摘要
苏鲁超高压变质带(UHP)中的芝麻坊超基性岩体是一个不寻常的地幔岩块,由高度亏损的橄榄岩组成,因地幔交代作用而富集REE和LILE.岩体由交替出现的石榴石橄榄岩和不含石榴石橄榄岩层组成.2种类型的岩石具有许多共同的成分特征,并在相同的温压p-T条件下形成.岩石记录了数期地质事件,但没有进变质作用的证据.平衡矿物对计算出的p-T条件为6~7GPa.发生在中元古代的部分熔融的差异造成了互层状的2种岩石类型.橄榄岩中锆石220Ma的U-Pb同位素年龄记录了扬子板块和北中国板块碰撞,但此年龄也许并不代表岩石UHP变质峰期的年龄,而可能记录了俯冲带中地幔楔的碎块从深部折返时的年龄.我们认为石榴石可能不是由于俯冲变质作用形成,有可能是岩石从地幔深部上升过程中从斜方辉石中出溶而成.
The Zhimafang ultramafic body in the Sulu ultrahigh pressure (UHP) metamorphic belt is a block of unusual mantle composed of highly depleted peridotite, which has been enriched in REE and LILE by metasomatism. It consists of interlayered garnet peridotite and garnet-free peridotite, which have many common compositional features and formed under the same p-T conditions. The rocks record several geological events; however, evidence of progressive metamorphism has not been confirmed. Calculated p-T conditions from equilibrium mineral pairs suggest pressures of 6-7GPa. Partial melting of the sequence in the Mesoproterozic was probably responsible for the interlayering of the two rock types. A U-Pb zircon age of 220Ma records the collision between Yangtze and North China blocks, but this age may not represent the time of peak UHP metamorphism. More likely, it records exhumation of the block from the deep portion of the subduction zone. The available data suggest that the garnet was exsolved from enstatite as rocks rose from the deep mantle.