Zircon and Monazite as Geochronometers

Abstract

The paper discusses the results of U-Pb isotope dating of monazites and zircons from granitoids formed under PT conditions of granulite and amphibolite facies and gneisses, and crystalline shales that are like xenoliths in these granitoids of the Ukrainian Shield. In some cases, such as the Berdychiv-type granitoids and in the Zhezhelivsky quarry, the age values of monazites and outer edges of zircons coincide well; in others, such as the granites of the Ivanivsky quarry, they differ by more than the error. In the Odessa quarry, even greater differences were found between the age of zircon (1.95-1.96 ± give errors Ga) from mafic granulite, and the age of zircon (1.99 Ga) and the age of monazite (1.89-1.83 ± Ga) of a crosscutting enderbite. The U-Pb isotope system of monazite from a vein of biotite granite common in this quarry does not meet the conditions of the Ahrens-Wetherill model. The 207Pb / 206Pb age of monazite ranges from 1909.5 to 1867.0 ± 5 Ma, i.e. by more than 40 Ma. For the Sabarivsky quarry, significant differences in the 207Pb / 206Pb age values (2015.8-1984.8 Ma), which significantly exceed the analytical error, do not allow the use of the concordia U-Pb diagram to interpret the analytical data obtained for zircons from the enderbite. It is concluded that monazite is the better mineral geochronometer, compared to zircon, to determine the age of palingenetic-anatectic granitoids. Unlike zircon, which usually inherits the relict nuclei of parent rocks, relict nuclei in monazites are an exception. However, monazite is not an ideal mineral geochronometer for determining the time of endogenous geological processes. Since the crystallization of monazite is depends on the activity of calcium in the environment, in the same rock association in rocks of different composition, monazite may crystallize at different times. The combination of radiometric studies of zircon and monazite, together with zoning of their crystals, provides ample opportunities to determine the time and duration of endogenous geological processes.