The geochemistry of Dinantian volcanism in south Kintyre and the evidence for provincialism in the southern Scottish mantle

Abstract

Scottish Dinantian transitional to mildly alkaline volcanism is represented by abundant outcrops in the Midland Valley, Southern Uplands and Highlands provinces. Dinantian volcanic rocks from Kintyre in the Scottish Highlands range in composition from basalt through basaltic hawaiite, hawaiite, mugearite and benmoreite to trachyte, the compositions of the evolved types being largely due to differentiation from the basaltic parents.Recent geochemical investigations of Scottish Caledonian granitoids, Siluro-Devonian Old Red Sandstone (ORS) lavas and xenolith suites from numerous vents and dykes of Permo-Carboniferous to Tertiary age have revealed that the Scottish crust and upper mantle both increase in age and are increasingly enriched in incompatible elements towards the north and northwest. The upper mantle and lower crust below the Highlands province are therefore largely considered to be more enriched and in parts older than those below the Midland Valley and Southern Uplands. Dinantian alkali basalts from these latter two provinces have eNd values predominantly in the range + 3 to +6, initial S7Sr/86Sr values of 0.7029-0.7041 and 2~ 2~ values of 15.48-15.60. However, similar basalts from Kintyre and Arran in the Highlands have lower eNd (+ 0A to +3.4) and 2~176(for given 2~176 ratios; 15.49-15.51) and slightly higher 87Sr/86Sr (0.7033-0.7046). This regional variation correlates well with the differences seen between Midland Valley and Highland magmas in the ORS calc-alkaline suite (Thirlwall 1986) and it is suggested that both the ORS and Dinantian basic rocks are derived from similar types of mantle, although no lithospheric slab component is present in the later Dinantian suites. Isotopically-distinct portions of mantle therefore appear to have been present below the Highland and Midland Valley-Southern Upland provinces from at least Caledonian to Carboniferous times. The combined incompatible element and Sr-Nd-Pd isotopic evidence from Kintyre and Arran basaltic rocks does not allow unequivocal distinction between a lithospheric mantle and a sublithospheric mantle source. The observed correlation between isotopic composition of Dinantian basalts and the chemical composition of the lithosphere, together with the apparent involvement of long-term separated source reservoirs suggests that Kintyre and Arran lavas were derived largely from a lithospheric mantle source. On the other hand, the isotopic enrichment of Kintyre basaltic rocks is not extreme; trace element and isotopic compositions are still comparable to modern OIB. Sublithospheric mantle could therefore also be a viable source for Kintyre and Arran Dinantian volcanism.

Sr‐Nd‐Pb isotope systematics of the Banda Arc, Indonesia: Combined subduction and assimilation of continental material

“…Sr and Nd isotopes were separated with standard ion exchange techniques and measured on a fixed-multicollector Finnigan MAT261 mass spectrometer at the Free University of Amsterdam. Double-jump experiments were applied [Smedley, 1988].…”

Primitive olivine melanephelinite dykes from the Orkney Islands, Scotland

“…4) the data plot on the mantle array with e Nd = 3.4-4.4, suggesting a magma source similar to that of ocean island basalts. The samples lie within the field defined by the majority of Carboniferous lavas from the Midland Valley of Scotland, suggesting an isotopically similar source (Smedley, 1988, fig. 7), but distinct from the lavas of Kintyre and Arran.…”

“…e Nd v. e Sr diagram showing Orkney dyke data(Table 4)in relation to: (A) Lower Carboniferous (Dinantian) basalts of the Scottish Midland Valley; (B) Lower Carboniferous (Dinantian) basalts of Kintyre and Arran; and (C) Upper Carboniferous (post-Dinantian) and Permian basalts of south and central Scotland. (A), (B) and (C) after data inSmedley (1988) and S. M. Wallis (unpub. Ph.D. thesis, Univ.…”

Carboniferous and Permian magmatism in Scotland

“…Smedley attributed this to the absence of any contribution to the Dinantian mantle sources from a relict subducted slab, indicating effective convective stirring in the mantle source in the 60-70 Ma interval between the two periods of magmatism. In a subsequent paper, Smedley (1988b) noted small but significant differences between the basalts from north of the Highland Boundary Fault and those from the SMV; basalts from Kintyre have slightly lower eNd and slightly higher 87Sr/S6Sr ratios than those from south of the fault. She suggested that the isotopic distinction implied that the Highland mantle was slightly enriched relative to that beneath the SMV, and that there may be a lithospheric component to the magmatism north of the fault; nevertheless, she still regarded the principal magma sources as being sub-lithospheric.…”