Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a numerical simulation

The evolution of a subduction channel and orogenic wedge is simulated in 2D for an active continental margin, with P-T paths being displayed for selected markers. In our simulation, subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years, with four zones from the trench into the forearc: (i) an accretionary complex of low grade metamorphic sedimentary material, (ii) a wedge of nappes with alternating upper and lower crustal provenance, and minor interleaving of oceanic or hydrated mantle material, (iii) a megascale melange composed of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks extruded from the subduction channel, and (iv) the upward tilted frontal part of the remaining lid. The P–T paths and time scales correspond to those typically recorded in orogenic belts. The simulation shows that HP/UHP metamorphism of continental crust does not necessarily indicate collision, but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction.     

Kampelite,Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O,a new very complex Ba-Sc phosphate mineral from the Kovdor phoscorite-carbonatite complex (Kola Peninsula,Russia)

Mineralogy and Petrology - Kampelite, Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O, is a new Ba-Sc phosphate from the Kovdor phoscorite-carbonatite complex (Kola Peninsula, Russia). It is orthorhombic, Pnma,...     

Petrology and retrograde P-T path in granulites of the Kanskaya formation, Yenisey range, Eastern Siberia

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.
Biotite-garnet-cordierite-sillimanite-quartz as well as spinel±biotite-garnet°Cordierite±sillimanite-quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net-transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P-T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P-T diagram these trajectories are subparallel lines, whose P-T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P-T evolution of the Kanskaya formation is proposed.     

P–T–fluid evolution in the Mahalapye Complex, Limpopo high-grade terrane, eastern Botswana

Metapelites, migmatites and granites from the c. 2 Ga Mahalapye Complex have been studied for determining the P–T–fluid influence on mineral assemblages and local equilibrium compositions in the rocks from the extreme southwestern part of the Central Zone of the Limpopo high‐grade terrane in Botswana. It was found that fluid infiltration played a leading role in the formation of the rocks. This conclusion is based on both well‐developed textures inferred to record metasomatic reactions, such as Bt ? And + Qtz + (K₂O) and Bt ± Qtz ? Sil + Kfs + Ms ± Pl, and zonation of Ms | Bt + Qtz | And + Qtz and Grt | Crd | Pl | Kfs + Qtz reflecting a perfect mobility (Korzhinskii terminology) of some chemical components. The conclusion is also supported by the results of a fluid inclusion study. CO₂ and H₂O ( = 0.6) are the major components of the fluid. The fluid has been trapped synchronously along the retrograde P–T path. The P–T path was derived using mineral thermobarometry and a combination of mineral thermometry and fluid inclusion density data. The Mahalapye Complex experienced low‐pressure granulite facies metamorphism with a retrograde evolution from 770 °C and 5.5 kbar to 560 °C and 2 kbar, presumably at c. 2 Ga.     

Structural and P-T Evolution of a Major Cross Fold in the Central Zone of the Limpopo High-Grade Terrain, South Africa

The Central Zone (CZ) of the Limpopo Complex of southern Africais characterized by a complex deformational pattern dominatedby two types of fold geometries: large sheath folds and crossfolds. The sheath folds are steeply SSW-plunging closed structureswhereas the cross folds are north–south-oriented withnear-horizontal fold axes. In the area south of Messina thiscomplexly folded terrain grades continuously towards the southinto a crustal-scale ENE–WSW-trending ductile shear zonewith moderate dip towards the WSW. All sheath folds documentconsistent top-to-the-NE thrust movement of high-grade material.The timing of this shear deformational event (D₂) and thus ofthe gneissic fabric (S₂) is constrained (at     

Analyzing the commercial activities of a street network by ranking their nodes: a case study in Murcia,Spain

Urban researchers and planners are often interested in understanding how economic activities are distributed in urban regions, what forces influence their special pattern and how urban structure and functions are mutually dependent. In this paper, we want to show how an algorithm for ranking the nodes in a network can be used to understand and visualize certain commercial activities of a city. The first part of the method consists of collecting real information about different types of commercial activities at each location in the urban network of the city of Murcia, Spain. Four clearly differentiated commercial activities are studied, such as restaurants and bars, shops, banks and supermarkets or department stores, but obviously we can study other. The information collected is then quantified by means of a data matrix, which is used as the basis for the implementation of a PageRank algorithm which produces a ranking of all the nodes in the network, according to their significance within it. Finally, we visualize the resulting classification using a colour scale that helps us to represent the business network.     

