Dolerite-pegmatites from the Whin Sill near Barrasford, Northumberland

Abstract

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.