Omphacite breakdown reactions and relation to eclogite exhumation rates

Anderson, Eric D.; Moecher, David P.

doi:10.1007/s00410-007-0192-x

Cited by 54 publications

(36 citation statements)

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“…Isothermal decompression is indicated by estimates of 10–13 kbar at 725–825°C (Figure 8c, field 3) based on garnet‐rim plagioclase‐hornblende equilibria (reactions (R6)–(R12). The high temperatures (∼800°C) indicated from garnet rim equilibria may be directly related to the formation of coarse‐grained diopside‐plagioclase symplectites after omphacite: the breakdown of omphacite and development of coarse Pl + Di symplectites (Figures 2f and 3e) may have been enhanced by isothermal decompression at elevated temperature [ Anderson and Moecher , 2007]. P‐T estimates are compiled in Figure 8d to produce nested P‐T paths interpreted as a record of HP Taconic metamorphism in the central Blue Ridge and EBR that has survived widespread Acadian and Alleghanian overprinting.…”

Section: Results and Interpretationmentioning

confidence: 99%

“…Omphacitic and aegirine‐augite clinopyroxene show some degree of breakdown to form symplectitic rods of plagioclase, hornblende, and minor quartz enclosed by diopsidic pyroxene (Figure 3a). Microbeam reintegrations indicate that these Cpx‐Pl‐Hbl‐Qtz symplectites after omphacite and aegirine‐augite are isochemical with adjacent unreacted clinopyroxene [ Anderson and Moecher , 2007]. Garnet is subhedral to euhedral, typically 0.5–1.5 mm in diameter, and contains inclusions of clinopyroxene, Cl‐rich hornblende, epidote, plagioclase, titanite, quartz, and apatite (Figure 3b).…”

Section: Petrography and Mineral Chemistrymentioning

confidence: 99%

“…Other occurrences of metabasites identified as retrograde eclogite or HP granulite/amphibolite occur within the EBR of the southern Appalachian Blue Ridge proximal to the WBR terrane boundary in western North Carolina and Georgia (Figure 1). These include retrograde eclogite exposed north of the Grandfather Mountain window [ Abbott and Raymond , 1997; Abbott and Greenwood , 2001; Anderson and Moecher , 2007] (Figure 1), garnet (Grt) granulites of the Lake Chatuge mafic‐ultramafic complex [ Dallmeyer , 1974], and clinopyroxene (Cpx)‐garnet granulites and amphibolites in the eastern Great Smoky Mountains [ Anderson and Moecher , 2007] (Figure 1). Only one definitive occurrence of blueschist occurs in the Appalachians (New Brunswick [ van Staal et al , 2008]), but it is younger (Late Ordovician–Early Silurian) than the southern Appalachian eclogites.…”

Section: Introductionmentioning

confidence: 99%

“…Thermobarometric calculations for the most common Lick Ridge assemblages indicate that omphacite and garnet formed at 13–17 kbar and 650–730°C [ Willard and Adams , 1994]. P‐T estimates based on compositions of inclusions in garnet, retrograde phases, and metamorphic assemblages in the surrounding schists and amphibolites provide a well‐constrained, clockwise P‐T path with a component of isothermal decompression [ Page et al , 2003; Anderson and Moecher , 2007]. Zircon ID‐TIMS U‐Pb ages for the Lick Ridge eclogite (459.0 +1.6/−0.6 Ma [ B. V. Miller et al , 2000, 2006]) indicate that eclogite facies metamorphism was a Taconian event (Table 1).…”

Section: Introductionmentioning

confidence: 99%

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Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Anderson

Moecher

2009

Tectonics

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Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

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Section: Results and Interpretationmentioning

confidence: 99%

Section: Petrography and Mineral Chemistrymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Anderson

Moecher

2009

Tectonics

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“…The common occurrence of symplectites in felsic gneisses from terranes that have decompressed from pressures N 20 kbar at temperatures N650°C ('diffusion-efficient' conditions) provides an opportunity to test the effects of high-temperature recrystallization on element remobilization (e.g. Heinrich, 1982;Wain et al, 2000;Gilotti and Krogh-Ravna, 2002;Anderson and Moecher, 2007). Of specific interest to metamorphic 40 Ar/ 39 Ar geochronology is the breakdown of phengite to symplectites of biotite plus plagioclase, which provides an excellent opportunity to assess the transport distances, pathways, sources and sinks of 40 Ar in K-rich assemblages as high temperature minerals break down and recrystallize during exhumation.…”

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Recycling argon through metamorphic reactions: The record in symplectites

McDonald

Regis

Warren

et al. 2018

Lithos

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The 40 Ar/ 39 Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40 Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20-25 to c. 8-12 kbar at~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely 'resets' the Ar clock, or whether there is some inheritance of 40 Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40 Ar/ 39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40 Ar recycling can be tracked, thus potentially allowing 40 Ar/ 39 Ar dates to be linked more accurately to metamorphic history.

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