Metaluminous Granitoids Research Articles

Petrography and Geochemistry of the Farchana-Hadjerhadid Granitoids (Ouaddaï Massif, Eastern Chad)

Work in the Farchana-Hadjerhadid sector is focused on petrography, with the main aim of mapping the geological formations of the eastern Ouaddaï Massif in order to identify petrographic types and gain an understanding of the geodynamic history of this area. The fieldwork showed that the Farchana-Hadjerhadid granitoids consist mainly of granites and diorites and are intersected by numerous pegmatite veins. Microscopic observation shows that the diorite is made up of plagioclase, amphibole, biotite, and quartz. The granite is composed of alkali feldspar, quartz, amphibole, plagioclase, and biotite. The granitoids in the study area are moderately differentiated (SiO2 = 55.62% to 71.67%) and belong to the medium to highly potassic calc-alkaline series. They are ferriferous type I metaluminous granitoids (A/CNK < 1.1). The parent magma was derived from the partial melting of the metabasalts, and the petrographic types evolved by assimilation and fractional crystallization.

Petrological models of magma evolution on the formation of mafic-felsic igneous complexes of the Emeishan large igneous province, SW China

Many Late Permian world-class Fe-Ti oxide-bearing layered intrusions in the Pan-Xi region, SW China are commonly considered to be fractionation products of the Emeishan mantle plume-related high-Ti basaltic melts. These layered intrusions coexist spatially and temporally with A-type granitoids, which consist of syenites and granites and are further classified into isotopically depleted metaluminous to peralkaline and isotopically enriched peraluminous to metaluminous groups. The petrogenesis of these mafic-felsic igneous complexes has been controversial. This study investigates geochemical correlations of the microgabbros at the margin of the Baima layered intrusions and the intruding isotopically enriched granitoids. The amphiboles at both sides of the contact zone are calcic (CaO = 10.4 to 12.5 wt.%) and display obvious mixing trends. Thermodynamical modeling using the MAGEMin program indicates that fractionation of basaltic melts compositionally similar to the Baima microgabbros seems incapable to crystallize olivine compositions comparable to those of the layered intrusions, but can be reconciled by early separation of 7.5 to 10 wt.% Fe-Ti-rich melts. Continued differentiation of the basaltic melts may lead to formation of the isotopically depleted granitoids, among which perthite accumulation in water-rich environments could result in the formation of metaluminous granitoids with high Na, K and Al contents and positive Eu anomalies. The isotopically enriched granitoids, on the other hand, can be produced by anatexis of the juvenile mafic lower crust together with the Yangtze ancient lower crust. Our study therefore provides a panoramic view on the complicated petrogenesis of the igneous complexes related to Emeishan mantle plume impingement through thermodynamic modeling.

The Peña do Seo W-Sn deposit, NW Iberia: Petrology, fluid inclusions and O-H-S isotopes

The Peña do Seo W-Sn ore deposit in NW Iberia consists mainly of quartz veins hosted in schists. Vein mineralogy comprises wolframite, cassiterite and minor molybdenite. Peraluminous S-type granites and minor metaluminous granitoid breccias crop out in the same location. Whole rock geochemistry, mineral characterization, fluid inclusions and stable isotope geochemistry have been combined to infer hydrothermal mineralization conditions. A pre-ore stage (0) involving the alteration of host rocks, three stages of mineralization (I, II and III), and a supergene stage (IV) have been identified. Stage I (oxide-halide-sulfide stage) consisted of a cassiterite-pyrite-fluorite mineralization in granitoid breccias. Stage II (main oxide stage) involved wolframite-rich selvages and wolframite-cassiterite-(molybdenite) quartz veins. Stage III (main sulfide stage) consisted of a sulfide mineralization in quartz veins comprising pyrite and minor arsenopyrite and chalcopyrite. Three types of fluid inclusions were found in vein quartz: (1) aqueous two-phase inclusions, with homogenization temperatures (Th) between 445 °C and 280 °C and moderate salinities (9-14 wt.% NaCl eq.), (2) aqueous-carbonic three-phase fluid inclusions, with Th from 340 °C to 260 °C and low salinities (2-7 wt.% NaCl eq.), and (3) aqueous two-phase fluid inclusions, with Th from 270°C to 155 °C and low salinities (0-6 wt.% NaCl eq.). δ18O values in quartz from mineralized veins range from +11.2‰ to +13.4‰, and between +15.0‰ and +15.4% in quartz from mineralized granitoid breccias. δ34S values in sulfides (pyrite, arsenopyrite and chalcopyrite) range between +13.0 ‰ and +37.1 ‰, thus suggesting a marine source of sulfate and possible equilibration with host-rock sulfides. δD values in muscovite and chlorite from quartz veins range between -105.7 ‰ and -71.5 ‰ and between -69.4 ‰ and -67.1 ‰, respectively, indicating a transition from magmatic to magmatic-metamorphic conditions. An aqueous (H2O–NaCl) magmatic-hydrothermal fluid led to the W-Sn mineralization, as deduced from the studied fluid inclusions and isotopic signatures. High W and Sn contents in the peraluminous granites indicate that the W-Sn mineralization in Peña do Seo could partially be related to the granites and granitoid breccias. Whereas the Sn (± W) likely derived from hydrothermal fluids exsolving from the crystallizing magmas, host quartz schists would have supplied other elements required for wolframite deposition such as Fe and Mn. Sulfide mineralization in stage III was probably driven by fluid dilution.

