Petrological studies of the lavas and associated rocks of the Arthur's Seat volcano

Abstract

Arthur's Seat is the remains of a dissected Lower Carboniferous volcano, from which were erupted a series of basaltic rocks representative of the province to which it belongs. The rocks occur as lavas, intrusions and tuffs, and include basalts of Dalmeny, Jedburgh, Dunsapie, Craiglockhart and Markle types, in addition to mugearite. The Dunsapie basalts have been subdivided into Normal Dunsapie and Feldspathic Dunsapie types.The volcano has been remapped on a scale of 15 inches to 1 mile, and a few minor departures from the 6- inch Geological Survey map have been recorded. These include changes in the numbering and classification of some of the lavas, an alteration to the position of a fault, and slight adjustment of a vent boundary.The petrographic characters of the various volcanic rocks have been described in detail, with modal and chemical analyses. Particular attention has been given to the augite phenocrysts of Craiglockhart basalt, and a method of cutting serial sections through single crystals has been devised. Evidence indicates that the augites have been corroded, and then have re grown prior to eruption.The parental magma of the volcano was probably of a composition very similar to that of Normal Dunsapie basalt. Markle and Craiglockhart types were produced as complementary differentiates of the parental liquid by a process of gravity differentiation, in which sinking of ferromagnesian crystals, particularly augites, played an important part.The Dalmeny and Jedburgh basalts, which have strong chemical resemblances, probably represent only slightly differentiated parental magma. Some sinking of olivine may have occurred in the pre-eruptive history of these microporphyritic types, and transfer of feldspar components by volatiles and establishment of a composition gradient under gravity may also have had some effect in producing the slight differences in composition which have been noted. The less pyroxenic Dalmeny and Jedburgh rocks are ophitic, and their plagioclase crystals show a well developed flow structure which passes through the ophitic augites. In such cases it is inferred that the augite did not crystallise until a late stage in the crystallisation history of the lava.Concentrations of soda-rich volatiles produced intense albitisation in the# Markle basalts and the mugearites. It is considered that the mugearites of Arthur's Seat initially crystallised as Jedburgh type, and were altered to their present state during the deuteric stage.The magma chamber believed to have underlain the volcano was charged with parental magma at least twice during the period of activity. The first and last eruptions were of Dunsapie basalt; the albitised products appeared at a fairly late stage,(see Table summarising Eruptive History).A macroscopic method of fabric analysis has been evolved, and fabric analyses of representative specimens made. both linear and planar fabrics, and sometimes both, have been shown to occur.