Petrology and geochemistry of Cambrian volcanic rocks from the Avalon zone in Newfoundland and New Brunswick

Greenough, John David (1984) Petrology and geochemistry of Cambrian volcanic rocks from the Avalon zone in Newfoundland and New Brunswick. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis examines the petrology and geochemistry of Cambrian volcanic rocks from the Avalon Peninsula (Newfoundland) and southern New Brunswick as well as Silurian sills and Devonian (?) dikes from the Avalon Peninsula. The geochemistry of these rocks provides new information on Early Paleozoic tectonism affecting Avalonian terrane in eastern North America. The effects of low grade metamorphism on the primary geochemistry of each rock group are also discussed. -- Middle Cambrian basalts on Cape St. Mary's (Avalon Peninsula) were affected by two phases of alteration that resulted in the formation of chlorite and carbonate. The effects of the chlorite formation on element concentrations resemble those observed for basalt/seawater interaction, whereas the effects of carbonate addition are similar to those produced in high CO₂ systems. Basalts in the Beaver Harbour area of New Brunswick show similar two-phase alteration, but phosphorous and yttrium were added during the carbonate addition phase suggesting that there were differences in the metasomatic solutions between the two areas. -- Silurian sills on Cape St. Mary's show primary textural, mineralogical and bulk-rock geochemical characteristics indicative of high volatile contents. Thermogravitative processes probably involving volatile complexing of some elements caused enrichment of these elements in the upper portions of the sills, or removal from the sill system as a whole. The parental sill magma was geochemically somewhat evolved, tholeiitic, and closely resembled compositions observed in flood basalt and rift provinces. -- Dikes in the Cape St. Mary's study area show a range of bulk rock compositions representative of at least two batches of magma that underwent varying degrees of evolution. Some of the rocks appear to have alkaline characteristics whereas others have tholeiitic attributes, but all were probably produced in a tensional environment. These rocks resemble Devonian dikes around Bonavista Bay and could be time equivalent. -- The Cape St. Mary's Cambrian basalts and their feeder pipes display (primary) whole-rock and mineralogical compositions representative of evolved alkali basalts. Lower to Middle Cambrian volcanic rocks in New Brunswick form a bimodal suite, the basaltic portion of which shows compositions ranging from relatively unevolved to highly evolved tholeiites. -- A review of information on the Precambrian stratigraphy, Cambrian lithologies, fauna, stratigraphy, and paleomagnetism in the Acado-Baltic province shows that most of western Europe and eastern North America probably formed a large continental block that remained at lower latitudes throughout the Cambrian. The bimodal nature of the volcanic rocks over this terrane along with their relatively small volume indicates they formed in a continental rifting environment. Basaltic rocks in southern New Brunswick, Cape Breton, Norway, Poland, and on the Avalon Peninsula show compositions consistent with this interpretation. The small volume of the volcanic rocks and platformal character of the associated sedimentary rocks suggest that the area experienced relatively stable tectonic conditions during the Cambrian with small amounts of extension causing the rift-type volcanism. The tension affected a broad area and resulted in a number of narrow basins but few rift valleys. Volcanism was much more common during the Early and Middle Cambrian than during the Late Cambrian suggesting that the tensional tectonic regime may have waned in the Late Cambrian in direct or indirect response to processes which led to (Ordovician) closing of the Iapetus Ocean. Sills and dikes on Cape St. Mary's provide local evidence that the stable tectonic conditions with small-scale tension persisted throughout the Early Paleozoic.

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