Bachelor of Arts
Mark E. Brandriss
Strontium isotopes, Cuillin, Skye, Gabbro, Layering, British palaeogene
The mafic to ultramafic plutonic rocks of the Cuillin Center on the Isle of Skye provide an opportunity to track sources of magma in an open magma system. One particular area of interest is a 175m stratigraphic section in the Outer Bytownite Gabbros, consisting of rhythmically layered coarse-grained massive gabbros and finer-grained laminated gabbros that were contaminated to varying degrees by the underlying Archean gneissic crust. The massive gabbro layers contain abundant coarse crystals of plagioclase with Ca-rich cores, which previous studies interpreted to have been carried into the Cuillin Center by repeated influxes of plagioclase-phyric magmas. Between these influxes, the laminated gabbros cystallized in-situ and contain few coarse plagioclase crystals. Twenty-six 87Sr/86Sr isotope analyses were made of the cores, intermediate zones, and rims of coarse plagioclase crystals from both gabbro types in the section. The analyses reveal that the plagioclase cores from both gabbro types have lower 87Sr/86Sr ratios than the whole rocks (analyzed in previous studies), indicating that the plagioclase crystals are antecrysts that formed in bodies of magma that were less crustally contaminated than the Cuillin Center. The isotopic compositions of the plagioclase cores are quite variable, suggesting that they originated in multiple and distinct magma bodies prior to being flushed together into one magma chamber. The overgrowths of the plagioclase were found to have gone through two additional processes of crystallization. The first process of crystallization was in a more crustally contaminated and evolved intermediate magma, where the plagioclase was then flushed into the Cuillin Center where the rim zones crystallized. The plagioclase antecrysts were flushed into the Cuillin as multiple influxes of magma, which mixed with the more contaminated magma that was already in the Cuillin chamber. The multiple influxes of magma into the Cuillin resulted in the rhythmic layering of the gabbros. This inference is supported by the 87Sr/86Sr ratios of the III plagioclase overgrowths in the massive gabbros, which have higher 87Sr/86Sr ratios than the plagioclase cores. The 87Sr/86Sr ratios of the overgrowths also increase upward through the stratigraphic section, implying an increase in crustal contamination of the magma bodies that fed the Cuillin Center over time. The cores of the plagioclase antecrysts from massive gabbros have similarly high anorthite contents as the sparse phenocrysts found in the Preshal Mor basalts, which are MORB-like primitive basalts that form the final part of the Skye lava sequence. The antecrysts, however, have higher 87Sr/86Sr ratios than the Preshal Mor basalts, suggesting that the antecrysts may have formed in a Preshal Mor-type magma that had been contaminated by the Archean crust. Two coarse plagioclase crystals from a gabbroic dike of the Oseitic group on Skye had 87Sr/86Sr ratios as low as the Preshal Mor basalts, along with equally high anorthite contents. This suggests that they are Preshal Mor-type magmas that crystallized coarse plagioclase without undergoing contamination by the Archean crust, unlike the magmas of the Cuillin Center. Overall, the large range in 87Sr/86Sr of the plagioclase cores implies a complicated magmatic plumbing system beneath Skye, with multiples magma batches of distinct magmatic evolutions and crustal contamination mixing together at various points and feeding into the main Cuillin intrusion.
©2019 Sofia Jean Johnson Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.
Johnson, Sofia Jean, "Tracking magma evolution using strontium isotopes in plagioclase antecrysts in the Cuillin Center, Scotland" (2019). Honors Project, Smith College, Northampton, MA.
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