The Petrogenesis of the High-temperature Peridotite Intrusion in the Lizard Area, Cornwall

Abstract

The Lizard peridotite produced a high-temperature metamorphic aureole during diapirie emplacement in a period of amphibolite facies regional metamorphism. The peridotite preserves a coarse-grained primary core within a cataclastic, finely foliated and recrystallized marginal shell. Chemical analyses demonstrate the constancy of rock composition in the primary and recrystallized mineral assemblages. The primary mineral assemblage of the peridotite consists of olivine (Fo89), aluminous enstatite, aluminous diopside, and green aluminous spinel. In contrast the mineral assemblage on recrystallization is olivine, enstatite (normal Al2O3 content), diopside (normal Al2O3 content), plagioclase, and brown chromite. A third assemblage of olivine, pargasite, and brown chromian spinel is developed locally from the plagioclase-bearing assemblage, particularly in contact areas. Major and trace element analyses of twenty minerals and eight rocks are given. These are compared in detail with peridotites and minerals occurring as accumulates from basaltic magma, as peridotite nodules in basalts and as ’intrusive' peridotites in orogenic areas. It is concluded that the primary minerals of the Lizard peridotite have not crystallized and accumulated from a basaltic magma but have crystallized in a similar environment to that of peridotite nodules in basalts. It is further concluded that the cause of the differences between the primary and recrystallized assemblages of the peridotite is primarily a difference in load pressure at crystallization. In particular the high alumina content of both enstatite and diopside and the coexistence of pyroxenes+ aluminous spinel instead of olivine+anorthite are considered to be a direct consequence of the high load pressure at the initial crystallization of the peridotite.