Late Cretaceous dacitic dykes swarm from Central Iran, a trace for amphibolite melting in a subduction zone

Abstract

Late Cretaceous Bayazeh dyke swarm is situated in the western part of the Central-East Iranian Microcontinent (CEIM). These dykes with a dominant northeast-southwest trend occur in the Eastern margin of the Yazd block. They cross cut the Lower Cretaceous sedimentary rocks. The length of the Bayazeh dykes occasionally reaches up to the 2 km. Rock forming minerals of these dykes are plagioclase (andesine and oligoclase), amphibole (magnesio-hastingsitic hornblende, magnesio-hornblende and tschermakitic hornblende), quartz, K-feldspar (orthoclase), zircon and apatite. Secondary minerals are chlorite (pycnochlorite), albite, magnetite and calcite. The main textures are porphyritic, glomeroporphyritic and poikilitic. The felsic character of the Bayazeh dacitic dykes is shown by their high SiO₂ (62.70 to 64.60 wt %) and low [Fe₂O₃* + MgO + MnO + TiO₂] (average 4.64 wt %) contents. These dykes represent the peraluminous to metaluminous nature and their Na₂O and K₂O values are 5.20–7.14 and 1.51–2.59 wt %, respectively, which reveal their sodic chemistry. The trace element characteristics are the LREE enrichment relative to HREE, [La/Yb]_CN = 13.27–22.99, and slightly negative or positive Eu anomaly. These geochemical characteristics associated with low Nb/La (0.16–0.25), Yb/Nd (0.04–0.05) and high Zr/Sm (37.60–58.25) ratios indicate that the melting of a metamorphosed subducted oceanic crust is occurred where the residual mineral assemblage is dominated by garnet amphibolite. The chemical compositions of the Bayazeh dykes resemble those of slab-derived tonalite-trondhjemite-granodiorite (TTG) series. They were formed by subduction of Mesozoic Neo-Tethys -related Nain and Ashin oceanic crusts.