MESOZOIC GRANULITE FACIES METAMORPHISM OF THE PITKA MAFIC-ULTRAMAFIC COMPLEX, NORTHERN ALASKA

Granulite facies mafic and ultramafic rocks of the Pitka complex are exposed as a klippe above the Tozitna terrane in northern Alaska. Metamorphosed mafic rocks contain plagioclase (pl)-clinopyroxene (cpx)-garnet (grt)-orthopyroxene (opx)-hornblende (hb). Scapolite is present locally but quartz (qtz) is absent. Metamorphosed ultramafic rocks contain olivine-cpx-opx-spinel. Grt-cpx Fe-Mg exchange temperatures (T) range from 770 to 860°C at 10 kbar. Opx-cpx geothermometry yields T’s from 715 to 810°C at 10 kbar. Maximum pressure based upon grt-pl-cpx equilibria without qtz is near 10 kbar at 800°C. The activity of H2O is not well constrained but is less than 1.0, based upon grt-hb-cpx-pl-H2O equilibria. An Ar/Ar date from a granulite facies hornblende yields a plateau age of 169.5± 0.3 Ma. Retrogression to epidote amphibolite facies hydrous assemblages is locally important and is associated with anastomosing shear zones. Ar/Ar plateau dates of hornblendes from the retrograde shear zones are 166 ± 0.6 and 164.8 ± 1.1 Ma. Many rocks show little evidence of retrogression (symplectites are rare) and apparently underwent rapid, relatively dry cooling. The Pitka complex appears to correlate with the Kanuti complex and other mafic/ultramafic complexes exposed as klippe above very low grade basalts, gabbros and cherts of the Angayucham and Tozitna terranes. These terrane are themselves structurally above blueschist and greenschist facies rocks of the Ruby terrane and Brooks Range. The structural stack is interpreted to have formed when the continental margin now preserved in northern Alaska subducted beneath an island arc.