Characteristics and geodynamic setting of the 2.7 Ga Yilgarn heterogeneous plume and its interaction with continental lithosphere: Evidence from komatiitic basalt and basalt geochemistry of the Eastern Goldfields Superterrane

摘要

A database of 1075 high-precision geochemical analyses of least-altered ultramafic-mafic units, predominantly flows, was compiled for the Eastern Goldfields Superterrane. Samples are divided into a high-Mg population at MgO≥10-24 wt% and a basaltic population where 4≤MgO<10 wt%. There are eight groups based on (La/Sm) _N and Nb/Th ratios. Five magma series are identified. Uncontaminated komatiitic basalts have MgO ~11-23 wt% and Nb/Th≥8, whereas contaminated counterparts have Nb/Th<8 corresponding to silicious high-Mg basalts (SHMB). A distinct second magma series with MgO ~5-18 wt% MgO has a narrow range of Nb/Th at 0.5-≤2 over a range of (La/Sm) _N from 0.7-5.5, unlike contaminated suites where (La/Sm) _N and Nb/Th are correlated; this series corresponds to the enriched Paringa Basalt representing shallow melts of heterogeneous domains of the plume with recycled ancient continental lithosphere, or an independent plume. Prevalent, crustally uncontaminated, tholeiitic basalt magma series three all have Nb/Th≥8, span Mg-rich to fractionated Fe-rich counterparts, and range from LREE-depleted to mildly LREE-enriched where high Nb/Th ratios stem from eclogite streaks in the asthenosphere plume; contaminated equivalents have Nb/Th<8. A fourth alkaline, high-Mg magma series has a narrow range of MgO at ~13-16 wt%, extends to elevated TiO _2 and Ni contents relative to komatiitic basalts at that MgO range, and features (La/Sm) _N ≥2. Two additional uncontaminated tholeiitic basaltic groups are defined respectively by high-Nb to 20 ppm akin to alkaline ocean island basalts, and high-ΣREE relative to the other basaltic groups. The former, a fifth magma series, reflect melts of an eclogite-rich domain of the plume. Contamination of all groups, when present, was dominantly by interaction with continental mantle lithosphere with a minor crustal component. Komatiitic basalts are fractionation products of komatiites erupted from the hot axis of a mantle plume whereas prevalent tholeitic basalts are liquids derived from the cooler plume annulus. In all cases melting was in anhydrous peridotite. Ratios of Nb/Th in uncontaminated samples span 8-24 signifying that the Neoarchean mantle was as heterogeneous in terms of this ratio as Phanerozoic asthenosphere. In contrast, the fourth alkaline magma series stems from decompressional melting of metasomatised, hydrous, continental mantle lithosphere at >90km. Komatiites and komatiitic basalts are most abundant proximal to terrane boundaries because mantle plumes are 'steered' to the margins of thin, rifted, continental lithosphere. Given that mantle plumes melt on impingement at the base of the lithosphere, (Gd/Yb) _N ratios are used as a proxy to 'map' the thickness of the contemporaneous lithosphere.