Geochronology of Some Alkalic Rock Provinces in Eastern and Central United States

Recent efforts to understand the ongm of igneous rocks have emphasized the association between petrographic suites and geologic environment. Plate tectonic theory has been particularly useful in relating certain types of calc-alkalic rocks to their position near active plate boundaries. In contrast, alkalic rocks are often emplaced after cessation of orogenic forces or in completely anorogenic settings of the craton. The role of plate tectonics in dictating such magmatism is not readily understandable. One obviou s prerequisite to any effort at iden tification of under­ lying causes is the accurate dating of the igneou s rocks. My objective in this paper is to review our prescnt state of knowledge concerning a chronology for some alkalic rocks of eastern and central United States. Although'representing less than five percent of the volume of all igneous rocks, the alkalic clan includes a variety of interesting petrographic types, such as syenite, alkalic granite, alkalic gabbro, lamprophyre, and mica peridotite. These rocks typically occupy discordant stocks, plugs, dikes, sills, and diatremes at relatively shallow levels in the crust; and they may also include eruptive counterparts. The literature abounds with contributions regarding their structure, petrography, mineralogy, and geochemistry; and I do not stray too far from my intended objective into these areas. My working definition of an alkalic rock corresponds essentially to that of Sorensen (1974), but I also include certain u ltram afic rock s (Wyllie 1967) that are often found in petrologic and spatial association. Generally, rock n ames are accepted as the original authors used them without defending the specific genetic implication sometimes att�ched to the name. This paper focuses upon several petrographically cohcrent groups of alkalic rocks, or provinces, that occur within the eastern and central U nited States. Although each province contains some felsic and some mafic members, two rather different