THE PETROLOGY OF THE LITTLE WHIN SILL, COUNTY DURHAM

Summary Core from the Rookhope borehole and suites of samples from natural outcrops have been used in a detailed investigation of the petrology and chemistry of the Little Whin Sill. Orthopyroxene (Fs 23–27 ), plagioclase, clinopyroxene and olivine (Fa 15 ) occur as phenocrysts; this is the first report of fresh olivine. The groundmass is composed of clinopyroxene (Ca 45 Mg 51 Fe 4 -Ca 38 Mg 38 Fe 24 ), plagioclase (An 70–35 ) and iron-titanium ores. Eleven new analyses for major elements are presented, together with a new analysis of the White Whin. Statistical analysis of trace element data suggests that only Mn and Sr show significant variation within a given section across the sill, but these elements and Rb show significant variation between sections. A comparison of the chemistry of the Great and Little Whin Sills suggests that the latter is poorer in SiO 2 , K 2 O, and Rb than the Great Whin Sill but carries more CO 2 . The White Whin, consisting of Ca-Mg-Fe-carbonates, illite, kaolin, plagioclase, anatase and quartz, is shown to be chilled whin to which CO 2 , H 2 O and K 2 O have been added, and from which Na 2 O, CaO, MgO and Fe have been lost. It is suggested that the Little Whin Sill represents the initial magma of the whole Whin Sill complex, the high CO 2 content being a result of later alteration. The chemistry of the whin suite of intrusions has characteristics intermediate between tholeiitic and alkaline dolerites, and the initial magma may itself be a differentiate. Although crystal settling is absent, a trend towards iron enrichment is found, with an insrease in the ratio FeO : Fe 2 O 3 towards the centre of the Little Whin Sill. In addition there may be another trend toward silica enrichment in a lateral direction.