Aluminous Xenoliths in Miocene Andesite, Central California Coast Ranges: Magma-Crust Interaction in a Subduction-Transform Transitional Setting

Abstract

Non-foliated, Al-rich inclusions in Miocene andesite dikes and plugs cropping out in the Dowdy Ranch area, central Diablo Range, record two stages of high-temperature metamorphism, as well as complex chemical interactions between the inclusions and their mafic magma host. Both xenoliths and andesitic host show marked compositional zoning and partial resorption of minerals. The first stage of subsolidus contact metamorphism produced andalusite + alkali-feldspar + Na-plagioclase + muscovite + biotite + quartz (± actinolite or epidote?) in xenoliths of the originally quartzofeldspathic wall rock. P-T conditions attending this event were approximately 600°C, ˜2 kbar, with Pfluid approaching Ptotal. Capture of fragments of metamorphosed wall rock by ascending mafic magma resulted in partial fusion of the xenoliths, loss of silica and alkalis to the liquid, and calcium-enrichment in the restitic, increasingly aluminous inclusions. Second-stage neoblastic assemblages are sillimanite + hercynite-rich spinel + corundum + Ca-plagioclase ± Mg-rich biotite or phlogopite ± orthopyroxene. Physical conditions in the xenoliths accompanying decompression partial melting were approximately 700-750°C and ˜1 kbar (with Pfluid < Ptotal). The presence of corundum and ZnO-poor spinel is consistent with a restitic residue produced by dehydration melting under high-temperature, silica-deficient conditions. The parageneses developed in the Al-rich inclusions were an upper crustal thermal response to Miocene passage of the Mendocino triple junction at this latitude.