Peraluminous Granites Research Articles

The A-type Pirrit Hills Granite, West Antarctica: an example of magmatism associated with the Mesozoic break-up of the Gondwana supercontinent

The Mesozoic geology of West Antarctica is largely related with the break-up of the Gondwana supercontinent and offers a good example for understanding magmatism associated with the continental break-up process. West Antarctica can be divided into five crustal blocks with relatively thin crust. The blocks are separated by deep rift zones and have moved during the Mesozoic break-up of Gondwana. The Pirrit Hills granite occurs as an isolated pluton in the Ellsworth-Whitmore Mountains block, which is the center of five blocks in the present configuration. The granite consists of quartz, perthitic alkali feldspar, and plagioclase with minor amounts of interstitial biotite and muscovite. The granite is a highly homogeneous, strongly fractionated, and mildly peraluminous granite and belongs to A-type granites with A2-type characteristics, suggesting its generation in an anorogenic environment. The strong enrichment of HREE and significant negative Eu anomalies suggest that the granitic magma was produced by a small degree of partial melting of a garnet granulitic source in the unusually hot lower crust. A weighted mean 206Pb/238U age of zircons is 164.5 ± 2.3 Ma (MSWD = 1.3), which is 8 to 9 Mys younger than a former Rb-Sr whole rock age (173 ± 3 Ma), and corresponds to the first rifting stage of the break-up of Gondwana (at 165 Ma). We suggest this age to be the emplacement age of the Pirrit Hills granite. The A-type Pirrit Hills granite was emplaced in the Middle Jurassic accompanying crustal thinning due to the break-up of Gondwana.

Evaluating the Controls on Tourmaline Formation in Granitic Systems: a Case Study on Peraluminous Granites from the Central Iberian Zone (CIZ), Western Spain

Evaluating the Controls on Tourmaline Formation in Granitic Systems: a Case Study on Peraluminous Granites from the Central Iberian Zone (CIZ), Western Spain

Peraluminous leucogranites of the Cordilheira Suite: A record of Neoproterozoic collision and the generation of the Pelotas Batholith, Dom Feliciano Belt, Southern Brazil

The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quitéria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butiá and Três Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70°E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event. The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K2O/Na2O and CaO/Na2O ratios are approximately 1 and less than 0.3, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 °C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Várzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite facies metamorphism were recorded in the Várzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 °C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic.

Textural control on the weathering of basic igneous rocks: A micromorphological approach

The Puno minettes (Eastern Andean Cordillera, southern Peru) are phlogopite–phyric rocks with a groundmass consisting mainly of phlogopite, alkali feldspar and apatite microphenocrysts and/or microlites. They display the strong large-ion-lithophile-element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan (Rb, Ba, Sr up to 103× Primordial Mantle, REE up to 6.102× CI-chondrites). In addition to their close association with granitoids, the rocks of this series show many features of minettes occurring in arc-related collisional zones, i.e., absence of mantle xenoliths, up to 30 vol.% felsic xenocrysts and strong negative Nb and Ti anomalies. However, they are distinguished by unusual phenocryst phases, whole-rock compositions and groundmass mineralogy. Some minettes contain only a few per cent of biotite±quartz±plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite (biotite+quartz+plagioclase+alkali feldspar+cordierite+zircon+pyrite) in a high-K latitic to trachydacitic groundmass. Field relations, radiometric ages, whole-rock data and felsic xenocrysts all indicate that the Puno minettes result from a mixing process involving Al-rich lamproites and peraluminous monzogranites. The most common phenocryst assemblage (highly magnesian phlogopite+olivine±titanian Cr-spinel±Ca-rich enstatite±magnesian ilmenite±Al- and Ti-poor diopside) is similar to that reported in Al-rich phlogopite lamproites from southeastern Spain. Likewise, the host rocks have diagnostic features of Spanish phlogopite lamproites (e.g., K2O/Al2O3 ratios >9, CaO contents <2 wt.%, negative Ba and Sr anomalies in primitive mantle-normalized multi-element plots, LaN/NdN<1.5). Graphite in the groundmass and Cr-spinel and ilmenite compositions devoid of Fe3+ suggest strongly reduced conditions of formation. Another phenocryst assemblage (Ca–Al-rich enstatite+Ca-deficient highly magnesian augite+magnesian phlogopite±ilmenite±discrete olivine) is observed in one sample which derives from an Al-rich leucite lamproite.

