云南新平县双沟蛇绿岩中地幔岩初始熔融物

INITIAL MELTING OF MATAPERIDOTITE IN SHUANGGOU OPHIOLITE FROM XINPING COUNTY, YUNNAN PROVINCE

本文描述了野外和镜下所见到的双沟二辉橄榄岩中初始部分熔融现象,研究了初熔物的矿物、主元素、微量元素以及稀土元素。它们源于LREE亏损的地幔岩,其成分是超镁铁质的,它们向上运移,经过某种分离过程才演化为玄武质岩浆,进入洋壳底部的岩浆房,因此,初始熔体应是所谓玄武质或苦橄质原始岩浆的源泉。文中还讨论了初始熔体形成条件以及铬铁矿的成因。

The Shuanggou ophiolite is exposed in the southern part of Ailaoshan ophiolite zone and is composed of three petrologic units, from bottom to top they are metaperidotite (mainly Pl-lherzolite and Sp-lherzolite, then Spharzburgite, while dunite is rarely found), gabbro and basalt. It is very interesting that initial melting took place in Pl-lherzolite. It is seen in the outcrops that the rodingite vein and myrmekitic streak composed of mineral aggregation are running parallel to the foliation of lherzolite. The rodingite veins are less than 20 cm in width and the myrmekitic streaks only 0.2～1cm in width and less than 8cm in length. Both rodingite vein and myrmekitic streak are believed to have crystallized from a melt or to be a product of equilibrium with a melt. The melting textures are exhibited in three types in thin section: (A)synneusis aggregation of clinopyroxene, (B)dark trapped pocket(mainly composed of grossular and hydrogarnet), and (C)light-coloured pocket(mainly chlorite with a small, amount of serpentine and glass). Expect for the minerals mentioned above, a few orthopyroxene, spinel and epidote also occur in melt aggregation, It can be seen in Table 4 that the compositions of initial melt from both rodingite vein and calculated minerals aggregation are consistent with each other in Shuanggou lherzolite, and may be compared with that of initial melt of mantle inclusion in basalt(Maalφe and Printzlau, 1979), but both of them are differnt from the generally recognized composition of primary magma.There are diversified arguments on the primary magma, one is that the most known MORB and basaltic glass, represent the primary magma, on the contrary, another is that the picrite is the primary magma derived from mantle, but MORB was only derived from picritic magma after separation of olivine when pressure droped. However, the Shuanggou initial melt is distinctly different from both of the primary magmas mentioned above.The MgO content of Shuanggou initial melt is up to 24.28(wt)％, Mg'=0.88, Cr and Ni are enriched, and LREE is depleted, belonging to a kind of ultramafic magma. It is estimated that the initial melt of ultramafic composition was a product of low degree partial melting(less than 13％) and was derived from LREE-depleted mantle source under low pressure and high temperature conditions. It has been argued whether ultramafic magma existed or not. In fact, the rocks formed from crystallization of truly ultramafic magmas, such as komatiites, were widespread in Archean, and there is at least one Phanerozoic example(Gorgona Island, Colombia). The most abundant magma in the modern world—oceanic tholeiite—probably is not primary, as it apparently represents a low-density derivative magma derived from a two-ormore stage differentiation of a more ultramafic magma. The example in Shuanggou area shows that the initial magma is an ultramfic one, when mantle began to be melted at depths. It is probable that fractionation of olivine and spinel (forming dunite and chromite, respectively) during ascent of ultramafic magma produced a mafic magma at the surface.