The structural, metamorphic and igneous history of the area around Braemar

Abstract

The area is made up of Lower Dalradian metasedimentary rocks intruded by granite ("Younger Granite") and minor intermediate to basic igneous rocks, the order of intrusion being basic to acid. Although the succession of the metasediments cannot be proved, enough evidence exists to show that these rocks were folded to form two primary recumbent nappe structures, partially bound by slides0 The attitude of these structures is compatible with the general Caledonoid pat-tern and they are thought to belong to part of the hinge region of the Ben Lui Syncline. Intense smaller-scale folds in the metasediments are the products mainly of two subsequent deformations, the earlier producing "cross-folds", the later producing folds on Caledonoid axes. Both nappes, however, have a complicated history of deformation, this being borne out by petrofabric studies of the quartzite and granite intrusions of each nappe. The deformation fabric of the granite is often similar to that of the adjacent quartzite despite the fact that the granite appears in the field to post-date most of the folding The fabric of both rock-types corroborates other evidence that the general structure of the upper nappe is simpler than that of the lower. Metamorphic changes in the pelitic rocks can to some extent be correlated with the main phases of deformation. The rocks were firstly regionally metamorphosed, and then (in the area around and to the north-east of Braemar) thermally altered. The resulting zone of foliated horn-felses corresponds with the limit of granite intrusion around the margins of the Lochnagar Granite Complex. The "thermal" minerals are themselves affected by even later deformation - probably contemporaneous with the straining of the granite.