Metamorphism Of The Spinel Granulite In The Larseman Hills,East Antarctica

The Prydz Bay Belt in the East Antarctica shield is closely related to the evolution of Rodinia and Gondwana supercontinent.It is one of the most classic regions with high-ultrahigh temperature metamorphism on the earth.Previous studies have shown that this area suffered from two periods of tectono-thermal events to different extents.However,due to the lack of research,there is still controversy on the characteristics and tectonic settings of these two periods.The Larsemann Hills of the Prydz Bay Belt have undergone high grade metamorphism of granulite facies,and have different types of granulites,such as garnet-sillimanite granulite,amphibole granulite,garnet-cordierite-spinel granulite and spinel granulite.Among them,spinel granulite often contains metal oxides such as spinel,magnetite,ilmenite and corundum with disproportionate.Previous studies on spinel granulite are few,and the dissolution characteristics and temperature indication of metal oxides such as spinel are also lack of depth research.Therefore,the study of spinel granulite metamorphism not only provides important limits on the metamorphic conditions and metamorphic evolution in this area,but also further helps identify the tectonic and thermal event properties in this area.In this paper,petrography,mineral chemistry,phase equilibrium simulation and solid solution temperature estimation are used to comprehensively study the spinel granulites in the Larsemann Hills.Spinel granulites are mainly divided into two types:the first type is the silica-undersaturated garnet-bearing spinel granulites,which lacks quartz in the mineral combination,mainly including spinel+magnetite+ilmenite+corundum+cordierite+sillimanite+garnet.The second type is quartz-bearing spinel granulite,which does not contain garnet in the mineral combination,mainly including spinel+magnetite+ilmenite+cordierite+sillimanite+quartz.But the content of each mineral varies from sample to sample.Spinel,magnetite,ilmenite and other metal oxides are aggregates.A large number of spinel and quartz direct contact phenomenon developed in the rocks,spinel in composition belongs to the hercynite,and has the typical characteristics of exsolution:magnetite in fine strip dendritic exsolution from the spinel grains;part of spinel is dissolved from the magnetite in the form of hemiidiomorphic and idiomorphic particles.A small amount of spinel is dissolved from ilmenite in thin strips.THERMOCALC 3.45 program was used to simulate the thermodynamics of the typical sample of spinel granulite(LSM5-1),under the system of Mn O-Na₂O-Ca O-K₂O-Fe O-Mg O-Al₂O₃-Si O₂-H₂O-Ti O₂-O(Mn NCKFMASHTO).The mineral combination of the peak stage is garnet+cordierite+spinel+biotite+K-feldspar+sillimanite+ilmenite+melt.The metamorphism condition in the corresponding phase diagram is about4.7-6.9kbar,805-910?.In the retrograde metamorphic stage,the mineral assemblage is garnet+cordierite+magnetite+biotite+K-feldspar+sillimanite+ilmenite.The metamorphism conditions in the corresponding phase diagram is about 2.4-5.4kbar,650?-810?.The results show that the LSM5-1 had recorded low pressure and high temperature metamorphism.While the presence of a large number of mineral combinations of spinel and quartz in the samples suggests that the region have experienced high temperature,and even may imply ultrahigh temperature metamorphism.Combined with the P-T trajectory of previous studies,it is suggested that the metamorphic evolution trajectory of the rocks in this region may be a clockwise P-T trajectory of post-peak decompression and cooling.The spinel granulites are rich in Fe-Ti oxides,including granular spinel,ilmenite,magnetite,etc.Among them,striped magnetite is developed in the spinel to dissolve.Based on the original composition regression and mapping of spinel-magnetite dendritic demineralized fine stripe in LSM5-1,it is concluded that the temperature range of spinel-magnetite solid solution composition metamorphism is 760-810?,which belongs to the range of retrograde metamorphism reaction stage in phase equilibrium and may correspond to a certain stage of retrograde metamorphism.However,the higher temperatures that early granular exsolution may imply need to be further defined.Combined with the existing regional data and chronological results,the spinel granulites reflect a high thermal background,which should be the product of Pan-African tectono-thermal events,and may be related to the cracking process of Gondwana continent.