HR: 08:45h
AN: V51E-04 INVITED     [Abstracts]
TI: Incremental Assembly and the Nature of Granitic Plutons
AU: * Glazner, A F
EM: afg@unc.edu
AF: Univ. of North Carolina, Dept. of Geological Sciences, CB# 3315, Chapel Hill, NC 27599 United States
AU: Bartley, J M
EM: jbartley@mines.utah.edu
AF: Univ. of Utah, Dept. of Geology and Geophysics, Salt Lake City, UT 84112 United States
AU: Coleman, D S
EM: dcoleman@unc.edu
AF: Univ. of North Carolina, Dept. of Geological Sciences, CB# 3315, Chapel Hill, NC 27599 United States
AB: Plutonic rocks are generally mapped on the basis of observable petrographic features such as mineralogy and texture. It is commonly assumed that these features distinguish magma bodies and that contacts between mappable phases separate different magma bodies; indeed, this assumption tacitly underlies the last century of research on how plutons form and are emplaced. However, recent work demonstrates that many superficially homogeneous, well-defined plutons (e.g., Half Dome Granodiorite of the Tuolumne Intrusive Suite, California) grew over millions of years by amalgamation of numerous small intrusive increments, many of which solidified before the next increment was emplaced. Within such intrusive suites, contacts between plutons may range from sharp to highly gradational; mappable contacts may bound different intrusive increments along their lengths, are likely diachronous, and can represent time gaps much smaller than the growth times of the plutons that they bound. The characteristic properties (texture, mineralogy, composition) of a pluton formed by protracted assembly of small intrusive increments thus cannot reflect in situ processes within a single magma body, but rather must be produced by repeated formation of the same magma type (e.g., in the lower crust) before injection, and repeated thermal history thereafter. This is clear in the Sierra Nevada, where the "Tuolumne triad" of nested plutons (outer foliated, medium-grained, mafic granodiorite; medial equigranular, coarse-grained granodiorite; inner megacrystic granodiorite/granite) was repeated several times in the Late Cretaceous along $>$400 km of arc length. These intrusive suites cannot have been derived from a single magma body yet comprise identical sequences of nearly identical rocks. A pluton is thus analogous to a stratigraphic lithofacies; the processes by which parts of a pluton formed were fundamentally similar across the scale of an entire arc, but different parts of any given pluton were likely added independently and as much as millions of years apart. Mappable contacts thus do not necessarily distinguish intrusive pulses but rather may mark changes in petrogenetic processes.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8035 Pluton emplacement
DE: 8110 Continental tectonics--general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting