Mineral chemistry and crystal size distributions of mafic dikes and sills on the eastern border of the Parnaíba Basin, NE Brazil

Abstract

Crystal size distributions (CSDs) and mineral chemistry of plagioclase and clinopyroxene are used to quantify textures and estimate timescales of Mesozoic mafic sills and dikes exposed on the eastern margin of the Parnaíba Basin (NE Brazil). The diabases consist of plagioclase, clinopyroxene (augite and pigeonite) and FeTi oxides and show textures that vary from holocrystalline, to locally porphyritic, glomerophyric and hypocrystalline. Plagioclase and clinopyroxene CSDs are similar in sills and dikes, with clinopyroxene slopes systematically steeper than plagioclase slopes. Plagioclase usually records nearly log-linear negative slopes that tend to steepen approaching the wall rock contact. Plagioclase typically shows normal zoning with anorthite contents varying (An84-25) from the center to the grain margin. Calculated residence times for the log-linear segments vary from ca. one month (τ = 10−7 mm·s−1) for samples situated 20 cm from the contact to ca. four months for samples ~3 m away from the contact. Clinopyroxene consists of augite (dominant) and pigeonite and shows relatively uniform compositions. Unlike plagioclase, clinopyroxene CSDs tend to be flat or slightly positive towards the finer grains suggesting that the texture was modified during the crystallization of the residual melt. Plagioclase microlites (L < 0.5 mm), in contrast, show kinked CSDs with the steeper negative slopes including minute crystals of sanidine (orthoclase component) and alkali-sodic feldspar in addition to CaNa plagioclase. The modified textures and high nucleation density of microlites in mostly static, deep-seated magmatic reservoirs therefore resulted from chemical disequilibrium between largely solidified tholeiitic magma and late, highly fractionated alkali-rich residual melts. The estimated time for the full crystallization of the largest sills is one and one-half years.