Anatectic Event Research Articles

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H2O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H2O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al2O3, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO2), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO* +TiO2 less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.

Monazite solubility and dissolution kinetics: implications for the thorium and light rare earth chemistry of felsic magmas

A series of monazite dissolution experiments was conducted in a hydrous (1–6 wt.%) granitic melt at 8 kbar over the temperature range 1,000–1,400° C. A polished cube of monazite was immersed in a natural obsidian melt and allowed to partially dissolve. Electron microprobe traverses perpendicular to the crystal-melt interface revealed concentration gradients in the LREEs and P. Diffusivities of the LREEs and P were calculated from these profiles, yielding the following Arrhenius relations for the LREEs: D=0.23 exp(−60.1 kcal mol−1/RT) at 6% water D=2.30×107 exp(−122.1 kcal mol−1/RT) at 1% water These results demonstrate the importance of dissolved water on REE diffusion. Phosphorus diffusivities are nearly identical to those of the rare-earths, suggesting that P diffusion charge-compensates REE diffusion. The concentration of LREEs required for monazite saturation in these melts is given by the level of dissolved LREEs at the crystal-melt interface. These values also show a dependence on dissolved water, with LREEsat=60 ppm at 6% H2O when extrapolated down to 700° C, and LREEsat=30 ppm at 1% H2O. Calculated dissolution rates based on the above parameters indicate that minute ( 2% H2O), whereas larger (> 50 μm) crystals will likely be residual over the duration of an anatectic event. The low solubility of monazite in this melt suggests that the LREE depletion observed in some felsic differentiation suites may be the result of monazite crystallization. Limited experimental and geochemical/petrologic evidence indicates that compositional changes in the melt accompanying differentiation decrease the solubility of monazite drastically. Kinetic and chemical constraints may also lead to localized monazite saturation and inclusion in major phases or even other accessories. Variations in the REE composition of monazite from different parageneses probably reflects the REE pattern of the parent melt, and may be due to gradational differences in the stability of individual or subgroup REE-complexes as a function of melt composition. Particularly important in this regard seems to be the lime+alkali/alumina balance of the melt and its volatile content.

Precambrian deformed granites of possible basement in the Himalayas

Abstract Granitic rocks are abundant in the Axial zone, the core of the Himalayas. The Lesser Himalayan unfossiliferous metasedimentary sequence lying to the south of the Axial zone also contains concordant sheets of granitic rocks which were earlier described either as intrusive or metamorphosed volcaniclastic sediments. These granites have yielded Precambrian radiometric ages. The Lesser Himalayan deformed granites have been demarcated and we suggest that they represent thrust wedges of the Precambrian (older than 1000 Ma) basement rocks of the Himalayas. The deformed granite sheets of the Eastern Himalayas show the development of various types of mylonite. The chemical characters of these granites vary within reasonable limits. Geochemistry and Sr isotope ratios suggest their derivation by melting from crustal material and sediments, probably greywacke, to variable degrees. From geochemical evidence it is suggested that the central crystalline rocks of the Axial zone may contain reworked Precambrian basement components. The studied granites provide evidence of a Precambrian metamorphic and anatectic event in the Himalayas.

Les gneiss rubanés à cordiérite et grenat d'ihosy: Un marqueur thermo-barométrique dans le sud de Madagascar

The Precambrian of Madagascar appears to be a part of the Mozambiquian mobile belt. Some authors consider the southern part of the island to be constituted, from east to west, by more and more recent formations, i.e., the ultrametamorphic ‘Androyan’ sequence with an Archean reworked crust as the oldest one, first followed by the ‘Graphite’ sequence, then by the ‘Vohibory’ sequence. The cordierite and garnet banded gneisses and leptynites are widely distributed in the Androyan sequence. These gneisses result from an incipient partial melting of paragneisses. Garnet—plagioclase, garnet—cordierite and garnet—biotite geothermo- and geobarometers allow the estimation of the conditions of this migmatisation ( T > 700°C, P T = 5−5.5 kbar, PH 2O = 0.3−0.4 P T). Some cordierite and garnet leptynites represent the leucosomes of this anatectic event. Since previous experiments showed that the seven-phase assemblage (quartz + plagioclase + K feldspar + garnet + biotite + sillimanite + cordierite) only occurs in a limited range of pressure and temperature ( T = 690−710°C, P T = 4−5.7 kbar, PH 2O = 0.2−0.5 P T), we can consider these gneisses as very precise thermobarometric reference levels in southern Madagascar. This migmatic event may be contemporary with the Pan-African orogenesis.

