SIGNIFICANCE OF ELEVATED IRIDIUM AND PLATINUM GROUP ELEMENT RATIOS IN UPPER TRIASSIC-LOWER JURASSIC CONTINENTAL STRATA OF THE NEWARK SUPERGROUP

We report new data on Ir concentrations and platinum group element (PGE) ratios in strata of the Newark Supergroup rift basins. At the former Tilcon Quarry in Clifton, New Jersey (Newark Basin), a dark gray mudstone containing abundant plant debris occurs in the uppermost Passaic Formation ca. 1.8 m below the Orange Mountain Basalt. Analysis by nickel-sulfide fire assay with ICP-MS reveals Ir concentrations up to 780 pg/g. The Ru/Ir and Pt/Pd ratios, 1.88 and 1.94, respectively, are nearly identical to chondritic values. However, the Pt/Ir and Pd/Ir ratios, 6.9 and 3.55, are elevated with respect to chondritic ratios. Similarly, published XRF analyses of mafic intrusions in the Newark Basins produce a near chondritic Pt/Pd ratio and elevated Pt/Ir and Pd/Ir ratios. Other occurrences of elevated Ir (determined by neutron activation analysis) have been reported from the Fundy Basin, Nova Scotia (Blomidon and McCoy Brook formations), Hartford Basin, Massachusetts (unassigned strata presumed correlative with the New Haven Formation) and Deerfield Basin, Massachusetts (Sugarloaf Formation). Results from new fire assay analyses of the previous samples demonstrates that the PGE ratios differ from those of the Passaic Formation in that the Pt/Pd ratios are substantially lower than the chondritic ratio, but the Pt/Ir and Pd/Ir ratios are elevated compared to chondrite, as in the Passaic Formation. Furthermore, we note that REE profiles from the high-Ir horizon in the Newark and Deerfield basins are relatively flat and strongly resemble profiles produced by analyses of the Newark Basin mafic intrusives. Thus, tentatively we associate the origin of the elevated Ir concentrations with the volcanism of the Central Atlantic Magmatic Province. Further, we propose that the horizon of elevated Ir below the Newark Extrusive Zone is correlative across the Newark Supergroup basins and may have resulted from an early phase of the eruption that produced fallout via either ash or aerosol deposition.