The volcanism of the Arasbaran region, northwest Iran, is characterized by multiple magmatic pulses from Cretaceous to Quaternary related to the consumption of the Neotethys oceanic basin and the subsequent continental collision between the Arabia and the Eurasian plates. In this paper we focus on the Eocene igneous products, which show wide compositional variations, ranging from shoshonite to tephrite and phonolite. They may be further grouped into leucite (analcime)-bearing and leucite-free rock types on the basis of their rock-forming minerals. Leucite-bearing and leucite-free Eocene magmatic rocks are geographically separate outcropping in the WNW and ESE part of the Ahar-Arasbaran region, respectively. K–Ar dating show leucite-bearing rocks (39.4–39.6 +/− 1.0 Ma) being slightly younger with respect to leucite-free rocks (41.0–41.9 +/− 1.0 Ma). The two rock types are differentiated by each other in terms of silica saturation degrees but display similar incompatible trace elements distributions, typical of subduction-related volcanic rocks. Indeed, clear depletions in HFSE (e.g., Nb, Ta, Ti, Zr) and enrichments in LILE (e.g., Ba, K) and Pb are shown. The leucite-bearing volcanic rocks are strongly silica-undersaturated (Δq ca. -35) and show higher LILE/HFSE, LILE/REE, Ba/La (30–90) and Ba/Th (up to 520) values with respect to leucite-free rocks (Δq from 0 to −15; Ba/La up to 30). Leucite-bearing and -free groups also show distinct Sr-Nd-Pb isotopic compositions, with the former having less radiogenic Sr (87Sr/86Sr 0.704424–0.704634) and Pb (206Pb/204Pb 18.58–18.65, 207Pb/204Pb 15.57–15.60, 208Pb/204Pb 38.63–38.71) and more radiogenic Nd (143Nd/144Nd 0.512695–0.512791), with respect to the latter ones (87Sr/86Sr 0.704481–0.705669, 206Pb/204Pb 18.65–18.75, 207Pb/204Pb 15.61–15.64, 208Pb/204Pb 38.65–38.87, 143Nd/144Nd 0.512572–0.512623). Geochemical and isotopic compositions, coupled with the strong silica-undersaturated character, of leucite-bearingrocks suggest in their mantle source the involvement of metasomatizing partial melts from subducted altered oceanic crust and subordinate carbonate-bearing sediments. On the other hand, the compositions of leucite-free igneous rocks are compatible with the involvement of a relatively higher contribution of partial melts from terrigenous (carbonate-poor) subducted sediments. The close spatial association and the relative geographical/stratigraphic position of these products indicate diachronous metasomatic events in the mantle wedge underlying the Arasbaran area that could have been originated by the late arrival of carbonate-rich sediments at depth during slab steepening and incipient roll-back preceding the continental collision. K –Ar dating indicates that the Arasbaran magmatism was triggered by a late geodynamic event, during middle Eocene, plausibly consisting of re-adjusting of isotherms that heated the veined mantle wedge following the slab migration after roll-back. The slightly younger age of leucite-bearing rocks with respect to leucite-free rocks, coupled with the lower melting degree of the former may suggest an evolution of the local thermal regime with the progressive involvement of portions of the mantle wedge closer to the subducted plate.
Ages, geochemistry and Sr-Nd-Pb isotopes of alkaline potassic volcanic rocks from the Ahar-Arasbaran region (NW Iran): Evidence for progressive evolution of mantle sources during the Neotethyan subduction system
Bianchini G.;
2024
Abstract
The volcanism of the Arasbaran region, northwest Iran, is characterized by multiple magmatic pulses from Cretaceous to Quaternary related to the consumption of the Neotethys oceanic basin and the subsequent continental collision between the Arabia and the Eurasian plates. In this paper we focus on the Eocene igneous products, which show wide compositional variations, ranging from shoshonite to tephrite and phonolite. They may be further grouped into leucite (analcime)-bearing and leucite-free rock types on the basis of their rock-forming minerals. Leucite-bearing and leucite-free Eocene magmatic rocks are geographically separate outcropping in the WNW and ESE part of the Ahar-Arasbaran region, respectively. K–Ar dating show leucite-bearing rocks (39.4–39.6 +/− 1.0 Ma) being slightly younger with respect to leucite-free rocks (41.0–41.9 +/− 1.0 Ma). The two rock types are differentiated by each other in terms of silica saturation degrees but display similar incompatible trace elements distributions, typical of subduction-related volcanic rocks. Indeed, clear depletions in HFSE (e.g., Nb, Ta, Ti, Zr) and enrichments in LILE (e.g., Ba, K) and Pb are shown. The leucite-bearing volcanic rocks are strongly silica-undersaturated (Δq ca. -35) and show higher LILE/HFSE, LILE/REE, Ba/La (30–90) and Ba/Th (up to 520) values with respect to leucite-free rocks (Δq from 0 to −15; Ba/La up to 30). Leucite-bearing and -free groups also show distinct Sr-Nd-Pb isotopic compositions, with the former having less radiogenic Sr (87Sr/86Sr 0.704424–0.704634) and Pb (206Pb/204Pb 18.58–18.65, 207Pb/204Pb 15.57–15.60, 208Pb/204Pb 38.63–38.71) and more radiogenic Nd (143Nd/144Nd 0.512695–0.512791), with respect to the latter ones (87Sr/86Sr 0.704481–0.705669, 206Pb/204Pb 18.65–18.75, 207Pb/204Pb 15.61–15.64, 208Pb/204Pb 38.65–38.87, 143Nd/144Nd 0.512572–0.512623). Geochemical and isotopic compositions, coupled with the strong silica-undersaturated character, of leucite-bearingrocks suggest in their mantle source the involvement of metasomatizing partial melts from subducted altered oceanic crust and subordinate carbonate-bearing sediments. On the other hand, the compositions of leucite-free igneous rocks are compatible with the involvement of a relatively higher contribution of partial melts from terrigenous (carbonate-poor) subducted sediments. The close spatial association and the relative geographical/stratigraphic position of these products indicate diachronous metasomatic events in the mantle wedge underlying the Arasbaran area that could have been originated by the late arrival of carbonate-rich sediments at depth during slab steepening and incipient roll-back preceding the continental collision. K –Ar dating indicates that the Arasbaran magmatism was triggered by a late geodynamic event, during middle Eocene, plausibly consisting of re-adjusting of isotherms that heated the veined mantle wedge following the slab migration after roll-back. The slightly younger age of leucite-bearing rocks with respect to leucite-free rocks, coupled with the lower melting degree of the former may suggest an evolution of the local thermal regime with the progressive involvement of portions of the mantle wedge closer to the subducted plate.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