Ras ed Dom migrating ring complex: A-type granites and syenites from the Bayuda Desert, Sudan

A major cluster of anorogenic A-type, subvolcanic ring complexes of Silurian to Jurassic age is emplaced into Proterozoic rocks of the basement complex in the Bayuda Desert of northern Sudan. The complexes belong to the Younger granite association which is widespread throughout the Sudan and environs. The Ras ed Dom complex is a striking example of migrating centres in which intrusive activity shifted eastwards and declined in strength with time. Intrusive rocks predominate and consist of metaluminous and peralkaline syenites and granites. Field relations indicate that the sequence metaluminous syenite-peralkaline syenite-peralkaline granite is a recurrent fractionation series representing high level fractionation of a parental metaluminous syeno-monzonitic magma. The metaluminous syenites contain rare early-formed plagioclase (oligoclase-andesine) but otherwise the suite is typically hypersolvus. The metaluminous rocks are characterised by an early pyroxene zone from ferrohedenbergite to aegirine-augite and enclosed in ferroedenitic amphibole. Microprobe analysis reveals a broad range of amphibole compositions in the peralkaline rocks, comprising ferrorichterite-arfvedsonite-riebeckite solid solutions. Late aegirine, exhibiting marked enrichment in the acmite component, typically replaces the arfvedsonite, especially in the granites. Extremely pure albite accompanies aegirine and the development of these phases is attributed to limited interaction between the rocks and late sodic fluids. Geochemical variation in the suite includes marked depletions of CaO, Ba and Sr with increasing silica, consistent with fractionational removal of alkali-feldspar. The peralkalinity of the later rocks has been enhanced by sodic autometasomatism, reflected in their high contents of REE and HFS elements such as Zr, Nb and Ga, and by the erratic distribution of these elements. LREE-enrichment in the suite is marked and suggests equilibrium with a residual HREE-rich phase such as garnet in the source rocks. The Younger Granite magmas are attributed to melting of a lower crustal mafic-rich source such as garnet-eclogite. The heat for this anatectic event is thought to have come from mantle-derived basalt magma, the uprise of which was largely checked in the lower crust, only minor amounts reaching the surface.

U-Pb investigations on zircons from pre-Variscan gneisses—I. A study from the Schwarzwald, West Germany

Field observations in the Schwarzwald indicate that pre-Variscan rocks were formed by the following events: 1. (1) Intrusion of granitic to granodioritic magma into sediments. 2. (2) A sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity. The absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved. We collected several samples of a coarse-grained, homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the Einetwald forest north of Steinach (Kinzig valley). The metasedimentary rock corresponds to the biotitepoor paragneisses in the Schwarzwald. The zircons in the latter sample are detrital and only very small specimens (⩽ 45μm) are euhedral. The zircons from the orthogneiss samples are euhedral. Some are zoned and some have dark inclusions. Analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 Myr and 524 ± 20 Myr, respectively. The 2000 Myr value is interpreted as the average age of the sedimentary source rocks for this material. The 524 Myr intercept defines an event responsible for most of the lead-loss in the zircons as well as new zircon growth. Analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons. Stepwise leaching of two zircon fractions shows newly grown U-rich zircon rims around old cores. A 207Pb 206Pb - age of 517 ± 15 Myr was found for the first dissolution steps (about 50% of the zircons dissolved). In subsequent steps with progressive dissolution of the residue. 207Pb 206Pb - age increased to 950 Myr. The U-Pb zircon data for the paragneiss and the orthogneiss establish an age of 520 ± 15 Myr for the magma intrusion.