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

S. S. Rathore ¹, A. R. Vijan ¹, M. P. Singh ¹, B. N. Prabhu ², K. N. Misra ¹

Affiliations
1 Geochemistry Division, KDM Institute of Petroleum Exploration, Oil and Natural Gas Corporation Ltd., Dehra Dun - 248 195, IN
2 Regional Chemical Laboratory, EKBC, ONGC, Sibsagar, Assam - 785 640, IN

Abstract

Precambrian basement from well HBM-1 in the Heera oil field of Mumbai offshore basin has been dated by Rb-Sr and K-Ar methods. Five granulitic basement samples from three conventional drill cores have yielded Rb-Sr isochron age of 502±25 Ma with an initial Sr ratio of 0.70855±0.00013. This age has been interpreted as the time of granulite facies metamorphism of the basement rocks in the region. Two whole rock samples from the basement of this well have yielded mutually concordant K-Ar ages of 505±16 Ma and 507±17 Ma. The K-Ar ages are significantly similar to Rb-Sr age obtained from this well, suggesting complete isotopic reequilibration around 500 Ma ago. The time of secondary thermal heating around 500 Ma ago in the basement of Heera field coincides with the widespread Neoproterozoic (Pan-African) thermo-tectonic event extending from the Arabian Peninsula and eastern Africa covering Madagascar, southern India, Sri Lanka and East Antarctica. This study widens the limit of the Pan-African zone, which hitherto was thought to be confined to the western part (presently southern part) of the Indian subcontinent, towards further east.

Keywords

Neoproterozoic, Pan-African Granulite Metamorphism, Rb-Sr and K-Ar Geochronology, Heera Oil Field, Mumbai Offshore Basin.

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A. R. Vijan ¹, S. S. Rathore ¹, K. C. Vig ¹, M. Bansal ¹, M. P. Singh ¹, B. N. Prabhu ²

Affiliations
1 Geochemistry Division, KDM Institute of Petroleum Exploration, Oil and Natural Gas Corporation Ltd., Dehra Dun - 248 195, IN
2 Regional Chemical Laboratory, ERBC, ONGC, Sibsagar, Assam - 785 640, IN

Abstract

The importance of dating glauconites is that they are formed under low sedimentation rates connected with marine transgression and are commonly associated with unconformities and other stratigraphic breaks. This paper presents the first quantitative age estimation of glauconite samples separated from subsurface cores of Goru and Pariwar Formations of Jaisalmer basin, Rajasthan, India. SEM studies, X-ray diffraction patterns and high K₂O content (6.8 to 7.4%) of these samples indicate that they are highly evolved and are ideal for isotopic dating.

The Rb-Sr age of 108 ± 5 Ma and average K-Ar age of 103 ± 3 Ma obtained for the gtauconites from the Goru Formation are indistinguishable within error and agree with their biostratigraphic age. Glauconites from the upper most part of the Pariwar Formation give an average K-Ar age of 106 ± 3 Ma (early Albian), which is slightly lower than their biostratigraphic age.

Keywords

Geochronology, Glauconite, Goru and Pariwar Formations, Jaisalmer Basin, Rajasthan.

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Sandeep Singh ¹, Biswajayee A. Patra ¹, A. R. Vijan ², A. K. Jain ³

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN
2 Geochronology Division, KDMIPE, ONGC, Kaulagarh Road, Dehradun - 248 001, IN
3 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667

Abstract

Biotite and K-feldspar Rb-Sr ages from three granitoid intrusives within the Karakoram Metamorphic Belt (KMB), eastern Ladakh yield indistinguishable ages between 12-10 Ma. Since the closure temperature of biotite for Sr diffusion is 300±50°C, we interpret these ages due to exhumation at 10 to 7 km depth within the crust. This implies that the Pangong Metamorphic Belt of the KMB, located at the southern termination of the Eurasian Plate, was undergoing a fast exhumation in a transpressional regime during Middle Miocene. These data also indicates that the southern edge of the Eurasian Plate has exhumed rapidly in comparison to the Higher Himalayan Crystallines within the Indian Plate segment.

Keywords

Rb-Sr Ages, Granitoids, Karakoram Complex, Eastern Ladakh.

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