Metamorphism And Chronology Of The Dengfeng Complex In The Songshan Area,Southern Margin Of The North China Craton

The Dengfeng Complex,located in the southern section of the North China Craton(NCC),is composed of the tonalite-trondhjemite-granodiorite(TTG)gneisses,metavolcano-sedimentary rocks,metadiorites,and felsic-mafic intrusions.In this contribution,detailed metamorphic evolution has been carried on the garnet-bearing metapelites,and the geochronological of metapelites and amphibolites in the Songshan area is studied.Petrological studies indicate that garnet porphyroblasts show obvious growth zoning from the core to inner rim and they are separated by a prominent chemical discontinuity narrow edge.Garnet-bearing metapelites preserve two generations of metamorphic mineral assemblages,the prograde mineral assemblages(M1)are mainly composed of plagioclase + chlorite + quartz + ilmenite,preserved as mineral inclusions within the garnet porphyroblasts.The metamorphic peak mineral assemblages(M2)are mainly composed of garnet porphyroblasts without the narrow and bright edge and the matrix mineral assemblages including muscovite + plagioclase + quartz + chlorite ± biotite.By pseudosection modeling calculations,the P-T conditions of sample 20DF17-C determined by the isopleths of XGrs and XPrp of the garnet core and inner rim are ~530 °C/6.7 kbar and 560 °C/5.1 kbar,respectively;while the P-T conditions of sample 20DF24 determined by the isopleths of XGrs and XPrp of garnet core and rim are ~530 °C/~5.2 kbar and 550 °C/4.5 kbar,respectively.The garnet porphyroblasts in the metapelites of the Dengfeng Complex preserve the clockwise P-T paths containing decompression with slight heating segment.Through detailed microscopic observation,the garnet-bearing amphibolites can be divided into the “dark domain” and the “light domain”,and the peak metamorphic mineral assemblages of garnet + hornblende + plagioclase + epidote + quartz is recognized in the garnet-bearing amphibolites.By the traditional geothermobarometer calculations,the peak metamorphic P-T conditions of the “dark domain” and the “light domain” from the sample 20DF22 are constrained at 620 ℃/6.4 kbar and 620 ℃/8.6 kbar,respectively.The peak metamorphic P-T conditions of the “dark domain” from the sample 20DF52 are constrained at 630 ℃/6.9 kbar.The peak metamorphic P-T conditions of the “light domain” in the sample 20DF56 are constrained at 630 ℃/8.0 kbar.Laser ablation inductively coupled with plasma mass spectrometry(LA-ICP-MS)zircon U-Pb dating from the Dengfeng metapelites suggests that the maximum depositional age is~2.52 Ga.These metapelites were affected by the hydrothermal event under the conditions of fluid participation immediately after diagenesis.And the metamorphic time is dated at ~1.92 Ga.LA-ICP-MS zircon U-Pb dating from the Dengfeng metabasites indicates that the protolith age of the amphibolites is ~2.55–2.56 Ga,and the polymetamorphic ages are limited at ~2.51–2.53 Ga and ~1.94–1.87 Ga.LA-ICP-MS titanite U-Pb dating from the amphibolites suggests that the metamorphic time of the Dengfeng Complex is ~2.45–2.53 Ga.Combined with the occurrence features of the amphibolites,TTG gneisses,metapelites and quartzites,the metamorphic zircon age of ~2.45–2.53 Ga is related to the collision of the Xuchang Block(XCH)and the Qianhuai Block(QH)welded by the Dengfeng Greenstone Belt(DFGB)in the Late Archean.The metamorphic age of ~1.94–1.87 Ga indicates that the Dengfeng Complex underwent a complex and long metamorphic process.The clockwise P-T paths occurred at the Paleoproterozoic were likely related to a subduction-collision event in the NCC.We inferred that all the Dengfeng metapelites and Songshan Group involved a Paleoproterozoic orogenic event along the Trans-North China Orogen(TNCO)/Jinyu Mobile Belt(JMB).