P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

López-Carmona, Alicia; Kusky, Timothy; Santosh, M.; Abati, Jacobo

doi:10.1016/j.lithos.2010.10.007

Cited by 17 publications

(6 citation statements)

References 52 publications

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“…However, the homblende-albite symplectitic intergrowth possibly represents replacement of former crystals of omphacitic pyroxene (e.g. Miller et al 1980;Ortega-Gutierrez et al 2004;Vega-Granillo et al 2007 ;Lopez-Carmona et al 2011 ) or glaucophanic amphibole (e.g. Miller et al 1980;Liou 1981;Engvik et al 2000).…”

Section: Amphiboles In the Matrixmentioning

confidence: 99%

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

López-Carmona

Abati

Pitra

et al. 2014

Contrib Mineral Petrol

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Blueschist facies terranes in the V ariscan lbero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Cea'n Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica-Tui Complex (NW lbe-rian Massif). The Cea'n Unit is a highly condensed meta-morphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane-chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composi-tion that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P-T space through pseudosection approach and is characterised by H 2 0-undersaturated pro-grade evolution induced by the crystallisation oflawsonite. Peak conditions in the blueschist/LT-eclogite fades have been constrained at ea. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly iso-thermal decompression from the lawsonite-bearing fields to fields with stable albite at P ::::; 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite-greenschist fades overprint at P < 0.8 GPa and T ::::; 440-480 ° C. The preservation of the lawsonite crystal shape despite com-plete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blue-schist 40 host rock were not affected by penetrative deformation during exhumation.Ar/3 9 Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ea. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to re15resent some point relatively close to peak conditions at the onset of the iso-thermal decompression. ArJ3 9 Ai dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre-Cea'n detachment, at the base of the Cea'n Unit, yields an age of ea. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21-45, 2005) support the equivalence of the Cea'n Unit and the Upper Unit of Ile de Groix along the lbero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ea. 360-370 Ma, that is inferred to represent the Late Devonian-Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

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Section: Amphiboles In the Matrixmentioning

confidence: 99%

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

López-Carmona

Abati

Pitra

et al. 2014

Contrib Mineral Petrol

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“…These are interpreted as an oceanic assemblage of basalts and marine sedimentary rocks that were underthrust and metamorphosed during the Jurassic-Cretaceous (Plafker and Berg, 1994;López-Carmona et al, 2011). The mélange unit, which is known as the McHugh Complex in the Chugach and Kenai Mountains, is located outboard of the blueschist unit, but in some localities where blueschists are absent it lies directly against the Border Ranges fault (Fig.…”

Section: Chugach Terranementioning

confidence: 99%

Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska

et al. 2011

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The Upper Cretaceous Valdez Group represents the fl ysch facies of the Mesozoic Chugach terrane accretionary complex in southern Alaska. The Valdez Group is dominated by litharenite sandstone and argillite deposited as coherent beds, unlike the older McHugh Complex mélange and massive sandstones. Detrital zircons from fi ve sandstones sampled along an ~55 km transect through the Valdez Group were dated using U-Pb laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). The youngest populations from the two oldest samples, located along strike from each other, were 82-81 Ma. Three samples across strike and outboard of the others are separated by ~50 km, but each has a youngest population dated at ca. 68 Ma. All of these samples have major grain population ages that suggest erosion from the Coast Mountains Batholith, consistent with petrography and grain modes suggesting an arc source. No apparent age gap exists between the youngest McHugh Complex samples and the oldest Valdez Group samples, suggesting continuous deposition despite the different depositional and tectonic style. We propose a model in which the onset of coherently bedded fl ysch marks the transition from deposition in the trench or trench slope to deposition on the oceanic plate beyond the trench after it was fi lled at ca. 84 Ma, i.e., the time of the youngest mélange sedimentation. Preservation of coherent bedding resulted as large coherent blocks of Valdez Group rocks were imbricated into the subduction complex during continued subduction in Paleogene time.For permission to copy, contact editing@geosociety.org |

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“…1B). The mantle rocks of the Tonsina complex in juxtaposition with blueschist-facies rocks formed at pressures comparable to mantle depths (López-Carmona et al, 2011), suggest the Liberty Creek schist preserves a vestige of the mantle-level megathrust. Younger events both to the north and south have isolated this deeply exhumed fragment relative to adjacent areas.…”

Section: Paleogeographic Implications Of the Liberty Creek Schist Witmentioning

confidence: 93%

“…1A). Previous studies have applied K/Ar, 40 Ar/ 39 Ar, and Rb/Sr analysis and a pressure-temperature (P-T) pseudosection approach to these rocks and have provided a reasonable description of peak blueschist-facies metamorphism for southern Alaskan terranes containing blueschist-facies rocks (e.g., Decker, 1980;Sisson and Onstott, 1986;Roeske et al, 1989;Plafker et al, 1994;Bradley et al, 1999;López-Carmona et al, 2011;Fig. 1A) Detrital zircons reveal Early Cretaceous blueschist-facies metamorphism in southern Alaska | RESEARCH data from these assemblages reveal a ca.…”

Section: Introductionmentioning

confidence: 99%

Detrital zircon ages indicate an Early Cretaceous episode of blueschist-facies metamorphism in southern Alaska: Implications for the Mesozoic paleogeography of the northern Cordillera

2016

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Detrital zircon U-Pb ages are presented from the Liberty Creek schist in the central Chugach Mountains that indicate two distinct periods of preservation of blueschist-facies metamorphism along the southern Alaskan margin. A maximum depositional age (MDA) of 136 Ma demonstrates that the Liberty Creek schist was deposited long after the Early Jurassic cooling ages (196-185 Ma) recorded in other western Alaskan schist bodies containing blueschist-facies rocks, thus revealing two distinct blueschist-facies preservation events: an Early Jurassic event and a post-Early Cretaceous event. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage (MDA of 169-156 Ma), directly south of the Liberty Creek schist, is an older but more shallowly exhumed assemblage. Strike-slip faulting has rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 140-130 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholith immediately to the east; the Firvale suite of the Coast plutonic complex, ~1500 km to the southeast near Vancouver, British Columbia; or our preferred source, the southern Mexican Guerrero terrane, ~3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of kilometers of strike-slip movement along the western Cordillera since Cretaceous time, consistent with the Baja-British Columbia hypothesis.

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P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

Cited by 17 publications

References 52 publications

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska

Detrital zircon ages indicate an Early Cretaceous episode of blueschist-facies metamorphism in southern Alaska: Implications for the Mesozoic paleogeography of the northern Cordillera

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