Collisional Granitoids Research Articles

Dzhida island-arc system in the Paleoasian Ocean: structure and main stages of Vendian-Paleozoic geodynamic evolution

We propose a model of the geodynamic evolution of the Dzhida island-arc system of the Paleoasian Ocean margin which records transformation of an oceanic basin into an accretion-collision orogenic belt. The system includes several Vendian-Paleozoic complexes that represent a mature oceanic island arc with an accretionary prism, oceanic islands, marginal and remnant seas, and Early Ordovician collisional granitoids. We have revealed a number of subunits (sedimentary sequences and igneous complexes) in the complexes and reconstructed their geodynamic settings. The tectonic evolution of the Dzhida island-arc system comprises five stages: (1) ocean opening (Late Riphean); (2) subduction and initiation of an island arc (Vendian-Early Cambrian); (3) subduction and development of a mature island arc (Middle-Late Cambrian); (4) accretion and formation of local collision zones and remnant basins (Early Ordovician-Devonian); and (5) postcollisional strike-slip faulting (Carboniferous-Permian).

Foreland-forearc collisional granitoid and mafic magmatism caused by lower-plate lithospheric slab breakoff: The Acadian of Maine, and other orogens

During collisional convergence, failure in extension of the lithosphere of the lower plate due to slab pull will reduce the thickness or completely remove lower-plate lithosphere and cause decompression melting of the asthenospheric mantle; magmas from this source may subsequently provide enough heat for substantial partial melting of crustal rocks under or beyond the toe of the collisional accretionary system. In central Maine, United States, this type of magmatism is first apparent in the Early Devonian West Branch Volcanics and equivalent mafic volcanics, in the slightly younger voluminous mafic/silicic magmatic event of the Moxie Gabbro‐Katahdin batholith and related ignimbrite volcanism, and in other Early Devonian granitic plutons. Similar lower-plate collisional sequences with mafic and related silicic magmatism probably caused by slab breakoff are seen in the Miocene‐Holocene Papuan orogen, and the Hercynian‐Alleghenian belt. Magmatism of this type is significant because it gives evidence in those examples of whole-lithosphere extension. We infer that normal fault systems in outer trench slopes of collisional orogens in general, and possibly those of oceanic subduction zones, may not be primarily due to flexural bending, but are also driven by whole-lithosphere extension due to slab pull. The Maine Acadian example suggests that slab failure and this type of magmatism may be promoted by preexisting large margin-parallel faults in the lower plate.

Svecofennian magmatic and metamorphic evolution in southwestern Finland as revealed by U-Pb zircon SIMS geochronology

Zircons from six samples collected from igneous and metamorphic rocks were dated using the NORDSIM ion microprobe, in order to investigate the tectonic evolution of the Palaeoproterozoic Svecofennian Orogen in southwestern Finland. These rocks represent pre-collisional, collisional and post-collisional stages of the orogeny. The ion microprobe results reveal two age groups of granodioritic–tonalitic rocks. The intrusions have different tectonic settings: the Orijärvi granodiorite represents pre-collisional 1.91–1.88 Ga island-arc-related magmatism and yielded an age of 1898±9 Ma, whereas the collision-related Masku tonalite was dated at 1854±18 Ma. The latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to ≈1.87 Ga. This is interpreted to reflect the collision between the Southern Svecofennian Arc Complex with the Central Svecofennian Arc complex and the formation of a suture zone between them during D2 deformation. Granulite facies metamorphism in the Turku area was dated at 1824±5 Ma using zircons from leucosome in the Lemu metapelite. This age constrains D3 folding related to post-collisional crustal shortening in this area. Crustal melting continued until ≈1.81 Ga, as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons. New metamorphic zircon growth took place in older granitoids at granulite facies, but not at amphibolite facies. Detrital zircons with ages between 2.91 and 1.97 Ga were found in the mesosome of the Lemu metapelite and 2.64–1.93 Ga inherited cores were found in the 1.87 Ga Masku tonalite.

Tectonic discrimination of granitoids

Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids. The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot. The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.

Types and genetic model of Precambrian granitoids of South China

Precambrian granitoids in South China, including more than 20 bodies, are distributed on the Jiangnan ancient continent covering Guangxi, Jiangxi, Anhui and Zhejiang Provinces. The total area of granitoids is about 4300 km 3, less than 2.5% of whole area of the Jiangnan ancient continent. According to the features of the representative bodies the Precambrian granitoids in South China may be divided into four genetic types: (1) mantle-derived granitoids crystallized from differentiated basic magma; (2) cordierite-bearing magmatic granitoids whose source rocks are mainly oceanic igneous rocks; (3) cordierite-bearing magmatic granitoids originated mainly from pelitic metamorphosed basement; (4) metasomatic granitoids caused by metasomatism of metamorphosed pelitic rocks. Except for the first type, the other three types essentially belong to the S-type granite in the current classification of granites. Tectonically, they are of collision granitoids irrelevant to subduction.