Mafic–Ultramafic Sills: New Insights from M- and S-shaped Mineral and Whole-rock Compositional Profiles

Abstract

The origin of mafic and ultramafic sills exhibiting different wholerock compositional profiles (e.g. I-, C-, D-, M-and S-shaped profiles) remains controversial. We have addressed this issue by revisiting three $100 m thick Siberian dolerite sills (Vavukansky, Kuz'movsky and Vilyuysky) that display remarkable internal differentiation. The Vavukansky sill has an M-shaped profile with prominent basal and top reversals showing inward increases in whole-rock MgO, Mg-number [100Mg/(Mg Fe)] and normative An content [100An/(An Ab)], followed by the Layered and Upper Border Series with inward decreases in these indices. The Kuz'movsky and Vilyuysky sills both show S-shaped profiles similar to the Vavukansky sill, but lack a top reversal. These whole-rock M-and S-shaped profiles are accompanied by similar profiles in mineral compositions. Plagioclase and, to a lesser extent, olivine show systematic inward increases in An content and Mg-number, respectively, across basal and top reversals.These compositional trends are followed by inward decreases in these ratios in the interiors of the Vavukansky and Kuz'movsky sills. Currently accepted models attribute whole-rock M-and S-shaped compositional profiles to crystal settling, compositional convection or compaction operating in closed systems. Our observations challenge these traditional interpretations because variations in mineral compositions observed in marginal reversals cannot result from closed-system fractionation. We suggest instead that initially the sills evolved as open systems that were slowly inflated by magmas that became gradually more primitive with time. The inflation was accompanied by in situ crystallization that preserved the preceding fractionation history of the injected magmas by forming basal and top reversals with minerals becoming more primitive inwards. This process culminated with rapid inflation of the sills to their current size owing to a major influx of primitive magma.