Sedimentary xenoliths and dolerite patch pegmatites from an analcite basalt intrusion

Abstract

Sandstone and shale xenoliths of the Triassic Hawkesbury formation occur in the analcite basalt plug at Peats ridge, New South Wales. Reactions between the magma and the xenoliths produced trachyte, dolerite, and dolerite-pegmatite modifications adjacent to the xenoliths. A possible sequence of events is postulated to explain the observed relations. Thermal and chemical gradients caused the magma immediately surrounding the xenoliths to be enriched in alkali-bearing volatiles and hence altered the normal cooling history.