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Continental flood basalt (CFB) provinces are products of the largest known volumetric eruptions on Earth (~104 km3), with individual flow fields commonly covering well over 10 000 km2 with a mean lava thickness of over 5 m. Studies focusing on the emplacement style of such lava flows have relied extensively on morphological observations and comparisons with modern lava flows and experimental analogs. In the present study, we compare the textures of flood basalt lavas with those from different eruption settings all over the world using data collected from pre-existing literature to gain detailed insights into the style of eruption. Comparison of crystal size distribution data indicates that the eruption style of CFBs is similar to those of modern-day fissure eruptions (e.g. Iceland). This matches inferences based on observations of morphology. We also use a 1D thermal model to estimate the depth-dependent cooling rates within a single lava lobe and test the validity of assumptions built into the formulation of these models for large scale flood basalt lavas. The results reveal that, on average, flood basalt lavas need to conductively cool much faster than we would expect (up to order of ~102 times faster) to match the textural observations. The model is also frequently unable to replicate the observed depth-wise relative variations in length with depth for CFB lavas. Furthermore, the calculated cooling rates from crystal shapes also do not match those calculated from crystal lengths, indicating the assumptions in cooling flow models need to be modified for large CFB flow fields. Given the large areas of CFB flow fields and the relatively long eruption times inferred for the emplacement of individual flow fields, we hypothesize that inflation of lobes and formation of new lobes via breakouts combined with variable eruption rates are key processes that are missing when modeling the cooling of these flow fields. Accounting for these processes is essential to derive accurate cooling rates, which is important to better understand the environmental impact CFBs have at the time of emplacement. [ABSTRACT FROM AUTHOR]

The Corbyn's Cove area in south Andaman Island exposes a ~ 30 m thick sequence of sheet lavas, pillows and hyaloclastite deposits related to the Andaman Ophiolite, India. The upper sheet lavas are emplaced as tabular forms with thick, jointed core, vesicular crust and patchy hyaloclastite. The sheet lavas are criss-crossed by a network of calcite veins. The pillows constitute a modest flow field at the base of the sheet lavas and contain small (4.55 x 5.18 cm) to large (60.89 x 113.86 cm), spherical, non-vesicular, glassy entities. Pillow interiors contain radial and concentric joints. Absence of vesicles suggests that the pillows have been emplaced subaquaeously at depths greater than 4 km. The pillow lavas are fed by dykes. Thick hyaloclastite deposits containing sporadic patches of pillow breccias are intricately associated with the pillows in mound-like progradational bird foot deltas. In thin sections, the pillow lavas are characterized by lanceolate plagioclase and fretted pyroxene quenched morphologies in a holohyaline texture indicating high degrees of undercooling (ΔT ~ 100 to 200°C). Plagioclase compositions range from Anorthite (An97-99) in the core to Albite (An1-9) in the glassy margin. Individual plagioclase phenocryst in the dyke and pillow interior show tabular forms (ΔT ~ 50°C), with effects of albitization of the margin (An1-10; S-D index - 1.89 to 65.57) and variable alteration in the core (PI - 20.84 to 37.14; CIA - 70.92 to 99.34). Geochemically, the dyke-pillow lavas are hypersthene-normative tholeiites that have undergone variable degrees of spilitization and alteration. The sheet lavas have higher concentrations of Cs, Rb, Ce, U, K, Sr, P, Zr and Yb relative to N-MORB due to post emplacement cryptic metasomatism related to the calcite stockwork. These episodes of alteration have influenced the mineralogy and geochemistry of the original lavas. The geochemistry of the lavas indicates that they were deposited in a MORB-like environment, with a possible back-arc affinity. We envisage that the entire sequence from Corbyn's Cove was emplaced subaqueously during the Late Cretaceous in the Indo-Sino Burmese sea along the flanks of a rising mid oceanic ridge where the magma (1222 to 1230°C; 6 to 8 kbar) was generated at shallow depths (~ 20 km) and erupted to the surface as dykes that fed the pillow lava flow fields. Spilitization of the pillow lavas to various degrees occurred at temperatures between 1218 to 1164°C and even lower while the hydothermal alteration took place at temperatures well below 300°. [ABSTRACT FROM AUTHOR]