Numerical models of the thermomechanical evolution of planetesimals: Application to the acapulcoite‐lodranite parent body

The acapulcoite‐lodranite meteorites are members of the primitive achondrite class. The observation of partial melting and resulting partial removal of Fe‐FeS indicates that this meteorite group could be an important link between achondrite and iron meteorites, on the one hand, and chondrite meteorites, on the other. Thus, a better understanding of the thermomechanical evolution of the parent body of this meteorite group can help improve our understanding of the evolution of early planetesimals. Here, we use 2‐D and 3‐D finite‐difference numerical models to determine the formation time, initial radius of the parent body of the acapulcoite‐lodranite meteorites, and their formation depth inside the body by applying available geochronological, thermal, and textural constraints to our numerical data. Our results indicate that the best fit to the data can be obtained for a parent body with 25–65 km radius, which formed around 1.3 Ma after calcium‐aluminum‐rich inclusions. The 2‐D and 3‐D results considering various initial temperatures and the effect of porosity indicate possible formation depths of the acapulcoite‐lodranite meteorites of 9–19 and 14–25 km, respectively. Our data also suggest that other meteorite classes could form at different depths inside the same parent body, supporting recently proposed models (Elkins‐Tanton et al. 2011 ; Weiss and Elkins‐Tanton 2013 ).     

Protocore destabilization in planetary embryos formed by cold accretion: Feedbacks from non-Newtonian rheology and energy dissipation

It is suggested that a distinct population of Mars-sized planetary embryos can form by cold accretion in the outer part of the terrestrial zone. A possible radial structure for such embryos consists of an outer silicate-rich layer and an undifferentiated solid protocore. The protocore is divided in a cold undifferentiated central region, a layer of iron, and an outer shell of undifferentiated material. This structure is gravitationally unstable. We investigate the influence of the rheology of the silicate-rich material and of the thermal effects on the protocore destabilization and on the iron core formation. We use a 2D thermomechanical numerical model of a self-gravitating planetary body. The planetary surface is treated as a free surface by imposing a massless weak medium. Our calculations include a non-Newtonian temperature-, pressure-, and strain rate-dependent viscoplastic rheology and thermal contributions from shear heating. We explored the influence of rheology by varying the activation volume, friction angle, and the maximum yield strength of silicate-rich layers. We distinguished three different core formation regimes: exposure mode, fragmentation mode, and transition mode. Like in previous models with Newtonian rheology, the core experiences large deviations from the spherical shape and is temporarily exposed at the surface in the exposure mode. In contrast to the Newtonian models the destruction of the protocores in the fragmentation modes is driven by (i) the spontaneous strain localization along planetary-scale shear zones forming inside the protocore, and/or (ii) descending localized iron diapirs/sheets penetrating the protocore. Feedback from energy dissipation notably modifies the thermal structure of the deforming planetary body. In particular it causes a temperature increase of up to several hundred Kelvin (i) around the moving and deforming protocore and (ii) along planetary scale rupture zones that form inside the protocore. If the protocore is large and has a high viscosity, a large fraction of the dissipated heat is used to increase the temperature of iron.     

Alumovesuvianite,Ca19Al(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group member from the Jeffrey mine,asbestos, Estrie region,Québec,Canada

Mineralogy and Petrology - Alumovesuvianite (IMA 2016–014), ideally Ca19Al(Al,Mg)12Si18O69(OH)9, is a new vesuvianite-group member found in the rodingite zone at the contact of a gabbroid...