Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA

Abstract

Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure-temperature-fluid history of the domes. Three northern Cordilleran migmatite domes (ThorOdin and Valhalla ⁄ Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg-Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphic conditions and pressure-temperature (P-T) path information not preserved in the host migmatite. These Mg-Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg-Al-rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm-to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and ⁄ or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high-T-low-P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720-750°C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P-T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene-Eocene time. In contrast to orthoamphibolecordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scale layer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P-T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…Schreyer and Seifert, 1969a;Seifert, 1974;Ackermand et al, 1975;Hensen, 1987;Kelsey et al, 2005). However, the presence of sapphirine in a rock does not immediately indicate that the rockdand terranednecessarily records temperatures in excess of 900 C. For example, Kruckenberg and Whitney (2011) and Goergen and Whitney (2012b) reported symplectic sapphirine in quartzabsent MgeAl-rich rocks that attained temperatures of approximately 750 C. In addition, there are a number of occurrences of sap phirineespinelebiotite-bearing rocks (AE corundum AE ortho pyroxene AE plagioclase AE sillimanite AE garnet AE chlorite, and all quartz-absent) that appear to record temperatures less than 900 C (e.g. Oliver and Jones, 1965;Kelsey et al, 2005;Prakash and Sharma, 2008;Raith et al, 2008;Sharma and Prakash, 2008;Nasipuri et al, 2009;Prakash et al, 2013;Sarbajna et al, 2013).…”

Short-lived high-temperature prograde and retrograde metamorphism in Shaerqin sapphirine-bearing metapelites from the Daqingshan terrane, North China Craton

“…Sapphirine occurring in a silica-saturated but oxidizing system recording non-UHT conditions sets a precedent, although numbers of Spr-Spl-Bt-bearing, but Qz-absent rocks, have been obtained temperatures less than 900 • C (e.g. Nasipuri et al, 2009;Prakash et al, 2013;Raith et al, 2008;Sharma and Prakash, 2008;Sarbajna et al, 2013), and even down to 750 • C (Kruckenberg and Whitney, 2011), as reviewed in Kelsey and Hand (2015).…”

Low‐temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia

“…Previous metamorphic studies of the middle and lower units determined maximum metamorphic conditions of ~650–800°C at 7–9 kbar and ~750–800°C at ≥ 10 kbar, respectively [ Ghent et al ., ; Duncan , ; Nyman et al ., ; Norlander et al ., ; Teyssier et al ., ; Hinchey and Carr , ; Goergen and Whitney , , ]. Thermobarometric data and reaction textures in high‐grade rocks of the lower unit have been interpreted to indicate near‐isothermal decompression of the dome rocks under melt‐present conditions [ Norlander et al ., ; Hinchey and Carr , ], similar to other domes in the Shuswap MCC [e.g., Marshall and Simandl , ; Kruckenberg and Whitney , ].…”

“…Many field‐based investigations and numerical modeling studies have documented the pressure‐temperature‐time and strain history of migmatite dome rocks to understand the relationship between partial melting and crustal dynamics [ McKenzie et al ., ; Beaumont et al ., ; Tirel et al ., ; Fayon et al ., ; Hinchey et al ., ; Thiede et al ., ; Rey et al ., , , , ; McFadden et al ., ; Brown et al ., ; Little et al ., ]. Metamorphic reaction textures indicating near‐isothermal decompression occur in dome migmatites and associated rocks [ Norlander et al ., ; Kruckenberg and Whitney , ; Goergen and Whitney , , ], consistent with the tight range of ages recorded by high‐to‐medium temperature chronometers (zircon U‐Pb to mica 40 Ar/ 39 Ar) in dome rocks; these are interpreted to indicate rapid cooling (≥50°C/km) [ Vanderhaeghe et al ., ; Robinson et al ., ; Kruckenberg et al ., ; Gordon et al ., , ; Little et al ., ; Stübner et al ., ].…”