Potassic volcanism near Mono Basin, California; evidence for high water and oxygen fugacities inherited from subduction

• Published in: Geology (Boulder) 1993-10, Vol.21 (10), p.949-952
• Main Authors: Lange, Rebecca A, Carmichael, Ian S. E, Renne, Paul R
• Format: Article
• Language: English
• Subjects: absolute age; California; Cenozoic; crystallization; dates; east-central California; fugacity; geochemistry; Geology; Geophysics; hydration; igneous and metamorphic rocks; K/Ar; lava; Long Valley Caldera; magmas; mantle; Mono Basin; Mono County California; Neogene; Oxidation; oxygen; Petrology; plate tectonics; Pliocene; potassic composition; Quaternary; rock, sediment, soil; solid Earth (tectonophysics); subduction; Tertiary; United States; upper mantle; volcanism; Volcanoes; Water
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/0091-7613(1993)021<0949:PVNMBC>2.3.CO;2

Exposed around the margins of Mono basin and the periphery of Long Valley caldera are ∼100 km3 of potassic, basic to intermediate lavas that record high water and oxygen fugacities. Most eruptions occurred between 4 and 2 Ma, although sporadic potassic volcanism continued into the Quaternary with a pulse between 0.1 and 0.5 Ma, ∼15 km northeast of Mono basin. The lava types include absarokite, minette, hornblende lamprophyre, trachybasalt, and trachyandesite. Estimated water contents for lavas without hydrous phenocrysts range between 2 and 3 wt%, considerably more than mid-ocean ridge, oceanic island, or back-arc magmas. Calculated f O2 values fall between -0.4 and +1.2 log units of the Ni-NiO buffer. Although this potassic suite was erupted in an extensional tectonic setting at the western margin of the Basin and Range province, its high K2O/TiO2 and low Zr/Ba ratios suggest a subduction-modified mantle source. If subduction was the process that enriched the lithospheric mantle in large-ion lithophile elements (K, Ba, etc.), it was also a mechanism for mantle oxidation and hydration. October 1993