C–O–H–N fluids in quartz segregations from a major ductile shear zone: the Berzosa fault, Spanish Central System

Abstract

Abstract The Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher‐grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al‐silicates) segregations, rich in fluid inclusions. Host rocks are medium‐grade staurolite schists and sillimanite gneisses.Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late‐peak metamorphic conditions and lasts until late in the uplift history of the zone.Three types of inclusions have been found, which in a time sequence are: CO2± H2O; H2O+salt (B‐type); and, N2+CH4. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic‐recovery phase (B1‐type), formed later as intergranular trails (B2‐type), or were trapped apparently along with N2+CH4 in clusions from a heterogeneous fluid (B3‐type).Considerations from isochores confirm that CO2± H2O inclusions were trapped during late‐peak and high‐T retrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N2+CH4 inclusions, along with the B3‐type of inclusions, formed at low‐pressures (<1 kbar) and temperatures (± 300°C). B2‐type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. Inferred P‐T paths for the area are convex to the T‐axis.