Quartz, mica, and amphibole exsolution from majoritic garnet reveals ultra-deep sediment subduction, Appalachian orogen

Abstract

Diamond and coesite are classic indicators of ultrahigh-pressure (UHP; ≥100-kilometer depth) metamorphism, but they readily recrystallize during exhumation. Crystallographically oriented pyroxene and amphibole exsolution lamellae in garnet document decomposed supersilicic UHP majoritic garnet originally stable at diamond-grade conditions, but majoritic precursors have only been quantitatively demonstrated in mafic and ultramafic rocks. Moreover, controversy persists regarding which silicates majoritic garnet breakdown produces. We present a method for reconstructing precursor majoritic garnet chemistry in metasedimentary Appalachian gneisses containing garnets preserving concentric zones of crystallographically oriented lamellae including quartz, amphibole, and sodium phlogopite. We link this to novel quartz-garnet crystallographic orientation data. The results reveal majoritic precursors stable at ≥175-kilometer depth and that quartz and mica may exsolve from garnet. Large UHP terranes in the European Caledonides formed during collision of the paleocontinents Baltica and Laurentia; we demonstrate UHP metamorphism from the microcontinent-continent convergence characterizing the contiguous and coeval Appalachian orogen.(11), eaay5178. 6 Sci Adv ARTICLE TOOLS

Kyanite preserves prograde and retrograde metamorphic events as revealed by cathodoluminescence, geochemistry, and crystallographic orientation

“…380 Ma (Gatewood et al., 2015; Vance & Holland, 1993). Within the Northern Appalachians, localized bodies were metamorphosed at UH T conditions (Ague & Eckert, 2012; Ague et al., 2013; Keller and Ague, 2020); evidence of UHP metamorphism also has been proposed (Keller and Ague, 2020; Peterman, Snoeyenbos, et al., 2016; Snoeyenbos et al, 2011), but neither coesite nor diamond have been reported in the Northern Appalachians. Samples of metapelites from the Northern Appalachians, USA, were collected from the Cobble Mountain Formation, the Gassetts Schist, the Straits Schist, and the Slashers Ledges Formation.…”

High-pressure, halogen-bearing melt preserved in ultrahigh-temperature felsic granulites of the Central Maine Terrane, Connecticut (U.S.A.)

“…Miocene in age, have been also recently reported (Pownall et al, 2018). More recent studies of other rocks in the CMT document high-pressure granulite facies metamorphism (~1050 °C, ~1.8 GPa) of silicaundersaturated garnet-spinel-corundum gneisses (Keller and Ague, 2018), as well as UHP metamorphism (Keller and Ague, 2020).…”

Metamorphic diamond from the northeastern margin of Gondwana: Paradigm shifting implications for one of Earth’s largest orogens

“… Proposed pressure-temperature (PT) conditions of the UHP garnet from the Running River Metamorphics. Exsolution mineralogy fields from ( 4 ). Minimum peak P-T conditions (blue triangle) of 3.5 to 4 GPa at 800° to 900°C are constrained by metamorphic diamond inclusions and Zr in rutile thermometry.…”

“…The identification of ultrahigh pressure (UHP) metamorphism is important for the tectonic reconstruction of ancient terranes. UHP metamorphism is typically interpreted to indicate collisional orogenesis and deep subduction (>70 km) of continental material (1)(2)(3)(4). Continental material may be subducted to depths >200 km (5-7) before exhumation and emplacement along continental suture zones.…”

“…The existence of coesite and metamorphic diamond in continental rocks is widely regarded as robust evidence for UHP metamorphism ( 6 , 10 , 11 ). The mineralogy of exsolution phases in metamorphic garnet and pyroxene, including rutile, ilmenite, apatite, amphibole, clinopyroxene, and quartz, has also been used to constrain HP/UHP metamorphic conditions ( 12 , 13 ), with silicate exsolution in garnet considered evidence of former supersilicic, majoritic garnet and therefore extreme pressure conditions ( 4 , 6 ).…”