Young bimodal volcanism at Medicine Lake volcanic center, northern California

Details Young bimodal volcanism at Medicine Lake volcanic center, northern California Young bimodal volcanism at Medicine Lake volcanic center, northern California

Aug 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39.1165c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 39, Issue 8, pp.1165-1178

Computer Science

13

Scientific paper

The last 10,000 years of activity at the Medicine Lake volcanic center in northern California is characterized by bimodal mafic and siliceous volcanism. Interflow element variations are complex and exhibit a discontinuity for most elements between 57 and 62 per cent SiO 2 . No simple linear or curvilinear element trends exist between the mafic (Modoc) and siliceous (glass) volcanics. The geochemical variation patterns exhibited by volcanic rocks from the Medicine Lake volcanic center preclude any simple model for magma origin involving either varying degrees of melting or of fractional crystallization. A model is tentatively invoked for the andesites and basalts involving 35 per cent melting of eclogite (of altered rise tholeiite composition) in a descending slab followed by varying amounts of fractional crystallization and perhaps magma mixing. Up to 20 per cent of shallow fractional crystallization of plagioclase and minor Ti-magnetite seems to be required by the Sr, Eu anomaly, and TiO 2 distributions. Compositional variation and high O 18 values in most dacite glass flows are best interpreted in terms of a crustal origin involving up to 50 per cent partial melting of average continental crust. Rhyolite glasses may have formed by small degrees of melting (20-30 per cent) of this crust followed by 5-10 per cent of shallow fractional crystallization (removing dominantly plagioclase) or by 40-50 per cent fractional crystallization of a dacite parent (~63 per cent SiO 2 ) produced in the crust. The shallow fractional crystallization is necessary to explain the low Sr contents and large negative Eu anomalies in the rhyolites. Dacites from the Composite Flow are tentatively interpreted to have formed by shallow mixing of a hybrid magma (composed of varying amounts of andesite and dacite) with rhyolite prior to and during eruption.

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