MELT-CRYSTAL INTERACTION DURING CRUST-MANTLE MAGMA MIXING: THE ZIRCON U-PB, HF-O ISOTOPE AND MINERAL MICROANALYSIS OF GRANITOIDS AND MAFIC ENCLAVES IN KARAMAY PLUTON, WEST JUNGGAR, NW CHINA

Massive magma mixing rocks and mafic microgranular enclaves in the Karamay pluton of West Junggar, this paper study mineral microanalysis of the zircon and multiple minerals. In-situ U-Pb, Hf and O isotope in core, mantle and rim of the zircons from monzonitic granite and mafic microgranular enclaves, show δ¹⁸O values varied from 4.72‰ to 7.41‰ and 4.84‰ to 7.67‰, respectively. Combined with the morphology and structure of zircon, two styles of zircons: ①Crustal zircons, characterized by Short columnar with significantly oscillatory zoning, δ¹⁸O weighted mean in cores are respectively 6.56‰, 6.67‰, decreasing in the mantle and rim; ②Mantle zircons, characterized by Long needle zircon, the δ¹⁸O weighted mean in cores are respectively 5.32‰, 5.53‰. Inversely, ε_Hf(t) values in cores, mantles and rims fo zircons all show analogous ranges, +9.3 ～ +16.0 in monzonitic granite and +9.2 ～ +13.6 in mafic microgranular enclaves. These results indicate that Hf isotope should be more active than O isotope in the magma mixing of Karamay pluton, O isotope retained, but Lu-Hf isotope has been strongly diffused in initial phase of magma mixing , subsequently the Lu-Hf isotope unifunctional exchanged, and O isotope begin to mutual diffuse during magma mixing. The analysis of electron probe and back scattered images(BSI) of minerals shows that plagioclase with dissolution belt and reversed zoning, amphibole with pyroxene reversed zoning, pyroxene with growth zoning in host rocks, plagioclase phenocryst with dissolution border and reversed zoning, amphibole phenocryst with growth zoning and apatite with hollow structure in mafic microgranular enclaves. During magma evolution experienced high-temperature mafic magma injection at least three times, and zircons, plagioclases, amphiboles and pyroxenes intermigrationed between host granitoids and mafic enclaves due to magma mixing. In addition, we rough estimate that the ratio of crust and mantle material in monzonitic granite is about 1:10, and about 2:3 in pyroxene dioritic enclaves according to the numbers of crustal and crustal zircons zir in rocks.