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Prehi\Ite.Pt'mpellyite Facies Metamorphism of Late Prtcambrian Rocks of the Avalon Peninsula, Newfot]Ndland

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Prehi\Ite.Pt'mpellyite Facies Metamorphism of Late Prtcambrian Rocks of the Avalon Peninsula, Newfot]Ndland Canadian Mineraloglst Vol. 12, pp. 463468 (1974) PREHI\ITE.PT'MPELLYITE FACIES METAMORPHISM OF LATE PRTCAMBRIAN ROCKS OF THE AVALON PENINSULA, NEWFOT]NDLAND V. S. PAPEUK Department ol Geology, Memorial Universitt of Newfoundland, St. John's, Newfoundland. ABsrRAcr Papezik (L972) rcported mineral assemblages characteristic of the prehnite-pumpellyite facies A preliminary study of mineral assemblagesat in Hadrynian sandstonesnear St. John's. Subse- several selected localities shows that the late quent work, summarized in this paper, shows Precambrian volcanic and sedimentary rocks of are developed at many the Avalon Peninsula of New'foundland have been that such assemblages affected by metamorphism grading from the preh- other localities, and are in fact characteristic of nite-pumpellyite to the greenschist facies. A broad the Late Precambrian rocks of the central and prehnito zone in the eastern part of the peninsula eastern part of the Avalon Peninsula from the is followed by a relatively narrow zone of prebnite westetn shore of Conception Bay eastwards, and pumpellyite south of Conception Bay, which in across a distance of 40 km (Fig. 1). turn grados westward into an actinolite zone with- out pumpellyite on the Istlmus of Avalon and in tho western part of tho Avalon Platform. The com- GnNener- GnorocY bined width of tlte prehnite and pumpellyite zones is an area of consider- is about 40 km. Pumpellyite is developed best The Avalon Peninsula in petrological the basalt flows at the western flank of the Harbour able structural, chemical and com- Main volcanic belt. The metamorphic zones seem plexity. In general, it conshts of an elongated to follow the northerly regional trend typical of northerly-trending dome, with the predominant- the Avalon rocks; the metamorphic grade increases ly volcanic rock$ of the Harbour Main Group in the direction of tighter folding and increasing forming its core and the dominantly sedimentary penetrative deformation, which indicate increasing rocks of the Conception, Cabot, Hodgewater tectonic stress. A diffuse therrral aureole, indicated and Musgravetown Groups its flanks. The grey by local development of actinolite, follows the con- siliceous siltstone and sandstone of the Concep- tacts of the granitoid pluton Holyrood in the cen- Group can be traced around the southern tral part of the peninsula. tion part the overlying coaner' The formation of prehnite Dear St. John's was of the central core; (with contemporatreous with the tectonic stresses that re- grained sandstone and conglomerate minor sulted in the open folding of the Late Precambrian shale) of the Cabot Group in the east and the sediments. As these are believed to have been related Hodgewater and Musgravetown Groups in the to the Acadian orogenic events, the age of the low- west are probably stratigraphically equivalent, grado metamorphism is tentatively considered to be but because of their physical separation cannot Acadian (Devonian). be correlated with certainty. The volcanic core is intruded by a highJevel granitoid pluton, the INrnoDucuoN Holyrood batholith. These Late Precambrian rocks are locally overlain by discontinuous The Late Precambrian rocks of the Avalon patches of Cambro-Ordovician sediments, gen- Peninsula have been systematically mapped and erally flat-lying or gently dipping in the east, described by several workers (e.9. McCartney tightly folded in the west, south of Trinity Bay 1967; Rose L952), but little attention has been (Fig. 1). paid so far to tleir metamorphic grade. Mc- In detail, the geology is relatively compl€x. Cartney (1967), noting tle lack of penetrative The Avalon rocks are both folded and intricately deformation and the excellent preservation of faulted; block-faulting, from outcrop to regional primary structures, believed the rocks to be dimensions, is one of the dominant tectonic generally unmetamorphosed, although locally features of the volcanic core. The intensity of modified by thermal and metasomatic effects. structural deformation increases in general from Rose (1952) stated in passing that the volcanic east to west. Two northerly-trending fault zones rocks of the Harbour Main Group were "re- of regional extent divide the Harbour Main gionally metamorphosed" (p. 15); his brief rock volcanic belt into three blocks. The eastern descriptions (e.5. p, 16) appear to imply a green- block consists mainly of massive to pillowed schist facies of nrctamorphism. More recent\, basaltic flows and pyroclastics,with a few rhyo- 463 464 THE CANADIAN MINERALOGIST lite domes. The western block is characterized lain by amygdaloidal basaltic flows. It is prob- by subaerial pyroclastic rocks interbedded with able that the two blocks represent different fluvial sediments; the volcanic rocks include levels of the original volcanis pile. The central several belts of well-preserved ignimbrites, over- block is dominated by the granitoid pluton, the ffi Comlro-OrdcvlGl66 l.dld.nt.. Bull Arm Fm- IN ,|lallc and taltla vol.o.l.t. I Cabot Gp.(C), HodgswaterGp.(Hw), & eq ! :E Sqndatcn., b rhol., (caglohardtr_ z t:l 9.glceptionGp.4 Connectingpoint cp. >r Slltr?on., gr.yrd.l.; 6tnor tutt, G!1. o- + fsjl Holtrocd Oronltcld tluton. t_ Harbour Main Go. l':v.vl I ,{otl. I t.trl. tlort, pyrorlotilar. 3adlhcntt. t----.i-----lf--i5 -r. o I lo rl 6 t!r- K dr" H' tI :?' l> L I:tc. 1. Geology of the northern part of the Avalon Peninsula, Newfoundlancl; sim- plified from McCartney (1967) and Rose (1952). Numbers in circles refer to Io- calities discussedin text. Note: x-pattern near loc. 72 - Powder Horn intrusive 'M" complex. - Musgravetown Group. Characteristic minerals at numbered localities: l. Prehnite, qz, cc, chl, ab 2, Ab, chl, ep, cpx 3. Stilbite, qz 4. Act, ep, chl, ab, qz 5. Pyrophyllite, sericite, qz 6. Prehnite, cc, qz 7. Celadonite, chl, cc, hem 8. and 9. Pumpellyite, prehnite, cc, ep, chl, qz, adularia 10, 11, and 12. Act, ep, chl, ab, qz PRBHNITE.PUMPELLYITE FACIES METAMORPHISM OF LATE PRECAMBRIAN ROCKS 465 intrusion of which caused metasomatic altera- et al. 1959) and are considered to be of late, tion of the adjacent volcanic rocks. The contact post-tectonic, low-temperature origin. auteole, varying geatly in width and intensity, In the central block, volcanic rocks adjacent includes several lenses of massive pyrophyfite. to the Holyrood granitoid pluton have been Most of the Avalon rocks show very little modified by thermal metamorphism and meta- penetrative deformation; even such delicate somatism related to the intrusion. Within the strucfures as shards, flow-banding, etc. are per diffuse contact aureole, which may be several fectly preserved at many localities. In general, km wide, actinolite appears in addition to the schistosity is confined to relatively narrow sheat- ubiquitous epidote and chlorite (Loc. 4). Clino- zones of local extent. pyroxene, where present, is partly 19 almost The rapid variations in lithology even over completely altered to fibrous pale greenish acti- short distances greatly complicate the tracing nolite, in contrast to the generally unaltered of regional distribution of low-grade metamor- pyroxene of the western and eastern blocks. phic assemblages.A systematic study of the me- Metasomatism has played a prominent role tamorphism of the Avalon locks, now in pro- n'ithin the contact aureole. Rhyolitic flows and gress, will thus require several years for its pyroclastics in the vicinity of the pluton have completion. This preliminary report describes been altered along local shear-zones to a fine- only the characteristic metamorphic assemblages grained assemblageof quartz, sericite and pyro- at a number of selected localities across the phyllite (Loc. 5); this alteration was probably Avalon Peninsula, and draws several tentative caused by acid (sulphate-bearing) solutions orig- conclusions from their distribution. (All locali- inating in the granite, in the temperature range ties mentioned below are shown on Fig. 1). of 200-250"C and under Pszo of about 2 kb Keats 1970). To the west of the Holyrood batholith, preh' MEtattonPHrslvl nite reappean in a few thin (I-2 cm) veins cut. ting a thick section of reddish sandstones and Prehnite zone conglomerates of the Harbour Main Group (Loc; 6). The milky white prehnite is here asso' On the (Loc. eastern outskirts of St. John's 1) ciated with minor quartz and calcite. About 1 prehnite is found in fine-grained feldspathic kq to the west (Loc. 7), an oxidized porphyritic sandstone of the part Signal Hill Formation, of sill of basaltic composition contains abundant the Cabot Group. The mineral appears in a sys- small (0.2 to 1 mm) amygdulesfilled with grass- tem of veins, together with varying amounts of green celadonite and minor amounts of calcite, quartz, calcite and chlorite, and as an interstitial chlorite and hematite. Celadonite, a relatively mineral in the sandstone matrix where it forms common mineral in sub-greenschistmetamorphic numerous patches. small difftrse It is restricted assemblages, is considered to be characteristic to a stratigraphic layer about 150-250 meters of the znolite facies, disappearing before the thick, close to the contact between Signal Hill pumFellyite stage is reached (e.g. Coombs et al. sandstone and the underlying St. John's shale; 1.959;Brown 1961). Although no zeolites have abovo this layer the vein assemblagechanges ra- been found here, a very low grade of metamor' pidly quartz-calcite-prehnite -+- from chlorite to phism is indicated at this point. quartz-chlorite-albite. Pumpellyite has not been found in this area (Papenk 1972). Pumpellyite zone The predominantly basic volcanic rocks of tho eastern block, studied in some detail near Pumpellyite appears first at the western flank Cape St. Francis (Loc. 2) show only a non- of the Harbour Main volcanic belt, near Col- diagnostic mineral assemblageof albitized pla- liers Bay (Loc. 8). The rocks at this locality gioclase, minor clinopyroxene, chlorite, epidote, consist of a series of steeply dipping ash-flow sphene and iron oxides, with some carbonate. tuffs, facing west, interbedded with fluvial and Similar assemblages have been reported by lacustrino sediments and overlain by basaltic Maher (1972).
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