北祁连山中段早中奥陶世蛇绿岩中席状岩墙杂岩的发现及其地质意义

THE DISCOVERY AND GEOLOGICAL SIGNIFICANCE OF SHEETED DYKE COMPLEX IN THE EARLY-MIDDLE ORDOVICIAN OPHIOL1TE IN THE CENTRAL SECTION OF THE NORTH Q1L1ANMOUNTAINS, CHINA

作为蛇绿岩套重要组成部分之一的席状岩墙杂岩(Sheeted dyke complex)近来在北祁连山中段肃南县大岔大坂北坡的早中奥陶世蛇绿岩中被发现。这一发现对于祁连山早古生代蛇绿岩来说尚属首次,而且对于研究蛇绿岩的发展演化及探讨奥陶纪时洋底扩张都有重要意义。席状岩墙杂岩由一系列具单向冷凝边的辉绿岩墙组成,以一墙挨一墙的形式产出,岩墙间无任何填充物。席状岩墙杂岩在矿物组合上,常量元素、稀土元素及痕量元素地球化学特征和配分模式,甚至金属硫化物矿化作用方面都有类似之处。这些证据表明席状岩墙杂岩是连通其下岩浆房与其上枕状熔岩的通道。席状岩墙的单向冷凝边为岩浆上升方式和扩张洋脊的存在提供了令人信服的证据。根据Zr/Y—Zr关系图式,得出该区洋脊的扩张速率大约为2cm/a。

A sheeted dyke complex as one of the essential components of ophiolite has been discovered in the Early-Middle Ordovician ophiolite on the north slope of Mount Dachadaban of Sunan county in the central section of the North Qilian Mountains. This discovery is the first for the Early Paleozoic ophiolite in the Qilian Mountains, and of important significance for the study of the evolution of ophiolite and ocean-floor spreading during the Ordovician. The sheeted dyke complex consists of a series of diabase dykes with single-side chilled borders, one against the other and without any fillings between these dykes. The sheeted dyke complex bears some resemblance to its underlying isotropic gabbro and its overlying pillow lavas and diabase dykes in pillow lavas with respect to the mineral assemblages, geochemical characteristics and distribution patterns of major elements, rare earth elements and trace elements and even metal sulfide mineralization. The above-mentioned lines of evidence show that the sheeted dyke complex represents a channel linking the magma chamber below and. the pillow lavas above. The single-side chilled borders of the sheeted dyke complex provide convincing evidence for the way of magma ascent and existence of spreading oceanic ridges. According to the Zr/Y-Zr diagram, it is estimated that the spreading rate of the oceanic ridge is about 2 cm/year.