Trace Element Composition Of Zircon From Myanmar Jadeitite, The Daixian Rutile Deposit And The Huangbaikeng Rhyolite Porphyry

Zircon, a common accessory mineral in nature with high resistant to alteration, acts a valuable archive of geochemical information. Benefiting from the rapid developments of mineral synthetic and in-situ analyses techniques, trace element characteristics of zircons have provided plentiful of information on melting sources, magma mixing, the temperature of diagenesis and ore formation, oxygen fugacity and so on. In this paper, trace element compositions of zircons from Myanmar jadeitite, the Daxian rutile deposit and the Huangbaikeng rhyolite porphyry were investigated in detail.The trace element composition of igneous zircon varies systematically with the trend of increasing trace element abundance in zircons from ultramafic through mafic to granitic rocks. Mantle-affinity zircons have lower trace element abundance, and the REE pattern show slight or without significant Eu-anomaly and relatively flat HREE patterns. Crust-affinity zircons have higher trace element abundance, and the REE pattern shows a steeply-rising slope torn the LREE to the HREE with an intensively positive Ce-anomaly and negative Eu-anomaly. Variations of trace element composition within single zircon grains and between populations may reflect variations of magma composition Crystallization of newly-grown zircon or modification of inherited zircons occurred under a wide range of temperature and pressure during pro grade, retrograde and peak metamorphism. Growth mechanisms include solid state recrystallization, dissolution recrystallization, and nucleation-crystallization related with Zr released from metamorphic reactions. Metamorphic zircons have complicated internal textures, and their trace element characteristics show correlation to growth mechanisms, coexisting minerals and forming environment. Hydro thermal zircon may exhibit oscillatory zoning that is also typical for igneous zircon. Fluid and hydro thermal mineral inclusion can clearly define its hydro thermal origin. Trace element composition of hydrothermal zircon reflects the features of hydrothermal mineral assemblage and the composition of fluids.Two groups of zircons from Myanmar jadeitite exhibited hydrothermal process in association with the subduction zone. Group-I zircon at-163 Ma (previously studied), showing a large positive Ce-anomaly and steep MREE-HREE pattern without a Eu-anomaly, is suggested to be typical of igneous zircon crystallizing from H2O-rich basaltic melt at relatively high pressure. Combining its high positive sHf(t) values and Ti-in-zircon temperature of 742±141℃, the presumed protolith for Group I zircon in Myanmar jadeitite was a basaltic rock associated with fluid-metasomatic mantle wedge, allowing for an assumption that enviroment for the protolith pointing to the mantle wedge. Group-Ⅱ zircon at-147 Ma shows lower total trace element concentrations, (Sm/La)N, Ce/Ce* but elevated (Yb/Gd)N, which hydrothermal zircon from other jadeitite localities also have.Trace element compositions of four types of zircon from the Daxian rutile deposit were investigated, exhibiting variable trace element features of metamorphic zircon with different forming mechanisms. Type I zircon is inherited cores with rounded shape and shows variable trace element compositions related to specific host rocks. Type II zircon with partially preserved oscillatory zoning is considered as solid-state or replacement recrystallizations of protolith zircons. Solid-state recrystallized zircons exhibit the lowest degrees of resetting and inherit geochemical features from the protolith zircons. Thus, the oldest upper intercept age could represent the time of protolith. Relative to solid-state recrystallized zircon, replacement recrystallization mainly show lower (Sm/La)N and Ce/Ce* which indicate alteration under presence of fluid. Type III zircon is bright CL rims or sealed fractures formed by dissolution recrystallization ofprotolith zircons, and show the highest degrees of reworking and have concordant U-Pb ages for the prograde metamorphism, lowered contents of trace elements. Type IV euhedral single grains are considered as metamorphically new grown zircons from the aqueous fluid. It is characterized by the lowest trace element contents and Th/U ratios. Variable REE patterns may indicate the variable composition of fluid. Solid-state recrystallized zircon plot predominately in the U/Yb versus Hf (and Y) field of modern ocean crust zircon and this result is consistent with the inference of genesis of anthophyllite geiss by hydrothermal alteration and subsequent high grade metamorphism of submarine intermediate-basic volcanic rocks.Trace element compositions of typical igneous zircon (-158Ma) from the Huangbaikeng rhyolite porphyry were investigated. Chondrite-normalized REE patterns show significant positive Ce anomalies and negative Eu anomalies, and steep MREE-HREE patterns. The positive correlations between U and Th, Nb and Ta, indicate these zircon crystallize from the cogenetic magma.Zircon data plot predominately in the U/Yb versus Hf (and Y) field of continental zircon.Using zircon oxygen fugacity empirical equation, the data points of zircon mainly fall within the field above FMQ. In the discriminant diagrams of (Sm/La)N-La and Ce/Ce*-(Sm/VLa)N, five data points plot on or near the field of hydrothermal zircons indicating hydrothermal alteration after formation.In summary, the ability to correlate internal textures, trace element signatures, isotopes and inclusions within single crystals makes zircon a useful tool in a wide range of geologic investigations.