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Abstract
Outcrops of leucitite lavas occur as scattered remnants up to 40 m thick in the El Capitan area, northeast of Cobar in western New South Wales. Two eruption sites have been located for these lavas. Preserved volcanic features indicate that the lavas were erupted on to a relatively low-relief, Early Miocene land surface, flowed along a shallow valley and underwent inflation. Geochemical analyses of the leucitites indicate only limited fractionation. Remnant outcrops of the leucitite lavas represent a very important time marker in the geomorphic history of the Cobar region, preserving evidence of both Early Miocene and post-Early Miocene landscape evolution and weathering conditions. A deep-weathering profile, similar to those common throughout the region and characterised by a ferruginous mottled zone and underlying bleached saprolite, is preserved beneath a dissected flow at one eruption site. Other deposits beneath the leucitite flows include baked soils, silcretes, and quartz-rich gravels and grits. Palaeomagnetic dating of the upper part of the deep-weathering profile indicates an Early to Middle Miocene age for hematite fixation. A new 40Ar/39Ar age on the volcanic plug at this site (17.14 ± 0.20 Ma, 2σ) refines the minimum eruption age for the leucitites and supports an Early Miocene age for ferruginisation of the deep-weathering profile. Topographic inversion of the basal contact of one of the leucitite flows indicates an average minimium erosion rate for the area of 1 m per million years. Weathering profiles on the leucitites are thin and lack significant ferruginisation or chemical leaching, indicating that post-Early Miocene weathering in the region has been very limited. These profiles also contain a significant aeolian component including abundant quartz dust.
Acknowledgements
The work reported here was supported by the Australian Government's Cooperative Research Centre Program in CRC LEME. Much of the field mapping was conducted by OG as part of an Honours project supported by a CRC LEME Scholarship. Hugh Patterson conducted differential GPS measurements. John Pyke from Geoscience Australia carried out the geochemical analyses, and Robin Westcott and Belinda Crowther from the Department of Geology, Australian National University assisted with the XRD analyses. Irradiation of the WT6 sample was facilitated by the Australian Institution of Nuclear Science and Engineering, and the Australian Nuclear Science Technology Organisation, through a generous grant provided to W. J. Dunlap. Property owners Colin Pate and Roger Broughton are thanked for allowing access to the field sites. Constructive comments by reviewers F. L. Sutherland and I. McDougall helped improve the manuscript.