大别山北麓尖晶石橄榄岩中石榴辉石岩包体的成因矿物学信息

大别山北坡霍山饶拔寨等地的超基性岩中含有石榴辉石岩的包体。石榴辉石岩为草绿色致密块状 ,呈分米级的块体出现于蛇纹石化强烈的橄榄岩中。运用成因矿物学的方法 ,研究对比了石榴辉石岩的主要矿物组成石榴子石 ( Prp2 5— 3 5 )和钠质普通辉石 ( Jd1 0— 2 5 )等。岩石结构显示退变质作用有两期 :榴辉岩相退变形成的麻粒岩相矿物组合明显地被角闪岩相所切割。石榴辉石岩的寄主岩是尖晶石橄榄岩类 ,包括尖晶石方辉橄榄岩和尖晶石二辉橄榄岩。由于强烈的蛇纹石化 ,残余的橄榄石 ( Fo92— 93 )仅占 5%～ 4 0 % ,斜方辉石富镁 ( En87— 93)并有解理弯曲等韧性变形现象。采用 Ellisand Green的石榴子石单斜辉石 Fe-Mg交换平衡温度计 ,可计算出石榴辉石岩的 Fe-Mg分配系数 ( KD)为 4 .0 6～ 5.2 8。变质温度 t=84 1～ 94 3℃ ,估算压力 p=1 .5GPa,可以推测该橄榄岩体是从深度约 60 km的地幔 ,固态侵位于下地壳 ,而后与之一起隆升到地表。显然 ,此种石榴辉石岩应属 Coleman所划分的 A型榴辉岩 ,它与地幔岩浆作用有密切关系。石榴辉石岩和橄榄岩的岩石化学特征和稀土配分形式 ,说明它们的化学性质相当于地幔部分熔融所形成的玄武岩熔体及其残留体。在侧重探讨石榴辉石岩及其有关岩石中主要造岩矿物的成因矿物?

ON THE GENETIC MINERALOGICAL INFORMATION FROM GARNET PYROXENITE ENCLAVES IN SPINEL PERIDOTITE, NORTH DABIESHAN, CHINA

Garnet pyroxenite enclaves are found in the ultramafic bodies around Raobazhai,Huoshan County,in the north of Dabieshan.The garnet pyroxenites are grass green massive rocks occurred as small pods decimeters in size,embedded in highly serpentinized peridotites.The genetic mineralogical approach on minerals of the garnet pyroxenite reveals:garnet(Prp 25 35 ),sodium augite(Jd 10 25 ) with small amounts of ilmenite.The rocks are apparently retrograded.There are two stages of retrogression: the retrogressive granulite facies are overprinted by retrogressive amphibolite facies.The host rocks of the garnet pyroxenite are spinel peridotites,including spinel harzburgite and lherzolite.Due to intensive serpentinization olivine relicts(Fo 92 93 ) are scarce,only 5%—40%.The orthopyroxenes are magnesium rich(En 87—93 ) and had undergone ductile deformation,showed as bending of cleavages and others.Using garnet clinopyroxene Fe Mg exchange equilibrium geothermometer(Ellis and Green,1979;Krogh,1988),the Fe Mg distribution coefficients ( K D=4 06—5 28 )and metamorphic temperatures ( t =841—943℃) are obtained,provided p =1 5 GPa.It is suggested that the peridotites were uplifted from some 60 kms in the depth of upper mantle into the lower crust.It is obvious that the garnet pyroxenites belong to the A type eclogite of Coleman(1965).They are closely related to the magmatism in the upper mantle.The petrochemical study and REE distribution pattern indicate that the chemical nature of the enclaves and the host are equivalent to those of basaltic partial melts and the ultramafic residua respectively .The present paper is concentrated at the genetic mineralogical characteristics of the major rock forming minerals from these rocks and the variety of the origin of eclogites from Dabieshan region.