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
Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece
Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.
more »
« less
- Award ID(s):
- 1725110
- NSF-PAR ID:
- 10341061
- Date Published:
- Journal Name:
- Solid Earth
- Volume:
- 12
- Issue:
- 6
- ISSN:
- 1869-9529
- Page Range / eLocation ID:
- 1335 to 1355
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract High‐pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr‐in‐rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high‐pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.
-
more » « less
Abstract 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 asP ‐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 rimP ‐T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU. -
more » « less
Abstract In subduction‐related tectonic mélange, thermobarometry on individual blocks can in principle constrain the scale of lithological mixing along the subduction interface. Previous thermobarometric investigation of the tectonic amphibolite facies mélange unit in the Catalina Schist, Santa Catalina Island, California, USA, has suggested relatively limited mixing among blocks (≤12 km). Here we further investigate scales of mixing among metamorphically disparate (‘exotic’) blocks within epidote‐amphibolite and lawsonite‐blueschist facies mélange of the Catalina Schist using field and petrographic observations, Zr‐in‐rutile thermometry, and quartz‐in‐garnet elastic barometry. A new statistically based method is presented for calculating elastic barometry maximum pressures. The exotic blocks record peak metamorphic temperatures between 580 and 735°C and peak pressures between 1.16 and 1.65 GPa. Temperatures primarily fall in between those recorded by rocks in the amphibolite facies and epidote amphibolite facies units (643–735°C and 553–596°C respectively). The pressure estimates encompass those recorded by blocks from the amphibolite facies mélange (1.34–1.44 GPa), although the exotic blocks record a much larger range of pressures. The large range of recorded temperatures and pressures suggests that blocks within the epidote amphibolite unit were sourced from and mixed along a 20–30 km region of the subduction interface while an exotic block from the lawsonite blueschist facies unit appears to have been sourced from at least 70 km deeper than the unit it is hosted in. Metre‐ to kilometre‐scale variations in matrix mineral rheology likely control strain partitioning at the interface and permit differential transport of mélange blocks over variable length scales.
-
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
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/se-12-1335-2021