Zircon U–Pb age, whole-rock geochemistry and Nd–Sr–Pb isotope constraints on petrogenesis of the Eocene Zajkan gabbro–monzogranite intrusion, Tarom-Hashtjin magmatic belt, NW Iran

The late Eocene high-K to shoshonitic Zajkan intrusion, situated in the Tarom-Hashtjin magmatic belt (THMB), NW Iran, ranges in composition from gabbro to pyroxene quartz monzodiorite, pyroxene quartz monzonite and monzogranite, and is classified as an I-type metaluminous granitoid. On the primitive mantle-normalised spider diagram, these rocks demonstrate negative high field strength element (HFSE) anomalies, large-ion lithophile element (LILE) enrichment, and positive Pb anomaly, indicating a subduction-related signature. Furthermore, the granitoids are characterised by coherent chondrite-normalised rare earth element patterns with high light rare earth elements/heavy rare earth elements (LREE/HREE) and moderate to weakly negative Eu anomalies (Eu/Eu* = 0.6–0.86). LA-ICP-MS zircon U–Pb dating demonstrates that the Zajkan intrusion crystallised at ca 36.5 Ma. Relatively low to moderate 87Sr/86Sr ratios (0.7046–0.7061) and positive εNd(t) values (1.52–1.88) of the Zajkan samples are consistent with an enriched lithospheric mantle source that was metasomatised by an earlier subducted slab. Geochemical features show that the parental magma of the Zajkan intrusion likely resulted from a partial melting of metasomatised amphibole-bearing mantle source. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of a mantle-derived magma and ∼20% of a juvenile lower crustal-derived magma. Partial melting of the metasomatised mantle may have resulted from late Eocene lithospheric thinning and hot asthenospheric upwelling through slab roll-back or lithospheric delamination. KEY POINTS The late Eocene high-K to shoshonitic Zajkan intrusion is located within the Tarom-Hashtjin magmatic belt, NW Iran. U–Pb zircon dating revealed an age of ca 36.5 Ma for emplacement of the Zajkan intrusion. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of mantle-derived magma and ∼20% of juvenile lower crustal-derived magma.

U-Pb zircon geochronology and geochemical constraints on the Ediacaran continental arc and post-collision Granites of Wadi Hawashiya, North Eastern Desert, Egypt: Insights into the ~600 Ma crust-forming Event in the northernmost part of Arabian-Nubian Shield

