Metamorphism Of The Xinghuadukou Complex In The Erguna Massif

The study area is in the Lulin Forest within the Erguna Massif of the eastern section of the Central Asian Orogenic Belt(known as the Xingmeng Orogenic Belt).As the metamorphic basement of the Erguna Massif,the Xinghuadukou Complex is primarily distributed in areas of the Qingsong Forest-Lulin Forest to the north of the Baikalu Mountain.The Xinghuadukou Complex is scattered as xenoliths in the meta-granitic plutons of different shapes and sizes because of the effect of multiple granitic magmatic events and structural deformation,which is important to investigate the metamorphic features of Precambrian units in the Erguna Massif.The Xinghuadukou Complex is composed of meta-supracrustal and meta-basic rocks,including Bt-Pl gneiss,Bt-Hbl-Pl gneiss,Bt-Pl-Kfs gneiss,garnet bearing Bt-Pl-Kfs gneiss,garnet bearing Sil-Bt geniss,garnet bearing Sil-Bt schist,Grt-Sil-Bt schist,Grt-Sil-Mica schist and amphibolite.This thesis aims to resolve metamorphic evolution and geocharonological framework of the study area by carrying out detailed study on the metapelites which recorded multiple metamorphic stages from the Xinghuadukou Complex.Geochemical features show that the metapelites within the Xinghuadukou Complex were originally Al-rich claystone coming from weathered upper crustal felsic rocks which experienced high weathering.The protolith of meta-basic rocks are characterized by island-arc calc-alkaline basalts and form in an island arc or transition tectonic setting from island arc to the continental environment.Based on the petrographic study,phase equilibrium modeling and geothermobarometry method,three metamorphic evolution stages were determined:(1)an early prograde metamorphic stage(M1)which characterized by high Mg~#and low Ti biotite inclusions in garnet yield P-T range of 650°C and 6 kbar,and the mineral assemblage is Bt+Mus+Grt+Sil+Q+fluid;(2)a near-peak high amphibolite facies stage(M3)yield P-T range of 710-720°C and 5-6 kbar with mineral assemblage of Bt+Grt+Sil+Kfs+Q+liquid;(3)retrograde cooling stage(M3)is featured by retrograde reaction of muscovite breakdown yield P-T range of 640°C and 4-5 kbar with mineral assemblage of Bt+Mus+Grt+Pl+Sil+Q+fluid.These three stages constitute a typical clockwise P-T path.Numerical modeling results of garnet show that the Carlson(2006)model which experienced 15 Ma metamorphic at the near peak temperature is perfect match with the analysis results.This indicates that the metapelites experienced high amphibolite metamorphism last up to 15 Ma.Zircon U-Pb geochronology study results define the maximum depositional age of~600 Ma of the protolith of metapelites within the Xinghuadukou Complex and document the~500 Ma Pan-African metamorphism.In summary,the Xinghuadkou Complex in Lulin Forest experienced high amphibolite facies metamorphism during the pan-African period of~500 Ma and could be related to the subduction-collision and exhumation processes.