The Black Stockarton Moor subvolcanic complex, Galloway

Abstract

The Black Stockarton Moor subvolcanic complex is a composite of intersecting porphyrite dyke swarms, granodioritic sheet intrusions, small granodiorite stocks and breccia pipes of late Caledonian age, emplaced within the Lower Palaeozoic turbidite sequence of the Southern Uplands of Scotland. Evolution from the subvolcanic to the plutonic igneous regimes can be traced in a first phase of activity with the successive intrusion of dykes, sheets, stocks and the adjacent Bengairn and Criffel plutonic complexes. The minor intrusions were partially preserved from obliteration by the subsequent plutonism, as the centre of igneous activity shifted progressively from W to E. A second phase of intrusion, post-dating most, if not all, of the plutonic activity, took place within an entirely different stress field producing en echelon sigmoidal porphyrite dykes with maximum dilation roughly at right angles to that of the earlier intrusions. Deep crustal shear is postulated as the source of the tension fractures, which were subsequently filled by the second phase dykes, and also of 2 of the 3 sets of wrench faults which cut the area.