Muscovite dehydration melting: Reaction mechanisms,microstructures, and implications for anatexis

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K⁺–Na⁺ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H₂O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.     

Low pressure cordierite-bearing migmatites from Kelly's Mountain,Nova Scotia

The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H₂O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks.     

Melt Extraction during Formation of K-Feldspar + Sillimanite Migmatites, West of Revelstoke, British Columbia

An exposure of sillimanite-rich, strongly deformed, stromatic,K-feldspar-bearing migmatites in the Monashee Terrane west ofRevelstoke, British Columbia, has been examined to determinethe process of migmatization and to evaluate whether the systemwas open or closed during leucosome formation. An anatecticorigin for the migmatites is supported by: (1) the minimum meltcomposition of the leucosomes; (2) textures suggesting a fluidbehavior of the leucosomes and local pegmatitic textures; and(3) P–T estimates (720–820C; 75–9 kbar)above vaporabsent melting conditions of muscovitt + quartz. To establish whether melt was extracted or added during migmatization,measured volume percents of leucosome were compared with estimatesof melt production modeled by muscovite + quartz dehydrationmelting. Quantitative estimates of volume percent of leucosomeat present in the outcrop are between 20 and 30%. The amountof melt produced from the model muscovite dehydration meltingreaction is constrained by measured modal percent of sillimanite(15–25%) in the outcrop and is dependent on modal proportionof muscovite in the unmelted protolith and the melt water contentUsing a muscovite-rich protolith and a melt water content of4 wt%, complete dehydration melting of muscovite results ina production of 54 vol % melt and 25 vol % sillmanite, indicatinga melt loss of 29 vol %. A melt water content of 6 wt% resultsin production of 41 vol % melt and 23 vol % sillimanite, indicatinga melt loss of 16 vol %. Melt loss may have occurred by meltmovement along foliation planes during flattening, during formationof shear bands or locally along subvertical fractures. Spatialproximity of the outcrop to the Monashee dcollement suggeststhat thrusting was localized to zones of high melt production,which in turn facilitated melt migration. KEY WORDS: migmatites; British Columbia; Monashee Tarrane; anatexis; melt extraction ^*Corresponding author. Present address: Department of Earth and Planetary Sciences, The Univenity of New Mexico, Albuquerque, NM 817131, USA     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

Trace element behaviour during migmatization. Evidence for a complex melt-residuum-fluid interaction in the St. Malo migmatitic dome (France)

The St. Malo migmatitic dome represents an interesting example wherein migmatites arise from the anatexis of the surrounding gneisses. Petrographical and chemical data suggest that leucosome compositions are compatible with partial melting of the quartzo-feldsphathic fraction of the parent gneiss. The contribution of the incongruent melting of biotite to the melt does not exceed 5% of the parent rock.Petrogenetic modelling based on experimental data and assuming non modal batch melting show that the K, Rb, Ca, Sr, U and Th chemical patterns of these migmatites result in fact from the interaction of several mechanisms, namely: equilibrium partial melting, mixing between melts and refractory minerals (biotite and accessories), melt removal and late hydrothermal alteration. Zr, Y and Th which are mostly hosted in accessory minerals are significantly withheld from the melts and remain stored in melanosomes (metatexites) except when leucosomes are affected by mixing (diatexites). U is frequently enriched in the leucosomes as well as in some melanosomes suggesting external supply.     

Petrogenesis of Al–silicate-bearing trondhjemitic migmatites from NE Sardinia, Italy

The migmatites from Punta Sirenella (NE Sardinia) are layered rocks containing 3–5 vol.% of centimeter-sized stromatic leucosomes which are mainly trondhjemitic and only rarely granitic in composition. They underwent three deformation phases, from D₁ to D₃. The D₁ deformation shows a top to the NW shear component followed by a top to the NE/SE component along the XZ plane of the S₂ schistosity. Migmatization started early, during the compressional and crustal thickening stage of Variscan orogeny and was still in progress during the following extensional stage of unroofing and exhumation.

The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite.

The leucosomes have higher SiO₂, CaO, Na₂O, Sr and lower Al₂O₃, Fe₂O₃, MgO, TiO₂, K₂O, P₂O₅, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly.

The trondhjemitic leucosomes were generated by H₂O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes.     

Can trace element distributions in migmatites be used as analogues in understanding petrogenesis of anatectic granites

Trace element concentrations in leucosomes of migmatites in the Black Hills, South Dakota, USA, were examined to determine if their compositions are analogous to those of pelite-derived granites. Melanosomes in the migmatites are dominated by biotite, sillimanite, and quartz. Leucosomes have constant Si/Al that corresponds to a peraluminous granite; however, they have variable proportions of (sil+qtz)/alkali feldspar that are attributed to instability of feldspar relative to sillimanite due to high a_HF in partial melts. There are strong positive correlations of Sr, Ba, Rb, and Cs concentrations with the proportion of feldspar in the leucosomes. The average concentrations of Sr and Ba are higher and of Rb and Cs lower in the leucosomes than in pelite-derived leucogranites. A reaction progress method is used to demonstrate that partitioning of these trace elements between melanosomes and leucosomes represent mineral-mineral equilibrium rather than residue-melt equilibrium. This implies that leucosomes in migmatites may crystallize while maintaining equilibrium with melanosomes and the resulting trace element compositions may not be analogous to those of partial melts.     

Behaviour of boron in the generation of an anatectic complex: The Pen˜a Negra complex, central Spain

The Pe˜na Negra anatectic complex (central Spain) was formed from a pelitic protolith low in boron (3–80 ppm), by dehydration reactions, melting and fluid loss, which expelled much of the boron; residual B is present in sillimanite and muscovite. Migmatite melanosomes and restites (sillimanite + biotite + ilmenite) are relatively richer in B (mean 29 ppm), while the leucosomes are poorer (13 ppm). Late shears, fractures and zones of retrograde alteration have permitted some reintroduction of B. Experiments demonstrated that B cannot be removed from these rocks by heating or by leaching, indicating that very little is present in superficial adsorption and that the element occurs in mineral structures.     

