PROVENANCE RECONSTRUCTION OF ORDOVICIAN TURBIDITES IN THE SOUTHERN UPLANDS OF SCOTLAND

The Southern Uplands accretionary complex grew southwards from the Mid-Ordovician subduction flip to the Late Silurian closure of Iapetus. Heavy mineral assemblages in Ordovician greywacke turbidites that become younger southwards allow reconstruction of the provenance and evolution of the Complex and the interplay between tectonics and the flux from varying source provinces. Each formation is characterized by distinctive heavy mineral suites, indicating that the sediments of successive or laterally-equivalent formations were sourced from differing and rapidly changing lithologies. From north to south: The Marchburn/Corsewall Formations, rich in Fe, Ti, and La, contain little metamorphic detritus but large amounts of epidote, hornblende and serpentine, suggesting the unroofing of the upper part of an ophiolite complex (probably Ballantrae and/or its lateral equivalents). In the Kirkcolm Formation, the zircon, apatite and tourmaline content rises with increasing amounts of chrome spinel and garnet, indicate the stripping of deeper parts of an ophiolite sheet and larger areas of Grampian metamorphics. In the overlying/interfingering Blackcraig Formation, epidote and hornblende dominate with increased amount of serpentine and very little metamorphic detritus, suggesting a very complex drainage pattern across an eroded ophiolite sheet with some distributaries disgorging mainly ophiolite, others mainly Grampian metamorphic detritus. Some Blackcraig turbidites were generated probably by long run-out submarine landslides initiated by the sector collapse of accreting seamounts. The Portpatrick and Shinnel Formations are high in Cr, MgO and Ni with increasing amounts of metamorphic detritus but also strongly calc-alkaline clinopyroxenes, indicating, at least partial, derivation from an arc. High-pressure index lawsonite, prehnite, pumpellyite and glaucophane in the Portpatrick Formation indicate an exhumed subduction complex in the source province (probably Newfoundland or western Ireland). The Shinnel Formation records two separate provenance events, one conveying sediments with higher amount of metamorphic components, the other with material from ophiolitic parentages.