The Kahak mafic volcanic rocks in the central part of the Urumieh-Dokhtar Magmatic Arc are composed of basalts and basaltic andesite and show sub alkaline to transitional affinity. They are calc-alkaline based on the tholeiitic index (THL). U-Pb zircon dating yields almost 60 (Middle Paleocene) and 24 to 19 Ma (Late Oligocene–Early Miocene) for andesitic basalt and basaltic rocks respectively. These rocks are identified by LREE and LILE enrichment and HFSE depletion with relatively negative or without Eu anomalies and E-MORB like pattern in multiple spider diagrams that.attributed to the subduction of the Neotethyan oceanic slab beneath the central Iranian microcontinent. Based on petrography, trace and rare earth elements, and isotopic features, fractional crystallization played a significant role during magma evolution in these rocks. Trace element modeling suggests that the studied mafic rocks were derived by partial melting within the spinel lherzolite mantle. Isotopic ratios also show that they resulted from lithospheric mantle metasomatized by released fluids from subducted slab sediments. The studied samples might have formed in the extensional regime followed by slab rollback and undergone a continental arc to back-arc basin transition during the Paleocene to Miocene. This basin might have been closed in the middle Miocene.
Post-collision Pliocene-Quaternary basaltic rocks outcrop in the Kerman Cenozoic Magmatic Arc (KCMA) to the northwest and east of Shahr-e-Babak city. These porphyritic and vesicular basaltic rocks are composed essentially of clinopyroxene, olivine, and plagioclase. These basalts display alkaline affinity and negative Ta, Zr, Rb anomaly, but slightly negative Nb anomaly, relative to elements with similar compatibility, and positive Ba, K, Sr anomaly, suggesting their magma source related to subduction-accretion with implication of subducted slab derived components to the source. In the primitive mantle and chondrite normalized diagrams, these rocks show trace elements (except depletion in Nb, Ta) and Rare Earth Element (REE) patterns similar to the Ocean Island Basalts (OIB) and share trace and major element characteristics similar to High-Nb Basalts (HNBs). Geochemical analyses for major and trace elements suggest that the Shahr-e-Babak HNBs have undergone insignificant crustal contamination and minor olivine + Fe-Ti oxide ±clinopyroxene fractional crystallization. These HNBs derived from a partial melting (~5%) of garnet-peridotite mantle wedge, which have already metasomatized by overlying sediments, fluids, and adakitic (slab-derived) melts as major metasomatic agents in post-collision setting in the KCMA. We conclude that asthenospheric upwelling arising from slab break-off followed by the roll-back of subducting Neotethys slab also triggered metasomatized peridotite mantle wedge and caused its partial melting in the subduction zone.
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