Fluid-present Partial Melting Research Articles

Petrogenesis of A-type leucocratic granite magmas: An example from Delbegetei massif, Eastern Kazakhstan

The genesis of leucogranite magmas is important issue of geodynamics, petrology and ore geology because leucogranites are associated with collisional belts, partial melting of sedimentary source rocks, and may host rare metal (Sn, W, Li, NbTa, and Be) mineralization. To establish the petrogenesis of large leucogranite intrusions, detailed studies of petrography, mineralogy, fluid regime, and the material and isotopic composition of rocks are required. The paper reports results of the studies of the Delbegetei massif of the intrusion in Eastern Kazakhstan. The massif is composed predominantly of leucocratic granites, while syenogranites are subordinate. Rocks of the massif belong to shoshonitic and high-K calc-alkaline series; demonstrate a predominance of K over Na, high ferroan and high contents of LREE and HFSE; which allows them to be classified as A-type granites. The age of Delbegetei massif, estimated by the UPb zircon dating, varies in the range 249–240 Ma, which correspond the Early-Middle Triassic. The differences in rock composition and in temperatures of zircon saturation allow supposing that syenogranites and leucogranites formed from different parental magmas. Syenogranite magma formed as a result of partial melting of metamorphosed volcanic rocks (andesidacites or dacites) with possible influence of mafic magmas. Leucogranite magma formed as a result of fluid-present partial melting of metaterrigenous sedimentary rocks. Leucogranite magma underwent the feldspars differentiation in the fluid-present conditions. This led to composition variations of leucogranites. Analysis of the geological position, age and composition of the rocks allows concluding that the Delbegetei massif formed at the Early Triassic in an intraplate geodynamic setting and that the activity of the Siberian mantle plume is the most probable reason for their formation.

Petrogenesis of the Early Paleozoic S-type granites in southern Alxa, Northwest China and its implications on fluid flourishing in the middle and lower crust

Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO2 (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO2 (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO2 = 72.50–76.04 wt%) are different from the two-mica granites in having higher Na2O (3.38–4.58 wt%) and lower K2O (3.70–4.90 wt%) with higher Na2O/K2O ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.

Partial melting of a depleted peridotite metasomatized by a MORB-derived hydrous silicate melt – Implications for subduction zone magmatism

Recent geodynamic models and geothermometers suggest that slabs in intermediate to hot subduction zone cross the water-saturated basalt solidus, indicating that hydrous silicate melts are important agents of mass transfer from slab to mantle wedge beneath arcs. Yet the effects of basaltic crust-derived hydrous melt fluxing on mantle wedge melting are poorly known. Here we present the melting phase relations of a depleted peridotite + a MORB-derived hydrous silicate melt at a melt:rock mass ratio of 0.1 and 0.05 (3.5 and 1.7 wt.% H2O, respectively) to simulate fluid-present partial melting of a depleted peridotite, which has been metasomatized by a hydrous silicate melt derived from subducting basaltic crust. Experiments were performed at 2–3 GPa and 900–1250 °C in a piston cylinder, using Au and Au75Pd25 capsules. Amphibole (7–10 wt.%) is stable up to 1000 °C at 2 and 3 GPa coexisting with an assemblage dominated by olivine and opx and with minor fractions of cpx and garnet at 3 GPa. The apparent fluid-saturated solidus of our bulk composition is located at 1000–1050 °C, coinciding with the exhaustion of amphibole at 2 and 3 GPa. Amphibole is exhausted between 0 and 5 wt.% melting at 2 and 3 GPa and dominates the melting reactions in this melting interval along with opx, generating SiO2 and Al2O3-rich, and FeO*- and MgO-poor primitive andesites under fluid-saturated conditions. The melting reactions during low-degree, fluid-saturated melting are incongruent, consuming opx and producing olivine + SiO2-rich melts and is observed over a wide range of starting compositions and pressures from this study and others. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 – 4.2 wt.% H2O can reproduce the major oxide spread and trends of primitive arc magmas globally. From this comparison, it is clear that differences solely in the pressure of hydrous mantle melting, where the partial melts are fluid-under saturated, can account for the first order trends observed in experimental and natural data, with differences in temperature and composition contributing to the compositional spread within these trends. The ubiquity of andesite genesis over a wide range of pressures and bulk compositions during aqueous fluid-saturated melting suggests that the relative rarity of primitive andesitic melt flux through the crust could be related to the fact that such melts are only produced at the base of the mantle wedge where temperatures are relatively low. As fluid-saturated andesitic melts ascend into the hotter core of the mantle wedge, they are likely consumed by higher-degree, fluid-undersaturated melting generating more common hydrous basaltic melts.

Zircon U-Pb Dating and Petrogenesis of Multiple Episodes of Anatexis in the North Dabie Complex Zone, Central China

The North Dabie complex zone (NDZ), central China, is a high-T ultrahigh-pressure (UHP) metamorphic terrane. It underwent a complex evolution comprising of multistage metamorphism and multiple anatectic events during the Mesozoic continental collision, characterized by granulite-facies overprinting and a variety of migmatites with different generations of leucosomes. In this contribution, we carried out an integrated study including field investigation, petrographic observations, zircon U-Pb dating, and whole-rock element and Sr-Nd-Pb isotope analysis for the migmatites in the NDZ and their leucosomes and melanosomes. As a result, four groups of leucosomes have been recognized: Group 1 (garnet-bearing leucosome), strongly deformed leucosomes with coarse-grained peritectic garnet; Group 2 (amphibole-rich leucosome), weakly deformed to undeformed amphibole-rich leucosomes with coarse-grained peritectic amphibole and no garnet; Group 3 (amphibole-poor leucosome), weakly deformed to undeformed amphibole-poor leucosomes with minor fine-grained amphibole; Group 4 (K-feldspar-rich leucosome), K-feldspar-rich leucosomes mainly composed of coarse-grained quartz, plagioclase and K-feldspar. Zircon SHRIMP and LA-ICPMS U-Pb dating suggest that the Group 1 leucosomes formed at 209 ± 2 Ma whereas the rest of the leucosome groups (Groups 2–4) occurred between 145–110 Ma, in response to decompression under granulite-facies conditions during the early stage of exhumation, and to heating during post-orogenic collapse, respectively. Furthermore, the garnet-bearing leucosomes were resulted from fluid-absent anatexis related to biotite dehydration melting, while the other three groups of leucosomes were formed during large-scale fluid-present partial melting and coeval migmatization. This migmatization comes from heating from the mountain-root removal and asthenosphere upwelling, together with the influx of fluids derived from country rocks at mid-upper crustal levels. However, all the leucosomes and melanosomes display Pb-isotopic compositions similar to those observed for the NDZ UHP rocks (eclogites and granitic gneisses), suggesting a common source from the Triassic subducted Neoproterozoic lower-crustal rocks. In addition, the Cretaceous partial melting and migmatization began at 143 ± 2 Ma with three age-peaks at 133 ± 3 Ma, 124 ± 3 Ma and 114 ± 7 Ma, respectively.

Fluid-present anatexis of Neoarchean tonalite and amphibolite in the Western Shandong Province

Metatonalite and amphibolite from the Taishan region of the Western Shandong Province in the North China Craton record c. 2.60 Ga fluid-present partial melting via the breakdown of biotite, plagioclase and quartz to produce peritectic hornblende and anatectic melt. Eight paired leucosome–melanosome samples from metatonalite and three paired samples from amphibolite were investigated to evaluate the composition of the melt. Hornblende, biotite and plagioclase in the leucosomes and hornblende and plagioclase in melanosomes from both rock types have similar compositions. Two leucosome samples from the metatonalite were influenced by the removal of heavy rare earth element-rich hornblende and the accumulation of plagioclase. The other leucosomes are interpreted to represent near initial melt compositions with a minor component of peritectic hornblende and are enriched in Si, Na and Sr and depleted in K, Ca, Ba and Rb relative to melanosomes. The enrichment of Na in the melt is inconsistent with experimental results of fluid-present melting of tonalite, but is broadly consistent with the experimental results of fluid-fluxed melting of amphibolite. The absence of K-feldspar in both rock types is a critical control on the composition of anatectic melt and initial melt compositions were probably similar for both rock types. Leucosomes inherited rare earth element patterns from their sources, which suggests that some trace element diagrams used to infer tectonic settings and depths of melting are not appropriate for reworked components of Archean grey gneisses. Whole-rockδ18O suggest that the fluids responsible for inducing local melting were derived from the intrusion and crystallization of ~2.60 Ga tonalites and trondhjemites in the Taishan region. One amphibolite sample has a relatively low δ18O suggestive of interaction with meteoric water or seawater possibly related to crustal extension and asthenosphere upwelling at ~2.60 Ga. Fluid-present partial melting reworked 2.75–2.60 Ga tonalites and amphibolites, generating ~2.60 Ga sodium-rich components of grey gneisses in the Western Shandong Province.

Prograde infiltration of Cl-rich fluid into the granulitic continental crust from a collision zone in East Antarctica (Perlebandet, Sør Rondane Mountains)

Utilizing microstructures of Cl-bearing biotite in pelitic and felsic metamorphic rocks, the timing of Cl-rich fluid infiltration is correlated with the pressure-temperature-time (P-T-t) path of upper amphibolite- to granulite-facies metamorphic rocks from Perlebandet, Sør Rondane Mountains (SRM), East Antarctica. Microstructural observation indicates that the stable Al2SiO5 polymorph changed from sillimanite to kyanite+andalusite+sillimanite, and P-T estimates from geothermobarometry point to a counterclockwise P-T path characteristic of the SW terrane of the SRM. In situ laser ablation inductively coupled plasma mass spectrometry for U–Pb dating of zircon inclusions in garnet yielded ca. 580Ma, likely representing the age of garnet-forming metamorphism at Perlebandet.Inclusion-host relationships among garnet, sillimanite, and Cl-rich biotite (Cl>0.4wt%) reveal that formation of Cl-rich biotite took place during prograde metamorphism in the sillimanite stability field. This process probably predated partial melting consuming biotite (Cl=0.1–0.3wt%). This was followed by retrograde, moderately Cl-bearing biotite (Cl=0.1–0.3wt%) replacing garnet. Similar timings of Cl-rich biotite formation in different samples, and similar f(H2O)/f(HCl) values of coexisting fluid estimated for each stage can be best explained by prograde Cl-rich fluid infiltration. Fluid-present partial melting at the onset of prograde metamorphism probably contributed to elevate the Cl concentration (and possibly salinity) of the fluid, and consumption of the fluid resulted in the progress of dehydration melting. The retrograde fluid was released from crystallizing Cl-bearing partial melts or derived externally. The prograde Cl-rich fluid infiltration in Perlebandet presumably took place at the uppermost part of the footwall of the collision boundary. Localized distribution of Cl-rich biotite and hornblende along large-scale shear zones and detachments in the SRM supports external input of Cl-rich fluids through tectonic boundaries during continental collision.

Geochemical constraints on the source nature and melting conditions of Triassic granites from South Qinling in central China

