Geochemistry of volcanic rocks from the Tetagouche Group, Bathurst, New Brunswick, Canada: Discussion

Abstract

A recent paper by Whitehead and Goodfellow (1978) presented much needed petrochemical data on volcanic rocks of the Ordovician Tetagouche Group in northern New Brunswick. Although analytical results were not tabulated, numerous plots of major and trace elements confirm an earlier suggestion by Davies et al. (1973) that the Tetagouche volcanic rocks are essentially bimodal, and that andesites are rare or absent. The mafic rocks are subdivided into three distinct groups, group A being either tholeiitic or calc-alkaline basalts, groups B and C being alkaline basalts. The dacitic to rhyolitic rocks belong to the calc-alkaline clan. The following model for the evolution of the Tetagouche Group is proposed in their paper: 1. An ocean-floor sequence of basalts formed as a result of the opening of the Proto-Atlantic. Tholeiitic basalts (group A) generated at a ridge are overlain by alkaline basalts (group B) believed to have been deposited in an off-axis environment. 2. During the Ordovician Period subduction started. Tholeiitic and calc-alkaline mafic and intermediate rocks expected to be extruded at the early stages of a developing island arc have not been recognized. They are thought to have been either subducted subsequently to their formation or uplifted and eroded; or they may underlie Carboniferous rocks in the southeast. 3. During continued evolution of the island arc calc-alkaline dacitic to rhyolitic pyroclastic rocks and lavas were deposited directly on the ocean floor (groups A and B). This discussion is not about the petrochemical classifications presented, but intends to show some of the problems arising from geological models established on the basis of petrochemical models without considering existing geological constraints. Although all workers familiar with the Bathurst district would agree that the whole stratigraphic and structural story of the Tetagouche Group has not been told yet, the published literature leaves considerably less room for modeling than the authors have taken. The model ignored the fact that the volcanic rocks of the Tetagouche Group are underlain by a quartz-rich sedimentary sequence (Smith 1957; Poole 1967; Helmstaedt 1971, 1973; Fyffe 1976), which is mainly of Early Ordovician age. Whereever the contact with the overlying volcanic rocks can be observed, the volcanic rocks are acidic, and basic volcanic rocks follow higher in the sequence (Helmstaedt 1971, 1973; Luff 1977). In the northeastern part of the district, where basic volcanic rocks predominate, the stratigraphic relationships to surrounding sedimentary rocks are not always clear. However, the basic volcanic rocks of sample areas 2 and 3 (Whitehead and Goodfellow 1978) are structurally continuous (Skinner 1974), and in area 3 contain interlayered limestones with fossils (Helmstaedt 1971 ; Skinner 1974) younger than those of the lower sedimentary sequence (Fyffe 1976). The basic volcanic rocks near the city of Bathurst (sample area 4) are either similar in age to those in areas 2 and 3 (Skinner 1974), or even younger, as suggested recently by Pajari et al. (1977). Is the model of Whitehead and Goodfellow (1978) compatible with this information? 1. The ocean-floor sequence (group A) in area 2, inferred to be related to the rifting of the ProtoAtlantic and therefore presumably of preOrdovician age, postdates Lower Ordovician metasediments as well as much of the acidic volcanic rocks making up the major volume of the Tetagouche Group. If the group A volcanic rocks represent an environment of rifting, this must be related to a later episode, perhaps back-arc spreading, as the main arc had been established already. So far, no Lower Ordovician ophiolites have been recognized in the Tetagouche Group; however, they are thought to exist in the Elmtree-Fournier inlier north of the Rocky Brook Millstream fault (Pajari et al. 1977). C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y Q ue en s U ni ve rs ity o n 05 /0 2/ 14