The Wadi Hawashiya, North Eastern Desert of Egypt, is a mountainous granitic terrain exposing two Ediacaran granitoid suites, namely the Hawashiya Older Granites (HOG) and the Hawashiya Younger Granites (HYG). The HOG comprise normal to high-K, calc-alkaline, metaluminous granitoids (mainly quartz diorites), while the HYG are strongly high-K, calc-alkaline, slighty peraluminous syeno- and monzogranites. Zr is strikingly high (av. 335 ppm) in the HOG and is notably depleted (av. 108 ppm) in the more evolved HYG. The HYG have very low Nb contents almost matching those of the HOG (Nb ~ 6 ppm), which is typical of I-type granites generated at arc-type settings. Although the REE patterns of the HOG and HYG are comparably LREE-enriched with moderately steep slopes, the patterns of the HYG have significant deep negative Eu anomalies of variable magnitudes, whereas those of the HOG usually exhibit very shallow negative and occasional positive Eu anomalies. The assessed apatite crystallization temperatures for both granitoid suites are higher than those estimated from the zircon and monazite thermometers indicating that apatite crystallized before zircon and monazite from the melt. HOG evolved by assimilation-fractional crystallization processes from mantle-derived magmas, which subsequently interacted with crustal sources during ascent and residence in higher crustal levels. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments. The HYG were emplaced (586–601 Ma) after the collision between the juvenile crust of the Arabian-Nubian Shield (ANS) and the older pre-Neoproterozoic continental blocks of Saharan Metacraton. The emplacement of HOG (622-611 Ma) had occured earlier (ca. 10 Ma) in a normal mature continental arc. The geochemical signatures and age data suggest that the terminal collision between East and West Gondwanaland in the northern tip of the ANS occurred before ~620 Ma, most likely at ~630 Ma, and transitioned to an extensional setting at ~600 Ma.

Geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Late Permian–Early Triassic granitoids in Eastern Kunlun Orogen, Northwest China: petrogenesis and implications for geodynamic setting

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB) in Northwest China, is an ideal area for understanding the tectonic evolution of the Palaeo-Tethys Ocean. The EKOB is marked by widespread Palaeozoic to Early Mesozoic magmatism. However, the Late Permian–Early Mesozoic tectonic evolution of the Palaeo-Tethys Ocean tectonic regime is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Late Permian–Early Triassic intrusive rocks in the Xiangyanggouxi and Dageledong areas in the EKOB, Northwest China, to discuss their petrogenesis and geodynamic setting and reconstruct the tectonic evolution of Palaeo-Tethys oceanic tectonic regime. Zircon U-Pb dating results indicate that these granitoids crystallized during ca. 256–248 Ma, i.e. the Late Permian to Early Triassic. These granitoids are mainly comprised of granodiorites and monzonitic granites that possess high concentrations of SiO2 (66.81–75.98 wt.%), K2O+Na2O (6.21–7.92 wt.%), and low contents of MgO, CaO, Cr, Ni, and Mg# (<0.44). They are classified as medium- to high-K calc-alkaline and metaluminous I-type granitoids. These granitoids are rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) with arc affinity. In addition, combined with the εHf(t) values of −2.4 to +2.4 with two-stage model ages ranging from 1409 Ma to 1027 Ma, and the εNd(t) values from −3.73 to −2.95, it is suggested that the primary magma of these granitoids was derived from the partial melting of the Late Meso–Proterozoic mafic lower crust. These findings, combined with spatio-temporal distributions of regional magmatism, reveal that the Late Permian and Early Triassic granitoids formed in a continental magmatic arc setting related to the northward subduction of the Palaeo-Tethys Ocean plate beneath the EKOB-Qaidam terrane.

Geochemical constraints on the origin and tectonic setting of Chargar intrusions in the Alborz orogenic belt, NW Iran

Chargar area is located in the southern border of Tarom subzone within the Alborz magmatic belt of NW Iran. Two types of intrusions, mainly present in the southern part of the area, have been identified. These bodies intruded into the Karaj Formation volcanic and volcaniclastic rocks. Type-I intrusions occur in the south and include two magmatic bodies: (a) Gabbro-pyroxene quartz monzodiorite-quartz monzodiorite and (b) quartz syenite. Type-II crops out in the west and has a gabbro – gabbro-diorite composition. Geochemically, Chargar intrusive rocks belong to the high-K calc-alkaline to shoshonitic series and are classified as I-type and metaluminous granitoids. These intrusive rocks show insignificant different distribution patterns of trace elements and REEs, but generally they are characterized by highly enriched large ion lithophile elements (e.g., K and Ba) and depleted Nb, similar to that of continental arc magmatism. Type-I intrusions and Type-II mafic intrusion were originated from mantle source with amphibole-bearing spinel peridotite composition. These intrusions formed in a continental arc to post-collisional tectonic setting.