Textural evolution in the transition from subsolidus annealing to melting process, Velay Dome, French Massif Central

Petrographic analysis is a useful, but underused tool to aid in distinguishing between subsolidus and anatetic-related textures in migmatites. This study focuses on assessing the relative contributions of these two processes in the development of migmatitic orthogneiss textures in the Velay Massif, French Massif Central. The results of this study show that subsolidus processes are more important in the development of migmatitic textures in the orthogneiss than anatectic leucosome development. Four textural stages are identified from the mylonitic non-anatectic orthogneiss, annealed, migmatitic orthogneiss to diatexite. The monomineralic K-feldspar and plagioclase–muscovite banding was transformed with increasing temperature to polymineralic plagioclase–quartz–muscovite and K-feldspar–quartz–muscovite layers by the wetting of feldspar boundaries during heterogeneous nucleation of quartz from a fluid phase at high surface energy triple points. A further increase of temperature led to the growth of K-feldspar probably related to production of small amounts of melt in plagioclase rich aggregates, controlled by muscovite abundance. Solid state annealing processes in conjunction with incipient anatexis resulted in the formation of apparent granitic-like textures in plagioclase dominated aggregates. By contrast, in K-feldspar dominated aggregates exclusively subsolidus processes prevail, leading to the development of coarse grained leucosome. With the onset of biotite dehydration melting the plagioclase-dominated aggregates are destroyed by the melt whereas the K-feldspar aggregates may be preserved.     

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region,east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12 kbar and 825–845 °C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30 Ma and lasted to ca. 20 Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.     

Palaeoproterozoic high-T, low-P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

A sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al₂SiO₅ polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.     

Petrology and U/Pb geochronology of the Telohat migmatites,Aleksod, Central Hoggar,Algeria

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth's, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material CRPG Contribution no 782     

P -T paths from anatectic pelites

A relatively simple petrogenetic grid for partial melting of pelitic rocks in the NCKFMASH system is presented based on the assumption that the only H₂O available for melting is through dehydration reactions. The grid includes both discontinuous and continuous Fe-Mg reactions; contours of Fe/(Fe+Mg) for continuous reactions define P-T vectors along which continuous melting will occur. For biotite-bearing assemblages (garnet+biotite + sillimanite + K-feldspar + liquid and garnet + biotite + cordierite + K-feldspar + liquid), Fe/(Fe+Mg) contours have negative slopes and melting will occur with increasing temperature or pressure. For biotite-absent assemblages (garnet + cordierite + sillimanite + K-feldspar + liquid or garnet + cordierite + orthopyroxene + K-feldspar + liquid) Fe/(Fe + Mg) contours have flat slopes and melting will occur only with increasing pressure. The grid predicts that abundant matrix K-feldspar should only be observed if rocks are heated at P < 3.8 kbar, that abundant retrograde muscovite should only be observed if rocks are cooled at P > 3.8 kbar, and that generation of late biotite + sillimanite replacing garnet, cordierite, or as selvages around leu- cosomes should be common in rocks in which melt is not removed. There is also a predicted field for dehydration melting of staurolite between 5 and 12 kbar. Textures in migmatites from New Hampshire, USA, suggest that prograde dehydration melting reactions are very nearly completely reversible during cooling and crystallization in rocks in which melt is not removed. Therefore, many reaction textures in “low grade” migmatites may represent retrograde rather than prograde reactions. Received: 5 March 1998 / Accepted: 7 August 1998     

The pressure–temperature–time path of migmatites from the Sikkim Himalaya

A combined metamorphic and isotopic study of lit‐par‐lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre‐decompression garnet growth at 23 ± 3 Ma and near‐peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr^?1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near‐isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.     

Dehydration melting of micas in the Chilka Lake Khondalites: The link between the metapelites and granitoids

Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na₂O-K₂O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.     

Wet or dry? The difficulty of identifying the presence of water during crustal melting

Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H₂O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water‐saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab‐rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all indicators have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660–750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.     

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone,eastern China

Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)_N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated ε_Hf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.     

Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada,California

We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial ⁸⁷Sr/⁸⁶Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125–0.7221). However, the leucosomes have a much wider range of initial ε_Nd values, which range from −6.0 to −11.0, as compared to −8.7 to −11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K₂O contents, Rb/Sr ratios (0.15<Rb/Sr<1.0), and initial ε_Nd values. Group B leucosomes have relatively low Rb, Pb, Ba, and K₂O contents, Rb/Sr ratios (<0.15), and initial ε_Nd values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H₂O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K₂O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb–Sr and Sm–Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities.     

P–T–t evolution of granulite facies metamorphism and partial melting in the Winding Stair Gap,Central Blue Ridge,North Carolina,USA

The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet–biotite–sillimanite–plagioclase–quartz, garnet–hornblende–biotite–plagioclase–quartz ± orthopyroxene ± clinopyroxene and orthopyroxene–biotite–quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P–T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of 850 ± 30 °C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K‐feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase. Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as five compositional zones. A few monazite inclusions in garnet, as well as Y‐rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites that grew in the main foliation plane yield dates of 370–330 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14 Ma, 438 ± 5 Ma and 360 ± 5 Ma. These data suggest that peak‐temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic orogeny. Following near isobaric cooling, a second weaker thermal pulse possibly related to intrusion of nearby igneous bodies resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian orogeny.     

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and a_H2O < 0·3. The pressure estimate impliesan emplacement depth of