Roll-back, extension and mantle upwelling triggered eocene potassic magmatism in NW Iran

Abstract

Igneous activity in the rear-arc of the Paleogene Urumieh-Dokhtar Magmatic Belt of Iran has to date been poorly studied. An example of such activity, Late Eocene potassic mafic to intermediate magmatic rocks in the Lahrud area of NW Iran, is the focus of this work. These lavas include olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite-clinopyroxene-phyric basalts, andesites, and trachytes, and Paleocene-Early Eocene pyroclastic rocks. Monzo-syenite plugs (dated here at ~37 Ma), clinopyroxene-phyric basaltic dikes, and leucite-bearing clinopyroxene- phyric basaltic dikes intrude older lavas and pyroclastic rocks. Olivine-bearing clinopyroxenephyric basalts and analcime-bearing leucite-clinopyroxene-phyric basalts are characterized by large phenocrysts of olivine, clinopyroxene, leucite, and analcime. Clinopyroxene-rich enclaves and partially resorbed mantle xenoliths also occur. Olivine phenocrysts are zoned from high-Mg# cores (Mg# = 90) to Fe-rich rims (Mg# = 66). Clinopyroxene phenocrysts from the olivine-bearing clinopyroxene-phyric basalts, analcime-bearing leucite-clinopyroxene-phyric basalts and clinopyroxene crystals in the enclaves show complex oscillatory zoning, sieve textures, and resorption textures, but with systematic core-rim compositional trends. Their 87Sr/86SrSr isotopic compositions measured in situ range from 0.7037 to 0.7068 (mean = 0.705260±0004), suggesting negligible crustal assimilation during fractional crystallization. The Lahrud lavas are potassic and are enriched in light rare earth elements and large ion lithophile elements such as Th, Rb, K and U. High field strength elements (HFSE), such as Nb, are depleted in the olivine-bearing clinopyroxene-phyric basalts and analcime-bearing leucite-clinopyroxene-phyric basalts, but enriched in the trachytes and trachytic ignimbrites. The isotopic compositions vary: 87Sr/86Srt from 0.7045 to 0.7052, eNd(t) from +2.8 to +3.3, and eHf(t) from +7.2 to +7.7. The rocks have radiogenic lead 206Pb/204Pb from 18.66 to 18.76, 207Pb/204Pb from 15.58 to 15.62, and 208Pb/204Pb from 38.73 to 38.81. Modeling of major and trace elements using the MELTS algorithm indicates that the geochemical variations in the basaltic to andesitic rocks are reasonably explained by shallow fractional crystallization with some complications owing to source heterogeneity, crustal assimilation, and magma mixing. The isotopic data imply that partial melting of old sub-continental lithospheric mantle was not responsible for the Lahrud potassic magmas; Hf isotopes and Zr/Nb ratios suggest derivation from an enriched mantle wedge, whereas ratios of incompatible trace elements (e.g. La/Yb, Ba/La, Ce/Pb, Th/Yb) and high 87Sr/86Sr suggest mantle metasomatized by slab-derived fluids or melts dominated by a sediment component. Geochemical modeling using the Arc Basalt Simulator version 5 reveals that the HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts originated from a high-temperature mantle wedge (2.2 GPa, 1310°C) fluxed intensively (5%) by melts from a deep hot slab (6 GPa, 1000°C). The moderately HFSE-depleted, olivine-bearing, clinopyroxene-phyric basalts reflect melting of a lower-temperature mantle wedge (2.2 GPa, 1300°C) with a lesser amount of slab melt flux (4%) from a lower temperature and shallower slab (3 GPa, 866°C). In contrast, the leucite- clinopyroxene-phyric basalts and andesites are from a similar source to the moderately HFSEdepleted, olivine-bearing, clinopyroxene-phyric basalts (3 GPa, 866°C) but with a contribution from a lower temperature mantle wedge (2.2 GPa, 1270°C). During Late Eocene times, slab retreat and upper-plate extension occurred in the rear-arc region of the Urumieh-Dokhtar Magmatic Belt. The Lahrud potassic magmas were generated from a high-temperature mantle wedge, which resulted in melting of the slab; this slab melt flux further promoted melting of the mtle wedge.