Post-collisional ultrapotassic volcanic rocks and ultramafic xenoliths in the Eslamieh Peninsula, NW Iran: Petrological and geochemical constraints on mantle source and metasomatism

The Eslamieh Peninsula in NW Iran exposes Miocene potassic to ultrapotassic volcanic rocks that are often associated with a variety of ultramafic xenoliths. Based on geochemical features, the rocks investigated in this study can be subdivided into two main groups: 1) MgO-rich (8.0-13.9 wt%) basic (SiO2 = 46.1-50.9 wt%) rocks, which comprise mafic lamprophyres and pyroxenite xenoliths, and 2) MgO-poor (1.5-6.4 wt%) basic to intermediate (SiO2 = 48.2-61.2 wt%) rocks, mainly consisting of trachytes, felsic lamprophyres, and analcimebearing tephriphonolites and phonotephrites. Rocks in both groups have variable but generally low Ni (10-115 ppm; mostly <60 ppm) and Cr (6-628 ppm; mostly <200 ppm), and display enrichment in LREE over HREE (e.g., La/Yb = 17-55), with a large overlap between the MgO-rich and MgO-poor types. These features are coupled with enriched LILE/HFSE ratios (e.g., Ba/Nb mostly 16-116), negative Nb-Ta-Ti anomalies and positive Pb peaks in primitive mantle-normalised diagrams (e.g., Nb/Nb* mostly <0.7). Both high-and poor-MgO samples have strongly radiogenic (Sr-87/Sr-86)i ratios (0.7078-0.7086), (Nd-143/Nd-144)i below the Chondritic Uniform Reservoir (CHUR) = 0.51235-0.51244, and epsilon Hfi ranging from-3.1 to 11.1%, suggesting involvement of a lower continental crust component in their genesis. The major oxide and trace element contents the high-MgO and high-CaO rocks indicate derivation from a subcontinental lithospheric mantle source, variably metasomatised by fluids and melts released by a subducted slab in a mantle wedge. Primitive melts evolved via mafic mineral fractionation and possible assimilation of lower crust, generating the SiO2 higher and CaO-MgO-poorer group. We suggest that small degree partial melting of the enriched mantle sources could have been triggered by Neo-Tethys slab roll-back during the Arabian-Iranian collision.