EMPLACEMENT AND DIFFERENTIATION OF THE YORK HAVEN DIABASE SHEET, PENNSYLVANIA

Many of the high-Ti quartz-normative tholeiitic intrusive sheets in th e early Mesozoic rift basins of the Eastern USA exhibit lateral differ entiation from mafic cumulate units, through diabase, to relatively ev olved iron-rich rock types. We have investigated a representative exam ple in detail, the York Haven sheet in the Gettysburg basin of south-c entral Pennsylvania. It ranges in thickness from 330 m to 675 m, and w e have sampled it from base to top along four separate stratigraphic s ections evenly spaced over the extent of the intrusion. The easternmos t section (York Haven) is entirely basaltic bronzite cumulate (average 15 vol. % bronzite), whereas the westernmost (Reesers Summit) consist s of diabase and low-MgO diabase with a middle to upper 'sandwich zone ' of ferrogabbro. The intervening sections feature rock types transiti onal between the two end-member sequences. Chemically, the rock series shows a gradual east to 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), an d rare earth elements (REEs)]. We suggest two main processes for the t rends observed in the York Haven sheet. First, flow differentiation du ring ascent and lateral injection of the parental magma produced a ton gue of basaltic bronzite cumulate that thins from southeast to northwe st and passes laterally into diabase, and, at the distal end of the in trusion, into low-MgO diabase. Then, in the latter stages of crystalli zation, density-driven hydrothermal fluids transported incompatible el ements westward, into structurally higher parts of the intrusion. Reac tion of this residual aqueous fluid with partly crystallized low-MgO d iabase produced a zone of ferrogabbro rich in hydrothermal replacement products (eg., Cl-amphibole, biotite, ferrohypersthene, and skeletal ilmenite) and precipitates (e.g., quartz, fayalite, Cl-apatite, sulfid es, and PGE minerals).