GENERATION OF PALEOARCHAEAN TTG UNDER CHANGING TECTONIC CONDITION:EVIDENCE FROM SINGHBHUM CRATON, EASTERN INDIA

Widespread crustal growth in Paleoarchaean, represented by Tonalite-Trondjhemite- Granodiorite (TTG) associaion, is evident from cratons all over the world. Singhbhum craton is no exception with occurance of banded gneissic rocks of TTG clan widely known as Older Metamorphic Tonalitic gneiss. “Archaean flat subduction of hot oceanic plateau basalt” and “melting of thick oceanic crust” are the two distinct mechanisms for the generation of Paleoarchaean TTG has been widely discussed till date by many authors. Absence of subduction related phenomena like elongated batholithic bodies, high magnesium diorites (Sanukitoids), high niobium basalts imply that possibly subduction is not the dominant mechanism for generation of TTG magma in Singhbhum craton, at least in Paleoarchaean. Thus melting of older protocrust may be contributed the TTG magmatism in Singhbhum Craton. TTG rocks from Singhbhum craton has been treated as a single episode of TTG magmatism by earlier researchers. However, our recent geochemical study on TTG rocks show notable difference in Heavy Rare Earth (HREE) pattern in chondrite normalized REE plot. Accordingly, two groups are named as High HREE TTG [low SiO2; high HREE avg. (Gd/Er)n=2.23; less fractionated REE avg. (La/Yb)n=27.9 and low Sr/Y avg. Sr/Y=53.59] and Low HREE TTGs [high SiO2; depleted HREE avg. (Gd/Er)n=3.23; steeply fractionated REE avg. (La/Yb)n=46.11 and high Sr/Y avg. Sr/Y= 95.49] with average total HREE content 9.84 and 4.66 respectively. There is little difference in the major element composition and mantle compatible elements of High HREE and Low HREE TTGs. The two types mainly differ in pressure sensitive sigantures indicating their generation at different pressures, in turn at different crustal levels. In spite of having similar Srontium content, Sr/Y is higher in low HREE type owing to low Y concentration than the high HREE with no significant Eu anomaly in both the types. Thus in both kind of TTGs garnet is the main residual phase that controls the HREE content but minor variation in HREE is controlled by gradual changes in depth of melting as a function of changing tectonic setup. Studies involving U-Pb dating of zircons from both kind of TTGs are being undertaken to understand the time gap in the generation of two different kind of TTGs, if at all exists.