DECIPHERING THE PETROGENETIC HISTORY OF THE SCOTTISH LATE CALEDONIAN GRANITES BY INTEGRATED IN-SITU TRACE ELEMENT AND ISOTOPIC ANALYSES OF ZIRCONS

The late Caledonian ‘I-type' granites of the Scottish Grampian Highlands, were emplaced between 430 Ma and 400 Ma. Three granite suites have been recognized from whole-rock geochemical and isotopic data. This study focuses on granites and diorites of the Lochnagar and Etive plutons, representing the Cairngorm and Argyll suites. Integrated in-situ REE / trace element, and oxygen, U-Pb and Lu-Hf isotope analyses of zircons have been used to constrain their crystallisation ages, crustal residence ages, tectonic setting, magma sources, and relative mantle and crustal contributions. This robust zircon record may be accessed at the intra- and inter-grain scale by integrating SEM imaging with SIMS (U-Pb dating, ä18O, REE), LA-ICP-MS (Lu-Hf isotopes) and EPMA. SIMS analyses may be made on the same ~20µm analysis spot in different components (e.g. growth zones, inherited cores) of single zircon crystals, providing new insights into the genesis and evolution of granitic rocks. In both plutons in-situ high-precision SIMS oxygen isotope analyses with an analytical precision of ±0.2-0.3‰ enable us to recognise statistically distinct zircon populations (e.g. Lochnagar diorite: population 1: 6.6±0.3‰, population 2: 7.4±0.3‰), indicating different source components. Mantle zircon populations (ä18O (Zrc) = 5.3±0.3‰) were not observed in any of samples, indicating that none of the granites and diorites have a recognisable pure mantle component. All facies must have experienced several magma mixing or assimilation events before and during zircon growth. The combination with U-Pb and REE helps to further characterise the zircons and allows identification of inherited, metamict and/or hydrothermally altered material.