Abstract

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 (M3), or uplift and cooling of a high-grade, static, pre-Arenig (M2) 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 (M4) metamorphism.

Whole rock analyses of metasediments from widely differing structural and stratigraphic levels show that the majority of sediments had a relatively uniform Sr87/Sr86 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 Sr87/Sr86 ratios in most of the granites reflect remobilization of crustal material enriched in radiogenic Sr. A close grouping of initial Sr87/Sr86 ratios around 0.717 from most of the granites, regardless of age, is interpreted as indicating a common source region. High initial Sr87/Sr86 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 Sr87/Sr86 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 Sr87/Sr86 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.

First Page Preview

First page PDF preview
You do not currently have access to this article.