A showcase of igneous processes in the Urumieh-Dokhtar magmatic arc. The Miocene-Quaternary collisional magmatism of the Bijar-Qorveh Area, Northwest Iran

The Cenozoic Arabia–Iran continental collision was associated with emplacement of a large variety of magmatic rock types. This aspect is particularly evident in the Bijar-Qorveh area of NW Iran, where Miocene andesitic to rhyolitic rocks and Quaternary basic alkaline rocks crop out. The Miocene intermediate to acid products show radiogenic Sr and Pb isotopic compositions (87Sr/86Sri 0.70531–0.71109, 206Pb/204Pb 18.71–19.01, 207Pb/204Pb 15.66–15.73, 208Pb/204Pb 38.76–39.14), coupled with unradiogenic Nd isotopic ratios (143Nd/144Ndi 0.51223–0.51265). These characteristics, together with primitive mantle-normalised multielemental patterns resembling ‘subduction-related’ geochemical fingerprints, are considered ultimately derived from the Iranian plate mantle wedge, metasomatised during previous NE-directed Neothetyan Ocean subduction. Thealkali-richandesiticanddaciticrocksevidencebothclosed-andopen-systemdifferentiation,astypicallyobservedforcollisionalsettingsingeneral.BothrocktypesdisplayahighSr/Y(37–100)andLa/Yb(29–74)‘adakitic’signaturethatitisinterpretedherewithplagioclase(±amphibole)accumulationormeltingoflocalmaficcrustalrocks.Open-systemprocessesinvolverecyclingofcrustalcumulatesforpyroxene-richandesiteandbiotite-richdacitevarietiesandlow-degreepartialmeltingofthelocalcrustforrhyolites.AradicalchangeoccurredduringtheQuaternary,whenSiO2-understaturatedtoSiO2-saturatedpoorlyevolvedrocks(basanites,tephrites,alkalineandsubalkalinebasalts)wereemplaced.ThecompletechangeofmantlesourcessuggestsaphaseoflocalextensionaltectonicsrelatedwithWNW-ESEright-transcurrentfaults.Themajoroxide,aswellasincompatibletraceelementandSr-Nd-Pbisotopicfingerprintoftheseyoungerrocksismoreakintothatofintraplatemagmas,butstillbearingsomeevidencesforavariablecontributionfroma‘subduction-modified’mantlesource.TheNW-trendofincreasinginvolvementofthissubductioncomponentisindicativeofthestrongtectoniccontrolonmagmatism.Additionallithotypesindicatethepresenceofopen-systemdifferentiationandremeltingprocessesintheyoungestphaseofmagmaticactivity.