Long-lived, stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus, Armenia and Nakhitchevan

Highlights

• Geodynamics, magmatism and metallogeny of the Tethys belt, southern Lesser Caucasus

• Eocene subduction to Oligo-Miocene post-collision magmatism in a stationary setting

• Subduction and collision/post-collision porphyry deposits in a composite pluton

Abstract

The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.

Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, ²⁰⁶Pb/²⁰⁴Pb vs. Th/Nb, Th/Nb vs. δ¹⁸O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.

A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.

Introduction

The Tethyan orogenic belt was formed during convergence of the African–Arabian and Eurasian plates, and included abundant microplates (e.g. Golonka, 2004, Barrier and Vrielynck, 2008, Adamia et al., 2011, Rolland et al., 2012). This complex converging system resulted in Jurassic–Cretaceous and Paleogene subduction-related magmatism and ore formation, followed by various collision to post-orogenic magmatic and ore forming events throughout the Cenozoic (e.g. Marchev et al., 2005, Von Quadt et al., 2005, Perelló et al., 2008, Yigit, 2009, Moritz et al., 2014, Hou et al., 2015, Richards, 2015).

The Zangezur–Ordubad region of the southernmost Lesser Caucasus, along the Armenian and Nakhitchevan borders with Iran (Fig. 1), is a unique location along the Tethyan orogenic belt where magmatism remained stationary from an Early Paleogene subduction setting to a Neogene post-collision environment, in a place where a Gondwana-derived terrane collided with the Eurasian margin (Fig. 2). This resulted in a long-lived, Eocene to Pliocene pulsed magmatic system generating the composite Meghri–Ordubad and Bargushat plutons at the contact of the Gondwana-derived South Armenian block with the Kapan zone (Fig. 3). With an area of about 1400 km², they form the largest pluton cluster along the Lesser Caucasus (Karamyan et al., 1974, Tayan et al., 1976, Babazadeh et al., 1990, Melkonyan et al., 2008). The composite Meghri–Ordubad and Bargushat plutons are also exceptional because they host within a small area several stages of Eocene to Miocene precious and base metal epithermal and porphyry Cu–Mo deposits (Bagdasaryan et al., 1969, Babazadeh et al., 1990, Tayan, 1998, Melkonyan et al., 2010, Moritz et al., 2013). The evolution and setting of the Zangezur–Ordubad region of the southernmost Lesser Caucasus is comparable to the Himalayan setting, where protracted Mesozoic to Cenozoic magmatism was also accompanied by pulsed porphyry deposit emplacement (e.g. Hou et al., 2015).

The Lesser Caucasus is a key area to understand the lateral connection of the Western and Central Tethyan orogenic and metallogenic belt (Jankovic, 1977, Jankovic, 1997, Richards, 2015), including the Balkans, Rhodopes and Taurides–Anatolides with the Iranian belts. In particular, the Meghri–Ordubad and Bargushat plutons, studied in this contribution in the southernmost Lesser Caucasus, constitute the northern extension of the Iranian Alborz and Urumieh–Dokhtar magmatic and metallogenic belts (Fig. 1). The link of the Lesser Caucasus with the later magmatic arcs is still not well understood, as well as the nature of the widespread Eocene to recent magmatic activity along the Lesser Caucasus (Sosson et al., 2010).

This study is based on a comprehensive lithogeochemical study from the Zangezur–Ordubad region, spanning Eocene to Pliocene magmatic rocks, complemented by a whole-rock Sr, Nd, Pb and O isotope geochemistry investigation, and high-precision TIMS U–Pb zircon dating of selected and representative magmatic events. The new major, trace element, isotope and radiometric age data of this study allow us to reconstruct the Eocene to Pliocene magmatic evolution and its link to major, pulsed ore formation events in the Zangezur–Ordubad region. This evolution is discussed within the Tethyan framework, in particular, with respect to the Iranian magmatic belts and the overall geodynamic and metallogenic evolution during the final subduction stages of the Neotethys, and the ultimate collision between Arabia and Eurasia.