Abstract
The Ossa-Morena Zone (OMZ) has a complex geological history including both Cadomian and Variscan orogenic events. Therefore, the OMZ plays an important role in understanding the geodynamic evolution of Iberia. However, the P–T–t evolution of the OMZ is poorly documented. Here, we combine structural and metamorphic analyses with new geochronological data and geochemical analyses of mafic bodies in Ediacaran metasediments (in Iberia known as Série Negra) to constrain the geodynamic evolution of the OMZ. In the studied mafic rocks, two metamorphic stages were obtained by phase equilibria modelling: (1) a high-pressure/low-temperature event of 1.0 ± 0.1 GPa and 470–510 °C, and (2) a medium-pressure/higher-temperature event of 0.6 ± 0.2 GPa and 550–600 °C. The increase in metamorphic temperature is attributed to the intrusion of the Beja Igneous Complex (around 350 Ma) and/or the Évora Massif (around 318 Ma). New U–Pb dating on zircons from the mafic rocks with tholeiitic affinity yields an age between 815 and 790 Ma. If the zircons crystallised from the tholeiitic magma, their age would set a minimum age for the pre-Cadomian basement. The ca. 800 Ma protolith age of HP-LT tholeiitic dykes with a different metamorphic history than the host Série Negra lead us to conclude that: (1) the HP-LT mafic rocks and HP-LT marbles with dykes were included in the Ediacaran metasediments as olistoliths; (2) the blueschist metamorphism is older than 550 Ma (between ca. 790 Ma and ca. 550 Ma, e.g., Cadomian).
Highlights
Peri-Gondwanan terranes involved in the Variscan amalgamation of Europe show evidence of a late Neoproterozoic arc-related tectono-thermal history starting around 750 Ma, and can be divided into two groups: (i) Cadomian terranes, Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Switzerland including northern Armorica, Ossa-Morena, Saxo-Thuringia, Moldanubia, and (ii) Avalonian terranes, including West Avalonia, East Avalonia, Carolina, Moravia-Silesia and NW Iberia (Murphy et al 2006)
From the late Neoproterozoic to the earliest Cambrian (Sánchez-Garcıa et al 2003), the Ossa-Morena Zone (OMZ) was involved in the Cadomian collision (e.g., Eguiluz et al (2000); Linnemann et al (2008)), which included the accretion of a continental arc (OMZ) against the passive margin of the Iberian Autochthon (Northern Gondwana)
We present a new age of ca. 815–790 Ma obtained by U–Pb dating on zircon from the mafic rocks intruded in marbles found in the Série Negra
Summary
The peak pressure conditions are estimated using the XMg in garnet core (Table 4) and the albitic plagioclase relics This corresponds to the stability field in the upper left corner of the diagram: Amp–Grt–Zo–Pg–Ab–Qz field around 510 °C and 1.00 GPa. The contours of XMg of the garnet rim and XAn of oligoclase in the matrix (Table 4) cross in the stability field: Chl–Amp–Pl–Grt–Qz (Fig. 10b). The contours for XMg of the garnet rim and XAn in plagioclase (Table 4) correspond to the P–T conditions of about 550–600 °C and 0.5–0.6 GPa. The calculated phase diagram section shows a clear distinction between amphibole-bearing high temperaturelow pressure corner and clinopyroxene-bearing low temperature and high-pressure corner (Fig. 11a). The probability diagram (based on the 206/238 age of 32 measurements) shows a concentration of ages around 330–370 Ma (Fig. 13d)
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