A petrogenetic study of harrisite in the Isle of Rhum Pluton, Scotland

Abstract

There is textural evidence in the Rhum pluton for crystallization of layered ultrabasic rocks ('harrisite') under conditions of massive olivine supersaturation. Some of these rocks are comb layered. This field, petrographic and experimental investigation of harrisite is aimed at determining (1) the degree of supercooling at which different shapes of olivine crystallized in harrisite, (2) how magma in a plutonic environment became massively supersaturated in olivine and (3) whether the parent magma of harrisite was basaltic or ultrabasic. The results have particular relevance to the conditions under which magmas crystallize close to, or on, their margins and enclosures, sometimes to produce comb layers. Harrisite is here defined as "an ultrabasic, plutonic, igneous rock in layered intrusions which contains >25 vol. percent of olivine. The olivine crystals are characterized by skeletal or dendritic shapes and have larger size than olivines in contiguous layers. Hydrous minerals are essential phases in the rock," Harrisite is a varietal type of peridotite, allivalite or olivine eucirite. Harrisite layers are mainly confined to the Western Layered Series of the Rhum pluton. They range in thickness from 7 cm to 10 m and have a preferred thickness of about 1 m. Individual layers vary laterally in thickness and olivine crystal shape. A few harrisite layers are intrusive into the Layered Series, but the majority crystallized in conformable sequence with cumulate layers. Five percent of the ultrabasic rock of the island is harrisite; sixty percent of the Transition Series is harrisite. The shapes of olivines in igneous rocks are classified into thirteen categories (those found in harrisite are underlined): polyhedral, porphyritic, granular, chain-like, parallel-growth, plate, branching (non-crystallographic, crystallographic, linked parallel-unit), randomly oriented, radiate, dendritic, lattice-work, swallow-tail and irregular olivines. Elongate olivines in harrisite can grow preferentially upwards, downwards, horizontally or in random orientation. There is a gradual, systematic upward change in layers from granular or polyhedral cumulus olivines to skeletal porphyritic olivines, to branching olivines (present only in some layers). This change is accompanied by up to a 10 vol. percent decrease in olivine content and by increases in contents of both chrome magnetite and hydrous minerals; also, olivine crystals increase up to 1000 times in size and plagioclase crystals increase in size up to 10 times. Olivine crystals in harrisite grew as fast as 10-4 cm/s. Olivine in harrisite has a composition range of Fo84- 73. Some crystals are reverse zoned (2 - 2.5 mol. percent Fo). Olivines in cumulate layers are up to 2 mol. percent poorer in Fo than those in contiguous harrisite layers. Augite (e.g., Wo46En46FS9) and rare orthopyroxene crystals (e.g., Wo1.3En79.4 Fs 19.3) indicate crystallization from a transitional magma. Plagioclases range from An87 in the generally broad, unzoned cores of crystals, to approximately An60 at in the extremity of the normally-zoned mantles. Cr-rich kaersutite and phlogopite crystallized by reaction of magma with pyroxene, whereas Cr-deficient varieties of the same minerals crystallized directly from the magma. Chrome magnetite composition is dependent on the enclosing silicate mineral- crystals enclosed in olivine are generally richer in Fe and poorer in Al, Cr and Mg than those enclosed by plagioclase and pyroxene. Variations in olivine, plagioclase and chrome magnetite compositions in one comb layer of harrisite are consistent with upward crystallization of the layer along a thermal gradient in the magma of increasing temperature with height. (Abstract shortened by Proquest)