GRANITE INCLUSIONS IN HOLOCENE LAVAS OF MEDICINE LAKE VOLCANO, CALIFORNIA, USA: CLUES TO THE SUBSURFACE GEOLOGY

1LOWENSTERN, J.B., 1DONNELLY-NOLAN, J., 2GROVE, T.L., 1WOODEN, J., 1LANPHERE, M.

1US Geological Survey, Menlo Park, CA 94025 USA, 2Dept. of Earth, Atmospheric and Planetary Science, MIT, Cambridge, MA 02139 USA

We studied inclusions of granite found in Holocene lava flows from Medicine Lake volcano, a large Quaternary shield volcano that lies about 50 km ENE of Mt. Shasta, in northern California. One subgroup is enriched in d18O (whole-rock and quartz separates: 7.8 - 9.8 ?: n=7) and 87Sr/86Sr (0.7046 - 0.7050). This group shows evidence for incipient melting prior to or during transport to the surface. Other inclusions are lower in d18O (quartz separates: 5.1 - 7.5 ?: n=8) and 87Sr/86Sr (0.7037 - 0.7041), and have been found in the Glass Mtn. domes and in three Holocene mafic flows where they are 30-50% melted and partly vesiculated. All inclusions are fine- to medium-grained; individual clasts may have hypidiomorphic granular or porphyritic textures, the latter usually with a micrographic groundmass. Mafic minerals include biotite and orthopyroxene, the latter more common where melting is greatest. Primary, vapor-rich fluid inclusions are common, as are glassy melt inclusions, consistent with relatively rapid cooling at shallow depths.

None of the inclusions are likely to have been tapped from depths < 3 km. Geothermal drillholes, up to ~3000-m-deep and sited near the Holocene rhyolites, traverse abundant altered silicic volcanic rocks before reaching granites similar in appearance to the inclusions. However, samples from drillholes are hydrothermally altered and have low whole-rock d18O values from -0.8 to -4.7 (n=4). The absence of similarly altered granite inclusions in Holocene lavas implies that recent magma storage, fractionation and attendant crustal melting occurred at depths > 3 km.

Shallow depths implied by inclusion textures may reflect their crystallization prior to initial growth of the overlying Medicine Lake volcano, at ~0.5 Ma, or intrusion early in its history. The granites may be late Miocene to Pleistocene intrusions, possibly remelted from Sierran terranes inferred by seismic data to underlie the volcano.


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