Structural setting and petrogenesis of Silurian granites in the Caledonides of northern Scotland
In the Ordovician-Silurian Caledonian orogenic belt of northern Scotland, two major thrust
nappes, the Moine and Naver nappes, contain a series of granitic plutons. The Rogart,
Strath Halladale and Helmsdale granites have been studied using field and microstructural
fabric analysis. Their emplacement mechanisms and pre, syn or post-tectonic status with
respect to phases of Caledonian ductile deformation have been detailed. The petrogenesis
of the Strath Halladale and Helmsdale granites has been examined within the framework of
a regional geochemical study involving nine Caledonian high Ba-Sr granitoids.
The foliated Strath Halladale Granite comprises a series of easterly-dipping sheets that
were intruded into structurally high parts of the Naver Nappe in northeastern Sutherland.
The granite contains magmatic-state shear zones that consistently show top-to-the- W-to-
NW sense of movement; it was emplaced during Caledonian (02) west-directed thrusting.
The Rogart Granite was emplaced into the footwall of the Naver Thrust in southeastern
Sutherland. It comprises early, sheeted quartz diorites that carry magmatic to solid-state
fabrics formed by thrust-related, west-directed ductile deformation and a central igneous
quartz monzodiorite-granodiorite-granite complex that ballooned into a tectonically created
void formed along the Strath Fleet Lineament after D2 thrusting. The emplacement of the
Rogart Granite encompassed the switchover from west-directed thrusting to strike-slip
tectonics in this area. The Helmsdale Granite was emplaced into the highest parts of the
Naver Nappe in eastern Sutherland. It does not contain any emplacement-related or
tectonically-induced fabrics and is post-tectonic with respect to Caledonian deformation.
Its emplacement mechanism and hence the Caledonian significance of the adjacent
Helmsdale Fault remain speculative.
Geochemistry confirms that the Strath Halladale and Helmsdale granites are part of the
Caledonian high Ba-Sr granites (sensu Tarney & Jones 1994) and indicates melt evolution
by assimilation fractional crystallization (AFC) involving Moine metasediment.
Associated mafic rocks do not form parents to the granites but represent a variety of
genetically-related cumulates. Nine Caledonian high Ba-Sr plutons in northern Scotland
show a systematic isotopic variability. Plutons with the highest 87Sr/86Sr(i)have the lowest
ENd(i) and the highest 8180, whereas plutons with lower 87Sr/86Sr(i)have complementary
higher ENd(i) and lower 8180. Syenite-dominated complexes evolved by assimilation
fractional crystallization (AFC) involving depleted granulite-facies basement whereas the
granites assimilated Moine metasediment. Combining available age data with the observed
isotope systematics suggests, that the syenites recorded progressive (source) enrichment
over c. 30 Ma followed by a short-lived, isotopically-diverse, magmatic pulse at c. 425 Ma.