Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India

Abstract

Monazites of five samples (one leptynitic garnet–biotite gneiss, one khondalite, one augen gneiss and two pegmatites) from the central and northern part (Ponmudi Unit) of the Kerala Khondalite Belt (KKB) in southern India were analyzed with the electron microprobe dating technique. Monazites in the augen gneiss and the pegmatites yield grain sizes between 200–800 μm, Th abundances are rather low (<10 wt.%) and the distribution of Th, U and Pb within single grains is fairly homogeneous. Contrasting to this, monazites in the leptynitic gneiss and the khondalite are small (<150 μm). They often display very complex Th–U–Pb patterns and contain high Th concentrations up to 20 wt.%. The statistical treatment of individual ages from the investigated samples revealed three populations of Lower Proterozoic (∼1.9 Ga), Upper Proterozoic (∼580 Ma) and Ordovician age (∼470 Ma) as well as Mid Proterozoic ages between 0.8–1.7 Ga which are not regarded to be of geological significance. Lower Proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites. They fairly agree with Sm–Nd model ages for similar rocks of the KKB and give a minimum age for first monazite growth or complete homogenization. The prominent Pan-African population with mean values between 540 and 580 Ma is present in the leptynitic gneiss, the khondalite and the augen gneiss and in line with published isotope ages for the KKB. The Ordovician population finally marks the emplacement of granitic pegmatites subsequent to the Pan-African high-grade metamorphic event. There is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of Lower Proterozoic ages. While these are abundant in the former, often rimmed by Upper Proterozoic ages, they are completely absent in the latter. It appears unlikely that Lower Proterozoic ages were completely reset during a Pan-African event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites. It is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites. Thus, the Upper Proterozoic age of 605±37 Ma calculated for the investigated sample approximates the time of magma emplacement, which slightly precedes the peak stage of Pan-African high-grade metamorphism in the KKB, and of monazite crystallization from the granitic melt. A characteristic feature of the investigated monazites is the resetting of Lower Proterozoic and Pan-African ages to significantly younger values due to partial Pb loss. Monazites not shielded by other minerals (e.g., garnet) suffered selective mobilization of Pb along fractures or at their rims while Th and U concentrations remained almost unchanged. The results presented in this study indicate that this was mainly due to fluid-rock interaction. It is concluded that thermal diffusion of Pb even at the suggested temperatures of 900°C only had minor influence on the Th–U–Pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed (∼750°C).