Raymond A. Duraiswami

Slabby pahoehoe from the western Deccan Volcanic Province: evidence for incipient pahoehoe–aa transitions

Abstract The pahoehoe–aa transition for a flow exposed near Bodshil village from the western part of the Deccan Volcanic Province (DVP) is reported for the first time. The ∼1-km-long Bodshil flow issued as a small sheet from a pre-existing lobe. Near the source, the crust is characterised by numerous squeeze-ups. A number of gaping fractures, parallel to sub-parallel to the flow direction, are exposed on the surface in the medial portion of the flow. About 800 m away, the flow completely transforms to slabby pahoehoe. The terminal portion of the flow is characterised by concentrations of slabs, blocks and lava balls. The size and concentrations of the slabs and lava balls appear to increase along the length of the flow. Petrographic studies reveal a dominant hypohyaline texture. The flow core is coarse and is characterised by plagioclase set in a glassy matrix. The presence of clinopyroxene in addition to plagioclase and glass distinguishes the crust and interslab crust from the core. On the basis of mineralogy, a temperature range of 1146±15°C to 1169±15°C is inferred for the Bodshil flow. Increased vesicle deformation across the transition is discernible and an average D -value of

A brief comparison of lava flows from the Deccan Volcanic Province and the Columbia-Oregon Plateau Flood Basalts: Implications for models of flood basalt emplacement

The nature and style of emplacement of Continental Flood Basalt (CFB) lava flows has been a matter of great interest as well as considerable controversy in the recent past. However, even a cursory review of published literature reveals that the Columbia River Basalt Group (CRBG) and Hawaiian volcanoes provide most of the data relevant to this topic. It is interesting to note, however, that the CRBG lava flows and their palaeotopographic control is atypical of other CFB provinces in the world. In this paper, we first present a short overview of important studies pertaining to the emplacement of flood basalt flows. We then briefly review the morphology of lava flows from the Deccan Volcanic Province (DVP) and the Columbia-Oregon Plateau flood basalts. The review underscores the existence of significant variations in lava flow morphology between different provinces, and even within the same province. It is quite likely that there were more than one way of emplacing the voluminous and extensive CFB lava flows. We argue that the establishment of general models of emplacement must be based on a comprehensive documentation of lava flow morphology from all CFB provinces.

Recycling of Flow-Top Breccia Crusts into Molten Interiors of Flood Basalt Lava Flows: Field and Geochemical Evidence from the Deccan Traps

Thick flood basalt lava flows cool conductively inward from their tops and bases, usually developing columnar jointing. Although relatively rapid cooling in such flows due to meteoric water circulation has been previously demonstrated, mixing of the surface crust with the interior – as observed in active lava lakes – has not been shown. Here we report large radial columnar jointing structures (rosettes) with cores of highly brecciated, weathered and amygdaloidal material within Deccan flood basalt lava flows. The morphology of such breccia-cored rosettes, petrographic observations, and geochemical data, particularly Nd–Sr isotopic ratios, all suggest that the features formed due to the sinking of the flow-top breccia crusts into these flows’ molten interiors and the resultant warping of isotherms around these “cold anchors”. Thus, cooling in some thick flood basalt lava flows may be accelerated by sinking of cooler upper crusts into hotter, molten interiors.

Possible lava tube system in a hummocky lava flow at Daund, western Deccan Volcanic Province, India

A hummocky flow characterised by the presence of toes, lobes, tumuli and possible lava tube system is exposed near Daund, western Deccan Volcanic Province, India. The lava tube system is exposed as several exhumed outcrops and is composed of complex branching and discontinuous segments. The roof of the lava tube has collapsed but original lava tube walls and fragments of the tube roof are seen at numerous places along the tube. At some places the tube walls exhibit a single layer of lava lining, whereas, at other places it shows an additional layer characterised by smooth surface and polygonal cracks. The presence of a branching and meandering lava tube system in the Daund flow, which represents the terminal parts of Thakurwadi Formation, shows that the hummocky flow developed at a low local volumetric flow rate. This tube system developed in the thinner parts of the flow sequence; and tumuli developed in areas where the tube clogged temporarily in the sluggish flow.

