Subaqueous early eruptive phase of the late Aptian Rajmahal volcanism,India:Evidence from volcaniclastic rocks,bentonite,black shales,and oolite

The late Aptian（118-115 Ma） continental flood basalts of the Rajmahal Volcanic Province（RVP） are part of the Kerguelen Large Igneous Province,and constitute the uppermost part of the Gondwana Supergroup on the eastern Indian shield margin.The lower one-third of the Rajmahal volcanic succession contains thin layers of plant fossil-rich inter-trappean sedimentary rocks with pyroclasts,bentonite,grey and black shale/mudstone and oolite,whereas the upper two-thirds consist of sub-aerial fine-grained aphyric basalts with no inter-trappean material.At the eastern margin and the north-central sector of the RVP,the volcanics in the lower part include rhyolites and dacites overlain by enstatite-bearing basalts and enstatite-andesites.The pyroclastic rocks are largely felsic in composition,and comprise ignimbrite as well as coarse-grained tuff with lithic clasts,and tuff breccia with bombs,lapilli and ash that indicate explosive eruption of viscous rhyolitic magma.The rhyolites/dacites（〉68 wt.%） are separated from the andesites（〈 60 wt.%） by a gap in silica content indicating their formation through upper crustal anatexis with only heat supplied by the basaltic magma.On the other hand,partially melted siltstone xenoliths in enstatite-bearing basalts suggest that the enstatite-andesites originated through mixing of the upper crust with basaltic magma,crystallizing orthopyroxene at a pressure-temperature of ~3 kb/1150℃.In contrast,the northwestern sector of the RVP is devoid of felsic-intermediate rocks,and the volcaniclastic rocks are predominantly mafic（basaltic） in composition.Here,the presence of fine-grained tuffs,tuff breccia containing sideromelane shards and quenched texture,welded tuff breccia,peperite,shale/mudstone and oolite substantiates a subaqueous environment.Based on these observations,we conclude that the early phase of Rajmahal volcanism occurred under predominantly subaqueous conditions.The presence of grey and black shale/mudstone in the lower one-third of the succession across the entire Rajmahal basin provides unequivocal evidence of a shallow-marine continental shelf-type environment.Alignment of the Rajmahal eruptive centers with a major N—S mid-Neoproterozoic lineament and the presence of a gravity high on the RVP suggest a tectonic control for the eruption of melts associated with the Kerguelen plume that was active in a post-Gondwana rift between India and Australia-Antarctica.

Early terrestrial and lunar anorthosites:Comparative geochemistry and evolutionary processes

In a paper in 1970,Brian Windley first recognised that early terrestrial and lunar anorthosites both have calcic plagioclase,and low TiO_(2)and high CaO and Al_(2)O_(3)contents.Despite these similarities,the geochemistry of early terrestrial and lunar anorthosites has not been rigorously compared and contrasted.To this end,we compiled 425 analyses from 212 early terrestrial anorthosite occurrences and 306 analyses from 16 lunar anorthosite occurrences.This was supplemented by a compilation of plagioclase anorthite(An)contents and pyroxene Mg#from early terrestrial and lunar anorthosites.Early terrestrial anorthosites have lower whole-rock An contents but similar Mg#to lunar anorthosites.The CaO contents of lunar anorthosites are higher than those of early terrestrial anorthosites for a given MgO and Al_(2)O_(3)content,early terrestrial anorthosites have higher SiO_(2)contents than lunar anorthosites at a given MgO content,and lunar anorthosites have higher Eu/Eu*anomaly ratios yet broadly similar La/Yb and Nd/Sm ratios than early terrestrial anorthosites.Some early terrestrial anorthosites have less fractionated chondrite-normalised rare earth element(REE)patterns and less prominent positive Eu anomalies than lunar anorthosites.Lunar anorthosites have higher plagioclase An contents,yet a similar range of pyroxene Mg#compared to their early terrestrial counterparts.Some early terrestrial anorthosites are more fractionated than some lunar anorthosites.Our interpretations imply that most early terrestrial anorthosites crystallised from basaltic parental magmas that were generated by high-degree partial melting of sub-arc asthenosphere mantle wedge sources that were hydrated by slab-derived fluids,with the remainder being associated with mid-ocean ridge and mantle plume settings.Some of the arc-related early terrestrial anorthosites were influenced by crustal contamination.In addition,early terrestrial anorthosites originated from partial melting of the mantle at various depths with variable garnet residua,whereas lunar anorthosites formed without any significant garnet residua.Lower plagioclase CaO contents and pyroxene Mg#in early terrestrial anorthosites can be explained by higher proportions of clinopyroxene and olivine fractionation in terrestrial magma chambers than in the lunar magma ocean where orthopyroxene and olivine fractionation occurred.Low TiO_(2)contents in both terrestrial and lunar anorthosites reflect rutile and/or ilmenite fractionation.

Geological Evidence for the Operation of Plate Tectonics throughout the Archean:Records from Archean Paleo-Plate Boundaries

Plate tectonics describes the horizontal motion of rigid lithospheric plates away from midoceanic ridges and parallel to transforms, towards deep-sea trenches, where the oceanic lithosphere is subducted into the mantle. This process is the surface expression of modern-day heat loss from Earth. One of the biggest questions in Geosciences today is ＂when did plate tectonics begin on Earth＂ with a wide range of theories based on an equally diverse set of constraints from geology, geochemistry, numerical modeling, or pure speculation. In this contribution, we turn the coin over and ask ＂when was the last appearance in the geological record for which there is proof that plate tectonics did not operate on the planet as it does today＂. We apply the laws of uniformitarianism to the rock record to ask how far back in time is the geologic record consistent with presently-operating kinematics of plate motion, before which some other mechanisms of planetary heat loss may have been in operation. Some have suggested that evidence shows that there was no plate tectonics before 800 Ma ago, others sometime before 1.8–2.7 Ga, or before 2.7 Ga. Still others recognize evidence for plate tectonics as early as 3.0 Ga, 3.3–3.5 Ga, the age of the oldest rocks, or in the Hadean before 4.3 Ga. A key undiscussed question is： why is there such a diversity of opinion about the age at which plate tectonics can be shown to not have operated, and what criteria are the different research groups using to define plate tectonics, and to recognize evidence of plate tectonics in very old rocks？ Here, we present and evaluate data from the rock record, constrained by relevant geochemical-isotopic data, and conclude that the evidence shows indubitably that plate tectonics has been operating at least since the formation of the oldest rocks, albeit with some differences in processes, compositions, and products in earlier times of higher heat generation and mantle temperature, weaker oceanic lithosphere, hotter subduction zones caused by more slab-melt generation, and under different biological and atmospheric conditions.