In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.

Experimental determination of the fluid-absent melting relations in the pelitic system

A classification is proposed, based mainly on major element analytical data plotted in a coherent set of three simple chemical-mineralogical diagrams. The procedure follows two complementary steps at two different levels. The first is concerned with the individual sample: the sample is given a name (e.g. granite, adamellite, granodiorite) and its chemical and mineralogical characteristics are determined. The second one is more important: it aims at defining the type of magmatic association (or series) to which the studied sample or group of samples belongs. Three main types of association are distinguished: cafemic (from source-material mainly or completely mantle-derived), aluminous (mainly or completely derived by anatexis of continental crust), and alumino-cafemic (intermediate between the other two types). Subtypes are then distinguished among the cafemic and alumino-cafemic associations: calc-alkaline (or granodioritic), subalkaline (or monzonitic), alkaline (and peralkaline), tholeiitic (or gabbroic-trondhjemitic), etc. In the same way, numerous subtypes and variants are also distinguished among the aluminous associations using a set of complementary criteria such as quartz content, colour index, alkali ratio, quartz–alkalies relationships and alumina index.Although involving a new approach using partly new criteria, this classification is consistent with most of the divisions used in previous typologies. The method may also be used in the classification of the volcanic equivalents of common plutonic rocks.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300010117

A chemical–mineralogical classification of common plutonic rocks and associations

Experimental investigation of the fluid absent melting reaction biotite+plagioclase+A12SiOs+quartz~ garnet + K-feldspar + melt at 10 kbar, indicated that melt- ing began between 760 and 800~ and was extensive at 850 ~ C This reaction apparently has a positive dP/dT at least up to 10 kbar, compared to a calculated backbending in a simplified system Natural biotites are stabilised to higher dehydration melting temperatures probably by Ti Consequently, metapelites undergo two-stages of fluid-ab- sent melting even in thickened, continental crust The two stages firstly as muscovite, and secondly as biotite react, appear to persist to at least 17 kbar, depending upon biotite composition This lies well within the stability field of kya- nite migmatites Thus, complex petrogenetic grids can be computed from the little available data over a wide range of P-T-Xroek-XH20 conditions Microprobe analyses indicate good correlations with the Fe/Mg garnet + biotite exchange thermometer and the garnet +plagioclase +A12SiO5 + quartz barometer at 850 ~ C, 10 kbar Furthermore, micro- probe analyses show that vGar'" Liq ~yBio in fluid-absent ~x Fe f Fe t- l Fe as well as in HzO-saturated pelite anatexis Thus intermedi- ate Fe/Mg compositions, containing Bio, Gar, Als, Plg, Ksp, Qtz will eutectically melt before Fe-richer or Mg-richer metapelites If the partition VG,r/vLiq reverses with increas- " Fe /~x Fe

Fluid-absent (dehydration) melting of biotite in metapelites in the early stages of crustal anatexis

The lanthanide tetrad effect and its correlation with k/rb, eu/eu*, sr/eu,y/ho, and zr/hf of evolving peraluminous granite suites

Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture, structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation, mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, ﬂow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated (including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sign indicating compositional maturity of the continental crust, and they are also closely related to the rare-elemental (metal) mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, REEs, etc.

Highly fractionated granites: Recognition and research

Peraluminous granitoid rocks are defined chemically as those in which the ratio (molecular proportions) ALOg/ (CaO + NarO + KnO) (= A/ CNK) > l. Mineralogically these sranites consist of quartz * two feldspars { one or more characteristic phases such as biotite, muscovite, garnet, cordierite, the aluminosilicates Al2SiOu, mullite. topaz, tourmaline, spinel and corundum. The factors that control the appearance of these phases are temperature, pressure, Pnora/Pro*,, and compositional variables such as l(O,), f(CO,), f(HCl), l (HF), :(MgO * FeO), MgO/FeO and minor elements (B. Be, Mn, Li. Cs, erc.). Various origins for each of the characteristic phases have been proposed for different batholiths. A genetic classification of these phases is proposed as follows: 1. exogenic: (a) restite, (b) xenocrysts from highJevel country rock: 2. endogenic: (a) nucleation from silicate melt with A/CNK > l, (b) nucleation from watersaturated, alkali-depleted silicate melt with A/CNK )) 1, (c) metasomatic replacement of silicate rock by vapor phase, (d) nucleation directly from aqueous fluid. Rocks formed by processes in categories 1(a), l (b) and 2(a) are termed "normal peraluminous" because they involve only meltphase compositions that can be reached by silicate melt crystal equilibria. Those in the remaining categories, 2(b), 2(c) and 2(d). are termed "hyperaluminous" because they represent melt or wholerock compositions with A/CNK )) I that result from the interaction of fltrids with silicate melts' or solid rocks.

The mineralogy of peraluminous granites; a review

Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And ¼ Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T–P–X controls on andalusite formation, using eight textural and chemical criteria: size— compatibility with grain sizes of igneous minerals in the same rock; shape—ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation—single grains or clusters of grains; association with muscovite—with or without rims of monocrystalline or polycrystalline muscovite; inclusions—rare mineral inclusions and melt inclusions; chemical composition—andalusite with little significant chemical variation, except in iron content (008–171 wt % FeO); compositional zoning—concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions;
compositions of coexisting phases—biotites with high siderophyllite–eastonite contents (Aliv 268 007 atoms per formula unit), muscovites with 057–401 wt % FeO and 002–
285 wt % TiO2, and apatites with 353 018 wt % F.
Coexisting muscovite–biotite pairs have a wide range of F contents, and FBt ¼ 1612FMs þ 0015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic—(a) prograde metamorphic (in thermally
metamorphosed peraluminous granites), (b) retrograde
metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic—(a) peritectic (water-undersaturated, T") associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T#), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T#) direct
crystallization from a silicate melt, and (d) pegmatitic (watersaturated, T#), associated with aplite–pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic—(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O3, water concentration and fluid evolution, high Be–B–Li–P, high F, high Fe–Mn–Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the
evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And ¼ Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may
completely replace andalusite leaving little or no evidence of its former existence.

/pdf/occurrence-and-origin-of-andalusite-in-peraluminous-felsic-4l0vmfc0uo.pdf

Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks

Geochemical evolution of the South Mountain Batholith, Nova Scotia; rare-earth-element evidence

Occurrence and origin of garnets in the South Mountain Batholith, Nova Scotia

Hypothetical liquidus relationships in the subsystem Al 2 O 3 -FeO-MgO projected from quartz, alkali feldspar and plagioclase for a(H 2 O) < or = 1

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