EVIDENCE FOR DYNAMIC MAGMATIC PROCESSES: PLAGIOCLASE TEXTURES AND COMPOSITIONS IN ADAKITIC VOLCANIC ROCKS, CENTRAL CALIFORNIA COAST RANGES

Five dacite and andesite samples were collected from small, fault-related dikes cropping out near the border of the Mississippi Creek and Mt. Stakes 7.5-minute quadrangles in the Red Creek/Robinson Creek area of Henry Coe State Park. These ~10 Ma rocks are inferred to be part of a northward younging sequence of volcanic rocks that may be related to migration of the Mendocino Triple Junction (MTJ) and formation of a slab window. Petrography, electron microprobe analysis, and whole rock chemistry were used to investigate the origin of these volcanic rocks.

The samples are porphyritic (~ 25-30% phenocrysts) with a holocrystalline groundmass comprised primarily of plagioclase microcrysts. Phenocrysts include plagioclase (most abundant), biotite, and scarce amphibole; glomerocrysts are common. Disequilibrium textures including coarse and fine sieving, resorption features, oscillatory zoning, and patchy zoning are observed in plagioclase phenocrysts. Plagioclase phenocryst rims (An 46-59) are more than calcic than the cores (An 27-35). The samples plot in the high-K calc-alkaline field and are enriched in large-ion lithophile elements and depleted In Nb and Ta, as is typical of subduction-related magmas. Based on the Sr/Y vs Y discrimination diagram, these rocks are adakitic and may be associated with slab melting, possibly at the edge of a slab window.

Plagioclase microtextures and compositions provide evidence for complex magma chamber processes, including convection, decompression, and recharge by a less evolved magma. The K-rich composition of the samples and presence of abundant quartz xenocrysts are evidence for assimilation of crustal material.

More in-depth geochemical analysis is needed to evaluate the hypothesis that slab melting was involved in producing the Red Creek area dacite/andesite samples. Additional petrographic and geochemical analysis will allow comparison with dacite and andesite samples collected from the Quien Sabe volcanic field to the south.