Bachelor of Science (Honours)
School of Earth & Environmental Sciences
Webb, Keira, The age and tectonometallogenic setting of the Fifield “Alaskan-type” intrusive suite, Lachlan Orogen, NSW., Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, 2014.
The Fifield suite of ultramafic-mafic intrusions occur as an N-S linear belt within the Lachlan Orogen extending from north of Condobolin to south of Bourke, NSW and are associated with Australia’s only significant platinum production from nearby placer deposits. These Alaskan-type or ankaramitic plutons intrude the highly deformed quartz-rich turbidites of the Middle to Late Ordovician Girilambone Group, which were deformed during the Late Ordovician-Early Silurian Benambran Orogeny. The lack of any internal fabric or significant alteration indicates the Fifield intrusive suite is a post-Benambran phase of extension related magmatism. U-Pb SHRIMP dating of zircons extracted from a hornblende gabbro of the Tout intrusion give an age of 439.6 ± 8.5 Ma (Early Silurian), providing the first accurate date for the intrusions and by association an age constraint to the termination of the Benambran Orogeny. Thirty-six samples were collected from outcrop and drill core from the Owendale, Tout, Murga and Murrumbogie intrusions for use in petrographic and whole-rock geochemical analysis. Petrographic analysis yielded a crystallisation sequence typical of Alaskan-type intrusions of olivine-clinopyroxene-orthopyroxene-amphibole-plagioclase-K-feldspar-quartz with minor biotite, magnetite, chromite and sulfide phases. This was supported by geochemical results, with the ultramafic association having >40 wt. % MgO and the cumulate association having an average of 12.66 wt. % MgO (pyroxenite – 18.83 wt. %, gabbro – 12.68 wt. %, monzonite/diorite – 6.49 wt. %). Harker diagrams reveal that two distinct suites of igneous rocks are present within the Fifield igneous suite; 1) an ultramafic association characterised by serpentinised peridotite (hazburgite) with an Mg number of 80 and REE patterns that indicate a depleted chondrite-normalised signature; and 2) the cumulate association characterised by progressive evolution from pyroxenite-hornblendite to gabbro, diorite and monzonite compositions that show clear geochemical fractionation trends from a single alkali basaltic melt source. Chromite compositions from the ultramafic rocks are consistent with Alaskan-type or island arc origins. The ultramafic rocks are interpreted to represent the residual depleted hazburgitic material from which the alkali basaltic melt was extracted via partial melting then undergo fractional crystallisation to produce the mafic felsic cumulate rocks. The Fifield intrusive suite is concluded to have been derived from a fertile mantle source which was emplaced as a solid but ductile diapir into the wet sediments of the Girilambone Group, where the addition of water into the diapir initiated partial melting. The cumulate association was then formed by fractional crystallisation. This is concluded to have occurred in an extensional setting similar to that associated with post-collisional, extension related alkali volcanism in northern Taiwan. Emplacement of the Fifield intrusive suite is interpreted to have occurred during a brief period of regional extension that occurred immediately after the accretion of the Macquarie Arc and is coeval with ‘Phase 4’ magmatism associated with the alkali ‘shoshonitic’ porphyry mineralisation at Cadia and Northparkes.