Joe Hiess

Setting of the ∼2560 Ma Qôrqut Granite Complex in the Archean crustal evolution of Southern West Greenland

The Archean gneiss complex of West Greenland contains packages of unrelated rocks created during relatively short periods of time in arc-like magmatic environments, and having similarities to rocks formed at Phanerozoic convergent plate boundaries. The terranes of new Archean crust were amalgamated by collisional orogeny and then partitioned by post-assembly tectonic processes. Having summarized the origin of West Greenland Archean crust in arc-like environments, this paper then focuses on new data concerning the latest Neoarchean post terrane-assembly “intra-continental” tectonic and magmatic evolution of the region. Following the youngest documented high pressure metamorphism in a clockwise P-T-t loop at ∼2650 Ma, attributed to tectonic thickening of the crust, there is in West Greenland a 150 million year record of intermittent production of crustally-derived granite, shearing and folding under amphibolite facies conditions. This is exemplified by the SSW-NNE orientated Neoarchean Qôrqut Granite Complex (QGC) which forms a myriad of closely spaced coeval sheets NE of Nuuk town. SHRIMP U-Pb zircon dating of a homogeneous gray granite sheet gives a magmatic age of 2561±11 Ma, with 3800 to 3600 and 3070 to 2970 Ma zircon xenocrysts. The >40 km long Fjringehavn straight belt, a lower amphibolite-facies vertical shear zone, runs from the QGCs SE margin and contains strongly deformed granite sheets with a U-Pb zircon age of 2565±8 Ma but is cut by undeformed granite sheets dated at 2555±12 Ma. The >60 km long Ivisaartoq fault consisting of lowermost amphibolite-facies mylonite runs from the northwestern end of the main mass of granite. It formed post-2630 Ma, because granites of that age are truncated by it. Near the QGCs northeastern extent, 2559±3 Ma granitic lithons in a folded mylonite are cut by 2521±72 Ma granite sheets. At a deep structural level at the northern end of the QGC, deformed granitic neosomes give ages of 2567±9 and 2563±5 Ma. Therefore, at ∼2560 Ma, the ages within error for strongly deformed to non-deformed granite bodies shows that the QGC is not a largely post-kinematic intrusion as previously thought, but was coeval with lowermost amphibolite-facies metamorphism and shear zones with an important strike slip component, late in the development of regional non-cylindrical upright folds. The main body of the QGC appears to be essentially post-kinematic only because it was emplaced in a node of dilation during the heterogeneous predominantly strike slip deformation. Melting at this node may have been triggered by meteoric water percolating down dilational fractures, causing metasomatism. Melting of these altered rocks gave rise to the low δ18O signature of QGC igneous zircons. Due to the hydrous nature of the melting event, the QGC was emplaced immediately above its migmatitic generation zone. These late Neoarchean shear zones of the Nuuk region partition and disrupt the earlier-formed mosaic of amalgamated terranes of unrelated rocks. Such tectonic patterns are seen more recently, for example in Holocene Asian intra-continental tectonics along the north side of the Himalayas.

U-Th-Pb zircon and monazite geochronology of Western Province gneissic rocks, central-south Westland, New Zealand

Abstract Ion-microprobe (Sensitive High Resolution Ion MicroProbe or SHRIMP) zircon and monazite U-Th-Pb geochronology of amphibolite-facies orthogneiss and paragneiss units, mostly from the Fraser and Granite Hill Complexes of Westland, New Zealand, has constrained the ages of protolith rocks and metamorphic overprints. Paragneiss zircon (6 samples, 175 analyses) comprise detrital grains of Paleozoic to Archean age and match patterns from metasedimentary (Ordovician) Greenland Group rocks of the Western Province, which are characteristic of early Paleozoic flysch units around the southwest Pacific margin of Gondwana. Orthogneiss zircon (9 samples, 138 analyses) reflect intense Devonian and Cretaceous magmatism associated with emplacement of the Karamea-Paparoa and Hohonu Batholiths within the Western Province. Both paragneiss and orthogneiss monazite analyses (10 samples, 116 analyses) are dominated by Devonian and Cretaceous ages, reflecting thermal pulses related to regional and localised granitoid intrusions. These results indicate that the Fraser and Granite Hill Complex gneisses are metamorphosed parts of the Western Province Buller Terrane with Karamea-Paparoa and Hohonu Batholith intrusive components.

Influence of the crust and crustal structure on the location and composition of high-alumina basalts of the Taupo Volcanic Zone, New Zealand

Abstract High‐alumina basalts (HABs) that occur throughout the central part of the Taupo Volcanic Zone (TVZ) are associated particularly with faulting, and many occur where faults intersect caldera margins. For convenience, the basalts are described in terms of three geographic‐tectonic segments: Okataina in the north, Kapenga in the middle, and Taupo in the south. Evidence for mixing and mingling between rising basaltic magmas and rhyolitic rocks and magmas is common, including the frequent occurrence of xenocrysts and xenoliths, quench textures, and melting around the rims of inclusions. Chemically, the basalts are similar in terms of major element compositions, suggesting relatively homogeneous PT conditions in the mantle source, but variation between some trace elements suggests different processes are operating in the crust with variable degrees of contamination. The model presented for HAB generation in the TVZ is for partial melting of mantle peridotite in the upper mantle, with the melt rising into the lower crust via dike swarms. In the upper crust, the distribution of HAB is strongly influenced by location and structure. In the Kapenga segment, there is little evidence for interaction between basaltic and rhyolitic magma, other than at very shallow levels, perhaps because the rhyolitic magma chambers (or pods) were solid, allowing brittle deformation and rapid intrusion of basalt dikes. At Okataina there is much greater mixing and mingling, suggesting there was still partially molten rhyolitic magma chambers beneath this area during basalt intrusion. Basalt in the Taupo segment occurs outside the Taupo caldera complex and may be related to the earlier Whakamaru caldera complex. The basalt is thought to rise through the crust as a network of unrelated melt batches into a plexus of discrete magma chambers and conduits, many of which are sited along fault zones causing fissure eruptions at the surface.