HR: 15:10h
AN: V43G-07    [Abstracts]
TI: Igneous origin of K-feldspar Megacrysts in Granitic Rocks of the Sierra Nevada Batholith
AU: * Moore, J G
EM: jmoore@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Study of the four principal K-feldspar megacrystic granitic plutons and related porphyrys in the Sierra Nevada composite batholith indicates that the included megacrysts are phenocrysts that grew in contact with granitic melt in long-lasting magma chambers. These 89-83 Ma plutons or intrusions are the youngest in the range, and represent the culminating magmatic phase of the batholith. They are the: Granodiorite of Topaz Lake; Cathedral Peak Granodiorite, Mono Creek Granite, Whitney Granodiorite, Johnson Granite Porphyry, and Golden Bear Dike. The zoned megacrysts in each of these igneous bodies attain 4-10 cm in length and all display oscillatory zoning with each zone beginning with a sharp increase followed by a gradual decrease in the concentration of BaO - commonly from 3 to 1 weight percent. Some of the more pronounced zones overlie resorption and channeling features on the underlying zone. Trains of small mineral inclusions (plagioclase, biotite, hornblende, quartz, sphene, and accessory minerals) are parallel to the BaO-delineated zones. The long axes of the inclusions are preferentially aligned parallel to the zone boundaries and inclusions are sorted by size from zone to zone. The growth temperature of sphene included in K-feldspar megacrysts is estimated by use of a Zr-in-sphene geothermometer. The sphene grains all yield igneous temperatures, mainly 735 - 760 °C. Sphene grains in the granodiorite host marginal to the megacrysts range to lower growth temperatures, in some instances into the subsolidus range. The zoning of the megacrysts, their presence in quenched porphry dikes, and the limited range and igneous values of growth temperatures of sphene inclusions within them, support the interpretation that the megacrysts formed as igneous sanidine phenocrysts, and that intrusion temperatures varied by only small amounts while the megacrysts grew. Each Ba- enriched zone was apparently formed by a repeated surge of new, hot melt injected into the large magma chamber represented by the plutons. Each recharge of hot magma offset cooling, maintained the partially molten or mushy character of the chamber, and induced convective currents that would loft settling megacrysts back up into the chamber. These processes continued long enough to provide the necessary conditions for the growth of these extraordinarily large K-feldspar phenocrysts.
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting