RHYTHMIC LAYERING FORMED BY INFLUXES OF PLAGIOCLASE-PHYRIC MAGMA IN THE CUILLIN COMPLEX, ISLE OF SKYE, SCOTLAND

The Cuillin mafic-ultramafic complex on the Isle of Skye is the exposed remnant of a large shallow crustal magma system. Zone II of the Outer Layered Eucrite Series of the complex includes several hundred meters of rhythmically layered gabbro section consisting of alternating layers of medium-grained laminated gabbro and coarse-grained massive leucogabbro. This rhythmic layering occurs on scales of meters to tens of meters. Large crystals of normally-zoned plagioclase with Ca-rich cores (An 83-91) constitute the bulk of the coarse-grained leucogabbros and are also present as sparse phenocrysts in the medium-grained gabbros. The coarse-grained leucogabbro layers typically have sharp lower boundaries but grade upward continuously into overlying laminated gabbros. Some of the leucogabbro layers have scalloped bases that resemble sedimentary load casts, with layering in the underlying laminated gabbros having been deformed into parallel with the scalloped layer boundaries. The leucogabbros are rich in incompatible elements and contain olivine and augite that are poorer in Mg than the same minerals in the laminated gabbros, indicating that the leucogabbros trapped relatively large proportions of intercumulus liquid. The rhythmic layering is interpreted to be the product of frequent replenishment of the Cuillin magma chamber by small batches of plagioclase-phyric magma. The plagioclase phenocrysts accumulated rapidly by gravitational settling, forming magmatic sedimentary structures and trapping large amounts of intercumulus liquid in the rapidly aggrading crystal pile. These interpretations support petrogenetic models in which Skye magmatism is inferred to have been dominated by the migration of small-volume magma pulses through a complex intracrustal plumbing system.