Geochronology,Petrogenesis and Tectonic Setting of the Late Jurassic I-type Granites in the North Qinling Orogenic Belt,Central China

The North Qinling Orogenic Belt(NQOB)is a composite orogenic belt in central China.It started evolving during the Meso-Neoproterozoic period and underwent multiple stages of plate subduction and collision before entering intra-continental orogeny in the Late Triassic.The Meso-Cenozoic intra-continental orogeny and tectonic evolution had different responses in various terranes of the belt,with the tectonic evolution of the middle part of the belt being particularly controversial.The granites distributed in the Dayu and Kuyu areas in the middle part of the NQOB can provide an important window for revealing the geodynamic mechanisms of the NQOB.The main lithology of Dayu and Kuyu granites is biotite monzogranite,and the zircon U-Pb dating yielded intrusive ages of 151.3±3.4 Ma and 147.7±1.5 Ma,respectively.The dates suggest that the biotite monzogranite were formed at the end of the Late Jurassic.The whole-rock geochemistry analysis shows that the granites in the study areas are characterized by slightly high SiO_(2)(64.50-68.88 wt%)and high Al_(2)O_(3)(15.12-16.24 wt%)and Na_(2)O(3.55-3.80 wt%)contents.They are also enriched in light rare earth elements,large ion lithophile elements(e.g.,Ba,K,La,Pb and Sr),and depleted in high field strength elements(HFSEs)(e.g.,Ta,Nb,P and Ti).Additionally,the granites have weakly negative-slightly positive Eu anomalies(δEu=0.91-1.19).Zircon Lu-Hf isotopic analysis showedε_(Hf)(t)=-6.1--3.8,and the two-stage model age is T_(2DM(crust))=1.5-1.6 Ga.The granites in the study areas are analyzed as weak peraluminous high-K calc-alkaline I-type granites.They formed by partial melting of the thickened ancient lower crust,accompanied by the addition of minor mantle-derived materials.During magma ascent,they experienced fractional crystallization,with residual garnet and amphibole for a certain proportion in the magma source region.Comprehensive the geotectonic data suggest that the end of the Late Jurassic granite magmatism in the Dayu and Kuyu areas represents a compression-extension transition regime.It may have been a response to multiple tectonic mechanisms,such as the late Mesozoic intra-continental southward subduction of the North China Craton and the remote effect of the Paleo-Pacific Plate subduction.

Zircon U-Pb ages and Hf isotopic characteristics of the Dehe biotite monzonitic gneiss pluton in the North Qinling orogen and their geological significance

The Dehe granitic pluton intruded the Xiahe Group which is in the core complex of the North Qinling Orogenic Belt(NQOB).It shows gneissic bedding and possesses typical S-type granite minerals such as muscovite and garnet.LA-ICP-MS U-Pb isotopic dating of the Dehe granite yielded a weighted average age of 925±23 Ma which represents the emplacement age of the pluton.Most of the εHf(t) values are negative,and the two-stage model ages are consistent with the age of the Qinling Group.The isotope data show that the Dehe granite was formed in the following geological setting:in the syn-collision setting of the NQOB in the Neoproterozoic,crustal thickening induced partial melting of materials derived from the Qinling complex,and then the maga upwelled and intruded into the Xiahe Group.The formation of the Dehe S-type granite implied the occurrence of a convergent event in the QOB during the Neoproterozoic.

The Paleozoic Huoshenmiao iron skarn deposit in the Tongbai area of North Qinling Orogen,China:Insights from garnet U-Pb dating and geological constraints

The Qinling Orogen is a composite orogenic belt that can be subdivided into the North and South Qinling,broadly separated by the Shangdan suture zone.These two orogenic belts were generated by subduction-collisional processes in the Early Paleozoic and Late Triassic,respectively.During the Late Jurassic to Early Cretaceous,the eastern portion of the Qinling Orogen was tectonically reactivated due to westerly subduction of the Izanagi plate underneath the East China continental margin.The Qinling Orogen is well-endowed with numerous Au,Mo,Ag-Pb-Zn deposits that predominantly formed in the Late Triassic to Early Cretaceous,with rare Paleozoic varieties documented.In this study,we present garnet and zircon U-Pb dating results to show that the Huoshenmiao iron skarn deposit in the Tongbai area of North Qinling orogenic belt formed in the Early Silurian and is genetically related to subduction-related magmatism.The Huoshenmiao deposit consists of lenticular and stratiform orebodies that contain massive or densely disseminated magnetite variably associated with garnet,diopside,epidote,hornblende,and actinolite.Garnets from iron ores are andradite and grossular in composition(Ad_(83.1)Gr_(13.3)-Ad_(86.5)Gr_(10.2))and characterized by enrichment in light rare earth elements(ΣLREE=57.85-103.82 ppm)and depletion in heavy rare earth elements(ΣHREE=5.50-11.34 ppm),with significantly positive Eu and Ce anomalies(δEu of 1.09-1.89 andδCe of 1.39-1.69).These compositional signatures are distinctly different from those of garnets in the ore-hosting metamorphic rocks that are typically dominated by almandine,spessartine and grossular(Al_(47.4)Sp_(30.4)Gr_(13.8)-Al_(51.9)Sp_(24.8)Gr_(17.6)),depleted in LREE(0.14-0.69 ppm),enriched in HREE(95.68-841.60 ppm)and have pronounced negative Eu anomalies(δEu=0.24-0.51).In addition,garnets from iron ores of the Huoshenmiao deposit contain abundant daughter mineral-bearing,multiphase fluid inclusions,further confirming their hydrothermal origin.Two samples of those hydrothermal garnets yield U-Pb dates of 437±9 Ma and 437±10 Ma(2σ),revealing a Paleozoic mineralization event that has long been ignored and rarely documented.These garnet dates agree well with zircon U-Pb dates of 438±4 Ma to 436±3 Ma for a gabbroic diorite and 430±4 Ma for a granite intrusion in close proximity of the mine,supporting a possible genetic link between the iron mineralization and Early Silurian magmatism.The Paleozoic intrusions are enriched in large ion lithophile elements(LILEs:Ba,K,Sr)and LREEs,depleted in high field strength elements(HFSEs:Nb,Ta,P,Ti),have whole-rock(^(87)Sr/^(86)Sr)i,ε_(Nd)(t),and zirconεHf(t)values of 0.7039-0.7042,3.32-4.33,and 13.0-14.9,respectively.These geochemical and isotopic characteristics suggest that the Paleozoic intrusions were affiliated with arc magmatism triggered by subduction of the Shangdan oceanic plate in the Early Paleozoic.Recognition of the Silurian Huoshenmiao iron skarn deposit opens a new window for exploration of Paleozoic mineral resources in the Tongbai area and other portions of the North Qinling Orogen.