Generation of coeval metaluminous and muscovite-bearing peraluminous granitoids in the same composite pluton in West Qinling, NE Tibetan Plateau

There is considerable controversy over the nature of (strongly) peraluminous granitoids and their relationship with coeval metaluminous granitoids. The origin of peraluminous granitoids is of great importance for an understanding of granite petrogenesis, as well as the formation and evolution of the continental crust. This study reveals the presence of contemporaneous metaluminous granitoids and muscovite-bearing peraluminous granitoids in the Guomaying composite pluton (GCP) at approximately 243–242 Ma in West Qinling in the NE Tibetan Plateau. The Middle Triassic granitoids in the GCP can be petrographically and geochemically divided into two groups. The group I samples are characterized by slightly higher K2O/Na2O ratios and are metaluminous with lower Zr-saturation temperatures (740–756 °C). In contrast, the group II samples have lower K2O/Na2O ratios and mostly strongly peraluminous compositions, with marginally higher Zr-saturation temperatures (762–778 °C). Both groups share similar trace elements and Sr–Nd–Pb–Hf isotopic compositions, pointing to middle to lower crustal origins with an amphibolitic residue. The metaluminous granitoids are interpreted as partial melting of amphibolites in the middle to lower crust beneath West Qinling, and the muscovite-bearing peraluminous granodiorites were also generated by partial melting of an amphibolitic source, with addition of a feldspar-rich component in the source. The hydrous basaltic melts derived from partial melting of a metasomatized mantle wedge source were also incorporated into the magma mush. It is inferred that disparate magma sources, entrained peritectic assemblages, and various degrees of antecryst recycling collectively exerted influence on the granitic magma chemistry of the GCP. Consequently, our data suggest that muscovite-bearing strongly peraluminous granitoids, which are widely regarded as the products of recycled metasedimentary rocks, can also originate from an amphibolite-dominated source with involvement of a feldspar-rich component.

EVALUATION OF SOIL GEOCHEMISTRY DATA OF CANCA AREA (GÜMÜŞHANE, TURKEY) BY MEANS OF INVERSE DISTANCE WEIGHTING (IDW) AND KRIGING METHODS- PRELIMINARY FINDINGS

In Northwest Anatolia, widespread magmatism developed due to collision between Anatolide-Tauride platform and Sakarya continent during Late Cretaceous-Early Tertiary period. The granitoids in Biga Peninsula are products of post-collisional magmatism following the convergence of the northern branch of Neotethyan Ocean and developed in two different stages as Eocene and Oligo-Miocene. Eocene Karabiga, Gureci, Kuscayir and Dikmen granitoids are granite and diorite-granodiorite; Oligo-Miocene Sarioluk, Yenice, Kestanbol, Eybek, Evciler, Camyayla and Alankoy granitoids are diorite, granodiorite, monzonite and Q-monzonite in composition. Metaluminous and peraluminous granitoids have similar geochemical variations and exhibit post-collisional geochemical signatures. Trace element patterns are almost similar to those observed in upper crust and GLOSS (Global Subducting Sediment) patterns with depletion in high field strength (HFS) elements (Nb, Ta, Ti, Zr, Hf). But, Oligo-Miocene Sarioluk, Yenice-Cakiroba, Kestanbol, Evciler, Camyayla, Alankoy and Eocene Karabiga, Gureci and Kuscayir granitoids have higher Th and U contents relative to upper crust and GLOSS. Dikmen, Yenice-Hamdibey, Yenice-Eskiyayla and Eybek granitoids have lower Th content. Geochemical variations indicate that partial melting and fractional crystallisation- crustal contamination processes are effective in their genesis and evolution. Trace element ratios also indicate subduction signatures in their genesis and Rb/Ba, Rb/Sr ratios suggest mantle melting rather than crustal melting. Accordingly, post-collisional Biga Peninsula granitoids were derived from a previously metasomatised lithospheric mantle source, which was enriched during northward subduction and closure of the northern branch of Neo-Tethys Ocean beneath the Sakarya continent, since variations in Rb, Cs, Th, La and Sm reveal that lithospheric mantle was mesomatised by both aqueous fluids and sediment melts.

