Publication Date:
2015-08-18
Description:
The Variscan Upper Allochthon is a continental-affinity terrane that recorded a Cambrian – Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision-related high-pressure metamorphism during the Devonian – Carboniferous amalgamation of Pangea. The objective of this paper is to decipher which continental margin subducted in the Devonian HP–HT event. To do so, a provenance study is presented using combined U – Pb (n = 613) and Lu – Hf (n = 463) isotopic LA – ICP – MS zircon analyses and Sm – Nd whole–rock (n = 5) determinations. These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex, NW Iberia), which forms part of the HP–HT bottom member of the Upper Allochthon. Paleozoic–Neoproterozoic zircon ages (34.7%) have a maximum abundance at 522–512 Ma, peaks at 575, 561, 545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu–Hf compositions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu–Hf pattern it is proposed that this population is also West Africa Craton derived. The Paleoproterozoic population (39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle derived magmas intruded an Archean craton margin. This craton is represented by the Archean population (22.8%), which is grouped at 2.52 - 2.48 Ga, 2.68 - 2.61 Ga and 3.0 Ga, and shows long term reworking processes and at least two juvenile magma intrusions. These data show that the Variscan Upper Allochthon has a West-African provenance and therefore it strongly suggests that the NW Iberian allochthonous complexes, and their correlative European terranes, are also West-Africa derived. These results allow us to finally clarify that the first high-pressure event, recorded during the eo-Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia. This article is protected by copyright. All rights reserved.
Print ISSN:
0263-4929
Electronic ISSN:
1525-1314
Topics:
Geosciences
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