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Fluidal-Clast Breccia Generated by Submarine Fire Fountaining, Trooper

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Fluidal-Clast Breccia Generated by Submarine Fire Fountaining, Trooper Journal of Volcanology and Geothermal Research 109 22001) 339±355 www.elsevier.com/locate/jvolgeores Fluidal-clast breccia generated by submarine ®re fountaining, Trooper Creek Formation, Queensland, Australia K. Simpson, J. McPhie* Centre for Ore Deposit Research, University of Tasmania, G.P.O. Box 252-79, Hobart, Tasmania 7001, Australia Received 25 May 2000; accepted 18 December 2000 Abstract A distinctive monomictic breccia, composed of ¯uidal and blocky basaltic andesite clasts, occurs in a Cambro-Ordovician submarine volcanic succession in northern Queensland, Australia. Associated with this ¯uvial-clast buccia facies are coherent facies and coarse and ®ne breccia facies of the same composition. The ¯uidal-clast breccia facies is internally massive and .250 m thick, varying only in the ratio of ¯uidal clasts to blocky clasts. Fluidal clasts range in size from 2 cm to 170 cm, and have moderately to highly vesicular cores and thick 2up to 1 cm), non-vesicular, formerly glassy rims. Blocky clasts are highly vesicular to non-vesicular, ,2 cm, angular, dominantly equant or splintery in shape and identical in composition to the ¯uidal clasts. The ¯uidal clasts strongly resemble subaerial volcanic bombs and are interpreted to be the products of submarine ®re fountaining of relatively low-viscosity lava. The blocky clasts were mainly derived from disintegration of the ¯uidal clasts, by means of quench fragmentation. Coherent basaltic andesite intercalated with the ¯uidal-clast breccia represents co-genetic lavas, dykes and irregular shallow intrusions. The coarse and ®ne breccia facies is very thickly bedded, monomictic 2basaltic andesite), poorly sorted and clast supported. This facies is inter- preted to have been generated by periodic gravitational collapse of unstable accumulations of the ¯uidal-clast breccia facies. Subaqueous ®re-fountain breccias are distinguished from subaerial ®re-fountain breccias by thick glassy margins on ¯uidal clasts, the lack of welding and agglutination, and the distinctive association of highly vesicular, ¯uidal clasts with non- vesicular, angular, blocky clasts. Recognition of submarine ®re-fountain breccias in volcanic successions constrains the eruption style, proximity 2tens of metres) to source and environment of deposition. q 2001 Elsevier Science B.V. All rights reserved. Keywords: submarine ®re fountain; ¯uidal clasts; volcanic bombs; basaltic andesite; submarine volcanic succession; vesicularity; fragmentation processes 1. Introduction several other submarine volcanic successions, in Japan 2Yamagishi, 1987; Cas et al., 1996), Sweden 2Allen Volcanic breccias characterised by distinctive ¯uid- et al., 1996, 1997), Canada 2Mueller and White, ally shaped clasts occur in a Cambro-Ordovician 1992), Germany 2Schmincke and Sunkel, 1987) and submarine succession in northern Queensland, Austra- Iceland 2Kokelaar and Durant, 1983). The juvenile lia. Similar ¯uidal-clast breccias have been identi®ed on clasts are typically basaltic andesite or basaltic in the modern sea¯oor 2Smith and Batiza, 1989), and in composition; however, Archean rhyolitic examples have been recognised in Canada 2Mueller and White, * Corresponding author. 1992). As the ¯uidal clasts resemble bombs, these brec- E-mail address: [email protected] 2J. McPhie). cias are generally attributed to ®re-fountain eruptions. 0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-0273201)00199-8 340 K. Simpson, J. McPhie / Journal of Volcanology and Geothermal Research 10932001) 339±355 Subaerial ®re-fountain deposits are common and In this paper, we describe the facies and facies asso- there are numerous examples in the recent volcanic ciations of the BBA and use textural characteristics to record 2Macdonald, 1972; Head and Wilson, 1987; constrain the modes of fragmentation and deposition, Head and Wilson, 1989; Wilson et al., 1995; Mattox and the facies architecture. The ¯uidal-clast breccia and Mangan, 1997). The fountain structure and erup- facies at Brittania is compared with submarine and tive products are controlled by magma viscosity, vola- subaerial ¯uidal-clast breccias, also thought to be tile content, magma volume ¯ux, accumulation rate, ®re-fountain deposits, and with other texturally simi- local temperature, and height and vigour of fountain- lar facies of different origins. ing 2Head and Wilson, 1989). In subaqueous settings, the controls on fountaining are likely to be the same, 2. Geological setting with additional important in¯uences exerted by hydrostatic pressure and magma±water interactions. The BBA is part of the Seventy Mile Range Group There is some evidence that these eruptions can