Recurrent intrusive episodes in the Paleozoic metasedimentary upper crust during the Early Carboniferous time: The Veladero granitoid stock and the peraluminous andesite

The Veladero hill is a well-exposed upper crustal block located between the basement of Sierra de Umango (Western Sierras Pampeanas), and the Devonian - Carboniferous sedimentary sequence of Precordillera, at western Argentina. The hill is formed by schists, metaluminous granitoids and a subalkaline peraluminous andesite studied in this work. The country rocks are schists with mineralogy typically developed during a thermal event (e.g., cordierite, andalusite, biotite). The magmatic assemblage of the peraluminous andesite consists of K-feldspar + plagioclase + quartz + biotite + cordierite, with zircon, monazite, and oxides as accessory minerals. One representative sample from this rock was analyzed and has contents of SiO2 = 61%, Fe2O3t + MgO = 9.03, Na2O + K2O = 6.6%, and aluminium saturation index = 1.5, consistent with the mineralogy. A U-Pb zircon Concordia age of 347 ± 4 Ma was acquired by LA-MC-ICP-MS from zircon hosted in the peraluminous andesite, which together with an age of 342 ± 2 Ma recently published for the metaluminous Veladero granitoid stock, indicate that these two ages overlap within error. Combined mineral assemblage and Al-in-hornblende geobarometric data from the Veladero granitoid stock reveal that the granitic magma passed through ∼10 km of continental crust before reaching its final emplacement level. Field evidence indicates that the Veladero granitoid stock was emplaced under dominantly brittle deformation mechanisms and that the peraluminous andesite magma was generated and subsequently intruded into a previously emplaced magma mush close to solidification, making space by the displacement of large blocks of the host granite. Documented mingling relationships between granite and peraluminous andesite also support emplacement while the granite remained partially crystallized so that the time interval between the two was relatively short, probably on the scale of hundreds of thousands of years. We propose that the peraluminous magma was generated during the ascent of the metaluminous Veladero granitoid stock magma and that the most of the zircon crystallized in high middle crust from the Veladero granitoid stock. These zircons were subsequently recycled and incorporated in the peraluminous andesite. Zircons in the Veladero granitoid stock and the peraluminous andesite have indistinguishable morphology and support this interpretation. In situ Hf isotope data were obtained from igneous zircons in the peraluminous andesite and reveal a dominantly continental source (average εHf,t = −8.0, excluding one just positive value). εHf,t and TDM values from zircon in the Veladero granitoid stock and hosted in the peraluminous andesite are indistinguishable, which is consistent with their original crystallization in the Veladero granitoid stock. TDM values mainly range from 1.5 to 1.9 Ga (average = 1.7 Ga) and reveal a continental Gondwana signature. Consistently, detrital zircon ages from schists in which the igneous rocks were emplaced indicate Ordovician depositional ages and original Gondwanan provenance. However, based on previous studies a Devonian depositional age cannot be ruled out.

Geochemistry and U–Pb SHRIMP zircon geochronology of microgranular enclaves and host granitoids from the South Khasi Hills of the Meghalaya Plateau, NE India: evidence of synchronous mafic–felsic magma mixing–fractionation and diffusion in a post-collision tectonic environment during the Pan-African orogenic cycle

Abstract Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km ( c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K 2 O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO 2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207 Pb/ 206 Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.

Calcic amphibole thermobarometry in metamorphic and igneous rocks: New calibrations based on plagioclase/amphibole Al-Si partitioning and amphibole/liquid Mg partitioning

Dependencies of plagioclase/amphibole Al-Si partitioning, DAl/Siplg/amp, and amphibole/liquid Mg partitioning, DMgamp/liq, on temperature, pressure and phase compositions are investigated employing robust regression methods based on MM-estimators. A database with 92 amphibole-plagioclase pairs – temperature range: 650–1050°C; amphibole compositional limits: >0.02 apfu (23O) Ti and >0.05 apfu Al – and 148 amphibole-glass pairs – temperature range: 800–1100°C; amphibole compositional limit: CaM4/(CaM4+NaM4) >0.75 – compiled from experiments in the literature was used for the calculations (amphibole normalization scheme: 13-CNK method).Statistical analysis reveals a significant dependence of DAl/Siplg/amp on pressure, temperature, Al fraction in amphibole T1-site, XAlT1, and albite fraction in plagioclase, XAb, leading to the barometric expression: Pkbar=8.3144TKlnDAl/Siplg/amp−8.7TK+23377XAlT1+7579XAb−11302/−274 (precision: ±1.5 to ±2.3kbar; expressed at 1s).The DMgamp/liq was found to have a significant dependence on temperature and the logarithm of XCaliq/(XCaliq+XAlliq) ratio (glass composition expressed as anhydrous mole fraction of cation components) that led to the thermometer: T℃=71975−11896lnXCaliq/XCaliq+XAlliq8.3144lnDMgamp/liq+58−273 (precision: ±35 to ±45°C).Amphibole-liquid compositional relationships revealed that the temperature dependence of DMgamp/liq was mostly controlled by a positive correlation of lnXMgliq with temperature that led to the thermometer: T(℃)=107 lnXMgliq−108ln[XCaliq/(XCaliq+XAlliq)]+1184 (precision: ±37 to ±42°C).The plagioclase-amphibole barometer is suitable for a large diversity of amphibole-plagioclase-bearing assemblages from metamorphic (amphibolites and mafic granulites) and igneous (metaluminous granitoids to gabbros) rocks, whereas the liquid-only and the amphibole-liquid thermometers are applicable to alkaline and subalkaline igneous rocks.The calibrated expressions yield P-T estimates consistent with those obtained with widely used barometers and thermometers. Plagioclase-amphibole barometry extends pressure determinations to quartz-free and/or garnet-free assemblages.

Mantle-derived sources of syenites from the A-type igneous suites — New approach to the provenance of alkaline silicic magmas

Granite is generally dominant in A-type igneous suites but these frequently include also alkali feldspar- and peralkaline- syenite and quartz syenite. Such syenites can provide essential information about magma sources and mode of generation of A-type silicic magma. This paper addresses the petrogenesis of syenites based on comparisons between the Mongolian–Transbaikalian Belt, Russia (MTB), and the northern Arabian–Nubian Shield (ANS) as exposed in the Sinai Peninsula, Egypt and adjacent areas of southern Israel. The syenitic rocks from MTB and ANS are characterized by high alkali content (Na2O+K2O=10.5 to 12.5wt.%) and are assigned to alkaline metaluminous and peralkaline granitoids. Peralkaline syenites are generally richer in Na and contain slightly less K and Ba than are metaluminous granitoids. REE abundances are similar in all types of syenites. The Eu/Eu* ratios range commonly from 0.35 to 0.65, although higher values, up to 1.15, attributed to presence of accumulated Afs and minor Pl, also occur in some plutons. The geochemical and Sr–Nd isotope characteristics of associated syenite, granite and monzogabbro from five igneous suites (~80 samples) suggest that the main rock types, silicic and mafic, are cogenetic in each suite. Syenite magmas were produced from mantle-derived source with little, if any silicic crustal component. The generation of abundant A-type granite and syenite magmas in the young juvenile crust (ANS) argues that old continental crust is not required for generation of highly alkaline silicic magmas, as commonly advocated. The most probable source of both syenite and granite was mantle-derived K-rich shoshonitic monzogabbro. The bimodal character of the A-type suites suggests that partial melting of monzogabbro, rather than fractional crystallization of basic magma, accompanied with enrichment of a cumulate phase in the mafic units, was the dominant mode of granitoid magma formation. Granite magmas were produced in the lower crust at ~20% of melting, whereas syenite–quartz syenite magmas could be generated by melting of ~25–30%.

