洋壳俯冲过程中的地幔楔上升对流：来自芝麻坊石榴子石二辉橄榄岩早期变质的证据

苏鲁超高压变质地体中发现了大量包裹在超高压（UHP）变质片麻岩和混合岩中的造山带石榴橄榄岩。根据它们的野外产出特征和全岩地球化学成分，其中一部分石榴橄榄岩的原岩来自于亏损地幔，后来被卷入俯冲陆壳并经受过俯冲陆壳产生的熔/流体的交代。但是，对这些岩石早期的亏损过程尚缺乏清晰的认识。本文报道了东海芝麻坊石榴子石二辉橄榄岩早期变质演化的新证据。根据详细的变质反应结构观察和矿物成分研究，芝麻坊石榴子石二辉橄榄岩在经历高压低温俯冲带型超高压变质之前经历了至少两期变质演化。其原岩矿物组合由石榴子石变斑晶的高Ca-Cr核部及其中包裹的高Mg单斜辉石、高Al-Cr斜方辉石和高Mg-Ni橄榄石所记录；指示芝麻坊石榴子石二辉橄榄岩的原岩为高温-高压的富集石榴子石二辉橄榄岩。第二期矿物组合为包裹在低Cr变斑晶石榴子石幔部和细粒新生石榴子石核部的大量富Al铬铁矿和高Mg低Ni橄榄石以及少量高Mg斜方辉石。该期组合未发现单斜辉石，表明岩石随后被转变为高温低压的难熔尖晶石方辉橄榄岩或尖晶石纯橄岩。芝麻坊石榴子石二辉橄榄岩的早期变质演化记录了它们被卷入大陆板片俯冲带之前的地幔楔上升对流过程。笔者认为芝麻坊石榴子石二辉橄榄岩的原岩来源于早期俯冲大洋板片之上的深部高温富集地幔楔，洋壳俯冲过程中的地幔楔对流导致其上升到弧后或岛弧之下的地幔楔浅部，减压部分熔融使原本富集的石榴子石二辉橄榄岩转化为难熔的尖晶石方辉橄榄岩或尖晶石纯橄岩。

Upward mantle wedge convection recorded by Zhimafang orogenic garnet-lherzolite, Sulu UHP terrane, eastern China

Many orogenic garnet peridotite bodies are enclosed in ultrahigh-pressure (UHP) gneisses and/or migmatites in Sulu UHP terrane. On the basis of their field occurrence and bulk rock geochemistry, a group of garnet peridotites have been derived from depleted mantle and were subsequently metasomatized by melts and/or fluids derived from the subducted continental crust. However, their early stage depletion processes are still unclear. New evidence on early stage metamorphism of the garnet lherzolite from Zhimafang, Donghai County, Sulu UHP terrane are reported. Based on detailed analysis of reaction textures and mineral compositions, the Zhimafang garnet lherzolite have experienced at least two stages of metamorphism before they were evolved in high P-T subduction zone metamorphism. The protolith of the Zhimafang garnet lherzolite was a high-pressure and high-temperature fertile garnet lherzolite, which is defined by the high Ca-Cr core of garnet porphyroclast and inclusions of high-Mg clinopyroxene, high-Al-Cr orthopyroxene and high-Mg olivine. The rocks were then transformed to a high-temperature and low-pressure refractory spinel-hurzbergite or spinel-dunite, as indicated by the presence of abundant relict high-Al chromite and high-Mg, low-Ni olivine, as well as high-Mg orthopyroxene included in the low-Cr mantle of the porphyroclastic garnet and core of fine-grained neoblastic garnet, clinopyroxene is absent in this stage. The early stage metamorphic evolutions of the Zhimafang garnet lherzolite suggest that it experienced upward mantle wedge convection during the subduction of previous oceanic slab before they were evolved in the high P-T subduction zone metamorphism due to subsequent continental subduction. We propose that the Zhimafang garnet lherzolite were originated from deep, hot and fertile mantle wedge above the previously subducted oceanic slab, thermal buoyancy in the mantle wedge above the subducted oceanic slab resulted in their convection to shallower back arc and sub-arc mantle wedge, and decompressional melting transformed the enriched garnet-lherzolite to depleted spinel-hurzbergite or spinel-dunite.