大别山北坡尖晶石橄榄岩中的高温石榴辉石岩包体:兼论上地幔部分熔融  

The High-temperature Garnet-pyroxenite Enclaves in the Spinel-bearing Peridotite: Evidence for Partial Melting of the Upper Mantle in Northern Dabie Mountain, China

作　　者：游振东[1] 钟增球[1] 索书田[1] 周汉文[1] 

机构地区：[1]中国地质大学,武汉430074

出　　处：《地质学报》2004年第1期89-96,共8页Acta Geologica Sinica

基　　金：国家重点基础研究发展项目(编号G1999075506)

摘　　要：出露于饶拔寨一带的含尖晶石橄榄岩体是大别山区最大的一个上地幔残片,固态侵位于北大别深成片麻岩中,呈无根侵入体的形式产出.岩体内的石榴辉石岩和榴辉岩包体,具高温成因的特点.属于上地幔部分熔融的产物,在Pl-CIPW值对Al2O3图解上落于拉斑玄武岩区;榴辉岩、石榴辉石岩和含尖晶石橄榄岩的REE配分形式相似,均属平坦型,只是尖晶石橄榄岩的∑REE较低.在Ni-Co-Sc三组分图解上,石榴辉石岩投绘于深位玄武岩熔体的趋势线附近,而尖晶石橄榄岩的投点则接近超镁铁质残余的趋势,清楚揭示出地幔部分熔融的迹象.尖晶石的Cr#=12～21,而共生橄榄石Fo=92～93,说明部分熔融程度不高,估计约15%.尖晶石贫TiO2暗示熔融过程氧逸度低.饶拔寨含尖晶石橄榄岩的出现表明:伴随超高压岩石单元的折返和隆升,会有大陆地幔残片被携带上来.软流圈的上涌或板片的断离使侵出的岩石圈板片得以维持较高的温度,这也就是石榴辉石岩早期深位退变质为麻粒岩相的原因,是后续的进一步抬升,才出现以韭角闪石+斜长石组合为特征的高角闪岩相的退变质作用.The spinel-bearing peridotite massif occurring near Raobazhai, Huoshan County of northern Dabie Mountain, is the largest upper mantle relict slab in the Dabie Mountain area. The massif was emplaced as a rootless lenticular intrusion of solid state into the plutonic gneiss complex of Dabie Mountain. The eclogite and garnet pyroxenite enclaves are of high-temperature genesis and are products of partial melting of the upper mantle. In the PI-CIPW versus A12O3 diagram the high-temperature eclogites are plotted into the tholeiite field rather than that of calc-alkaline basalts. The REE distributions of the garnet pyroxenite and the spinel-bearing peridotite are of similar flat patterns, only the total REE of the latter are lower than those of the high temperature eclogites. The plots in the Ni-Co-Sc ternary diagram show that the eclogite and garnet pyroxenites are plotted close to the trend line of deep-seated basalts, while the spinel-bearing peridotites are plotted along the trend of ultramafic relics, which clearly reveals the partial melting of the upper mantle in northern Dabie Mountain. The spinel in the peridotite is defined as pleonaste, the Cr# of which is rather low (Cr# = 12~21), while the associated olivine are Fo-rich (Fo=92~93), indicating that the partial melting was just at its initial stage, estimated at 15%. The low content of TiO2 in spinel suggests a low oxygen fugacity during the melting of the mantle. The occurrence of the Raobazhai spinel-bearing peridotite massif shows that accompanying the exhumation and uplift of the UHP rock units there must be some upper mantle fragments being carried up to the shallower levels. Due to the upwelling of the asthenosphere or the break-off of the subducting slabs, the extruded rock slab could maintain a high temperature regime. This had caused the earlier deep-seated granulite-facies retrometamorphism, which was later shifted to high amphibolite-facies retrometamorphism during the subsequent uplift.

关 键 词：尖晶石橄榄岩 地幔 榴辉岩 深位退变质 

分 类 号：P588.3[天文地球—岩石学]