Major types and time–space distribution of Mesozoic ore deposits in South China and their geodynamic settings

The ore deposits of the Mesozoic age in South China can be divided into three groups, each with different metal associations and spatial distributions and each related to major magmatic events. The first event occurred in the Late Triassic (230–210 Ma), the second in the Mid–Late Jurassic (170–150 Ma), and the third in the Early–Mid Cretaceous (120–80 Ma). The Late Triassic magmatic event and associated mineralization is characterized by peraluminous granite-related W–Sn–Nb–Ta mineral deposits. The Triassic ore deposits are considerably disturbed or overprinted by the later Jurassic and Cretaceous tectono-thermal episodes. The Mid–Late Jurassic magmatic and mineralization events consist of 170–160 Ma porphyry–skarn Cu and Pb–Zn–Ag vein deposits associated with I-type granites and 160–150 Ma metaluminous granite-related polymetallic W–Sn deposits. The Late Jurassic metaluminous granite-related W–Sn deposits occur in a NE-trending cluster in the interior of South China, such as in the Nanling area. In the Early–Mid Cretaceous, from about 120 to 80 Ma, but peaking at 100–90 Ma, subvolcanic-related Fe deposits developed and I-type calc-alkaline granitic intrusions formed porphyry Cu–Mo and porphyry-epithermal Cu–Au–Ag mineral systems, whereas S-type peraluminous and/or metaluminous granitic intrusions formed polymetallic Sn deposits. These Cretaceous mineral deposits cluster in distinct areas and are controlled by pull-apart basins along the South China continental margin. Based on mineral assemblage, age, and space–time distribution of these mineral systems, integrated with regional geological data and field observations, we suggest that the three magmatic–mineralization episodes are the result of distinct geodynamic regimes. The Triassic peraluminous granites and associated W–Sn–Nb–Ta mineralization formed during post-collisional processes involving the South China Block, the North China Craton, and the Indo-China Block, mostly along the Dabie-Sulu and Songma sutures. Jurassic events were initially related to the shallow oblique subduction of the Izanagi plate beneath the Eurasian continent at about 175 Ma, but I-type granitoids with porphyry Cu and vein-type Pb–Zn–Ag deposits only began to form as a result of the breakup of the subducted plate at 170–160 Ma, along the NNE-trending Qinzhou-Hangzhou belt (also referred to as Qin-Hang or Shi-Hang belt), which is the Neoproterozoic suture that amalgamates the Yangtze Craton and Cathaysia Block. A large subduction slab window is assumed to have formed in the Nanling and adjacent areas in the interior of South China, triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic W–Sn mineralization. A relatively tectonically quiet period followed between 150 and 135 Ma in South China. From 135 Ma onward, the angle of convergence of the Izanagi plate changed from oblique to parallel to the coastline, resulting in continental extensional tectonics and reactivation of regional-scale NE-trending faults, such as the Tan-Lu fault. This widespread extension also promoted the development of NE-trending pull-apart basins and metamorphic core complexes, accompanied by volcanism and the formation of epithermal Cu–Au deposits, granite-related polymetallic Sn–(W) deposits and hydrothermal U deposits between 120 and 80 Ma (with a peak activity at 100–90 Ma).

Precambrian geology of China

Precambrian geology of China

Diversity of Rhyacian granitoids in the basement of the Neoproterozoic-Early Cambrian Gurupi Belt, northern Brazil: Geochemistry, U–Pb zircon geochronology, and Nd isotope constraints on the Paleoproterozoic magmatic and crustal evolution

The Gurupi Belt, in northern Brazil, is a Neoproterozoic-Early Cambrian orogen developed at the south-southwestern margin of the São Luís-West African Craton. Several plutonic bodies are exposed as basement units of the Gurupi Belt and represent a variety of granitoid types emplaced at different ages and all show zircon inheritance and chemical and isotopic features that imply participation of reworked Archean to Paleoproterozoic crust in the magma genesis in clear contrast with the juvenile characteristics of the predominant magmatic unit of the neighboring São Luís cratonic fragment, the Tromaí Intrusive Suite of 2167–2147Ma. The weakly peraluminous, high-K, calc-alkaline biotite-bearing Cantão Granodiorite of 2163±4Ma has TDM model ages of 2.21–2.92Ga and ɛNd values of +2.7 to −7.1. The 2142±9Ma-old, weakly peraluminous, biotite- and muscovite-bearing Jonasa Granodiorite underwent metamorphism and deformation at 525Ma and shows TDM model ages of 2.14–2.40Ga, with ɛNd values of +3.9 to −0.2. Both are, probably, related to a continental arc setting. A relatively large event of strongly peraluminous granitic magmatism took place between 2116 and 2079Ma, and comprises the crust-derived, biotite- and muscovite-bearing granites and leucogranites of probable collisional setting belonging to the Japiim, Tamancuoca, Moça and Maria Suprema units. Despite broadly similar, these units show some chemical differences that are interpreted to result from mixtures of variable amounts of sedimentary and igneous sources of Archean to Paleoproterozoic ages. These granites show TDM model ages of 2.19–2.62Ga and the ɛNd values range from +3.9 to −0.2, and probably represent collision rocks. Associated in space and time with part of the strongly peraluminous granites, the 2100±21Ma-old, metaluminous to slightly peralkaline granites, quartz-monzonites and quartz-syenites of the Anelis Intrusive Suite have characteristics of potassic to shoshonitic post-collisional rocks. The suite shows TDM model ages of 2.27–2.48Ga and ɛNd values of +0.1 to −2.6. The last known event is represented by the post-orogenic, 2085±4Ma-old, weakly peraluminous, high-K, calc-alkaline, biotite-bearing Timbozal Monzogranite, with TDM model ages of 2.27–2.58Ga and ɛNd values of +1.1 to −2.8.The strongly peraluminous and different generations of potassic granitoids represent magmatic events with characteristics that may be related to both continental arc subduction and collision processes and indicate that the granitoid bodies represent disrupted fragments of a Rhyacian orogenic belt (continental arc and collision belt) that remains today as basement inliers within the Gurupi Belt in the margin of the São Luís-West African Craton. These sequences, together with the juvenile rocks of the São Luís cratonic fragment, are part of a protracted Rhyacian orogen that evolved from ∼2240Ma to ∼2050Ma, involving accretionary and collision processes. These sequences also correlate in time and several aspects of evolutionary processes with Eburnean-Birimian terranes of the West-African Craton and Trans-Amazonian belts of the Amazonian Craton.

Contrasted crustal sources for peraluminous granites of the segmented Montes de Toledo Batholith (Iberian Variscan Belt)

The Variscan Montes de Toledo Batholith (MTB) is an E–W linear array of peraluminous granite plutons which is chemically segmented. The study is focused on the western segment of the MTB (W-MTB), mainly composed of granites with slightly lower CaO and higher P2O5 contents than associated eastern plutonic units and nearby S-type granites, giving them a more pronounced peraluminous nature. The chemical contrast is also observed in isotopic composition, especially in radiogenic Nd and Pb ratios. The W-MTB granites have higher initial eNd (–5.0 to –5.9) and lower 206Pb/204Pb and 208Pb/204Pb ratios than peraluminous types from the E-MTB segment. A mixed pelitic–greywackeous derivation from regional Neoproterozoic formations is suggested, whereas lower crustal and meta-igneous sources were involved in the origin of the easternmost MTB granites. The presence of igneous muscovite together with coexisting andalusite and sillimanite in some of the studied granites suggests that solidus was reached at 650–700 oC and depth corresponding to the pressure of 2–3 kbar.

Ordovician metagranites and migmatites of the Svratka and Orlice-Sněžník Units, northeastern Bohemian Massif

*Corresponding author Metagranites and migmatites of the Svratka and the Orlice–Sněžnik units, northeastern Bohemian Massif, exhibit a number of similarities in their lithological, mineralogical and geochemical features. Both of these units were affected by intense migmatitization accompanied by intrusion of peraluminous granites during the Cambrian to Ordovician. The chemical composition and mineralogy of all the studied rocks correspond to crustal melts. From the geochemical point of view, both rock groups (migmatites and metagranites) exhibit progressive geochemical fractionation (increase in the SiO 2 , Rb, X FeO , W and Sn contents, accompanied by a decrease in the concentrations of Ba, Sr, Mg and Ca) as a result of varying degrees of partial melting and fractional crystallization. The main rock-forming minerals are influenced by metamorphic recrystallization under amphibolite-facies conditions during the Variscan orogeny.

Geochemical and structural constraints on the magmatic history of the Chandman Massif of the eastern Mongolian Altay Range, SW Mongolia

2� CzechGeologicalSurvey,�Leitnerova�22,�658�69�Brno,�CzechRepublic� 3� SchoolofGeologyandPetroleumEngineering,�MongolianUniversityofScienceandTechnology,�Box�520,�P.O.�46,�Ulaanbaatar,�� � Mongolia�210646 4� InstitutfurGeowissenschaften,�GoetheUniversitat,�Altenhoferallee�1;�60438�FrankfurtamMain;�Germany * �Correspondingauthor In the Mongolian Altay range, immediately south of the Bogd fault zone, the Chandman Massif intruded the Chandman Khayrkhan Crystalline Complex to the NE. This complex consists of migmatized biotite gneisses, orthogneisses and amphibolites. To the south, the Massif cuts the chlorite schists and quartzites of the Tugrug Formation. The Massif mainly consists of diorites to granites with rare gabbro bodies interspersed with metamorphic host rock screens, generally of amphibolite with calc-silicate lenses. Granitoids are further categorized based on petrography and whole-rock geochemical data into peraluminous and metaluminous groups. Granites comprise the peraluminous group, and possess high (71-77 wt. %) silica contents with abundant modal K-feldspar. Metaluminous granitoids span a wide range in silica contents, from diorite to granite. Also their feldspars are compositionally variable. Both these granitoid units are of volcanic-arc character and display a calc-alkaline geochemical trend. Granodiorites of the metaluminous group contain widespread planar rhythmic schlieren layering. Metaluminous units slightly predate the intrusion of peraluminous granites, but the two suites are occasionally co-magmatic. The Al-in-hornblende barometric analysis combined with plagioclase thermo- metry reveals a depth of emplacement of 11.5-13.7 km and magma temperatures of 725-775 oC. Chandman Khayrkhan metamorphic foliations strike NNW. Magmatic fabrics in the Chandman Massif roughly E-W and subparallel E-W solid-state foliations overprint magmatic foliations. All foliations are moderately to steeply dipping. Fabric analysis shows a discordance of structures to the north and south of a fault that cross-cuts the field area. This fault is interpreted to be related to motion on the Bogd Fault. Thus, this area experienced 1) amphibolite-facies metamorphism of the Chandman Khayrkhan Crystalline Complex, 2) exhumation to higher crustal levels, 3) juxtaposition against the greenschist-facies Tugrug Formation, 4) intrusion of the Chandman Massif under tectonic strains that continued through the solidification of plutons, and 5) late block-style rotation related to motion on recent faults. These age and geological constraints iden- tify the Chandman Massif as an intrusion of substantially younger age than the Caledonian association into which it was previously placed. It is thus far the only arc-type intrusion in the earliest Hercynian age range identified in the Gobi-Altay Terrane. Its metamorphic and magmatic history of migmatization followed by intrusion of metaluminous and peraluminous plutons are similar to those of rocks to the west, in the Tseel Terrane, and may represent its easternmost counterpart. The exact juxtaposition mechanism for metamorphic units of different grade and the formation of schlieren layering in the Chandman Massif granodiorites remain enigmatic.

Space–time relation of post-collisional granitic magmatism in Santa Catarina, southern Brazil: U–Pb LA-MC-ICP-MS zircon geochronology of coeval mafic–felsic magmatism related to the Major Gercino Shear Zone

LA-MC-ICP-MS U–Pb zircon dating was performed on syntectonic, early post-collisional granitic and associated mafic rocks that are intrusive in the Brusque Metamorphic Complex and in the Florianópolis Batholith, major tectonic domains separated by the Neoproterozoic Major Gercino Shear Zone (MGSZ) in south Brazil. The inferred ages of magmatic crystallization are consistent with field relationships, and show that the syntectonic granites from both domains are similar, with ages around 630–620 Ma for high-K calc-alkaline metaluminous granites and ca. 610 Ma for slightly peraluminous granites. Although ca. 650 Ma inherited zircon components are identified in granites from both domains, important contrasts on the crustal architecture in each domain are revealed by the patterns of zircon inheritance, indicating different crustal sources for the granites in each domain. The granites from the southern domain (Florianópolis Batholith) have essentially Neoproterozoic (650–700 Ma and 900–950 Ma) inheritance; with a single 2.0–2.2 Ga inherited age obtained in the peraluminous Mariscal Granite. In the northern Brusque Metamorphic Complex, the metaluminous Rio Pequeno Granite and associated mafic rocks have scarce inherited cores with ages around 1.65 Ga, whereas the slightly peraluminous Serra dos Macacos Granite has abundant Paleoproterozoic (1.8–2.2 Ga) and Archean (2.9–3.4 Ga) inherited zircons. Our results are consistent with the hypothesis that the MGSZ separates domains with distinct geologic evolution; however, the contemporaneity of 630–610 Ma granitic magmatism with similar structural and geochemical patterns on both sides of this major shear zone indicates that these domains were already part of a single continental mass at 630 Ma, reinforcing the post-collisional character of these granites.

Geochemistry and petrogenesis of Neoproterozoic A-type granites at Nakora in the Malani Igneous Suite, Western Rajasthan, India

The Nakora Ring Complex (NRC) (732 Ma) occurs as a part of Malani Igneous Suite (MIS) in the Western Rajasthan, India. This complex consists of three phases (volcanic, plutonic and dyke). Geochemically, the Nakora granites are peralkaline, metaluminous and slightly peraluminous. They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content. The peralkaline granites show higher concentrations of SiO2, total alkalies, TiO2, MgO, Ni, Rb, Sr, Y, Zr, Th, U, La, Ce, Nd, Eu and Yb and lower concentrations of Al2O3, total iron, Cu and Zn than metaluminous granites. AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites. Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths (16–28 km and 480–840°C) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites. The primitive mantle normalized multi-element profiles suggest that Nakora granites (peralkaline, metaluminous and peraluminous) are characterized by low La, Sr and Eu and relatively less minima of Ba, Nb and Ti which suggests the aspects related to crustal origin for Nakora magma. The Nakora granites are characterized as A-type granites (Whalen et al., 1987) and correspond to the field of “Within Plate Granite” (Pearce et al., 1984). Geochemical, field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.

Geochemistry and petrogenesis of the Mesoarchean granites from the Canaã dos Carajás area, Carajás Province, Brazil: Implications for the origin of Archean granites

Four Mesoarchean (2.93 to 2.83Ga) granite units, which encompass the Canaã dos Carajás, Bom Jesus, Cruzadão and Serra Dourada granites, were recognized in the Canaã dos Carajás area of the Archean Carajás Province. The Mesoarchean units are composed dominantly of biotite leucomonzogranites. They are compared with the Neoarchean Planalto suite (2.73Ga) which encompasses biotite–hornblende monzogranites to syenogranites. The Canaã dos Carajás, Bom Jesus and the variety of the Cruzadão granite with higher (La/Yb)N are geochemically more akin to the calc-alkaline granites, whereas the other varieties of the Cruzadão granite are transitional between calc-alkaline and alkaline granites. The Serra Dourada granite has an ambiguous geochemical character with some features similar to those of calc-alkaline granites and other peraluminous granites. The Planalto granites have ferroan character, are similar geochemically to reduced A-type granites and show a strong geochemical contrast with the Mesoarchean studied granites. The Mesoarchean granites described in the Canaã dos Carajás area are geochemically distinct to those of the Rio Maria domain of the Carajás Province. The Canaã dos Carajás and Bom Jesus granites are similar to the high-Ca granites, whereas the Cruzadão and Serra Dourada are more akin to the low-CaO granites of the Yilgarn craton. The geochemical characteristics of the Mesoarchean studied granites approach those of the biotite granite group of Dharwar but the latter are enriched in HFSE and HREE compared to the Canaã dos Carajás granites. The Neoarchean Planalto suite granite has no counterpart in the Mesoarchean Rio Maria domain of the Carajás Province, neither in the Yilgarn and Dharwar cratons. Geochemical modeling suggests that partial melting of a source similar in composition to an Archean basaltic andesite of the Carajás Province could give origin to the Bom Jesus and Cruzadão granites. In the case of the Bom Jesus granite the residue of melting contained, in similar proportions, plagioclase, clinopyroxene, amphibole, and garnet, with subordinate ilmenite. The variations in (La/Yb)N and Sr/Y of the Cruzadão granite are controlled by changes in the residual melting phases. Garnet and amphibole are abundant in the residue of the variety with higher (La/Yb)N, whereas in the rocks with moderate to lower (La/Yb)N garnet is absent, magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest (La/Yb)N. In the Canaã dos Carajás and Serra Dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited. A crustal environment and a variable pressure from 10 to 7–8kbar are estimated for the generation of the granite magmas. The Bom Jesus and Cruzadão granites of Carajás derived from basaltic andesite sources and not of TTG and their geochemical contrasts can be explained by variation in the pressure of melting. The sources of Archean granite magmas are more diversified and could be more mafic than generally admitted.

