Subduction channel fluid–rock interaction and mass transfer: Constraints from a retrograde vein in blueschist (SW Tianshan, China)

Abstract

One widely neglected part of the subduction zone fluid cycle is the retrograde fluid flow along the plate interface. This fluid flow may facilitate the exhumation of slab-derived rocks within the subduction channel. However, our understanding on the nature and behavior of these retrograde fluids is still insufficient. Retrograde veins preserved in HP–UHP rocks can provide valuable information about fluid activities and mass transfer at the plate interface and exhumation processes. In this study, we present a petrological and geochemical investigation of a typical retrograde vein and its blueschist host rock, which occurs throughout the whole Akeyazi (U)HP terrane in the Chinese southwestern Tianshan. The magnetite-bearing albite–calcite vein, representing an external fluid pathway, crosscuts the host blueschist that mainly consists of glaucophane, garnet, epidote and titanite. Along the fluid conduit the immediate wall-rock experienced greenschist-facies overprinting as displayed by an albite-rich selvage that grades into a chlorite–biotite selvage towards the host rock. The replacements of garnet by chlorite and glaucophane by albite are thought to have occurred due to an interface-coupled dissolution-precipitation mechanism. Mass balance calculations show that during fluid–rock interaction large amounts of LILE (K–Rb–Ba–Cs and Pb), Cu and CO2 were added to the selvages (mass gain >200%), whereas the altered wall-rock released significant amounts of Ca, Li, Sr, REE and transition metal elements (V–Mn–Co–Ni–Zn) (mass loss mainly between 40 and 100%) due to the dissolution of glaucophane, garnet and epidote. The chlorite–biotite selvage experienced additional SiO2 and Na2O mass losses as well as FeO, MgO and H2O mass gains, whereas the albite-rich selvage underwent exactly the opposite mass transfer with respect to these components. The Al, Ti, Nb, Ta and Cr contents were constant during the alteration. Therefore, the retrograde fluid causing greenschist-facies overprinting of the blueschist host in the subduction channel (at ca. 30km depth) is proposed to be oxidized and enriched in LILE, Cu and CO2 components, whereas Ca, Li, Sr, REE, Y and transition metal elements (V–Mn–Co–Ni–Zn) are thought to have been mobilized during fluid-rock interaction. In addition, the main fluid sources (sediments, oceanic crust or serpentinites) diversely contribute to retrograde fluids at different depths in the subduction channel.