月球花岗岩——比较行星学意义

LUNAR GRANITES∶IMPLICATIONS FOR COMPARATIVE PLANETOLOGY

与大陆地壳广泛出露的花岗岩不同,在月球表面仅发现了少量细小的花岗岩碎屑,此外有长英质组分以熔体包裹体形式出现于月球玄武岩的矿物中。月球花岗岩碎屑的主要矿物为石英、钾长石和钙质斜长石,具花斑状结构;含少量铁橄榄石、单斜辉石、钛铁矿、锆石、磷灰石、白磷钙矿等矿物,缺少含水矿物。月球花岗岩富K2O,富Ba,相容元素(Cr、Sc、Co、V)含量比其它月岩低,具有平坦或V型的REE型式,负Eu异常明显。它们的化学特征可以用硅酸盐液态不混熔来解释。月球花岗岩的结晶年龄在4.4～3.9Ga间,具有至少8个年龄峰,可能代表了与花岗岩形成相关的8次独立的岩浆事件。由于月球花岗岩成因和分布对于认识月球演化和岩浆作用历史至关重要,在新一轮的深空探测中,应更加重视对月球花岗岩的研究。

Granite, a major component of the Earth's continental crust, was found as small clasts, polymict breccias on lunar surface, or as felsic melt inclusions in minerals of lunar basalts. Major minerals of lunar granitic clasts are quartz,K-feldspar and calcic plagioclase with granophyric intergrowths.Anhydrous mafic and accessory minerals include fayalite,clinopyroxene,ilmenits,zircon,apatite,whitlockite and others,but hydrous minerals are not present.All reported lunar granites contain elevated K_2O and Ba abundances,and have lower compatible element contents(Cr,Sc,Co and V)than other lunar rocks.Generally,the REE profile is flat or 'concave upward' with a strongly negative Eu anomaly.Chemical features of these granites are consistent with silicate liquid immiscibility. The crystallization of lunar granites ages range from 4.4 to (3.9 Ga) with at least eight age peaks,probably indicating eight separate magmatic events leading the formation of these granites. The understanding of the origin and distribution of lunar granites is critical for understanding the history of lunar evolution and magmatism,so much attention should be paid to the studies of lunar granites in new deep space exploration programs.