前寒武纪基性岩墙群的地球化学特征与岩石成因讨论

GEOCHEMISTRY AND PETROGENESIS OF PRECAMBRIAN MAFIC DYKE SWARMS

基性岩墙群广泛出现在全世界前寒武纪地盾中 ,它的发育往往与区域性伸展构造环境有关 ,并可能与地幔柱有密切联系。前寒武纪出现的基性岩墙群以拉斑玄武质占绝对优势 (>80 % ) ,但在新太古代—古元古代高Mg的苏长质组分和苦橄质岩石也占重要位置 ,碱性岩类则相当稀少 ;而在新元古代碱性岩墙则变得很重要。基性岩墙群总体以富集LREE和大离子亲石元素为特征 ,其中在苏长岩中更为富集。基性岩墙群的岩浆有多种来源 ,其中苏长质岩浆主要来源于太古宙的陆下岩石圈 ,而拉斑玄武质岩浆多来源于主要由主体地幔组分 (PREMA)构成的地幔柱。地壳混染总体说来不是造成基性岩墙同位素及地球化学特征的主要机制。

Usually mafic dyke swarms occur in most Precambrian shields. They usually intrude into extensional regional tectonic setting and have close relationship with mantle plume. Most of the mafic dyke swarms are of tholeiite composition. High\|Mg noritic dykes and picritic dykes are important from Late Archaean to Early Proterozoic. In contrast alkali dykes are rather scarce, and are important in Late Proterozoic. The dyke swarms are characterized by LREE and LILE enrichments, especially in norites. The magmas responsible for the dyke swarms are derived from different sources: noritic magma is associated with Archaean subcontinental lithosphere, and tholeiitic magma with mantle plume. Crustal contamination is not an important mechanism in explaining the geochronology and geochemistry of the dykes in general.\;