23 Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism (Posters)

We welcome diverse submissions on the petrology, structure, geochemistry, and geophysics of subduction zones from all approaches. Example topics include initiation, evolution, volatile cycling, ophiolites, arc tempos, high-P metamorphism, mélanges, seismicity, and accretionary wedges.

Thursday, 23 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Petrology of a Franciscan garnet lawsonite blueschist from the southern Diablo Range (California, USA) and Automated mapping of inclusions in garnets utilizing ArcGIS and SEM-EDS phase maps

Paper number: 23-4
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Petrology of high-grade tectonic blocks hosted by the Franciscan Formation are largely focused on samples from the more northern reaches of the formation. Here, we report on a garnet lawsonite blueschist found as a small boulder weathered out of Franciscan mélange near Pacheco Pass in the southern Diablo range (California, U.S.A.). Thin sections were analyzed petrographically and by SEM-EDS Samples are composed of glaucophane, garnet, sodic pyroxene (Jd62Ac11Quad27), quartz, lawsonite, epidote, sphene, rutile, and apatite. Small (< 1mm) euhedral to subhedral garnets predominantly almandine, while displaying minimal major cation zoning other than in Mn (Sps9.8 cores and Sps0.1 rims). Glaucophane has thin rims slightly richer in Fe. Garnets contain inclusions of jadeite, glaucophane, lawsonite, quartz, rutile, and sphene, ranging in size from 10 to 50 μm. Lawsonite is present as subhedral crystals in the matrix as well as part of late veins with quartz, which cross cut some garnets. Chlorite is notably absent. Some matrix sphene crystals preserve relict rutile in their cores. Modeling using Equilibrium Assemblage Diagrams (EADs) unsurprisingly does not reconstruct the full observed assemblage. However, a garnet, glaucophane, lawsonite, pyroxene, rutile, quartz assemblage is predicted at ~600˚C, >20kbar, surprisingly high for a garnet blueschist. Further modelling to assess the effects of changing fO2 and Ca-metasomatism are ongoing.

In order to better understand the distribution and textures of inclusions in garnet, SEM-EDS phase maps were analyzed using custom scaling in a Geographic Information System (GIS). The position of inclusions were measured from garnet centroids and also matched to adjacent Mn-composition in the host garnet. By scripting the procedure in Python within ArcGIS, 25,000 inclusions were analyzed in a single thin section. Probability density functions for all five inclusion phases relative to host garnet Mn content have single peaks with nearly identical mode lines at Sps5.3. The asymmetry of the inclusion distribution is very similar to the distribution of melt inclusions in migmatites as measured by X-ray computed microtomography (Parisatto et al., 2018). Ongoing research will apply microscopic computed tomography scanning to analyze radial distances of garnet inclusions in 3D in these samples.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410946

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