Jurassic subduction zone tectonics of the Rhodope Massif in the Thrace region (NE Greece) as revealed by new U–Pb and ⁴⁰Ar/³⁹Ar geochronology of the Evros ophiolite and high-grade basement rocks

Highlights

• Evros ophiolite has magmatic life between 176-164 Ma south to the Rhodope margin.

• The timing of (UHP) HP event coincides with the Evros ophiolite magmatic history.

• Basement granitoid magmatism overlaps the ages of HP event and Evros ophiolite.

Abstract

In the Thrace region of NE Greece, the crystallization and post-solidification cooling below 600 °C of the intrusive suite of the Circum-Rhodope Belt arc-related Evros ophiolite span from 176.4 ± 0.93 to 163.5 ± 3.85 Ma. In the underlying Rhodope high-grade metamorphic basement, (ultra-) high-pressure metamafic rocks are cross-cut by orthogneisses with granitoid protoliths and crystallization ages between 160 ± 0.69 and 154 ± 1.5 Ma. This new U–Pb LA–ICP-MS zircon geochronological data provides evidence that the timing of the evolution of the Evros ophiolite coincides, within analytical error, with granitoid magmatism in the underlying high-grade basement. One of the dated metagranitoid rocks cross-cuts amphibolitized eclogite, indicating that the high-pressure conditions have a maximum age of 160 Ma postdating one of the proposed ages of ultra-high pressure conditions in the Rhodope Massif. The currently earliest recognised Rhodope high/ultra-high pressure metamorphic event is therefore related to the subduction setting which formed the Evros ophiolite. Based on these new temporal constraints and the regional tectonic–geochronologic framework, we discuss and update the geodynamic context connected to the Jurassic subduction–collisional setting at the Rhodope continental margin of Eurasia.

Introduction

In the Hellenides, Jurassic ophiolites are remnants of the Pindos–Mirdita and the Vardar oceanic basins (Robertson, 2002, Beccaluva et al., 2005, Dilek et al., 2007, Dilek et al., 2008, Saccani et al., 2011), where the Vardar Ocean ophiolites are exposed along the Vardar suture zone of the north Aegean region (Fig. 1, inset). Recent studies have addressed the debate surrounding the different branches of the Vardar Ocean, which are located between continental blocks (Stampfli and Borel, 2002, Schmid et al., 2008, Robertson et al., 2009), but also involve arc systems (Bonev and Stampfli, 2003, Robertson et al., 2013). The Vardar suture zone throughout its easternmost part is represented by the Circum-Rhodope Belt. This is located close to the Serbo-Macedonian and Rhodope crystalline massifs (Papanikolau, 2009) (Fig. 1) with which a common Mesozoic geodynamic evolution is shared. The latter massifs consist of Late Cretaceous south-verging ductile nappe stacking in the hanging wall of the north-dipping subduction system, which is now expressed as the Vardar suture zone (Ricou et al., 1998). The 92–67 Ma-old Late Cretaceous Magmatic Belt (von Quadt et al., 2005, Marchev et al., 2006) was formed during the Cretaceous north-directed Vardar subduction onto the Rhodope Massif, Sredna Gora and Balkan zones.

Studies of the eastern Circum-Rhodope Belt Evros ophiolite have demonstrated its arc-related origin (Magganas et al., 1991, Bonev and Stampfli, 2008). A single age of 169 Ma (Koglin et al., 2007) however poorly constrains the timing of magmatic evolution of the Evros ophiolite. Recent studies have revealed Late Jurassic north-directed thrusting of the Circum-Rhodope Belt and accretion to the Rhodope continental margin along a south-dipping subduction zone (Bonev et al., 2010, Bonev and Stampfli, 2011). Accurate and precise crystallization ages of the Evros ophiolite are necessary to constrain the age of tectonic emplacement of the Evros ophiolite and for improving our knowledge about the Jurassic subduction setting along the Rhodope continental margin of the Eurasian plate.

The Rhodope high-grade metamorphic basement underlying the eastern Circum-Rhodope Belt contains relics of ultra- and high-pressure events (Kolcheva and Eskenazy, 1988, Kozhoukharova, 1998, Liati and Mposkos, 1990, Mposkos and Liati, 1993, Mposkos and Kostopoulos, 2001, Cornelius, 2008) (Fig. 1). Although there is widespread geochronologic evidence for multiple high-pressure subduction events spanning the Middle Jurassic-Eocene, the timing of the ultra-high pressure event remains poorly constrained. Studies have linked both ultra-high pressure and high-pressure events to the Cretaceous north-dipping subduction of the Vardar Ocean (Mposkos and Krohe, 2006, Krenn et al., 2010), whereas other authors have proposed that the ultra-high pressure and high-pressure relics predate a granulite-facies overprint at 170–160 Ma (Bauer et al., 2007). A possible ultra-high pressure event has been dated around 150 Ma (Liati et al., 2011), which coincides temporally with ca. 150–152 Ma granitoid magmatism in the metamorphic basement (Cornelius, 2008, Liati et al., 2011). This granitoid magmatism which started in the Jurassic (ca. 163 Ma), persisted into the Early Cretaceous (ca. 134 Ma) in the metamorphic basement of the Central Rhodope Massif (Turpaud and Reischmann, 2010), where relics of ultra- and high-pressure events have been also reported by Mposkos and Kostopoulos (2001), Liati et al. (2005) and Schmidt et al. (2010).

The temporal range and spatial distribution of the Evros ophiolite, ultra-high pressure and high-pressure relics and granitic magmatism (Fig. 1) are fundamental clues for a better understanding of the subduction environment associated with the Mesozoic geodynamic evolution of the Rhodope orogen.

This contribution documents the Jurassic subduction history related to ultra-high pressure and high-pressure events and granitoid magmatism in the eastern Rhodope Massif using new U–Pb zircon and ⁴⁰Ar/³⁹Ar amphibole ages derived from the Circum-Rhodope Belt Evros ophiolite and the Rhodope high-grade basement in Thrace region of NE Greece. We discuss the relevance of this new age data in the context of tectono-magmatic processes and geodynamic models.