2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

36
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.19
Did Laurentia and Gondwana play terrane tennis in the Palaeozoic? The
implications of a Iapetus convergent margin in the Merida Terrane,
Venezuela
Van der Lelij Roelant 1 , Spikings Richard 1 , Ulianov Alexey 2 , & Chiaradia Massimo 1
1Départment de Minéralogie, Section des Sciences de la Terre et de l’Environnement, Université de Genève, Rue des Maraîchers 13,
CH-1205 Genève, roelant.vanderlelij@unige.ch
2Institute of Mineralogy and Geochemistry, University of Lausanne, Switzerland
The tectonic, metamorphic and palaeogeographic origin of the basement terranes of the circum-Maracaibo region,
Venezuela and Colombia, are poorly understood. A majority of the reconstructions of western Pangaea place the Merida
Terrane of western Venezuela between the Guyana Shield, Mexican and Central American terranes, and hence unravelling
its tectonic history is a key step to reconstructing western Pangaea. We aim to develop a robust framework for the age and
tectonic origin of the Merida Terrane by using new, in-situ LA-ICP-MS zircon U-Pb geochronology on isotopically and geochemically
characterised igneous and metamorphic rocks of the basement complexes.
Widespread I- and S-type arc magmatism and amphibolite facies metamorphism of sedimentary rocks of the Iglesias
Complex occurred between the late Cambrian and early Devonian (500-415 Ma), corresponding to a north facing Iapetus
continental arc. Detrital zircons from these metasedimentary rocks are early Cambrian and older, which provide a maximum
age for deposition. A magmatic hiatus in the Merida Terrane spans the Devonian to Permian, from ~415- 75 Ma.
A compilation of previous work suggests that faunal assemblages from several locations in the Merida Terrane reveal significant
assemblage transitions. Faunal affinities change from Gondwanan in the Cambrian, to Acado-Baltic in the
Ordovician, to Appalachian in the Silurian. This shift is also characteristic of the Avalonia microcontinent, which rifted
off Gondwana in the Cambrian and accreted to Baltica and finally Laurentia in the Silurian.
To the south of the Merida Andes, the Apure Fault separates the Early Palaeozoic igneous and metamorphic belt from the
Guyana Shield. The latter exhibits extensive Cambrian rifts which were reactivated in the Jurassic, and K/Ar dates obtained
from biotite and feldspar from the basement rocks of the Guyana Shield suggest that the Palaeoproterozoic crust has not
experienced a significant thermal event since ~1Ga. Therefore, we propose that the Apure Fault is the south-eastern
boundary of the Merida Terrane.
We propose two contrasting scenarios for the evolution of NW Gondwana and the Iapetus and Rheic oceans:
A) The Merida Terrane may be autochthonous or para-autochthonous to Gondwana, which would imply that the NW
corner of Gondwana was a Cambrian to Silurian active margin. This implicitly means that the rift to drift transition
which led to the opening of the Rheic Ocean in the Cambrian-Ordovician did not occur this far west, precluding the
Rheic Ocean separating NW Gondwana from Laurentia, which in turn suggests that these two supercontinents were
close or perhaps connected by the Silurian. A simultaneous back-arc to rift-to drift transition, as required for the opening
of the Rheic Ocean in the Cambrian-Ordovician, would lead to scenario B:
B) The Merida Terrane may be an allochthonous, Avalonia type terrane which separated from Gondwana in the Cambrian-
Ordovician due to back-arc basin becoming an oceanic spreading centre (Figure 1). In this scenario, NW Gondwana
would become the southern passive margin of the Rheic Ocean; the Merida Terrane would at the same time be the
southern Iapetus active margin and the Rheic Ocean northern passive margin (Figure 1). In the Silurian, the Merida
Terrane would, like Avalonia, collide with Laurentia during as the Iapetus closed, resulting in the Acadian orogeny in
the Appalachians. Following the Amalgamation of Pangaea it would have been re-accreted to Gondwana.
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT