The geochemistry of the Mull-Morvern Tertiary lava succession, NW Scotland: an assessment of mantle sources during plume-related volcanism

• Published in: Chemical Geology
• Main Author: Kerr, Andrew Craig
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 1995
• Subjects: GC Oceanography; QE Geology; Mull; North Atlantic
• Online Access: https://orca.cardiff.ac.uk/id/eprint/9566/; https://doi.org/10.1016/0009-2541(95)00009-B

Palaeocene basaltic magmatism on the northwestern coast of Britain, represents one of the earliest manifestations of the arrival and decompression melting of the North Atlantic plume below the region. This study is concerned with the geochemistry of the 1800-m-thick Mull lava succession, and attempts to assess mantle sources and degree of partial melting during volcano development. After the effects of fractional crystallisation, lithospheric and crustal contamination have been accounted for, three successive magma types of asthenospheric origin can be identified: 1. (1) The oldest of these, the Mull Plateau Group, is composed of transitional tholeiitic-alkalic basalts-trachytes with (La/Sm)n≤ 1 and (Sm/Yb)n > 1. Modelling suggests these basalts are the result of variable (6–10%) partial melting of depleted (relative to bulk Earth) garnet-bearing asthenosphere, over a range of depths. The estimated potential temperature (Tp) of the upwelling mantle which underwent decompression melting to produce these basalts, was of the order of 1420–1460°C, confirming the presence of a mantle plume beneath Mull in the early Tertiary. 2. (2) Overlying the Plateau Group are a suite of basalts, the Coire Gorm type, with similar major-element chemistry to the Plateau Group, but with flatter rare-earth element (REE) patterns, and lower levels of incompatible trace elements, than the earlier lavas. These appear to be the product of shallower, more extensive (8–12%) asthenospheric melting, almost entirely within the spinel lherzolite stability field. 3. (3) The Central Mull Tholeiites, the youngest lavas found on Mull, show greater depletion of incompatible trace elements than the Coire Gorm type, and have flat to LREE-depleted REE patterns. Geochemical modelling, using a depleted spinel lherzolite source, suggests that these lavas require at least 12–17% partial melting. Alternatively, they may represent remelts of the mantle source region which had already produced the Coire Gorm type. The transition from melting in the garnet ...