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Maritime Sediments and Atlantic Geology Maritime Sediments and Atlantic Geology Vol. 20 AUGUST, 1984 No. 2 Geochemistry of Silurian-Devonian alkaline basalt suites from the Gaspe Peninsula, Quebec Appalachians R. LamiejvL and BelxingeJi, de,pcudbnejrvL o-fL Qe.otogy, Laval. llnlvesiAity, Ste. Toy, Qu£JLe.c Q1H 7P4 Products of Late Silurian and Early Devonian volcanism in the Northern Appalachians are well ex- posed in the Chaleurs Bay area of the Gaspe Peninsula. The volcanic association is dominated by plagioclase basalts and andesites, and includes pyroclastic flows of dacitic and rhyolitic composi- tion. Major and trace-element chemical analyzes of 174 rocks show that the volcanic association de- rives from alkaline and high-alumina basalt magmas. The basalts are usually olivine normative and high in A!203, TiOz, P20s and in incompatible elements such as Y and Zr. They follow the course of a calc-alkaline differentiation trend. This volcanism is interpreted to have been related genetically to a regime of tectonic compression, and controlled structurally by fault zones. The Quaternary volcanism associated with the major transcurrent fault zones of northern Anatolia and Iran within the Alpine Orogen provides a modern analog to the Silurian-Devonian volcanism of the Gaspe Peninsula and Newfoundland. Les produits du volcanisme silurien et devonien des Appalaches du Nord sont bien exposes dans la region de la Baie des Chaleurs de la peninsule gaspesienne. L'association volcanique comprend sur- tout des basaltes plagioclasiques et des andesites, tout en incluant une quantity plus faible de coulees pyroclastiques de composition dacitique et rhyolitique. Les analyses chimiques des elements majeurs et traces de 174 roches indiquent que l'association volcanique derive de magmas de type basalte alcalin et basalte alumineux. Les basaltes ont frequemment de l'olivine normative, et ils sont riches en A1203, Ti02, P20s , ainsi qu'en elements incompatibles tels que Y et Zr. Ils suivent une tendance de differenciation de type calco-alcaline. Ce volcanisme est interprete comme etant genetiquement lie a un regime tectonique de compres- sion, et sa localisation est contrSlee structuralement par des zones de failles. Le volcanisme qua- ternaire associe aux grandes failles de decrochement du nord de l'Anatolie et de l'Iran, a l'inter- ieur de I'orogene alpin, est considers comme un analogue moderne du volcanisme siluro-devonien de la p6ninsule de Gaspe et de l'ile de Terre Neuve. INTRODUCTION Devonian stratigraphic boundary in the Although volcanic rocks make up a sig- Chaleurs Bay area (Fig. 1). Two volcanic nificant amount of the geological record groups are distinguished on the basis of of Gaspe Peninsula, their chemistry and structure and stratigraphic position: (1) petrology have remained poorly known the Late Silurian rocks of the Mont Alex- until recently. This information, however, andre syncline, and (2) the Earliest Devon- is critical for an understanding of the tec- ian volcanic rocks of the Restigouche area. tonic environment that characterized the These two groups are separated by a evolution of this segment of the Appala- stratigraphic unconformity. chian Orogen in mid-Paleozoic time. We The volcanic rocks of the Mont Alex- report herein a summary of our work car- andre syncline are interbedded with sedi- ried out since 1980 on the Silurian-Devon- mentary rocks that laterally grade in com- ian volcanic rocks and which involved position from clastic to calcareous. At detailed mapping as well as petrological Lake McKay, in the western part of the studies (B§langer et al. 1981). syncline, the volcanic rocks are interbed- The Silurian-Devonian sedimentary and ded within the Saint-L6on mudstone and volcanic rocks of southern Gasp€ Peninsula siltstone whereas at Mont Observation, in lie at the northeastern end of the Gasp6- the eastern part of the syncline, they are Connecticut Valley Synclinorium. Thick se- interbedded within the reef limestone of quences of shallow marine and subaerial the West Point Formation (Fig. 1). Both volcanic rocks occur at the Silurian- sequences are of Late Ludlovian and Pri- dolian age (Bourque and Lachambre 1980). MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 20, 67-78 (1984) 0711-1150/84/020067-12$2.80/0 68 Laurent and Bfelanger Fig. 1 - Outline map of the Gaspfe Peninsula showing then Silurian-Devonian outcrop and the location of the volcanic rocks studied. In the Ristigouche area, most of the vol- from 1 to 20 mm long, forming from less canic rocks occur above the regional un- than 3% to more than 25% of their conformity and are in close association volume. These plagioclase basalts are high- with the Indian Point Formation of Gedin- ly vesicular, the vesicles being partly fill- nian age (Bourque and Lachambre 1980). ed by calcite, chlorite and quartz. Swarms The latter consists of basal conglomerates, of dikes consanguinous with these volcanics made up of Late Silurian limestone and are intrusive within the underlying Silurian lava pebbles, overlain by sandstone and sedimentary rocks of the syncline. mudstone. The 5 km thick volcanic belt of Risti- The volcanic sequence of the Mont gouche is part of the northern limb of the Alexandre syncline has a maximum thick- Chaleurs Bay syncline, the southern limb ness of about 1200 m in the Mont Obser- of which crops out in New Brunswick. The vation area and of about 1000 m in the volcanic sequence consists of lava flows Lake McKay area (Bourque and Lachambre and pyroclastic rocks varying in composi- 1980). It consists of basaltic lava flows tion from basaltic to rhyolitic. Andesites ranging in thickness from a few to 50 m. predominate. The lava flows are vesicular Most flows are massive. Some flows are and massive or brecciated; locally they dis- brecciated (flow breccias), others locally play columnar jointing but never pillowed display columnar jointing or pillowed struc- structures. The pyroclastic rocks occur as tures. Lenticular beds of volcaniclastic thin layers of well-bedded pumice or scoria conglomerate and coarse-grained sandstone and ash deposits, and as thick units of un- are common within this sequence, but sorted block and ash deposits. Blocks in pyroclastic tuffs and breccias are rare. this type of deposit are up to 3 m long, Textures of lavas vary from aphyric to por- though most of them are less than 40 cm phyria All porphyric lavas are character- in diameter. The matrix consists of lithic ized by ubiquitous plagioclase phenocrysts and crystal fragments of the same com- MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 69 position as the blocks. This type of deposit Devonian. Although there is a continuum is commonly oxidized and purple-red in in the composition of this volcanic associ- color. Textures of lavas are most fre- ation, the sequence is divided on a strati- quently aphyric or microporphyric. The graphic basis into a Late Silurian and an porphyric lavas are characterized by 5 to Early Devonian group (Table 1). This sub- 30% plagioclase phenocrysts from 2 to 20 division allows a better understanding of mm long. Small amounts of clinopyroxene the evolution of the volcanism through phenocrysts occur in some lavas. Lava time. vesicles contain calcite, quartz, chalce- The silica content varies from less than dony, chlorites, leucoxene, pumpellyite 50% to more than 70%, which justifies the and zeolites. Consanguinous intrusive practice of using silica content in classify- bodies are few and always of small size. ing rocks with these compositions. The The Black Cape volcanic member (Fig. 1) samples have been classified on the basis also belongs to the Chaleurs Bay syncline. of silica content into basalt (<53% Si02), Though structurally related to the Risti- andesite (53-63% Si0 ), dacite (63-70% gouche volcanic belt, the Black Cape ba- 2 Si02), and rhyolite (>70% Si02) following saltic and andesitic lavas are stratigraphi- the silica limits proposed by Taylor et al. cally equivalent to the Late Silurian Mont (1969). Alexandre volcanic rocks. They are also The Late Silurian volcanic group con- included in the present study. sists of 75% basalt, 24% andesite, and 1% rhyolite; the Early Devonian volcanic group PRESENTATION OF DATA consists of 28% basalt, 45% andesite, 22% The major and trace element chemical dacite and 5% rhyolite (Table 1 and Fig. analyzes of 174 rocks have been used in 2). No group is bimodal. The Late Silurian this paper to identify the main features volcanism was predominantly basaltic while of the volcanic association. The sampling the Early Devonian volcanism was mainly was systematically made from four contin- andesitic and- represents a wider spectrum uous sections studied in detail, and is as- of compositions. It was also characterized sumed to be representative of the four lo- in the Ristigouche belt by several cycles calities listed on Table 1. We believe that of igneous activity that locally led to these volcanic rocks were extruded into accumulations of more than 3 km of vol- a distinctive tectonic setting during an canic rocks. essentially continuous 10 Ma time interval Most basalts are aphyric or micropor- spanning the Late Silurian and the Earliest phyric, especially in plagioclase. Average Table 1 DISTRIBUTION OF SAMPLES BY LOCALITY AND BY AGE LOCALITY Number of samples: Percentage Basalt Andesite Dacite Rhyolite Basalt Andesite Dacite Rhyolite Mont-Observation 27 8 85.4 14.6 - 43 : : Lake McKay 4 Black Cape 8 12 32.6 48.9 14.1 4.4 Ristigouche 22 32 14 4 AGE Number of samples: Percentage Basalt Andesite Dacite Rhyolite Basalt Andesite Dacite Rhyolite Late Silurian x 82 27 - 1 75 24 1 Early Devonian y 18 29 14 3 28 45 22 5 Total number of samples: 174 x = samples from Mont-Observation, Lake McKay, Black Cape and Ristigouche y = samples from Ristigouche 70 Laurent and Bfelanger COMPOSITION B¥ AGE siluriaN Jw devonian tvmr/m -and.^ac coMposition Fig.
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