INCREMENTAL PLUTON ASSEMBLY IN THE SCOTTISH LATE CALEDONIAN GRANITES: NEW RESULTS FROM AN INTEGRATED MINERALOGY AND ZIRCON ISOTOPE STUDY

Pluton assembly models have been under debate for many decades. Recently, on the basis of numerical modelling and U-Pb zircon TIMS dating, the model of incremental pluton assembly has been proposed. To test this model, samples from the Scottish late Caledonian Etive Granite, a major composite pluton comprising distinct mappable units of granite to monzodiorite/diorite were collected. Automated mineralogy (QEMSCAN), and ion microprobe (SIMS) analysis of zircon (O and U-Pb isotopes), were used to detect melt batches with differing major mineral abundances, plagioclase compositions, O isotope signatures and crystallisation ages.

Automated mineral analysis reveals that samples collected from single mappable units vary largely with respect to grain size, texture and major mineral modal abundance. In addition, plagioclase crystals display a range of anorthite contents and zoning patterns between and within samples. SIMS analyses show that zircon δ¹⁸O values also differ not only between samples, but between zircons of a single sample, and within zircons. O isotope data distributions within samples are found to be unimodal, bimodal or scattered. Similarly, U-Pb isotope data show that the pluton’s emplacement history is more protracted than previously recognised, with magmatic activity lasting for approximately 16 m.y. (423-407 Ma).

To generate the complexity observed in the Etive Granite we propose that melt generation occurred in a ‘deep crustal hot zone’ where batches of intruding mantle melts differentiated and mixed with lower/middle crust partial melts with more evolved δ¹⁸O compositions. On the basis of O isotope variations observed within single zircon crystals, magma mixing not only occurred before zircon crystallisation, but continued during zircon growth. Small magma batches, each showing distinct δ¹⁸O values, were subsequently emplaced incrementally over c. 16 m.y.. The results of this study are consistent with and provide further evidence for the model of incremental pluton assembly. Furthermore, by integrating automated mineralogy with O and U-Pb isotope data of zircons an even more complex petrogenetic evolution and emplacement history was revealed for the Etive Granite than previously recognised.