The Alborz range is part of Alpine- Himalayan orogenic system which preserved important magmatic features related to the evolution of Gondwana-Eurasia junction realm. Several well-developed sub-marine Cretaceous volcanic units are exposed in Central to Northern Alborz e.g. in Marzan-Abad and Lahijan areas. The volcanics display mainly basaltic composition. Study of chemical signatures of these rocks is a key issue to constrain the tectonomagmatic environment of melt formation and paleogeographic reconstructions. In this research we aim to study the volcanic outcrops located to the northeast of Tehran (south of Amol), Central Alborz. Stratigraphic relationships show that the volcanics are interlayered with marly limestone, limestone and marls of Late Cretaceous. Here, we present new geochemical data on these rocks to identify the origin and condition of magma genesis which helps to infer paleotectonic setting of Alborz magmatism during Late Cretaceous. The major and trace element contents of whole rock samples have been obtained by XRF and ICP-MS at Ferrara University, Italy. Using recalculated anhydrous major elements, the samples display wide range of variation in SiO2 (42-68 wt.%), TiO2 (0.45- 4.7 wt.%), Al2O3 (12.6-18.8 wt.%), Mg# (17.8-49) and Na2O + K2O (2.9-11.7 wt.%), likely reflecting various mineralogy and different degrees of fractionation. Based on Nb/Y (1.8-3.4) and Zr/Ti (0.01-0.29) ratios the rocks plot within alkali basalt, trachy-andesite and trachyte fields. High Zr (83-738 ppm) and Nb (43-111 ppm) contents is also consistent with alkaline nature of the samples. Other incompatible element values shown by normalized multi-element patterns are enriched as well, which is similar to typical within-plate alkaline basalts, such as ocean island basalts (OIB). Furthermore, no negative Nb and Ta relative depletion can be observed in these patterns. In chondrite- normalized REE patterns, the rocks display elevated LREE enrichment with respect to HREE, exemplified by high (La/Yb)N ratio (15-33). Overall, the REE enrichment of the rock suites is 155- 375 and 8.5- 17 times chondrite value for La and Yb, respectively. Using trace element abundances such as Zr, Y, Nb and Ti contents and also the ratios of Zr/P2O5, Nb/Y, Zr/Y, Ti/Y, Th/Yb and Ta/Yb, the basaltic rocks plot in the fields of alkaline and subduction-unrelated OIB-like magmas. Moreover, theoretical modeling using REE ratios e.g. Sm/Yb and La/Yb displays that the parental melt originated from partial melting of deep garnet-bearing enriched mantle with low (<15%) partial melting. Therefore, we conclude that the tectonic environment of melt formation is unrelated to subduction process and mostly corresponds to within-plate magmatism in an extensional setting. This conditions is more probably consistent with hot spot/mantle plume activity and continental rift-related events.
Chemical constraints on the genesis and geodynamic setting of Late Cretaceous magmatism in northeast of Tehran, Central Alborz
Saccani E.Membro del Collaboration Group
;Barbero E.Membro del Collaboration Group
;
2019
Abstract
The Alborz range is part of Alpine- Himalayan orogenic system which preserved important magmatic features related to the evolution of Gondwana-Eurasia junction realm. Several well-developed sub-marine Cretaceous volcanic units are exposed in Central to Northern Alborz e.g. in Marzan-Abad and Lahijan areas. The volcanics display mainly basaltic composition. Study of chemical signatures of these rocks is a key issue to constrain the tectonomagmatic environment of melt formation and paleogeographic reconstructions. In this research we aim to study the volcanic outcrops located to the northeast of Tehran (south of Amol), Central Alborz. Stratigraphic relationships show that the volcanics are interlayered with marly limestone, limestone and marls of Late Cretaceous. Here, we present new geochemical data on these rocks to identify the origin and condition of magma genesis which helps to infer paleotectonic setting of Alborz magmatism during Late Cretaceous. The major and trace element contents of whole rock samples have been obtained by XRF and ICP-MS at Ferrara University, Italy. Using recalculated anhydrous major elements, the samples display wide range of variation in SiO2 (42-68 wt.%), TiO2 (0.45- 4.7 wt.%), Al2O3 (12.6-18.8 wt.%), Mg# (17.8-49) and Na2O + K2O (2.9-11.7 wt.%), likely reflecting various mineralogy and different degrees of fractionation. Based on Nb/Y (1.8-3.4) and Zr/Ti (0.01-0.29) ratios the rocks plot within alkali basalt, trachy-andesite and trachyte fields. High Zr (83-738 ppm) and Nb (43-111 ppm) contents is also consistent with alkaline nature of the samples. Other incompatible element values shown by normalized multi-element patterns are enriched as well, which is similar to typical within-plate alkaline basalts, such as ocean island basalts (OIB). Furthermore, no negative Nb and Ta relative depletion can be observed in these patterns. In chondrite- normalized REE patterns, the rocks display elevated LREE enrichment with respect to HREE, exemplified by high (La/Yb)N ratio (15-33). Overall, the REE enrichment of the rock suites is 155- 375 and 8.5- 17 times chondrite value for La and Yb, respectively. Using trace element abundances such as Zr, Y, Nb and Ti contents and also the ratios of Zr/P2O5, Nb/Y, Zr/Y, Ti/Y, Th/Yb and Ta/Yb, the basaltic rocks plot in the fields of alkaline and subduction-unrelated OIB-like magmas. Moreover, theoretical modeling using REE ratios e.g. Sm/Yb and La/Yb displays that the parental melt originated from partial melting of deep garnet-bearing enriched mantle with low (<15%) partial melting. Therefore, we conclude that the tectonic environment of melt formation is unrelated to subduction process and mostly corresponds to within-plate magmatism in an extensional setting. This conditions is more probably consistent with hot spot/mantle plume activity and continental rift-related events.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
