The Implications of the Compositional Dependence of Texture in Spinel Lherzolite Xenoliths

Abstract

Spinel lherzolites of the xenolith population of Nunivak Island, Alaska, may be classified on textural criteria into the categories: coarse-equant, coarse-tabular, and granuloblastic-equant. This textural variation is correlated with increasing titanium and aluminum in the lherzolite pyroxenes and decreasing Cr/(Cr + Al) in coexisting spinel. This correspondence between chemical composition and texture resembles that frequently documented in garnet lherzolite suites from kimberlites. Unlike the latter, however, there is no systematic relation between temperature and texture in the Nunivak suite. The change from the coarse-equant to coarse-tabular lherzolite is gradational in both texture and chemistry and is interpreted to reflect progressive deformation and metasomatism. The granuloblastic-equant xenoliths are distinct and may represent annealed, highly deformed, coarse-tabular lherzolites. Both the coarse-equant and coarse-tabular textural categories can be divided into two common sub-groups. Lherzolites of the first are characterized by equant interstitial spinel, relatively magnesium-rich silicates, and jadeite-poor clinopyroxene; while in lherzolites of the second, the spinel is interpreted to have been inclusions in pyroxene, the silicates are iron-rich, and the clinopyroxene is jadeitic. Xenoliths of the latter sub-group are visualized as garnet-bearing lherzolite which rose as a diapir and partially re-equilibrated in a spinel lherzolite upper mantle; represented by lherzolites of the first sub-group. Deformation and metasomatism associated with this diapirism are believed responsible for the observed textural and correlated chemical variations in the Nunivak xenolith suite.