The Qinling orogen marks the continental collision between the North China Block and the South China Block in the Triassic, but the Triassic collisional orogeny is only prominent in the South Qinling zone with the occurrence of voluminous granites. In order to understand the petrogenesis of these granites, a combined study of petrology, geochronology and geochemistry was carried out for them from outcrops at Guangtoushan (GTS) and Huayang (HY) in South Qinling. The results indicate that the studied granites were derived from partial melting of metasedimentary rocks rather than metaigneous rocks as suggested by previous studies. LA-ICPMS zircon U–Pb dating yields concordant ages of 207±3Ma to 212±4Ma for their magma emplacement. The GTS granites are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.15 and K2O/Na2O ratios of 0.5 to 1.32 and show high whole-rock δ18O values of 9.52 to 11.20‰. The HY granites are peraluminous with high A/CNK values of 1.04 to 1.14 and K2O/Na2O ratios of 1.01 to 2.82, and have high zircon δ18O values of 10.71 to 11.43‰ and whole-rock δ18O values of 12.42–13.77‰. On the other hand, the GTS granites exhibit low Rb/Sr ratios but high Sr/Ba ratios, variable Eu anomalies (Eu/Eu⁎=0.29–1.05), in association with low zircon saturation temperatures. In contrast, the HY granites have high Rb/Sr ratios but low Rb/Ba ratios, conspicuous negative Eu anomalies (Eu/Eu⁎=0.52–0.77) and relatively high zircon saturation temperatures. An integrated interpretation of all these results is that the GTS granites were produced by fluid-present partial melting of a mixture source of metapelite and metagreywacke at low temperatures, whereas the HY granites were generated by fluid-absent partial melting of a metagreywacke at relatively higher temperatures. Furthermore, the two granite plutons show variably high whole-rock (87Sr/86Sr)i ratios of 0.7047 to 0.7115, negative whole-rock εNd (t) values of −9.8 to −6.8 and zircon εHf (t) values of −13.1 to −2.0, with two-stage Nd-Hf model ages of Mesoproterozoic to Paleoproterozoic. Their εNd (t) values are comparable with those for sedimentary rocks in the southern margin of the South Qinling zone and the northern margin of the South China Block, suggesting a genetic link between them. Therefore, the GTS and HY granites originated from the sedimentary rocks rather than igneous rocks, and thus belong to S-type rather than I-type. It is inferred that these sedimentary rocks were subducted to the lower crustal depth during the Late Triassic collisional orogeny in the Qinling orogen for partial melting to produce the S-type granites.

Partitioning of Nb and Ta between rutile and felsic melt and the fractionation of Nb/Ta during partial melting of hydrous metabasalt

In order to fully assess the role of rutile in fractionation of Nb/Ta during partial melting of hydrous metabasalt, we have measured rutile – felsic melt partition coefficients ( D values) for Nb and Ta with tonalitic to trondhjemitic compositions at 1.5–3.5 GPa, 900–1350 °C and ∼5.0–20 wt% H 2O. D Nb, D Ta and D Nb/ D Ta range from 17 ± 1 to 246 ± 13, 34 ± 2 to 232 ± 25 and 0.51 ± 0.04 to 1.06 ± 0.13, respectively. For the compositions investigated, melt composition appears to have no observable effect on the partitioning; the effect of pressure is also slight; whereas temperature and H 2O have marked effects. D Nb, D Ta and D Nb/ D Ta increase with decreasing temperature and H 2O content, showing a reversal of D Nb/ D Ta from <1.0 to >1.0. Using the data that approached equilibrium and obeyed Henry’s law, expressions describing the dependences of D Nb, D Ta and D Nb/ D Ta on temperature, pressure and melt H 2O content were obtained: (1) ln ( D Nb ) rutile/melt = - 2.846 ( ± 0.453 ) + 9621 ( ± 470 ) / T + 0.207 ( ± 0.101 ) P - 0.042 ( ± 0.009 ) H 2 O (2) ln ( D Ta ) rutile/melt = - 0.775 ( ± 0.398 ) + 6954 ( ± 413 ) / T + 0.140 ( ± 0.089 ) P - 0.009 ( ± 0.007 ) H 2 O (3) ln ( D Nb / D Ta ) rutile/melt = - 2.075 ( ± 0.289 ) + 2657 ( ± 301 ) / T + 0.075 ( ± 0.065 ) P - 0.033 ( ± 0.006 ) H 2 O where T is in K, P is in GPa, and H 2O are in wt%. These expressions are applicable to calculate Nb and Ta partitioning coefficients between rutile and felsic (highly siliceous) melts and Nb/Ta fractionation during partial melting of metabasalt. The derived models suggest that fluid-present partial melting of metabasalt always leads to D Nb/ D Ta lower than 1.0, whereas dehydration melting may lead to D Nb/ D Ta higher than 1.0 at temperatures <1000 °C and melt H 2O contents <10 wt%. Low-degree melting cannot substantially change the Nb/Ta ratio of rutile-bearing eclogite residues relative to their basalt precursors, whereas high-degree melting leads to lower Nb/Ta ratios in the residues. These results thus confirm that partial melting of hydrous metabasalt cannot produce rutile-bearing eclogites with superchondritic Nb/Ta. Fractionation of Nb/Ta for the melts relative to their basalt precursors can occur during low-degree melting due to the high D Nb and D Ta and variable D Nb/ D Ta. The fractionation direction depends on melting degree (temperature) and protolith initial H 2O content (assuming that hydrous phases are consumed entirely during partial melting), whereas the fractionation extent depends on protolith initial H 2O and TiO 2 contents. Fluid-present melting at <20 wt% melt fractions will significantly elevate melt Nb/Ta ratio, whereas dehydration melting with low initial H 2O (<1.2 wt%) may reduce melt Nb/Ta ratio. In general, rutile in eclogite residues accounts for the highly variable Nb/Ta ratios in the Archean TTG magmas. In particular, the very high Nb/Ta ratios of some TTG magmas may only be explained by low-degree (<20 wt%) melting in the presence of both rutile and a hydrous fluid.

