Rise and fall of a basalt-trachyte-rhyolite magma system at the Kane Springs Wash Caldera,Nevada |
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| Authors: | Steven W Novak Gail A Mahood |
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| Institution: | (1) Geology Department, Stanford University, 94305 Stanford, CA, USA;(2) Present address: Charles Evans and Associates, 301 Chesapeake Drive, 94063 Redwood City, CA, USA |
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| Abstract: | Magmas erupted at the Kane Springs Wash volcanic center record the buildup and decay of a silicic magma chamber within the upper crust between 14.1 and 13.2 Ma ago. Intrusion of a variety of mantle-derived basaltic magmas into the crust sustained the system thermally, but only alkali basalts appear to be parental. Fractionation of alkali basalt, together with 10–20% contamination by partial melts of the lower crust, generated trachyandesite magmas. Mafic trachytes, with magma temperatures of  1,000° C, were initially generated from trachyandesites at depths greater than 15 km. Continued fractionation combined with assimilation of upper crustal melts at a depth of 5–10 km produced more evolved trachytes and high-silica rhyolites. These silicic magmas erupted as the Kane Wash Tuff 14.1 Ma ago from a chamber zoned from fayalite-bearing alkali rhyolite near 820° C at the roof to a trachytic  dominant volume . Initial ash flows of the Kane Wash Tuff, Member V1, are metaluminous, whereas later cooling units, Members V2 and V3, are mildly peralkaline and have higher Fe, Zr, and Hf and lower Ca, Th/Ta, Rb/ Zr, and LREE/HREE. Less than 1 % upper crustal component was involved in generation of Members V2 and V3 from trachytic magma. Eruption of 130 km3 of magma resulted in collapse of the Kane Springs Wash caldera. Trachytic magma from deeper levels of the system was extruded onto the caldera floor shortly afterward, forming a central trachyte/syenite complex. Replacement of this magma by hotter, more mafic magma may have induced additional melting of the already heated chamber walls, as high-silica rhyolites that erupted in the moat surrounding the central complex have a large crustal component. Early moat rhyolites had temperatures near 800° C and, in contrast to the Kane Wash Tuff, are ferroedenite-bearing, have higher Al, K/Na, Th/Ta, and Ba, and have lower Fe, REE, and Zr. Fractional crystallization of this magma within the cooling and crystallizing magma chamber formed biotite-bearing rhyolite in isolated pockets. The most evolved of these had temperatures near 700° C, elevated F contents, H2O contents of 5 wt.%, Rb> 500 ppm, chondrite-normalized LREE/HREE <1, and formed vapor-phase topaz. Declining temperatures and Cl/ F from the Kane Wash Tuff through the moat rhyolites may reflect decreasing basalt input into the base of the system and increasing proportions of upper crustal melts in the silicic magmas. |
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