Multi-stage metamorphism in the South Armenian Block during the Late Jurassic to Early Cretaceous: Tectonics over south-dipping subduction of Northern branch of Neotethys

https://doi.org/10.1016/j.jseaes.2014.07.018Get rights and content

Highlights

  • Multi-stage metamorphic evolution preserved in the SAB Crystalline Basement.

  • North-verging shearing followed by radial extension.

  • South-dipping subduction of below the South Armenian Block.

Abstract

The geologic evolution of the South Armenian Block (SAB) in the Mesozoic is reconstructed from a structural, metamorphic, and geochronologic study including U–Pb and 40Ar/39Ar dating. The South Armenian Block Crystalline Basement (SABCB) outcrops solely in a narrow tectonic window, NW of Yerevan. The study of this zone provides key and unprecedented information concerning closing of the Northern Neotethys oceanic domain north of the Taurides-Anatolides platform from the Middle Jurassic to the Early Cretaceous. The basement comprises of presumed Proterozoic orthogneiss overlain by metamorphosed pelites as well as intrusions of granodiorite and leucogranite during the Late Jurassic and Early Cretaceous. Structural, geochronological and petrological observations show a multiphased evolution of the northern margin of the SAB during the Late Jurassic and Early Cretaceous. A south-dipping subduction under the East Anatolian Platform-South Armenian Block (EAP-SAB) is proposed in order to suit recent findings pertaining emplacement of relatively hot subduction related granodiorite as well as the metamorphic evolution of the crystalline basement in the Lesser Caucasus area. The metamorphism is interpreted as evidencing: (1) M1 Barrovian MP–MT conditions (staurolite–kyanite) at c. 157–160 Ma and intrusion of dioritic magmas at c. 150–156 Ma, (2) near-adiabatic decompression is featured by partial melting and production of leucogranites at c. 153 Ma, followed by M2 HT–LP conditions (andalusite–K-feldspar). A phase of shearing and recrystallization is ascribed to doming at c. 130–150 Ma and cooling at 400 °C by c. 123 Ma (M3). Structural observations show (1) top to the north shearing during M1 and (2) radial extension during M2. The extensional event ends by emplacement of a thick detrital series along radial S, E and W-dipping normal faults. Further, the crystalline basement is unconformably covered by Upper Cretaceous-Paleocene series dated by nannofossils, evolving from Maastrichtian marly sandstones to Paleocene limestones.

Introduction

The Crystalline Basement of the South Armenian Block (SAB) (Fig. 1A and B) has a geological history poorly constrained. The SAB was involved in a long and complex tectonic evolution including a presumed Panafrican high-grade metamorphism (Baghdasarian and Ghukasian, 1983, Aghamalyan et al., 2011a, Aghamalyan et al., 2011b). However, contrary to other occurrences of Paleozoic sedimentary rocks SE of Yerevan (Sosson et al., 2010), the presumed Paleozoic sediments of the Tsaghkuniats crystalline massif are metamorphosed. Both the metamorphic basement and the cross-cutting intrusions are unconformably covered by Upper Cretaceous deposits (Fig. 2A–C). This observation leads us to question the Precambrian age forwarded for the metamorphism, as well as the tectonic setting and its geodynamic cause. Indeed, several geodynamic processes can explain the metamorphic history of this basement, including (1) a rifting stage leading to Neotethys ocean opening and drifting from Gondwana in the Permian to Early Mesozoic times (Sosson et al., 2010 and references therein), (2) oceanic closure by subduction of Paleotethys from the Early to late Mesozoic and (3) subsequent continental subduction/ocean crust obduction in the middle-Late Cretaceous (Galoyan et al., 2009, Rolland et al., 2009b), followed by (4) continental collision or accretion to the Eurasian margin in Late Mesozoic/Early Cenozoic times (Sosson et al., 2010) and depending on authors the Arabia-Eurasia collision from middle to late Eocene up to Oligocene ((Hempton, 1985, Robertson et al., 2012, Rolland et al., 2009a, Rolland et al., 2012, Yılmaz, 1993). Moreover, the influence of the Cimmerian orogeny, known more eastward in Iran, is also a possible cause of this metamorphism (Philippot et al., 2001, Kazmin, 2006, Saintot et al., 2006, Zanchi et al., 2009). Consequently, the SAB Crystalline Basement (SABCB) needs to be investigated by detailed geochronological and petro-metamorphic (P–T–t) studies to evaluate the importance of these post-Paleozoic events in this part of the Caucasus region.

In order to constrain the timing of these geological events, field geology and sampling were undertaken in the SABCB, which outcrops in a narrow tectonic window to the NW of Yerevan, in the Tsaghkuniats massif (Aghamalyan, 1983; Figs. 1B and 2A). This study presents new U–Pb and 40Ar/39Ar age data to temporally constrain the metamorphism and magmatism of the basement. This study is complemented by a petrologic and Pressure–Temperature (P–T) analysis to interpret the metamorphic significance of these rocks. Subsequently, we propose a tectonic and geodynamic reconstruction of the evolution of the SAB throughout the closing of the northern Neotethys oceanic domain.

Section snippets

Origin of the SAB

Palaeomagnetic analyses indicate palaeo-latitudes for the SAB during the Early and Middle Jurassic at least 2000 km farther south than its current position (Bazhenov et al., 1996, Meijers et al., 2013). This argues a Gondwanian origin of the SAB, as also suggested by the dating undergone by Baghdasarian and Ghukasian (1983) and palaeogeoraphic reconstructions (Barrier and Vrielynck, 2008, Knipper and Khain, 1980, Monin and Zonenshain, 1987, Robertson and Mountrakis, 2006, Şengör et al., 1988).

New field observations

The crystalline basement of the Tsaghkuniats massif is made up of metamorphic rocks characterized by para- and ortho-gneisses with micaschists cross-cut by leucogranite and granodiorite intrusions (Baghdasarian and Ghukasian, 1983, Aghamalyan, 1998, Shengelia et al., 2006). A stack of metasedimentary rocks mainly comprised by skarnified limestones is also evidenced. These metasedimentary rocks (Fig. 2A–C) could belong to the Proterozoic formations (Aghamalyan, 1998) or to the Upper

Mineralogy and pressure–temperature path of metamorphic rocks

The chemical compositions of minerals were obtained by Electron Microprobe Analysis (EPMA) in order to verify the homogeneity of mineral compositions from core to rim (see Supplementary Data 1). A detailed mineralogical analysis of the micaschist (metapelite) sample AR-03-62M (Table 2; Fig. 4C, D, G, H, I, J, K1, K2, 5C1, C2, D1, D2, E, F1 and F2) allows us to construct the Pressure–Temperature (PT) path followed by part of the SAB crystalline basement (Fig. 6C), using the grid established by

40Ar/39Ar dating

Pure minerals less than 1 mm in diameter (between 800 μm and 500 μm) were obtained using a mortar and pestle and multiple sieving. Datable minerals were separated by careful hand-picking under a binocular microscope to avoid altered grains or inclusions. All samples were irradiated for around 70 h (J1) and 10 h (J2, J3) in the nuclear reactor at McMaster University in Hamilton (Canada), in position 5c along with Hb3gr hornblende fluence monitor (1073.6 Ma ± 5.30 Ma; Jourdan et al., 2006) and Fish Canyon

Discussion

The origin of the different rock types found in the Tsaghkuniats region is unconstrained in time as well as the geodynamic processes responsible for their formation. Before the present paper, only the age obtained by Baghdasarian and Ghukasian (1983) obtained using the Rb–Sr isochron method on the Bjni migmatite–granite suggests a Proterozoic (610 ± 36 Ma) origin for the metamorphism and magmatism undergone by the SABCB. The metamorphic evolution of the SABCB is investigated here for the first

Conclusion

We show here for the first time an amazingly young multi-stage metamorphic evolution for this region preserved in the SABCB, fully occurring during the Late Jurassic and Early Cretaceous times. Geochronological data show it to be unrelated to the Cimmerian orogeny. (1) First M1 metamorphism is featured by Barrovian (staurolite–kyanite) MP–MT conditions at 157–160 Ma and intrusion of dioritic magmas at 150–156 Ma. (2) Near-adiabatic decompression PT path is featured by partial melting and

Acknowledgements

This work was supported by the MEBE (Middle East Basin Evolution) and DARIUS programs jointly supported by a consortium including oil companies, UMPC and the INSU/CNRS. Fieldwork was facilitated by the support of the Armenian National Academy of Science (Institute of Geological Sciences). We wish to thank J.L. Devidal in Clermont-Ferrand for their involvement during data acquisition. The technical help of G. Delanoy for thin sections as well as the help from S. Gallet and C. Verati in utilizing

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