Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing

The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO₂ and K₂O (72–74 wt.% and5?4–6?4 wt.%, respectively), moderate P₂O₅ and REE (0?22–0?24%and 85?0–95?7 ppm), and low Fe₂O₃^* and Zr contents (1?3–1?5%and 80–90 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (5–25%),complexly zoned plagioclase, and high Fe₂O₃ (2?72–6?99%),CaO (0?56–1?95%), Zr (101–213 ppm), and Ce (39?8–98?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.     

Rheological Transitions During Partial Melting and Crystallization with Application to Felsic Magma Segregation and Transfer

We consider the rheological behaviour of felsic magma in thezone of partial melting and during subsequent crystallization.We also introduce and combine concepts (mushy zone, percolationtheory, granular flow, shear localization) derived from thenon-geological literature and apply them to field observationson migmatites and granites. Segregation and transportation offelsic magmas is commonly observed in association with non-coaxialdeformation, suggesting that gravity forces have limited influenceduring magma segregation. Solid to liquid and liquid to solidtransitions are shown to be rheologically different, which infirmsthe concept of a unique rheological critical melt percentagefor both transitions. Four stages are examined, which dependon the melt fraction present. (1) A minimum of 8% melt by volume must first be produced toovercome the liquid percolation threshold (LPT) above whichmelt pockets can connect, thus allowing local magma displacement.Transport of the liquid phase is amplified by deformation towarddilatant sinks and is restricted to a very local scale. Thiscorresponds to partially molten domains illustrated by incipientmigmatites. (2) When more melt (20–25%) is present, a melt escapethreshold (MET) allows segregation and transport of the meltand part of the residual solid phase, over large distances.This corresponds to segregation and transfer of magma towardsthe upper crust. (3) Segregation of magma also occurs during granite emplacementand crystallization. In a flowing magma containing few particles(20%), particles rotate independently within the flow, defininga fabric. As soon as sufficient crystals are formed, they interactto construct a rigid skeleton. Such a random loose packed frameworkinvolves 55% solids and corresponds to the rigid percolationthreshold (RPT). Above the RPT, clusters of particles can sustainstress, and the liquid fraction can still flow. The only remainingpossibilities for rearranging particles are local shear zones,often within the intrusion rim, which, as a consequence, developsdilatancy. This stage of segregation during crystallizationis totally different from that of magma segregation during incipientmelting. (4) Finally, the system becomes totally locked when random closepacking is reached, at 72–75% solidification; this isthe particle locking threshold (PLT). The introduction of four thresholds must be viewed in the contextof a two-fold division of the cycle that generates igneous rocks,first involving a transition from solid to liquid (i.e. partialmelting) and then a transition from liquid to solid (i.e. crystallization).Neither transition is simply the reverse of the other. In thecase of melting, pockets of melt have to be connected to afforda path to escaping magma. This is a bond-percolation, in thesense of physical percoloation theory. In the case of crystallization,randomly distributed solid particles mechanically interact,and contacts between them can propagate forces. Building a crystalframework is a site-percolation, for which the threshold ishigher than that of bond-percolation. For each transition twothresholds are applicable. The present approach, which basicallydiffers from that based on a unique critical melt fraction,expands and clarifies the idea of a first and a second percolationthreshold. One threshold in each transition (LPT and RPT, respectively)corresponds to a percolation threshold in the sense of physicalpercolation theory. Its value is independent of external forces,but relies on the type and abundance of minerals forming thematrix within which melt connectivity is developing. The exactvalue of the second threshold (MET or PLT) will vary accordingto external forces, such as deformation and the particle shape. KEY WORDS: migmatites; partial melting; granites; magma segregation; magma solidification ^*Corresponding author. Telephone: 33 03 83 44 19 00. Fax: 33 03 83 44 00 29. e-mail: jlv{at}cregu.cnrs-nancy.fr     

High-Pressure Dehydration Melting of Metapelites: Evidence from the Migmatites of Yaounde (Cameroon)

