SUMMARY
Four varieties of dolerite-pegmatite occurring in the Whin Sill at Barrasford are described. They are (1) normal pegmatite (NP) which is similar to the dolerite-pegmatites of Teesdale occurring as concordant lenticular lenses parallel to the sill’s upper surface and confined to the upper portion of the sill; (2) intrusive pegmatite (IP) a discordant vein containing autoliths of dolerite; (3) pegmatite (QP) below partially filled vesicles containing quartz and calcite and (4) pegmatite (PP) below amygdales completely filled with pectolite. QP and PP only occur below the amygdales and have the form of basal layers about 10mm thick. The pegmatites developed at an early stage in the consolidation of the sill and QP and PP had vesicular space above them. The filling of the vesicles took place later at lower temperature in two separate hydrothermal episodes yet there is an invarient relationship between pegmatite composition and amygdale fill. A chemical study of the four pegmatites, the normal dolerite (ND), intrusive basalt (IB) and aplite veins (IA) and their component minerals shows that all the rocks except PP lie on the normal differentiation trend of the Whin Sill.
Final consolidation temperatures as indicated by feldspar geothermometry are:- ND 1025°C; NP 713°C; IP 748°C; QP 572°C and IP 590°C. A temperature of feldspar equilibration of 460°C for PP supports the conclusion that PP is the metasomatic product of QP, altered by the solutions that deposited the pectolite. Some of the pectolite has been subsequently replaced by stevensite which has X-ray characteristics of heat treated (400°C) stevensite.
SummaryA stevensite occurring as an alteration product of pectolite in amygdale-like structures within the Whin Sill is described. Data given for the stevensite include optical properties, stain reactions, chemical analysis, X-ray powder data, and D.T.A. curves. The close similarity between many of the properties of sepielite and stevensite is indicated and D.T.A. is shown to be the most satisfactory method of distinguishing between the two minerals.
Synopsis
A regional gravity survey of part of Galloway has defined two major negative anomalies, one centred over the Cairnsmore of Fleet granite and the other over Wigtown Bay. The former indicates that the granite probably continues to a depth of 11 km, has outward dipping contacts, and a sub-surface cupola-like protruberance to the south-west of the exposed portion of the massif. The gravity low in Wigtown Bay is thought to be indicative of an unexposed sedimentary basin which has a depth between 220 and 720 m. This basin extends in a north-westerly direction at least as far as the Cree estuary.
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