Petrology of the alaskites of the Boulder batholith, Montana

The Boulder batholith outcrops in the Northern Rookies of western Montana. It is exposed over an area of 5700 km^2 and extends about 112 km along the continental divide from Mullen Pass on the north to 32 km south of Butte. The Boulder batholith represents an invasion of the upper zones of the crust by a large volume of fluid granodioritic quartz monzonitic magma emplaced at the close of a main orogenic episode during late Cretaceous and Paleocene time. Eight alaskite bodies were chosen for sampling from previous field experience and from geologic maps published by the U.S.O.S. Textural descriptions, modal analyses, and chemical analyses were done on eighty-eight samples. The primary minerals of these alaskites are alkali feldspar, quartz, and plagioclase. Accessory minerals include biotite, apatite, sphene, zircon, allanite, tourmaline, rutile, and opaques. The main alteration minerals include chlorite, sericite, and hematite. In comparison with the quartz monzonites, the alaskites are typically higher in Si02 and K20 and lower in il203, Fe203+FeO, MgO, CaO, Ti02, P20.5, and MnO. The alaskite occurs as dikes and as irregularly-shaped lenses or pods, primarily within the Butte quartz monsonite. Most of the contacts between the Butte quartz monzonite and alaskite are sharp, but a few are gradational. The variation across the individual bodies and the variation of each body' relative to the others was compared. The most consistent patterns across the bodies were shown by below average concentration or Na2') 1n the center of the bodies, and an above average concentration of intergrown albite content 1n the perthite and K20 near the margins. The proposed model suggests a mechanical separation of a water rich rest magma from the quartz monzonite magma. With decreasing temperature and increasing crystallization the rest magma becomes saturated with water. The saturated rest magma eventually separates into a rest magma and an aqueous phase. The textural, modal, and chemical relationships observed in the alaskites may be the result of interaction between the rest magma and the aqueous phase.