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

Vertical transitions from pillowed to pillow breccia are explained by changing discharge rates during single eruptive events, whereas lateral changes represent slope differences andor dimin- ishing magma supply at the distal parts of flows as a result of increasing surface area Cas, 1992. The lobe to pillow transition in the Beaulieu River volcanic belt indicates decreasing flow rate Griffiths and Fink, 1992. Autoclastic pillow breccia and disorganized hyaloclastite in the Peltier Formation is interpreted to have developed at flow tops and fronts from quench fragmenta- tion during the late stages of an eruption in a remote part of a volcanic edifice where lava sup- ply was sufficiently decreased Dimroth et al., 1978; Busby-Spera, 1987. Stratified hyaloclastite, or hyalotuff, in the Beaulieu River volcanic belt represents the reworked deposits of pillowed flows and autoclastic pillow breccia and hyaloclastite Dimroth et al., 1978. Bedded tuffs interstratified with massive flows indicate syn-volcanic deposi- tion of localized subaqueous eruption material. Shale units intercalated with pillow breccia and hyaloclastite mark the boundaries between sepa- rate flow events, indicating periods of volcanic quiescence.

5. Discussion

Volcanic facies characteristics and associations are contingent on eruption style, depositional pro- cesses and environment, and tectonic setting. In contrast to studies of modern seamounts, identify- ing and describing Archean pillow volcano or seamount sequences is facilitated by cross-sec- tional exposures through the edifice and unam- biguous contact relationships. Problems may arise in areas that have undergone extensive metamor- phism, deformation, and erosion of the seamount summit. Notwithstanding, a model reconstructing a seamount based on the facies architecture in all three localities is attempted because 1 examples of Archean seamounts are lacking, 2 using a model facilitates interpretation of eruptive pro- cesses in addition to lateral and vertical facies distribution, and 3 the study areas contain well- exposed homoclinal sequences. 5 . 1 . Seamount construction Abundant pillowed flows, pillow breccia, and hyaloclastite in the Peltier Formation and Beaulieu River volcanic belt attest to a sub- aqueous environment in which effusion rates were generally low Dimroth et al., 1978; Ballard et al., 1979. Although all three study areas document mafic subaqueous volcanism, differing facies types, structures, and percentage of vesicularity, are characteristic of specific locations on a typical seamount. Initial seamount construction in water depths below 500 – 2000 m is represented by locality A in the Peltier Formation, as suggested by the thick pillowed sequence, dykesill complex, absence of explosive debris, and low vesicularity Moore and Schilling, 1973; Fisher, 1984; Staudigel and Schminke, 1984; Kokelaar, 1986; Cas, 1992. Dykes and sills, salient components of seamount architecture, are interpreted to represent settings proximal to the magma source i.e. vent because dyke percentage decreases with increasing dis- tance from a volcanic centre Walker, 1993; Mc- Phie, 1995. These intrusions have high preservation potential, especially in ancient rocks where erosion of the subaerial portions of seamounts is extensive Walker, 1993; Sohn, 1995. Primary eruption in seamount development necessitates a point source that may be fed di- rectly by pipes or from an initial fissure that has collapsed, leaving only a few central conduits through which lava is supplied to the edifice Smith and Cann, 1992; Chadwick and Embley, 1994. At locality A, magma originating from a point source is inferred to have been distributed and emplaced through a complex dyke-sill system, leading to the development of a pillow volcano or mound Fig. 13A. The minimal pillow breccia and hyaloclastite component at locality A sup- ports a proximal setting with respect to the magma source Wells et al., 1979; Fisher, 1984; Busby-Spera, 1987. The low vesicularity of flows is suggestive of a deeper water environment than those envisaged for locality B and the Beaulieu River volcanic belt because vesicularity typically decreases with increasing depth Moore and Schilling, 1973; Staudigel and Schminke, 1984. An increase of pillow breccia and hyaloclastite with interstratified sedimentary deposits at local- ity B represents growth of a seamount outward from the source Fisher, 1984 and Fig. 13B. Following quench fragmentation of pillowed flow tops and fronts, periods of volcanic quiescence ensued, resulting in the accumulation of very fine- grained sedimentary deposits between pillow brec- cia and hyaloclastite Fig. 13B. Massive hyaloclastite units typically accumulate at vents relatively remote from the magma source where discharge temperatures are significantly less Lonsdale and Batiza, 1980; Busby-Spera, 1987. Effusive volcanism ensued following hyaloclastite emplacement, resulting in the deposition of mas- sive flows. Local interstratified bedded tuffs were deposited from turbidity currents following lim- ited subaqueous pyroclastic eruptions Fig. 13B. Volcaniclastic deposits are typically more common in remote parts of a volcanic edifice, away from the near-vent setting McPhie, 1995; Orton, 1996. The increase in vesicularity 0 – 27 with respect to locality A 0 – 5 suggests that the pillowed and massive flows, pillow breccia, and hyaloclastite at locality B were deposited in water B 500 – 800 m deep, but no shallower than 200 m based on the presence of non-stratified pillow breccia and interstratified shale, indicating a be- low wave base setting Fig. 13B. Stratified pillow breccia and hyaloclastite and increased vesicularity in the Beaulieu River vol- canic belt are characteristic features of the upper- most part of a subaqueous edifice Staudigel and Schminke, 1984, Fig. 13C. Scoriaceous, glass shard-rich, stratified hyaloclastite formed by wave reworking and redeposition along the flanks of an edifice in shallow water B 200 m. Based on the lateral and vertical facies associations with coher- ent pillowed flows, the stratified pillow breccia is interpreted to have developed at the head of steep flow fronts in shallow water Dimroth et al., 1978, Fig. 13C. In addition, high vesicularity 20 – 49 in pillowed-lobate and massive flows, and in pillow breccia, supports a shallow water setting Moore and Schilling, 1973. The study area in the Beaulieu River volcanic belt is a good repre- sentative of the topmost portion of a seamount Fig. 13C. 5 . 