雅鲁藏布江蛇绿岩带东段泽当蛇绿岩起源及演化

摘要

In this paper,based on a study of mineral chemistry of mantle peridotites as well as petrochemistry and geochemistry of mantle peridotitses,basalts,gabbros from Zedang ophiolite,combined with previous research of chronology and tectonic setting in study area,we discuss the origin and evolution of Zedang ophiolite.①Based on a resrarch of Fo values of olivine,En and Mg# values of clinopyroxene and orthopyroxene,contents of Al2 O3 of clinopyroxene and orthopyroxene,Mg# and Cr# values of spinel,we found that Zedang mantle peridotites originated from depleted mantle source in a MOR(mid-ocean ridge) environment,then was modified by later-stage melts and fluids in a SSZ (Suprasubduction Zone)setting, and that the molten degree of harzburgites is higher than lherzolites.② Based on a resrarch of petrochemistry and geochemistry of mantle peridotitses,we found the contents of most major elements and trace elements in mantle peridotites are lower than that in primitive mantle,the distribution patterns of trace-elements well coincide with abyssal peridotites,which proved again mantle peridotitses originated from depleted mantle source in a MOR environment.③We found that gabbros from Zedang ophiolite had the feature of N-MORB (Normal type Mid-Ocean Ridge Basalts)and originated from a source region which was much more depleted than N-MORB,the gabbros also had the feature of IAT(Island Arc Tholeiites) and probably ascribe it modified in a SSZ setting.④There are two types basalts in Zedang ophiolite, N-MORB and E-MORB(Enriched type Mid-Ocean Ridge Basalts),because the source region might have the feature of inhomogeneity,which include enriched magma.N-MORB type basalts also have the feature of IAT,indicating them modified in a SSZ setting later,whereas E-MORB type basalts not have the feature of IAT.⑤Combined with previous research of chronology and tectonic setting in study area, Zedang ophiolite originated from depleted and inhomogeneous mantle source in a MOR environment in c. 170Ma,then was modified in a fore-arc setting in c.150 Ma.%本文通过对泽当蛇绿岩中地幔橄榄岩主要矿物的矿物化学，以及蛇绿岩中地幔橄榄岩、玄武岩和辉长岩的岩石化学和地球化学的研究，结合前人对泽当蛇绿岩年代学和构造背景的认识，讨论泽当蛇绿岩的起源和演化。①通过对泽当蛇绿岩地幔橄榄岩中橄榄石的Fo值，斜方辉石和单斜辉石的En值、Mg＃值和Al2 O3含量，尖晶石的 Mg＃和C r＃值的讨论，发现泽当蛇绿岩地幔橄榄岩起源于洋中脊下亏损地幔域，方辉橄榄岩的熔融程度比二辉橄榄岩高，后期都受到俯冲带的改造。②通过对地幔橄榄岩岩石化学和地球化学分析，绝大多数的主量元素和微量元素的含量低于原始地幔，微量元素的原始地幔标准化曲线与深海橄榄岩曲线重合，说明它们是亏损的原始地幔熔融残留物，起源于洋中脊环境。③通过对泽当蛇绿岩辉长岩的岩石化学和地球化学研究发现辉长岩起源于比N-MORB更亏损的源区，具有N-MORB的性质，后期可能因为受到俯冲带的改造又具有了岛弧拉斑玄武岩的性质。④泽当蛇绿岩玄武岩包含N-MORB型和E-MORB型两类，说明泽当蛇绿岩起源于包含富集地幔的岩浆源区。N-MORB型玄武岩还具有岛弧拉斑玄武岩的性质，说明N-MORB型玄武岩后期受到俯冲带的改造，而 E-MORB型玄武岩没有受到俯冲带影响。⑤通过总结前人对泽当蛇绿岩年代学和构造背景的研究成果以及本文的发现，作者认为泽当蛇绿岩是170Ma左右起源于混合不均一的洋中脊下亏损地幔域，150Ma 左右在弧前环境受到俯冲带的改造。