The Distribution of Trace Elements in a Scottish Permo-Carboniferous Teschenite and its Lugaritic Differentiate

Introduction

In a recent description of a composite teschenite-picrite sill at Saltcoats in Ayrshire (Patterson, 1945), mention was made of a potash-rich variety of lugarite which occurs as a differentiate of biotite-teschenite. The latter rock was emplaced at an intermediate stage in the intrusion cycle of the sill, being preceded by a flow-banded teschenite and followed by hornblende-picrite. Differentiation of the biotite-teschenite took place in situ and had proceeded to a considerable extent before the picrite was intruded along the middle portion of the former rock, splitting it into an upper and a lower layer. Richness in volatile constituents was a characteristic of the biotite-teschenite magma and facilitated the process of differentiation (Tomkeieff, 1937, p. 85; Smyth, 1913, p. 33). The separation of an alkali-rich fraction which resulted, is seen in the field by the appearance of pink analcitic patches or “blebs” in the teschenite at a distance of a foot or so from the margins. Further from the contacts, where cooling conditions were more gradual, these analcitic areas increase in size and frequently coalesce to form veins, which themselves intrude adjacent areas of biotite-teschenite. The veins are pink in colour, and contain varying amounts of conspicuous acicular crystals of barkevikite and titanaugite. Petrologically they are similar to the lugarites of the Scottish Permo-Carboniferous suite (Tyrrell, 1917). In chemical composition they also resemble the type lugarite, with the important exception that the Saltcoats rock is much richer in potash, although the sum of potash and soda is little different. A close analogy is provided by covite, from Magnet Cove in Arkansas (Washington, 1901). On account of the close relationship of the Saltcoats rock to the Scottish teschenites, it was preferred to name the rock potash-lugarite.