Lead enrichment in Neotethyan volcanic rocks from Iran: The implications of a descending slab

Abstract

Basic-intermediate volcanic rocks from the Karaj-Danesfahan area in the Neotethyan magmatic belt of Iran show typical island arc geochemical signatures. The rocks demonstrate calcalkaline affinity with low abundances of "high field strength elements" (HFSE) such as Nb, Ta, Hf and Ti, and highly distinctive spiked trace element patterns at "large ion lithophile elements" (LILE) such as Ba, K, and Sr. Comparing the geochemical characteristics of two sets of the volcanic rocks sampled from two parts of the study area 100 km apart, however, indicate subtle but significant geochemical differences which bear important petrogenetic implications. One set of the volcanic rocks is abnormally enriched in Pb and shows consistently higher abundances of HFSE, particularly Ti, than the other set. These two series are called HTL series (after high titanium and lead) and LTL series (after low titanium and lead), respectively. HTL series volcanic rocks are further from the subduction axis, so they should have had a deeper descending slab contributing to their mantle wedge magmatism. To be able to release Pb-enriched fluids, the subducting slab should not have undergone earlier dehydration. It is because Pb is highly incompatible. It appears that in the early stage, subduction proceeded at a higher rate (cold slab), so slab-dehydration occurred at greater depths triggering HTL series magmatism. Subsequently the subduction rate decreased which in turn raised isotherms and promoted partial melting at shallower depths leading to LTL series magmatism.

A Paleogene extensional arc flare‐up in Iran

“…Although there are only two Eocene samples in this subset of primitive basalts, the trace element patterns of all of the Eocene andesites (Figure 6a) and Eocene shoshonites (Figure 6f) have Nb depletions. In short, the trace element composition of Eocene magmatism is typical of continental arcs, a finding in agreement with the vast majority of previous geochemical studies conducted on Iranian Eocene volcanism [e.g., Jung et al , 1974; Berberian et al , 1982; Ghorbani , 2006; Omrani et al , 2008]. However, in contrast to Eocene volcanism, the Oligocene samples we analyzed (which are from basalts in the lower part of the Qom Formation/Gand Ab limestones and mafic dikes intruding the Eocene stratigraphy) have overall flat trace element patterns and do not have depletions in Nb.…”

“…Eocene metamorphic core complexes from Moritz et al [2006] and Verdel et al [2007]. Previous geochemical studies: A, Alberti et al [1979]; B, Boccaletti et al [1976]; C, Ghorbani [2006]; D, Amidi et al [1984]; E, Amidi and Michel [1985]; F, Förster et al [1972]; G, Hassanzadeh [1993]; H, Spies et al [1984]; I, Jung et al [1984]; J, Dupuy and Dostal [1978]; K, Omrani et al [2008]. the flare-up was minor to absent within both the Urumieh-Dokhtar belt and the Alborz Mountains, despite a history of subduction that may have begun as early as the Late Triassic, as discussed below.…”

Tafresh intrusive rocks within the Urumieh‐Dokhtar Magmatic Arc: Appraisal of Neo‐Tethys subduction

“…The subduction of the Neo‐Tethys oceanic floor underneath the Iranian continental plate resulted in two distinctive arc magmatic events in the NW–SE trending Sanandaj‐Sirjan magmatic arc (SSMA) and UDMA (Figure ) in the Mesozoic and Cenozoic, respectively. The 1,700‐km‐long and 4‐ to 50‐km‐wide UDMA consists of calc‐alkaline volcanic, pyroclastic, and intrusive rocks (Alavi, ; Berberian & King, ; Ghorbani, ; Stocklin, ).…”

Slab partial melts from the metasomatizing agent to adakite, Tafresh Eocene volcanic rocks, Iran

“…The gray areas (Tafresh data) are derived from Figures 2 and 4 (this study). The Danesfahan and Karaj data (seven samples each) are from Ghorbani (2006), data for Naragh, Kashan, Koohpayeh, and Ardestan (10 samples each) are from Ghorbani et al . (in prep., 2011).…”