Geochemistry and petrogenesis of potassic monzonites in the Lar igneous suite, north of Zahedan, eastern Iran: Constraints on the origin of C-type adakites

The Lar igneous suite occurs in the central part of the Sistan suture zone, about 22 km north of Zahedan. The suite comprises varied intrusive, hypabyssal and extrusive silica under saturated to silica saturated rock types including syenite, shonkinite, lamprophyres, monzonite, tuff and breccia. Stock-like monzonitic bodies crops out mainly in the south-southeast and northern parts of the suite. The study rocks are quartz monzonite to monzonite in modal composition with porphyritic texture including alkali feldspar, plagioclase, quartz, amphibole and biotite. Their high SiO2, Al2O3, K2O and K2O/Na2O classify them as silica saturated metaluminous, shoshonitic and Roman Province type potassic monzonites. Decreasing in FeO, CaO, MnO, TiO2, P2O5 and MgO concentrations, during magma evolution, represent the fractionation of Ca-plagioclase, apatite and titanite along with mafic crystals from their parental magma. Geochemical studies reveal that partial melting, in comparison to differentiation processes, has been more effective on their geochemical features. Monzonites in the Lar igneous suite show LREE and LILE (except Ba) enrichment and HFSE (Nb, Ta, Ti and Zr) depletion. Negative anomalies in Ba, Nb, Ta and Ti, and high Ba/Nb ratio in these rocks represent their relation to subduction processes. They classify, due to their high K2O and K2O/Na2O ratio, Sr/Y adakitic ratio and post collisional affinities, as continental or C-type adakites. Listric-shaped REE profile, Nb/Ta ratios similar to continental crust and positive correlations between La/Yb and Sr/Y ratios all indicate partial melting of amphibole-bearing lower continental crust. Moreover, high Th and Ni concentrations, Ce negative anomaly, Nb/U ratios similar to subducted sediments and also Mg# higher than 45 probably reveal that not only magmas with geochemical signatures similar to composition of subducted sediments have been contributed in the formation of lower continental crust but also the later have been delaminated or foundered and it’s resulted melts, en route to surface, have inherited some peridotite geochemical signatures.