Major shear zones and autochthonous Dalradian in the north-east Scottish Caledonides
The Buchan area within the Grampian Caledonides of north-east Scotland is a classic locality for the study of low-pressure regional metamorphism. Using detailed geophysical surveys, pits and boreholes to augment sparse outcrop data, we have revealed a system of steep shear belts of Grampian age. When combined with a reassessment of the stratigraphy, the new information leads us to the view that the Banff nappe does not exist. We also argue that the Tay nappe is of limited extent in Buchan, and that the tract of uninverted Southern Highland Group (upper Dalradian) is autochthonous.
Age Relations and Provenance of the Dalradian Series of Scotland
Rb-Sr and K-Ar isotopic measurements from Dalradian metasediments and associated intrusions of the Caledonian belt of Scotland indicate a magmatic and metamorphic history extending over 130 m.y. Magmatic events took place approximately 530, 440, and 400 m.y. ago. The mid-Cambrian activity included the formation of minor granitic bodies (previously referred to as the Older Granites), as well as nonfoliated granites and pegmatites. Microcline granites and pegmatites were emplaced 440 m.y. ago during Ordovician times. The last phase of Caledonian magmatic activity took the form of forcefully and passively emplaced postorogenic granites 400 m.y. ago. Two regional thermal events at 440 to 460 m.y. and 400 to 420 m.y. ago are recorded. The former event is discussed in terms of either a widespread retrograde metamorphism (M 3 ), or uplift and cooling of a high-grade, static, pre-Arenig (M 2 ) event. The 400 to 420 m.y. event was restricted to the westerly margins of the present Dalradian outcrop and is thought to be associated with a younger (M 4 ) metamorphism. Whole rock analyses of metasediments from widely differing structural and stratigraphic levels show that the majority of sediments had a relatively uniform Sr 87 /Sr 86 ratio of about 0.719 at time of deposition. A whole rock isochron of 670 ± 30 m.y. is interpreted as either an approximate or maximum depositional age for the bulk of the Dalradian sedimentary rocks. Unusually high initial Sr 87 /Sr 86 ratios in most of the granites reflect remobilization of crustal material enriched in radiogenic Sr. A close grouping of initial Sr 87 /Sr 86 ratios around 0.717 from most of the granites, regardless of age, is interpreted as indicating a common source region. High initial Sr 87 /Sr 86 ratios of approximately 0.730 from two granitic masses may reflect remobilization of the Dalradian sedimentary rocks during regional metamorphic activity. Although the 400 m.y. passively and forcefully emplaced granites cannot be distinguished chronologically from one another, distinct differences in their initial Sr 87 /Sr 86 ratios of 0.717 and 0.707, respectively, may indicate distinct petrogenetic differences. Because of the variable distribution of radiogenic Sr between the Dalradian sediments and lower crustal levels, the initial Sr 87 /Sr 86 ratios of the magmatic intrusions can be used as an indicator of their relative depth of formation. A working hypothesis is outlined for the region including a series of magmatic and metamorphic events 530, 440, and 400 m.y. ago.
Polyphase generation of nappe structures in the Dalradian rocks of the southwest Highlands of Scotland
Synopsis A regional synthesis of the Dalradian structures in the South-west Highlands of Scotland is attempted, based on the conclusion that the Iltay Boundary Slide passes into the Ballachulish Slide as it is traced around the Glen Orchy Antiform and the Stob Bhan Synform. On this framework the primary and secondary folds are traced throughout the South-west Highlands within the constraints imposed by the stratigraphy and structural knowledge. The primary folds associated with the development of the Tay Nappe at a high structural level are themselves affected by secondary folds at a deeper level. These folds define the Glen Orchy Nappe with an amplitude exceeding 25 km, underlying the Tay Nappe. The primary structures are traced through the secondary structures to root at depth within the Moine rocks lying between the Glen Orchy Antiform and the Kinlochleven Antiform. It is concluded that the polyphase development of nappes in the SW Dalradian is essentially a superficial phenomenon.