ELECTRON-MICROPROBE DATING OF MONAZITES FROM HIGH-GRADE GNEISSES AND PEGMATITES OF THE KERALA KHONDALITE BELT, SOUTHERN INDIA

Monazites of five samples tone leptynitic garnet-biotite gneiss, one k hondalite, one augen gneiss and two pegmatites) from the central and n orthern part (Ponmudi Unit) of the Kerala Khondalite Belt (KKB) in sou thern India were analyzed with the electron microprobe dating techniqu e. Monazites in the augen gneiss and the pegmatites yield grain sizes between 200-800 mu m, Th abundances are rather low (< 10 wt.%) and the distribution of Th, U and Pb within single grains is fairly homogeneo us. Contrasting to this, monazites in the leptynitic gneiss and the kh ondalite are small (< 150 mu m). They often display very complex Th-U- Pb patterns and contain high Th concentrations up to 20 wt.%. The stat istical treatment of individual ages from the investigated samples rev ealed three populations of Lower Proterozoic (similar to 1.9 Ga), Uppe r Proterozoic (similar to 580 Ma) and Ordovician age (similar to 470 M a) as well as Mid Proterozoic ages between 0.8-1.7 Ga which are not re garded to be of geological significance. Lower Proterozoic ages are pr eserved in the cores of monazites from leptynitic gneisses and khondal ites. They fairly agree with Sm-Md model ages for similar rocks of the KKB and give a minimum age for first monazite growth or complete homo genization. The prominent Pan-African population with mean values betw een 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 granit ic pegmatites subsequent to the Pan African high-grade metamorphic eve nt. There is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning t he presence of Lower Proterozoic ages. While these are abundant in the former, often rimmed by Upper Proterozoic ages, they are completely a bsent in the latter. It appears unlikely that Lower Proterozoic ages w ere completely reset during a Pan-African event exclusively in the aug en gneisses while they were preserved in leptynitic gneisses and khond alites. It is further concluded that the augen gneisses are of magmati c origin and were derived from porphyritic granites. Thus, the Upper P roterozoic age of 605 +/- 37 Ma calculated for the investigated sample approximates the time of magma emplacement, which slightly precedes t he peak stage of Pan-African high-grade metamorphism in the KKB, and o f monazite crystallization from the granitic melt. A characteristic fe ature of the investigated monazites is the resetting of Lower Proteroz oic and Pan-African ages to significantly younger values due to pal-ti al Pb loss. Monazites not shielded by other minerals (e.g., garnet) su ffered selective mobilization of Pb along fractures or at their rims w hile Th and U concentrations remained almost unchanged. The results pr esented 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 degrees C only had minor influence on th e Th-U-Pb composition in monazite and that the closure temperature for this system must br significantly higher than previously assumed ( si milar to 750 degrees C). (C) 1998 Elsevier Science B.V. All rights res erved.