Generation of Nb-enriched mafic rocks and associated adakitic rocks from the southeastern Central Asian Orogenic Belt:Evidence of crust-mantle interaction

Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated.Here we focus on the Early Mesozoic mafic rocks and granite porphyry,which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean.The granite porphyry samples are calc-alkaline and distinguished by high Sr contents,strong depletion of heavy rare earth elements,resulting in high(La/Yb);and Sr/Y ratios,and negligible Eu anomalies.Based on their high Na_(2)O and Mg O,low K_(2)O contents,positiveε_(Hf)(t)andε_(Nd)(t)and low(^(87)Sr/^(86)Sr)ivalues,we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust.The Nb-enriched mafic rocks are enriched in Rb,Th,U,Pb and K,and depleted in Nb,Ta,Ba,P and Ti,corroborating a subduction-related origin.Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts.Trace element and isotope modeling results and elevated zirconδ^(18)O values suggest variable subducting sediments input into the mantle wedge,dominated by terrigenous sediments.Synthesizing the widely-developed bimodal rock associations,conjugated dikes,thermal metamorphism,tectonic characteristics,paleomagnetic constraints,and paleogeographical evidence along the Solonke-Changchun suture zone,we identify a slab window triggered by slab break-off,which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.

U-Pb Geochronology, Elemental and Sr-Nd Isotopic Geochemistry of the Houyaoyu Granite Porphyries： Implication for the Genesis of Early Cretaceous Felsic Intrusions in East Qinling

The Early Cretaceous Houyaoyu granite porphyries are located in the south margin of the North China Craton. Field observations, petrography, geochronology, major and trace elemental and Sr-Nd isotopic compositions are reported to elucidate the genesis of the Houyaoyu granite porphyries. SIMS zircon U-Pb analyses for the Houyaoyu granite porphyries yield two concordant ages of 133.2±2.3 （2σ） and 131±1.1 （2σ） Ma, respectively. Major and trace elemental compositions indicate that these porphyries are high-K I-type granites with high contents of SiO2, K20, Rb, U, Pb, low Nb, Ta, Ti, and P. Initial S：Sr/S6Sr ratios range from 0.708 3 to 0.709 7, and εNd（t） values range from -9.13 to -12.3, with corresponding two-stage depleted-mantie Nd model ages （T2DM） varying from 1.57 to 1.91 Ga. This suggests that the Houyaoyu granite porphyries were predominantly derived from ancient lower conti- nental crust, with minor involvement of mantie-derived components. On the basis of the tectonic evolution of the Qinling Orogen and geochemical characteristics of the Houyaoyu granite porphyries, it is proposed that they were formed in an extensional tectonic setting related to lithospheric destruction of the North China Craton, and produced Mo and Pb-Zn mineralization in East Qinling Orogen. KEY WORDS： East Qinling, granite porphyries, ancient lower continental crust, destruction of North China Craton.