HR: 0800h
AN: V41E-1520    [Abstracts]
TI: Trace element and Nd-Hf-Sr-Pb Isotope Geochemistry of Proterozoic Lamproites from Southern India: Subducted Komatiite in the source?
AU: * Chakrabarti, R
EM: ramananda@earth.rochester.edu
AF: University of Rochester, Department of Earth and Environmental Sciences, 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Basu, A R
EM: abasu@earth.rochester.edu
AF: University of Rochester, Department of Earth and Environmental Sciences, 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Paul, D K
EM: dalimpaul@yahoo.co.in
AF: Presidency College, 86/1 College Street, Calcutta, WB 700073 India
AB: The Proterozoic Krishna lamproites of South India, thought to be the possible sources of the famous Indian diamonds - Kohinoor, Hope, Regent, Nizam etc., occur in the northeastern part of the Cuddapah sedimentary Basin, sandwiched between Archean Dharwar cratonic rocks to the west and Grenville age-granulites of the Eastern Ghat mobile belt to the east. Published ages for these lamproites range from 1224 ± 14 Ma to 1402 ± 5 Ma. We present trace element and Nd, Sr, Hf, Pb isotopic data for these lamproites. The chondrite-normalized REE plots of the lamproites show a nearly uniform, parallel distribution pattern showing extreme LREE enrichment (La_N/Yb_N = 41-88) like most global lamproites, with La concentrations as high as one thousand times chondrite values. These rocks show flattened HREE patterns and are characterized by the absence of Eu anomaly. Rb and Ba show a wide range in concentration but have lower concentrations compared to Th when normalized to primitive mantle. The lamproites have variable but high U/Pb and Th/U ratios and show a negative Sr and Pb anomaly. However, no negative Nb anomaly is seen. The lamproites have high Ni (average ~ 301ppm, highest 819ppm) and Cr (average ~183ppm, highest 515ppm) concentrations that show a positive correlation with MgO indicating presence of olivine in their source. The lamproites have high Nb/U (average 59) and Nb/Th (average 9) ratios similar to those of komatiites. They also have high Ce/Pb (6.7-65.3, average 32.5) ratios, much higher than crustal values and MORB/OIB mantle. Initial Sr-isotopic ratios vary from 0.70096-0.70665. The Sr isotopic composition is quite radiogenic and variable and clearly show post-emplacement Rb enrichment. Nd-Hf isotopic composition of the lamproites is similar to group II kimberlites and plot on the `terrestrial array'. The present day υNd values range from -30.4 to -22.6 while the present day υHf values range from -41.1 to -30.1 indicating their derivation from ancient crustal sources with low Sm/Nd and Lu/Hf ratios similar to Al-depleted komatiites. Depleted mantle Nd model ages range from 1.9-2.1 Ga (average 2.0Ga) while depleted mantle Hf model ages range from 2.16-3.13 Ga (average 2.77Ga). The Krishna lamproites have characteristically high initial ²⁰⁷Pb/²⁰⁴Pb (15.11-16.66) at correspondingly low initial ²⁰⁶Pb/²⁰⁴Pb values (10.78- 26.52), which could be due to segregation of sulfides associated with subducted komatiites in the mantle. Previous models for lamproite genesis suggest their generation from a metasomatised mantle with no crustal contribution although, they typically have strongly negative υNd values. We propose a model in which lamproites are generated by low degree melting of a subducted komatiite bearing mantle peridotite source. The lamproitic melt generated this way would have high Nb/U and Nb/Th ratios and unradiogenic Nd and Hf isotopic signatures.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005