Thermobarometric and fluid expulsion history of subduction zones

Abstract

Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite‐pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high‐P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high‐rank blueschists, high‐P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low‐T/high‐P parageneses. Inclusion studies suggest that two‐phase immiscible volatiles (liquid H2O, and gaseous high‐hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher‐grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote‐amphibolite facies assemblages. Alpine‐type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low‐density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high‐P minerals, and lack a low‐pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high‐grade blocks, reflect somewhat higher pressures and lower temperatures. Subducted sections constituting portions of the Franciscan‐type of metamorphic belt evidently moved slowly back up the inclined lithospheric plate junction during continued convergence and sustained refrigeration. Upward motion due to isostatic forces was produced by tectonic imbrication of fault suces, laminar return flow in melange zones, and lateral extension of the underplated accretionary prism. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low‐permeability mudstones tend to maintain fluid values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone‐rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes). Because of substantial updip expulsion of volatiles during prograde recrystallization, only small amounts of H2O and CO2 are available to support hydration and carbonation of the accretionary complex during its return toward the surface; thus limited back reaction takes place and occurs at low Pfluid/Plithostatic ratios, unless an abundance of volatiles is introduced during uplift.