Distinct Source Compositions and Genetic Environments for Archaean and Proterozoic Charnockites Across a Major Collisional Boundary in South India: Evidence From Geochemical and Sr-Nd Isotope Systematics
In the Southern Granulite Terrain (SGT), South India, a crustal scale shear zone system described as the Palghat- Cauvery Shear Zone (Pa-Ca) delineates the boundary between Late Archaean (ca. 2.52 Ga) and Neoproterozoic (ca. 0.7-0.55 Ga) high-grade terrains. In recent Gondwana Supercontinent models, the Pa-Ca is contiguous with the limit of an extensive zone of reworking of ancient crust within the Pan-African (0.75-0.5 Ga) East African Orogen which includes vast terranes covering parts of East Africa, Madagascar, southernmost India, Sri Lanka and parts of Antarctica (eg., Collins and Windley, 2002, J. Geol. 110, 325-339). A new dataset of major-trace element compositions and Sr-Nd isotopic systematics of about 180 charnockite samples has been utilized here to model source characteristics and magma genesis for the protoliths of the Archaean and Proterozoic charnockites across the Pa-Ca. The charnockites and enderbites of the Archaean domains show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic υNd0 and initial-87Sr/86Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. Their source could be either dominated by a mantle component produced by subduction process during the latest Archaean with variable extent of incorporation of mid-Archaean (upto 3.6 Ga) crustal components or may have involved an Archaean granulitic lower crust typically with low\-initial 87Sr/86Sr. On the other hand, sources of charnockites from the Proterozoic domains show greater recycling of older (upto 3.0 Ga) crustal components. Modeled at ca. 1.8 Ga and 0.8 Ga, the genesis of charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, is consistent with significant intracrustal melting processes within a thickened crust at temperatures between 800 and 1000° C. Our results ascribe further fundamental importance to the Pa-Ca, and also help rationalize the evidence for some enigmatic Archaean remnants in the region south of it, which play an important role (as integral source- components) in the Proterozoic crustal accretion processes of the SGT. This interpretation is consistent with the modeled position of the SGT south of the Pa-Ca within the East African Orogen in the zone characterized by reworked pre-1.5 Ga crust.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1040 Radiogenic isotope geochemistry;
- 1065 Major and trace element geochemistry