Mount Stuart Batholith Research Articles

Principal stress directions from a natural occurrence of stress-induced clinoenstatite

Lamellae of clinoenstatite are common in enstatite grains in the high-grade ultramafic hornfelses around the Mount Stuart Batholith in the Central Cascades of Washington. Laboratory deformation experiments on enstatite have shown that shear stress on (100) planes parallel to [001] drastically promotes the formation of clinoenstatite. Petrofabric studies of a sample of the hornfels reveal a strong preferred orientation of clinoenstatite-bearing enstatite grains that is unrelated to the fabric of the complete set of enstatite grains. This preferred orientation is well explained by supposing that the clinoenstatite was produced by shearing due to N-S compression. Local and regional geological features are also consistent with N-S compression. These results suggest that there is considerable potential for the application of stress-induced clinoenstatite as a geopiezometer in rocks.

Petrology and petrogenesis of the Mount Stuart batholith — Plutonic equivalent of the high-alumina basalt association?

The Mount Stuart batholith is a Late Cretaceous calc-alkaline pluton composed of rocks ranging in composition from two-pyroxene gabbro to granite. Quartz diorite is most abundant. This batholith may represent the plutonic counterpart of the high-alumina basalt association. A petrogenetic model is developed in which this intrusive series evolved from one batch of magnesian high-alumina basalt, represented by the oldest intrusive phase, by successive crystal fractionation of ascending residual magma. However, the possibility that this intrusive suite originated from an andesite (quartz diorite) parent by fractionation cannot be excluded.

Contact Metamorphism of Serpentinite, Chloritic Blackwall and Rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington

The 2 km wide contact aureole produced from serpentinite by the intrusion of the Mount Stuart Batholith into the Ingalls Complex at Paddy-Go-Easy Pass contains the following ultramafic assemblages, in order of increasing grade: serpentine-forsterite-diopside, serpentine-forsterite-tremolite, forsterite-talc, forsterite-anthophyllite, forsterite-enstatite-anthophyllite, forsterite-enstatite-chlorite, forsterite-enstatite-spinel. Associated metarodingites display five metamorphic zones, the diagnostic assemblages of which are, in increasing grade: grossular-idocrase-chlorite, grossular-diopside-chlorite, epidote-diopside-chlorite, epidotediopside-spinel, plagioclase-grossular-diopside. Mafic hornfels in the aureole contains no orthopyroxene, indicating that the conditions of pyroxene hornfels facies were not reached. The breakdown of chlorite is best displayed in aluminous blackwall reaction zones around mafic inclusions in the peridotite. At temperatures above those of the anthophyllite-out isograd, but within the field of forsterite+tremolite, these chlorite-rich rocks react to form the assemblage: forsterite-enstatite-spinel. Calculations show that cordierite did not form as a result of chlorite breakdown in the natural system because impurities, such as iron and chromium, displaced the equilibrium: forsterite+cordierite = enstatite+spinel to much lower pressures than the three kilobars found in the pure system. The primary chromite of the peridotite has been altered to chrome-magnetite in the serpentinite. This alteration seems to be isochemical over the whole rock, as true chromite, formed by metamorphism, occurs at grades above that of the forsterite-enstatite-anthophyllite assemblage. Calcic amphibole in high-grade metaperidotite is tremolite, even in the presence of aluminous chromite, whereas that in metamorphosed blackwall rock grades from tremolite into hornblende. The pattern of substitution appears to be: Mg2Si3rlhar2;(Na,K)(AlVI)2(AlIV)3.

Concerning the Differentiation of a Lamprophyric Magma at Corbaley Canyon, Washington

Evidence is presented to prove that the dike rocks of Corbaley Canyon represent a series of intrusions injected at intervals during the differentiation of an original lamprophyric magma. A very complete series of differentiates, with a quartz-hornblende kersantite and a spherulitic granophyre as the poles, has been found. That differentiation did not proceed by the splitting of a magma of intermediate or of granitic composition into acidic and basic poles to form "complementary dikes" is proved by the fact that the basic dikes were intruded earlier than dikes of intermediate and granitic composition. It is thought possible that the original lamprophyric magma from which the differentiates were formed may represent one of the lamprophyric offshoots of the Mount Stuart Batholith which lies about 20 miles to the southwest.