Some aspects of the petrology of the Isle of Rhum

The acid rocks of the west of Rhum, previously considered to be one petrological unit designated 'granophyre', have been divided into two petrographically and geographically distinct units; a core of microgranite is surrounded by an outer zone of graphophyre. Along rost of its northern boundary the graphophyre has been thrust over Torridonian Sandstone. Along a short length of the contact, however, no faulting has occurred and the graphophyre grades into the Torridonian Sandstone through a belt of transitional rocks. These transitional rocks are divisible into five zones which are interpreted as marking successive stages in the conversion of the Torridonian into graphophyre. The graphophyre grades inwards into the microgranite which is believed to represent a still more advanced stage of metasomatism. The production of the microgranite from the Torridonian has been achieved by the enrichment of the latter In Al, Na, Fe, Ca, K, Ti, 1 and P, at the expense of Si. The excess Si has been driven outwards and fixed as an acid front in the bleached and indurated Torridonian Sandstone to the north. Seven small dolerite sills and several basalt dykes had been intruded into this part of the Torridonian before Its conversion to microgranite and graphophyre. These basic intrusions have suffered considerable metamorphism involving the addition of various constituents, principally Na. The petrography and internal structures of the basic plutonic rocks of the west of Rhum are described; the harrisite mass is divided into six zones and the gabbro into two. The lower zone of the gabbro conistc of an earlier coarse gabbro and a later fine gabbro. The fine gabbro was injected from below into almost horizontal parting planes in the coarse gabbro and forms a series of sheets the members of which are a few inches thick and a few inches apart. This alteration of fine and coarse layers gives the lower zone of the gabbro a highly banded appearance. The contact between the grahophrre and the basic plutonic rocks to the east is everywhere marked by a narrow zone of hybrid rocks against which both the basic lutonic rocks and the graphophyre have been metamorphosed. These hybrids are interpreted as highly metamorphosed fragmental rocks occurring along a faulted juntion up which streams of hot gases were passing. The hybrid rocks are believed to have been of both cataclastic and pyroclastic derivation; although many fragments were derived prom the wall rocks locally, some were transported for considerable distances. The southern and central parts of the narrow zone of hybrid rocks are believed to mark the continuation of the ring fault which is well known in eastern Rhum. The northern hybrid rocks were probably developed along a fault which intersected the western continuation of the ring-fault tangentially. A small Tertiary vent, which cuts the ring-fault in the east of Rhurm, is described. The hyrbrid rocks which occupy this vent are very similar to those found between the graphothyre and the basic plutonic rocks in the west of the island but they are somewhat finer in grain. The hybrids in the vent are fragmental rocks highly metamorphosed by rising gases at temperatures in excess of 1,0000C. These gases have enriched the vent rocks in K, Na, C, Al, Si, and P. and the contiguous Torridonian Sandstone in the same elements togeher with Fe, Mg and water. The rocks of the four Tertiary volcanic outliers in the west of Rhum have been correlated and a succession, nearly 1,700 feet in thickness, has been synthesised from the combined data. The volcanic rocks have been shown to lie unconformably upon the grathophyre. The post-granitic minor intrusions of the west of Rhum have been divided into Six distinct rock types. Two of these, viz. mugearite-tachylyte and augite-andesite, have not previously been recognised in this part of the island