yb)PM(2.42～0.56)、(Th/Ta)PM (1.21～0.43)和(La/Nb)pM (1.55～0.27)比值,说明岩浆演化过程中发生了含铬矿物的分离结晶,且没有遭受地壳物质的混染.变玄武岩稀土、微量配分型式及构造背景判别显示,岩石的化学成分与E-MORB及N-MORB相当.野外考察显示,与变玄武岩共生的岩石主要为二叠系大理岩,暗示变玄武岩原岩很可能形成于成熟的弧后盆地环境.结合二叠系还发育超基性岩的事实,本文认为该套变玄武岩可能是古特提斯洋的洋壳残余.另外成分对比显示,松潘-甘孜造山带南缘的二叠系变玄武岩并非峨眉山玄武岩浆活动的产物.