EVIDENCE FOR COMPACTION OR FLOW BASED ON PLAGIOCLASE CRYSTAL ORIENTATIONS IN THE MORGANTOWN SHEET, WESTERN NEWARK BASIN, PENNSYLVANIA

The Morgantown sheet is a Jurassic diabase intrusion composed of interconnected sills and dikes located in the Newark Basin, southeast PA. The PA Granite Quarry (PAGQ) is located in the SE corner of the mapped sheet and is the deepest part of the intrusion exposed at the surface. The PAGQ has ~50-m vertical exposure of layered plag-rich and opx-rich diabase. Rock chemistry and mineralogy indicates that PAGQ diabase consists of accumulated crystals that lost some evolved liquid and dissolved fluids. Mechanisms to produce diabase crystal accumulations include gravity settling and compaction or adcumulus growth with expulsion of evolved liquid. Aligned plagioclase crystals could result from compaction or flow.

These models were tested in 3 rock cores drilled at the top, middle, and bottom of the PAGQ. Each core was cut into two sets of orthogonal, vertical thin sections, one looking parallel to the strike of the sill and one looking perpendicular to strike. In each section, the angle of the long dimension of plagioclase crystals relative to vertical was measured along several traverses (n = 114-246 angles for each section), and kernel smoothing was applied to the angle distributions. Comparing measured distributions to simulated distributions helps resolve ambiguities resulting from measuring angles in only 2 sections. The null model is that magmatic crystallization results in a random distribution of angles. Adcumulus growth would not change orientations and also would produce random distributions. Compaction would rotate crystals toward horizontal, leading to a clustered distribution of angles. Flow would result in a clustered distribution skewed toward the direction of flow. The symmetry of the distribution shape also provides information about flow direction.

Preliminary data from the very top and very bottom sections show clustered, non-random distributions, ruling out adcumulus growth as the only cause of the cumulus mineralogy and chemistry. Each section has a different distribution, but the strike-perpendicular sections have the greatest skewness consistent with post-magmatic tilting of the sill toward the North. Data from all 20 sections will be compared with the simulations and other evidence to evaluate the effects of tilting, and plag textures are being examined to evaluate flow.