Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny

The Rogart igneous complex is unique within the northern Scottish Caledonides because it comprises an apparent continuum of magma types that records a progressive change in emplacement mechanisms related to large-scale tectonic controls. Syn-D2 leucogranites and late-D2 quartz monzodiorites were emplaced during crustal thickening and focused within the broad zone of ductile deformation associated with the Naver Thrust. In contrast, emplacement of the post-D2 composite central pluton was controlled by development of a steeply dipping dextral shear zone along the Loch Shin Line, interpreted as an anti-Riedel shear within the Great Glen Fault system. The mantle-derived nature of the late-to-post-D2 melts implies that the Naver Thrust and the Loch Shin Line were both crustal-scale structures along which magmas were channelled during deformation. A U–Pb zircon age of 425±1.5 Ma for the outer component of the central pluton provides an upper limit on regional deformation and metamorphism within host Moine rocks. These findings are consistent with the view that a fundamental change in tectonic regime occurred in the Scottish Caledonides at c. 425 Ma, corresponding to the switch from regional thrusting that resulted from the collision of Baltica and Laurentia, to the development of the orogen-parallel Great Glen Fault system.

The structural age and possible origin of the Vagastie Bridge granite and associated intrusions, central Sutherland

The Vagastie Bridge granite, central Sutherland, belongs to a regional suite of igneous intrusions within Moine psammites adjacent to the base of the east Sutherland migmatite complex. An isotopic date of 405 ± 11 Ma, obtained using U–Pb determinations on zircon and sphene taken from the granite, has previously been interpreted as an intrusion age. However, a reassessment of the field relationships demonstrates that intrusion was syn-tectonic with respect to the later stages of ductile Caledonian deformation affecting the Moine rocks. It is suggested that the granite suite magmas originated at depth and that they were channelled into the still active Caledonian ductile thrust zone (the Naver Thrust) which forms the tectonic base of the Naver Nappe. On regional grounds, the interpretation of the c. 405 Ma date as an intrusion age is untenable, which implies that the isotope systematics of the Vagastie Bridge granite require re-investigation.

The structural evolution of the Dalradian of the Central Highlands of Scotland

ABSTRACTThe first section of the paper is concerned with a detailed account of the structure of the classic Schiehallion area. The account is extended to the flat belt via the Glen Lyon-Ben Lawers area. It is demonstrated that major D1 folds involve the whole of the Appin and Argyll groups and now face SE. However, the original D1 folds were probably open upward-facing folds which have been refolded and modified by the dominant D2 deformation. D2 is represented by an antiformal complex which culminates downwards in the Boundary Slide, a zone of strong deformation associated with NW translation of Dalradian over the ‘young Moine’ rocks. D3 has been responsible for the rotation of the originally flat D2 structures into their present ‘steep’ attitudes N of Glen Lyon. D4 is represented by the strike-swing of the lower Dalradian formations, possibly related to early movements on the two major faults that bound the area. The concept of a D1 Tay Nappe which roots in the ‘young Moine’ to the N is retained, although the scale and flat attitude of that fold is regarded as in part a product of D2.

Moine─Dalradian relationships and their palaeotectonic significance

Regional geochemical and lithogeochemical data, when reviewed in relation to the development of the Caledonian orogen in Scotland, indicate that the Moine-Dalradian boundary coincides with a long-lived crustal discontinuity. It is suggested that this boundary follows the original southeastern margin of an old continental slab in which the Lewisian basement was overlain by a thick layer of ‘Old Moine’ metasediments affected by pre-Caledonian (750 Ma or over) deformation and metamorphism. Early Caledonian ‘Young Moine’ sediments on this slab resemble the underlying metasediments in lithofacies but are somewhat poorer in Zr and Y. To the southeast of the boundary, a thick Dalradian succession accumulated in a marine ensialic basin on a thinned basement of granulites and gneisses. In the Dalradian, elements of basic-ultrabasic association are high, especially in the upper Argyll and Southern Highland Groups where they are associated with products of basic volcanicity. Stratabound Ba, Pb and Zn mineral deposits occur widely not far below the volcanic horizon, and magmatism and mineralization are attributed to the opening of a palaeo-oceanic rift within the Dalradian basin. The geochemistry of the Torridonian, Old Moine and Young Moine detrital sediments suggests derivation from a common source dominated by intermediate-acid calc-alkaline rocks. Low to moderate large-ion litho-phile (l. i. l.) element levels suggest variable degrees of depletion caused by deep metamorphism of the source rocks. Isotopic data show that the sourceland was Archaean and early Proterozoic, and suggest that it may have resembled the Ketilidian and pre-Ketilidian of southern Greenland. The geochemical influence of this ancient western sourceland can be recognized throughout the Dalradian succession, constraining models that involve the availability of young island arc or exotic tectonic materials during the filling of the Dalradian basin. The mechanical strength, relatively low density and high heat production of the Old Moine rocks enhanced the contrast between the crustal slab incorporating a thick Moine layer and the main Dalradian basin beneath which this layer is absent. Tectonic and metamorphic develop­ments differed in the adjacent regions during orogeny, and deep discon­tinuities at the Moine-Dalradian boundary acted as conduits for Late Caledonian appinitic and metalliferous granites rising from sub-crustal sources.