HR: 0830h
AN: V51I-0397    [PDF]
TI: Magmatic construction and duration of solidification of Searchlight pluton, Eldorado Mountains, Nevada (USA)
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: 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: 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: Wooden, J L
EM: jwooden@usgs.gov
AF: USGS-SUMAC, Stanford Univ, Stanford, CA 94035-2220 United States
AU: Means, M A
AF: Dept. of Geology, San Jose State Univ., San Jose, CA 95192-0102 United States
AU: Ericksen, S
EM: shellyericksen@hotmail.com
AF: Dept. of Geology, San Jose State Univ., San Jose, CA 95192-0102 United States
AB: The process of chamber construction and the residence time of magma in mid- to upper crustal magma bodies have been illuminated by recent advances in high-resolution geochronology. Most work has so far concentrated on young volcanic systems; few geochrononlogic data have addressed magma chamber longevity and history from studying plutons. Plutons offer an important complementary record of magma processing and solidification, and can therefore reveal much about the internal workings of magma chambers. The Miocene Searchlight pluton (Nevada, USA) is a spectacular example of a very thick magma chamber (10-12 km). Crystal accumulation (mafic quartz monzonite cumulate), coupled with roof-down solidification (upper quartz monzonite) resulted in segregation of evolved felsic melt in the chamber interior (middle granite). Initially horizontal and gradational internal contacts and coplanar magmatic fabrics between all major units, geochemical mass balance, and very limited isotopic variation among major units, suggest that the entire pluton was molten at one time. However, the time inteval over which the chamber solidified was not well known. New TIMS and {\it in situ} ion microprobe U/Pb dating of zircon, combined with our earlier and ongoing field and isotopic studies, now appear to document a protracted magma chamber history. TIMS and ion microprobe dating (Stanford/USGS SHRIMP-RG) was done on two samples, and a third sample was dated by ion microprobe only. A multi-grain, multi-fraction discordia lower intercept age of 16.7$\pm$0.5 Ma (MSWD=22) was obtained by TIMS from the lower mafic quartz monzonite cumulate. Ion probe dating of zircons from the same sample yielded a $^{206}$Pb/$^{238}$U age of 16.9$\pm$0.2 Ma (MSWD=1.3; N=24) in agreement with the TIMS lower intercept age. A discordia lower intercept age of 15.7$\pm$0.4 Ma (MSWD=3.7, one concordant point at 15.8 Ma) was obtained from the middle granite unit. Ion probe dating of zircons from the same sample yielded a $^{206}$Pb/$^{238}$U age of 16.2$\pm$0.2 (MSWD=3.5; N=28). Distinct $^{206}$Pb/$^{238}$U age maxima at 16.8 Ma for the lower cumulate unit and 15.8 Ma for the middle granite are also apparent on cumulative age-probability plots. Ion probe dating of zircons from a third sample of fine-grained quartz monzonite near the roof yielded a $^{206}$Pb/$^{238}$U age of 16.0$\pm$0.3 (MSWD=1.4; N=20) with a broad cumulative age probability maximum at 16.2 Ma. High MSWD of the discordia for the TIMS data are attributable to Proterozoic inheritance but are well-anchored at the lower intercepts. Proterozoic cores are occasionally found during ion probe analysis but these are not included in the ion probe ages. The age results taken together argue for 700,000 to 1 million years of separation between crystallization of the oldest and youngest zircons in the Searchlight magma chamber. Although melts may have been added to the chamber incrementally from deeper sources, the new age data above are consistent with all other data and observations, which indicate protracted magma residence and monotonic solidification and fractionation from relatively uniform trachydacite magma.
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