Two High-Pressure Metamorphic Events in NE Oman: Evidence from<sup>40</sup>Ar/<sup>39</sup>Ar Dating and Petrological Data

El‐Shazly, A. K.; Lanphere, Marvin A.

doi:10.1086/629625

Cited by 52 publications

(51 citation statements)

References 52 publications

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“…K-Ar ages of 100 ± 4 and 80 ± 4 Ma from phengites were obtained for the As Sifah blueschist (Lippard, 1983). These age data, together with structural and petrological criteria, suggest that parts of the Oman continental margin may have experienced two high-P/T metamorphic events during: (1) Early Cretaceous subduction beneath a microcontinent of Gondwanaland; and (2) the Late Cretaceous emplacement of the Semail ophiolite (El-Shazly and Lanphere, 1992).…”

Section: Oman Blueschists Blueschists and Minor Eclogites Have Been mentioning

confidence: 85%

“…Just beneath the Oman ophiolite in the northernmost part of the schist unit, a downward increase in metamorphic grade is observed, from lawsonite-albite subfacies to epidote glaucophane subfacies; this zonation formed in the range from 3-6 kbar and 250-300° C to >6.8 kbar and >310° C (ElShazly, 1994). 40 Ar/ 39 Ar dating of whole rocks and white mica separates from region I and the lowest-grade blueschist of region III yielded plateau ages of 72 to 80 Ma, whereas white micas from Zone-C blueschist and eclogite yielded ages of 106 to 111 Ma (El-Shazly and Lanphere, 1992). K-Ar ages of 100 ± 4 and 80 ± 4 Ma from phengites were obtained for the As Sifah blueschist (Lippard, 1983).…”

Section: Oman Blueschists Blueschists and Minor Eclogites Have Been mentioning

confidence: 95%

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Blueschists and Eclogites of the World and Their Exhumation

Maruyama

Liou

Terabayashi

1996

International Geology Review

580

364

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Section: Oman Blueschists Blueschists and Minor Eclogites Have Been mentioning

confidence: 85%

Section: Oman Blueschists Blueschists and Minor Eclogites Have Been mentioning

confidence: 95%

Blueschists and Eclogites of the World and Their Exhumation

Maruyama

Liou

Terabayashi

1996

International Geology Review

580

364

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“…The exhumation mechanism of these high‐ P / T rocks is also controversial (e.g. El‐Shazly & Lanphere, 1992; Michard et al ., 1994; Chemenda et al ., 1996; Miller et al ., 1999).…”

Section: Introductionmentioning

confidence: 99%

“…One of the main reasons for the continued controversy over the tectonic history of NE Oman is that the age of peak metamorphism and the cooling rates of the high‐ P / T rocks are poorly constrained. High‐ P / T metamorphic rocks from Saih Hatat have so far been dated by K–Ar or 40 Ar/ 39 Ar methods on phengite separates, which yielded ages ranging from 131 to 72 Ma (Lippard, 1983; Montigny et al ., 1988; El‐Shazly & Lanphere, 1992; Searle et al ., 1994; Miller et al ., 1999). El‐Shazly & Lanphere (1992) interpreted ages as indicating two discrete high‐ P / T metamorphic events: an Early Cretaceous event resulting from the subduction of the north‐eastern part of the Oman continental margin, and a weaker Late Cretaceous event related to ophiolite emplacement.…”

Section: Introductionmentioning

confidence: 99%

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

El‐Shazly¹,

Bröcker²,

Hacker³

et al. 2001

Journal Metamorphic Geology

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Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the 40Ar/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.

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“…In this model the older (pre-95 Ma) 40 Ar/ 39 Ar ages of white micas from the As Sifah eclogites (Montigny, Le Mer & Whitechurch, 1988;El-Shazli & Lanphere, 1992;Searle et al 1994;Miller et al 1999) are regarded as unreliable, and probably affected by excess 40 Ar. The distinctive high-pressure rocks (carpholite, blueschist and eclogite facies) crop out around the Saih Hatat culmination stretching from the Muscat area southeastwards to beyond Quriat ( Fig.…”

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confidence: 99%

Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

2002

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The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet + diopside amphibolites, enstatite + cordierite + sillimanite + spinel ± sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet + diopside + scapolite + plagioclase ± wollastonite. P-T conditions of granulite facies metamorphism are in the range 800-860°C and 10.5 ± 1.1 kbar to 14.7 ± 2.8 kbar. Garnet + clinopyroxene + hornblende + plagioclase amphibolites from the metamorphic sole record peak P-T conditions of 840 ± 70°C and 11.6 ± 1.6 kbar (THERMOCALC average P-T mode) and 840-870°C and 13.9-11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57-46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. 40 Ar/ 39 Ar hornblende ages from the amphibolites range from 95-93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian-Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic-Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100-1500°C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.

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Two High-Pressure Metamorphic Events in NE Oman: Evidence from⁴⁰Ar/³⁹Ar Dating and Petrological Data

Cited by 52 publications

References 52 publications

Blueschists and Eclogites of the World and Their Exhumation

Blueschists and Eclogites of the World and Their Exhumation

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

Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

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Two High-Pressure Metamorphic Events in NE Oman: Evidence from40Ar/39Ar Dating and Petrological Data

Cited by 52 publications

References 52 publications

Blueschists and Eclogites of the World and Their Exhumation

Blueschists and Eclogites of the World and Their Exhumation

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

Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

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Two High-Pressure Metamorphic Events in NE Oman: Evidence from⁴⁰Ar/³⁹Ar Dating and Petrological Data

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