Fluid-rock interaction in the Kangankunde Carbonatite Complex, Malawi: SEM based evidence for late stage pervasive hydrothermal mineralisation

The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO2-rich and alkali chloride-carbonate-bearing fluid at ∼250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis.

Formation of atoll garnets in the UHP eclogites of the TSO Morari Complex, Ladakh, Himalaya

The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, idiomorphic and enclose inclusions of amphibole, phengite and zoisite within the cores, and omphacite and quartz/coesite towards the rims. Detailed electron microprobe analyses and back-scattered electron images show well-preserved prograde zoning in the whole garnets with an increase in Mg and decrease in Ca and Mn contents from the core to the rim. The atoll garnets commonly consist of euhedral ring over island/peninsular core containing inclusions of phengite, omphacite and rarely amphibole between the core and ring. Compositional profiles across the studied atoll grains show elemental variations with higher concentrations of Ca and Mn with low Mg at the peninsula/island cores; contrary to this low Ca, Mn and high Mg is observed at the outer rings. Temperature estimates yield higher values at the Mg-rich atoll garnet outer rings compared to the atoll cores. Atoll garnet formation was favoured by infiltration of fluid formed due to breakdown of hydrous phases, and/or the release of structurally bounded OH from nominally anhydrous minerals at the onset of exhumation. Infiltration of fluids along pre-existing fracture pathways and along mineral inclusion boundaries triggered breakdown of the original garnet cores and released elements which were subsequently incorporated into the newly-grown garnet rings. This breakdown of garnet cores and inward re-growth at the outer ring produced the atoll structure. Calibrated geo-thermobarometers and mineral equilibria reflect that the Tso Morari eclogites attain peak pressures prior to peak temperatures representing a clockwise path of evolution.

Soft sediment deformation structures from Khari River section of Rudramata member, Jhuran Formation, Kutch: A testimony of Jurassic seismites

Soft sediment deformation structures such as slump folds, clastic dyke, syn-sedimentary faults and convolute bedding are present in the coarse–fine grained yellowish buff coloured sandstone, and interbedded reddish brown fine grained sandstone and yellowish–white siltstone at the Khari River section belonging to Rudramata member of Jhuran Formation (Upper Jurassic), Kutch. These soft sediment deformation structures are confined to lower and middle parts of the section and are invariably underlain as well as overlain by undeformed beds that have restricted lateral and vertical extent and occur in close proximity of Kutch Mainland Fault, thereby suggesting that these structures were formed by seismic activity and therefore represents seismites.

Volcanology and lava flow morpho-types from the Koyna-Warna region, western Deccan Traps, India

Flow mapping and physical volcanology of 15 basaltic lavas exposed in three critical road pass sections (ghats) in the Koyna-Warna region of the western Deccan Traps is presented in this paper. Transitional lavas like rubbly pahoehoe are most common morpho-type exposed in these ghat sections. Sinking of rubbly breccia into flow interiors and formation of breccia-cored rosette are common in some lava flows. Few rubbly lavas exhibit slabby tendencies. The amount and nature of the associated rubble is variable and result from the mechanical fracturing and auto-brecciation of the upper vesicular crust in response to distinctive stages in the cooling, crystallization and emplacement history of individual lava flows. Occurrence of aa and pahoehoe morpho-types in the lava flow sequence is subordinate. Three prominent pahoehoe flows separated by red bole horizons are seen in the upper parts of the Kumbharli ghat. These are thick, P-type sheet pahoehoe. The pahoehoe lavas represent compound flow fields that grew by budding, endogenous lava transfer and inflation. Presence of pahoehoe lavas in the Koyna-Warna region hints at possible hitherto unrecorded southern extension of Bushe-like flow fields. This study reconfirms the existence of pahoehoe-slabby-rubbly-aa flow fields and transitions even in the upper echelons of the Deccan Trap stratigraphy. The study of morphology and internal structure of lava flows exposed at the ghat sections in the Koyna-Warna region could guide subsurface core-logging that is critical in deciphering the physical volcanology and emplacement dynamics of basaltic lava flows penetrated by drill holes sunk under the scientific deep drilling programme.