The origin, evolution and provenance of the Northern Belt (Ordovician) of the Southern Uplands Terrane, Scotland: a heavy mineral perspective

Abstract

The heavy mineral components of Ordovician greywacke turbidites of the Southern Uplands (subduction-accretion prism) Terrane were studied using high-resolution heavy mineral analysis. Diagnostic heavy minerals and the discovery of hitherto unreported species provide a new insight into the interplay of sediment fluxes, tectonic controls and provenance. Each formation is characterized by distinctive heavy mineral suites, indicating that the sediments of successive or laterally-equivalent formations were sourced from different and rapidly-changing lithologies. Appreciable variations were detected even in greywacke packages within an individual formation, indicating that these changes were taking place at a very fast rate. Detritus came from either an eroding Dalradian or a similar Barrovian metamorphic complex and its superjacent obducted ophiolite complex to the northeast, and from oceanic island arcs and collapsing seamounts to the south. The rapid appearance of metamorphic detritus, following a short period of ophiolite stripping, indicates that post-subduction-flip exposure of and erosion into the metamorphic complex was fast, suggesting extensional unroofing. The chemical similarity of detrital garnet with Scottish Grampian garnet may suggest derivation from that complex and, therefore, that the Southern Uplands Terrane has not travelled, sinistrally, more than about 200 km with respect to the Scottish Highlands. The absence of detrital staurolite in the greywackes raises the possibility that stripping either did not reach abundant staurolite levels or that staurolite was a relatively uncommon mineral in the source area. The former explanation is unlikely because, in western Ireland, a similar Dalradian stripping sequence involves the appearance of staurolite in the first arrival of metamorphic detritus. However, possibly, in western Ireland, the Dalradian source was extensionally unroofed to a much deeper structural level than was Scotland in the same time interval. Only the Shinnel Formation shows spatial detrital consistency, indicating that, by Shinnel times, the Dalradian hinterland had achieved a relatively uniform level of erosion. The discovery of lawsonite, as part of a ferroglaucophane-glaucophane blueschist assemblage in the Portpatrick Formation, is important yet enigmatic. Ordovician crosstie-winchite-ferroglaucophane-phengite assemblages are known throughout the northwestern Appalachian/Caledonian belt and may have provided the source for the Portpatrick blueschist-facies detritus. However, a possible source was from the Cadomian blueschist belt of Anglesey, then on the southern margin of the Iapetus Ocean. This is supported by garnets with sodic/calcic amphibole inclusions, unknown in the Dalradian. Perhaps Cadomian ‘terrane fragments’ were rifted from the southern margin of Iapetus to drift towards and collide with Laurentia to shed detritus into the Laurentian margin trench.