High‐pressure‐low‐temperature metamorphism of granitic orthogneiss in the Brooks Range, northern Alaska

• Published in: Journal of Metamorphic Geology
• Main Author: PATRICK, B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1995
• Subjects: Brooks Range; Alaska
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1995.tb00208.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1995.tb00208.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1995.tb00208.x

Abstract Granitic orthogneiss is widespread throughout the metamorphic core of the Brooks Range in both the ductilely deformed blueschist/greenschist facies Schist Belt and the lower grade Central Belt (= Skajit allochthon) to the north. Orthogneiss occurs as large metaplutonic massifs and in small bodies enclosed within metasedimentary rocks. Crystallization ages for the granitic protoliths range from Proterozoic through Devonian (U‐Pb zircon); the K‐Ar system was reset during Cretaceous metamorphism. Mineral assemblages of the orthogneisses reflect nearly complete re‐equilibration during Jurassic‐Cretaceous collisional orogenesis in northern Alaska. The most common metamorphic paragenesis in orthogneiss is: Qtz + Kfs + Ab + Phe + Bt ± Ep, Ttn, Rt, Ap, Chl, Cal. Constituent minerals from 16 Brooks Range orthogneiss samples were analysed with the electron microprobe. Phengite from the Schist Belt samples is highly enriched in Al‐celadonite, with Si values up to 3.50 per formula unit (on an 11‐oxygen basis). Central Belt samples contain phengite with lower Si content (±3.38 p.f.u.). In nearly all samples, Si content of phengite varies considerably, reflecting partial re‐equilibration to lower pressure and/or higher temperature conditions. Metamorphic conditions were estimated using the Phe‐Bt‐Kfs‐Qtz barometer and the two‐feldspar solvus thermometer. The results indicate that the Schist belt underwent high‐pressure/low‐temperature metamorphism (generally 9‐12 kbar at 375‐430° C), consistent with the widespread development of glaucophane + epidote/clinozoisite and lawsonite pseudomorphs in other rock types. The Central Belt also experienced a relatively high P‐T metamorphism, with most samples yielding pressure estimates in the range 5‐8 kbar (at 325‐415° C). These results confirm the existence of two metamorphic belts in the core of the Brooks Range that differ in metamorphic conditions by up to 5 kbar. The range in Si content in phengite from Schist Belt samples is consistent with isothermal decompression of up ...