Synkinematic contact metasomatism: implications for the timing of pluton emplacement and regional deformation in the Scanlon shear zone, south‐eastern California

Structures in rocks altered by the infiltration of magmatic fluids provide key constraints on the relative timing of plutonism and regional deformation in the central Old Woman Mountains, south‐eastern California. In this well‐exposed area the Scanlon shear zone, a belt of attenuated, shallowly dipping, amphibolite facies Palaeozoic strata, is in contact with two tabular, Late Cretaceous (∼74 Ma) granitoid plutons. The shear zone contains massive wollastonite‐bearing skarns ∼40 m above the contact with the Old Woman granodiorite. Field evidence, petrological data and stable isotope compositions indicate that the mineral assemblages in these skarns formed when the granodiorite crystallized and exsolved water‐rich, magmatic fluids. Structural features in the skarns, which include transposed wollastonite foliations, syntectonic vesuvianite and garnet porphyroblasts, and quartz ± wollastonite veins, afford an opportunity to monitor the regional strain at the time of pluton crystallization. These structures yield a broad range of timing relationships that indicate synkinematic mineral growth with deformation ending first, at c . 74 Ma. The metasomatic structures are kinematically compatible with the regional deformation observed in other lithologies and no post‐tectonic features overprint the fabrics in the skarns. Observations of isoclinally folded and boudinaged dykes and pegmatites, granodiorite saddle reefs in isoclinal fold hinges, and undeformed, locally pegmatitic dykes also yield a broad range of timing relationships indicative of synkinematic plutonism, with deformation ending first. Our results show that coupling petrological and stable isotope data with structural analysis is effective for unravelling the relative timing of heat and mass transfer processes in pluton‐wallrock systems.