|Title||Breccia-cored columnar rosettes in a rubbly pahoehoe lava flow, Elephanta Island, Deccan Traps, and a model for their origin|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Sheth H., Pal I., Patel V., Samant H., D'Souza J.|
|Type of Article||Article|
|Keywords||alkaline; Basalt; basalt province; Columnar jointing; Deccan Traps; direction; emplacement; flood; Flow-top breccia; kilauea volcano; morphology; Rubbly pahoehoe; stratigraphy; transitions; volcanic province; volcanism; western india|
Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of these Deccan "breccia-cored columnar rosettes" ruled out explanations such as volcanic vents and lava tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incorporated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly pahoehoe flow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) thick rubbly pahoehoe flows probably form by rapid inflation (involving many lava injections into a largely molten advancing flow), and (2) such flows are transitional to 'a'(a) over bar flows (which continuously shed their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We suggest that the Deccan flows under study were shedding some of their FTB in front of them as they advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the 'a'(a) over bar flows of the Deccan. (C) 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.