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

Abstract

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.