中国大陆科学钻探工程主孔榴辉岩的岩石学研究

Petrology of eclogites from the main hole of the Chinese Continental Scientific Drill Project

中国大陆科学钻探工程5000m主孔位于苏鲁超高压变质带南部。该钻孔0～2000m主要由榴辉岩、片麻岩、石榴石橄榄岩和少量片岩和石英岩组成。累积厚度达1000多米的榴辉岩具有不同的矿物组成、不同的矿物含量和不同的全岩化学成分,可划分成富Si的石英榴辉岩、富Ti的金红石榴辉岩、富Al的多硅白云母和蓝晶石榴辉岩、富Mg的双矿物榴辉岩和具有正常玄武岩成分的普通榴辉岩。榴辉岩的原岩包括基性层状侵入岩和变质表壳岩。榴辉岩全岩成分对石榴石和绿辉石中某些化学组分的含量有明显控制,而且直接影响到变质条件估算的准确性。扩散成分环带的广泛发育表明超高压矿物在早期退变质过程中发生了成分再平衡。这一事实以及具有成分生长环带石榴石变斑晶的存在,为榴辉岩形成在更高温度(>940℃)和更高压力(>4.5GPa)条件下提供了有力的证据。

The drill site of the 5000 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD) is located in the southern Sulu ultrahigh-pressure (UHP) metamorphic belt. The 0 to 2000 m interval of the CCSD main hole consists of eclogite, gneiss, garnet peridotite and minor schist and quartzite. The eclogites with a cumulative thickness of about 1000 m have different mineral assemblages and model contents, as well as variable petrochemical compositions. They can be divided into five types:Si-rich quartz eclogite, Ti-rich rutile eclogite, Al-rich phengite and kyanite eclogite, Mg-rich two-mineral eclogite and normal eclogite. Protoliths of eclogites include layered mafic to ultramafic cumulates and supracrustal rocks. The whole-rock composition of eclogites has an apparent controlling effect on the contents of some components in garnet, omphacite and phengite and directly influences the P-T estimates of UHP metamorphism. The extensive development of diffusional compositional zoning in the eclogitic garnet, omphacite and phengite indicates that these UHP minerals were re-equilibrated during the early-stage retrograde metamorphism. This fact and the existence of garnet porphyroblasts with growth zoning suggest that the eclogites formed at higher temperature (940℃) and pressure (>4.5GPa) than those estimated by previous workers.