Metamorphic Anatexis And Tectonic Significance Of Early Precambrian Garnet Granite In The Northern Margin Of North China Craton

The Daqingshan-Jining-Liangcheng area that locates in the central-eastern segment of Khondalite Belt of Northern margin of North China Craton?NCC?,is a typical exposure of the Paleoproterozoic early precambrian basement,where paleoproterozoic khondalite belt and garnet bearing granite are widespread,there are relatively few Daqingshan supracrustal rocks,gabbro and charnockite.The extensive exposure of anatetic garnet bearing granite in the khondalite be makes it as a natural laboratory for the study of anatexis.On the basis of detailed field geological investigations,this paper carried out SHRIMP zircon U-Pb dating,Electronic probe mineral geochemical analysis and whole-rock geochemical study on the Paleoproterozoic basement rocks in Daqingshan-Jining-Liangcheng area.The mainly achievements are as follows:1)The surrounding rocks of anatectic garnet granite consists of Daqingshan supracrustal rocks?sillimanite-cordierite?garnet-biotite gneisses which are closely symbiosis and intertwined with each other in space.There are also many different size supracrustal inclusions in the garnet bearing granite and similar geochemistry characteristics to each other,described as a certain family resemblanc.?sillimanite-cordierite?garnet-biotite gneisses,anatetic?sillimanite-cordierite?garnet-biotite gneisses and garnet granite are successively observed in a geologic section of Hademengou in Daqingshan area.Zircon U-Pb dating results show that the age of the metamorphic origin zircon is 2367±8Ma in?sillimanite-cordierite?garnet-biotite gneisses and anatetic origin zircon is 2434±6Ma in garnet granites,indicating the metamorphic anatexis of garnet granites take place in 2.45-2.37Ga,which is related to the tectonic thermal event at2.45-2.37Ga.The zircon samples of the Pingfanggou charnockite obtain the anatexis ages at 2466±8Ma.Based on the previously study,it is considered that the anatexis of garnet granite and Pingfanggou charnockite may occur in the same tectonic thermal event at2.43-2.37Ga,and it is related to the regional basic magmatic underplating event.2)Quantitative phase equilibria modeling studies show that the?sillimanite-cordierite?garnet-biotite gneisses and garnet granites contain three distinct metamorphic assemblages during the tectono-thermal event.The core of a garnet porphyroblast in the?sillimanite-cordierite?garnet-biotite gneisses and garnet granites,along with fine-grained inclusions of Biotite+Plagioclase+K-feldspar+Quartz,defines the prograde metamorphic?M₁?stage.The peak metamorphic?M₂?stage assemblage consists of Garnet?mantle?+Biotite+K-feldspar+Plagioclase+Quartz+Sillimanite+Ilmenite+Melt.Peak metamorphism was followed by a Post-peak near-isothermal decompression and cooling?M₃?the development of coronae of Garnet+Biotite+Cordierite+Plagioclase+Quartz+Melt?M₃?in the?sillimanite-cordierite?garnet-biotite gneisses during the following garnet breakdown reactions:Gt+Sil+Melt?Crd+Bio.Cordierite is associated with a large number of plagioclase and contains fine-grained quartz inclusions in cordierite,through the dehydration and melting reaction of biotite as follows:Bio+Sil+Qtz+Pl?Crd+Melt.Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature?P–T?trajectories from 650–750°C/5.3–8.9kbar?M₁?up to 800–830°C/9.8–11.2 kbar?M₂?,to 760–830°C/5.0-6.0 kbar?M₃?,forming a P-T path with clockwise characteristics.Based on the previously study,it is considered that the anatexis occurred at peak metamorphic stage and post-peak near-isothermal decompression of the clockwise P-T path of the?sillimanite-cordierite?garnet-biotite gneisses,mainly the decompression melting under granulite facies involves K-feldspar melting and biotite dehydration melting.3)Using the U-Pb dating of zircon by SHRIMP,three porphyritic medium-fine-grained garnet granite samples from Jining Liang area yielded metamorphic anatectie ages at 1.90-1.93Ga,three Alumina-rich gneiss samples from Jining Liang area yielded metamorphic ages at 1.89-1.92Ga.Combined with previous study,the results show that the metamorphism anatexis of garnet granite occurred in 1.90-1.93Ga,and there was also a 1.90-1.92Ga age of metamorphism tectono-thermal event.One large-porphyritic garnet granite sample from Liangcheng yielded metamorphic anatectie ages at 1915±5Ma,and two fine-grained garnet granite obtained metamorphic ages 1909±6Ma and inconsistent line ages at 1911±23Ma,one orthopyroxene-bearing orthosite gneiss sample obtained metamorphic ages at 1925±6Ma.In conclusion,there is at least one stage of 1.90-1.93Ga tectonic event in Jining Liangcheng area,wiht corresponding to a stage of anatectic garnet granite.Comparative with the geochemistry and geochronology