| Summary: | Mineralogical and geochemical effects of alteration were studied in three early Mesozoic diabase/host-rock systems: (1) the Rocky Ridge dike and aureole, Gettysburg basin, Maryland; (2) the West Gainesville and Nokesville diabase sheets and subjacent host rocks, Culpeper basin, Virginia; and (3) the East Gainesville diabase sheet and associated host rocks, Culpeper basin, Virginia. The Rocky Ridge diabase is relatively unaltered and its thermal aureole formed under predominately isochemical conditions. The West Gainesville/Nokesville system exhibits apparent isovolumetric, complementary exchange of volatiles and alkalis(?) from the host rocks with Si, Al, Fe(?), Mn, Mg, Ca, Co and V(?) from the diabase sheet. Mass-balance calculations for this system indicate that, except for loss of Si and gain of volatiles and perhaps alkalis, it behaved isochemically. The East Gainesville diabase sheet is characterized by multiple injection of two magma types from different mantle sources. Alteration in this system is more complex and aureole temperatures were lower than those in the West Gainesville/Nokesville system. === Observed mineral reactions in the diabase are consistent with relative mobilities; the dominant mechanisms for this mass transfer are apparently infiltration metasomatism and intergranular diffusion. Relative mobilities of cations can be correlated with ionic radius. Rare-earth elements and Zr were immobile with respect to the diabase sheets, but Ti, commonly used to distinguish magma types in these and other mafic rocks, was not. In all studies Si, Al, Fe, Ca, and V were lost from the diabase during metasomatism, whereas volatiles, Rb and K were gained. === Source: Dissertation Abstracts International, Volume: 55-04, Section: B, page: 1344. === Director: Paul C. Ragland. === Thesis (Ph.D.)--The Florida State University, 1994.
|
|---|
