Macro to micro-scale insights into granite petrogenesis; a case study from the Caledonian Ross of Mull granite (ROMG), Argyllshire, Northwest Scotland

Abstract

The Ross of Mull Granite (ROMG) occupies the largest part of the Pre-Paleogene inlier of the Ross of Mull peninsula, Isle of Mull, Argyllshire, Scotland and is the most south-westerly of the Northwest Highland Caledonian granitoid complexes. Re-mapping of the entire intrusion (Zaniewski et al. 2006) has revealed a reverse zonation of facies resulting from mafic-felsic magma interaction prior to ascent at the proposed intersection of the Sound of Iona Fault and Great Glen Fault with north-eastward emplacement of flattish sheets into high-grade lithologies of the Moine Supergroup. Subsequent to mapping, Quantitative X-Ray Diffractometry (QXRD) has re-enforced modal classifications of Zaniewski et al. (2006). To further examine if this phenomenon is also encoded at the micro-scale (crystal), and if a correlation exists with macro-scale (field) studies, in-situ elemental and Pb-Pb isotopic zoning in magmatic feldspars from all ROMG components were analysed. The following micro-scale studies have been undertaken: (a) detailed textural study of feldspars using cold Cathodo-Luminescence (CL) and Back-Scatter Electron (BSE) imaging; (b) acquisition of feldspar major and minor element abundances by Electron Probe Micro Analysis (EPMA) and Pb-Pb isotopes by Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). Feldspars throughout the components display a three-fold CL magmatic zonation associated with dissolution at the core followed by elemental spike zones of Ba, Sr (0.00 - 1.00 wt. %) and Ab [mol. %] with elemental re-equilibration back towards the rims. Such resorption surfaces correspond to rim-ward increase of 206Pb/204Pb and 208Pb/204Pb in alkali feldspars in host granitoid and andesine in early phase MME advocate dissolution attributed to punctuated mafic magmatic recharge from a mantle-derived component interacting with a crustal component. Bulk Pb-Pb isotope data also highlight the importance of magma mixing and mingling in the petrogenesis of the complex and define a simple binary mixing array reflecting quasi-contemporaneous hybridisation between such components. A temporal sequence of macro-scale and micro-scale events is proposed and it is evident that the process of mafic-felsic magma interaction seen at the outcrop-scale is encoded at the crystal-scale.