Tectonic significance of dykes in the Sarnu-Dandali alkaline complex,Rajasthan, northwestern Deccan Traps

Whether swarms of preferentially oriented dykes are controlled by regional stress fields, or passively exploit basement structural fabric, is a much debated question, with support for either scenario in individual case studies. The Sarnu-Dandali alkaline complex, near the northwestern limit of the Deccan Traps continental flood basalt province, contains mafic to felsic alkaline volcano-plutonic rocks and carbonatites. The complex is situated near the northern end of the 600 km long, NNWe SSE-trending Barmer-Cambay rift. Mafic enclave swarms in the syenites suggest synplutonic mafic dykes injected into a largely liquid felsic magma chamber. Later coherent dykes in the complex, of all compositions and sizes,dominantly strike NNWe SSE, parallel to the Barmer-Cambay rift. The rift formed during two distinct episodes of extension, NWe SE in the early Cretaceous and NEe SW in the late Cretaceous. Control of the southern Indian Dharwar structural fabric on the rift trend, as speculated previously, is untenable,whereas the regional Precambrian basement trends（Aravalli and Malani） run NEe SW and NNEe SSW.We therefore suggest that the small-scale Sarnu-Dandali dykes and the much larger-scale BarmerCambay rift were not controlled by basement structure, but related to contemporaneous, late Cretaceous regional ENEe WSW extension, for which there is varied independent evidence.

Breccia-cored columnar rosettes in a rubbly pahoehoe lava flow,Elephanta Island,Deccan Traps,and a model for their origin

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 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 flows of the Deccan.