HR: 0830h
AN: V51I-0398    [PDF]
TI: Mafic injection and local hybridization in a nearly frozen magma chamber: failed (?) rejuvenation in the Searchlight pluton, Eldorado Mountains, Nevada (USA)
AU: * Means, M A
EM: molball9@hotmail.com
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Miller, J S
EM: jsmiller@email.sjsu.edu
AF: Dept of Geology, San Jose State Univ, San Jose, CA 95192-0102 United States
AU: Miller, C F
EM: calvin.miller@vanderbilt.edu
AF: Geology Dept., Vanderbilt Univ, 2301 Vanderbilt Place, VU Sta B, Nashville, TN 37235-1805 United States
AU: Cates, N L
EM: nicole.l.cates@Vanderbilt.Edu
AF: Geology Dept., Vanderbilt Univ, 2301 Vanderbilt Place, VU Sta B, Nashville, TN 37235-1805 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: USGS-SUMAC, Stanford Univ, Stanford, CA 94305-2220 United States
AU: Koteas, C
EM: chris.koteas@vanderbilt.edu
AF: Geology Dept., Vanderbilt Univ, 2301 Vanderbilt Place, VU Sta B, Nashville, TN 37235-1805 United States
AB: The Miocene Searchlight pluton (Nevada, USA) is a well-exposed, 12 km thick magma chamber. Previous work indicates it solidified in simple monotonic fashion. Crystal accumulation on the chamber floor, and downward propagation of a solidification front from the roof, resulted in felsic melt segregation in the interior. However, one small (3 km$^{2}$) relatively mafic part of the pluton (called Guzzler Gulch) has abundant chemical and textural variation at a scale not observed elsewhere in Searchlight pluton (cm to m). This unit intrudes through the gradation between the lower cumulate and the middle granite. The main rock type is an equigranular hornblende diorite, but fine-grained hornblende gabbro is also common. Contacts of mafic rocks with surrounding granite are gradational, and mutually crosscutting relations are seen where gabbros and diorites intrude granite and are back-veined by granitic dikes. Later gabbros are also seen cutting earlier gabbros. A number of observations suggest that the mafic injections occurred when the Searchlight magma chamber was waning: (1) Guzzler Gulch unit is intruded into granite that is at least partly cumulate, (high modal alkali feldspar displaying originally horizontal layering); (2) inclusions of the feldspar-rich, cumulate granite, up to 5 m in diameter, are completely surrounded by hornblende diorite, and are inferred to be autostoped blocks of mostly solidified granite; (3) irregular-shaped gabbroic pods several m's in diameter are also intruded into the cumulate granite, but hybrid zones are narrow (a few m's), and consist of heterogeneous light and dark banded rocks with abundant mafic enclaves; (4) the originally horizontal cumulate fabric in the granite is locally disrupted by, and sometimes reoriented, into parallelism with the contact between the Guzzler Gulch mass and the surrounding granite; (5) geochemical data are consistent with local hybridization, but hybrids are chemically quite heterogeneous on the scale of the entire Guzzler Gulch body; (6) Ion-microprobe $^{206}$Pb/$^{238}$U zircon ages from a hybrid diorite and a coarse-grained gabbro are 16.2$\pm$0.2 Ma (MSWD = 2.7; N=17) and 15.9$\pm$0.3 (MSWD = 2.2; N=19), respectively, which overlap within error the 15.8$\pm$0.4 Ma zircon age of the late-crystallizing middle granite unit of the main Searchlight pluton. The Guzzler Gulch area of the Searchlight pluton appears to capture the process of failed rejuvenation of an intermediate to felsic magma chamber that was nearly solid. Physical evidence for physical disruption of crystal-rich, or even near-solidus magma by repeated mafic input (e.g. autostoping and engulfment of cumulate blocks) was accompanied by quite localized remelting of mush and/or reheating of the felsic cumulates and spatially restricted hybridization. Thus during the time of intrusion of the Guzzler Gulch mass, the Searchlight magma chamber was incapable of thorough mixing.
DE: 1035 Geochronology
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting