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

Miguel Cisneros,John P. Platt,Whitney M. Behr,Robert Anczkiewicz

doi:10.1007/s00410-021-01876-4

Miguel Cisneros, John P. Platt + Show 2 more

Open Access

https://doi.org/10.1007/s00410-021-01876-4

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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.

Highlights

High pressure–low temperature (HP–LT) rocks provide insight into the thermal (e.g., Penniston-Dorland et al 2015), mechanical (e.g., Agard et al 2018), fluid (e.g., Collins et al 2015), and temporal (e.g., Baldwin et al 2004) evolution of subduction zones
Because previous studies suggest that they preserve the maximum pressure conditions reached by the Franciscan Complex
We focus on eclogite blocks that are found within the Central belt of the northern Franciscan (Junction School, Ring Mountain, and Jenner Beach), and a single eclogite from Catalina Island

Summary

Introduction

High pressure–low temperature (HP–LT) rocks provide insight into the thermal (e.g., Penniston-Dorland et al 2015), mechanical (e.g., Agard et al 2018), fluid (e.g., Collins et al 2015), and temporal (e.g., Baldwin et al 2004) evolution of subduction zones. California represents an exhumed accretionary complex that preserves exceptional HP–LT rocks that are well-known for their block-and-matrix relationship, with high-grade blocks embedded within metasediments or an ultramafic matrix (serpentinites, chlorite, talc, actinolite) These rocks have been used to understand: (a) the evolution of the Franciscan complex (e.g., Wakabayashi 2015), (b) the formation conditions of blueschists and eclogites (e.g., Tsujimori et al 2006; Page et al 2007; Ukar et al 2012), (c) interactions among multiple deep subduction zone processes (e.g., Viete et al 2018), and (d) the mechanisms responsible for exhumation of HP–LT rocks (e.g., Cloos 1982; Platt 1986; Horodyskyj et al 2009). Eclogites present a thermobarometric challenge owing to the high number of components and the low number of phases

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