Petrogenesis of the A-type, Mesoproterozoic Intra-caldera Rheomorphic Kathleen Ignimbrite and Comagmatic Rowland Suite Intrusions, West Musgrave Province, Central Australia: Products of Extreme Fractional Crystallization in a Failed Rift Setting

• Published in: Journal of petrology 2015-03, Vol.56 (3), p.493-525
• Main Authors: Medlin, C C, Jowitt, S M, Cas, R A F, Smithies, R H, Kirkland, C L, Maas, R A, Raveggi, M, Howard, H M, Wingate, M T D
• Format: Article
• Language: English
• Subjects: Chambers; Evolution; Fractional crystallization; Fractionation; Geochemistry; Magma; Plagioclase; Rhyolite
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egv007

The Pussy Cat Group rhyolites of the Mesoproterozoic west Musgrave Province of central Australia, a constituent part of the Bentley Supergroup, were deposited during the c. 1085-1040Ma Ngaanyatjarra Rift and Giles events, and are related to the Warakurna Large Igneous Province. This study focuses on the two silicic components of the Pussy Cat Group, the Kathleen Ignimbrite and the Rowland Suite. These silicic rocks are A-type, metaluminous (to slightly peraluminous) rhyolites and are enriched in the rare earth elements (REE) relative to average crustal abundances. The rhyolitic Kathleen Ignimbrite records an explosive caldera fill-sequence and contains, amongst others, a thick ( less than or equal to 500m), initially subaqueously emplaced, rheomorphic, intra-caldera ignimbrite unit, whereas the Rowland Suite consists of a number of mineralogically and geochemically related porphyritic rhyolites that intrude throughout the Pussy Cat Group. Whole-rock geochemistry, Rb-Sr, Sm-Nd and in situ zircon Lu-Hf isotope data are indicative of a dominantly mantle-derived source for the magmas that formed the Pussy Cat Group rhyolites. Secondary ion mass spectrometry U-Pb dating of these units yields ages of 1062 plus or minus 8, 1071 plus or minus 5, 1076 plus or minus 5, and 1078 plus or minus 5Ma. The magmas that formed these units were formed by extreme fractional crystallization of a mantle-derived basaltic magma, with minimal crustal contamination, during a failed intra-plate extensional rifting event. This involved three main stages of fractional crystallization: early fractionation of plagioclase, olivine, clinopyroxene and magnetite from a basaltic magma to reach an intermediate composition, subsequent fractionation of plagioclase, K-feldspar and quartz to form a proto-Rowland Suite-type magma at mid- to upper-crustal levels that migrated into the shallow upper crust and formed a magma chamber, and final fractionation of quartz, K-feldspar, plagioclase, magnetite and biotite plus or minus minor REE-enriched accessory phases from the Rowland Suite magma resulting in the evolved Kathleen Ignimbrite magmas. This final phase of fractionation generated the most evolved silicic rock suite identified to date within the entire west Musgrave Province. The new petrographic, geochronological, geochemical, and isotopic data presented within this study indicate that these two units are coeval and comagmatic, suggesting a common source for the Kathleen Ignimbrite and the ent