Metamorphic and basin fluids in quartz-carbonate-sulphide veins in the SW Scottish Highlands: a stable isotope and fluid inclusion study

Metalliferous (Fe-Cu-Pb-Zn) quartz-carbonate-sulphide veins cut greenschist to epidote-amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE-SW to NW-SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite-ankerite and clays. Both quartz-sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing. Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz-precipitating fluids were moderately saline (4.0-12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz 18O (+11.7 to +16.5) and sulphide 34S (13.6 to 1.1) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low-temperature (TH = 120-200°C) fluids, probably associated with secondary carbonate, barite and clay formation, were also moderately saline (3.8-9.1 wt.% NaCl equivalent), but were strongly enriched in 18O relative to host Dalradian lithologies, as indicated by secondary dolomite-ankerite (18O = +17.0 to +29.0, 13C = 1.0 to 3.0). Compositions of carbonate-forming fluids were externally buffered. The veins record the fluid-rock interaction history of metamorphic host rocks during cooling, uplift and later extension. Early vein quartz precipitated under retrograde greenschist facies conditions from fluids probably derived by syn-metamorphic dehydration of deeper, higher-grade rocks during uplift and cooling of the Caledonian metamorphic complex. Veins are similar to those of mesothermal veins in younger Phanerozoic metamorphic belts, but are rare in the Scottish Dalradian. Early quartz veins were reactivated by deep penetration of low-temperature basin fluids that precipitated carbonate and clays in veins and adjacent Dalradian metasediments throughout the SW Highlands, probably in the Permo-Carboniferous. This event is consistent with paragenetically ambiguous barite with 34S characteristic of late Palaeozoic basinal brines

Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins

The discovery of exhumed continental mantle and hyper-extended crust in present-day magma-poor rifted margins is at the origin of a paradigm shift within the research field of deep-water rifted margins. It opened new questions about the strain history of rifted margins and the nature and composition of sedimentary, crustal and mantle rocks in rifted margins. Thanks to the benefit of more than one century of work in the Alps and access to world-class outcrops preserving the primary relationships between sediments and crustal and mantle rocks from the fossil Alpine Tethys margins, it is possible to link the subsidence history and syn-rift sedimentary evolution with the strain distribution observed in the crust and mantle rocks exposed in the distal rifted margins. In this paper, we will focus on the transition from early to late rifting that is associated with considerable crustal thinning and a reorganization of the rift system. Crustal thinning is at the origin of a major change in the style of deformation from high-angle to low-angle normal faulting which controls basin-architecture, sedimentary sources and processes and the nature of basement rocks exhumed along the detachment faults in the distal margin. Stratigraphic and isotopic ages indicate that this major change occurred in late Sinemurian time, involving a shift of the syn-rift sedimentation toward the distal domain associated with a major reorganization of the crustal structure with exhumation of lower and middle crust. These changes may be triggered by mantle processes, as indicated by the infiltration of MOR-type magmas in the lithospheric mantle, and the uplift of the Brianconnais domain. Thinning and exhumation of the crust and lithosphere also resulted in the creation of new paleogeographic domains, the Proto Valais and Liguria-Piemonte domains. These basins show a complex, 3D temporal and spatial evolution that might have evolved, at least in the case of the Liguria-Piemonte basin, in the formation of an embryonic oceanic crust. The re-interpretation of the rift evolution and the architecture of the distal rifted margins in the Alps have important implications for the understanding of rifted margins worldwide, but also for the paleogeographic reconstruction of the Alpine domain and its subsequent Alpine compressional overprint