北大巴山早古生代双模式火成岩套:以高滩辉绿岩和蒿坪粗面岩为例

EARLY PALAEOZOIC BIMODAL IGNEOUS SUITE ON NORTHERN DABA MOUNTAINS GAOTAN DIABASE AND HAOPING TRACHYTE AS EXAMPLES

北大巴山辉绿岩类和粗面岩属碱性系列。它们是加里东晚期形成的双模式火成岩套。两类岩石均以富集REE、Zr、Hf、Nb、Ta、Th、U、Rb、Sr、Ba等不相容元素,亏损Cr、Ni等相容元素为特征,具相近的(^(87)Sr/^(86)Sr)i初始比值(辉绿岩为0.705995,粗面岩为0.705096)。地质产状、岩石学、同位素、地球化学等资料表明,辉绿岩和粗面岩系由同源原生岩浆经液态不混溶作用生成的两个独立的派生岩浆——富钛富铁的镁铁质熔体和高硅富钾的硅铝质熔体,再经不同程度演化冷凝结晶而成。

Gaotan diabase bodies had intruded as a sill swarm into the Early Palaeozoic deposits situated in the northern periphery of Yangtze plate. Haoping trachyte occurs in Silurian Daguiping Formation. The diabase is high-Ti ferrobasalt of a mildly alkaline composition and the trachyte is alkaline and oversaturated. The age, geological position,maior and trace element characteristics suggest that the trachyte is genetically related to the basalt. The rocks in this area display strong bimodal composition distribution: rocks with 50～60％ SiO_2 are rare. The predominant composition of the rocks is of diabase, and the trachyte is secondary. The diabase and the trachyte have initial Sr isotopic ratios 0.705995 and 0.705096. It indicates that they are comagmatic rocks. Primary magma should be generated by partial melting in the continental mantle. The 'Daly gap' may be a consequence of the silicate-liquid immiscibility process.