| The Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt,also named the Central China Orogenic Belt(CCOB),records the processes of amalgamation between the North China Block(NCB)and the South China Block(SCB)during the Paleozoic to Mesozoic.It not only contains one of the largest and best preserved ultrahigh-pressure(UHP)metamorphic terranes in the world,but also is one of the regions with the most profound occurrence of magmatism during and after arc-continent and continent-continent collision.The Dabie-Sulu orogenic belt in the east was built by a simple continental collision between the NCB and SCB,preserving the evidence of continental deep subduction during the early Mesozoic.In contrast,the Qinling-Tongbai orogenic belt in the west is of composite property,recording a series of tectonic processes from the oceanic subduction,continental subduction and arc-continent collision in the early Paleozoic to the continent-continent collision in the early Mesozoic.During the oceanic subduction and subsequent continental subduction,various crustal materials would be transported into subduction channels,leading to interaction with the overlying mantle wedge in different ways and proportions,and then recycled back into the orogenic crust through mafic magmatism.Therefore,studying the origin of mafic igneous rocks in different stages in the composite orogens has important implications for understanding the recycling of subducted crustal materials,the nature of orogenic lithospheric mantle and the tectonic evolution of composite orogens.This study focuses on the petrology and geochemistry of early Paleozoic mafic igneous rocks in the Qinling-Tongbai orogens,the results provide new insights into the recycling of subdcuting paleo-oceanic crust,the mechanism of crust-mantle interaction,the nature of orogenic lithospheric mantle,and the tectonic evolution of this composite orogenic belt.An integrated study of geochronological and geochemical study has been performed for the mafic igneous rocks from the Fushui intrusive complex in the Qinling orogen,including in situ zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Sr-Nd isotopes.SIMS zircon U-Pb dating yields consistent late Cambrian ages of 488 ± 4 to 490 ± 4 Ma for magma emplacement.These ages are slightly later than the zircon U-Pb ages of 500 to 490 Ma for the UHP metamorphism of the North Qinling microcontinent(NQMC),indicating that the Fushui mafic igneous rocks is the product of syn-exhumation magmatism during the continent-continent collision.These mafic igneous rocks exhibit arc-like trace element distribution patterns,with enrichment in large ion lithophile elements(LILE),Pb and light rare earth elements(LREE)but depletion in high field strength elements(HFSE).They display enriched whole-rock Sr-Nd isotope compositions,with high(87Sr/86Sr)i ratios of 0.7094 to 0.7144 and negative ?Nd(t)values of-5.5 to-3.4.Zircon Hf-O isotope compositions show large variations that can be categorized into two groups.Group I zircons have low ?180 values of 2.0 to 2.9‰ and positive ?Hf(t)values of 0.1 to 1.6,indicating recycling of the oceanic basalt that underwent seawater-hydrothermal alteration at high temperature,into their mantle source.In contrast,Group ? zircons exhibit high ?18O values of 5.1 to 7.9‰ and low ?Hf(t)values of-3.7 to 0.9,suggesting involvement of the terrigenous sediment in their mantle source.Together with whole-rock geochemical features,the Fushui complex was originated from fertile,enriched mantle sources beneath the North Qinling microcontinent,which would be generated by metasomatic reaction of subducting oceanic crust-derived liquid phases with the overlying mantle wedge peridotite during subduction of the Shangdan oceanic slab at a time before the subduction of the NQMC to subarc depths for the UHP metamorphism.The model calculation results for trace elements and Sr-Nd isotopes indicate that the mantle sources can be generated by metasomatism of the mantle wedge peridotite by approximate 5-10%AOB(altered oceanic basalt)-derived aqueous solutions and 0.5-2.5%sediment-derived hydrous melts,respectively,at subarc depths,and their partial melting can produce mafic melts with trace element and Sr-Nd isotope compositions similar to the Fushui mafic complex.The melt-peridotite reaction would generate phlogopite-bearing ultramafic metasomatites beneath the NQMC.At the tectonic transition from the Late Cambrian to Early Ordovician,the metasomatites underwent partial melting to produce the mafic melts in response to extensional exhumation of the UHP metamorphic slices,giving rise to the Fushui complex in the North Qinling microcontinent.Therefore,the syn-exhumation Fushui intrusive complex in the Qinling orogen provides petrological and geochemical insights into the nature of orogenic lithospheric mantle and the tectonic evolution of the collisional orogen.A combined study of in situ zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Sr-Nd isotopes was carried out for Paleozoic mafic igneous rocks in the Tongbai orogen.SIMS zircon U-Pb dating for the mafic igneous rocks yields Early Paleozoic ages of 436 ± 4 to 449 ± 4 Ma for magma crystallization,which postdate the timing of arc-continent collision between the NQMC and the Erlangping arc,but are synchronous with the northward subduction of the Shangdan oceanic crust beneath the NQMC.These mafic rocks are characterized by arc-type trace element distribution patterns,with enrichment in LILE and LREE but depletion in HFSE,and by depleted Sr-Nd isotope compositions,with low(87Sr/86Sr)i ratios of 0.7037 to 0.7047 and positive ?Nd(t)values of 2.7 to 5.0.In addition,they also display positive zircon ?Hf(t)values of 8.9 to 14.8 and variable?18O values of 4.4 to 5.9‰.These geochemical features indicate their origination from fertile mantle sources enriched in LILE and LREE but depleted in radiogenic isotopes.The mantle sources are suggested to be part of the orogenic lithospheric mantle,which was generated by metasomatic reaction of the overlying mantle wedge with previously subducted Erlangping oceanic crust-derived fluids.These mafic igneous rocks have two groups of Ba/Th,Th/Nb,(La/Yb)N and(87Sr/86Sr)i ratios,suggesting that the metasomatic agents include both basaltic oceanic crust-derived aqueous solutions and sediment-derived hydrous felsic melts.Model calculations confirm that the geochemical compositions of the Tongbai mafic igneous rocks can be explained by reaction of the depleted mid-oceanic ridge basalt(MORB)mantle wedge peridotite with 1-10%of subducting Erlangping oceanic crust-derived aqueous solutions plus 0.05-0.1%of sediment-derived hydrous melts.This metasomatic reaction occurred during the subduction of the Erlangping oceanic crust prior to the collision between the Erlangping arc and the NQMC,generating pyroxene-rich ultramafic metasomatites.During the Late Ordovician to Early Silurian,the rollback of the subdcuting Shangdan oceanic slab and the subsequent upwelling of the asthenospheric mantle led to back-arc extension of the Erlangping arc.This caused partial melting of the metasomatites,giving rise to the Late Ordovician to Early Silurian mafic igneous rocks.Thus,the Paleozoic mafic igneous rocks in the Tongbai orogen record the recycling of subdcuting paleo-oceanic crust.In summary,the orogenic mafic igneous rocks provide petrological and geochemical records for the recycling of subdcuting paleo-oceanic crust and its associated crust-mantle interaction.The fertile,enriched mantle sources would be generated by metasomatic reaction of the overlying mantle wedge peridotite with fluids derived from the different layers of subducted oceanic crust.Partial melting of such mantle sources would give rise to the orogenic mafic igneous rocks.Resolving the origin of these mafic igneous rocks enables us to understand the crust-mantle interaction and the nature and source of involved metasomatic agents within ancient subduction channels,the nature of orogenic lithospheric mantle and the tectonic evolution of composite orogens. |
PDF Full Text Request