The Fox Tor Diorite, a newly recognised intrusion within the New England Batholith, northern New South Wales

Abstract

The Fox Tor Diorite is a newly recognised intrusion with an I‑type intermediate composition. It is totally enclosed by the S‑type Pringles Monzogranite, a member of the Bundarra Supersuite of the New England Batholith. The intrusion was initially detected by its strong aeromagnetic signature and subsequent mapping showed that there are six separate outcrops covering 84 hectares within an area of 2.6 km north–south and 1.2 km east–west. Field relationships imply intrusive contacts against the Pringles Monzogranite, with localised contamination of the diorite at the contact. Modelling of magnetic data indicates that the intrusive mass is steep‑sided and extends to considerable depth. K–Ar geochronology on the Fox Tor Diorite gave an age of 239.7±6.7 Ma, a result that overlaps with age determinations for the Uralla and Moonbi Supersuites in the southern New England Orogen, but is approximately 50 Ma younger than granitoids of the Bundarra Supersuite. Rocks of the Fox Tor Diorite are medium‑grained, with early crystallised orthopyroxene, clinopyroxene and plagioclase, and subsequent crystallisation of minor hornblende, biotite, K‑feldspar and quartz. The intrusion is characteristically magnetic, with susceptibilities of 900×10‑5 to 2800×10‑5 SI. Although ilmenite is present, magnetite is more abundant and the magnetic (and oxidation state) characteristics are more typical of the Moonbi Supersuite granitoids, rather than those of the Uralla Supersuite. On the other hand, geochemical characteristics of the Fox Tor Diorite, such as contents of K2O, P2O5, Rb, Sr, Nb and Zr, accord better with mafic members of the Uralla Supersuite. Geochemical and mineralogical criteria indicate that the Fox Tor Diorite magma had a significant mantle‑derived component and that it has undergone fractionation, perhaps largely by precipitation of pyroxenes and possibly plagioclase.