Differences Of Petrogenesis And Metallogenesis Between The Daocheng And The Pulang Granitoids In The Yidun Arc,SW China

During the formation of the Yidun arc in the Late Triassic,a large number of granitoids and related mineralization were developed,the Yidun arc was divided into two sections by the 28°30?N,the time limit of magmatic activity,petrogenesis and tectonic setting of the north-south Yidun island arc are similar,however,the mechanism and controlling factors of regional metallogenic differences in the Late Triassic are still unclear.This paper focuses on the difference in metallogenesis of the north-south Yidun island arc in Late Triassic,the petrogenesis,magma origin and tectonic setting of the Daocheng granitoid and MMEs are discussed,magma mixing is analyzed,the physical and chemical conditions of magma formation are calculated,comparing the geochemical and isotopic characteristics of the Daocheng bareen rock and the Pulang ore-forming rock,it is revealed that the difference of oxygen fugacity in the magma source is the fundamental reason for the difference of regional metallogenesis in Late Triassic.The Daocheng batholith in the north Yidun arc was formed in subduction arc setting,which consists of granite,granodiorite and K-feldspar megacrystic granite.LA-ICP-MS zircon U-Pb dating of the host granodiorite yields ²⁰⁶Pb/²³⁸U ages varying from 223 to 210 Ma,with a weighted mean ²⁰⁶Pb/²³⁸U ages of 215.3±1.8 Ma.Zircons from the mafic microgranular enclaves?MMEs?yield ²⁰⁶Pb/²³⁸U ages ranging from 218 to 209 Ma,with a weighted mean ²⁰⁶Pb/²³⁸U ages of 214.2±1.4 Ma.Geochemical analyses showes that the granitoid belongs to high-K,calc-alkaline and I-type granite,with SiO₂ contents of 67.90%-70.54%.These rocks are metaluminous to slightly peraluminous?A/CNK=0.98-1.00?,moderately rich in alkalis with K₂O contents of 3.28%-4.59%and Na₂O contents of 3.18%-3.20%,with low MgO?1.08%-1.29%?,Cr?12.70-16.80 ppm?,Ni?5.19-6.16 ppm?and Mg^#?35-49?values.The MMEs have relatively lower SiO₂ contents?56.34%-60.9%?,higher Al₂O₃contents of 16.06%-17.98%,with are enriched in magnesium and iron,belonging to metalumnious series?A/CNK=0.82-0.98?.Furthermore,both the MMEs and the host granitoids are enriched in light rare-earth elements?LREEs?relative to heavy rare-earth elements?HREEs?,with slightly negative Eu anomalies,enriched in Th,U and large ion lithophile elements?LILEs,e.g.,K,Rb and Rb?and depleted in high field strength elements?HFSEs,e.g.,Nb,Ta,Zr,Hf,P and Ti?,showing affinities of typical arc magmas.The Zr/Hf ratios?32.69³6.90?of the host granitoid were closely to the crustal-and mantle-derived rocks?Zr/Hf=33.0³6.3?,negative zircon?_Hf?t?values?-9.1 to-2.7?and two-stage Hf model ages of 1.92 to 1.25 Ga,these features indicates that the magma genetically related to the crust-derived materials,with the input of minor mantle-derived materials.Abundant MMEs mainly developed within the granodiorite and K-feldspar granite,which are characterized by the quartz eye structure,quenched apatite and plagioclases phenocrysts,the relatively low silica contents and high Mg^#values,with the linear patterns of the MgO,Al₂O₃ and Fe₂O₃ with SiO₂ contents from the MMEs and host granitoid,indicating that the MMEs are generated by the mixing of the mafic magma with felsic magma.The formation temperature of the barren granitoid in Daocheng batholith is slightly higher than that of the Pulang ore-bearing quartz monzonite porphyry,The Dacheng barren granitoids are shallower and lower in their emplacement depth and oxygen fugacity than the Pulang ore-bearing quartz monzonite porphyry.The Daocheng granodiorite in the north Yidun arc is characterized with much lower zircon Ce⁴⁺/Ce³⁺ratio?average value of 3.53?and low fO₂ value?average of?FMQ=-10.84?,whereas the ore-bearing quartz monzonite porphyry of the Pulang copper deposit in the south Yidun arc are characterized with the much higher zircon Ce⁴⁺/Ce³⁺ratio?average value of 52.10?and high fO₂ value?average of?FMQ=2.8?.These features indicate that the ore-bearing porphyry pluton had much higher fO₂ of magma than the barren intrusions,and the high oxygen fugacity of the magma is conducive to mineralization.