Cyclic units in the Somerset Dam layered gabbro intrusion, southeastern Queensland, Australia

Abstract

The well-preserved Somerset Dam intrusion probably represents a small, relatively shallow, subvolcanic magma chamber. The 500-m-thick exposed sequence consists of 22 macrolayers which are defined by sharp phase, modal and textural contacts. At least six cyclic units, 30–150 m thick, are exposed, and the sequence from the base to the top of a cyclic unit is inferred to be leucogabbro (plagioclase cumulate), troctolite (plagioclase-olivine cumulate), olivine gabbro (plagioclase-augite-olivine cumulate), and oxide gabbro (plagioclase-augite-(olivine)-magnetite-ilmenite cumulate). Mineral compositions in a typical cyclic unit show a reversed fractionation trend in the sequence leucogabbro-troctolite, and a normal fractionation trend from troctolite (the least fractionated rock type) to the oxide gabbro (the most fractionated rock type). The most sensitive parameters for defining the cryptic trends are An in plagioclase, Fo and Ni in olivine, and Cr in magnetite and augite. Whole-rock compositions also show marked changes, and Fe, Ti, V, S and Cu increase and Al, Mg Fe , Cr and Ni decrease from troctolite to oxide gabbro. Despite the remarkable similarity of successive cyclic units, significant differences exist between them in the sequences of layers, thicknesses of individual layers and of the cyclic units, mineral compositions and cryptic patterns, average level of fractionation and the size of the reversals. Unit 3 is particularly unusual because it is the least fractionated and consists of two incomplete subunits. Unit 1, the lowest exposed, is the most fractionated. These differences between the units cannot be explained in terms of a closed system, and are strong evidence for an open system involving periodic injections of magma. The formation of a cyclic unit appears to reflect the dominant control of the order of crystallisation from a batch of replenished magma, which is essentially plagioclase first, followed by olivine, augite, magnetite and ilmenite, and brown hornblende. The parent magma is inferred to have been a high-alumina olivine tholeiite. The gradual rather than sharp reversals in the cryptic trends in the lower part of each cyclic unit may reflect some magma mixing, though postcumulus processes could have been operative.