Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

Abstract

Determining pressure and temperature variations between high-pressure/low-temperature (HP–LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-garnet elastic barometry to constrain formation pressures of eclogites from the northern (Junction School, Ring Mountain, Jenner Beach) and southern Franciscan Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and a single eclogite from the other localities. By comparing garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. We compare the mean, median, and max pressures between different garnet zones and eclogites. Pressures sometimes exhibit systematic changes across garnet zones; however, some eclogites exhibit no systematic pressure variations across garnet zones. Pressures from northern Franciscan eclogites range from $$\sim $$ ∼ 1.4–1.8 GPa, at an estimated temperature of 500 $$^{\circ }$$ ∘ C; pressures from the Catalina eclogite range from $$\sim $$ ∼ 1.2–1.5 GPa, at an estimated temperature of 650 $$^{\circ }$$ ∘ C. Mean and maximum pressures of different eclogites from the northern Franciscan exhibit negligible differences (< 0.1 GPa). The results are inconsistent with models that propose exhumation of metamorphic blocks from different structural levels, and suggest that now exposed HP–LT eclogites from the northern Franciscan Complex may represent rocks that were coherently underplated, and exhumed from similar structural levels.

A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high‐pressure rocks

“…Despite these clear differences and complexities, there are significant commonalities among different eclogite (and related) blocks in the Franciscan, including the consistent temperatures recorded by rutile in this study, similar pressures recorded by quartz inclusions in garnet (Cisneros et al, 2022) and perhaps a similar shared early exhumation history (Rutte et al, 2020). Most garnet and rutile that formed after fluid infiltration point to a similar fluid composition in the subduction channel (Figures 8 and 9), resulting in δ 18 O Grt = 6-7‰ and δ 18 O Rt = 4-6‰, regardless of the initial δ 18 O.…”

Is the inverted field gradient in the Catalina Schist Terrane primary or constructional?

“…older than the MDA of the lawsonite-blueschist facies rocks. U-Pb ages on metamorphic zircon and titanite and Lu-Hf ages on garnet from the high-grade rocks all cluster around 115-112 Ma [Mattinson, 1986;Anczkiewicz et al, 2004;Page et al, 2019;Cisneros et al, 2022], suggesting that this was the time of peak-temperature metamorphism.…”

Computing Rates and Distributions of Rock Recovery in Subduction Zones

“…A major uncertainty in the markers data set presented below stems from an implicit assumption that the plate interface behaves like a distributed shear zone composed of hydrated ultramafic material mixed with fragments of dehydrating oceanic crust and seafloor sediments (i.e., a tectonic mélange). Field evidence for tectonic slicing of HP rocks does not necessarily support this view of plate interface behavior (e.g., Agard et al., 2018; Angiboust et al., 2009, 2014; Angiboust, Langdon, et al., 2012; Gilio et al., 2020; Locatelli et al., 2018; Monié & Agard, 2009; Poulaki et al., 2023), while other well‐studied mélange‐like structures do (e.g., Festa et al., 2019; Harvey et al., 2021; Hsü, 1968; Kusky et al., 2013; Penniston‐Dorland & Harvey, 2023; Platt, 2015; Wakabayashi & Dilek, 2011), and still other localities exhibit field relations that are more ambiguous (e.g., Bonnet et al., 2018; Cisneros et al., 2022; Kotowski et al., 2022; Kusky et al., 2013; Platt, 1975). Such a variety of different structures interpreted as former plate interfaces highlight the fact that large uncertainties exist in the rock record—in addition to large experimental and theoretical uncertainties—all of which challenge our understanding of plate interface mechanics in subduction zones.…”