EVALUATING THE CONSTRUCTION OF AN UPPER-CRUSTAL PLUMBING SYSTEM BY COMPARING ROCK AND MINERAL COMPOSITIONS IN A BASALT AND UNDERLYING PLUTON CHILL MARGINS

The Newark basin (PA-NJ-NY) is part of the Central Atlantic Magmatic Province and contains mafic igneous rocks that formed at about 201 Ma during Mesozoic rifting of Pangaea. The western basin experienced greater tilting, folding, and erosion that exposed natural cross-sections of almost 6 km of an upper-crustal plumbing system composed of interconnected sills, inclined sheets, and dikes including three potential feeder structures. We are testing the idea that one large pulse of magma built the entire plumbing system by comparing compositions of the same minerals in 3 samples of the lava flow and 16 chilled margins of the igneous plutons.

The chill margin and lava flow samples contain phenocrysts of orthopyroxene and augite with a narrow range of core compositions (opx Mg# 85-82, and aug Mg# 84-82) and similar textures. This suggests that the magma that built the plumbing system and erupted as the basalt was carrying phenocrysts of the same composition, possibly from the same sub-volcanic magma chamber at depth > 6km.

Matrix cores of aug and plagioclase, however, are not the same in all samples: aug Mg# = 84-65 and plag An = 82-68. The basalt matrix has high-Mg aug and high-An plag consistent with a high-T, more primitive magma. High-Mg aug and high-Ca plag could be microphenocrysts that served as seed crystals for matrix mineral growth, but this does not explain all observed variations. Aug and plag compositions show no correlation with each other or with intrusion depth, location, or associated feeder into the system. This suggests there is no simple process such as fractionation of a common host magma that can explain the differences.

Matrix rim compositions also vary from Mg# = 63-48 for aug and An = 61-31 for plag; this may be due to different proportions of evolved liquid and/or different extents of mineral zoning during crystallization. Samples with low-Mg# aug also have low-An plag which follows expected fractionation trends for the liquid after emplacement.

We will compare our data with whole-rock compositions and the results of MELTS models to test if different matrix minerals could result from different pressure or water content. Otherwise, our matrix mineral results may indicate that a narrow spectrum of host magma compositions were generated and modified at depth and emplaced in the upper crust.