МОНГОЛ ОРНЫ ЗАРИМ ТАЛБАЙНУУДАД ТАРХСАН МАГМЫН БОЛОН ТУНАМАЛ ЧУЛУУЛГУУДЫН ЦИРКОНЫ TH/U-НЫ ХАРЬЦАА БОЛОН ДОТООД БҮТЦИЙН СУДАЛГАА

Abstract

We investigated the zircon Th/U ratios and their internal textures from more than 60 magmatic rocks and a few sedimentary rocks which are collected from different areas of Mongolia such as the Tavt ore field, the Dzuun mod areas, and others. All these zircon
samples were photographed in SEM images, backscattered (BSE) and cathodoluminescence (CL) images that are used to examine the internal texture of the analyzed zircons and with petrographic observations guided the selection of analytical spots, using a JEOL JSM-6610LV scanning electron microscope at KBSI (Korea Basic Science Institute). High‐precision data on U and Th contents and Th/U ratios of zircon obtained using a SHRIMP IIe and a LA-MCICPMS housed at KBSI. A long experience, modern instrumentation, and techniques have provided the “zircon community” means to image and interpret preserved textures, and to decipher the history and evolution of rock. The zircon internal texture images are categorized according to their inferred genetic context. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. Th/U values in zircon are commonly used to discriminate between metamorphic
(Th/U < 0.1) and magmatic (Th/U > 0.1) origin for zircon overgrowths. We test this hypothesis in the rocks from some areas of Мongolia. Higher temperature, more-fractionated, short-duration, bimodal magmatism in extensional magmatic systems may produce highly variable and elevated zircon Th/U. In compressional magmatic systems, lower temperature, long-lived, granitoid, oxidizing melts
are more conducive to low Th/U zircon crystallization. To test the utility of this correlation, we evaluated Th/U ratios in our samples. Most of them have the low Th/U ratios between 0.1–1.0 that corresponds to overall compressional phases (i.e., terrane accretion and orogenesis).