Cracking at the magma-hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus

The nature of the magma-hydrothermal transition in oceanic hydrothermal sys tems is poorly understood, in part because the geological relations in this critical region have rarely been observed in modern ocean crust. Detailed mapping was conducted in the Troodos Ophiolite, Cyprus, where a gabbronorit e sequence intrudes the sheeted dyke complex, which is truncated at its bas e by a thin contact aureole composed of massive hornfels. Geothermometric d ata for hornblende and pyroxene hornfels show that hydrated sheeted dykes w ere recrystallized at amphibolite to granulite facies conditions (778-986 d egrees C). Quartz diorite veins and apophyses, and monomineralic amphibole veins cross-cut the contact aureole and show no preferred age relationships . Geothermometric data indicate that quartz diorite was injected at 817-919 degrees C and that fractures were filled with amphibole at 575-750 degrees C. Phase relations of quartz-hosted, halite-bearing fluid inclusions in qu artz diorite veins constrain minimum entrapment temperatures of 225-520 deg rees C (average 402 degrees C) and minimum pressures that span lithostatic and hydrostatic conditions. We believe that these characteristics are indic ative of a conductive boundary layer that separates an active hydrothermal system from the heat source that drives it. Field and petrological data ind icate that transient fracturing caused oscillations in temperature and pres sure conditions within the conductive boundary layer, and mixing of hydroth ermal and magmatic fluids at the magma-hydrothermal interface. Cross-cuttin g relations between magmatic and hydrothermal vein networks show that fract uring occurred prior to the cessation of magmatic activity. We explore plau sible models for the causes and consequences of fracturing that consider th e role of dyke injection, thermoelastic stresses, and volatile build-up. (C ) 1999 Elsevier Science B.V. All rights reserved.