HR: 09:00h
AN: V21A-03    [Abstracts]
TI: The Spirit Mountain Batholith: Documenting Magma Storage in the Upper Crust One Pulse at a Time
AU: * Walker, B A
EM: barry.a.walker@vanderbilt.edu
AF: Dept. of Earth and Environmental Sciences, Vanderbilt University, VU Station B #351806 2301 Vanderbilt Place, Nashville, TN 37325-1805 United States
AU: Miller, C F
EM: calvin.miller@vanderbilt.edu
AF: Dept. of Earth and Environmental Sciences, Vanderbilt University, VU Station B #351806 2301 Vanderbilt Place, Nashville, TN 37325-1805 United States
AU: George, B E
EM: benjamin.e.george@vanderbilt.edu
AF: Dept. of Earth and Environmental Sciences, Vanderbilt University, VU Station B #351806 2301 Vanderbilt Place, Nashville, TN 37325-1805 United States
AU: Ludington, S
EM: slud@earthink.net
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: USGS/Stanford University, Rm. 89, Ion Probe Lab Green Building 367 Panama Street, Stanford, CA 94305-2220 United States
AU: Bleick, H A
EM: heather.a.bleick@vanderbilt.edu
AF: Dept. of Earth and Environmental Sciences, Vanderbilt University, VU Station B #351806 2301 Vanderbilt Place, Nashville, TN 37325-1805 United States
AU: Miller, J S
EM: jsmiller@email.sjsu.edu
AF: Department of Geology San Jose State University, Department of Geology San Jose State University, San Jose, CA 95192-0102 United States
AB: More than 500km² of Miocene intrusive rocks in the Newberry Mountains, southernmost Nevada, comprise a batholith that has been tilted west up to 50° (Hopson et al, 1994) and exposed due to late Miocene extension and crustal thinning in the Colorado River Extensional Corridor. New zircon U/Pb data (SHRIMP) demonstrate episodic crystallization of the batholith during four main episodes of magmatism that are consistent with field evidence: 17.2 Ma, 16.4-16.0 Ma, 15.8 Ma, and 15.6 Ma (errors are ±0.2 [2σ] with MSWD <2). Formerly referred to separately as the Spirit Mountain Pluton (SMP) and Mirage Pluton (Howard et al., 1996; Hopson et al, 1994), new geochronological, geochemical, and field evidence suggest that the Mirage is closely related and that the SMP itself is composite. Thus, we suggest these intrusions should be considered collectively as the Spirit Mountain Batholith. Rocks emplaced at 17.2 Ma include high-silica equigranular granites and porphyries (~77% SiO₂) at the shallowest levels of the batholith. The 16.4 - 16.0 Ma unit is the largest, consisting of multiple sequences of coarse to medium grained granite grading to magmatically foliated quartz monzonite with prominent accumulated feldspar crystals. These sequences range from ~60-73 % SiO₂, and appear to represent discrete intrusive episodes. Local inter-pluton cupolas of high-silica granite (~77% SiO₂) document fractionation and melt segregation within individual pulses. Minor mafic input into the pluton during this period is manifested by magmatic enclaves ranging from mm's to 2 m in size, locally abundant and highly flattened into pancake shaped discs parallel to the foliation of the host. The 15.8 Ma episode consist primarily of fine grained biotite granite (~73% SiO₂) that forms irregular, initially subhorizontal sheets (m's to <100 m thick). A large sheet of gabbro appears to have mingled and locally mixed with the biotite granite and yields an identical age. The 15.6 Ma pulse is a series of N-striking felsic (~73% SiO₂) to mafic dikes that cuts all other units (George et al, 2005). Field relations suggest that the two latest episodes of magmatism (15.8 Ma, 15.6 Ma) are associated with the Mirage Pluton. Xenolith relations along with sharp contacts between the granites show that these later magmas intruded the SMP after it had become rigid. Though many different pulses of magma are involved in this system, we contend that given their proximity in space, time, and composition, this series of intrusions constitutes a single composite batholith that formed in response to continuing input of very similar magma into the upper crust. Intrusion styles were likely dictated by the changes in regional tectonics. For example, at the onset of extension, diking may have ensued, as opposed to magma ponding in thick, slowly solidifying sheets. Because of tilting, this field area serves as a cross section, and thus a window into the processes of batholith construction, including magma injection, magma interaction, differentiation of individual magma pulses, and compaction of crystal mushes.
DE: 1035 Geochronology
DE: 3640 Igneous petrology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly