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Mantle magmas interact with surrounding rocks during their ascent and storage in the continental crust, leading to open system processes as wall rock partial melting. In this study, we have experimentally investigated the reactions between a leucosome depleted migmatite and a primitive K-basaltic of Campi Flegrei (Italy). Experiments were carried out at pressure of 0.8 GPa temperatures from 1250 °C to 1050 °C and constant temperature and thermal gradient conditions. The experimental products consist of biotite-free migmatite, glass and crystals of clinopyroxene, olivine, plagioclase and Cr-spinel with proportions that vary as a function of temperature. Open system isothermal experiments indicate that the chemistry of melts and phase relationships are controlled by the high Al2O3 content of leucosome depleted migmatite with the glass composition shifting from K-trachybasalt towards shoshonite as the temperature decreases from 1200 °C to 1125 °C. At temperatures 1150°C, migmatite assimilation is not exclusively due to the assimilation fractional crystallization process because evidence of mingling and mixing is observed. T-gradient experiment shows melt composition ranging from shoshonite to phono-tephrite moving from the slightly crystalline zone (T = 1250 –1210 °C) at the bottom of the capsule towards the highly crystalline zone (T = 1160 –1140 °C). This SiO2-constant trend indicates that at temperature below the basalt solidus, the assimilation of leucosome depleted crust is represented almost exclusively by the biotite breakdown, leading to the increase in Al, Mg, Fe, Ti, and K activities in the system. The shoshonitic composition obtained in our experiments could represent the parental magma for both Campi Flegrei volcanic district and Vesuvius magmatic systems, indicating modification in a deep storage zone through mixing with the partial melts derived from restitic continental crust.

We report contrasting pressure-temperature-time (P-T-t) paths of migmatites developed in the highest-grade metamorphic zone (Grt-Crd zone) and the contact metamorphic zone (Crd-Kfs zone) of the Mikawa area, Ryoke belt, SW Japan to discuss the complex P-T-D-t evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt-Crd zone, sillimanite-grade high-T metamorphic condition prevailed from ca. 97 Ma to 87 Ma, followed by cooling to ~500 oC, ~4 kbar. The intrusion of gneissose granitoids below the Grt-Crd zone isobarically reheated the Grt-Crd zone rocks again to the sillimanite-grade high-T condition at ca. 84 Ma. This was followed by ca. 71-70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt-Crd zone. On the other hand, ca. 74 Ma age of the Crd-Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn-tectonic pegmatite vein revealed that the intrusion of “75-69 Ma granitoids” caused partial melting and locally triggered low-strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono-metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.c. contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt.

This brief note presents geochemical data from rock samples from the Cabo Ortegal Complex (NW Spain). The samples belong to acid lithologies within the mainly basic to intermediate granulite unit that have been poorly investigated so far. For this communication, five samples of the migmatites and an amphibolitic enclave within them have been analysed. The whole-rock major and trace-element analyses were accomplished by means of Q-ICPMS. The dataset provides new and useful information relevant to the origin of the acid migmatites and can be used in addition to information from neighbouring lithologies to enhance understanding of the geological evolution of the Western Variscan Belt.

This brief note presents geochemical data from rock samples from the Cabo Ortegal Complex (NW Spain). The samples belong to acid lithologies within the mainly basic to intermediate granulite unit that have been poorly investigated so far. For this communication, five samples of the migmatites and an amphibolitic enclave within them have been analysed. The whole-rock major and trace-element analyses were accomplished by means of Q-ICPMS. The dataset provides new and useful information relevant to the origin of the acid migmatites and can be used in addition to information from neighbouring lithologies to enhance understanding of the geological evolution of the Western Variscan Belt.

This brief note presents geochemical data from rock samples from the Cabo Ortegal Complex (NW Spain). The samples belong to acid lithologies within the mainly basic to intermediate granulite unit that have been poorly investigated so far. For this communication, five samples of the migmatites and an amphibolitic enclave within them have been analysed. The whole-rock major and trace-element analyses were accomplished by means of Q-ICPMS. The dataset provides new and useful information relevant to the origin of the acid migmatites and can be used in addition to information from neighbouring lithologies to enhance understanding of the geological evolution of the Western Variscan Belt.

The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

The Seve Nappe Complex exposed in the Kittelfjäll area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 °C and 780 °C, which is in agreement with the c. 774 °C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 °C and 726 °C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475–469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (LuN/GdN = 0.49–38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (LuN/GdN = 3.96–65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values.

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values.

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values.

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values.

The Western and Median Segments of the Idefjorden terrane have been metamorphosed and migmatized during the Sveconorwegian orogeny (ca. 1 Ga), but the timing of these events between the two segments remains unclear. Here I show that Sveconorwegian metamorphism and associated migmatite formation is not coeval between the Western and Median Segments and that there is a contrast between the segments in terms of strain, migmatite abundance and retrograde metamorphic overprint. Magmatic crystallization ages were determined through U-Pb spot dating on zircon using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Zircon cores with Th/U >0.2 yields an age of 1551±6 Ma for the Arsjön migmatite in the Western Segment, 1349±5 Ma for the Tösslanda migmatite within the Göta Älv Shear Zone, and 1601±3 Ma and 1581±8 Ma respectively for the Hälltorp migmatites in the Median Segment. Zircon in leucosome yield dates of 1073±31 Ma for the Arsjön migmatite, 1046±21 Ma for the Tösslanda migmatite and 1019±27 Ma for the Hälltorp migmatites. The dates of the zircon rims are interpreted to be ages of metamorphism and migmatization and are supported by Th/U values around 0.02. The metamorphic ages are coeval with Sveconorwegian metamorphism in the Idefjorden terrane (1.14-0.90 Ga), but the results demonstrate that Sveconorwegian metamorphism and migmatite formation occurred earlier in the Western Segment than in the Median Segment., Det Västra segmentet och Mediansegmentet i Idefjordenterrängen omarbetades under metamorfos och migmatitbildning under den Svekonorvegiska orogenesen (ca. 1 Ga), men det råder fortfarande oklarheter kring tidpunkterna för dessa händelser inom och mellan de två segmenten. Denna studie visar att Svekonorvegisk metamorfos och associerad migmatitbildning inte är samtida i Västra segmentet och Mediansegmentet, samt att deformation, retrograd metamorf överprägling och mängden migmatiter kontrasterar mellan segmenten. Magmatiska kristallationsåldrar bestämdes med U-Pb-datering av zirkon genom laserablation och induktivt kopplad plasma masspektrometri (LA-ICP-MS). Zirkonkärnor med Th/U >0,2 ger en ålder av 1551±6 Ma för Arsjönmigmatiten i Västra segmentet, 1349±5 Ma för Tösslandamigmatiten i Göta Älv zonen, samt 1601±3 Ma respektive 1581±8 Ma för Hälltorpmigmatiterna i Mediansegmentet. Zirkon i leukosom ger 1073±31 Ma för Arsjönmigmatiten, 1046±21 Ma för Tösslandamigmatiten och 1019±27 Ma för Hälltorpmigmatiterna. Dateringarna av tillväxterna tolkas som åldern för metamorfos och migmatitbildning och stöds av Th/U runt 0,02. De metamorfa åldrarna är samtida med Svekonorvegisk metamorfos i Idefjordenterrängen (1.14-0.90 Ga), men resultaten visar att Svekonorvegisk metamorfos och migmatitbildning inträffade tidigare i Västra segmentet än i Mediansegmentet.

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values.

Biotite‐rich selvedges developed between mafic schollen and semipelitic diatexite in migmatites at Lac Kénogami in the Grenville Province of Quebec. Mineral equilibria modelling indicates that partial melting occurred in the mid‐crust (4.8–5.8 kbar) in the range 820–850°C. The field relations, petrography, mineral chemistry and whole‐rock composition of selvedges along with their adjacent mafic schollen and host migmatites are documented for the first time. The selvedges measured in the field are relatively uniform in width (~1 cm wide) irrespective of the shape or size of their mafic scholle. In thin section, the petrographic boundary between mafic scholle and selvedge is defined by the appearance of biotite and the boundary between selvedge and diatexite by the change in microstructure for biotite, garnet, plagioclase and quartz. Three subtypes of selvedges are identified according to mineral assemblage and microstructure. Subtype I have orthopyroxene but of different microstructure and Mg# to orthopyroxene in the mafic scholle; subtype II contain garnet with many mineral inclusions, especially of ilmenite, in contrast to garnet in the diatexite host which has few inclusions; subtype III lack orthopyroxene or garnet, but has abundant apatite. Profiles showing the change in plagioclase composition from the mafic schollen across the selvedge and into the diatexite show that each subtype of selvedge has a characteristic pattern. Four types of biotite are identified in the selvedges and host diatexite based on their microstructural characteristics. (a) Residual biotite forms small rounded red‐brown grains, mostly as inclusions in peritectic cordierite and garnet in diatexite; (b) selvedge biotite forms tabular subhedral grains with high respect ratio; (c) diatexite biotite forms tabular subhedral grains common in the matrix of the diatexite; and (d) retrograde biotite that partially replaces peritectic cordierite and garnet in the diatexite. The four groups of biotite

The continental crust, which started to form about 4 Ga ago, is petrologically very diverse. The main hypothesis that this research examines is that diversity is produced through anatexis of the continental crust resulting in the formation of granite magmas, which can eventually be transferred to shallower levels and form plutons there. This study uses field and microstructural observations combined with mineral and whole-rock composition to understand when and how a granitic continental crust, in the southern São Francisco Craton, Brazil, became petrologically diverse. The Kinawa migmatite represents a former Archean leucogranodiorite that has been reworked in a shear zone at upper amphibolite facies conditions (<730°C and 0.5-0.6 GPa). A wide variety of rocks were produced in this process, including three types of diatexite (grey, schlieren and homogenous), a series of leucosomes and minor amphibolites, metatexites and grey gneisses. Grey diatexites are residual with a microstructure consistent with H2O-fluxed melting (degree of partial melting ~0.35-0.40) via the reaction Pl + Kfs + Qz + H2O = melt. Schlieren diatexites alternate residuum-rich domains with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO2 and K2O contents and are coarse-grained, leucocratic rocks. Leucosomes are small but abundant rocks bodies that vary in compositions from tonalite to alkali-feldspar granite. Homogeneous diatexites, quartzo-feldspathic domains from the schlieren diatexites and leucosomes have, in different parts, plagioclase-dominated and K-feldspar-dominated framework microstructures. They are interpreted as melt-rich rocks and some of them contain evidence of tectonic compaction. In the active shear zone setting of the migmatite shearenhanced compaction, both during melting and crystallisation, provided an essentially continuous driving force for the segregation of melt from solids, and generated the petrological diversity observed in the mig

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been utilised to investigate the age and chemical composition of two texturally different types of monazite in leucosome and mesosome of polymetamorphic aluminous migmatitic gneisses at Stensjöstrand, Halland. Results of radiometric dating are compared with previously published zircon dates. Monazite grains were documented and investigated in three thin sections using polarization microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) images show that monazite grains in the mesosome have anhedral morphology, embayments, and complex internal zonation. Monazite grains in the leucosome are relatively small, euhedral to subhedral or ovoid, and have relatively distinct internal zoning. Anhedral and ovoid morphologies, embayments, and complex zoning indicate recrystallisation and resorption processes having affected monazite grains during metamorphism. Documented monazite grains were analysed for trace elements Y, Lu, Th, and U, rare earth elements (REE), and radiometrically dated. Results reveal two age groups, c. 1.43 Ga and c. 0.99 Ga, that relate to the Hallandian and Sveconorwegian orogenies, respectively. Both monazite generations occur in leucosome and mesosome, either as single- or double-generation grains. Weighted average calculations of concordant 207Pb/206Pb isotopic dates yield 1434 ± 14 Ma and 988 ± 11 Ma, which can be compared to previously published zircon dates of 1386 ± 7 Ma and 970 ± 5 Ma (Piñán-Llamas et al. 2015). The analytical results may have been affected by data drift owing to downhole fractionation, most distinctly observed in results from Thompson Mine Monazite (TMM) standard as steadily increasing dates throughout the day of analysis. Trends in trace element analyses show relative U enrichment and depletion of Y, Lu, and Th in the 0.99 Ga monazite generation compared to the 1.43 Ga generation, indicating t, Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) har använts för att undersöka ålder och kemisk komposition av två texturellt skilda typer av monazit i leucosom och mesosom i polymetamorf aluminös migmatitgnejs vid Stensjöstrand, Halland. Dateringsresultat jämförs med tidigare publicerade zirkondateringar. Monazitkorn dokumenterades och undersöktes i tre tunnslip med hjälp av polarisationsmik-roskopi och Scanning Electron Mikroskopy kopplat med Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) bilder visar att monasitkorn i mesosomen har anhedral morfologi med inskärningar och komplex intern zonering. Monazitkorn i leucosomen är relativt små, euhedrala till subhedrala eller äggformiga och har relativt distinkt intern zonering. Anhedrala och äggformiga morfologier, inskärningar samt komplex zonering indikerar att monazitkornen har utsatts för omkristallations- och resorptionsprocesser vid metamorfism. Dokumenterade monazitkorn analyserades för spårelementen Y, Lu, Th och U, sällsynta jordartsmetaller (REE) samt utfördes ratdiometrisk datering. Dateringar visar två åldersgrupper, ca 1.43 Ga och ca 0.99 Ga, som relaterar till den Hallandiska respektive den Sveconorvegiska orogenesen. Båda generationer av monazit förekommer i leucosom samt mesosom, antingen som enstaka eller dubbelgenerations korn. Viktade medelvärdesberäkningar från konkordant 207Pb/206Pb isotopiska datum gav 1434 ± 14 Ma och 988 ± 11 Ma som kan jämföras med tidigare publicerade zirkondata, 1386 ± 7 Ma och 970 ± 5 Ma (Piñán-Llamas et al. 2015). De analytiska resultaten kan ha påverkats av datadrift på grund av s.k. downhole fractionation. Detta syns tydligast i resultat från standarden Thompson Mine Monazite (TMM) i form av stadigt ökande dateringar under dagen som analyserna genomfördes. Trender i spårelementanalyser visar relativ U anriktning samt utarmning av Y, Lu och Th i 0.99 Ga monazitgenerationen jämfört med 1.43 Ga generationen. Detta indikerar att bulkmängde

The Åreskutan nappe complex consists of the partly molten rock migmatite, which originates from the subduction formed by the collision of continents Baltica and Laurentia. It is a so-called hot nappe, which has been deeply buried in the subduction zone, based on findings of high-pressure minerals in the migmatitic gneiss. As the nappe returned to shallower depths the rock was partially molten during the subsequent exhumation as the lithostatic pressure decreased. Tectonic forces led to thrusting of the nappe towards the east and the building of mount Åreskutan. It is generally accepted that the shear zone between the migmatite of the Åreskutan Nappe and the underlying Lower Seve Nappe is a mylonitic shear zone, but the question of whether similar shear zones can be found at other sites in the migmatite complex has now been raised. In this project two major shear zones have been identified and shear direction has been determined after detailed geological mapping. Many small shear zones have also been identified, but their sense of shear direction was more difficult to determine. The two major shear zones identified have been labelled the Eastern Major Shear Zone and Western Major Shear Zone. In these shear zones the original migmatite appearing on Åreskutan is deformed and sheared with a top to the east sense of shear. The strongest evidence for determining the shear sense are garnets found mantled by micas in a sigma-type shear microstructures, found during microscope analysis. A grade of mylonitization can be seen in the mineral microstructures, with the most fine-grained matrix in the centre of the shear zones. It indicates that ductile deformation dominates, even though some minerals tend to break in a brittle manner.

The Åreskutan nappe complex consists of the partly molten rock migmatite, which originates from the subduction formed by the collision of continents Baltica and Laurentia. It is a so-called hot nappe, which has been deeply buried in the subduction zone, based on findings of high-pressure minerals in the migmatitic gneiss. As the nappe returned to shallower depths the rock was partially molten during the subsequent exhumation as the lithostatic pressure decreased. Tectonic forces led to thrusting of the nappe towards the east and the building of mount Åreskutan. It is generally accepted that the shear zone between the migmatite of the Åreskutan Nappe and the underlying Lower Seve Nappe is a mylonitic shear zone, but the question of whether similar shear zones can be found at other sites in the migmatite complex has now been raised. In this project two major shear zones have been identified and shear direction has been determined after detailed geological mapping. Many small shear zones have also been identified, but their sense of shear direction was more difficult to determine. The two major shear zones identified have been labelled the Eastern Major Shear Zone and Western Major Shear Zone. In these shear zones the original migmatite appearing on Åreskutan is deformed and sheared with a top to the east sense of shear. The strongest evidence for determining the shear sense are garnets found mantled by micas in a sigma-type shear microstructures, found during microscope analysis. A grade of mylonitization can be seen in the mineral microstructures, with the most fine-grained matrix in the centre of the shear zones. It indicates that ductile deformation dominates, even though some minerals tend to break in a brittle manner.

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

In the Fosdick migmatite-granite complex of West Antarctica, U-Pb geochronology of monazite in migmatites and zircon in granites records two episodes of high-temperature metamorphism, one in the Devonian-Carboniferous and another in the Cretaceous. For the older lower-grade event, whole-rock and zircon isotope geochemistry of granites within the Fosdick complex are interpreted to record crustal reworking during metamorphism associated with continental arc magmatism along the East Gondwana convergent plate margin. By contrast, the geochemistry of correlative granites suites from along and across the same margin indicates a greater proportion of crustal growth. This suggests prominent arc-parallel and arc-normal variations in the mechanisms of crustal reworking versus growth in continental arc systems. Based on garnet Lu-Hf ages, the timing of peak metamorphism in the younger higher-grade event has been determined as c. 116-111 Ma. U-Pb ages of monazite from migmatites and zircon from anatectic granites suggest that exhumation of the complex as a gneiss dome occurred during the interval c. 107-100 Ma. Contemporaneous exhumation of high-grade metamorphic rocks in the Western Province of New Zealand suggests that intracontinental extension preceded the final breakup of New Zealand from West Antarctica by c. 25 Myr. Melt migration through and emplacement within the Fosdick complex during Cretaceous metamorphism was accomplished via a self-organized melt network controlled by the regional stress field and anisotropy of the host rocks. Granites within this network and in sills at shallower crustal levels have microstructures and chemistry consistent with a cumulate origin, and are interpreted to record fractional crystallization during magma ascent and doming related to exhumation. Phase equilibria modeling of open system melting during prograde metamorphism is used to quantify the reduced fertility of source rocks during high-temperature exhumation and later overprinting

Nanogranites represent totally crystallized inclusions of anatectic melt trapped within peritectic minerals of migmatites and granulites. They have recently been discovered in several locations. This discovery opens new possibilities for investigating crustal melting processes, provided that an appropriate method for retrieving the information contained within nanogranite inclusions is available. Here, we describe a series of remelting experiments that have been performed at different temperatures and under dry, and H2O-added, conditions on nanogranite inclusions hosted in migmatitic garnet, using a piston-cylinder apparatus. The glasses obtained by quenching the sample from temperature that approaches the trapping temperature have compositions very similar to those of preserved glassy inclusions coexisting with nanogranites in the same cluster. No significant differences in H2O contents were observed for nanogranites rehomogenized under dry and wet conditions. Higher (50-100°C) experimental temperatures resulted in dissolution of the host into the melt and inclusion decrepitation with the loss of volatiles. Therefore, piston-cylinder remelting experiments may eliminate inclusion decrepitation, maintaining the primary fluid contents in the originally trapped melt. These volatiles would otherwise be lost during remelting experiments at ambient pressure. By preventing volatile loss, the inclusion does not have to be overheated to achieve homogenization, and the compositions of quenched glasses so obtained can be assumed to be those of melts produced (and trapped as inclusions) during crustal anatexis. The experimental approach described here represents a promising technique for the successful rehomogenization of crystallized melt inclusions from high-pressure environments, such as the mafic continental crust. © 2013 John Wiley & Sons Ltd.

The mid to lower crustal metamorphic field gradient through amphibolite and granulite facies rocks in the Ivrea Zone offers the potential to study partial melting and melt loss in the crust. Metapelitic rocks in Val Strona di Omegna show a progressive evolution in migmatite structures from metatexites with rare isolated leucosome veins in the amphibolite facies rocks to stromatic migmatites and diatexites in granulite facies rocks. Little field or petrographic evidence for melting can be seen on crossing the position of the modelled wet solidus, consistent with the small amounts of melt predicted to occur by H2O-saturated melting. The first field evidence for partial melting, in the form of narrow discontinuous leucosomes, coincides with the fluid-absent breakdown of muscovite and the prograde appearance of K-feldspar. The consumption of biotite, which is modelled to occur over a 50–100 °C wide field of coexisting garnet–sillimanite–biotite, led to more pronounced melting and the formation of abundant garnet-bearing leucosomes. At the highest grades, metagreywacke compositions contain leucosomes that are spatially focussed on orthopyroxene porphyroblasts. Calculated P–T pseudosections show that the metapelitic rocks could have produced up to 40 mol% melt and the metagreywackes up to 25% melt at peak metamorphic conditions of around 11 kbar, 900 °C. Modelling of granulite facies samples shows elevated solidi suggesting significant melt loss prior to cooling, consistent with depletion in SiO2, Na2O and K2O and enrichment in FeO, MgO and TiO2 relative to amphibolite facies samples. Zones of diatexite in the highest grade rocks indicate that, at least locally, melt loss was inefficient and/or accumulation of melt occurred. Zones of apparent accumulation of melt are common at the boundaries between metatexitic metagreywacke and diatexitic metapelite, and may indicate that the metagreywacke formed a low-permeability barrier that restricted melt flow.

The central region of the mainland Lewisian complex is dominated by granulite-facies tonalite–trondhjemite–granodiorite (TTG) gneisses that are highly depleted in some mobile trace elements (Cs, Rb, Th and U) relative to amphibolite-facies TTG gneisses elsewhere in the Lewisian complex and to the average composition of TTG gneisses worldwide. Over almost half a century of research there has been vigorous debate as to the origin of this depletion, in particular with respect to the role of partial melting and melt loss. Here we provide field and petrographic evidence that TTG gneisses across the central region partially melted during granulite-facies (Badcallian) metamorphism. Partial melting occurred largely by fluid-absent incongruent reactions consuming plagioclase, quartz, hornblende and biotite to produce melt and peritectic clino- and orthopyroxene. The preservation of dry, granulite-facies assemblages requires loss of melt, consistent with the presence of an interconnected network of leucosomes and larger felsic sheets that probably record segregation and transfer of melt to higher crustal levels. Regardless of whether or not partial melting and melt loss can explain fully the unusual geochemical signature of the central region TTG gneisses, these fundamental processes did occur.

Three lithologic assemblages exposed in the central Santa Fe Range, northern New Mexico include (1) a septum of Proterozoic low temperature (~700oC, 5.5 kbars), semi-pelitic anatectic migmatites surrounded by (2) deformed megacrystic biotite granite that is intruded by (3) discrete plutons of largely undeformed hornblende biotite tonalite. Metamorphic fabrics in the migmatite record three deformation events (D1 - D3); a composite S1/S2 fabric (D1 and D2) that formed contemporaneous with anatexis and is folded about a southwest plunging map-scale (D3) antiform. The megacrystic granite exhibits a pervasive S1 subsolidus foliation that parallels the S1/S2 fabric in the migmatite and is cut by discordant leucosomes. The hornblende tonalite cross-cuts both the S1 foliation of the megacrystic granite and the S1/S2 foliation of the migmatite. New U-Pb SIMS ages were collected on zircon from the two plutonic units and from a stromatic metasedimentary migmatite (leucosome-melanosome-paleosome). Megacrystic granite yielded a concordant U-Pb age of 1633 ± 12 Ma (2ó) and hornblende tonalite yielded a concordant U-Pb age of 1395 ± 7 Ma (2ó); both plutonic units have Th/U >0.1. The evaluation of migmatite zircon from leucosome-melanosome-paleosome have highly variable Th/U values (>0.1 to0.1 yielded two age populations: protolith cores with a Pb-Pb age = 1634 ± 16 Ma and new growth with a Pb-Pb age of 1398 ± 17 Ma. Three zircon analyses yield older Pb-Pb ages of 1791 ± 25 Ma. Migmatite zircon with Th/U values

The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic–ultramafic bodies that exhibit macroscopic features diagnostic of anatexis. In situ partial melting of metagabbroic rocks produced patches and sheets of coarse-grained plagioclase-rich leucosome containing euhedral peritectic clinopyroxene. These leucosomes connect with larger, laterally continuous tonalite or trondhjemite sheets that record segregation and migration of melt away from the metagabbro source rocks. This melt loss allowed wide-scale preservation of granulite-facies assemblages within the residual melanosome. Whole-rock major and trace element geochemistry is broadly consistent with the field evidence, but suggests contamination of the metagabbroic rocks by their host-rocks and a strong mineralogical control on trace element distributions, the consequence of large diffusive length scales during protracted ultrahigh-temperature metamorphism. Variations in the trace element composition of the felsic sheets reflect heterogeneities in the source rocks, the presence of material entrained from the melanosome and fractional crystallization dominated by plagioclase. The felsic sheets are largely cumulate, suggesting loss of the evolved melt fraction to higher crustal levels. Partial melting of felsic gneisses that surround the mafic–ultramafic bodies is inevitable at the implied metamorphic peak provided they contained hydrous phases, although the field evidence is largely obscured by later reworking. This study provides insights into the processes involved in intracrustal differentiation during the Neoarchaean, during which partial melting of mafic rocks is likely to have made a more significant contribution than durin

Hf-isotope analyses in previously dated zircon crystals from four Variscan granulite-facies rocks from central Iberia have been studied to examine crustal evolution processes. Two samples are from exposed migmatite terranes, the other two are granulite xenoliths from lower crustal levels. Hf model ages show that lower crustal granulites display a bigger contribution of Archean components than exposed highgrade rocks (14% of the zircon population instead of 6%), reinforcing ideas of preservation of ancient crustal material in the lower crust. The distribution of age-Hf isotope data from zircon suggests two juvenile crust formation events in central Iberia: the main one during the Neoproterozoic (540–620 Ma) and a possible second Mesoproterozoic event around 1.0–1.2 Ga. Magmatism appears in the Central Iberian Zone exclusively related to that younger juvenile addition. Hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central Iberia as a peri-Gondwanan terrane located between the West African craton and the Arabian shield, far from the Amazonia microplate., Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

Melt that crystallizes as granite at shallow crustal levels in orogenic belts originates from migmatite and residual granulite in the deep crust; this is the most important mass-transfer process affecting the continents. Initially melt collects in grain boundaries before migrating along structural fabrics and through discordant fractures initiated during synanatectic deformation. As this permeable porosity develops, melt flows down gradients in pressure generated by the imposed tectonic stress, moving from grain boundaries through outcrop-scale vein networks to ascent conduits. Gravity then drives melt ascent through the crust, either in dikes that fill ductile-to-brittle-elastic fractures or by pervasive flow in planar and linear channels in belts of steep structural fabrics. Melt may be arrested in its ascent at the ductile-to-brittle transition zone or it may be trapped en route by a developing tectonic structure.

Stenbrånet är ett område strax väster om Bastuträsk i utkanten av gruvdistriktet Skelleftefältet i Västerbotten, norra Sverige. Tidigare undersökningar gjorda av Boliden Mineral AB har visat på förekomst av en skivformad kropp kraftigt anrikad på Zn, Ag och Pb. Studier av tunnslip och borrkärnor som karterats i detta arbete visar på en decimeter- till metertjock sulfidhorisont med kraftig impregnation av zinkblände och/eller magnetkis som lokalt övergår till att bli massiv. Kartering av borrkärnor längs fyra profiler bekräftar tidigare utförda studier avseende bergartsrelationer. Området domineras av en gnejsig bergart, Sikträskgranitoiden, som är flankerad av kraftigt regionalmetamorfoserade suprakrustalbergarter som i sin tur överlagras av postmetamorfa graniter av Revsundstyp. De suprakrustala enheterna består av kraftigt förgnejsade till migmatiserade vulkaniter och sediment med spår av sulfider (främst magnetkis och pyrit). Det förekommer även metertjocka granitiska skivor (leukosom) av Skellefte-Härnötyp. Sulfidhorisonten har antagits finnas i kontakten mellan vulkaniterna och sedimenten, men existensen av en sådan kontakt har inte bekräftats i denna studie. De profiler som konstruerats utifrån borrkärnekartering visar att samtliga litologier stupar in mot Sikträskgranitoiden och bildar en domlik geometri. Sulfidhorisontens lutning stämmer väl överens med den allmänna lagerföljden i området. Den övergripande stupningen av sulfidhorisonten, gnejsigheten och kontakten mot Revsundsgraniten är cirka 30º ut från Sikträskdomen. Mina resultat baserade på kartering och tunnslipsstudier indikerar följande geologiska historia: 1. Avsättning av sediment och vulkaniter (1.90-1.88 Ga), 2. intrusion av Sikträskgranitoiden (1.88 Ga), 3. regionalmetamorfos (1.88- 1.80 Ga) och slutligen 4. intrusion av Revsundsgranit (1.80-1.78 Ga). Mineraliseringen har troligen bildats antingen som ett remobilisat av en äldre mineralisering eller som en koncentration av de metaller som fanns, Stenbrånet i situated west of Bastuträsk at the margin of the Skellefte mining district in Västerbotten, northern Sweden. Previous investigations carried out by Boliden Mineral AB reveal a disc-shaped orebody highly enriched in Zn, Ag and Pb. Studies of thin sections and mapping of drill cores verify the existence of a dm- to mthick sulphide horizon with a high impregnation of sphalerite and/or pyrrhotite which locally occurs as massive sulphide. Mapping of drill cores along four transects show the relation between various rock units at depth. The area is dominated by a gneissic rock, the Sikträsk granitoid, which is flanked by highly regional metamorphosed supracrustal rocks which are stratigrafically covered by postmetamorphic granites of Revsund type. The supracrustal rocks are composed of highly gneissic to migmatitic volcanics and sediments with traces of sulphides (mainly pyrrhotite and pyrite). There are also abundant metre-thick granitic layers (leucosome) of the Skellefte-Härnö suite in the supracrustals. The sulphide horizon has been assumed to exist along the contact zone between the volcanics and the sediments, but the existence of this contact has not been found in this work. The profiles based on drill core mapping show that all lithologies dip towards the Sikträsk granitoid forming a dome-like geometry. The dip of the sulphide horizon agrees well with the general structural layering in the area. The dip of the sulphide horizon, the gneissosity and the contact to the Revsund suite is approximately 30º out from the Sikträsk dome. My results from drill-core mapping and studies of thin sections indicate the following geologic history: 1. Deposition of sediments and volcanics (1.90-1.88 Ga), 2. the intrusion of the Sikträsk granitoid (1.88 Ga), 3. regional metamorphism (1.88 Ga) and, finally, 4. intrusion of the Revsund suite (1.80-1.78 Ga). The mineralization has probably been formed either by a remobilized older mineralization or as a concentration of th

Le Pluton granitique Wuluma, qui est le résultat de la fusion partielle des roches encaissantes, se situe dans la partie ouest des Wuluma Hills qui sont inclus dans le Complexe Métamorphique Strangways (SMC), en Australie Centrale. Une cartographie détaillée à l'intérieur de ces migmatites a permis de distinguer trois fabriques planaires reliées à deux événements tectoniques et principalement quatre unités lithologiques. Un premier événement tectonique, le Strangways précoce (1780-1770 Ma), a provoqué une importante foliation gneissique S[. Un deuxième événement tectonique, le Strangways tardif (1749 Ma), a premièrement engendré un plissement isoclinal F2. La fin du Strangways tardif est caractérisé par le développement de zones de cisaillement majeures d'orientation NW accompagnées d'un clivage de plan axial S3 orienté NW, qui est le résultat du plissement F3, responsable du patron des structures régionales dans les Wuluma Hills. La fusion partielle s'est développée de façon synchrone avec le plissement F3, à des conditions de 850-900°C pour une pression de 6 kbar. Les leucosomes, géochimiquement similaires avec le granite Wuluma, sont dérivés des gneiss quartzofeldspathiques, qui forment l'unité lithologique la plus abondante. Les autres unités lithologiques observées sont principalement les unités compétentes (de composition intermédiaire) et le matériel résiduel. Le magma issu de la fusion partielle a migré le long d'une variété de structures (plan axial, fractures, contours de boudins) principalement reliées à l'événement de déformation F3, qui a également contrôlé la migration du magma. L'accumulation du magma prend place dans les zones de faible pression reliées à la déformation locale et régionale, particulièrement là où il y a de grandes différences de compétence. La cartographie détaillée a également démontré une continuité entre l'accumulation du magma à petite échelle (dans les migmatites), les plutons granitiques d'échelle hectométrique et le granite Wu

Le district aurifère de Hetai est situé au sud de la Chine, dans le terrain métamorphique de Yunkai, qui est limité à l'est par le système de failles de Wuchuan - Sihui, et à l'ouest par le système de failles de Bobai - Wuzhou. Il représente le plus grand camp minier aurifère découvert à l'ouest de la province de Guangdong. Les dépôts aurifères avec des réserves prouvées incluent les gîtes de Gaocun, Yunxi, Taipingding, Shangtai, Kangmei, Taozhishan, Huojing et Hehai. À part le gîte de Hehai, tous ces gîtes sont de type hydrothermal dans une zone de mylonite. Le gîte de Hehai est constitué de veines de quartz. L'encaissant principal des gisements d'or est la strate Sinienne. Cette unité est composée de roches métamorphiques variées, avec les schistes, les quartzites schisteuses et les migmatites constituant les roches encaissantes immédiates des dépôts aurifères. Les intrusions granitiques de Wucun et de Yunluogan se rencontrent au nord et à l'ouest, alors que les granites (migmatitiques) de Shidong en occupent l'extrémité Nord - Ouest. 1. Métamorphisme régional et granites Les roches métamorphiques de la région de Hetai consistent principalement en des schistes, des quartzites schisteux, ainsi que de migmatites. On retrouve aussi des métasédiments de bas degré de métamorphisme au sud-est des dépôts aurifères. La paragénèse minérale caractéristique des schistes dominant est quartz + muscovite + biotite ± staurolite ± sillimanite ± almandine. Ces minéraux se sont développés dans des séries de faciès métamorphiques de moyenne à basse pression, au cours de l'orogénie Calédonienne. La température et la pression auxquelles les schistes se sont formés varient respectivement entre 550° - 670°C et 230 - 600 MPa. Les protolithes sont principalement des roches pélitiques avec des cherts intercalés. Les migmatites font partie intégrante des séries métamorphiques de Hetai et représentent les roches les plus intensément métamorphisées de la région. La formation des migmatites es

Les mécanismes et les conditions de migmatisation polyphasée ont été déterminés pour les ségrégations quartzo-feldspathiques du complexe migmatitique à l'est de Chicoutimi, Québec. Chronologiquement, trois groupes lithostructuraux sont distingués: 1) Les paragneiss, les amphibolites concordantes associées, le granite I et localement des dykes d'amphibolite I. Toutes ces roches contiennent des veines leucocrates minces (mobilisat I), qui suivent la schistosité S1. 2) Un gneiss charnockitique (granite IIa) et le granite gneissique oeillé IIb. Toutes les roches des groupes I et II ont été injectées par au moins 4 générations de dykes felsiques à mafiques. La mise en place de ces dykes coincide avec une période d'extension régionale. Une compression subséquente (déformation IIa) a crée une schistositê dominante (IIa) qui contrôle la localisation des segrégations leucocrates (mobilisat IIa) générées tôt pendant la migmatisation II. Ces mobilisats ont été" plissés (F2b) durant la déformation IIb, et peu après replissés par la déformation IIc. Durant cette déformation, le mobilisat IIb s'est accumulé le long des surfaces de "strain slip" (S2c), parallèles aux plans axiaux des plis F2c. Les leucosomes générés pendant les stages tardifs de la migmatisation II recoupent les plis F2b, les plans axiaux de F2c et les leucosomes antérieurs. Les leucosomes générés pendant les périodes de migmatisation polyphasés I et II le furent in situ. La minéralogie des leucosomes est contrôlée par le type de paleosome et les minéraux mafiques des mélanosomes présents sont les mêmes que ceux du paleosome correspondant. Les veines du mobilisat I consistent en assemblages minéralogiques subanatectiques qui sont considérés comme un des produits de ségrégation métamorphiques qui se sont formés sous des conditions estimées à 650-700 C; 3.5-4.5 kb. Les leucosomes générés durant la migmatisation polyphasée II montrent des comportements de fluide. Les plagioclases dans ces leucosomes (An 12-15) sont p