Long-term exhumation of an Aegean metamorphic core complex granitoids in the Northern Menderes Massif, western Turkey

The Egrigoz, Koyunoba, and Alacam plutons are located in the northern portion of the Menderes Massif, a region characterized today by large-scale extension. To gain a better understanding of their exhumation history, we acquired zircon ages, geochemical analyses, and cathodoluminescence (CL) images to search for evidence of micro- to macro-scales of deformation. The combination is a powerful means to decipher their tectonomagmatic history. In situ ion microprobe 238U/206Pb zircon ages of the granitoids range from 30.0 ± 3.9 Ma to 14.7 ± 2.6 Ma (±1σ) and indicate the plutons crystallized over ∼15 m.y. The dated zircons show CL zoning consistent with igneous crystallization and are only located adjacent to or as inclusions in biotite grains. The youngest ages are dominated by a blue color in CL, whereas a majority of older zircons are yellow. Higher Th/U contents are typically shown by zircons with black or dark green colors in CL. Most of the samples of these granitoids indicate they are magnesian, calc-alkalic and peraluminous granite to granodiorites, but variations exist, likely reflecting heterogeneity caused by magma mixing, partial melting, crustal contamination, and post-emplacement fluid interactions as evidenced by the CL images. CL images show that all samples experienced magma mixing, multiple episodes of brittle deformation, and fluid-mediated alteration. Sources for the generation of Northern Menderes Massif granitoids include simple slab induced upwelling from the subduction of the African plate along the Hellenic arc, adiabatic decompression as the northern edge of Turkey9s Anatolide-Tauride block delaminates, and upwelling asthenosphere as the continental lithosphere thins during extension. We present a model in which western Turkey is an amalgamation of stacked subduction zones that transition from north to south over time. Northern Menderes Massif granitoids document their complex geologic history in their ages, chemistry, and textures.

From granite to highly evolved pegmatite: A case study of the Pinilla de Fermoselle granite–pegmatite system (Zamora, Spain)

The Pinilla de Fermoselle pegmatite is a cap-like body with an asymmetrical vertical zoning, from a granitic facies at the bottom to the upper contact with the metamorphic country-rocks. The granite grades imperceptibly into the pegmatite, which includes three main zones with different degrees of enrichment in Li+F+B (±P, Rb, Cs, Be, Sn). The essential minerals are quartz, feldspar, Al-micas from the muscovite–lepidolite series, Fe-micas (biotite and zinnwaldite), tourmaline (schorl–elbaite–rossmanite) and Fe–Mn phosphates. Apatite, beryl, cassiterite and cookeite are the most significant accessory minerals.The trace elements Li, Be and Sr show similar trends in feldspar, micas and tourmaline, with an increase in the Li and Be contents and a decrease in Sr from the granite to the most evolved pegmatitic zone. Similar trends are shown by Rb, Cs and Ba for micas and K-feldspar, Rb and Cs increasing gradually from the granite to most evolved pegmatitic zones, simultaneously to the decrease of Ba. In tourmaline Nb and Ta contents increase upwards whereas Zn contents decrease in the same way. The Mn contents increase until intermediate degrees of evolution, and decrease again in the pinkish elbaite.Combined field, petrographic and geochemical data are consistent with a fractional crystallization model from a granitic melt, with a clear petrogenetic relationship between the underlying peraluminous granite and the pegmatite body. K-feldspar and, particularly, micas and tourmaline appear as good geochemical monitors using trace elements such as Li, Rb, Be, Sr and Ba, which offer intriguing insights into the petrogenesis of pegmatites.

Origin of the granulite enclaves in Indo-Sinian peraluminous granites, South China and its implication for crustal anatexis

Metasedimentary granulite enclaves hosted in the Indo-Sinian Darongshan–Shiwandashan granite belt from the southeastern Guangxi province, South China, experienced a three-stage metamorphic evolution: (1) the early prograde metamorphism in the stability field of sillimanite at ~800°C; (2) the peak metamorphism with a mineral assemblage of spinel+quartz at 7.5–8.0kbar and 950–1000°C (260–250Ma); and (3) a near-isothermal decompression stage with orthopyroxene+plagioclase and orthopyroxene+cordierite symplectic textures at 3.2–3.7kbar and 790–820°C (240–230Ma). The clockwise P–T–t path suggested that the granulite enclaves have undergone initial crustal thickening and subsequent rapid exhumation and cooling history.The granulite enclaves show peraluminous affinities (A/CNK=1.11–3.14, A/CNK=Al2O3/(CaO+Na2O+K2O) in molecular ratio) with Al2O3 up to 25.31%; chondrite-normalized REE patterns similar to those of upper continental crust; relative Rb, Ba, Th, LREE, K, Zr and Hf enrichment and Sr, Nb, Ta, P and Ti depletion in primitive mantle-normalized spidergrams; and highly radiogenic Sr (87Sr/86Sr(i)=0.71295–0.74273 ) and nonradiogenic Nd (εNd(t)=−14.8 to −11.2). Systematic elemental variation trends from the granulite enclaves to the host granites, and similar Sr–Nd isotope compositions in the enclaves and the host granites, indicate that the two have a coherent origin — the granulites were restites of the host granites. The granulite enclaves can also be subdivided into two groups: high-87Sr/86Sr(i) (>0.7303) and low-87Sr/86Sr(i) groups (<0.7135). The estimated magma source for the host granites may contain 10–40% low-87Sr/86Sr(i) and 60–90% high-87Sr/86Sr(i) granulites.P–T–t path and geochemical features of the granulite enclaves, temperature estimation for the host granites, and previous geochronological data, indicate limited dehydration melting of the metasedimentary protoliths during the prograde and peak metamorphic stages. Instead, large scale crustal melting may have occurred during the stage of exhumation and uplifting of the lower-middle crust.

Trace element behavior during partial melting of Iberian orthogneisses: An experimental study

The determination of the exact protolith from which every leucogranite is derived may be a hard task. The trace element composition of both the leucogranite and their likely source rock can be a way to establish this relationship. This hypothesis is checked in an experimental study on the partial melting of two Iberian orthogneisses, the Ollo de Sapo and the Tormes Dome Gneisses. Several runs between 700 and 900 °C at 0.6 and 1.0 GPa, with added water ranging from 2% wt. to 10% wt. have been carried out in piston-cylinder devices. Their results show that two gneisses of virtually the same major and trace element composition yield partial melts of closely similar major element composition but extremely different trace element composition, above all REE. These differences are produced because of several factors, mainly the ASI value of melts (which in turn depends on minor differences in the composition in the main oxides of the gneisses) and the phosphorus hosted in the feldspars of the gneisses. The consequence of these minor changes is that the dissolution of the trace-element-bearing accessories, mainly apatite, can be delayed until higher temperatures are achieved, so melts formed under these circumstances are depleted in REE, Th and Y. Gneisses of relatively low aluminosity and phosphorus-rich feldspars will produce REE-poor melts saturated in P 2O 5 at low temperatures because neither apatite nor other phosphates dissolve. In contrast, gneisses of relatively high aluminosity and phosphorus-poor feldspars will produce REE-rich melts subsaturated in P 2O 5 at low temperatures. For this reason, using the trace element composition of peraluminous granites as the only way to establish their protoliths is not recommended.

Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine

The ∼3 Ga North Caribou greenstone belt comprises a package of volcanic and sedimentary rocks metamorphosed from greenschist to upper amphibolite grade, surrounded by ∼2.7–3.0 Ga granitoids and gneisses and is host to the Musselwhite gold mine, a large orogenic gold deposit. New U–Pb (zircon, monazite), Ar–Ar (biotite) and Sm–Nd (garnet) ages for volcanic, sedimentary and intrusive rocks from the Musselwhite mine and surrounding plutons have been obtained. Felsic volcanic rocks yielded ages of 2972.4 ± 1.6 and 2982.4 ± 0.8 Ma, broadly consistent with existing ages for volcanic rocks and plutonism. An age of 2857.3 ± 1.4 Ma for the Schade Lake gneiss is consistent with published plutonic and volcanic ages. A younger, and previously unrecognized, plutonic event is represented by three granites dated at 2723.1 ± 0.8, 2727.7 ± 1.0 and 2729.4 ± 7.1 Ma. An Sm–Nd age of 2690 ± 9 Ma for garnets associated with gold mineralization is bracketed by peraluminous granites with ages of 2668.8 ± 1.1 and 2715.8 ± 1.6 Ma. Ar–Ar ages range from 2420 ± 11.97 to 2658 ± 13.46 Ma, broadly synchronous with the youngest intrusive age, and are consistent with progressive resetting due to metamorphism. The new geochronological data for the NCGB show that volcanism in the Opapimiskan Lake area spanned 10 million years from 2982.4 to 2972.4 Ma, allow for new stratigraphic interpretations of the belt and are broadly correlative with other greenstone belts in the North Caribou terrane. The new data for the NCGB and associated plutons are consistent with ∼3 Ga plume-related activity forming a proto-craton that was the site of successive sedimentary basins over the next ∼150 million years. Subsequent plutonism is consistent with supra-subduction zone processes at the margins of this proto-continent by ∼2.87 Ga and is best interpreted as the result of horizontal tectonic processes consistent with those operating in the Phanerozoic, with the plutons emplaced behind continental margin arcs on the edges of the craton. Sm–Nd ages of garnets that are coeval with mineralization suggest that the orogenic gold mineralization at the Musselwhite mine was broadly synchronous with the ∼2700 Ma orogeny caused by the collision of the Oxford-Stull domain with the North Caribou terrane ∼75 km northeast of the Musselwhite mine and furthermore argues that S-type granites may play a greater role in the formation of some orogenic gold deposits than has been previously recognized.

40Ar ∗ loss in experimentally deformed muscovite and biotite with implications for 40Ar/ 39Ar geochronology of naturally deformed rocks

The effects of deformation on radiogenic argon ( 40Ar ∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas. Infrared (IR) laser (CO 2) heating of individual 1.5–2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/ 39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar ∗ loss of 0–35% in muscovite and 2–3% 40Ar ∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/ 39Ar ages (±4–10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0–16% 40Ar ∗ loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/ 39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% 40Ar ∗ loss. No spatial correlation is observed between in situ 40Ar/ 39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar ∗ loss in the experimentally treated muscovite can be utilized to predict average 40Ar ∗ diffusion dimensions. Maximum 40Ar/ 39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for 40Ar ∗ diffusion ⩾700 μm. The UV laser ablation and IR laser incremental 40Ar/ 39Ar ages indicating 40Ar ∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ≪100 μm. These results support a hypothesis of grain-scale 40Ar ∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for 40Ar ∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both 40Ar ∗ retention and intragrain distribution in deformed mica. Intragrain 40Ar/ 39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.

Diversity in Earth's early felsic crust: Paleoarchean peraluminous granites of the Barberton Greenstone Belt

Earth9s oldest preserved granitoid crust dates back to the Paleoarchean and consists predominantly of sodic tonalite-trondhjemite-granodiorite (TTG) granitoids that arose through the partial melting of hydrated metabasalts. In contrast, granites (sensu stricto) typically appear relatively late in the plutonic record of the old cratons. However, the existence of Hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust; although we have direct evidence, this earliest felsic crust is not preserved. Here we present evidence of an unusual variety of markedly low-CaO, K 2 O-rich, rutile-bearing, peraluminous granite and rhyolite that was produced concurrently with TTG magmas during three magmatic cycles in the Barberton Greenstone Belt (BGB), southern Africa. This material is not preserved as in situ rock units, but occurs as clasts within a younger conglomerate. Within these rocks, plagioclase feldspar is a rare inclusion in zircon, relative to alkali feldspar, and has low anorthite contents (An 2 O and Sr content, as well as the peraluminous character of the magmas, is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability. This process contributed to each episode of continental crustal growth through the Paleoarchean to Mesoarchean in the BGB, despite leaving no plutonic record at the typical mid-crustal level of exposure that the TTG plutons around the belt represent.