The Negash pluton consists of monzogranites, granodiorites, hybrid quartz monzodiorites, quartz monzodiorites and pyroxene monzodiorites, emplaced at 608 ± 7 Ma (zircon U–Pb) in low-grade volcaniclastic sediments. Field relationships between mafic and felsic rocks result from mingling and hybridization at the lower interface of a mafic sheet injected into partially crystallized, phenocryst-laden, granodiorite magma (back-veining), and hybridization during simultaneous ascent of mafic and felsic magmas in the feeder zone located to the NW of the pluton. The rock suite displays low 87Sr/86Sr(608) (0·70260–0·70350) and positive εNd(608) values (+3·9 to +5·9), along with fractionated rare earth element patterns [(La/Yb)N = 9·9–17·7], enrichment in large ion lithophile elements (Ba, U, K, Pb and Sr) and depletion in Nb and Th compared with the primitive mantle. Monzogranites, granodiorites and hybrid quartz monzodiorites define a calc-alkaline differentiation trend, whereas the quartz monzodiorites have higher Fe/Mg ratios. The pyroxene monzodiorites show anomalously high Ti/Zr, Ti/Y and Ti/V ratios, suggesting that they are cumulates. Chemical modelling suggests that pyroxene and quartz monzodiorites could derive from a common gabbrodioritic parent by fractional crystallization. Structural and chemical data suggest that (1) the pluton results from the assembly of several, low-viscosity, melt-rich batches (sheeting/dyking), differentiated in intermediate chambers prior to their emplacement; (2) in situ differentiation is limited to the coarse-grained pyroxene monzodiorites; (3) mafic–felsic magma interactions at the emplacement level were essentially limited to mingling.
We consider the rheological behaviour of felsic magma in the zone of partial melting and during subsequent crystallization. We also introduce and combine concepts (mushy zone, percolation theory, granular flow, shear localization) derived from the non-geological literature and apply them to field observations on migmatites and granites. Segregation and transportation of felsic magmas is commonly observed in association with non-coaxial deformation, suggesting that gravity forces have limited influence during magma segregation. Solid to liquid and liquid to solid transitions are shown to be rheologically different, which infirms the concept of a unique rheological critical melt percentage for both transitions. Four stages are examined, which depend on the melt fraction present. (1) A minimum of 8% melt by volume must first be produced to overcome the liquid percolation threshold (LPT) above which melt pockets can connect, thus allowing local magma displacement. Transport of the liquid phase is amplified by deformation toward dilatant sinks and is restricted to a very local scale. This corresponds to partially molten domains illustrated by incipient migmatites. (2) When more melt (20–25%) is present, a melt escape threshold (MET) allows segregation and transport of the melt and part of the residual solid phase, over large distances. This corresponds to segregation and transfer of magma towards the upper crust. (3) Segregation of magma also occurs during granite emplacement and crystallization. In a flowing magma containing few particles (≤20%), particles rotate independently within the flow, defining a fabric. As soon as sufficient crystals are formed, they interact to construct a rigid skeleton. Such a random loose packed framework involves ∼55% solids and corresponds to the rigid percolation threshold (RPT). Above the RPT, clusters of particles can sustain stress, and the liquid fraction can still flow. The only remaining possibilities for rearranging particles are local shear zones, often within the intrusion rim, which, as a consequence, develops dilatancy. This stage of segregation during crystallization is totally different from that of magma segregation during incipient melting. (4) Finally, the system becomes totally locked when random close packing is reached, at ∼72–75% solidification; this is the particle locking threshold (PLT). The introduction of four thresholds must be viewed in the context of a two-fold division of the cycle that generates igneous rocks, first involving a transition from solid to liquid (i.e. partial melting) and then a transition from liquid to solid (i.e. crystallization). Neither transition is simply the reverse of the other. In the case of melting, pockets of melt have to be connected to afford a path to escaping magma. This is a bond-percolation, in the sense of physical percoloation theory. In the case of crystallization, randomly distributed solid particles mechanically interact, and contacts between them can propagate forces. Building a crystal framework is a site-percolation, for which the threshold is higher than that of bond-percolation. For each transition two thresholds are applicable. The present approach, which basically differs from that based on a unique critical melt fraction, expands and clarifies the idea of a first and a second percolation threshold. One threshold in each transition (LPT and RPT, respectively) corresponds to a percolation threshold in the sense of physical percolation theory. Its value is independent of external forces, but relies on the type and abundance of minerals forming the matrix within which melt connectivity is developing. The exact value of the second threshold (MET or PLT) will vary according to external forces, such as deformation and the particle shape.
Na Provincia Mineral de Carajas, um importante evento magmatico gerador de granitoides moderadamente alcalinos (Granito Old Salobo e Complexo Granitico Estrela) ocorreu no final do Arqueano (~ 2,56 Ga). Granitos com tal assinatura geoquimica (tipo A) sao muitas vezes originados pela fusao parcial de rochas metaigneas e/ou de granulitos. Os liquidos magmaticos produzidos sao normalmente de temperaturas elevadas, e baixa viscosidade, sendo colocados em niveis crustais rasos. Na regiao de Carajas, estes corpos graniticos tardi-arqueanos impuseram aumentos significativos nos gradientes geotermicos, afetaram os sistemas isotopicos Rb-Sr e criaram modificacoes das condicoes reologicas de suas rochas encaixantes metavulcano-sedimentares (grupos Salobo, Pojuca, Grao-Para) onde formaram aureolas metamorficas e tectonicas de comportamento ductil. Tres dominios foram observados nas rochas metavulcano-sedimentares encaixantes do Complexo Granitico Estrela: uma aureola externa (~ 450-550°C), uma interna (~ 550-650°C) e os xenolitos (~ 650-850°C). Na aureola externa as rochas metavulcano-sedimentares se apresentam pouco estruturadas, mostrando uma foliacao (S1) discreta e concordante ao acamadamento regional (S0). Na aureola interna as rochas encaixantes mostram uma forte foliacao (S2) subvertical e uma lineacao de alto ângulo de caimento. Veios preenchidos por anfibolio sao comuns nas metabasicas situadas na aureola interna e dos xenolitos. A distribuicao espacial das facies petrograficas e os dados geoquimicos, estruturais e aerogeofisicos mostram que o Complexo Granitico Estrela e formado por diferentes plutons. A organizacao estrutural do complexo e marcada por um bandamento magmatico (S0) de disposicao concentrica, ou seja, subvertical nas porcoes perifericas e concordante com os limites dos plutons, e subhorizontal nas partes centrais destes. A disposicao espacial do bandamento magmatico (S0) indica que a colocacao do Complexo Granitico Estrela foi controlada principalmente pelo mecanismo de ballooning. Uma foliacao (S1) e um bandamento magmatico secundario associado, de direcao geral E-W e mergulhos verticais, se desenvolve em resposta ao somatorio de esforcos ligados a colocacao (ballooning) e de esforcos regionais coaxiais de direcao N-S. Zonas miloniticas (S1m) resultaram de instabilidade mecânicas no final da consolidacao do complexo. Esta evolucao estrutural, em regime de deformacao progressiva e temperatura decrescente, comprova o comportamento sintectonico do Complexo Granitico Estrela.
Abstract The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca. 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca. 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al., 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregurà Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican (ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic (ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.
'/%3e%3cpath d='m450 224.315-44.467 136.87 116.401-84.559h-143.87l116.403 84.559z' style='fill:none;stroke:%23006233;stroke-width:14.62993431;stroke-miterlimit:4;stroke-dasharray:none'/%3e%3c/svg%3e)