班公湖-怒江缝合带西段洞错石榴石麻粒岩岩石学特征及其构造意义

Petrology and tectonic implications of garnet granulite from Dong Tso,western Bangong-Nujiang suture zone

班公湖-怒江缝合带是青藏高原中部一条重要的构造边界,将北部的羌塘地块与南部的拉萨地块分隔开,当前对班公湖-怒江洋壳的俯冲过程知之甚少,本文报道班公湖-怒江缝合带西段洞错地区的石榴石麻粒岩,将有助于提升班公湖-怒江缝合带构造演化过程的认识。石榴石麻粒岩呈5.0~_10mm的透镜体产出于斜长角闪岩内,后者呈构造岩片产出于班公湖-怒江缝合带内,与超基性岩片呈断层接触。石榴石麻粒岩的石榴石呈变斑晶产出,具有不规则的成分环带,石榴石核部成分较均匀,以富Ca和Mg,贫Fe和Mn为特征,石榴石边缘及内部裂隙成分则与之相反,富Fe和Mn,与绿泥石和黝帘石组成的细脉共生,反映了石榴石遭受后期流体的交代作用。单斜辉石为透辉石,在其内部发现早期进变质阶段的细小石榴石与角闪石颗粒,石榴石粒径<_10μm,该石榴石相对富Fe和Mn,贫Ca和Mg。角闪石均为钙质系列的角闪石,早期角闪石以相对富Na和Fe^(3+),含有微量Ba,而与晚期角闪石区分开。早期角闪石常产出于石榴石和透辉石变斑晶内部,脱水熔融生成富Ca、贫Fe^(3+)和Na的透辉石和以钠长石为主的熔融囊体,囊体内部发育斜长石和富Ba冰长石的包体/出溶体。晚期角闪石常与斜长石呈细小的后成合晶产出于石榴石边部或呈变斑晶产出于基质之中,表明斜长角闪岩系石榴石麻粒岩退变质的产物。此外,金红石斑晶常变成细粒、不规则的贫Al榍石和钛铁矿,基质中有相对富Al榍石产出。因此该麻粒岩进变质阶段矿物组合由早期石榴石(Grt__1)、早期角闪石(Amp_1)、相对富Al的榍石(Spn_1)等矿物组成。峰期矿物组合包括:核部石榴石(Grt2)+透辉石+斜长石(Pl_1)+石英+金红石+富Na和Ba的熔体,采用传统温/压计估算此峰期矿物组合形成条件为870℃、11.7kbar。退变质矿物组合有:边部石榴石(Grt3)+晚期贫Na和Fe^(3+)的角闪石(Amp_1)+黝帘石+绿泥石+斜长石(Pl_2)+白云母+贫Al榍石(Spn2)+钛铁矿。岩相学观察与地球化学示踪、变质温压估计表明洞错石榴石麻粒岩系热洋壳俯冲的产物,为前人提出的"班公-怒江洋盆包含次级小洋盆"之观点提供了变质岩石学的证据。从峰期到退变质阶段,角闪石的Fe^(3+)含量明显下降,铁铝榴石含量明显升高,指示该麻粒岩经历了还原反应,早期角闪石脱水熔融时被还原,生成了高氧逸度的埃达克质岩浆,促进了Cu、Au等成矿元素从俯冲热洋壳的活化与迁移至弧岩浆之中,因此本文的研究提供了一个探讨俯冲热洋壳与上覆地幔楔之间化学反应的研究案例,这有助于理解班公-怒江成矿带内众多斑岩型铜、金矿床的形成。

Separating the Qiangtang terrane in the north and the Lhasa block in the south,the Bangong-Nujiang suture zone is an important tectonic boundary within Central Tibet. However,subduction of the Bangong-Nujiang oceanic crust remains obscure. Here we report several garnet granulites in the vicinity of Dong Tso area,western Bangong-Nujiang suture zone,contributing to understand the tectonic evolution of Bangong-Nujiang suture zone. The garnet granulites as lenses in size of 5. 0 ~ 10 mm are distinguished in amphibolites,which occur as tectonic slices in the Bangong-Nujiang suture zone and structurally contacting with ultramafic and mafic rocks. The garnet grains within garnet granulites present as porphyroblasts with irregularly compositional zonation. The composition of garnet core is more homogeneous and characterized by higher Ca and Mg but lower Fe and Mn than the garnet rim. Moreover the rims and cracks of garnet show paragenetic relationship with fine veins composed of chlorite and clinozoisite indicating that the garnet grains suffered metasomatism by later fluids. Fine-grained early garnets and amphiboles were found as inclusions in diopside. These garnets are smaller than 10μm and characterized by higher Fe and Mn but lower Ca and Mg than the garnet core. All the amphiboles are calcic amphiboles. The early amphiboles are different from the late amphiboles by their enrichment in Na,Fe3 ＋,as well as trace amount of Ba. The early amphiboles normally as inclusions in garnet and diopside porphyroblasts have been transformed into diopsides（ rich in Ca but poor in Fe^（3＋） and Na） and melt pockets dominanted by albite. Inclusions / exsolutions composed of plagioclases and Ba-rich hyalophanes are widely distributed in the melt pockets. The late amphiboles occur as fine-grained symplectite with plagioclases surrounding the rim of garnet or present as porphyroblast in matrix,suggesting that they are retrograde products of garnet granulites. In addition,sphenes（ rich in Al） are found in the matrix and rutile grains have been replaced by fine and irregular sphene（ poor in Al）and ilmenite. Therefore the mineral assemblage of prograde stage is consisting of early garnet（ Grt_1）,early amphibole（ Amp_1）,Alrich sphene（ Spn_1） and so on. The peak metamorphic stage is documented by garnet core（ Grt_2） ＋ diopside ＋ plagioclase（ Pl_1） ＋quartz ＋ rutile ＋ Na-rich and Ba-rich melts,which formed at 870℃,11.7kbar on the basis of traditional geothermobarometer.Garnet rim / crack（ Grt_3） ＋ late amphibole（ Amp_2） ＋ clinozoisite ＋ chlorite ＋ plagioclase（ Pl_2） ＋ white mica ＋ Al-poor sphene（ Spn_2） ＋ ilmenite are regarded as the retrograde stage assemblage. In combination with petrographic and geochemical investigations and estimation of metamorphic P-T condition indicates that Dong Tso garnet granulites are products of warm subduction of oceanic crust,providing petrological evidence for the view that ‘the Bangong-Nujiang oceanic basin contains many sub-oceanic basin＇. The significant decrease of Fe^（3＋）（ a. p. f. u） in amphibole and marked increase of almandine from peak to retrograde stage reveals that the Dong Tso garnet granulites suffered reduction reaction and the amphiboles were redoxed during dehydration,generating adakite magmawith high oxygen fugacity which promotes the activation from subducted warm oceanic crust and migration to arc magma of ore-forming elements,such as Cu and Au. Therefore,our work provides a case study of the chemical interactions and material migration between subducted warm oceanic slab and overlying subarc mantle wedge,and also contributes to intensively understanding the formation of numerous porphyry-type copper and gold deposits within the Bangong-Nujiang metallogenic belt.