HR: 15:25h
AN: V43G-08 INVITED    [Abstracts]
TI: Questioning the Sedimentary Paradigm for Granites
AU: * Glazner, A F
EM: afg@unc.edu
AF: Dept. of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, United States
AU: Bartley, J M
EM: jbartley@earth.utah.edu
AF: Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, United States
AU: Coleman, D S
EM: dcoleman@unc.edu
AF: Dept. of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, United States
AU: Boudreau, A
EM: boudreau@duke.edu
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, United States
AU: Walker, J D
EM: jdwalker@ku.edu
AF: Dept. of Geology, University of Kansas, Lawrence, KS 66045, United States
AB: A critical question regarding volcano-pluton links is whether plutons are samples of magma that passed through on its way to eruption, or residues left behind after volcanic rocks were extracted. A persistent theme of recent work on granites sensu lato is that many are sedimentary accumulations of crystals that lost significant volumes of magmatic liquid. This view is based on observations of structures that clearly seem to reflect deposition on a magma chamber floor (e.g., flows of chilled mafic magma into silicic magma) and on the inference that many other structures, such as modal layering, truncated layering, and crystal accumulations, reflect crystal sedimentation on such chamber floors. There are significant physical and geochemical reasons to question this view, based on observations in the Sierra Nevada of California and similar results from other batholiths. First, few granites show the enrichments in Ba, Sr, and relative Eu that feldspar accumulation should produce. Second, sedimentary features such as graded bedding and cross-bedding form in highly turbulent flows, but turbulence is unachievable in viscous silicic liquids, where velocities on the order of 10⁴ m/s would be required to induce turbulence in a liquid with η=10⁴ Pa s. Third, tabular modally layered domains commonly cut surrounding modal layering on both sides, and orientations of modal layering and of the troughs of "ladder dikes" commonly scatter widely within hectare-sized areas; it is difficult to reconcile these features with gravity-driven settling. Fourth, accumulations of K-feldspar megacrysts are typically inferred to be depositional, but this is precluded by crystallization of most K- feldspar after rheologic lock-up occurs. Finally, accumulations of K-feldspar and hornblende are typically packed too tightly to be depositional. With analogy to layered mafic intrusions, many features attributed to crystal sedimentation in granites may be better explained by crystal aging and other in situ chemical processes. In particular, many of these features may record pore-melt flow paths rather than depositional processes.
DE: 3618 Magma chamber processes (1036)
DE: 3642 Intrusive structures and rocks
DE: 3643 Layered magma chambers
DE: 3675 Sedimentary petrology
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting