HR: 1330h
AN: V13A-01    [Abstracts]
TI: Newberry Mountains Dike Swarm, Southern Nevada: Final, Extension-Related Pulse of the Spirit Mountain Batholith
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 37235-1805 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 37235-1805 United States
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 37235-1805 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
AB: More than 100 large, mafic to felsic dikes comprise the Newberry Mountains dike swarm. The dikes strike N-S and dip 40-60 degrees E; paleomagnetic data indicate that they were tilted W from a near-vertical initial orientation (Faulds et al. 1992). The swarm cuts the Spirit Mountain batholith, extending approximately 15 km north to south; individual dikes can be traced up to several km. Felsic porphyries are mostly 3-20 m thick and contain abundant resorbed quartz, subordinate biotite, and in most cases euhedral Kfeldspar and plagioclase phenocrysts. They are very uniform in composition (73% SiO2). Basaltic andesite to andesite dikes are more variable in composition, mostly smaller (typically 3-5 m thick, rarely to 15 m), somewhat less abundant, and commonest in the eastern portion of the swarm. The more mafic dikes are aphanitic and dominated by plag, cpx, and hbl; intermediate dikes are typically coarser, with abundant hbl, plag, and biotite, and they contain small mafic enclaves. Zircon U/Pb (SHRIMP) analyses of two felsic dikes yielded an age of 15.6±0.3 Ma (2σ). The dikes sharply cross-cut and are chilled against all units of the batholith except for initially horizontal sheets of fine-grained biotite granite and the Mirage pluton. Contacts with the granite sheets are irregular ("soft"-looking), and the felsic dikes are similar to the Mirage pluton (73% SiO2, resorbed quartz phenocrysts) and appear to emanate from it. Emplacement of the dikes may have been facilitated by the onset of rapid E-W extension in this part of the Colorado River extensional corridor, suggested to have occurred at 15.5-16.0 Ma (e.g. Faulds et al., 2001). The bulk of the batholith solidified 16.0-17.2 Ma (Walker et al., 2005), and the biotite granite sheets and associated gabbro and diorite at 15.8 Ma. The felsic dikes are strikingly similar to the granite sheets in major and trace element concentrations and also to the probable input magmas that formed the more voluminous, coarser-grained, older portions of the batholith. The mafic dikes may reflect continued input of mafic-intermediate magma that is locally evident within earlier units. We suggest that the dike swarm was the final pulse of the magmatic system that formed the batholith, and that its age of 15.6 Ma marks the initiation of rifting in this region.
DE: 1020 Composition of the crust
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 8035 Pluton emplacement
DE: 8109 Continental tectonics--extensional (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly