Geochronology, Oxidization State and Source of the Daocheng Batholith, Yidun Arc: Implications for Regional Metallogenesis

• Main Authors: Rui-Gang Zhang, Wen-Yan He, Xue Gao
• Format: Article
• Language: English
• Published: MDPI AG 2019-10-01
• Series: Minerals
• Subjects: zircon u–pb dating; hf isotope; trace elements of zircons; oxygen fugacity; daocheng batholith; the yidun arc
• Online Access: https://www.mdpi.com/2075-163X/9/10/608

The Daocheng batholith consists of granite, granodiorite and K-feldspar megacrystic granite, which is located in the north Yidun Arc. It is a barren batholith in contrast to plutons of the same age that contain major copper deposits, such as Pulang to the south. In the Daocheng, abundant mafic microgranular enclaves (MMEs) mainly developed within granodiorite and K-feldspar megacrystic granite, which are characterized by quenched apatite, quartz eyes and plagioclase phenocrysts. LA-ICP-MS zircon U&#8722;Pb dating of host granodiorite yielded ages ranging from 223 Ma to 210 Ma, with a weighted mean of 215.3 &#177; 1.8 Ma. Zircons from MMEs yielded ages ranging from 218 Ma to 209 Ma, with a weighted mean of 214.2 &#177; 1.4 Ma. Geochemical analyses show that granodiorite is high-K, calc-alkaline and I-type, with SiO₂ contents ranging from 67.90% to 70.54%. These rocks are metaluminous to marginally peraluminous (A/CNK = 0.98&#8722;1.00) and moderately rich in alkalis with K₂O ranging from 3.28% to 4.59% and Na₂O ranging from 3.18% to 3.20%, with low MgO (1.08%&#8722;1.29%), Cr (12.7 ppm&#8722;16.8 ppm), Ni (5.19 ppm&#8722;6.16 ppm) and Mg^# (35&#8722;49). The MMEs have relatively low SiO₂ contents (56.34%&#8722;60.91%), higher Al₂O₃ contents (16.06%&#8722;17.98%), higher MgO and FeO abundances and are metaluminous (A/CNK = 0.82&#8722;0.83). The MMEs and host granodiorite are enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with slightly negative Eu anomalies, and enriched in Th, U and large ion lithophile elements (LILEs; e.g., K, Rb and Pb), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti), showing affinities typical of arc magmas. The zircon &#949;_Hf(t) values (&#8722;6.28 to &#8722;2.33) and ancient two-stage Hf model ages of 1.92 to 1.25 Ga, indicating that the magmas are generally melts that incorporated significant portions of Precambrian crust. The relatively low silica contents and high Mg^# values of the MMEs, and the linear patterns of MgO, Al₂O₃ and Fe₂O₃ with SiO₂ between the MMEs and host granodiorite, showing the formation of MMEs are genetically related to magma mixing. The Daocheng granodiorite is characterized by much lower zircon Ce⁴⁺/Ce³⁺ (average of 3.53) and low <i>f</i>O₂ value (average of ∆FMQ = &#8722;10.84), whereas the ore-bearing quartz monzonite porphyries in the Pulang copper deposit are characterized by much higher zircon Ce⁴⁺/Ce³⁺ (average of 52.10) and high <i>f</i>O₂ value (average of ∆FMQ = 2.8), indicating the ore-bearing porphyry intrusions had much higher <i>f</i>O₂ of magma than the ore-barren intrusions considering that the high oxygen fugacity of the magma is conducive to mineralization.