The subduction and exhumation history of the Voltri Ophiolite, Italy: Evaluating exhumation mechanisms for high-pressure metamorphic massifs

Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass‐transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high‐pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk‐rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg‐metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg‐rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si‐metasomatism of serpentinite in subduction zones is limited by pressure‐dependent increase in the silica activity buffered by the serpentine‐talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P‐T conditions. Alternatively, a different set of mineral assemblages, such as serpentine‐ or chlorite‐rich rocks, may be controlling the coupling‐decoupling transition of the plate interface.

Section: Resultsmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Fluid‐Mediated Mass Transfer Between Mafic and Ultramafic Rocks in Subduction Zones

Codillo

Klein

Dragovic

et al. 2022

“…Once crushed to a grain size of 75–100 μm, inclusion phases in garnet were removed using partial dissolution techniques from Starr et al. (2020). Several fine powders were also treated in order to preferentially remove inclusion phases.…”

Section: Methodsmentioning

confidence: 99%

“…Note. All garnet samples were cleaned using methods described in Starr et al (2020) unless indicated (labeled "dirty"). "Aliquots" represent garnet crushed to 75-100 μm grain sizes and "powders" are fine (<75 μm grain size) garnet residues collected during the crushing process.…”

Section: Quartz In Garnet Geobarometrymentioning

confidence: 99%

Insight Into the Early Exhumation of the Cycladic Blueschist Unit, Syros, Greece: Combined Application of Zoned Garnet Geochronology, Thermodynamic Modeling, and Quartz Elastic Barometry

Gorce

Caddick

Baxter

et al. 2021

Self Cite

Constraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samples can be safely combined into a single pressure‐temperature‐time (P‐T‐t) path that might then be used to infer tectonic context and mechanisms of exhumation. Here, we present new thermobarometric and geochronologic information preserved in a single sample from Syros, Greece, to deduce the conditions and rates of the earliest phase of exhumation as a part of the well‐preserved high‐pressure metamorphic rocks of the Cycladic Blueschist Unit (CBU). The sample studied here is a garnet‐bearing, quartz‐mica schist that records two distinct metamorphic events. Results from thermodynamic models and quartz‐in‐garnet elastic geobarometry show that metamorphic garnet cores formed as P‐T conditions evolved from ∼485°C and 2.2 GPa to 530°C and 2.0 GPa, and that garnet rims formed as conditions evolved from ∼560°C and 2.1 GPa to ∼550°C and 1.6 GPa. Sm‐Nd geochronology on garnet cores and rims yields ages of 45.3 ± 1.0 and 40.5 ± 1.9 Ma, respectively, thus indicating a 4.8 ± 2.1 Myr growth span. Given the decompression path calculated based on garnet core and rim P‐T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU.

“…Each separate was then hand crushed using an agate mortar and pestle to between 75 and 150 μm for chemical preparation. Powder fractions (grain sizes <75 μm) were also collected for analysis and are indicated in the results by “Pwd.” Partial dissolution (Baxter et al., 2002; Dragovic et al., 2012; Pollington & Baxter, 2011; Starr et al., 2020) was used to remove any recalcitrant micro‐inclusions such as apatite, epidote and hornblende that may significantly affect on the isotopic data. Aliquots between 40 and 130 mg of the garnet and garnet powder separates were first chemically leached in 2 mL of 7 N nitric acid (HNO 3 ) for 2 h at 120°C to remove easily dissolved non‐silicate phases.…”

Section: Methodsmentioning

confidence: 99%

A Mélange of Subduction Ages: Constraints on the Timescale of Shear Zone Development and Underplating at the Subduction Interface, Catalina Schist (CA, USA)

Harvey

Walker

Starr

et al. 2021

Mélange at the subduction interface controls many important physico-chemical processes, including mass transfer between the downgoing slab and overriding plate, large-scale channelization of fluids, and the rheologic evolution of the plate interface. Both fluid transport at the subduction interface and the mechanical behavior of rigid blocks within a deforming matrix have recently been proposed to catalyze seismic phenomena occurring below the megathrust including episodic tremor and slip (ETS; e.g.