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Detailed petrographic and geochemical analyses of samples collected from eight cores of the U.S. Army Corps of Engineers Passaic Tunnel project indicate that the three principal basalt formations of the early Mesozoic Newark basin in New Jersey are composed of multiple flow units that can be identified on the basis of physical and petrochemical criteria. The high-titanium, quartz-normative (HTQ) Orange Mountain Basalt (lowermost formation: at least three flow units) and high-iron and high-titanium, quartz-normative (HFTQ) Hook Mountain Basalt (uppermost formation: at least two flow units) include flows characterized by a limited range in composition and are indicative of eruptive cycles during which individual magma sources underwent little change in composition and were tapped repeatedly. In contrast, the wide chemical diversity of the flows in the Preakness Basalt (middle formation: possibly nine flow units) is indicative of a period of volcanism in which successive eruptions were derived either from (1) separate high-iron, quartz-normative (HFQ) and low-titanium, quartz-normative (LTQ) parental magmas; or (2) an LTQ parent that changed composition during the eruptive interval.

The HTQ composition and limited variability of the Orange Mountain Basalt indicate petrochemical correlation with the stratigraphically comparable Mount Zion Church (Culpeper basin, Virginia) and Talcott (Hartford basin, Connecticut) Basalts. The wide range in composition of the Preakness Basalt supports previous petrochemical correlation with the Hickory Grove and Sander Basalts of the Culpeper basin. The HFQ compositional type of the Preakness, Hickory Grove, and Sander Basalts is similar to the Holyoke Basalt of the Hartford basin (Connecticut and Massachusetts) and Deerfield Basalt of the Deerfield basin (Massachusetts). The HFTQ Hook Mountain Basalt is nearly identical to the stratigraphically comparable Hampden Basalt of the Hartford basin (Connecticut and Massachusetts) in both composition and limited degree of compositional variability.

These new data and conclusions drawn from recent cyclostratigraphic investigations of the intercalated sedimentary strata from these basins define three nearly synchronous eruptive intervals for the Eastern North America (ENA) province from Virginia to Massachusetts. Volcanic interval I, which followed an extended period of sedimentation in the Culpeper, Newark, and Hartford basins, was marked by repeated production of exclusively HTQ-type basalt and involved parental magmas that did not significantly change composition during the eruptive episode. Volcanic interval II, which followed a second, shorter period of basin sedimentation occurring throughout the province, began with eruption of the Hickory Grove Basalt in the Culpeper basin and, following a period of renewed terrestrial sedimentation in this basin, resumed with nearly simultaneous extrusion of HFQ-type basalt in all of the basins. In the northern basins (Hartford and Deerfield), where basalt eruption was uninterrupted by significant sedimentation, this interval involved only HFQ-type basalt. However, in the southern basins (Culpeper and Newark), where basalt production was interspersed with one or more periods of sedimentation, lavas defined a range of compositions, including LTQ-type and other hybrid varieties. Volcanic interval III, which followed another significant period of sedimentation in the Newark and Hartford basins, involved production of exclusively HFTQ-type basalt from parental sources that underwent slight change in composition during the episode. The similarity of inferred parental magma compositions from Virginia to Massachusetts implies the effect of large-scale petrologic control on melt production. The time-composition relationships of the basalts indicate that the volcanic sequences of the ENA province were produced by episodic tapping of compositionally distinct parental sources that varied in the extent of volcanic production on both a regional and intrabasin scale.

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