Evolved magmas, including highly explosive rhyolites, are mainly generated by extraction of viscous melts from solid residues either in (1) partial melting zones within the crust (dominantly up-temperature evolution with newly formed silicic melt), or in (2) long-lived crystallizing mush zones fed by mafic to intermediate magmas (dominantly down-temperature evolution with residual silicic melt). Although both processes undoubtedly occur and are generally coupled, allowing for mixing between mantle and crustal components, we argue that combined field, thermal, geochemical, and geophysical observations favor residual melt extraction from crystalline mushes as the likely scenario in all tectonic settings. Depending on the main melting process in the mantle, two end-member differentiation trends occur: (1) a dry lineage leading to hot-reduced rhyolites and granites in magmatic provinces fueled by decompression melting of the mantle; (2) a wet lineage leading to cold-oxidized rhyolites and granites in subduction zones dominated by flux melting of the mantle.

/pdf/rhyolites-and-their-source-mushes-across-tectonic-settings-2j4x2rd6jx.pdf

Rhyolites and their Source Mushes across Tectonic Settings

The formation, age and trace element composition of zircon and monazite were investigated across the prograde, low-pressure metamorphic sequence at Mount Stafford (central Australia). Three pairs of inter-layered metapelites and metapsammites were sampled in migmatites from amphibolite-facies (T 600 C) to granulite-facies conditions (T 800 C). Sensitive high-resolution ion microprobe U–Pb dating on metamorphic zircon rims and on monazite indicates that granulite-facies metamorphism occurred between 1795 and 1805Ma. The intrusion of an associated granite was coeval with metamorphism at 1802 ± 3Ma and is unlikely to be the heat source for the prograde metamorphism. Metamorphic growth of zircon started at T 750 C, well above the pelite solidus. Zircon is more abundant in the metapelites, which experienced higher degrees of partial melting compared with the associated metapsammites. In contrast, monazite growth initiated under sub-solidus prograde conditions. At granulite-facies conditions two distinct metamorphic domains were observed in monazite. Textural observations, petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak. Ages from sub-solidus, prograde and peak metamorphic monazite and zircon are not distinguishable within error, indicating that heating took place in less than 20 Myr.

/pdf/temperature-and-bulk-composition-control-on-the-growth-of-k6qxd267g3.pdf

Temperature and Bulk Composition Control on the Growth of Monazite and Zircon During Low-pressure Anatexis (Mount Stafford, Central Australia)

Petrogenesis and Experimental Petrology of Granitic Rocks

Corrosion of heavy minerals during weathering and diagenesis: A catalog for optical analysis

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

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

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Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks