Petrogenesis of the Scapolite-Bearing Kongurblak Shear Zone, Chinese Southwestern Tianshan: Implications for Rock-Buffered Fluid Flow Migration During Orogeny

Abstract

In the lower Kongurblak River, Chinese SW Tianshan, scapolite was first found in a narrow shear zone (the Kongurblak shear zone, KSZ), consisting of variably mylonitized sedimentary rocks, including marble, meta-sandstone and felsic–marly phyllonite. These rocks show varying scapolite textures and modes (from less than 1% to 15%), as a sporadic fine-grained matrix phase, porphyroblasts with abundant inclusions, or blocky aggregates. The Cl content of scapolite (0.90–2.79 wt.%) varies between different rock types, which is higher in meta-sandstone and marble than in felsic–marly phyllonite. Rare patchy lower Cl domains of scapolite and Ba zoning of K-feldspar in meta-sandstone and Cl zoning of porphyroblastic scapolite in marly phyllonite suggest pulsed fluid circulation. P–T estimates, thermodynamic modelling and mineral textures indicate scapolite formation through metasomatizing sodic feldspar and/or metamorphic reactions at amphibolite-facies conditions (470°C°C–600°C and 3–7 kbar). These findings suggest the NaCl–CO₂–H₂O fluids in the KSZ stem from an external source, not from evaporitic horizons, at ~200 Ma by apatite U–Pb dating. The chlorinity and X (CO₂) values of fluids, calculated based on the chemistry of the assemblage scapolite–biotite and T–X (CO₂) diagrams, correlate well with specific rock types they traverse, which show different mineral modes and compositions, prograde reactions, structural permeability and host rock cavities. This indicates that the chemistry of medium-grade infiltrative fluids in shear zones is strongly rock-buffered, with crucial implications for orogenic metallogenesis in specific rock types. Evidence that the lithologies within the KSZ and their tectonometamorphic histories are distinct from those of the two bounding metamorphic units suggests they represent exotic sedimentary blocks transported over long distances within a large-scale mélange, also providing new constraints on the tectonic collage of the Chinese SW Tianshan.