北祁连蛇绿岩的地质地球化学研究

Research for Geology and Geochemistry of SeveralOphiolites in the North Qilian Mountains, China

在北祁连广泛出露早中奥陶世蛇绿岩。本文较详细地研究了玉石沟、大岔大坂、塔墩沟和老虎山四个地区的蛇绿岩。这些蛇绿岩的地质和地球化学特征表明：蛇绿岩中的基性岩类都接近ＭＯＲＢ；它们的上地幔源区属亏损型；玉石沟、大岔大坂、塔墩沟和老虎沟蛇绿岩分别生成于弧前盆地（或小洋盆）、弧间盆地、近弧的弧后盆地和近陆的弧后盆地的扩张脊环境。早中奥陶世时该区曾出现一个具沟－弧－盆体系的成熟大洋，在大约４６０Ｍａ时由于洋内剪切引起的向北俯冲作用导致俯冲杂岩和岛弧的形成以及弧后扩张。玉石沟蛇绿岩是一个外来移置体，其余３个蛇绿岩则属原地或准原地岩系。

Ophiolites dating Early-Middle Ordovician period are widely outcroped in the North QilianMountains. Four well-investigated ophiolites, i. e. the Yushigou, the Dachadaban, the Tadungou, theLaohushan opiolites, have been selected and discribed in this paper. AFM diagrams of these basic rocks fromthese ophiolites plot into MORB area. Their REE distribution patterns near N-MORB patterns. Theirspiderdiagrams of trace elements are analogous to MORB, but contents of Ba, Rb, K, Th, Zr in them arecommonly higher than that in MORB. It is clearly shown that some metamorphic fluid entered into the magmachamber producing these basic rocks altered. Information suppleid by geology and geochemistry of theseophiolites is clearly shown as the followings: (l ) In the area the upper mantle is a depleted-type. (2) Theseophiolites generated in some environments of spreading ridge: the Yushigou, the Dachadabap, the Tadungou,and the Laohushan ophiolites were seperatelly formed in a fore--arc basin or asmall oceanic basin, an inter-arcbasin, a back--arc basin near an arc, and a back-arc basin near a continental massif. (3) During fEarly--MiddleOrdovician period a mature ocean with a trough-arc--basin system once occured in the area. A north-towardssubduction resulted from shearing within the ocean at about 460Ma led to forming of the subduction complex andthe arc, and spreading of the arc and the back--arc area. (4) The Yushigou ophiolite, which obducted onto thecentral Qilian massif, is an allochthone, but the other three ophiolites are autochthones or para--autochthones.