Geochronology by monazite veins of granite of the old Crimea quarry (Azov Region, Ukrainian Shield)

Abstract

The purpose. The aplitoid biotite granites with traces of rheomorphic transformations lying among gneisses and crystalline schists of the Central-Priazovska series of the Ukrainian shield, as well as the anatomy of accessory crystals of zircon and monazite from these granites, were researched.

Method. The time of granite intrusion was determined by the uranium-lead isotope dating method based on multi-grain measurements of accessory monazite. The anatomy of zircon crystals was studied in artificial slices by optical microscopy methods.

The results. The age of monazites was determined by multigrain weighing based on the 207Pb/206Pb isotopic ratio. The interest in these granites is related to the fact that they form a vein that breaks through the gneisses and crystalline slates of the Сentral-Priazovska series, discovered by the Oldcrimean granite quarry. The structure of the granite is heterogeneous, with grain sizes ranging from 0.1 to 3.5 mm. Based on the ratio between mineral grains and their shapes, the structure is identified as allotriomorphic and flattened. The configuration of the grains and their relationships indicate the coexistence of relict primary magmatic structure with dynamoblastic and brittle-plastic features caused by rheomorphism. These elements include granuloblastic phenomena and the flattening of quartz crystals; granuloblastesis of primary plagioclase grains; myrmekitization of early and crystallization of newly formed feldspars; microclinization of relict orthoclase and crystallization of newly formed microcline, and deformation of biotite flakes. Currently, the granites consist of (in %) acidic plagioclase - 30, microcline + orthoclase - 25, quartz - 45, chloritized dark mica - 1-5. Secondary transformations include biotite chloritization, as well as sericitization and pellitization of newly formed plagioclase. Accessory minerals are represented by zircon, monazite, apatite, possibly magnetite, and rutile, which forms during the replacement of biotite by chlorite. Weathering minerals include clay minerals, iron hydroxides, and carbonates. The anatomy of accessory zircon and monazite crystals has been studied. According to mineralogical research, zircon crystals have a complex structure. Zircon in synpetrogenic granites grows on heterogeneous cores of relict zircon, usually in the form of thin shells. A small number of crystals without cores are also present, consisting exclusively of fine-zonal zircon shells.The monazite is represented by two varieties: large (>0.1 mm) brownish-brown, dark brown to nearly black unevenly colored opaque crystals and small light brown, brown-yellow transparent crystals.

Scientific novelty and Conclusions. The age of the granite was determined using the uranium-lead isotope method on multi-grain samples of dark opaque monazite crystals. The weighted average age, based on the ²⁰⁷Pb/²⁰⁶Pb isotope ratio, is 1978.8 ± 6 million years. The age obtained from multi-grain samples of light brown and brown-yellow transparent crystals is 1959 ± 18 million years and theoretically represents the minimum timing of rheomorphism processes.