Petrology and geochemistry of upper carboniferous - lower permian volcanic rocks in Scotland

The volcanic rocks considered in this thesis represent the later stages of a volcanic event which affected southern Scotland throughout the Carboniferous and early Permian. Magmatism was dominated by alkaline to transitional compositions, with a short tholeiitic period in the late Stephanian - early Permian. Both groups are relatively unevolved: the alkaline samples range from basalt and basanite to hawaiite, and the quartz dolerites from basalt to rare basaltic andesite. Phenoncryst assemblages comprise olivine ± clinopyroxene with occasional plagioclase in the more evolved samples. Clinopyroxenes are diopsidic salites or augites and many show appreciable Ca-Tschermak's molecule substitution reflecting the degree of magma undersaturation. Al"/AI" ratios attest to a range of crystallisation pressures with the highest pressures (10-20kb) indicated by clinopyroxenes from the Highland dykes and Fife & Lothian basanites. The preservation of high-pressure phenocrysts and olivine and clinopyroxene xenocrysts indicates high rates of ascent for many of the magmas, with little or no residence time in high level magma reservoirs. Major and compatible trace element concentrations have been controlled by fractional crystallisation, constrained by calculations to <36%. Incompatible trace element and REE concentrations indicate a narrow range of melting for the alkaline samples. Y and HREE concentrations suggest buffering by residual garnet, and pronounced negative K and Rb chondrite-normalised anomalies Imply that phlogopite was also a minor residual phase. Flatter chondrite-normalised profiles for the quartz dolerites indicate a larger degree of melting for these magmas, although average Y and HREE contents are very similar to those of the alkaline samples, suggesting that melting had not proceeded to a point at which garnet was consumed. However, a trend of increasing Ce/Y with increasing Zr/Nb for these samples apparently contradicts this observation. Partial melting models suggest that the post-Dinantian suite must represent the result of mixing between melts from at least two sources. The absence of a hot-spot trail across the Midland Valley despite a long period of intraplate volcanism and rapid plate movements argues against the involvement of a deep-seated mantle plume. Lithospheric mantle sources require impossibly large degrees of melting to produce realistic incompatible element concentrations (and are unlikely to melt without melting of the asthenosphere). By elimination it is suggested that the post-Dinantian basalts represent partial melts from a heterogeneous (streaky) asthenospheric source. Relatively low ⁸⁷Sr/⁸⁶Sr ₜ ratios (0.7032-0.7049) and high ¹⁴³Nd/¹⁴⁴Nd ₜ ratios (0.5122-0.5126) indicate that crustal contamination has been of minor importance. Isotopic and elemental variations within the post-Dinantian samples are consistent with variable degree melting of a heterogeneous (streaky) asthenospheric mantle source. However, trends of increasing ⁸⁷Sr/⁸⁶Sr and decreasing ⁴³Nd/¹⁴⁴Nd with progressive melting suggest minor lithospheric contributions to the Mauchline melts. Comparison of the post-Dinantian compositions with data for the Dinantian groups allows the identification of a lithospheric component in many of these older lavas. It is suggested that they represent the result of mixing between small degree (<3%) partial melts from the asthenosphere and even smaller degree melts (<1%) from the lithosphere. A synthesis of magmatism in the Midland Valley throughout the Carboniferous and Permian suggests that during the Dinantian, asthenospheric melts were affected by significant lithospheric mantle interaction. The younger larger-scale melting event which produced the quartz dolerites effectively exhausted any remaining enrichments at the base of the lithosphere leaving only refractory phases less likely to contribute to the late Stephanian - Permian melts. The absence of quartz dolerites in Ayrshire allowed the preservation of isotopic enrichments at the base of the lithosphere which were sampled by some of the Mauchline magmas.