MESOZOIC INTRUSIVE ROCKS FROM LANCASTER, BERKS, AND CHESTER COUNTIES IN SOUTHEASTERN PENNSYLVANIA

Petrography, whole rock major and trace element geochemistry via XRF, and ⁴⁰Ar/³⁹Ar data are reported on an extensive suite of samples collected mainly from quarries owned and operated by Haines and Kibblehouse, Inc. Mangan and Others (1993) and Woodruff and Others (1995) report data from the York Haven Sheet which is located immediately to the west of the area investigated in this work. At the Douglassville Quarry four miles west of Pottstown the lower contact between gently northwestward dipping Mesozoic sedimentary rocks and an intrusive igneous sheet changes from an intersertal textured basalt (10 to 11% MgO) at the contact to diabase to an olivine gabbronorite 10 to 15 m above the contact (12 to 18% MgO). The near liquidus minerals are olivine, plagioclase, and pyroxene (OPX and CPX) with an opaque phase and Fe-rich biotite being near solidus minerals. There are no signs either on the outcrop or in thin section of interaction between the older sedimentary rocks and the igneous intrusion. Bronzite cumulate is the name used by Mangan and Others (1993) to describe the correlative rocks in the York Haven Sheet. The major point of petrographic distinction is the persistent presence of several percent olivine and olivine pseudomorphs in the Douglassville Sheet cumulate rocks. Once above the chilled margin only olivine-bearing bronzite cumulate equivalent is present. However, at Birdsboro and Silver Hill quarries located further to the west no bronzite or olivine bronzite cumulate has been detected. These plutonic rocks vary between gabbro and ferrogabbro (8 to 1.5% MgO) and have undergone significant differentiation. At these locations the pyroxene assemblage is almost exclusively augite, sub-calcic augite, and minor pigeonite. Also, in stark contrast to Douglassville, these gabbroic rocks have suffered varying degrees of hydrothermal alteration. Geochemical trends and the ⁴⁰Ar/³⁹Ar data will be presented in March, 2011.