Petrotectonic implications of metabasites of the Eastern Andean Metamorphic Complex at Lago O´Higgins-San Martin, southern Patagonia

<p>The Eastern Andean Metamorphic Complex (EAMC) in southwestern Patagonia (4°-52°S) is a 450 km long belt mainly composed by low-grade metasedimentary rocks of Upper Devonian-lower Carboniferous, and Permian-lower Triassic ages. Previous works have suggested a passive margin environment for the deposition of the protolith.  The EAMC comprise scarce interleaved tectonic slices of marbles, metabasites, and exceptional serpentinite bodies. At Lago O´Higgins-San Martin (48°30’S-49°00’S) the metasedimentary sucessions are tectonically juxtaposed with lenses of pillowed metabasalts and greenschists having OIB, N-MORB, BABB and IAT geochemical affinities. The Nd-isotopic composition of metabasalts is characterized by εNd<sub>(t=350 Ma)</sub> of +6 and +7. The metabasalts show no signal of crustal contamination, instead, the mantle source was probably modified by subduction components. New and already published provenance data based on mineralogy, geochemistry and zircon geochronology indicate that the quartz-rich protolith of metasandstones were deposited during late Devonian-early Carboniferous times (youngest single zircon ages around of latest Devonian-earliest Carboniferous times) sourced from igneous and/or sedimentary rocks located in the interior of Gondwana, as the Deseado Massif, for instance. Noticeable, the detrital age patterns of all samples reveal a prominent population of late Neoproterozoic zircons, probably directly derived from igneous and/or metaigneous rocks of the Brasiliano/Pan-African orogen or from reworked material from variably metamorphosed sedimentary units that crops out at the same latitudes in the extra-Andean region of Patagonia. We propose that the protolith of metabasites formed part of the upper part of an oceanic-like lithosphere generated in a marginal basin above a supra-subduction zone, where plume-related oceanic island volcanoes were generated. The closure of the marginal basin, probably in mid-Carboniferous times, or soon after. The oceanic lithosphere was likely underthrusted within an east-to-northeast-dipping subduction zone, where ophiolitic rocks and metasedimentary sequences were tectonically interleaved at the base of an accretionary wedge.</p>

Formation of a Late Cretaceous continental margin arc and an Early Cenozoic back-arc basin in the Kyrenia Range, northern Cyprus related to S-Neotethyan subduction

<p>Field, geochemical and geochronological research on Late Cretaceous and Early Cenozoic volcanic rocks in Kyrenia Range provide constraints on the tectono-magmatic evolution of the northerly, active continental margin of the Southern Neotethys. Field mapping in the western Kyrenia Range demonstrates that frontal (southerly) thrust sheets are dominated by felsic volcanogenic rocks. U-Pb zircon dating indicates that the felsic volcanics erupted at 72.9 ± 1.0 Ma (Late Campanian). These volcanics are interpreted as the products of sub-aqueous continental margin arc volcanism based on geochemical evidence. The exposed arc volcanics are somewhat younger than arc-derived volcaniclastic sediments in W Cyprus (80.1 ± 1.1 Ma), and are also younger than arc-related granitic rocks (88-82 Ma) cutting the Tauride active continental margin (Malatya-Keban platform) in SE Turkey. Structurally higher (more northerly) imbricate thrust sheets include Late Cretaceous (Maastrichtian) and Early Cenozoic basalts that are underlain by a Mesozoic continental carbonate platform (metamorphosed), and interbedded with pelagic and redeposited carbonates that formed in an active continental margin setting. The basalts have within-plate geochemical characteristics, although with a variable subduction influence in some areas (e.g., western Kyrenia Range) that could be either be contemporaneous or inherited from Late Cretaceous (c. 70-80 Ma) subduction. Modern and ancient comparisons (e.g., Tyrrhenian Sea) suggest that the basaltic rocks represent incipient, extensional marginal basin formation. Integration with comparable evidence of continental margin arc magmatism in SE Turkey and elsewhere provides a picture of arc magmatism and marginal basin formation along an active continental margin, prior to collision during the Miocene.</p>

Chapter 3 Pre-Late Cretaceous basement terranes of the Gondwana active margin of New Caledonia

The basement under the Late Cretaceous unconformity in New Caledonia consists of three amalgamated terranes. They are all oceanic, arc-related and developed offshore from the eastern Gondwana active margin during periods of marginal basin development. Téremba Terrane is composed of deep sea Permian to Mesozoic arc-derived volcanic rocks and greywackes. The Koh–Central Terrane includes at its base an ophiolite with island arc tholeiites and boninites (Koh Ophiolite) of Late Carboniferous to Early Permian age overlain by a thick sequence of greywacke (Central Range Volcaniclastic Rocks) of Permian to Late Jurassic age. The Téremba Terrane and the Koh–Central Terrane may be part of the same forearc basin, with the rocks from the Koh–Central Terrane deposited in a deeper environment. The Boghen Terrane is a metamorphic complex composed of schists, broken formations and mafic–ultramafic mélange, derived from mixed terrigenous and volcanic sources. The overall fine grain size and laminar bedding suggest deep sea and more distal deposition than the other terranes. The maximum depositional ages from detrital zircons suggest deposition during the Early Jurassic to Early Cretaceous. The terrane is interpreted as a metamorphosed subduction complex that includes blueschist and greenschist facies metamorphic rocks exhumed through the Koh–Central Terrane. At a regional scale, the nature of these three pre-Late Cretaceous terranes confirms the existing palaeogeographical reconstructions, which locate New Caledonia outboard the ocean–continent subduction that surrounded Gondwana during the Paleozoic and Early Mesozoic. A detailed analysis of these terranes and their relationship with East Australian terranes of the same age shows that a marginal basin system probably existed between mainland Gondwana and proto-New Caledonia and closed before the Late Cretaceous. A tentative detailed reconstruction of this margin during the Carboniferous–Early Cretaceous period is proposed.