Age and significance of felsic dikes from the UHP western gneiss region

Abstract

Twenty-one plagioclase-bearing dikes were analyzed to place firmer limits on the end of (ultra)high-pressure (UHP) metamorphism across the Western Gneiss Region (WGR). Nineteen dikes were analyzed with laser ablation split-stream petrochronology to tie the U-Pb dates to zircon rare earth element (REE) chemistry, and a few key samples were analyzed by chemical abrasion thermal ionization mass spectrometry to provide high-precision constraints. All analyzed dikes yielded zircons with REE chemistry consistent with low-pressure crystallization. Approximately half of the dikes yield Precambrian dates; nondeformed dikes of this age support previous interpretations that much of the WGR underwent limited deformation during Caledonian subduction and exhumation. The oldest Caledonian dikes have dates that overlap with the circa 420-400 Ma eclogite dates from the region; this discrepancy indicates that either (1) cryptic structures separate early exhumed material from later exhumed material, (2) some of the dike dates are not low-pressure crystallization ages, or (3) post-406 Ma dates from eclogites are posteclogite-facies retrogression ages.

Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

“…During the Caledonian orogeny, subductionrelated magmatism related to closure of the Iapetus ocean ended at 430 Ma [Corfu et al, 2006], marginal basin ophiolites and island arcs were emplaced onto the Baltica craton in the Wenlock [Andersen et al, 1990], and the previously hyperextended continental margin [Andersen et al, 2012] began to contract. The combined thrust stack was subducted, and eclogites [Krogh, 1977;Gee, 1980;Andersen et al, 1991;Hacker and Gans, 2005] and (U)HP peridotites formed between~425 and~405 Ma [Glodny et al, 2008;Kylander-Clark et al, 2009;Krogh et al, 2011;Beckman et al, 2014;Kylander-Clark and Hacker, 2014].…”

“…2) [Terry et al, 2000;Hacker et al, 2003;Labrousse et al, 2004;Walsh and Hacker, 2004;Root et al, 2005;Johnston et al, 2007a;Spencer et al, 2013;Fauconnier et al, 2014]. This was accompanied by partial melting that was locally extensive in the hinterland, but progressively more feeble toward the foreland [Ganzhorn et al, 2014] [Hacker et al, 2010 #5959] [ Kylander-Clark and Hacker, 2014].…”

“…Thus, a reasonable explanation for the persistence of Precambrian monazite in some WGC samples is that those samples were converted in the Precambrian to a relatively dry, granulite-facies metamorphic mineral assemblage that was resistant to resetting [Hacker, 1996], and the colder Caledonian metamorphism [Corfu et al, 2013] was unable to induce monazite recrystallization. The preservation of Precambrian monazite is yet another indication that that large portions of the WGC did not deform during subduction and exhumation [Krabbendam et al, 2000;Austrheim et al, 2003;Peterman et al, 2009;Hacker et al, 2010;Spencer et al, 2013;Kylander-Clark and Hacker, 2014].…”

“…1) Walsh et al, 2007;Young et al, 2011;Walsh et al, 2013]; these are inferred to record westward-propagating exhumation of the Western Gneiss Region [Walsh et al, 2013]. Exhumation was accompanied by melting from 400 to 385 Ma [Austrheim et al, 2003;Gordon et al, 2013;Ganzhorn et al, 2014], with the youngest and most-extensive melting along the western edge of the WGR [Spencer et al, 2013;Kylander-Clark and Hacker, 2014].…”

Recycling argon through metamorphic reactions: The record in symplectites

“…This study focuses on the Nordfjord-Stadlandet domain, where the recorded/preserved P-T gradient ranges from the quartz eclogite-facies in the southeast (~23 kbar and~600°C) to coesite-stable eclogite-facies in the northwest (~30 kbar and~700°C) across 2500 km 2 (Root et al, 2005;Young et al, 2007). Peak conditions were reached between c. 410-400 Ma (Hacker et al, 2010;Kylander-Clark and Hacker, 2014;Root et al, 2004;Young and Kylander-Clark, 2015). Following (near) isothermal decompression from (U)HP conditions, the entire WGR experienced retrograde amphibolite-facies recrystallization at~700°C and 10 kbar by c. 399-379 Ma (Spencer et al, 2013;Walsh and Hacker, 2004).…”