Northmaven Igneous Complex

Significance: the Northmaven plutonic rocks cover an area of 129 square km and were emplaced around 350 Ma (early Carboniferous) after the Caledonian Orogeny had ended. The complex is faulted against older 365 Ma volcanic rocks of the Eshaness peninsula to the west and is truncated to the east by the Walls Boundary Fault.

Emplacement of the complex into Moine and Dalradian rocks involved a long period of magmatic activity during which successive component parts of the complex were intruded. In the North Roe area the complex is composed mainly of the Ronas Hill and Muckle Roe granophyres along with smaller units of diorites and gabbros. Emplacement of these smaller masses occurred before the granophyre so now they are seen as large enclaves within the granophyre.

Locality 1. Ell Wick to Mavis Grind. HU 345 680 to HU 341 683. Diorite and gabbro. Roadside quarry.

At the south end of the cuttings the red leucocratic Eastern Granite has been faulted against diorite. The granite is composed of red feldspar plates and clear quartz blebs cemented by a matrix of granular quartz and feldspar which is sprinkled with smaller specks of dark minerals (aggregates of biotite, chlorite and magnetite). A sill of a basic rock (dolerite or basalt) can be seen cutting the granite.

            Quarrying has mainly been into the fine-grained grey diorite but in part of the quarry coarse gabbro is seen to overlie red granite. The quarry face is also cut in places by veins of pink granite. The diorite is considered to be a hybrid rock produced from a basic (gabbroic) magma by reaction during its emplacement. This reaction within the basic magma was partly from more acid fluids produced by magmatic differentiation processes and with injected material from adjacent reservoirs of granitic magma. The adjacent reservoirs were later intruded as dykes, sheets, the Eastern and Colla Firth granites and the Muckle Roe and Ronas Hill granophyres.

            The rock is frequently broken and crushed due to the proximity of faulting and bands of white or pinkish scapolite are seen to be associated with these NNE-SSW trends in the dislocation of the rock.