Eastern North American (ENA) Tholeiitic Magmatism
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 87Sr/86Sr and 143Nd/144Nd 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 flood-basalt provinces.