MINERALOGIC AND GEOCHEMICAL ANALYSIS OF A TRIASSIC DIABASE: LIVERPOOL, PA

The geochemistry and mineralogy of a Triassic diabase from Liverpool, PA indicate assimilation and differentiation following its emplacement in Mississippian-Devonian age sedimentary rock. The presence of sedimentary xenoliths (shaley, quartz-rich sandstone) along its contacts indicate an influence by the host rock and potenital altered magma chemistry. Both the geochemistry and mineralogy differ from the southern extensions of the same intrusion, located along the Pennsylvania Turnpike in the Cumberland Valley and near Gettysburg, PA. Based on TiO₂ values of 0.73% to 0.85% this magma represents the Rossville type.

The diabase in Liverpool is holocrystalline and consists mostly of anhedral to subhedral pyroxene (augite and enstatite) and subhedral to euhedral plagioclase (albite). Accessory minerals: magnetite, rutile, ilmenite, fayalite are also present. Secondary minerals, due to weathering, were chlorite, serpentenite, goethite, and limonite. Crystal size increases toward the center of the intrusion; here minerals are phaneritic and ophitic; near the upper and lower contacts of the intrusion, the samples appear more aphanitic and subophitic. The normalized elemental geochemistry of major elements was found to be elevated in Al₂O₃, SiO₂, Fe₂O_3(total), MgO, P₂O₅ and K₂O in comparison to the diabase samples from Cumberland Valley and Gettysburg. Trace elements Sr, BaO and PbO were also elevated compared to other Rossville-type magmas. Though these studies found elevated CaO, MnO, and Na₂O₃values, the Liverpool diabase had undetectable levels of CaO and low levels of MnO.

The Triassic diabase near Gettysburg and in the Cumberland Valley interacted with siliciclastic and carbonate rocks, whereas the intrusion near Liverpool, PA interacted with siliciclastic sedimentary rocks. As it moved through this rock, and assimilated shales and sandstone the melt became enriched with Al₂O₃, SiO₂, Fe₂O₃, MgO, and K₂O in comparison to other magmas. The farther the melt moved from its magma source, the more depleted it became in CaO and MnO. The geochemistry and mineralogy of this Triassic diabase indicate it underwent both assimilation and differentiation.