Unravelling the hydration history of an inverted passive continental margin using epidote U[sbnd]Pb geochronology and Pb–Sr–O–H isotope geochemistry

Abstract

Inverted passive continental margins permit to comprehensively characterize fluid circulation in the continental crust in orogens from rifting to tectonic inversion. Since granitic rocks have low primary water contents and dominate the continental crust, their hydration exerts important control on metamorphism and deformation of the continental crust during orogeny. It is therefore of great interest to determine whether the granitic continental crust is hydrated during rifting phases or after tectonic inversion. Because fluid circulation may occur at both stages, a comprehensive picture of hydration of the continental crust in inverted passive continental margins can be obtained by the combination of different geochronometers and geochemical methods.In this study, we combine UPb geochronology and Pb–Sr–O–H isotope geochemistry of epidote in hydrothermal veins in the Albula Pass area (eastern Swiss Alps) as powerful tools to trace hydration events in the continental crust in orogens. The Albula Pass area is part of the inverted Adriatic passive continental margin, which was extensively affected by seawater infiltration during continental rifting in Jurassic times. Epidote geochronology elucidates the fluid circulation history in this rifted crustal section by revealing two hydration events that are not recorded by other datable minerals: (1) 85.2 ± 9.7 Ma and (2) 59.9 ± 2.7 Ma, showing that fluid circulation in the Adriatic passive continental margin continued after Late Cretaceous tectonic inversion. Epidote Pb–Sr–O–H isotope geochemistry characterizes pathways and fluid sources of the epidote forming fluids, which differ between the Late Cretaceous and the Paleocene fluid circulation events. The Late Cretaceous epidote-forming fluids were produced by compaction of sedimentary units beneath the Err nappe and release of modified seawater (i.e., formation/connate water) from marine sediments, while Paleocene veining was mainly mediated by syn-kinematically infiltrated meteoric water. The present geochronological and isotope data argue for multi-stage hydration of the continental crust in the inverted Adriatic passive continental margin. This work promotes the importance of the interplay of newly introduced and recycled fluid components in the hydrothermal alteration of the continental crust, and it highlights epidote as a powerful hygrochronometer and isotope tracer.