麻豆传媒

Textural and chemical characterization, and U-Th-Pb chemical dating were carried on monazites from upper greenschist to granulite facies rocks belonging to the Paleoproterozoic (ca. 1.8-1.9 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. Abundance and size of LBF monazite constantly increase with metamorphic grade. Transition from the Sil-Kfs zone to the migmatite zone coincides with a significant increase in size and abundance of monazite. Chemical zoning is generally weak in low- to medium grade samples. Monazites from migmatitic samples are the most abundant and they are commonly zoned in U, Th and Y.

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. Spread in values at higher grade suggests that factors other than metamorphic grade also exert control on monazite chemical composition. Sharp changes in monazite composition are observed at the transition from the Sil-Kfs zone to the migmatite zone, notably a steep increase in Y content. Highly resorbed garnets are reported from this transition.

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. Detrital grains were identified. Episodes of metamorphic monazite growth were dated at ca. 1880 Ma (E1), 1850 Ma (E2), 1830 Ma (E3), 1800 Ma (E4) and 1770 Ma (E5). Chemical age data suggest the existence of a northward younging age gradient for E2 and E3 events. These two events are mainly recorded in the southernmost transects. E4 is synchronously recorded throughout the field area and E5 is mainly recorded in the northern part of the study area. Constraints from petrological setting of monazite grains and chemical mapping were used to interpret the ages.

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Supervisor: Rebecca A. Jamieson