HR: 09:30h
AN: V21A-05    [Abstracts]
TI: Contrasting Modes for Granitic Batholith Construction: the Role of Tectonic Stress
AU: * Hogan, J P
EM: jhogan@umr.edu
AF: University of Missouri-Rolla, Department of Geological Sciences and Engineering, Rolla, MO 65409-0410
AB: Mental images of felsic magma chambers are inextricably linked to eruption of voluminous high silica ignimbrites which require the presence of even larger volume magma chambers within the crust. Systematic stratigraphic compositional variation within ignimbrites indicate compositionally stratified chambers, with a high silica cap, grading downward to intermediate compositions, all underlain by mafic magma. Influx of basaltic magma keeps the overlying felsic magma liquid and convecting, thus sustaining the large volume chamber. Too large a mafic replenishment can catastrophically extinguish the chamber by triggering an eruption. Silurian granitic batholiths of the coastal Maine magmatic province fit well with an origin as the plutonic roots for such volcanic systems e.g., Mount Desert Igneous Complex (Seaman, 1999), Vinalhaven Igneous Complex (Hawkins and Wiebe, 2004). Large volcanic eruptions are associated with these batholiths. In contrast to "open system" magma chambers, isotopic evidence indicates large "closed system" granitic batholiths can be assembled incrementally from coalescence of discrete smaller batches of felsic magma. Observed heterogeneity in initial isotopic compositions (e.g., Sr, Pb) from these batholiths is interpreted to be inherited form the source region. Preservation of this isotopic heterogeneity eliminates homogenization by convective mixing, indicating discrete batches of magma welded together to form large granitic batholiths. The Devonian Lucerne granite of the coastal Maine magmatic province fits this style of "closed system" batholith construction (Hogan and Sinha, 1991). It is indeterminate as to whether or not the Lucerne magma chamber erupted. The distinction between these two styles of batholith construction, and their magma chambers dynamics, is dependent upon the availability and volume of basaltic magma within the crustal column, which is linked to tectonic stress (Hogan et al., 1998). Increased magma driving pressures during periods of crustal extension enable higher density basaltic liquids to rise further in the crustal column, increasing the probability of intersection and eventual trapping within shallow felsic magma chambers, and the formation of "open system" granitic batholiths. Lower magma driving pressures during periods of relaxed extensional stress or compression promotes intraplating of mafic magma in the deep crust, leading to large scale crustal anatexis, the rise of buoyant felsic magmas, and the formation of "closed system" granitic batholiths.
DE: 1010 Chemical evolution
DE: 3640 Igneous petrology
DE: 8145 Physics of magma and magma bodies
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly