Volcanology of the Aries micaceous kimberlite, central Kimberley Basin, Western Australia Peter J. Downes a, ⁎ , Dale Ferguson b , Brendan J. Griffin c a Department of Earth and Planetary Sciences, Western Australian Museum, Francis St, Perth, Western Australia, 6000 b Hillgrove Resources, Level 2, 20 Howard St, Perth, Western Australia, 6000 c Centre for Microscopy and Microanalysis, University of Western Australia, Crawley, Western Australia, 6009 Received 18 October 2004; accepted 11 June 2006 Available online 8 August 2006 Abstract The Neoproterozoic (815.4±4.3 Ma) Aries kimberlite intrudes the King Leopold Sandstone and the Carson Volcanics in the central Kimberley Basin, northern Western Australia. Aries is comprised of a N–NNE-trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs. The diatremes are volumetrically dominated by massive, clast-supported, accidental lithic-rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform- to globular segregationary-textured groundmasses. Lower-diatreme facies, accidental lithic-rich breccias probably formed through fall-back of debris into the vent with a major contribution from the collapse of the vent walls. These massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme. Abundant, poorly vesicular to nonvesicular, juvenile kimberlite ash and lapilli, with morphologies that are indicative of phreatomagmatic fragmentation processes, occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence. This unit and overlying bedded accidental lithic-rich breccias are interpreted to be sediment gravity-flow deposits (including possible debris flows) derived from the collapse of the crater walls and/or tephra ring deposits that surrounded the crater. Diatreme-forming eruptions may have been initiated by magma–water interactions along fracture and joint-controlled aquifers within the King Leopold Sandstone. The current level of exposure of the diatremes probably extends from the lower-diatreme facies up into the base of a bedded upper-diatreme sequence. © 2006 Elsevier B.V. All rights reserved. Keywords: kimberlite; diatreme; pipe; phreatomagmatism; debris flow; erosion 1. Introduction The Aries kimberlite was emplaced in the central Kimberley Basin, northern Western Australia, at ap- proximately 815.4 ± 4.3 Ma (Figs. 1 and 2; Ar–Ar on phlogopite, weighted mean, 95% confidence, n =15; P. Downes, unpubl. data), and comprises a N–NNE- trending series of three steep-sided diatremes and associated hypabyssal kimberlite dykes and plugs. Aries is a micaceous kimberlite that has close mineral- ogical affinities to group 2 kimberlites (orangeites) and olivine lamproites (Edwards et al., 1992). However, trace element and isotope geochemistry–notably a Nd–Sr isotopic signature that suggests an asthenospheric source similar to that of group 1 kimberlites–indicates that it is petrogenetically distinct from these two groups of rocks (Edwards et al., 1992; Taylor et al., 1994). On the basis of Journal of Volcanology and Geothermal Research 159 (2007) 85 – 107 www.elsevier.com/locate/jvolgeores ⁎ Corresponding author. Tel.: +61 8 94272735; fax: +61 8 94272882. E-mail address: peter.downes@museum.wa.gov.au (P.J. Downes). 0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.06.004