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Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are finegrained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.

Keywords

Calc-Alkaline Granitoids, Mafic Magmatic Enclaves, Magma Mixing and Mingling, Jonnagiri Schist Belt, Andhra Pradesh.

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Microstructures and Compositional Variation in the Intra-Volcanic Bole Clays from the Eastern Deccan Volcanic Province: Palaeoenvironmental Implications and Duration of Volcanism

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Authors

J. P. Shrivastava ¹, Mansoor Ahmad ², Surabhi Srivastava ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi - 7, IN
2 Geological Survey of India, Eastern Region, Patna - 20, IN

Source

Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 80, No 2 (2012), Pagination: 177-188

Abstract

Clay minerals associated with intra-volcanic bole horizons of varied colours and thicknesses contain montmorillonite, halloysite and kaolinite, show distinct microstructures and microaggregates. In kaolinite, Fe³⁺ ions substitute for Al³⁺ at octahedral sites. Most of these clays are dioctahedral type, show balance between net layer and interlayer charges. The interstratified illite - smectite (I/S) mixed layers containing variable proportions of montmorillonite. Illite contains sheet-like, well oriented microaggregates. The parallel stacks of chlorite sheets show chlorite/smectite (C/S) mixed layers. Progressive enrichment of Fe and depletion of Al ions with the advancement of kaolinization process is observed. High order of structural and compositional maturity observed in these bole clays, indicate long hiatus between the two volcanic episodes.

It is observed that the bole horizons suffered rigorous weathering, approximately 5 fold higher than the respective parent lava flows under the alternate wet and dry spells of climatic changes. Translating the production time estimates of clays associated with the 21 bole horizons, found in the eastern Deccan volcano-sedimentary succession, it is assessed that the minimum time required for their formation is approximately 7 my. Clay minerals that occur across the stratigraphic sequence show cyclic changes in the climate, favouring longer duration of volcanic activity. Late Maastrichtian Lameta beds post-date Deccan volcanism to 70 Ma or earlier to this. Considering the formation of bole clays in terms of time, it is possible that the volcanic activity started much earlier in the late Maastrichtian, and continued after 65 Ma or even later. Thus, the clay stratigraphy based estimates lend support to a prolong duration.

Keywords

Lava Flows, Deccan Volcanic Province, Intra-Volcanic Bole Horizons, Clay Minerals, Maastrichtian.

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