辽宁复县地区古生代岩石圈地幔特征

CHARACTERISTICS OF PALEOZOIC MANTLE-LITHOSPHERE IN FUXIAN, LIAONING PROVINCE

本文通过对辽宁复县地区50号金伯利岩管的研究,根据金伯利岩的地球化学和其中地幔矿物及深源捕虏体特征初步推测,复县地区古生代的上地幔反映了该区自早古生代以来上地幔的各种深部事件。它的岩石圈地幔组成与南非金伯利岩发育地区不完全相同。本区是由二辉橄榄岩、纯橄岩、方辉橄榄岩、少量云母石榴长石岩以及金伯利岩的早期堆积物——各种金云母岩所组成。特别值得注意的是,与金刚石平衡共存的是含石榴石及铬铁矿的二辉橄榄岩和方辉橄榄岩,而南非地区与金刚石平衡共存的只是方辉橄榄岩,它是该区在克拉通化过程中通过岩石圈垫底作用增生于其底部的岩石类型。复县地区上地幔中部分橄揽岩仍保留有早期地幔熔体结晶时的火成结构,方辉橄揽岩为提取苦橄质玄武岩-玄武质科马提岩的难熔残余。17亿年本区转变为稳定克拉通后,11亿年(或更晚一些)有交代作用发生,使上地幔富集不相容元素,为熔融金伯利岩准备了源区条件。50号岩管的金伯利岩相当于南非Ⅰ型与Ⅱ型金伯利岩的过渡类型,表明上地幔富集程度较高。复县地区古生代岩石圈厚度至少为170km,其下部上地幔温度为1130℃左右,fo_2接近WM缓冲反应线,与西伯利亚和西非加纳金刚石结晶时的氧逸度相似。

The geochemistry of kimberlites, xenoliths and disaggregated minerals derived from the mantle in Pipe No. 50 has been used for study of the characteristics of mantle-Lithosphere in Fuxian area. The data serve as a basis for speculation of the igneous , metasomatic and tectonic processes from early Proterozoic to Palaeozoic. Mantle Lithosphere in Palaeozoic is composed of Iherzolites, dunites, harzburgites and phlogopi-tites. It should be noted that diamonds in Pipe No. 50 contain not only olivines and enstatites but also diopsides and both G9 and G10. Therefore they are considered to be of lherzolitic type. This suggests that diamonds can be in equilibrium with both Iherzolites and harzbugites, and that may be the major difference between Fuxian and South African diamonds (the latter is harzbugitic type). There is a few peridotite xenoliths which keep the igneous texture. For instance, a chromite-lher-zolite with poikilitic texture has been found in the Pipe. Harzbugites may have originated as residues of partial melting events in early Proterozoic. After cratonization of Fuxian area in 1. 7 b. y. ago, there might be a mantle metasomatic event 1. 1 b. y ago, (or later) which might be very important for an enriched source region of kimberlitic magma melting. The type of kimberlite in Pipe No. 50 is a transi-tionol one between Type Ⅰ and Ⅱ kimberlitse in South Africa, implying a more highly-enriched mantle beneath Fuxian. Equilibration temperature and pressure estimated for inclusion in diamonds reflect a depth of lithosphere greater than 170km with a temperature of 1130℃ and the fo2 approaching toward WM oxygen buffer.