Strontium Isotope Evidence for Numerous Small-Volume Inputs of Hybrid Magma During Growth of the Cuillin Layered Gabbro 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 of Paleocene age. It was fed by basaltic magmas that ascended through a thick Archean gneissic crust via a complicated magmatic plumbing 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. The layers of massive leucogabbro formed by accumulation of Ca-rich plagioclase phenocrysts (An 85-91) during episodic replenishment of the chamber by small batches of plagioclase-phyric magma. The initial 87Sr/86Sr ratios of massive layers are about 0.0002 lower than those of adjacent laminated layers, indicating that the plagioclase-phyric magmas underwent less crustal contamination than did other magmas in this part of the complex. Both types of layers are isotopically variable, implying that all of the layers are hybrids of at least two isotopically distinct magmas. In general, however, 87Sr/86Sr increases upward through the section, indicating that the proportion of material derived from the low-87Sr/86Sr end-member decreased through time. Much of the hybridization probably occurred in external magma reservoirs prior to emplacement in the currently exposed chamber, although some mixing may have occurred within the chamber itself. Earlier studies have shown that, in the context of the entire Cuillin layered mafic-ultramafic complex, the Outer Layered Eucrite Series represents an irregular transition from earlier influxes of isotopically heterogeneous and highly contaminated magmas to later influxes of isotopically homogenous and less contaminated magmas. The section described here appears to record the intermediate stages of this transition, with the massive layers representing numerous minor pulses of the less contaminated magmas that eventually came to dominate the late-stage growth of the complex. Temporary waning of these pulses produced episodic reversals in the larger trend toward lower degrees of contamination, with one such reversal being recorded by the upward increase in 87Sr/86Sr described in this study.