Emplacement and Differentiation of the York Haven Diabase Sheet, Pennsylvania

Many of the high-Ti quartz-normative tholeiitic intrusive sheetsin the early Mesozoic rift basins of the Eastern USA exhibitlateral differentiation from mafic cumulate units, through diabase,to relatively evolved iron-rich rock types. We have investigateda representative example in detail, the York Haven sheet inthe Gettysburg basin of south–central Pennsylvania. Itranges in thickness from 330 m to 675 m, and we have sampledit from base to top along four separate stratigraphic sectionsevenly spaced over the extent of the intrusion. The easternmostsection (York Haven) is entirely basaltic bronzite cumulate(average 15 vol. % bronzite), whereas the westernmost (ReesersSummit) consists of diabase and low-MgO diabase with a middleto upper ‘sandwich zone’ of ferrogabbro. The interveningsections feature rock types transitional between the two end-membersequences. Chemically, the rock series shows a gradual eastto west depletion of compatible elements (Mg, Ca, Ni, and Cr),and enrichment of incompatible elements Ti, Fe, Na, K, P, Cu,Zr, Th, Ta, Hf, Sb, Cs, As, platinum group elements (PGEs),and rare earth elements (REEs)]. We suggest two main processes for the trends observed in theYork Haven sheet. First, flow differentiation during ascentand lateral injection of the parental magma produced a tongueof basaltic bronzite cumulate that thins from southeast to northwestand passes laterally into diabase, and, at the distal end ofthe intrusion, into low-MgO diabase. Then, in the latter stagesof crystallization, densitydriven hydrothermal fluids transportedincompatible elements westward, into structurally higher partsof the intrusion. Reaction of this residual aqueous fluid withpartly crystallized low-MgO diabase produced a zone of ferrogabbrorich in hydrothermal replacement products (e.g., Cl-amphibole,biotite, ferrohypersthene, and skeletal ilmenite) and precipitates(e.g., quartz, fayalite, Cl-apatite, sulfides, and PGE minerals). ^* Present address: US Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718