HR: 0800h
AN: GP11A-0011    [Abstracts]
TI: In situ ion microprobe monazite and zircon ages from India`s Southern Granulite Terrain
AU: * Catlos, E J
EM: catlos@okstate.edu
AF: Oklahoma State University, School of Geology, 105 NRC, Stillwater, OK 74075 United States
AU: Sivasubramanian, P
EM: spsiva@hotmail.com
AF: V.O. Chidambaram College, SPIC Research Centre, Tuticorn, 628008 India
AU: Dubey, C S
EM: csdubey@vsnl.com
AF: University of Delhi, Department of Geology, Delhi, 110007 India
AB: Understanding the assembly and breakup of supercontinents requires interpretable ages. Insufficient geochronologic studies of India`s Southern Granulite Terrain have prevented the development of a better understanding of the evolution of South India and its role in supercontinent reconstructions. Approaches used to decipher its geologic history typically use methods that involve rock crushing and mineral separation, and the ensuing loss of textural information can lead to confusion. Here we present monazite and zircon ages from Southern Granulite Terrain rocks obtained in thin section (in situ) using an ion microprobe. Dating these minerals in situ is ideal because (1) the chemistry and the textural relationships of mineral grains are preserved, (2) inclusions in garnet are protected from daughter-product loss, and (3) analyses of small grains (~1μm) and zones within larger grains is feasible. For example, a South Indian rift-related carbonatite contains large (mm-sized) apatite surrounded by ~1μm-thick monazite rims. The age of the rim (715±42 Ma, Th-Pb, ±1σ) is consistent with ages of carbonatite complexes located to the north, suggesting that extensionally-controlled magmatism occurred during Mid-Neoproterozoic in both the Dharwar Craton and Southern Granulite Terrain. An anorthosite located ~5 km south of Oddanchatram contains a concordant zircon inclusion in garnet that is consistent with this age at 710±16 Ma (238U-206Pb). Monazite inclusions in garnet from a Southern Granulite Terrain rock found as a lens within a thick charnokitic unit range from 733±15 Ma in the core to 490±8 Ma near the rim, indicating protracted and possible polymetamorphic garnet growth within the region. Another garnet-bearing assemblage nearby contains monazite inclusions consistent with a single population at 502±14 Ma. In comparison with U-Pb ID-TIMS analysis, ion microprobe dating has the disadvantage of poorer chronologic resolution. However, U-Pb ID-TIMS analysis requires a relatively large amount of mineral, either as a whole grain or several grains. Clearly this is inadequate to distinguish ages of chemically different domains and provide unambiguous information about a sample`s geologic history. In addition, garnet-bearing assemblages allow the P-T conditions of the rock to be ascertained, and when combined with the ages of their monazite inclusions, are a powerful means for deciphering the tectonometamorphic history of the region.
DE: 1100 GEOCHRONOLOGY
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005