6. Data Report: Spinifex-Textured Basalt Xenoliths at PACMANUS, Papua New Guinea1

R.A. Binns2, 3

INTRODUCTION

A number of intensely altered, dark xenoliths with palimpsest quench textures were recorded within altered dacitic host rocks at Site 1189 (Roman Ruins, PACMANUS) during Ocean Drilling Program (ODP) Leg 193. Several of these displayed puzzling marginal fringes, apparently of altered plagioclase with variolitic texture, protruding into adjacent host rocks. Despite their alteration, the xenoliths were interpreted as fragments of rapidly chilled, possibly olivine-bearing basalts incorporated into the dacitic magmas either within the crustal plumbing system or during eruption at the seafloor (figures F15, F16, F17, F42, and F43 in Shipboard Scientific Party, 2002).

An additional example of formerly spinifex-textured xenolith, the first from Site 1188 (Snowcap) and the first from the upper cristobalite-bearing zone of alteration, has been revealed by postcruise studies. Furthermore, a pristine sample of the parent lithology has been found within a dredge haul (MD-138, Binatang-2000 Cruise of Franklin; 343.60S, 15140.35E, 1688 meters below sea level) from the Satanic Mills hydrothermal field at PACMANUS, near ODP Site 1191.

The purpose of this report is to document these discoveries and thereby to confirm and build on shipboard interpretations. To my knowledge, similar xenoliths have never before been found in modern island arc or backarc volcanic sequences. Spinifex textures are most common in Archean komatiites, some of which are bimodally associated with calc-alkaline felsic volcanic rocks (Barnes et al., 2002).

1Binns, R.A., 2004. Data report: Spinifex-textured basalt xenoliths at PACMANUS, Papua New Guinea. In Barriga, F.J.A.S., Binns, R.A., Miller, D.J., and Herzig, P.M. (Eds.), Proc. ODP, Sci. Results, 193 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/193_SR/212/212.htm>. [Cited YYYY-MM-DD]

2Division of Exploration and Mining, Commonwealth Scientific and Industrial Research Organisation (CSIRO), PO Box 136, North Ryde NSW 1670, Australia. Ray.Binns@csiro.au

3Department of Earth and Marine Sciences, Australian National University, Canberra ACT 0200, Australia.

Initial receipt: 31 October 2003
Acceptance: 28 September 2004
Web publication: 22 December 2004
Ms 193SR-212

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