Contact metamorphic reactions and fluid–rock interactions related to magmatic sill intrusion in the Guaymas Basin

Abstract. Igneous basaltic intrusions into young organic-rich sedimentary basins have a major impact not only on the carbon cycle but also on major and trace element transfers between deep and superficial geological reservoirs. The actively rifting Guaymas Basin in the Gulf of California, which was drilled by the International Ocean Discovery Program during Expedition 385, represents the nascent stage of an ocean characterized by siliceous organic-rich sediments (diatom ooze) intruded by a very dense network of basaltic sills. This study focuses on Site U1546 where the relatively high geothermal gradient (over 200 ∘C km−1) induces early diagenetic transformations in both pore waters and sediments, involving sulfide, carbonate and silica. Geochemical and mineralogical characterizations of the sediment at sill contacts indicate that sulfides and silica polymorphs are the main phases impacted by contact metamorphism, being evident by a transition from opal-CT to quartz and pyrite to pyrrhotite, respectively. Mass balance calculations have been used to estimate mass transfers in metamorphic aureoles. In the top contact aureole, predominantly isochemical metamorphism is reflected by the presence of authigenic quartz and disseminated 20–50 µm sized pyrrhotite crystals, filling primary interstitial space, and partial dissolution of detrital feldspar grains. In the bottom contact aureole, quartz and euhedral pyrrhotite crystals occur, which are up to 4 times larger than those at the top contact. Significant metamorphism of sediments is observed in the lower contact aureole, where plagioclase recrystallizes around the detrital feldspars and locally euhedral pyroxenes are included in patches of carbonate cement; this suggests precipitation from carbon-rich fluids at temperatures (T) higher than 300 ∘C. The lower contact aureole also is more enriched in CaO, Na2O, Fe2O3 and trace elements (Cu, As, Zn, etc.) compared to the upper contact. Based on these petrological investigations, a conceptual model of magma–sediment–fluid interaction is proposed distinguishing top and bottom contact processes. Initial contact metamorphism due to sill emplacement is characterized by dehydration reactions in sediments and crystallization of new minerals. It was followed by carbonate precipitation from the released fluids. At a final stage, the temperature re-equilibrated with the geothermal gradient and the rocks were further altered by hydrothermal fluids.