The occurrence of Zr-bearing amphiboles and their relationships with the pyroxenes and biotites in the teschenite and nepheline syenites of a differentiated dolerite boss, Islay, NW Scotland

摘要

The Cnoc Rhaonastil minor dolerite intrusion on Islay, NW Scotland represents a single body of alkali olivine basalt magma which differentiated in situ, from leucodolerite, through teschenite to minor nepheline syenite. The syenites occur as isolated nests and pegmatitic schlieren within the leucodolerite, and schlieren of gabbroic pegmatite also occur at the margin of the teschenite. The differentiated rocks contain pyroxene, amphibole and biotite of variable compositions which reflect both primary fractionation processes and late-stage deuteric alteration and reaction. Mafic phases within the gabbroic pegmatite, teschenite and syenite are typically rimmed and speckled with biotite, the composition of which is controlled by the local environment of crystallization. The nepheline syenites contain primary ferro-kaersutite which, where in contact with interstitial patches, has been altered to arfvedsonite, which occasionally contains up to 1.2 wt. percent ZrO_2. The occurrence of Zr-arfvedsonite (and of Zr-aegirine) in interstitial patches suggests that variably trace element-enriched domains existed within the residual melts on very small scales.