西藏班公湖存在MOR型和SSZ型蛇绿岩——来自两种不同地幔橄榄岩的证据  

Recognition of MOR- and SSZ-type ophiolites in the Bangong Lake ophiolite mélange, western Tibet: evidence from two kinds of mantle peridotites

作　　者：史仁灯[1] 杨经绥[1] 许志琴[1] 戚学祥[1] 

机构地区：[1]中国地质科学院地质研究所

出　　处：《岩石矿物学杂志》2005年第5期397-408,共12页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金重大国际合作项目(49732070);国土资源部国际合作与科技司十五重点项目(20010101)

摘　　要：在班公湖-怒江蛇绿岩带的西端日土出露两种不同的地幔橄榄岩:①角砾状方辉橄榄岩,由80%橄榄石(Fo=90.76～91.84,平均91.09)、15%斜方辉石(Mg#=90.97～91.41,平均91.16)、2%单斜辉石(Mg#=93.24～94.60,平均93.96)、3%棕色尖晶石(Cr#=0.20～0.25,平均0.23＜0.60)和磁铁矿组成,以低MgO(41.41%～42.02%)、高Al2O3(1.63%～1.94%)、CaO(1.34%～1.60%)和Ti(133.04～134.52μg/g)为特征,亏损REE,ΣREE为球粒陨石的17%～22%,估算其为原始地幔岩经过10%～15%部分熔融的残留物;②块状方辉橄榄岩,由85%橄榄石、13%斜方辉石、2%红褐色尖晶石(Cr#=0.69～0.74,平均0.71＞0.60)和磁铁矿组成,不含单斜辉石,相对于角砾状方辉橄榄岩,高MgO(42.96%～44.69%),低Al2O3(0.23%～0.61%)、CaO(0.08%～0.11%)和Ti(68.55～68.82μg/g),强烈亏损REE,ΣREE仅为球粒陨石的3%～5%,估算其为原始地幔橄榄岩经过30%～40%部分熔融的残留物.初步研究认为角砾状方辉橄榄岩是古大洋岩石圈在板块汇聚过程中构造侵位于缝合带中的残留物,是MOR型蛇绿岩的地幔橄榄岩;块状方辉橄榄岩是古大洋岩石圈在俯冲消减过程中再度发生熔融的残留物,是SSZ型蛇绿岩的地幔橄榄岩,这与本区发育MOR型蛇绿岩熔岩-洋中脊拉斑玄武岩(P-MORB)和SSZ型蛇绿岩熔岩-玻安岩(Boninite)是一致的.Two kinds of mantle peridotites are recognized in Rutog at the westernmost part of the Bangong Lake - Nujiang ophiolite mélange in Tibet. One is a brecciform harzburgite, comtmsed of about 80% olivine with Fo 90.76-91.84, 91.09 on average, 15% orthopyroxene with Mg^# 90.97-91.41, 91.16 on average, 2% clinopyroxene with Mg^# 93.24-94.60, 93.96 on average, and 3% magnetite and light brown chromium spinel with Cr^# 0. 20 - 0. 25 , 0.23 on average （lower than 0.60）, and characterized by low MgO （41.41% -42.02% ）, high Al2O3（ 1.63% - 1.94% ）, CaO （ 1.34% - 1.60 % ） and Ti （ 133.04- 134.52μg/g）, and depletion of REE with ∑REE being 0.17- 0.22 times that of chondrite. Major and trace element characteristics indicate that the brecciform harzburgite is restite resulting from 10 % - 15 % partial melting of primitive mantle. The other type is massive harzburgite without clinopyroxene, composed of about 85% olivine, 13% orthopyroxene, and 2% magnetite and red-brown chromium spinel with Cr~： 0.69-0.74, 0.71 on average （higher than 0.60）. In contrast to the brecciform harzburgite, the massive harzburgite is characterized by high MgO （42.96% -44.69% ）, low Al2O3 （0.23% -0.61% ）, CaO （0.08% -0.11% ） and Ti （68.55 -68.82μg/g）, and considerable depletion of REE, with ∑REE being 0.03-0.05 times that of chondrite. Major and trace element characteristics suggest that the nmssive harzburgke is restite formed by 30% -40% partial melting of the primitive mantle. It is preliminarily held that the brecciform harzburgite is restite of ancient oceanic lithosphere emplacement in the orogeny zone, and that the massive harzburgite is restite formed by another partial melting of residual peridotites during the subduction of ancient oceanic lithosphere. They are typical mantle peridotites of MOR- and SSZ-type ophiolite, respectively, and consistent with the two types of lavas in Bangong Lake ophiolite mélange, i.e., P-MORB and Boninite.

关 键 词：方辉橄榄岩 MOR型蛇绿岩 SSZ型蛇绿岩 班公湖 西藏 

分 类 号：P588.125[天文地球—岩石学]