Pb-isotopic constraints on the source of A-type Suites: insights from the Hiltaba Suite - Gawler Range Volcanics Magmatic Event, Gawler Craton, South Australia

The 1595–1575 Ma Hiltaba Suite intrusives and coeval Gawler Range Volcanics (GRV) represent a significant crustal component of the Mesoarchean-Mesoproterozoic Gawler Craton, South Australia, and one of the largest silicic igneous provinces in the world. They are also spatially and temporally-affiliated with world-class IOCG mineral deposits (i.e. Olympic Dam) thus making an understanding of these rocks important both economically, and petrologically. Here, we present the results of 177 in situ Pb-isotopic analyses of alkali feldspar in the Hiltaba Suite/GRV, which when integrated with existing SmNd isotopic and geochemical data, reveal a craton-wide isotopic zoning. Pb-isotopic zonation is controlled by the varying interaction of a parental melt derived from a sub-continental lithospheric mantle (SCLM)-like enriched mantle reservoir (²³⁸U/²⁰⁴Pb ≤ 9.4, ²³²Th/^238U ≤ 3.66) and twocrustal end-member reservoirs: an unradiogenic reservoir (²⁰⁷Pb/²⁰⁶Pb ≥ 0.97, ²⁰⁸Pb/²⁰⁴Pb ≥ 35.7) and a radiogenic reservoir (²³⁸U/²⁰⁴Pb ≥ 10, ²³²Th/²³⁸U ≥ 3.95). The SCLM-like reservoir exerts a strong control on Pb-isotopic compositions around the central-western Gawler Craton (Nuyts Terrane), while the unradiogenic reservoir which represents granulitic lower crust, appears to be spatially limited to the northwest of the Gawler Craton. The radiogenic reservoir, coincident with the IOCG province and probably represented by monazite-rich metasediments, is more prevalent in the eastern Gawler Craton. Correlation between melt-source ²³⁸U/²⁰⁴Pb_(t) and ²³²Th/²³⁸U_(t), with LREE (i.e. La, Ce etc) and HFSE (Zr, Nb etc) which are characteristically enriched in A-type magmas, suggest the importance of hybridisation between a crustal melt component and enriched-mantle-melt component in the formation of A-type magmas.

The isotopic and geochemical compositions of the ∼1592 Ma Lower GRV and ∼1588 Ma Upper GRV effectively bracket the Hiltaba Suite intrusive rocks, and thus provide an avenue for investigating the temporal Pb-isotopic development of a representative part of the Hiltaba Suite/GRV magmatic systems. The Pb-isotopic characteristics imply that either Lower and Upper GRV magma systems developed independently, or reflect a magmatic system evolving substantially over a ∼2 Ma period, dominated by mafic input at first (3 to 8% crustal melt; Lower GRV) and moving towards homogeneous, crustal, isotopic compositions with increased crustal melt proportions (15 to 20%) in the Upper GRV.

The consistency of these melt source constraints - isotopically, geochemically and proportionally - with other A-type suites around the world favours a unified petrogenetic model for A-types involving dominantly mantle-derived melts, with varying proportions of crustal material which impart characteristically LREE-HFSE enriched geochemical signatures.