Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. R. Basir ¹, S. Balakrishnan ²

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry - 605 014, IN

Abstract

A study of the distribution of REE in sphene from three granodiorite samples around NNW-SSE trending Hutti-Maski schist belt has been described here. Two distinct sets of sphene-melt K_d, values for REE have been observed among the three sphene-whole rock pairs studied. The chondrite normalized REE pattern of sphene from east (Gajalagatta) and northeast (Uti) granodiorites show LREE enriched and HREE depleted patterns with slight or no negative Eu anomaly. These REE patterns are parallel to the respective whole rock samples. But the chondrite normalized REE pattern of sphene from the sample to west of the schist belt (Ankasdoddi) has markedly depleted Ce and Nd with slight or no negative Eu anomaly and steeply fractionated HREE, resulting in a convex-up pattern. The variation of REE K_d, values of sphenes is not due to differences in the composition of host magmas (because they are all granodiorites with identical major element abundances), but could be a function of temperature of crystallization. Small degrees of fractional crystallization of sphene from granodiorite magmas would result in residual magmas having lower REE abundances either with parallel REE patterns or HREE depleted fan shaped REE patterns, depending on the K_dvalues.

Partial melting of granitoid rocks, leaving minor amounts of sphene in the residue gives rise to melts with lower REE abundances than the source. The REE patterns of the melts are parallel to the source if one set of REE K_d sphene-melt values are considered. If the other set of K_d, values are used, the partial melts have (fan shaped) REE patterns with more depletion for heavy REEs.

Keywords

Geochemistry, Granodiorite, Sphene (Titanite), Hutti-Maski Schist Belt, Dharwar Craton, Karnataka.

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S. K. Som ¹, P. Jana ¹, U. Bhattacharya ², Sandeep Bardhan ¹, S. R. Basir ³, A. K. Ghosh Roy ¹

Affiliations
1 Earthquake Geology Division, Geological Survey of India, Eastern Region, DK-Block, Sector-II, Salt Lake City, Kolkata - 700 095, IN
2 Earthquake Geology Division, Map Division, Geological Survey of India, Eastern Region, DK-Block, Sector-II, Salt Lake City, Kolkata - 700 095, IN
3 Earthquake Geology Division, TC-1 Division, Geological Survey of India, Eastern Region, DK-Block, Sector-II, Salt Lake City, Kolkata - 700 095, IN

Abstract

A moderate earthquake struck the Sikkim Himalaya on Tuesday, February 14, 2006 at 6.25 am. The earthquake was felt in North Bengal, Sikkim,nepal and Bhutan. Macroseismic survey was carried out for the accessible regions of Sikkim and the northern portion of West Bengal to assesses the intensity of ground motion, the damage caused by the earthquake and also to ascertain the possible source and its orientation.

The survey results show that the MSK (Medvedev-Sponheuer-Karnik) intensity varies between III to VII. The maximum intensity is found in Gangtok-Deorali-Tadonga areas, which are far from the instrumental epicenters identified by USGS and IMD. Statistical procedure has been adopted for numerical weighted intensity calculation to quantify the MSK intensity and for deducing the isoseismal lines more logically. Separate macroseismc epicenter is calculated from the high MSK- intensity sites, which falls in between Deoralt and Tadong. The probable orientation of the source (fault) is found to be E-W that passes through Makha village, north of Gangtok area. The damages to constructions on the terraces is much less than those constructed on other topographic forms, like slopes, ridges and low zones. Within the most damaged Gangtok-Deorail Tadong areas, most of the buildings are located along the ridge top or along the ridge axis.

Keywords

Earthquake, Macroseismic Survey, Sikkim.

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