Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and <sup>40</sup>Ar/<sup>39</sup>Ar dating

El‐Shazly, A. K.; Bröcker, Michael; Hacker, Bradley R.; Calvert, Andrew T.

doi:10.1046/j.1525-1314.2001.00309.x

Cited by 71 publications

(42 citation statements)

References 82 publications

(247 reference statements)

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“…Biostratigraphic and recent radiometric age constraints (Rb‐Sr [ El‐Shazly et al , 2001]; U‐Pb [ Warren et al , 2003, 2005]) lend support to the scenario of a single subduction zone active during ∼25 Myr, ending through continental subduction of the Arabian margin between 85 and 80 and 70 Ma. Despite ongoing controversy [ Gray et al , 2005; Searle et al , 2005; Warren and Miller , 2007], we note that the restoration of Searle et al [2004] was recently supported by high resolution P‐T data for the central Hulw unit [ Yamato et al , 2007].…”

Section: Geological Setting Of the Hp Nappe Stackmentioning

confidence: 90%

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

et al. 2010

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[1] The northeastern continental margin of Oman in the Saih Hatat region is characterized by high-pressure (HP) chloritoid-or carpholite-bearing metasediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper cover units of this HP nappe stack indicate that all of them underwent similar P conditions to the underlying Hulw structural unit (with a cooler exhumation pressure-temperature path). Early SSW directed crustal thickening during ophiolite emplacement created recumbent folds and strong schistose fabrics in these Permian-Mesozoic shelf carbonates and was followed by later NNE dipping normal sense shear zones and normal faults. The Mayh unit shows high strain in a 15-25 km long sheath fold that likely formed at carpholite grade pressures of 8-10 kbar. We show that there are no significant P differences across the Hulw shear zone (upper plate-lower plate discontinuity) or between the overlying Mayh, Yenkit-Yiti, and Ruwi units. Postpeak metamorphic exhumation of the HP rocks was therefore accomplished by bottom-to-SSW (rather than top-to-NNE) active footwall extrusion beneath a fixed, static, passive hanging wall. Footwall uplift beneath these passive roof faults resulted in progressive expulsion of the HP rocks from depths of ∼80-90 km (eclogites) and mainly 30-35 km (blueschists and chloritoid-/ carpholite-bearing units) during the Campanian-Early Maastrichtian. Oman thus provides a detailed record of how continental material (thick platform shelf carbonates) progressively jammed a subduction zone and emphasizes the contrasting behavior between cover units and their underlying basement. Citation: Agard, P., M. P. Searle, G. I. Alsop, and B. Dubacq (2010), Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman, Tectonics, 29, TC5018,

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Section: Geological Setting Of the Hp Nappe Stackmentioning

confidence: 90%

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

et al. 2010

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“…, 1999; Giorgis et al. , 2000; El‐Shazly et al. , 2001), and indicates that the age of high‐ P /low‐ T metamorphism (87–85 Ma) is at the Coniacian/Santonian boundary (Ogg et al.…”

Section: Geochronologymentioning

confidence: 99%

Late Cretaceous blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications

Topuz¹,

Okay²,

Altherr

et al. 2008

Journal Metamorphic Geology

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A blueschist facies tectonic sliver, 9 km long and 1 km wide, crops out within the Miocene clastic rocks bounded by the strands of the North Anatolian Fault zone in southern Thrace, NW Turkey. Two types of blueschist facies rock assemblages occur in the sliver: (i) A serpentinite body with numerous dykes of incipient blueschist facies metadiabase (ii) a well-foliated and thoroughly recrystallized rock assemblage consisting of blueschist, marble and metachert. Both are partially enveloped by an Upper Eocene wildflysch, which includes olistoliths of serpentinite-metadiabase, Upper Cretaceous and Palaeogene pelagic limestone, Upper Eocene reefal limestone, radiolarian chert, quartzite and minor greenschist. Field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the Miocene clastic rocks in a transpressional strike-slip setting, and represents an uplifted part of the pre-Eocene basement. The blueschists are represented by lawsonite-glaucophanebearing assemblages equilibrated at 270-310°C and 0.8 GPa. The metadiabase dykes in the serpentinite, on the other hand, are represented by pumpellyite-glaucophane-lawsonite-assemblages that most probably equilibrated below 290°C and at 0.75 GPa. One metadiabase olistolith in the Upper Eocene flysch sequence contains the mineral assemblage epidote + pumpellyite + glaucophane, recording P-T conditions of 290-350°C and 0.65-0.78 GPa, indicative of slightly lower depths and different thermal setting. Timing of the blueschist facies metamorphism is constrained to c. 86 Ma (Coniacian ⁄ Santonian) by Rb-Sr phengite-whole rock and incremental 40 Ar-39 Ar phengite dating on blueschists. The activity of the strike-slip fault post-dates the blueschist facies metamorphism and exhumation, and is only responsible for the present outcrop pattern and post-Miocene exhumation (2 km). The high-P ⁄ T metamorphic rocks of southern Thrace and the Biga Peninsula are located to the southeast of the Circum Rhodope Belt and indicate Late Cretaceous subduction and accretion under the northern continent, i.e. the Rhodope Massif, enveloped by the Circum Rhodope Belt. The Late Cretaceous is therefore a time of continued accretionary growth of this continental domain.

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“…Li et al. , 1994; Arnaud & Kelley, 1995; Hacker & Wang, 1995; Scaillet, 1998 and references therein; El‐Shazly et al. , 2001).…”

Section: Differences Between Ar–ar and Rb–sr Ages: General Consideratmentioning

confidence: 99%

High‐Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega‐detachments in the Aegean Sea

Bröcker¹,

Bieling²,

Hacker

et al. 2004

Journal Metamorphic Geology

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In the lower main unit of the Attic-Cycladic crystalline belt (Greece), white mica geochronology (Rb-Sr, K-Ar, 40 Ar-39 Ar) has established the timing of at least two metamorphic events: well-preserved highpressure/low-temperature (HP/LT) rocks yielded Eocene ages (c. 53-40 Ma) and their greenschist facies counterparts provided Oligocene-Miocene dates (c. 25-18 Ma). Marbles from Tinos Island contain high-Si phengite with Rb-Sr (phengite-calcite) and 40 Ar-39 Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and 40 Ar-39 Ar total fusion ages generally are 3-6 Ma older than corresponding Rb-Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older 40 Ar-39 Ar dates. This concept also suggests that the Rb-Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca-rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system. On Tinos, the lowermost part of the metamorphic succession has experienced a pervasive greenschist facies overprint. Si-rich phengite from marbles representing this lithostratigraphic level yielded Rb-Sr ages of c. 24 Ma. If the earlier metamorphic history is not taken into account, such data sets may lead to the erroneous conclusion of Miocene HP metamorphism. This study indicates that this phengite experienced pervasive rejuvenation of the Rb-Sr isotope system during overprinting, without significant changes in Si content, due to bulk-compositional constraints. This leads to the conclusion that in the absence of critical mineral assemblages the Si value of phengite is not a reliable indicator for metamorphic pressures in impure marbles. Recent studies have reported large displacements (>100 km) for detachment faults in the Aegean Sea. A critical parameter for such models is the age of HP metamorphism as deduced from white mica dating in the basal units of the Cyclades. We question the underlying idea of Miocene HP metamorphism and suggest, instead, that this age constrains the timing of the greenschist facies overprint and that the existence of mega-detachments in the study area requires further investigation.

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Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and ⁴⁰Ar/³⁹Ar dating

Cited by 71 publications

References 82 publications

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

Late Cretaceous blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications

High‐Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega‐detachments in the Aegean Sea

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Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and 40Ar/39Ar dating

Cited by 71 publications

References 82 publications

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

Late Cretaceous blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications

High‐Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega‐detachments in the Aegean Sea

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Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and ⁴⁰Ar/³⁹Ar dating