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

• Published in: Tectonics (Washington, D.C.) D.C.), 2014-12, Vol.33 (12), p.2342-2360
• Main Authors: Kylander-Clark, Andrew R. C., Hacker, Bradley R.
• Format: Article
• Language: English
• Subjects: Caledonides; Chemistry; Crystallization; Ionization; Isotopes; LA-ICPMS; Mass spectrometry; Metamorphism; Norway; Precambrian; rare earth elements; Western Gneiss Region; zircon
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1002/2014TC003582

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. Key Points Zircons from low‐pressure dikes are analyzed for age and chemistryHigh‐pressure metamorphism ended by 405 Ma in parts of the Western Gneiss RegionMany portions of the WGR did not transform/deform during Caledonian orogenesis