First ⁴⁰Ar-³⁹Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

“…Comparing this metamorphic peak age to that of other HP-LT localities in the region, it is slightly younger than the Middle Jurassic to Berriassian (169-147 Ma) metamorphic peak ages of high grade metamorphic blocks and low-temperature mafic blueschist from the Franciscan complex (Anczkiewicz et al 2004;Wakabayashi and Dumitru 2007;Ukar et al 2012), as well as the Middle to Late Jurassic (170-150 Ma) metamorphic peak age of epidote-amphibolite blocks within Cedros Island serpentinite mélange in Baja California (Baldwin and Harrison 1992). In contrast, it is slightly older than the Berriasian to Hauterivian (144-132 Ma) peak age of lawsonite-eclogites of the South Motagua Mélange (SMM) and the Hauterivian to Aptian (131--125 Ma) age of zoisite-eclogites in North Motagua Mélange (NMM) of the GSZ (see Figure 11B and C and Table 8, Tsujimori et al 2004;Harlow et al 2008;Brueckner et al 2009;Harlow et al 2011).…”

Detrital zircon U–Pb reconnaissance of the Franciscan subduction complex in northwestern California

“…Gucwa 1975;Phipps 1984;Wakabayashi 2011Wakabayashi , 2012Wakabayashi , 2015Aalto 2014;Platt 2015). Cloos (1982) argued that subduction-driven tectonic return flow of mélange was a key process, subducting matrix materials to depths of 6 to >25 km, mixing and shearing various types of blocks with the matrix, and returning the mélange to shallower depths (see also Shreve 1988a, 1988b;Ukar et al 2012). In the San Francisco Bay region, Blake et al (1984) speculated that the Central terrane was a more distal volcanicand chert-rich facies of the Great Valley forearc basin into which other Franciscan terranes were tectonically emplaced, including those comprised of fragments of oceanic crust, oceanic islands or plateaus, and continental margins.…”

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

“…The Sierran Jura-Triassic arc belt and Klamath chert-argillite-rich North Fork and Eastern Hayfork units were laid down at *175-165 Ma Scherer and Ernst 2008), whereas the near-shore Mariposa-Galice formations began accumulating by *165-160 Ma (Ernst et al 2009a). Transpressive plate convergence also generated HP/LT basaltic eclogites and garnet-glaucophane schists along the Middle and Late Jurassic convergent plate junction at *170-155 Ma (Cloos 1986;Wakabayashi 1990;Tsujimori et al 2006;Ukar et al 2012).However, except for the *174 Ma Red Ant blueschists (Hacker and Goodge 1990;Hacker 1994), such HP/LT metamafic rocks remained at depth, and fragments apparently returned surfaceward only during mid-and Late Cretaceous time as tectonic ± olistostromal blocks were sheared and gravity-fed into low-density Franciscan mud-matrix mélange. These relatively high-grade metamafic blocks formed much earlier during construction of the emergent arc.…”

“…These high-grade blocks of mid-to Late Jurassic metamorphic age are relatively well studied (Coleman and Lanphere 1971;Wakabayashi 1992;Anczkiewicz et al 2004;Wakabayashi and Dumitru 2007;Ukar et al 2012). …”

“…How the high-grade metamafic blocks and spatially associated Franciscan, chiefly metasedimentary section were exhumed is a matter yet debated (e.g., Ernst 1971;Platt 1986Platt , 1993Ring and Brandon 2008). Some HP metabasalts exhibit nearly complete rinds of actinolite ± chlorite ± talc that are slightly younger than the high-grade blocks (Moore 1984;Catlos and Sorensen 2003;Ukar 2012;Ukar et al 2012). In the rare cases where Jurassic metamafic blocks are clearly enveloped in surrounding fine-grained mud-matrix or serpentinite bodies, the latter are substantially younger (e.g., mid-to Late Cretaceous).…”