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1. Exhumed Serpentinites and Their Tectonic Significance in Non-Collisional Orogens

   Donoso-Tapia, Damián; Flores, Kennet E; Martin, Celine; Gazel, Esteban; Marsh, Jeffrey (February 2024, Geochemistry geophysics geosystems)

   Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid-rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non-collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non-collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in-situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene high-pressure (HP)–low-temperature metamorphic sequences. Whole-rock trace element data and in-situ B isotopes favor serpentinization by a crust-derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid-Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back-arc basins. Subsequent compressional phases trigger short-lived subduction in the back-arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non-collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back-arc basins. 

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2. Exhumed Serpentinites and Their Tectonic Significance in Non-Collisional Orogens

   Donoso-Tapia, Damián; Flores, Kennet E; Martin, Celine; Gazel, Esteban; Marsh, Jeffrey (February 2024, Geochemistry geophysics geosystems)

   Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid-rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non-collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non-collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in-situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene high-pressure (HP)–low-temperature metamorphic sequences. Whole-rock trace element data and in-situ B isotopes favor serpentinization by a crust-derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid-Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back-arc basins. Subsequent compressional phases trigger short-lived subduction in the back-arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non-collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back-arc basins. 

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3. Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)

   https://doi.org/10.1029/2022GC010395  

   Raia, Natalie_H; Whitney, Donna_L; Teyssier, Christian; Lesimple, Stéphane (August 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Owing to the importance of serpentinites for planetary geochemical and geodynamic processes, there has been much work discerning the origins of their parent rocks, including distinguishing between serpentinites derived from a subducting plate versus overlying mantle in exhumed subduction complexes. The island of New Caledonia (SW Pacific Ocean) provides a rare window into Cenozoic Pacific subduction processes. The island is unique in exposing both an exceptionally preserved high‐pressure, low‐temperature subduction complex and one of the largest supra‐subduction zone ophiolites in the world. Previous studies disagree on the origin of serpentinites in the subduction complex. In this study, we analyze 23 serpentinites from this complex for whole‐rock major and trace element geochemistry and stable isotope (δD, δ¹⁸O) compositions. Our data reveal two distinct groups of serpentinites: Group I samples in the northern portion of the complex are pervasively serpentinized, and exhibit enriched heavy rare earth element (REE) compositions and δ¹⁸O values between +6.7‰ and +10.2‰. In contrast, Group II serpentinites in the south preserve relict orthopyroxene and olivine, and show depleted trace element compositions and comparatively lower δ¹⁸O values between +5.1‰ and +8.0‰. We interpret Group I serpentinites to derive from downgoing plate mantle, whereas Group II serpentinites derive from overlying mantle wedge, exhibiting remarkable similarity to the REE geochemistry of the structurally overlying New Caledonia ophiolite. Our results establish the subduction complex in New Caledonia as an unusual natural record of the entrainment and exhumation of mantle from both the overlying mantle wedge and the downgoing plate in an oceanic subduction zone. 

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4. Record of de-serpentinization and re-serpentinization of an exhumed slab sliver: Implications for fluid circulation in subduction zones

   https://doi.org/10.1016/j.epsl.2025.119213  

   Donoso-Tapia, Damián; Flores, Kennet E; Martin, Celine; Hull, Sarah; Hernández-Uribe, David; Gazel, Esteban (January 2025, Earth and Planetary Science Letters)

   Fluid release associated with serpentinite dehydration (de-serpentinization) during subduction plays a key role in fundamental geological processes such as element transport and recycling, seismicity, and arc magmatism. Although the importance of these fluids is well-known, evidence of de-serpentinization remains scarce in the rock record. Here, we investigated the effects of de-serpentinization and fluid circulation in exhumed metaperidotites from the Raspas Complex (Ecuador). This Early Cretaceous complex records warm subduction (∼13.5 °C/km) and has been hypothesized to represent a coherent slab sliver that preserves the mantle-crust contact (moho) between eclogite-facies metaperidotites and the corresponding crustal section. Petrological observations reveal that titanian-clinohumite-bearing metadunites and banded metaperidotites underwent de-serpentinization after reaching peak pressure–temperatures (P–T) of ∼1.3–1.6 GPa and 620–650 °C. The peak paragenesis is partially obscured by a strong retrograde overprint, driven by crust-derived metamorphic fluids (δ11B ∼ -6 to +8 ‰) that infiltrated at varying fluid/rock ratios, triggering the re-serpentinization of metaperidotites during exhumation (P < 1.3 GPa and 320–400 °C). Thermodynamic forward modeling reveals that fluid release in the Raspas paleo-subduction zone is controlled by brucite breakdown and de-serpentinization, which occur at depths of 25–30 km and ∼50 km, respectively, accounting for a total of up to 10 wt. % H2O of water stored in the rock. Comparatively, dehydration of the crustal section, albeit a minor component, promotes enhanced fluid circulation between 25 and 45 km. During exhumation, circulating crust-derived metamorphic fluids heavily metasomatized the ascending slab sliver and effectively modified its geochemical signature. The depth range of the dehydration reactions overlap the depth of non-volcanic tremors and slow-slip events in warm, active subduction zones worldwide (25–65 km). Thus, the Raspas Complex offers an in-situ window into the fluids responsible for triggering these seismic events. 

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5. A c. 1900 Ma Tethyan-type ophiolite in the Penokean Orogen, Pembine, Wisconsin (USA): Insights from the volcanic stratigraphy

   https://doi.org/10.1016/j.precamres.2023.107223  

   Guice, George L.; Viete, Daniel R.; Holder, Robert M.; Roy, Supratik (December 2023, Precambrian Research)

   Orogenic ophiolites are a hallmark of Phanerozoic plate tectonics, containing igneous lithologies that provide constraints on fundamental tectono-magmatic processes. The c. 1900Ma Pembine Ophiolite (Wisconsin, USA) is associated with the Penokean Orogen and represents a rare example of a proposed Paleoproterozoic ophiolite. The Penokean Orogen shares broad characteristics with Phanerozoic (<541 Ma) orogens, but the origin of the Pembine Ophiolite remains unclear, with the mafic volcanic rocks interpreted as representing either an intra-oceanic arc or continental back arc setting. To test these hypotheses, we present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of 34 Pembine rocks, as well as U-Pb zircon geochronology for two samples. Based on trace elements established as immobile in the studied rocks, we demonstrate that mafic volcanism progressed (up-stratigraphic-section) from mid-ocean ridge-like to boninitic. The chemical evolution is identical to that observed in < 250 Ma ophiolites in the Himalayan–Alpine Orogen, which record forearc spreading during the nascent stages of subduction in the Tethys Ocean. We interpret the Pembine Ophiolite as forearc lithosphere formed during subduction initiation and obducted to the margin of the Superior Craton during the Penokean Orogeny. The processes responsible for forming (and preserving) this example of a Paleoproterozoic ophiolite may not have been dissimilar to those operating on the Phanerozoic Earth. 

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