Textural, chemical and age variation in monazites of the Paleoproterozoic Longstaff Bluff Formation, central Baffin Island, Nunavut.

摘要

Textural and chemical characterization and U-Th-Pb chemical dating were carried out on monazite from upper greenschist to granulite facies rocks of the Paleoproterozoic (ca. 1.92--1.90 Ga) Longstaff Bluff Formation (LBF) using an electron microprobe (EMP).; In the LBF, monazite appears early in the prograde metamorphic sequence. Upper greenschist rocks contain small but widespread metamorphic monazite. The abundance and size of LBF monazite constantly increase with metamorphic grade. The transition from the Sil-Kfs zone to the migmatite zone coincides with a significant increase in size and abundance of monazite.; Chemical data show that metamorphic grade has a strong influence on monazite composition. HREE, Ca, Si, Th, U, and Y increase with metamorphic grade, while LREE abundance decreases.; Electron microprobe U-Th-Pb chemical dating of monazite allowed recognition of 6 different age populations within the LBF monazites. Results correlate very well with available U-Pb isotopic ages.; The significance of E1 is not clear, although it is similar in age with the emplacement of granite at 1897+7/-4 Ma and ca. 1877--1874 Ma monazite ages obtained from by TIMS analysis. E2 likely corresponds to development of an S1 fabric and predates growth of cordierite. (Abstract shortened by UMI.)