北大别的高温超高压变质作用与多阶段折返

High-T/UHP metamorphism and multistage exhumation history of the North Dabie complex zone

近期的变质岩石学、地球化学及同位素年代学研究表明,北大别整体经历了高温超高压变质作用和多阶段折返历史,因而表现为广泛发育的多期减压结构和极少保留早期的超高压变质记录。北大别榴辉岩以高温变质作用以及折返期间因麻粒岩相和角闪岩相退变质变质作用而形成的多期后成合晶为显著特征。石榴子石中伴有放射状胀裂纹的单晶和多晶石英包体指示早期柯石英的转变结果,这已被锆石中发现的柯石英残晶所证实。结合北大别北部榴辉岩和片麻岩中发现的金刚石等超高压证据以及三叠纪变质记录,由此证明北大别整体经历过深俯冲和印支期超高压变质作用。北大别榴辉岩的多阶段高温条件主要来自石榴子石-绿辉石矿物对温度计、单斜辉石中紫苏辉石+石英针状矿物出熔体以及金红石中较高的Zr含量和变质锆石中较高的Ti含量等得出的温度证据。此外,多期后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个多阶段、快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩经历了长时间的高温变质演化与缓慢冷却过程。长时间的高温变质作用与缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压记录的重要原因。因此,这些成果为大别山三个不同超高压岩片的差异折返模型的建立提供了新的证据.

Recent investigations of metamorphic rocks from the North Dabie complex zone（NDZ）suggest that the NDZ wholly underwent high-T/UHP metamorphism and multistage exhumation history as a coherent slice,resulting in the common formation of peculiar multiple decompression textures and the extremely rare preservation of former UHP metamorphic record.The eclogites are characterized by high-T metamorphism and multiple symplectites that formed by granulite-and amphibolite-facies metamorphism during decompression.The occurrence of radial cracks around quartz inclusions and of polycrystalline quartz inclusions in garnet,most likely suggests that these inclusions formerly were coesite,now inverted to quartz.This hypothesis is also supported by the presence of relic coesite in zircon.These data,in combination with previously reported micro-diamond and related UHP evidence,and Triassic metamorphic ages from rocks the northern part of the NDZ,further demonstrate that the NDZ wholly experienced the Triassic deep subduction and UHP metamorphism.Multistage high-T conditions of the eclogites are indicated by garnet-omphacite thermometer and hypersthene＋quartz lamellae in clinopyroxene.It is also supported by high-T indicators including high Zr and Ti contents in metamorphically grown rutile and zircon,respectively.The multiple coronitic and symplectitic reaction textures and the occurrence of compositionally zoned minerals such as garnet and clinopyroxene record multistage rapid exhumation of the UHP eclogites.Their metamorphic P-T conditions and isotopic ages for various stages show that they experienced a protracted high-T period with slow cooling process.This may be one of the major reasons why the coesite and coherent UHP records are rarely preserved in the NDZ high-T/UHP rocks.These observations provide further insights into the processes of multi-slice successive exhumation,whereby the detachment and decoupling occurred at variable depths within the subducted continental crust and the multi-slices of UHP rocks in the Dabie orogen were successively exhumed during the continental collision between the South China and North China blocks.