THE DYNAMIC HISTORY OF A 201.5 MA, UPPER-CRUSTAL, BASALTIC MAGMA SYSTEM IN THE WESTERN NEWARK BASIN (PALISADES-ORANGE MTN CORRELATIVE), PENNSYLVANIA, USA

Recent work on active volcanic systems has led to a new paradigm – that magma reservoirs are mostly crystalline mush for most of their lifetimes – and to new questions about how crystal mush forms, evolves chemically and rheologically, and is mobilized for eruption. Plutonic rocks in sub-volcanic intrusions may be solidified crystal mush but they are overprinted, incomplete records that need to be interpreted within the context of their magmatic system. A suitable basaltic system, part of the 201.5 Ma Central Atlantic Magmatic Province associated with rifting of Pangaea, is exposed across about 6 km of upper crust in the western Newark basin, PA. Combining macro- and micro-structural evidence, whole-rock and mineral compositions, P-T estimates, thermodynamic modeling, and viscosity modeling, we develop a general history of the magmatic system with four major findings. 1) An upper-crustal plumbing system was established relatively early. Flood basalts (Orange Mtn equivalent) with viscosity about 30-300 Pa-sec were erupted after 10-30 volume % crystallization and did not require mobilization of highly-crystalline mush. 2) Eruption was followed by an interval of multiple magma inputs, transport, and crystallization within the sill-dike network. Viscosity increased from about 1000 to >30,000 Pa-sec as crystallinity increased to about 50 vol.%. 3) In the basal sill (emplaced at 5.5-6 km), crystal mush comprising hundreds of decimeter- to meter-scale lobate structures outlined by millimeter-scale plagioclase-pyroxene modal layers are beautifully exposed in a dimension stone quarry. Microstructural evidence indicates that multi-directional lateral flow formed the layered, lobate structures during later emplacement or rejuvenation of mush in the sill, as crystallinity increased to >75 vol.% (<25 vol.% Liquid remaining). Separation and alignment of plagioclase could have reduced viscosity by shear thinning, a positive feedback promoting both flow and layering. 4) Final basaltic magma inputs into the basal sill changed from lateral to vertical channelized flow structures, influenced by the highly viscous layered mush and perhaps waning magma flux. The change in flow regime may have prevented mush rejuvenation and basalt eruption in the last stages of the system history.