MINERALOGIC AND GEOCHEMICAL ANALYSIS OF A TRIASSIC DIABASE: LIVERPOOL, PA
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, Fe2O3(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.