The petrology of an adamellite-porphyrite from the New England Bathylith (New South Wales)

The paper presents an account of the occurrence, petrography, mineralogy, and geochemistry of several intrusions of adamellite-porphyrite, an early member of the New England Bathylith (N.S.W.). The rock is composed of phenocrysts of quartz, andesine, hornblende, biotite, augite (and minor orthopyroxene) which are set in a fine-grained quartzofeldspathic groundmass. Compositions of silicate phenocrysts (which reveal widespread evidence of cataclasis) accord with coexisting phases of an assemblage appropriate to intermediate rocks. Xenolith mineralogy is similar to equivalent minerals occurring as phenocrysts. The recalculated bulk composition of a rock represented by the modal andesine, hornblende, biotite, augite, and magnetite of the adamellite-porphyrite is similar to the composition of a biotite-diorite; the composition of the quartzo-feldspathic groundmass plots in the low-melting region of the NaAISi ₃O ₈-KAISi ₃O ₈-SiO ₂-H ₂O system. Evidence is presented to indicate a hybrid origin for the adamellite-porphyrite whereby disrupted biotite-diorite was invaded and mixed with low-melting silicic alkalic liquid.