The Petrology and Geochemistry of Cone-sheets from the Cuillin Igneous Complex, Isle of Skye: Evidence for Combined Assimilation and Fractional Crystallization during Lithospheric Extension
The origin and evolution of the magma(s) involved in the formation of the olivine tholeiite cone-sheets which invade the ∼60-Ma Cuillin Igneous Complex, Isle of Skye, have been investigated using mineral chemistry data together with whole-rock major- and trace-element and isotope (Sr and Nd) geochem...
| Published in: | Journal of Petrology |
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| Main Authors: | , , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1994
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/35/4/1055 https://doi.org/10.1093/petrology/35.4.1055 |
| Summary: | The origin and evolution of the magma(s) involved in the formation of the olivine tholeiite cone-sheets which invade the ∼60-Ma Cuillin Igneous Complex, Isle of Skye, have been investigated using mineral chemistry data together with whole-rock major- and trace-element and isotope (Sr and Nd) geochemistry data. The most primitive compositions identified are almost identical to those of the basalts being erupted at the present day along the spreading centre in Iceland. Rare examples of lavas from the slightly older lava pile of west-central and north Skye, together with a large number of dykes from the axial zone of the Skye Main Swarm, are of similar composition and are referred to as being of the Preshal More Basalt type (spelling is according to current Ordnance Survey of Great Britain maps). The intra-suite compositional variation exhibited by the cone-sheets can be explained in terms of relatively low-pressure fractionation of the three-phase assemblage olivine+clinopyroxene+plagioclase in their cotectic proportions of 10: 35: 55. Modelling of major- and compatible trace-element data indicates that the most evolved composition may be derived by 60% crystallization of the least evolved composition. Incompatible trace-element modelling implies greater degrees of crystal-liquid fractionation: Y and Zr indicate 64 and 68% crystallization, respectively, whereas the rare earth elements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization. This decoupled behaviour of compatible and incompatible elements is attributed to the cone-sheet magma(s) evolving in a subjacent magma chamber, before final emplacement in the overlying crust. Sr- and Nd-isotope data indicate that the cone-sheet magma(s) assimilated upper-crustal lithologies during fractional crystallization, most likely involving amphibolite facies gneiss of the Lewisian Gneiss Complex. This style of simultaneous assimilation and fractional crystallization (AFC) process in the upper crust for the Skye cone-sheet magma(s) is in direct contrast to the ... |
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