甘肃玉门市黑山北超高压变质带中蛇纹岩成因浅析

The Genesis of Serpentinite in North Heishan Superhigh Pressure Metamorphic Belt in Yumen, Gansu Province

甘肃玉门市黑山北超高压变质带中的蛇纹岩,其大地构造位置处于阿拉善地块与柴达木地块之间的巨型复合造山带,在研究区其与纯橄岩、辉橄岩等基性岩类伴生,矿物学和矿物化学研究表明它们的原岩为超基性堆晶岩,蛇纹岩全岩亏损Nb、Ti、Zr、Pr等元素,富集强相容元素富集大离子亲石元素Cs、Ba、Pb及U、Sr,并且在蛇纹岩Yb-Cs图解上样品点都落在俯冲带蛇纹岩的范围。由此认为,黑山北蛇纹岩为超基性堆晶岩成因,它们侵入到阿拉善地块的大陆地壳中,随其俯冲,在俯冲带上经历了超高压变质作用和退变质作用,之后发生蛇纹石化且折返至地表。

Located in a huge composite orogenic belt between Alxa terrane and Qaidam terrane,the serpentinite occurred in North Heishan superhigh pressure metamorphic belt is intergrowing with dunite,pyroxene peridotite and other basic rock series.On the basis of their mineralogy and mineral chemistry,the primary rocks of these serpentinite are belong to ultrabasic cumulates.Their geochemistry is featured with depleting in Nb,Ti,Zr and Pr and enriching in strong compatible elements and large ion lithophile elements such as Cs,Ba,Pb,U and Sr.In the Yb-Cs diagram,all of the serpentinite samples demonstrate the characteristic of subduction zone tectonic setting.So the genesis of North Heishan serpentinite could naturally be ascrible to ultrabasic cumulate formations.The cumulates intruded into the crust of the Alxa terrane then subducted with it,underwent superhigh metamorphism and retrograde metamorphism,exhumed to the surface after the serpentinization.