ALBERT

Beschreibung: The oldest rocks in New Zealand are the Mid- to Late Cambrian intra-oceanic island arc rocks of the Takaka terrane (Devil River arc). The provenance of Cambrian conglomerates stratigraphically above the exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental crust. The Dead Goat Conglomerate contains two distinct groups of igneous clasts: (1) intermediate to felsic volcanic clasts with moderately enriched light rare earth element (LREE) and high field strength element (HFSE) contents and positive ϵNd500 (+2.1) that were derived from a medium-K calc-alkaline source, probably the main sequence of the Devil River arc; (2) dioritic to metagranitic plutonic clasts strongly enriched in LREE and HFSE and with ϵNd500 of +3.5 to +5.9 that were derived from a high-K arc source, probably the uppermost units of the Devil River arc. This is consistent with a new U–Pb sensitive high-resolution ion microprobe age of 496 ± 6 Ma. The Lockett Conglomerate also contains two distinct groups of igneous clasts: (1) ultramafic to intermediate igneous clasts identified as boninitic to transitional low-K calc-alkaline arc-related rocks based on depleted REE and HFSE abundances; (2) ‘I’-type metagranitoid clasts derived from a distinct Andean type continental margin, as indicated by ϵNd500 as low as −7.1. Both conglomerates contain sandstone clasts derived from a common old, multi-cycle continental source with ϵNd500 of −14.2 to −15.7, and no suitable source has been found in present-day New Zealand. The new provenance data from these conglomerates constrain the time of accretion of the Devil River arc to the palaeo-Pacific Gondwana margin and provide new information on the structural evolution of the accretionary event.

Beschreibung: We investigated the Neoproterozoic–early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon analysis and dating of syn- and post-tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45–0.65 Ga and 0.9–1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474–442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician.