Basal Reversals in Mafic Sills and Layered Intrusions

Abstract

Fractional crystallization of ponded basaltic magma results in rocks that become increasingly more evolved in composition inwards. However, almost all mafic sills and layered intrusions show basal reversals tens to hundreds of meters thick in which minerals and rocks become compositionally more primitive upwards. Two main types of basal reversals can be distinguished with different relationships with the overlying Layered Series. Most common are fully-developed reversals that gradually pass into the overlying Layered Series through a crossover horizon with the most primitive rock and mineral compositions. Above the crossover, the Layered Series becomes progressively more evolved. This type of reversal presents a mirror image of the overlying Layered Series. In contrast, aborted reversals are separated from a more primitive or evolved Layered Series by a sharp textural and compositional break. Both types of reversal are attributed to three principal processes: (1) inflowing magma becomes progressively more primitive, (2) magma undercooling gradually decreases and (3) adcumulus growth becomes increasingly more effective. The major difference between these types is that fully-developed reversals result from a continuous evolution and therefore show a gradual transition into the overlying Layered Series. In contrast, aborted reversals form when the evolution is abruptly interrupted by a pulse of primitive magma that resets the crystallization history in the chamber. This generally results in basal reversals in sharp contact with a more primitive Layered Series. In some lopolithic layered intrusions, aborted basal reversals are discordant with the Layered Series and therefore can be in contact with more evolved rocks along the upper sections of the inward-dipping floor.