Rare Earth Elements in the Whitestone Anorthosite

Abstract

<p>An analytical procedure is adopted to determine eight rare earth elements in the Whitestone anorthosite, its constituent minerals, and the surrounding metamorphic rocks. Chondrite-normalized rare earth fractionation trends display the rare earth data.</p> <p>Distinctive rare earth fractionation trends for anorthosites of different origins are not expected to be found. The anorthosite rare earth fractionation trends reflect the crystal-chemically controlled uptake of rare earths by the mineral phases present, so the rare earth fractionation trends of the plagioclase, clinopyroxene, hornblende and garnet are stressed. The rare earth data are consistent with plagioclase and clinopyroxene being primary phases, and hornblende and garnet beine the products of autometamorphism involving the primary phases ond a residual, Fe-Ti, volatile-rich fluid phase.</p> <p>The country rock-anorthosite relationship is not elucidated by this rare earth study, except the suggestion that rare earths have been supplied to the metasomatic envelope from the anorthosite or a late stage, residual fluid phase associated with the anorthosite.</p> <p>The theories of anorthosite origin and their application to the Whitestone anorthosite are discussed in light of rare earth information and previous field and petrologic work. The most probable modes of origin of the Whitestone anorthosite are:</p> <p>1. early fractional crystallization and accumulation of a plagioclase-rich rock fran a dioritic (possibly contaminated) magma, partial melting and intrusion of the anorthosite into its present geological setting.</p> <p>2. crystallization of a gabbroic anorthosite magma formed by partial fusion of amphibolite,</p> <p>3. disruption (with partial melting) of a layered-type anorthosite and subsequent injection of this anorthosite into its present surroundings.</p>