T101 36th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Monday, 23 September 2024
8:00 AM - 5:30 PM
Hall D

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

Paper number: T101-1
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-402772

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