13-2 Early Metamorphism of High-Grade Blocks from the Subduction Mélange of Cedros Island, Baja California, Mexico: Insights from Garnet Lu-Hf Geochronology

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

Presenting Author:

Charlotte Connop

Authors:

Connop, Charlotte¹, Holder, Robert², Xu, Yiruo³, Neumann, Sam⁴, Hernandez-Uribe, David⁵, Wang, Jordan⁶, Kapp, Paul⁷, Worthington, James⁸, He, John⁹

Abstract:

High-grade metamorphic blocks within subduction mélanges provide key insight into subduction zone initiation and evolution. The Cedros Island subduction complex in Baja California, Mexico includes both serpentinite- and clastic-matrix mélange structurally underlying a c. 171 Ma suprasubduction zone ophiolite (Kimbrough and Moore, 2003). Metasedimentary and metamafic blocks within mélange include greenschist, epidote-and lawsonite-blueschist, as well as epidote-, epidote-garnet-, and garnet-amphibolite. Garnet Lu-Hf dates from five garnet-amphibolite blocks span 182.1 ± 1.5 Ma (n = 3, MSWD = 1.8) to 177.3 ± 1.1 Ma (n = 4, MSWD = 1.3), demonstrating that high-grade blocks underwent metamorphism by c. 180 Ma, prior to the formation of the overlying Cedros Island ophiolite. This mirrors geochronology from high-grade rocks and their structurally overlying ophiolites in the Franciscan Complex of California, USA (c. 176 Ma; Mulcahy et al, 2018) and Easton Metamorphic Suite of Washington, USA (c. 183 Ma; Cunetta et al., 2025), indicating a shared metamorphic and tectonic history over >3000 km along the North American Cordilleran margin. In both the Franciscan and Easton, these c. 180 Ma high-grade metamorphic events followed by overlying forearc ophiolite formation are interpreted to mark initiation of east-dipping subduction along the western margin. However, in Mexico, subduction is thought to have initiated >50 million years earlier based on the presence of older Triassic ophiolites, arc, and metamorphic rocks preserved in the upper plate (Busby, 2023). We discuss the potential trigger(s) of high-grade metamorphism well after subduction initiation and any connections to the initiation of subduction further north along the Cordilleran margin.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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