Mesozoic-Cenozoic mafic magmatism in Sanandaj-Sirjan Zone, Zagros Orogen (Western Iran): geochemical and isotopic inferences from Middle Jurassic and Late Eocene gabbros

Deevsalar, R., Shinjo, R., Ghaderi, M., Murata, M., Hoskin, P.W.O., Oshiro, S., Wang, K.-L., Lee, H. Y. and Neill, I. (2017) Mesozoic-Cenozoic mafic magmatism in Sanandaj-Sirjan Zone, Zagros Orogen (Western Iran): geochemical and isotopic inferences from Middle Jurassic and Late Eocene gabbros. Lithos, 284-85, pp. 588-607. (doi: 10.1016/j.lithos.2017.05.009)

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Abstract

One of the consequences of Neo-Tethys ocean subduction beneath the Central Iranian Micro-continent (CIMC) is the development of rare gabbroic intrusions in the Malayer-Boroujerd Plutonic Complex (MBPC) located in the Sanandaj-Sirjan Zone (SaSZ) of the Zagros Orogenic belt. The MBPC is a suite of extensive felsic and lesser mafic magmatic products in the northern SaSZ with geochemical signatures of arc-like magmatism during the Middle Jurassic (Ghorveh-Aligudarz arc) and intraplate type in the Late Eocene. Middle Jurassic gabbros (non-cumulate and cumulate) have low-Ti concentrations (< 1 wt. %) and quite uniform isotopic compositions (initial 87Sr/86Sr: 0.7035‒0.70593 and εNd(t): - 6.18‒-0.7), enriched LILE relative to HFSE, variable fractionation between the LREE and HREE ((La/Yb)cn: 2.27‒7.45) and both negative to positive Eu anomalies. These distinctive features of arc-type magmatism are consistent with a subduction-modified mantle source for these rocks. Trace element and REE models indicate ~ 15% melting of a metasomatized amphibole-bearing garnet-spinel lherzolite (garnet:spinel ~ 7:3) in the sub-arc mantle wedge. The cumulate gabbros and non-cumulates belong to common liquid line of descent, with complementary trace element patterns. Much of the variation between samples can be modeled by fractional crystallization (FC) of a common parent; only one cumulate gabbro from this suite exhibits isotopic evidence of contamination, probably by Rb-depleted crustal materials. The Late Eocene gabbros have relatively high Ti (>1 wt. %) and display isotopically depleted Sr-Nd values (initial 87Sr/86Sr: 0.7044-0.7087, εNd(t): 1.9-+3.2, barring one crustally contaminated sample). OIB-like trace element characteristics such as enriched HFSE, and only minor enrichment of LILE and LREE, reflect a within-plate character and asthenospheric source. Trace element modeling indicates small degree melting (fmelting: 0.05) of upper mantle lherzolite (garnet:spinel ~ 3:1) followed by higher degree melting (fmelting: 0.15) at shallower depths (garnet:spinel ~4.5:2). The Eocene parental magma underwent FC of olivine and clinopyroxene. We propose that Eocene asthenospheric upwelling was triggered by slab tearing in response to slab-rollback, which is elsewhere reported to have triggered a 'flareup' of extension-related magmatism across Iran. Three stages of tectonomagmatic evolution in the Ghorveh-Aligudarz arc segment of the N-SaSZ are represented by: 1) arc-like magmatism during active subduction of the Neo-Tethys seaway at Middle Jurassic, 2) magmatic quiescence during an interval of shallow-angle or highly oblique subduction during the Cretaceous‒Paleocene, and 3) asthenosphere melting during slab tearing shortly before the onset of the Arabia-Eurasia collision.

Item Type:Articles
Keywords:Zagros Orogen, Neo-Tethys Ocean, Ghorveh-Aligudarz arc, gabbro, trace element modeling, convecting enriched asthenosphere, mantle wedge.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Neill, Dr Iain
Authors: Deevsalar, R., Shinjo, R., Ghaderi, M., Murata, M., Hoskin, P.W.O., Oshiro, S., Wang, K.-L., Lee, H. Y., and Neill, I.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Research Group:Solid Earth
Journal Name:Lithos
Publisher:Elsevier
ISSN:0024-4937
ISSN (Online):1872-6143
Published Online:20 May 2017
Copyright Holders:Copyright © 2017 Elsevier B.V.
First Published:First published in Lithos 285-285: 588-607
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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