occur 2SMRG; Henderson, 1980; Fig. 1) which extends at depths .1000 m although the maximum water approximately 165 km east±west and comprises four depths may be much greater 2Batiza et al., 1984; regionally mappable formations. The middle two Smith and Batiza, 1989; Clague et al., 1990; Gill et formations 2Mount Windsor and Trooper Creek al., 1990; Clague et al., 2000). Formations) are submarine volcanic successions. Subaqueous ®re fountaining has been interpreted to The other two formations 2Puddler Creek and Rollston account for a variety of different volcanic facies and Range Formations) are also submarine, but domi- clast types including hyaloclastite 2Yamagishi, 1987), nantly sedimentary. Although compositionally varied, scoria 2Gill et al., 1990), reticulite 2Siebe et al., 1995), the volcanic formations have calc-alkaline af®nities spatter and highly irregular ¯uidally shaped bombs, and are dominated by rhyolite and dacite 2Henderson, lapilli, shards 2Kokelaar and Durant, 1983; Batiza et 1986; Berry et al., 1992). Ma®c to intermediate rocks, al., 1984; Schmincke and Sunkel, 1987; Mueller and although rare in the Mount Windsor Formation, are White, 1992; Cas et al., 1996; Allen et al., 1997) and locally abundant in parts of the Trooper Creek glass spheres 2Clague et al., 1990). However, there are Formation. few systematic descriptions for the textural and facies Regional metamorphism of the SMRG ranges from characteristics of submarine ®re-fountain deposits, subgreenschist facies to amphibolite facies and is nor have any contrasts with subaerial or shallow locally overprinted by contact metamorphism asso- water equivalents been identi®ed. Correct identi®ca- ciated with Ordovician to Devonian granitoids tion of submarine ®re-fountain breccias is critical in 2Berry et al., 1992). The present distribution of the reconstructing the facies architecture of ancient volca- SMRG is the surface expression of the subvertical nic successions. They can be used to identify paleo- and south-younging limb of an east±west trending sea¯oor positions although they cannot precisely fold 2Berry et al., 1992). Despite the metamorphism constrain the water depth. Fire fountaining can gener- and deformation, the preservation of primary textures ate substantial thicknesses in short periods, signi®- is excellent and a wide variety of volcanic and sedi- cantly modifying the local sea¯oor topography and mentary facies has been identi®ed. The presence of hence facies geometry. In addition, ®re-fountain brec- pillow lava, hyaloclastite, very thick graded volcani- cias rarely extend beyond tens of metres from the clastic mass-¯ow units, peperite and massive sul®de source vent 2Kokelaar and Durant, 1983; Kokelaar, ore deposits in the volcanic formations, and marine 1986) and are thus good indicators of proximity to a fossils 2trilobites and graptolites; Dear, 1974; McClung, volcanic centre. 1976; Henderson, 1983) in the two youngest formations The ¯uidal-clast breccia and associated facies collectively indicate a submarine environment of described herein are located near Brittania Homestead deposition for the SMRG 2Berry et al., 1992). south of Charters Towers 2Fig. 1) and will be referred to as the Brittania basaltic andesite 2BBA). The BBA 2.1. Setting of the BBA was initially identi®ed in drill core and subsequent ®eld mapping allowed de®nition of three main facies. The BBA occurs near the top of the Trooper Creek SEVENTY MILE Charters Towers RANGE GROUP Ravenswood A WATERLOO- AGINCOURT HIGHWAY,REWARD, THALANGA HANDCUFF WARRAWEE 7755000 B A MAGPIE K. Simpson, J. McPhie / Journal of Volcanology and Geothermal Research 10932001) 339±355 Pentland 25km LIONTOWN Location of Study Area Trafalgar Brittania Bore Highway/Reward G 7745000 Waterloo- Mine regory Deve Agincourt Prospect Sunrise l o Liontown pm Spur Mine en tRoad Trooper Creek Prospect 0 5 10 km 0000 39 0000 00000 7735000 41 420000 430000 4 440000 B TRIASSIC-TERTIARY Waddy's Cover rocks Mill 7755000 ORDOVICIAN-DEVONIAN Lolworth-Ravenswood batholith CAMBRIAN-ORDOVICIAN Rollston Range Formation - volcanogenic siltstone, greywacke, minor dacite Thalanga Mine Trooper Creek Formation - rhyolitic, dacitic, and andesitic volcanic facies Mt Windsor Formation - dominantly rhyolitic volcanic facies with lesser dacitic, and andesitic volcanic facies Highway rs CAMBRIAN de lin F Puddler Creek Formation - sandstone, siltstone, minor andesite and dolerite 0000 0000 fault 7745000 37 36 Fig. 1. Geological map of the Seventy Mile Range Group between Trafalgar Bore and Sunrise Spur 2A) and at Waddy's Mill 2B), also showing younger granitoid intrusions 2after Berry et al., 1992; Stolz, 1995). The Brittania basaltic andesite occurs near the top of the Trooper Creek Formation. Inset in 2A) shows the positions of 2A) and 2B) in relation to the Seventy Mile Range Group. 341 342 K.
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