The Granulite-facies Metamorphism And Its Dynamic Implications Of The Mashan Complex In The Jiamusi Massif

The Mashan Complex in the southeastern margin of the Jiamusi Massif has traditionally been interpreted to be the oldest stratigraphic sequence in eastern NE China.The nature and age of the Mashan Complex(or Mashan Group)in northeast China is key for determining the Precambrian geological evolution and origins of the Jiamusi Massif.Here we present petrological,mineralogical chemistry,geochemical analysis,geothermobarometry,phase equilibria modelling,in addition with zircon U–Pb dating of the Mashan Complex from the Liumao,Ximashan,and Shangsanyang area,Heilongjiang Province,China.Four aspects of achievements are obtained,i.e.,(1)estimate the granulite-facies metamorphic P–T conditions of different stage and plot the P–T–t path;(2)limit the metamorphism timing from peak to retrogradeof the granulite-facies metamorphism,as well as the depositional age of the supracrustal rocks within the Mashan Complex;(3)determine the relationship between the early Paleozoic magmatism and the Late Pan-African high-grade metamorphism;and(4)discuss the formation mechanism of the Mashan Complex and the tectonic attribute of the Jiamusi–Khanka Massif,and accordingly propose a tentative spatio-temporal evolution model for the Jiamusi/Khanka/Bureya massifs.1.Metamorphism of granulite facies in the Liumao areaThe metamorphic rocks of the Mashan Complex in the Liumao area consist mainly of two groups:granulite-facies Crd–Sil–Grt–Kfs–Pl and Grt–Opx–Bi–Amp–Pl gneisses;amphibolite-facies Sil–Grt–Bi,Sil–Grt,and Grt–Bi gneisses.Three metamorphic assemblages(M₂–M₄)were identified in the Crd–Sil–Grt–Kfs–Pl gneisses.The metamorphic P–T conditions of the peak granulite-facies stage(M₂),the post-peak decompression stage(M₃),and the final cooling stage(M₄)assemblages in the pelitic granulite were estimated as 9.2 kbar/845–865°C,4.7–5.1 kbar/820–865°C,and 4.0–4.8kbar/660–720°C,respectively.Similar results were obtained for the felsic granulite,with the metamorphic P–T conditions of the peak assemblage(M₂)evaluated as 9.0–9.2kbar/845–870°C.2.Metamorphism of granulite facies in the Ximashan areaThe metamorphic rocks of the Mashan Complex in the Ximashan area are mainly composed of four types:Spl-bearing Sil–Cord–Grt–Kfs–Pl gneiss,Spl-bearing Grt–Sil–Kfs–Pl gneiss,Spl-bearing Cord–Grt–Sil–Kfs–Pl gneiss and Grt-bearing Kfs–Pl gneiss.Three metamorphic assemblages(M₂–M₄)were recognized in the granulite-facies metamorphic rocks:a peak granulite-facies stage(M₂:?8 kbar/835–850°C),a post-peak near-isothermal decompression stage(M₃:?4 kbar/775–835°C),and a late near-isobaric cooling stage(M₄:4.5–5.3 kbar/730–775°C).3.Metamorphism of granulite facies in the Shangsanyang areaThe metamorphic rocks of the Mashan Complex in the Shangsanyang area are mainly three types:Grt–Sil–Cord–Bi–Kfs–Pl,Grt–Sil–Kfs–Pl,and Grt–Sil–Bi–Kfs–Pl gneisses.Four metamorphic assemblages(M₂–M₄)were recognized in the Grt–Sil–Cord–Bi–Kfs–Pl gneisses:a prograde metamorphism(M₁),a peak granulite facies metamorphism(M₂:6.5–8.5 kbar/815–825°C),a post-peak decompression stage(M₃:5.3–5.7 kbar/780–795°C),and retrograde stage(M₄:4.5–5.3 kbar/730–775°C).4.Metamorphic evolution of the Mashan ComplexIt is found that the peak metamorphic grade of the Mashan Complex in Liumao,Ximashan and Shangsanyang areas of the Jiamusi Massif all reached granulite facies.These metamorphic stages imply a collisional orogenic process with typical clockwise P–T paths with post-peak near-isothermal decompression(ITD)and late near-isobaric cooling segments.According to the results of the P–T condition estimation,it can be preliminarily inferred that the metamorphism intensity of the Mashan Complex in the Jiamusi Massif gradually weakened from northeast to southwest(Liumao?Ximashan?Shangsanyang).5.Geochemical and chronological characteristics of the Mashan ComplexThe protoliths of the metapelites from the Mashan Complex were clay rocks and sandstones,whose provenance is dominantly felsic rocks which formed from a continental island arc.Zircon U–Pb geochronology shows that the protolith of the Mashan Complex deposited during the Early Neoproterozoic(?898 Ma),and the earliest time might be Paleoproterozoic(?1927 Ma).The peak,post-peak near-isothermal decompression,and final cooling stages of the granulite-facies metamorphism in the Jiamusi Massif might have occurred at 530?500 Ma,530?490 Ma,and 476?453 Ma,respectively.These rocks have been intruded by syn-(530?500 Ma)and post-tectonic(490?476 Ma)granitic rocks.6.Tectonic affinity of the Jiamusi MassifTaking into account of the above research results,published regional geological data,and global late Pan-African metamorphic terranes correlations,it is inferred that the Jiamusi and Khanka massifs should be regarded as forming part of a contiguous massif from the late Neoproterozoic time.In summary,the Jiamusi–Khanka Massif cannot be directly derived from the North China Craton,South China Craton,and Tarim Craton.Instead,it might be a rifted portion of the Altai-Sayan–Baikal orogen and derived from the southern margin of the Siberia Craton during the late Pan-African period.