Fluid and deformation induced partial melting and melt volumes in low-temperature granulite-facies metasediments, Damara Belt, Namibia

Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study we report on granulite-grade metasediments from the Pan-African Damara Belt in Namibia that have undergone fluid-present biotite melting at a relatively low-temperature, yet appear to have lost a significant volume of melt. In situ anatectic features have been identified on the basis of the existence of new generations of cordierite and/or garnet produced as the solid products of incongruent anatexis within or adjacent to leucosomes. Leucosomes occur in lens-shaped pods that are orientated at high angles to the regional stretching lineation that formed during orogen-parallel extrusion of the rocks during the main collisional event in the Damara Belt. Within these sites biotite underwent incongruent melting via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd. Cordierite nucleated on pre-existing crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulite-facies peak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower than the accepted conditions for the onset of fluid-absent biotite melting. This, coupled to the focussing of anatexis on dilational sites, suggests that anatexis occurred through water-present biotite incongruent melting. In order to better understand this process, both fluid-absent and water-present partial melting experiments were conducted within the temperature interval 700 to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between 800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, in water-saturated experiments, biotite melted via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3. The garnet compositions produced in these low-temperature experiments match very well with the relatively high spessartine content (10%), low Mg# garnets formed with the leucosomes, confirming the water-saturated, low-temperature nature of anatexis in the study area. Mass balance calculations indicate that melting in typical dilational sites has produced 6 dm 3 of melt. This melt has been sourced from some 0.08 m 3 of rock, a volume that coincides well with the typical frequency of extensional fracture spacing. This was probably important in enabling efficient melt extraction which has caused a slight shift towards more refractory compositions in the granulite-facies metapelites over amphibolite-facies equivalents. However, the attendant expansion in chemical equilibration volume has resulted in a marked compositional homogenization of the high-grade gneisses over the rhythmically bedded amphibolite-facies metapelites.

Abrupt transition in H2O activity in the melt-present zone of a thermal aureole: Evidence from H2O contents of cordierites

The water content of cordierite was analyzed in seven samples of pelitic hornfels from successive contact-metamorphic zones in the Etive thermal aureole, Scotland, using secondary ion mass spectrometry. Together with independent estimates of peak metamorphic pressure and temperature, these results were used to calculate an H 2 O activity ( a H 2 O) in each sample. The calculated a H 2 O values are considered to reflect peak metamorphic conditions. In the middle aureole calculated a H 2 O values are high, close to unity, suggesting fluid-present conditions. In the innermost aureole they are low, a H 2 O appears to be abrupt and occurs at higher grade than the melt-in isograd. The results are compatible with the operation of an up-temperature sequence of reactions in pelitic and semipelitic rocks involving, progressively, (1) dehydration, followed by (2) fluid-present partial melting, followed by (3) fluid-absent partial melting. The consumption of H 2 O by fluid-present melting reactions provides a mechanism for reducing a H 2 O at the transition. A xenolith within the Quarry Diorite appears to have crystallized in the presence of magmatically derived hydrous fluid, but wall-rock hornfels as close as 15 m from the contact have not.

