Polymineralic hydrothermal veins in the Paleozoic Jandaq ophiolite gabbros (Central Iran); Evidence for ingression of high temperature seawater-derived fluids in to the gabbroic section of the Paleo-Tethys oceanic crust

The Paleozoic Jandaq ophiolite is situated in the western part of the Central-East Iranian microcontinent and is a remnant of the Paleo-Tethys oceanic crust. This ophiolite consists of mantle peridotites and serpentinized mantle peridotites, metagabbro, basic and ultrabasic metamorphosed dykes and lavas, metapyroxenite, amphibolite, rodingite and listwaenite which are covered by the Late Paleozoic schist and marble. The open spaces of joints and cracks of gabbroic dykes and stocks are filled by hydrothermal minerals such as calcite, prehnite, garnet (grossular-andradite), epidote, clinopyroxene (diopside), chlorite, albite and sericite. The calcite and garnet are the latest minerals in this association. The most predominant mineral is calcite with coarse-grained morphology. The main textures are granoblastic and poikiloblastic.The very low abundances of Al, Ti, Na, Cr and rare earth elements (REEs) in clinopyroxenes support their metamorphic origin. The high modal abundance of calcite, the presence of clinopyroxene, garnet, prehnite and epidote as the Ca-bearing minerals, as well as the positive anomaly of Eu in the chondrite-normalized REE patterns of minerals possibly point to the seawater-derived penetrating fluids ingression through the oceanic crust-covering sediments and the uppermost portion of oceanic crust into the gabbros. The involved fluids in the Jandaq area have leached the calcite-bearing sediments (e.g., limestone) and calcic plagioclase-rich lithologies (e.g., basic pillow lavas and sheeted dykes) before reaching to the gabbros, caused to the CO2, calcium and Eu enrichment of fluids. Accordingly, the hydrothermal veins in the Jandaq ophiolite are possibly formed by high temperature circulation of seawater-originated fluids throughout the uppermost oceanic crust.