阿尔卑斯超镁铁岩的演化及上地幔的局部熔融

THE EVOLUTION OF ALPINE ULTRAMAFIC ROCKS AND PARTIAL MELTING OF THE UPPER MANTLE

西藏新疆超镁铁岩是上地幔局部熔融的产物。超镁铁岩在矿物成分上的演化表现为两个平行的演化系列:一是随各亚类岩石的递变,硅酸盐矿物的100Mg,(Mg+Fe^(2+))值递增,即硅酸盐矿物向富镁方向演化;二是副矿物铬尖晶石的100Cr/(Cr+Al)值相应递增,向富铬方向演化;上述演化趋势与岩浆分异演化趋势相反。在组构上则由原粒结构向熔融残余、后成合晶,碎斑结构,然后递变为等粒镶嵌;板状镶嵌结构。二辉橄榄岩的熔融实验资料清楚表明、阿尔卑斯超镁铁岩的演化乃是地幔岩熔融的结果。各亚类岩石代表了不同程度的局部熔融,岩体所显示的上部偏基性,下部偏酸性的垂直分带实属地幔岩的熔融分带。

The equilibrium temperatures and pressures of Alpine ultramafic rocks (except for the ultramafic cumulates of ophiolite) separately coorespond to those of spinel lherzolite and garnet lherzolite of the pyrolite model of Ringwood. On the basis of the evolutions of minerals and fabrics of the ultramafic rocks in Tibet and Xinjiang, the authors hold the opinion that the Aipine ultramafic rocks are products formed by partial melting of the upper mantle.The evolution of the mineral compositions of Alpine ultramafic rocks is characterized by two paralled evolutionary series: one is the progressive rise of the 100 Mg/(Mg+Fe^(2+)) ratios of the silicate minerals (01, Opx, Cpx) in order of lherzolite→harzburgite→dunite, accompanied by the gradual decrease of FeO in olivine and Al_2O_3 in pyroxene, i. e. the increase in magnesium; the other is the increase of the 100Cr/(Cr+Al) ratios of accessory spinels in the same order, resulting in progressive enrichment in chromium.The evolutionary trends mentioned above are as opposed to that of magma differentiation.At the same time, the evolution of fabrics of the ultramafic rocks is characterized by a progressive variation in order of protogranular texture (lherzolite)→melted residual texture and symplectic texture as well as clastoporphyritic texture (harzburgite)→mosaic texture of equigranular or tabular (or called blastic texture, dunite).The experimental results of partial melting performed on lherzolite have convincingly suggested that the evolutions of Alpine ultramafic rocks result from the partial melting of pyrolite. Every subtype of them represents the product formed by varying degree of partial melting. The lherzolite represents that by lower degree of partial melting, but the dunite, by the highest degree of partial melting.The vertical zoning of the Alpine ultramafic rocks revealing the more basic rocks with higher magnesium in its upper part and the more acid rocks with lower magnesium in the lower part actually belongs to the melt zoning of pyrolite.