云开地块北缘MORB型火山岩的首次发现及其大地构造意义

First Discovery of MORB Volcanic Rock and Its Tectonic Significance on the North Margin of the Yunkai Block,Southeastern Guangxi

最新研究发现,在云开地块北缘陆川县石窝镇—北流市清水口镇一带的“云开岩群”中出露一系列变质基性火山岩,该火山岩总体呈北东向带状分布,主要岩性为斜长角闪岩和斜长角闪片麻岩,局部岩石中还夹有少量呈透镜状、布丁状或似层状产出的石榴辉石岩、透辉石岩、透辉角闪石岩、角闪石岩、(含石榴)斜长角闪岩、角闪绿帘石岩、石榴阳起石岩、石榴斜长辉石岩等。地球化学研究表明,其为典型的大洋拉斑玄武岩(MORB型玄武岩)。斜长角闪岩的锆石SHRIMP法喷发成岩年龄为(1462±28)Ma,表明其可能代表该区消失了的古洋壳岩石,是四堡期洋壳蛇绿岩的组成部分。

Recent field and in-door researches and investigations has made a first discovery of tabasic volcanic rocks in the Qingshui of Beiliu in Yunkai Yunkai Group-complex that northeastern stretching from Shiwo a series of meof Luchuan to area. The metabasic volcanic rock is composed mostly of plagioclase amphibolite, plagioclase-hornblende gneiss and a few of garnet-pyroxenite, diopsidite, diopside-hornblendite, hornblendite, garnet-plagioclase amphibolite, hornblende-epidote rock, garnet actinolitite and garnet pyriclasite which occur as lenses,the shape of pudding and like layer. The study of geochemistry indicates that these metabasic volcanic rocks are characterized with N-type MORB （mid-ocean ridge basalt）. The age of the plagioclase amphibolite is （1 462±28） Ma using the SHRIMP zircon U-Pb method. The age is interpretedas the time of the volcanic rock formation,which reveals that these metabaslc volcanic rocks are remainders of the palaeo-ocean crust,and components of Sibao period ophiolite.