Evolution of the continental crust in the Kerala Khondalite Belt, southernmost India: evidence from Nd isotope mapping, U–Pb and Rb–Sr geochronology

• Published in: Precambrian research 2004-10, Vol.134 (3), p.275-292
• Main Authors: Cenki, Bénédicte, Braun, Ingo, Bröcker, Michael
• Format: Article
• Language: English
• Subjects: Earth Sciences; Kerala Khondalite Belt; Monazite dating; Nd model ages; Rb–Sr age; Sciences of the Universe; Ultra-high temperature metamorphism
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2004.06.002

In order to decipher crustal genesis and evolution as well as to refine the metamorphic history of the Kerala Khondalite Belt (KKB), southernmost India, we have applied U–Pb monazite, Rb–Sr biotite-feldspar dating and Nd isotope geochemistry. The KKB belonged to an eastern Gondwana mobile belt that underwent ultra-high temperature metamorphism at Pan-African times. It is dominated by Nd model ages ranging between 2.0 and 3.0 Ga except at its northernmost boundary (the Achankovil Unit, AU), where Middle Proterozoic Nd model ages were recognised (1.3–1.6 Ga). The new data show, that these ages are not restricted to cordierite-bearing metasediments alone but can be found in other lithologies of the Achankovil Unit as well. Peak metamorphism occurred between 590 and 550 Ma as recorded by U–Pb dating of monazite from charnockites and garnet-biotite gneisses. Rb–Sr ages of minerals in pegmatite intrusions record cooling to ca. 400–500 °C by 490-470 Ma. Based on a systematic regional sampling, this contribution confirms and refines former models based on smaller datasets. The major issue is that the Kerala Khondalite Belt must have undergone an extremely slow cooling history (3–6 °C/Ma) comparable to the one described for Sri Lanka and Madagascar. Still such ultra-high temperatures associated to slow cooling processes cannot be satisfactorily fit into a common tectonic context.