Chapter 20: Geochemistry of Mio-Pliocene volcanic rocks from around Panamint Valley, Death Valley area, California
Drew S. Coleman, J. Douglas Walker, 1990. "Chapter 20: Geochemistry of Mio-Pliocene volcanic rocks from around Panamint Valley, Death Valley area, California", Basin and Range Extensional Tectonics Near the Latitude of Las Vegas, Nevada, Brian P. Wernicke
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Miocene-Pliocene volcanic rocks of the Nova Formation and Darwin Plateau near Death Valley, California, were erupted during extensional faulting and range in composition from basalt to rhyolite. Petrographic evidence, including sieved plagioclase, glass-bearing plagioclase, mafic and felsic xenoliths, and quartz, sanidine, and amphibole xenocrysts in samples of intermediate composition, suggest that they were derived through magma mixing or crustal assimilation. Chemical modeling indicates that rocks that show these disequilibrium textures can be derived through a combination of mixing between end-member basalt and rhyolite and fractionation of olivine, pyroxene, and plagioclase ± intergrown magnetite and ilmenite ± Cr-Al spinel. Pb and Sr isotopic data support these conclusions and may constrain the source of the contaminant to the lower crust.
The age (~4 Ma) and chemical similarity of rocks from Pinto Peak and Darwin Plateau and the apparent lack of vents and feeder dikes at Pinto Peak indicate that the lavas from these two areas probably shared a common source located at Darwin Plateau. This suggests that the two areas, now separated by Panamint Valley, were adjacent during the period of volcanic activity and that Panamint Valley has therefore opened in the last 4 m.y.
Volcanic activity in the study area progressed from exclusively basaltic to basaltic-andesitic-rhyolitic and shifted westward through time. These observations are consistent with a simple-shear mechanism for extension, because this mechanism better explains the “off-axis” nature of the volcanism and provides a more efficient means for crustal anatexis than a pure-shear mechanism.