More Like this

1. Distinct P-T histories in a subduction mélange reveal underplating/mixing processes at the plate interface (North Motagua Mélange, Guatemala)

   Bonnet, G; Flores, K; Martin, C; Harlow, G (December 2020, American Geophysical Union, Fall Meeting 2019)

   Relicts of subducted and exhumed ocean floor preserved in suture zones record the events occurring at the plate interface. In particular, underplating and exhumation are the two main processes required to recover rocks from mantle depths. High-grade blocks exposed in serpentinite mélanges of the Motagua Valley record evidence of past subduction events between the North American Plate and the Caribbean Plate. Previous works suggest the existence of two subduction zones during Cretaceous, with cold metamorphism (lawsonite eclogite and blueschists) in the South (South Motagua Mélange), and warmer eclogites and amphibolites in the North (North Motagua Mélange, NMM). Although little work as been done so far to characterize the P-T paths and variability of the metabasite blocks embedded within serpentinite matrix in the NMM. Here we present new thermobarometric estimates using conventional thermobarometry, pseudosection modeling and thermometry of carbonaceous matter on a set of metabasites of different grades. There a minimum of four kinds of P-T paths: (1) (lawsonite-bearing) garnet-blueschists with peak P-T around 2.1 GPa and 480°C, (2) "cold eclogites" at ~2.2 GPa and 550°C experiencing isothermal decompression and epidote-amphibolite overprints, (3) "warm eclogites" at ~2.3 GPa and 600°C exhumed in cold environments and affected by blueschist-facies overprints, and (4) garnet-bearing epidote-amphibolites that may represent either retrogression of some eclogites, or prograde metamorphism under warm conditions. We find that garnet fractionation has a limited impact on isopleth-derived P-T estimates and that lawsonite breakdown may drive retrograde metamorphism and rheological switches at the plate interface. These new P-T estimates suggest that high-pressure rocks of the NMM may be recovered from different depths of a unique subduction zone, between 65 and 80 km, and exhumed in a relatively cold (and serpentinized) environment. This suggests a more complicated story than previously described, and calls for additional geochronological evaluation (in process). 

   more » « less
2. Evidence and timing for multiple cooling mechanisms in a long-lived subduction zone from the Easton metamorphic suite in the northwest Cascades

   Mulcahy, S.R.; Schermer, E.L.; Cordova, J.; Gates, E. (January 2023, Geological Society of America Abstracts with Programs)

   Numerical models of subduction initiation and observations of exhumed subduction complexes indicate that the early stages of subduction are characterized by rapid cooling followed by a prolonged steady thermal state that can last tens of millions of years. Several mechanisms are proposed to drive cooling in subduction zones and include thermal relaxation, exhumation, and underplating, but determining the relative contribution of each mechanism in the history of an exhumed subduction complex can be difficult. The Easton metamorphic suite in the Northwest Cascades of Washington is a Jurassic-Cretaceous subduction complex that records the subduction and accretion of distinct units within a thermally maturing nascent subduction zone. The region preserves an inverted metamorphic sequence with metamorphic temperatures and ages that decrease structurally downward from an early accreted metamorphic sole to younger regionally extensive blueschist facies units. In the metamorphic sole Grt±Cpx amphibolite was metamorphosed at 750-800 C at 1.0 GPa prior to 167 Ma. The amphibolite is underlain by a high temperature Grt-Ab-Gln blueschist that was metamorphosed at ~530 C and 1.0 GPa at 165 Ma. The contact between the units is gradational and the general lack of deformation suggests initial cooling to lower temperatures may have been caused by cooling of the overall subduction zone. Retrograde Lws-Ep-Gln-Ms assemblages suggest that cooling of both the amphibolite and high-grade blueschist units to below 400-500 C was caused by exhumation to 0.7 GPa by 157 Ma. In the regionally extensive blueschist units, Ep-Ab-Chl-Ms±Grt±Lws phyllite was metamorphosed at 430-450 C and 0.7 GPa by 149 Ma. Retrograde fabrics in the phyllite record similar temperatures to peak metamorphic conditions in an underlying Ep-Ab-Gln/Act greenschist/blueschist unit that was accreted and metamorphosed at ≤350 C by ≤140 Ma. The contact between the phyllite and greenschist is marked by a high strain mylonite zone and the combined observations suggest that cooling of the phyllite was driven by underplating of the younger greenschist unit. The observed assemblages and fabrics within the Easton metamorphic suite record cooling as the result of thermal relaxation, underplating, and exhumation at distinctly different times in the subduction history. 

   more » « less
3. Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

   https://doi.org/10.1029/2020TC006528  

   Kotowski, Alissa J.; Cisneros, Miguel; Behr, Whitney M.; Stockli, Daniel F.; Soukis, Konstantinos; Barnes, Jaime D.; Ortega‐Arroyo, Daniel (June 2022, Tectonics)

   Abstract Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex. 

   more » « less
4. Structural evolution of subduction initiation and early cooling: insights from the Easton metamorphic suite, northwest Washington

   Cunetta, N.; Schermer, E.L.; and Mulcahy, S.R. (January 2023, Geological Society of America Abstracts with Programs)

   Exhumed high pressure/low temperature metamorphic belts provide evidence of subduction processes at depth, but the mechanisms by which subduction zones initiate and evolve to steady thermal state remain contested due to a paucity of recovered subduction infancy rocks. The Easton metamorphic suite in northwest Washington is a Jurassic-Cretaceous subduction complex that preserves a high-grade metamorphic sole inferred to have formed during subduction initiation. Near Gee Point, WA, coherent garnet amphibolite, hornblende-bearing white mica quartzose schist, and underlying garnet blueschist are characterized by their proximity to overlying serpentinized peridotite and the presence of blocks and layers of foliated and unfoliated metasomatic rock. These high-grade rocks are exposed in a series of northeast vergent, steeply inclined, tight-isoclinal folds that post-date regional high pressure/low temperature metamorphism. Two shear zones dip steeply to the southwest, concordant to adjacent blueschist-greenschist, quartzose schist, and amphibolite fabrics. Pervasive retrogression from amphibolite to blueschist-greenschist facies from high to low structural levels is documented at the map, outcrop, and micro scale. Amphibolites contain a foliation defined by aligned hornblende, or in places are weakly foliated. Retrogression of amphibolite is associated with a well-developed glaucophane-white mica-chlorite fabric that wraps older garnet and hornblende, with pargasite cores rimmed by glaucophane and chlorite. Both garnet blueschist and quartzose schist preserve at least one younger foliation. Prior 40Ar/39Ar thermochronology and thermobarometry are consistent with a cooling subduction zone, from amphibolite facies conditions of 760C at >167 Ma to <400C at ~163 Ma. Paired garnet 176Lu-176Hf geochronology and 40Ar/39Ar thermochronology samples will elucidate the timing of (1) proposed subduction initiation, (2) early subduction zone cooling rates, and (3) two shear zones that deformed the Easton metamorphic suite. 

   more » « less
5. Accessory phases as recorders of subduction redox: Sulfide–oxide– silica equilibria during high-pressure metamorphism

   Walters, JB; Cruz-Uribe, AM; Marschall, HR (December 2018, 2018 Fall Meeting, AGU)

   Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

   more » « less