2 . Analogues Volcanic facies in the Peltier Formation and Beaulieu River volcanic belt resemble those of Phanerozoic seamounts or pillow volcanoes that develop on the ocean floor in mid-oceanic or back-arc rift zones. Mid-oceanic analogues in- clude modern seamounts associated with the Mid- Atlantic Smith and Cann, 1992; Head et al., 1996 and Juan de Fuca Chadwick and Embley, 1994 ridges, and the East Pacific Rise Lonsdale and Batiza, 1980; Hekinian et al., 1989; Smith and Batiza, 1989, in addition to Mesozoic-Cenozoic seamounts in the Canary Islands Staudigel and Schminke, 1984 and Cyprus Eddy et al., 1998. Further comparisons can be made to modern arc-backarc-related seamounts in the Mariana Trough Fryer, 1995, and the Lau Basin Hawkins, 1995, and to Cenozoic seamounts in southwest Japan Kano et al., 1993 and in the Japan Sea Sohn, 1995. The pillowed flow-dominated sequence at local- ity A in the Peltier Formation is similar to B 45 m-thick pillow mounds on the Cleft segment of the Juan de Fuca Ridge Chadwick and Embley, 1994, 50 – 650 m-high pillow-dominated seamounts in the rift valley of the Mid-Atlantic Ridge Smith and Cann, 1992; Head et al., 1996, and the 200 m-high Alestos Hill seamount in an Fig. 13. Models illustrating the inferred location of Peltier Formation and Beaulieu River belt facies on a seamount. A Initial seamount construction on the ocean floor, represented by locality A in the Peltier Formation. Magma originating from a point source was transported through the edifice using dykes and sills as conduits and was extruded as pillowed flows in deep water. B Continued effusive volcanism of a seamount in shallower water is represented by locality B. Disorganized pillow breccia and hyaloclastite accumulated on the medial portions of the seamount. Interstratified shale units indicate periods of volcanic quiescence. During deposition of massive flows, local andesitic-dacitic eruptions occurred, depositing bedded tuffs. C A shallow water setting is represented by the volcanic facies in the Beaulieu River belt where stratified pillow breccia and hyaloclastite were deposited under wave action along the flanks of a seamount. Fig. 13. inter-graben zone of the Troodos ophiolite, Cyprus Eddy et al., 1998. Edifices in these set- tings are either inferred or proven to be fed by magma migrating through feeder dykesill com- plexes. Mafic edifices characterized by abundant feeder dykes and sills indicate extension associ- ated with rifting Walker, 1993. Extensive disorganized autoclastic pillow brec- cia and hyaloclastite associated with coherent massive and pillowed flows at locality B resemble the Trachyte I lithofacies of the inferred 2000 m-high, back-arc Tok Island volcano, Korea, which represents a subaqueous effusive episode before the onset of explosive shallow-water to emergent volcanism Sohn, 1995. The interstrat- ification of bedded tuffs and massive flows at locality B is comparable to the relationship be- tween distal volcaniclastic deposits and volcanic flows in the \ 1500 m-thick, inferred island-arc seamount on the Shimane Peninsula, southwest Japan, as described by Kano et al. 1993. Inter- stratified sedimentary deposits recording volcanic quiescence are comparable to those presently ac- cumulating on seamount summits 800 – 2500 m deep near the East Pacific Rise Lonsdale and Batiza, 1980; Smith and Batiza, 1989. Stratified hyaloclastite and pillow breccia asso- ciated with coherent pillowed and massive flows in the Beaulieu River volcanic belt are analogous to the intermediate-shallow water flank deposits of the 1800 m-thick, Pliocene seamount sequence at La Palma, Canary Islands Staudigel and Schminke, 1984. The Archean Slave Province examples are best compared with the seamount at La Palma, which is characterized by a deep water, basal pillowed sequence intruded by dykes and sills, overlain by in-situ hyaloclastite and pillow breccia, changing up-section into intermediate- shallow water, stratified, reworked deposits Staudigel and Schminke, 1984. The paucity of mafic explosive debris in the Peltier Formation and Beaulieu River volcanic belt indicates that either 1 none of the seamounts breached the water surface, or 2 the emergent portions of the seamounts have been eroded. Comparisons with Phanerozoic mafic sub- aqueous edifices have shown that distinguishing between Archean seamounts forming in different tectonic settings based solely on volcanology is problematic. Elucidating the tectonic setting would be facilitated by integrating volcanology and geochemistry. On a regional scale, the Point Lake and Beaulieu River belts are tentatively interpreted to represent remnants of arc systems that developed on continental crust based on geo- chemical results from Lambert et al. 1992 and preliminary results from Dostal and Corcoran 1998 which indicate that the rocks range from mainly tholeiitic to calc-alkaline in composition, and in the Point Lake belt display both negative and positive ond values. On a more detailed scale, the tholeiitic basalts in each of the study areas are similar to MORB, indicating that the seamounts may be associated with mid-oceanic or back-arc rifting. More in-depth geochemical results and implications from the Peltier Formation are the focus of a subsequent manuscript in preparation.

6. Conclusions