A comparison of Eastern North America and Coastal New England magma suites: implications for subcontinental mantle evolution and the broad-terrane hypothesis

Abstract

New England and Maritime Canada host two major suites of Mesozoic diabase dykes. The oldest is the Coastal New England dykes that were emplaced between 225 and 230 Ma. These rocks are dominantly alkaline with trace element and isotopic compositions indicative of a high-238 U/ 204 Pb mantle (HIMU) source. The oldest of the -200 Ma Mesozoic rift magmas is represented by the Talcott basalt of the Hartford basin and its feeder dykes. External to the basin is the compositionally equivalent Higganum dyke. The extension of the Higganum, the Onway dyke in New Hampshire, is identical in major and trace element and isotopic compositions indicating that the dyke system represented a feeder to flows of flood basalt proportions. The Talcott system rocks have some trace element similarities with arc basalts and have been interpreted as representing melts of a subduction zone modified mantle beneath the LaurentianGondwanan suture. Incompatible trace element ratios and Ba, Th, and U values are, however, unlike arc basalts and are more indicative of crustal contamination of the primary magma. The coastal New England magmas have oceanic island basalt signatures that are generally thought to represent plume-tail magmatism, which is antithetic to a plume-head origin for the younger eastern North America magmas. However, coastal New England rocks have the same trace element signatures as the alkaline rocks of the Loihi seamount, which represent the pre-shield stage to the voluminous tholeiitic magmatism in Hawaii.Résumé : La Nouvelle-Angleterre et les provinces maritimes canadiennes sont les hôtes de deux suites majeures de dykes de diabase datant du Mésozoïque. La plus ancienne est la suite de dykes de la côte de la Nouvelle-Angleterre; les dykes ont été mis en place il y a 225 à 230 Ma. Ces roches sont surtout alcalines avec des éléments traces et des compositions isotopes indiquent une source HIMU (rapport élevé 238 U/ 204 Pb). Le plus ancien des magmas de rift (~200 Ma, Mésozoïque) est représenté par le basalte Talcott du bassin Hartford et ses filons nourriciers. À l'extérieur du bassin on retrouve le dyke Higgamun de composition équivalente. L'extension du dyke Higganum, le dyke Onway au New Hampshire, est identique en ce qui concerne les compositions des éléments majeurs, des éléments traces et des isotopes, indiquant que le système de dykes représentait un filon nourricier pour des écoulements de basalte à l'échelle de plateaux. Les roches du système Talcott ont des similitudes avec les basaltes d'arc en ce qui concerne les éléments traces; elles représenteraient des fusions d'un manteau modifié d'une zone de subduction sous la suture Laurentia -Gondwana. Les rapports d'éléments traces incompatibles et les valeurs Ba, Th et U sont toutefois peu semblables aux basaltes d'arc et indiquent plus une contamination du magma primaire par la croûte. Les magmas de la côte de la Nouvelle-Angleterre ont des signatures de basaltes d'îles océaniques, lesquels représenteraient un magmatisme de queue de panache, ce qui est antithétique à un...

The Christmas Cove Dyke of coastal Maine, USA, and regional sources for Early Mesozoic flood basalts in northeastern North America

“…Only very minor differences are evident among the compositions of dyke samples from Hurd State Park (Connecticut) through New Hampshire to Swans Island (Maine). The dyke system is remarkably uniform both petrographically and chemically along its great length, with small variations possibly related to fractionation of orthopyroxene phenocrysts as well as minor assimilation of wall rocks, as also observed by Dorais et al (2005) at Onway, and Dunn and Stringer (1990) at Ministers Island.…”

“…4 and 5). Compositions are generally in the basaltic andesite field of LeBas et al (1986) but overlap into true basalt, as is common for quartz tholeiite derived from sub-continental lithospheric mantle (Dorais et al 2005). Rare earth elements normalized to chondrite (Sun and McDonough 1989) have relatively flat patterns with (La/Lu) N ratios about 3.0 to 7.5 (Fig.…”

“…for the Caraquet, Shelburne, and other dyke magmas (Dunn et al 1998;Dostal and Durning 1998;Dorais et al 2005).…”

Field Relations, Petrography and Provenance of Mafic Dikes, Western Maine

“…It would appear that the WM dikes are more closely related to the CNE suite than the ENA dikes. The discrimination diagrams, Figure 8, not surprisingly demonstrate that the WM dikes are within plate basalts with a spread of alkali and tholeiitic compositions McHone (1992) and Dorais (2005), to discriminate between the CNE (red field ) and ENA (blue field) dike suites.…”

“…Geochemical discrimination diagrams (Pearce and Cann, 1973) can aid in examining the tectonic setting of these intrusions. In addition, several geochemical parameters can be employed to discriminate between dikes of potential magma sources, i..e. the Coastal New England (CNE) suite and the Eastern North America (ENA) suite (Dorais et al, 2005 andMcHone, 1992). In particular the levels of TiO2 and Zr are higher in the CNE series compared to the ENA, and the CNE dikes are enriched in Nb whereas the ENA series have Nb and Ti anomalies 25 samples have been collected from western Maine and analyzed for 18 major, minor and trace elements, Analyses were performed in the UMF XRF laboratory using standard procedures of sample preparation and analysis, including USGS standard materials.…”