Middle Fork

Middle Fork of west-central Alaska is a plutonic complex covering ~125 km2, thought to be coeval with the Windy Fork pluton (174-00-151) located 8 km to the southeast. The complex intrudes limestones, shales, and volcaniclastics of the Dillinger terrane of the southern Alaska Range and is surrounded by a hornfelsed aureole 1-3 km wide. Middle Fork is composed of a range of lithologies mainly alkali biotite-olivine-pyroxene gabbro, miaskitic silica-saturated to over-saturated olivine-pyroxene syenite, and two varieties of granite. The first of which, the Windy Fork granite named for similarities to the granite of the nearby Windy Fork pluton, is a peralkaline perthite-arfvedsonite granite with secondary riebeckite and accessories of note including monazite(?), fluorite, apatite, carbonate, and eudialyte. The second, the eponymous Middle Fork granite, is perthite-hornblende-biotite granite characteristically pink in colour, and locally containing both riebeckite and arfvedsonite. Also, present is a heterogeneous unit of granite, quartz monzonite, and monzodiorite, with altered xenoliths commonly found entrained within. Geochemical data including whole rock major and trace element, and CIPW norms can be found in Solie (1983).