The migmatites of Yaound? consist essentially of anatectic metapelitickyanite-garnet gneisses characterized by granulite-facies mineralassemblages. Several types of migmatitic rocks have been recognized:(1) leucosomes associated with garnet-rich melanosomes, conformablewith the regional metamorphic layering; some leucosomes aregranitic in composition whereas some others are granodioriticand characterized by low K and Rb and by the lack of HREE fractionation;(2) quartzo-feldspathic differentiations without the relatedmelanosomes, occurring as veins conformable with or cross-cuttingthe regional metamorphic layering or along shear-zones, andcorresponding mineralogi-cally to granitic or quartz-rich v?ins;(3) garnet-rocks mainly composed of garnet with abundant accessories,occurring as intrusive bodies within the migmatitic series. Structural and petrographic data suggest that the migmatitesare not derived from the surrounding granulite-facies gneissesbut that both types of rock result from a single dehydrationmelting event. The formation of migmatites or gneisses, interpretedin terms either of absence of melt extraction or of shear-inducedmelt segregation, is ascribed to variations in strain distributionwithin the metamorphic pile. The chemical characteristics of the rocks and petrogenetic modellingsuggest that the migmatites of Yaounde arose from the superimpositionof the following events: (1) subsolidus differentiation of biotite-gneisses;(2) dehydration melting of biotite-gneisses at temperaturesaround 800?C (P=10–12 kbX leading to low amounts of melt(F<0?2), which was either tectonically segregated (migmatites)or not (granulite-facies gneisses); (3) injection of anatecticmaterial comprising both partial melts and garnet-rich residues,corresponding to high melt fractions (F>0?5) and probablyformed at higher temperatures (850?C) and at deeper structurallevels. The REE signature of equilibrium partial melts (9?3<Ce_N/Yb_N78;l?2<Yb_N<5?4) indicates that granitic magmas cannot bederived from dehydration melting of biotite-bearing metapelitesonly. Several other possibilities are discussed.     

A polycyclic two-stage corona growth in the Iforas Granulitic Unit (Mali)

Abstract Retrograde and prograde mineral assemblages from metapelitic and metabasic rocks of the Iforas Granulitic Unit (Mali) were generated by the superimposition of two granulite facies metamorphic events. They clearly result from a polycyclic evolution and can be related to a late Eburnean unroofing followed by a Pan-African burial.
Thermobarometry on Pan-African garnet-bearing assemblages yields ( P, T ) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite-facies metamorphism in the Pan-African. These P, T estimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust. A P-T-t path, during the Pan-African orogeny, is proposed and discussed for both the Iforas Granulites and Kidal Assemblage.     

Thermobarometry and granite genesis: the Hercynian low-P, high-T Velay anatectic dome (French Massif Central)

The Velay dome (French Massif Central) offers a quasi-continuous section across an anatectic domain comprising low- to high-grade schists, gneisses and granites. Two main tectonometamorphic events, and their related generation of granitic material, were recognized in addition to a major Barrrovian tangential event (D2) attributed to intracontinental collision tectonics: (i) a medium- to low- P , high- T event (D3) which gave rise to migmatites and syntectonic monzonitic granites and granodiorites, and (ii) a widespread melting event (D4) which led to the generation of migmatities, the Velay granite and post-anatectic granites.
Thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms: (i) a first, associated with the D3 event, characterized by P > 5 kbar, T ≤ 750° C and water-present melting (biotite remains stable) which led to large-scale migmatization but minor amount of granites; (ii) a second, associated with the D4 event and characterized by vapour-absent melting ( P = 4–5 kbar, T = 760–850° C) which gave rise to the Velay granites and late-migmatitic granites. The temperature increase during the D4 event is attributed to the intrusion of hot mafic magmas within the crust.
The time-integrated features of the different granitic rocks in the Velay dome can be directly related to a _H2O in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 Ma.     

Petrology and Isotope Geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic-Felsic Magma Interactions During the Construction of Shallow-level Calc-alkaline Plutons

The Negash pluton consists of monzogranites, granodiorites,hybrid quartz monzodiorites, quartz monzodiorites and pyroxenemonzodiorites, emplaced at 608 ± 7 Ma (zircon U–Pb)in low-grade volcaniclastic sediments. Field relationships betweenmafic and felsic rocks result from mingling and hybridizationat the lower interface of a mafic sheet injected into partiallycrystallized, phenocryst-laden, granodiorite magma (back-veining),and hybridization during simultaneous ascent of mafic and felsicmagmas in the feeder zone located to the NW of the pluton. Therock suite displays low ⁸⁷Sr/⁸⁶Sr₍₆₀₈₎ (0·70260–0·70350)and positive