Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue2-4.2000:

Precambrian Research 101 2000 211 – 235 Subaqueous, Paleoproterozoic, metarhyolite dome-flow-cone complex, Flin Flon greenstone belt, Manitoba, Canada L.D. Ayres a, , A.S. Peloquin b a Department of Geological Sciences, Uni6ersity of Manitoba, Winnipeg, Man., Canada R 3 T 2 N 2 b De´partement de Ge´ologie, Uni6ersite´ de Montre´al, CP 6128 , Succursale A, Montreal, Que´bec, Canada H 3 C 3 J 7 Abstract Two sparsely plagioclase-phyric rhyolite domes, 50 – 100 m thick and 125 – 250 m long as observed in 2-dimensional, vertically dipping exposures, are partly mantled by a rhyolite volcaniclastic complex that, on the south side of the domes, also forms a half-cone-like mound at least 60 m thick and 500 m long. The domes and mantling volcaniclastic complex are overlain by three aphyric to sparsely phyric rhyolite flows that are 70 – 120 m thick and 550 – \ 1200 m long. Domes and flows have a lower columnar-jointed, highly fractured, locally brecciated subfacies that grades upward into 40 – 95 crackled and disaggregated breccia. Crackled breccia is a highly fractured subfacies in which clast-like areas are bounded by closely spaced joints and minor matrix, but there is only limited rotation of clasts. Crackled breccia was produced by quench fracturing combined with hydraulic action of water converted to steam or supercritical fluid within the fractures. Overlying disaggregated breccia comprises rotated particles and 25 – 50 matrix. It is most abundant near flow margins and is a crumble breccia produced by flow advance or expansion. The volcaniclastic mound comprises lenticular, interlayered facies of 1 resedimented, phreatomagmatically generated, heterolithic, rhyolitic tuff and lapilli-tuff, 2 isolated to close-packed rhyolite lobes that are 0.5 – 5 m thick and 0.5 – \ 50 m long; the lobes are, at least in part, pillows, and 3 units comprising small B 2 m long rhyolite lobes and blocks enclosed in a monolithic, probably hyaloclastic tuff and lapilli-tuff matrix. The volcaniclastic mound represents a series of thin lava tongues both extruded over, and intruded into, coeval hyaloclastite and resedimented, pyroclastic deposits, and it is part of a cone. Once initiated, the cone, and nearby early domes, acted as a barrier to further flow advance and progressively grew upward with eruption of successive flows. © 2000 Published by Elsevier Science B.V. Keywords : Rhyolite; Subaqueous; Lava flows; Volcaniclastic rocks; Paleoproterozoic; Manitoba www.elsevier.comlocateprecamres

1. Introduction

Felsic volcanism is uncommon in oceanic areas; thus, there are few modern examples of sub- aqueous felsic lava flows and domes e.g. Reynolds and Best, 1976; Lonsdale and Hawkins, 1985. There are, however, a number of examples from ancient volcanic sequences ranging in age from Archean to Quaternary. These include flows and domes erupted in both subglacial lakes Fur- nes et al., 1980 and oceanic settings Cas, 1978; Corresponding author. Fax: + 1-204-4747623. E-mail address : ld –ayresumanitoba.ca L.D. Ayres 0301-926800 - see front matter © 2000 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 0 8 9 - 3 Bevins and Roach, 1979; De Rosen-Spence et al., 1980; Yamagishi and Dimroth, 1985; Cas et al., 1990; Kano et al., 1991 and shallow intrusive domes that locally broke through to the surface to feed lava flows Kokelaar et al., 1984; Allen et al., 1996. Many of the described subaqueous felsic flows and domes are characterized, at least in part, by pillow-like lava lobes, pods, and tongues enclosed within a volcaniclastic component of the same composition. The volcaniclastic component is generally believed to be hyaloclastite produced by quench brecciation of the lobes, but there is some disagreement on the emplacement of the lobes. For example, De Rosen-Spence et al. 1980 and Yamagishi and Dimroth 1985 have proposed that the lobes were lava extrusions at the front of an advancing lava flow or on the surface of a growing dome. As lobe extrusion continued, quench brecciation produced a hyaloclastite en- velope surrounding the lobes. Furnes et al. 1980, on the other hand, stated that most lobes were intrusions into a preexisting hyaloclastite cone, whereas Kano et al. 1991 described the lobe-vol- caniclastic facies as the subaqueous equivalent of block lava flows. In this paper, we describe a Paleoproterozoic rhyolitic flow-dome-cone com- plex, part of which is a discrete lobe-volcaniclastic facies that appears to be spatially separate from, although genetically related to, domes and flows.

2. Regional setting