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Siderophile enrichment of basement uplift granitoids, Woodleigh impact structure: shock metamorphism and vapor-mediated chemical alteration

A.Y. Glikson, S. Eggins, F. Pirajno1, R. Iasky1, A. Mory1 and T.P. Mernagh2 1 Geological Survey of Western Australia2 Australian Geological Survey Organization

Scanning electron microscopy, electron microprobe energy dispersive spectrometry, laser ICPMS spectrometry, Laser Raman spectroscopy (LRS), and whole-rock major and trace element XRF and ICPMS analyses allow resolution of shock-related micron-scale heterogeneities in granitoid core samples from the central uplift of the recently confirmed 120 km-diameter Woodleigh impact structure, Western Australia. The samples consist of interleaved zones dominated by quartz showing planar deformation features (PDF), feldspar with PDF, diaplectic feldspar showing fusion along and across PDFs which results in honeycomb-like textures, and microbrecciated pseudotachylite veins enriched in vapor-transferred components and classified as S-type pseudotachylites (Spray, 1998, Geol. Soc. London Sp. Publ., 140, 195—204). The pseudotachylite and amorphous intra-feldspar zones are enriched in refractory Al, Mg, Ca and siderophile elements and depleted in the volatile Si and K relative to whole rock compositions, which approximate adamellite. However the rocks are enriched in Mg, Fe, Ca, Ni, Co and Cr relative to low-Ca granites. High Ni/Co and Ni/Cr ratios militate for the importance of a meteoritic component. The bulk of the metals occur in penetrative pseudotachylite veins and in amorphous diaplectic zones within desegregated and resorbed feldspar. The origin and mode of transport of the trace metals require that melt and vapor originated from the exploding projectile. Evidence for a vapor phase is yielded by occurrence of texturally isolated micron-scale metal-enriched amorphous spots within feldspars. The enrichment in metals suggest the shocked granitoids were located either at high levels of the central uplift or, alternatively, represent subcrater breccia injected by impact melt and vapor.

Figure 1: SEM BSE image marking an EDS (Energy Dispersion Spectrometry) chemical profiles along a 98 micron-long traverse through a shock metamorphosed feldspar grain. The K-feldspar shows diaplectic shock-amorphisation along PDF (planar deformation features). The traverse starts in quartz (qz) at point A, passing through alternations of feldspar with diaplectic spots (fl) and parallel amorphous zones (dg) marking solid-state transformation of feldspar to glass along PDF planes. The chemical profiles indicate depletion of the amorphous material in volatile oxides (SiO2, K2O) relative to feldspar and enrichment of the amorphous material in more refractory oxides (CaO, MgO, FeO). Note the high levels of NiO, interpreted in terms of metal-volatile transfer from the impacting projectile.