HR: 1330h
AN: V22D-0607    [PDF]
TI: A Two-Stage Model for Origin of Al-rich Crustal Xenoliths in Miocene Andesite, Diablo Range, West-Central California
AU: * Metzger, E P
EM: metzger@geosun.sjsu.edu
AF: San Jose State University, Geology Department, San Jose, CA 95192-0102 United States
AU: Ernst, W G
EM: ernst@pangea.stanford.edu
AF: Stanford University, Geological Environmental Sciences, Stanford, CA 94305-2115 United States
AB: Miocene ($\sim$ 8-10.5 Ma) andesite exposed as small plugs and dikes in the Diablo Range of west-central California encloses scattered xenoliths with diverse compositions and textures. The andesite is part of the Diablo Range Volcanics (DRV), a mafic to intermediate suite that is broadly coeval with and may be erosional remnants of the more extensive Quien Sabe Field located to the south and east. The DRV suite is inferred to be part of a northwestwardly younging sequence of volcanic fields that may be related to migration of the Mendocino Triple Junction (MTJ). Two basic categories of xenoliths are present: (1) metasedimentary rocks including quartzite, biotite schist, garnet-clinopyroxene gneiss, and distinctive sillimanite-corundum rocks; and (2) gabbroic and dioritic rocks exhibiting plutonic textures. Preliminary analysis has focused on aluminous xenoliths in which blocky porphyroblasts consisting of intergrown plagioclase, corundum, and hercynite $\pm$ sillimanite $\pm$ alkali feldspar up to $\sim$ 2 cm in length are surrounded by a very fine-grained granoblastic matrix of plagioclase, orthopyroxene, and hercynite $\pm$ biotite $\pm$ alkali feldspar $\pm$ minor quartz. Glass is present both within the inclusions and in the surrounding matrix. The square to elongate outlines of the plagioclase-corundum inclusions suggest that they are pseudomorphic after andalusite. The corundum-bearing xenoliths are interpreted as the products of two stages of high T-low P metamorphism. The first event involved mid-crustal metamorphism (reflecting cessation of outboard subduction/refrigeration?) to produce andalusite-bearing hornfels; other phases probably included K-feldspar, Na-plagioclase, muscovite, biotite, and quartz. The second stage of recrystallization took place when the previously metamorphosed wall rock was incorporated in andesitic magma, possibly during passage of the MTJ. In response to heating by the magma, andalusite was replaced by corundum, plagioclase $\pm$ sillimanite, muscovite and quartz broke down to produce more K-feldspar and sillimanite, and most of the biotite decomposed, forming abundant hercynite.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting