HR: 13:55h
AN: V43G-02    [Abstracts]
TI: Caldera-related Tertiary Granitic Plutons in the Southern Rocky Mountain Volcanic Field, Western USA: Links to Subvolcanic Batholiths
AU: * Lipman, P W
EM: plipman@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo park, CA 94025, United States
AB: Granitic intrusions, ranging from small plugs coring precursor stratocones to composite batholith-scale bodies, are exceptionally exposed within and adjacent to the numerous Tertiary calderas (37-23 Ma) of the Southern Rocky Mountain volcanic field (SRMVF), as result of high regional topography, large-scale faulting associated with the Rio Grande rift system, and associated deep erosion. Such plutons are variable in age, texture, composition, and size relative to associated ignimbrite calderas. Large caldera subsidence structures are direct evidence for batholith-scale magma bodies in the upper crust, for which these plutons are inferred to provide samples of shallow solidified residua. Most exposed plutons are smaller than associated calderas, but large negative gravity anomalies (to –50 mgal) document upper-crustal presence of vertically extensive composite batholiths that are much larger than individual calderas. Some granitic plutons within or near SRMVF calderas are indistinguishable in age from the time of ignimbrite eruption, but others are millions of years younger. Pluton textures range from nearly aphanitic to medium-grained equigranular or megacrystic granitic, reflecting variable cooling and crystallization rates in shallow crustal environments. Compositions of SRMVF intrusions vary from diorite to highly evolved granite; those most closely related in age and location to calderas commonly are compositionally similar to more mafic late-erupted portions of associated ignimbrites. These relations are consistent with interpretation of pluton compositions as mushy residua from prolonged fractionation energized by sustained mafic magma inputs, during which more evolved upper parts of the subcaldera magma chamber were largely erupted as ignimbrite. Sparse recent U-Pb-zircon age data for granitic rocks elsewhere suggest that deeper-level plutons in Cordilleran arcs may have longer crystallization histories than shallower subvolcanic cupolas, contrasts inferred to reflect more prolonged open-system recharge, mixing, and crystallization at greater depths in the batholithic environment. A continuing uncertainty is the proportion of the eventual subvolcanic batholith present at times of peak ignimbrite volcanism, versus continued later- and post-volcanic enlargement.
DE: 8425 Effusive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting