Patterns of Nd and Sr isotopic ratios produced by magmatic and post-magmatic processes in the Shiant Isles Main Sill, Scotland

Abstract

The Shiant Isles Main Sill of the British Tertiary Igneous Province is a classic example of a differentiated, alkaline basic sill. Four separate intrusions, each emplaced internally in rapid succession, form a 165-m-thick sill hosted by Lower Jurassic sedimentary rocks. Extensive Nd and Sr isotopic studies were conducted on samples from a vertical section through the sill where the relationships of samples to one another are well defined. The results illuminate patterns of modification of isotopic ratios and clarify the petrogenesis (magma sources, crustal contamination), magmatic processes (bulk mixing, interstitial liquid mixing), and post-magmatic alteration (hydrothermal effects on Sr and Nd). Overall, the whole-rock initial 87Sr/86Sr ratios range from ∼0.7037 to 0.7061 while initial 143Nd/144Nd ratios vary from ∼0.51243 to 0.51286 (ɛNd∼−0.7 to +5.7) – values that contrast markedly with those of the country rock. Acid leaching (HCl) of the whole-rock samples that removes analcime indicates that most of the scatter in the 87Sr/86Sr is caused by the ubiquitous sub-solidus, aqueous alteration during which more-radiogenic Sr was introduced into the sill, especially along the margins, and also reveals magmatic isotopic ratios. In contrast, Nd was immobile during fluid interaction so that the sill 143Nd/144Nd ratios were not affected, even <1 m from the country-rock contact. Using leached rock values, 87Sr/86Sr and 143Nd/144Nd ratios are inversely correlated from magmatic processes. Magmas with two distinct isotopic compositions were involved: a more primitive one with 143Nd/144Nd ∼0.51285 and 87Sr/86Sr ∼0.7035 that produced the first two intrusions and a more evolved one (with 0.51252 and 0.7048) that produced the third intrusion. Mixing of the two magmas was very limited, restricted to near contacts between units, and apparently occurred by interstitial melt migration. The more evolved crinanitic magma was probably produced from a batch of the more primitive picritic melt by a small degree of crustal contamination and crystal fractionation during a short crustal residence prior to ascent and emplacement.