HR: 08:30h
AN: V21A-01    [Abstracts]
TI: Multiple Magmatic Lineages and the Source of K in High-K Calcalkaline Granitoid Suites: the Arrochar and Garabal Hill-Glen Fyne Intrusions, Scotland
AU: * Clemens, J D
EM: j.clemens@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AU: Darbyshire, F
EM: dpfd@nigl.nerc.ac.uk
AF: NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG United Kingdom
AB: The spatially associated, High-K, I-type Garabal Hill, Glen Fyne and Arrochar intrusions (ca 425 Ma) intrude Dalradian metasediments. Major rock types are porphyritic gabbro and K-feldspar-phyric granite. Minor rock types include ultramafic rocks, diorite, quartz monzodiorite, granodiorite, pegmatite, leucogranite and granophyre. The granitoids contain igneous, microgranular enclaves (typically rounded and < 10 cm across) that are more mafic than their hosts. Geochemical data, portrayed on Harker plots, suggest that all these rocks could be related by some fairly simple processes such as crystal fractionation and/or magma mingling and mixing. In such models, the mafic to intermediate enclaves are commonly interpreted as intermediates in the mixing process - hybridised remnants of the mafic end member. Here we pose the questions: are these simple models viable and what end members may have been involved? Backed up by detailed petrographic study, we have carried out geochemical and isotope (Rb-Sr and Sm-Nd) analyses of a large number of rocks from the area. These data show that, far from being simple, the magmas were derived from several different mantle source materials, and at least two crustal reservoirs. We show that the enclave suite of the Glen Fyne granite does not form part of the host magmatic lineage. The enclaves represent a group of accessory hybrid magmas produced by mixing between a mantle-derived mafic end member and a Glen Fyne granitic magma. The main trends for the Arrochar and Glen Fyne intrusions are also characterised by mixing between mafic and more felsic end members. Contrary to what might be expected in high-K calcalkaline series, the high K content in the magmas is derived from the mantle end member rather than directly from the crust. This could be a general feature in such suites. Finally, there is significant isotopic variation among the felsic Glen Fyne rocks. A good deal of the geochemical variation in all these magmas was either inherited directly from the protoliths (in the mantle and crust) or was produced by magma mixing. There is little evidence for crystal fractionation, though it locally produced some of the most felsic magmas.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly