Metamorphism And Geological Implications Of Paleoproterozoic HT-HP Granulites From Huai'an-Datong Area, The North China Craton

One of the key research progresses about Precambrain tectonic evolution of the North China Craton (NCC) is the discovery and its associated researches of Paleoproterozoic mafic high-pressure (HP) granulites (also named as garnet mafic granulites) and (ultra)high-temperature ((U)HT) granulites (also named as the khondalite series) during recent years.Accordingly various Early Precambrian continent-continent collision or micro-blocks amalgamation hypotheses were suggested. Field geological observations suggest that these mafic HP granulites not only occur as metamorphic dykes intruded to TTG-granitic gneisses, but also they are associated with HT metapelitic granulites in some areas. A key issue that whether the associated HTpelitic granulites suffered similar metamorphic history to the mafic HP granulites is hotly debated during recent years. It is very important for a deep understanding of early continental evolution and metamorphic mechanisms.Huai'an-Datong area, one of the areas for earliest discovery of mafic HP granulite terrains in the NCC, is located in the conjunction zone of the Fengzhen (Khondalite belt) and Jinyu mobile belt (Trans-North China Craton), where mafic HP granulites and khondalite series are closely exposed. It is one of the best area for study the petrogenetic relationships of these two granulite terrains. In this dissertation, detailed geological mapping, field observations, petrological studies, phase equilibrium modeling and geochronological work were conducted, and we draw four major conclusions as follow.(1) The garnet mafic granulites occur as lenticular bodies or dismembered dykes within the khondalite series and gray gneisses in the Huai'an-Datong area. All of these lithogical units shared their foliations. Three episodes deformation events (Di to D3) are recognized in the study area. The early deformation (D1) is represented mainly by rootless intrafolial folds preserved locally in some layers of the gray gneisses, khondalite series and garnet-bearing mafic granulites. It is suggested that the HP granulite facies metamorphism (M2) predates or during D1. It is then reworked by intensive secondary deformation (D2), which is characterized by penetrative foliations (S2) within all metamorphosed lithological units. The textures of oriented idioblastic laths of sillimanite and coronae of orthopyroxene and plagioclase around some garnets indicate the medium-pressure (MP) granulite facies stages (M3) predates or coincides with D2. D3 is characterized by large scale open folds accompanying with retrogression (M3 to M4) during uplift and cooling stages.(2) HP pelitic granulites are discovered in the khondalite series from Datong and Huai'an areas, which provide robust evidence that the pelitic granulites suffered comparable HP granulite facies metamorphism with garnet mafic granulites. Based on the microtextures and mineral compositions of the pelitic granulites from Huai' an-Datong, four generations metamorphic stages are recognized:prograde metamorphic stage (M1), HP granulite facies stage (M2), MP granulite facies stage (M3) and granulite to amphibolite facies stage (M4). Pre-peak assemblage (M1) is characterized by the mineral inclusions within garnet core, namely, garnet core+biotite+rutile+quartz ± muscovite ± plagioclase. Peak assemblage (M2) is composed of garnet mantle and its inclusions of kyanite+rutile+biotite+K-feldspar +quart ± muscovite ± gahnitic spinel, representing HP granulite facies stage. Post-peak decompression assemblage (M3) consists of garnet rim+sillimanite+quartz+biotite+ K-feldspar+rutile+plagioclase ± gahnitic spinel, representing the imprinted MP granulite facies stage. The mineral assemblages and reaction textures from granulite to amphibolite facies (M4) retrogression are not obvious, with minor intergrowth of biotite and quartz around embayed garnet, and replacement of rutile by ilmenite, or growth of secondary muscovite.(3) The results of phase equilibrium modeling show that the pelitic granulites from Manjinggou suffered HP granulite facies (M2) conditions of 11.5-15kbar/810-860?, M3 of ?9.5kbar/850?. Consistently, the peak metamorphic conditions of the pelitic granulites from Gushan area are?13.2kbar/848?, and the subsequent MP granulite facies conditions are ?7.8 kbar/?835?. It is different from previous views that pelitic granulites are low-to medium-P granulites. They define a near isothermal decompression P-T path, comparable to P-T results of associated garnet mafic granulites from Gushan and Manjinggou. Two similar episodes metamorphic ages (?1.95Ga and ?1.85Ga) are recorded by the HT-HP granulites from both two localites.(4) It is suggested that the khondalite series and garnet-bearing mafic granulites from Huai'an-Datong area suffered their similar metamorphic and deformation history. They were experienced HP granulite facies stages during ?1.96-1.90Ga, imprinted subsequently by MP granulite facies to the middle to lower crust level during ?1.88-1.82 Ga, and finally exposed to the upper crust level during-1.82-1.80 Ga, accompanying with amphibolite retrogression and intrusion of pegmatite veins. It differs from previous opinions that the khondalite series and garnet mafic granulites are different slabs with individual metamorphic history.(5) These HT-HP granulites from Huai'an-Datong record high apparent geothermal gradient of ?16-25?/km, within the thermal regime of medium-P/T facies series, and slow exhumation rate from HP to MP granulite facies stages (?0.15-0.2mm/yr), and then via low-P granulite facies to amphibolite facies (-0.15-0.45 mm/yr). It indicates that the metamorphic features of these HT-HP granulites have principle difference from Phanerozoic continental collisional belts.Based on regional geological and published data, we propose two corollaries as follow.(1) It is suggested that the khondalite series from the other regions of the NCC probably suffered similar metamorphic history with the associated garnet-bearing mafic granulites.(2) The widespread distribution, high apparent geothermal gradients and slow exhumation of these granulite terranes, as well as long dwell time at HT conditions of these HT-HP granulites from the NCC differ from the HP-UHP terranes formed in the Phanerzoic continental collisional belts. It seems to indicate that the thermal regimes and geological processes of Paleoproterozoic probably differ from those of Phanerozoic eons. More information is needed before a comprehensive understanding for the petrogenesis and geological implications of these HT-HP granulites.