Exhumation of Granulites Within a Transpressive Regime: an Example from Southern Madagascar

Granitic magmas extracted from crustal sources can form over a wide variety of P, T and aH2O conditions. Both fluid-present and effectively fluid-absent conditions can yield granitic magmas, though the majority are formed through high-T, fluid-absent reactions because, in the deep crust, most available H2O within rock bodies is contained within minerals rather than in free fluids. Fluid-present partial melting generally results in the formation of migmatites (and sometimes diatexites) under upper amphibolite-facies conditions. By definition, aqueous fluid-present melting begins at temperatures very close to the relevant ‘wet’ solidus. Consequently, studying mid-crustal migmatites, and the poorly mobile and mainly small intrusions that may result from such melting, provides no guide to the temperatures and fluid conditions that are involved in the genesis of highly mobile granitic magmas that facilitate crustal differentiation. Although zircon-saturation temperatures are commonly used to infer magma temperatures, and even melting temperatures, they seldom resemble actual magmatic temperatures. Recent notions about the persistence of granite-forming melts at conditions well below the experimentally determined H2O-saturated (‘wet’) solidi for granites (i.e., generally <650 °C) are commonly based on the results of TitaniQ (titanium-in-quartz) thermometry on silicic plutonic rocks. These results are problematic and inconsonant with TitaniQ thermometry on rhyolites, including those thought to represent erupted near-solidus crystal mushes. Thus, the hypothesis that low-T, aqueous fluid-fluxed granitic magmas have significance for the recycling and large-scale differentiation of the continental crust is poorly founded.

Migmatization and ?secondary? granitic magmas: effects of emplacement and crystallization of ?primary? granitoids in Southern Bohemia, Austria

The Sauwald and Muhl zones of the prebatholithic, Moldanubian, middle crust in northern Austria contain metapelites and metaluminous to weakly peraluminous metagreywackes, respectively. Both zones were affected by low-pressure, high-temperature metamorphism and anatexis. The metapelites of the Sauwald zone became in-situ diatexites, probably by fluid-absent reactions involving the breakdown of muscovite and the partial breakdown of biotite. The biotite-plagioclase-quartz gneisses of the Muhl zone experienced only slight melting. Following this event, and while the mid crust was still hot, additional heat was locally advected into the Muhl zone by the intrusion of the Weinsberg granite. This brought about fluid-present partial melting of the biotite-plagioclasequartz gneisses, producing relatively large volumes of metaluminous to weakly peraluminous, I-type Schlieren granite. This cool, wet, restite-rich magma remained close to its site of generation. Thus, infracrustal I-type granitoids may be formed anywhere in the crust, and not always at high T. Under special circumstances the heat and fluids from granitic magmas can spawn “secondary” granites. Also, relatively low initial 87Sr/86Sr values (of around 0.707) in I-type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle-derived magma. The Weinsberg granite magma came from the lower crust (P probably <700 MPa), where widespread fluid-absent breakdown of biotite-plagioclase-quartz assemblages occurred. The necessary high heat flow was probably provided by newly underplated mafic magmas. However, these seem not to have mixed or mingled with the crustally derived Weinsberg magmas. Deep equivalents of the Muhl-zone metagreywackes may have formed the Weinsberg protolith. Fluid-absent experiments show that the melting temperature probably exceeded 850°C and that a garnet-bearing, orthopyroxene-rich residue should be present in the lower crust. Fluid-present experiments demonstrate that the availability of free H2O can radically alter the characteristics of the partial melts, from apparent S-type mineralogy (with fluid-absent melting) to I-type mineralogy (with wet melting).