Boulder-bearing conglomerates from the Lower Old Red Sandstone (ORS) Crawton Group in east-central Scotland contain large granite clasts (0.4-1.0 m in diameter) which were derived from an area of the Midland Valley now concealed by younger rocks. These clasts provide new information concerning early Paleozoic magmatism along the margin of Laurentia. Rb/Sr mineral-whole-rock age dating for a series of calcalkaline tonalite, granodiorite, and granite boulders produces a cluster of late Wenlock-early Ludlow ages (ca. 420 Ma) which are interpreted as recording the original emplacement of hinterland plutons. The clast ages extend back to the late Ashgill (443 Ma), and the magmatism identified thus bridges a gap between an older Ordovician detrital record of coeval magmatism in the Midland Valley, and in situ ORS (Late Silurian/Early Devonian) volcanic rocks in this zone. Evidence from hornfels clasts and xenoliths within igneous boulders suggests that some of the granitoid intrusions were emplaced into an unseen, Lower Ordovician flysch sequence from which graywacke and limestone clasts were also derived. Textural and other evidence indicates that the source plutons rose to a high structural level in the crust. Both foliated and unfoliated granites have been distinguished, but these have no straight-forward relationship with Rb/Sr age. The foliated tonalite clasts are unlikely to have been derived from an autodeformed marginal facies, and the association of synchronous deformed and undeformed granitoids resembles the pattern of emplacement in areas undergoing transcurrent shear; this is consistent with evidence suggesting that juxtaposition of the northern Midland Valley and Grampian terranes was largely achieved during this interval. The cryptic igneous rocks share some features with in situ late Caledonian granites with which they overlap in age (for example, high Na, initial low 87Sr/86Sr), but differ in that tonalitic compositions and uniform positive ϵNd values were developed. The isotopic distinction between Silurian Midland Valley and Grampian Highlands magmas probably reflects the disparate lithospheric structure of the terranes now separated by the Highland Boundary fault and is a robust feature of subsequent magmatism, persisting for at least 70 m.y. Temporal changes in magmatism associated with the Midland Valley during the Ordovician to mid-Silurian may have been influenced by thinning of previously thickened arc crust.

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