Eastern North American flood basalts in the context of the incipient breakup of Pangea

Abstract

The eastern North America (ENA) Mesozoic flood basalt province, particularly the part of the province north of central Virginia, is dominated by quartz-normative tholeiites. High-titanium quartz-normative (HTQ) basalts are the first and most widespread Mesozoic extrusions that flowed across several North Atlantic margin rift basins, including the Fundy, Hartford, Newark, and Culpeper basins of North America and the Argana basin of Morocco, Africa. These initial flows are chemically uniform and appear to be largely unchanged by fractionation from their mantle source. Isotopic data for 87 Sr/ 86 Sr and ,43 Nd/ 144 Nd are consistent with a lithospheric subcontinental mantle source that was either undepleted or slightly enriched through previous subduction events. The mantle source presumably underwent adiabatic decompressive melting during Mesozoic North Atlantic rifting.The HTQ flood basalts are chemically the same as the dominant members of flood-basalt provinces that were penecontemporaneously extruding from rifts along the South Atlantic margin (the Lesotho basalts of South Africa and the adjacent Patagonian basalts of Argentina), and from the Siberian platform of Asia. This world-wide distribution of virtually identical basalts associated with Pangean rifting implies the uniform operation of some fundamental petrogenetic controls and a diminished likelihood that localized processes unique to any single basin were dominant controlling factors of the regional HTQ composition.The HTQ flood basalts also chemically resemble some of the least-evolved flows of several other continental flood-basalt provinces. Most world-wide continental provinces, however, are dominated by basalts that are relatively enriched in incompatible elements, as if they were contaminated or fractionated to varying degrees. Perhaps the Pangean rifting event responsible for the flood basalts of the North and South Atlantic margins and the Siberian platform was uniquely powerful enough to allow for the rapid delivery of large quantities of unfractionated and uncontaminated magma directly from shallow undepleted or enriched mantle sources after a prolonged period of insulation and thermal accumulation under the Pangean "supercontinent."The HTQ flows are chemically distinct from other quartz-normative tholeiites of the ENA Mesozoic flood-basalt province that quickly followed them. Flows overlying the HTQ flows are a chemically diverse array including several varieties of high-iron quartz-normative, low-titanium quartz-normative, and possibly olivine-normative types that were probably generated at mantle sources chemically modified by previous HTQ partial melting or from mantle sources independent of the HTQ source. The diverse array of extrusive basalts that followed the HTQ flows seems to have undergone the varying degrees of contamination and fractionation typical of other continental floodbasalt provinces.

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

“…8). Co-genetic dyke and basalt compositions fall along particular linear trends that are mainly from pyroxene fractionation, as has been confirmed with additional analyses in later studies by Puffer (1992), McHone (1996), and Jourdan et al (2009). In particular, Philpotts and Martello (1986) show that the Higganum dyke and the co-magmatic Fairhaven Dyke (separated by faulting) are the source for the Talcott Basalt, the earliest basalt of the basin.…”

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

“…Onway, Higganum (Asher 1995), and Shelburne (Papezik and Barr 1981;Dostal and Durning 1998) (Dostal and Greenough 1992;Puffer 1992;Tollo and Gottfried 1992). The Shelburne samples show the same (La/Yb) N values as the Onway, and while the Shelburne and Onway samples overlap, some Shelburne rocks are more REE-rich.…”

“…11-15). The isotopic values of CNE (Puffer 1992;Jourdan et al 2003;Salters et al 2003) overlap Onway values. Unlike major and trace element compositions, where the Shelburne dyke matches the Onway and Talcott-Higganum rocks, the Shelburne rocks are more radiogenic, possibly as a result of local crustal contamination as shown by the Shelburne sample with 61.9% SiO 2 , negative ε Nd values, and high 87 Sr/ 86 Sr i values.…”

“…Philpotts and Asher (1992) provided field evidence of similar contamination of the Higganum dyke. These isotopic values suggest that the Onway and other ENA rocks were derived from either (1) a mantle with chondritic compositions; (2) a depleted mantle that was slightly enriched from an earlier subduction zone process (Puffer 1992(Puffer , 2001); or (3) a depleted mantle followed by crustal contamination to produce the negative ε Nd and relatively high Sr i values.…”

“…Epsilon Nd versus initial Sr (Sr i ) values for New Hampshire CNE and Onway samples (Table 3), ENA data from the literature (Pegram 1990;Puffer 1992;Jourdan et al 2003), Shelburne dyke samples (Dostal and Durning 1998), and CNE from Nova Scotia (Pe-Piper and Reynolds 2000) are plotted in Fig. 15.…”