An upper-crustal intrusive network in the 201.5 Ma, rift-related Central Atlantic Magmatic Province is exposed in the western Newark basin (PA, USA). Alpha-MELTS modeling was used to track magma evolution starting with initial pyroxene crystallization at depth (1000-500 MPa); plagioclase crystallized during ascent in the upper crust. For magma emplaced at 5-6 km depth (170 MPa), six MELTS models were generated to bracket different composition, H2O (1-3 wt.%), and crystallinity (28-49 vol.%). Corresponding magma viscosities evolved from 3 to 1624 Pa-sec (predicted using Giordano et. al 2008; Moitra and Gonnermann 2014). Detailed crystal mush structures in a diabase sill are revealed in a dimension stone quarry. Ubiquitous asymmetric modal layers a few mm thick comprising plag-rich layers (PRL, 75% modal plag) overlying more pyx-rich layers outline the tops of hundreds of dm-m scale flow lobes in the quarry. Tabular plag in PRL show shape-preferred orientations, tiling, and pressure shadows around larger pyx that resemble analog experiments on particle slurries and indicate flow with limited mechanical compaction. During magma emplacement, recursive interactions of propagation, sorting, and crystallization self-organized as flow lobes with plag entrained and aligned along lobe tops. Our calculations show plag separation can reduce bimodal suspension viscosity; a positive feedback likely enhanced by shear thinning and crystal alignments. EDS analyses and X-ray maps show that plag has oscillatory-zoned cores (An82-67) with patchy-zoned mantles (An67) filled in by An66-63. In PRL, plag are cemented together by An62-55; Na-rich rims occur next to qtz-Kspar pockets. By the end of cementation, PRL liquid volume was significantly reduced to 11-18% compared with 28-45% in overall magma based on MELTS models for An62-55 plag. Diabase suspension viscosity increased to >6000 Pa-sec; PRL viscosity cannot be modeled by equations based on random packing. PRL with aligned interlocking crystals were more rigid and less permeable than surrounding diabase. Upward flow of magma after modal layer development was channelized into pipes truncated and deflected by PRL. Thus, lateral flow during emplacement developed sub-vertical heterogeneities that exemplify complex mush rheology over m-scale distances.

Magmatic structures are well-preserved in a 201.5 Ma diabase sill (PA, USA, equivalent to the Palisades sill) formed as part of the Central Atlantic Magmatic Province during rifting of Pangea. The sill was emplaced at ~6 km depth and tilted ~20° NNW by post-magmatic fault movement. Detailed mush structures are exposed in a dimension stone quarry with walls cut parallel and perpendicular to the strike of the sill. Light gray, plagioclase-rich layers (PLR) a few mm thick contain up to 75% modal plag and are underlain by more pyroxene-rich layers with larger orthopyx antecrysts up to 1 mm length. PLR are sub-parallel to sill margins, have dm-m lateral dimensions, and spaced 0.33 m apart on average. Magma replenishments < 1m thick cross-cut plag-pyx layers at low angles and have basal load-cast-like structures. Since mafic replenishments have PLR at their tops and similar thickness to PLR spacing, we interpret all PLR as having formed by emplacement of small-volume magma pulses bearing ~30% larger pyx and smaller plag antecrysts. This model is similar to Petford and Mirhadizadeh (R Soc Open Sci, 2017) for the Basement sill, Antarctica. Upward migration of mafic melts in pipe-like channels (cm to dm wide) disrupted plag-pyx layers to form dm-scale graben-like and slump-like structures that resemble sediment liquefaction. Channelized flow late in sill development may have been enhanced by seismicity (Davis et al., JVGR, 2007). Diabase micro-structures are similar to published experimental results and numerical simulations of flow and shear-thinning in particle-rich slurries (e.g., Cimarelli et al., G3, 2011; Ishibashi, JVGR, 2009; Deubelbeiss et al., G3, 2011). These include layers such as the plag-pyx couplets and orientations of euhedral plag around pyx phenocrysts. Plag long-axis orientations and tiling indicators in the PLR have strike-parallel and strike-normal components in vertical and plan views consistent with flow alignment in the plane perpendicular to the stress gradient. Plag chemical zoning patterns, limited deformation, and long-axis orientations parallel to inclined layer margins also indicate magmatic flow rather than compaction. Mineral x-ray maps are used to derive initial crystal fraction and aspect ratios for modeling relative viscosity and explore compositional aspects of layer development.