摘要
苏鲁超高压变质带有大量的超镁铁岩体,对这些岩体的成因的研究一直是一个热点。为了鉴别这个地区有无在俯冲进变质前经历过地壳浅部地质过程的超镁铁岩,本文利用BrF5气氛中用CO2红外激光熔样和质谱测试分析方法对中国大陆科学钻探工程主孔内603.20—683.53 m深度的三个石榴单辉橄榄岩样品的单矿物橄榄石、石榴石、单斜辉石的氧同位素进行了分析。橄榄石的δ18O值为 3.31‰- 3.82‰;石榴石的为 4.03‰~ 4.10‰;单斜辉石的δ18O值误差较大,平均值为 2.10‰。这些矿物的氧同位素组成总地低于典型的地幔值。研究表明,岩石单矿物低δ18O值是侵入到地壳浅部的超镁铁岩体与寒冷大气降水热液之间的氧同位素交换反应所造成,之后发生了与地壳围岩一齐的俯冲、超高压变质、折返退变质等过程。这是苏鲁地区首次发现的低δ18O值的岩浆侵入型超镁铁岩体,具有重要的大陆动力学意义,从超镁铁岩方面证明了苏鲁地体可能是大别地体的东延部分。
There are a lot of ultramafic bodies in Sulu ultrahigh pressure metamorphic belt and the research on the origin of these rock bodies has long been a hot topic. To identify if there is the ultramafic rocks that experienced shallow crustal process before subduction in this terrane, this study carried out oxygen extraction from mineral separates olivine, garnet and clinopyroxene of three peridotite samples between the depths 603. 20 -683. 53m of Chinese Continental Scientific Drilling (CCSD) bore hole using CO2 laser fluorination system and isotope analysis by mass spectrometer. The results are as follows.δ18O values of olivine are 3. 31‰- 3. 82‰, garnet 4. 03‰- 4. 10‰, and clinopyroxene 2. 10‰in average with larger error. Theδ18O values of these mineral separates are comprehensively lower than typical mantle values. Combined with other evidences, it is indicated that the lowδ18O values of ultramafic rock in shallow crust level was got from interaction between heated cold meteoric water and the rock body and experienced afterwards subduction, UHPM, retrogressive metamorphism during exhumation etc together with wall-rocks. This is the first case that the ultramafic rocks possess aaaaaaaa 18O depleted character found in Sulu terrane. This result bears even more important continental dynamic meaning and proves that Sulu terrane is the eastward extension of Dabie terrane in respect of ultramafic rocks.