Int J Earth Sci (Geol Rundsch) (2004) 93:13–22 DOI 10.1007/s00531-003-0376-1 ORIGINAL PAPER Ingo Braun · Michael Bröcker Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt, southern India: implications for Late Proterozoic crustal evolution in East Gondwana Received: 14 February 2003 / Accepted: 30 August 2003 / Published online: 23 January 2004 # Springer-Verlag 2004 Abstract Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khon- dalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak- metamorphism. Subsequently, leucogranites and leuco- tonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma em- placement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 0C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous asser- tions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic. Keywords U-Pb dating · Monazite · Southern India · Gondwana · Granulite-facies Introduction The Kerala Khondalite Belt (KKB) at the southern tip of India is a fragment of East Gondwana. Based on geochemical investigations, Chacko et al. (1992) con- cluded that the KKB represents a large supracrustal unit of predominantly high-grade metamorphic rocks, which were derived from sedimentary or volcanic protoliths. Geochronological studies have shown that the metamor- phic history is closely related to the Pan-African orogeny and that peak-metamorphism occurred ca. 550 Ma ago (Buhl 1987; Soman et al. 1995; Bartlett et al. 1998; Ghosh 1999). It was long believed that the KKB was not affected by major magmatic activity during this orogenic cycle, with the exception of some small and volumetrically subordinate syenite, leucogranite or pegmatite intrusions (Miller et al. 1997; Kovach et al. 1998). However, in the last years detailed field work showed that porphyritic granites, now present as coarse-grained granitic garnet- biotite gneisses, represent a major lithological component in the northern part of the KKB (Braun et al. 1998). Field observations clearly indicate that the granitoids were emplaced into the crystalline basement of the study area prior or contemporaneous to the penetrative Pan-African deformation. The focus of this contribution is on the geochronology of these magmatic rocks. Conventional U- Pb dating by means of thermal ionization mass spec- trometry (TIMS) and age determinations with the electron microprobe (EPMA) were carried out on monazites from granitic gneisses, leucogranites and a leucotonalite in order to address the following questions: Is it possible to place time constraints on the different stages of granitic magmatism? Is there any regional variation in the timing of magmatism? Is it possible to correlate the timing of magmatic and/or metamorphic processes in the KKB with other fragments of East Gondwana? I. Braun ( ) ) Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany e-mail: [email protected] Tel.: +49-228-733752 Fax: +49-228-732763 M. Bröcker Institut für Mineralogie, Zentrallabor für Geochronologie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany
Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt, southern India: implications for Late Proterozoic crustal evolution in East Gondwana
Jun 23, 2023
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