of porphyritic medium-fine-grained garnet granite,large-porphyritic garnet granite and fine-grained garnet granite shown that the anatexis was closely related to the underplating of basic magma in a post-collisional extensional tectonic setting at 1.96-1.90Ga.4)The garnet granite shows similiar geochemical characteristics of the source rocks,but with some variations,such as:strong depletion of high field strength elements Nb,Ta,P and Ti,and inriched in large ion lithophile element Ba.Showing variations in Eu anomalies,as indicated by Eu-enriched and Eu-depleted patterns,which is considerd to be a prominent feature of garnet granite of anatectic origin and may be related to the relatively high content of plagioclase and low content of garnet in Eu enriched samples.The characteristics of Eu anomaly have become one of the most typical features of these small-scale anatectic rocks.Upwarping of the tail of the REE distribution pattern curve is due to the influence of the minerals such as garnet,zircon and apatite content.The occurrence of anatexis has certain selectivity to the source rocks,and the silicic endmember may be more easily to anatexis.The inheritance of the source rocks in geochemistry characteristicsis is related to the origin of the In-situ and nearly In-situ formation,and the anatectic melting can keep the composition characteristics to their source rock as soon as possible,and without the redistribution of elements.However,the difference geochemistry characteristicsis of the anatectic granite and their source rocks suggest that the convergence of anatetic melting is related to the In-situ,nearly In-situ,semi In-situ and far semi In-situ migration.5)According to the characteristics of petrology,petrography,geochemistry,geochronology,and the migration distance of the anatectic melt,these anatectic granites in the khondalite belt in the northern margin of the NCC can be divided into In-situ,nearly In-situ,semi In-situ and far semi In-situ garnet granites.These In-situ to far semi In-situ garnet granites are characterized by the gradually simplification of local rock types,indicating that the rock composition is gradually uniform,and the differences between them and their source rocks are gradually enhanced.The morphology of zircon in cathodoluminescence image and the ages obtained from these ziorcons are similar to their source rocks.It is believed that the garnet granites in these area are the product of the source rocks under different degrees of anatexis,and the melt migration in different distances.The results show that the garnet granite is mainly the partial melting of the metamudstone and metagreywacke,which are consistent with the characteristics of its source rock,Daqingshan supracrustal?sillimanite-cordierite?garnet-biotite gneisses?The source rock of Hadmengou garnet granite?/khondalite series Aluminum-rich gneiss?The source rock of Jining Liangcheng garnet granite?.The occurrence of anatexis has certain selectivity to the source rocks,and the silicic endmember may be more easily to anatexis.6)Based on the existing achievement,the Daqingshan-Jining-Liangcheng garnet granite in NCC formed as the following evolution process:at².45-2.37Ga,the underplating and upwelling of the mantle derived basic magma in the Daqingshan area provided a hot spot,and the anatexis occurred in the?sillimanite-cordierite?garnet-biotite gneisses forming the Hademengou garnet granite by In-situ and nearly In-situ migration of anatectic melt.The Pingfanggou charnockite formed by the In-situ and semi In-situ migration of anatetic melt which separation from residual body and residual mineral phase.At 1.96-1.92Ga,with the underplating and upwelling of mantle derived basic magma in the Jining-Liangcheng area,and the anatexis of the alumina-rich gneiss of khondalite in the middle-lower crust formed a large-scale anatectic melt.At 1.92-1.90Ga,The In-situ crystallization and diagenesis of the anatectic melt,almost no separation between the residue and the melt,resulted in the formation of In-situ garnet granite located in the anatectic Aluminum rich gneiss.the porphyritic medium-fine-grained garnet granite was originated from the crystallization of the In-situ and nearly In-situ migration of the anatectic melt after the preliminary separation of the residual mineral and melt.Because of the fully contacting with the basic magma,a part of the anatectic melts changed their composition and formed the large-porphyritic garnet granite magma after the semi In-situ migration,with the most residual mineral is separated from the melts.The composition uniformity fine-grained garnet granite formed by crystallization fractionation or extraction of the far semi In-situ migration of the anatectic melt,with the almostly complete separation of residue body and melt.