The Geology and Petrology of the Cockburn Law Intrusion, Berwickshire

Abstract

X.SummaryThe Cockburn Law intrusion is a small sheeted boss of late Caledonian age, intruded into Silurian rocks of Llandovery age.The following are the chief stages in its formation:—1. The earliest igneous activity was the intrusion of basic rock, now only found as xenoliths. This intrusion was followed at some later stage by—2. The main plutonic magma which assimilated the basic rock and country rock to produce the following hybrids—(a) Hybrid hypersthene diorite, from contamination by the Silurian pelites and greywackes.(b) Hybrid diopside diorite, from contamination by the basic rock.(c) Hybrid pyroxene diorite, from contamination by material from both basic igneous and sedimentary rocks.3. After the consolidation of the marginal hybrids the main body of the magma began to solidify, but had time to differentiate in place to give a composition ranging from adamellite through granite to granophyric microgranite.4. The heat from the intrusion metamorphosed the Silurian sediments to biotite cordierite hornfels which grade insensibly into the hybrid hypersthene diorite.5. Following the consolidation of the plutonic mass there was considerable activity by vapours to form pneumatolytic products, while differentiation of the residual magma produced the aplitic, acidic, and lamprophyric dykes.The intrusion may form a cupola from a bathylith, which may possibly belong to a series of bathyliths lying in a Caledonian direction from Newry to the Firth of Forth.