Further Observations on the Ballantrae Igneous Complex, South Ayrshire

Extract

In the southern belt of the Ballantrae serpentine, stretching south-west from Millenderdale, there occur many exposures of altered and more or less recrystallized dolerite and gabbro. Some of these rocks have afforded considerable difficulty to previous observers and they have been variously interpreted. Thus Dr. G. W. Tyrrell, in an interesting paper published twenty-five years ago (Trans. Geol. Soc. Glas., xiii, 283), regarded some of the rocks on Littleton Hill as “relics of an older mass of dolerite or gabbro” that had “become enveloped by the later serpentine”. But this expression of opinion does not appear to have been founded upon conviction, since quite recently Tyrrell states (Proc. Geol. Assoc, xliv, 59) “contact metamorphism presumably by the enveloping serpentine seems to have been developed in rocks on Littleton Hill which may originally have been spilites and diabase porphyrites”. At the time of writing a previous paper (GEOL. MAG., LXIX, 1932, 107) I hadalready inferred that the metamorphic rocks seen on Littleton Hill must have been derived from dolerites and gabbros, but the nature of their association with the serpentine was then merely a matter of surmise. In continuing field work in the province, therefore, I keptin mind the requirement to determine, if possible, the true geological relationships of these rocks, and to that end I have re-investigated carefully the whole southern outcrop of the serpentine and its accompanying metamorphic masses. A short account of the results of the field and laboratory work is given below. Firstly, however, I take leave to acknowledge the very helpful character of Mr. A. G. Macgregor's researches on felspar clouding, “a characteristic effect that is often produced in the fresh plagioclase felspars of igneous rocks by thermal metamorphism” (see Min. Mag., xxii, 524).