Post-collisional Pan-African granitoids and rare metal pegmatites in western Nigeria: Age, petrogenesis, and the ‘pegmatite conundrum’

The Minna area of western Nigeria lies within a Pan-African orogenic belt that extends along the margin of the West African Craton, from Algeria southwards through Nigeria, Benin and Ghana, and into the Borborema Province of Brazil. This belt is characterised by voluminous post-collisional granitoid plutons that are well exposed around the city of Minna. In this paper we present new information about their age and petrogenesis. The Pan-African plutons around Minna can be divided into two main groups: a group of largely peraluminous biotite–muscovite granites that show varying levels of deformation in late Pan-African shear zones; and a younger group of relatively undeformed, predominantly metaluminous hornblende granitoids. Pegmatites, including both barren and rare-metal types, occur at the margins of some of the plutons. New U–Pb zircon dating presented here, in combination with published data, indicates an early phase of magmatism at c. 790–760 Ma in the Minna area. This magmatism could be related either to continental rifting, or to subduction around the margins of an existing continent. The peraluminous biotite–muscovite granites were intruded at c. 650–600 Ma during regional shearing in the orogenic belt, and are likely to have formed largely by crustal melting. Subsequent emplacement of metaluminous granitoids at c. 590 Ma indicates the onset of post-orogenic extension in this area, with a contribution from mantle-derived magmas. The rare-metal pegmatites represent the youngest intrusions in this area and thus are likely to have formed in a separate magmatic episode, post-dating granite intrusion.

Orogenic gold and rare metal deposits of the Upper Kolyma District, Northeastern Russia: Relation to igneous rocks, timing, and metal assemblages

The Upper Kolyma district is the most important part of the Yana–Kolyma gold province, administratively located in Magadan Oblast, Russia. To date, historic production of the region is about 90Moz of gold, mostly from placers, but with about 5% contribution from lode deposits. Two major types of gold deposits are distinguished in the region: (1) Au-quartz veins related to regionally metamorphosed rocks; and (2) granitoid-related Au. Besides gold, tin deposits are also wide-spread and can be distinguished as: (1) Sn-quartz veins, (2) Sn-silicate-sulfide veins, and (3) Sn-polymetallic veins. Quartz-molybdenite (porphyry Mo) and Ag-polymetallic (Ag-quartz) veins are less abundant. Metamorphic zones and major regional faults are the most important factors of control for the Au-quartz veins. Granitoid intrusions are the most important ore-controlling factor for other deposit types.Field relationships between Au-quartz vein, granitoid-related Au mineralization and intrusions, as well as the 40Ar/39Ar dating reveal several pulses in formation of the Upper Kolyma gold deposits. The majority of granitoid-related Au deposits and occurrences in the region are related to the oldest (150Ma) metaluminous granitoid plutons of the subduction nature. 40Ar/39Ar dates for Au-quartz mineralization in the Upper Kolyma district correspond mostly to 139–125Ma, which is a period of less intense igneous and metamorphic activity in the area. We interpret these ages to indicate vein formation by metamorphic fluids, boiling from uplift and decompression, following Early Cretaceous collision-induced metamorphism. Most Sn-quartz, Sn-silicate-sulfide, Sn-polymetallic veins, as well as Ag-polymetallic and U-REE mineralization relates to the ca. 80Ma plutons, emplaced in an extensional setting.The 40Ar/39Ar data, obtained for Au-quartz vein mineralization, were used as a basis to develop a genetic model that assumes a time interval (from 1 to 15m.y.) between the ore-mobilizing processes and the timing of ore deposition. Such an interval can be interpreted by virtue of the regional uplift and irregular drop in pressure of fluid systems as the main factors of ore deposition.Iron- and As-bearing sulfides are typical for all hardrock deposits of the Upper Kolyma district, and they reflect the event and/or remobilization of gold. Pyrite is the main sulfide in greenschist metaturbidities and igneous rocks. Metamorphism of turbidites resulted in pyrite–pyrrhotite transformations with sulfur, arsenic, and metal mobilization. Arsenopyrite assemblages, usually dominating in the ores of all deposit types of the district, are products of geochemical evolution of the above-mentioned mobilized sulfur and arsenic. The Au-quartz vein deposits are characterized by pyrite–arsenopyrite assemblage, and arsenopyrite has almost a stoichiometric composition; the granitoid-related Au- and Sn-quartz veins are characterized by lollingite–arsenopyrite assemblage and As/S>1 in arsenopyrite; the Sn silicate-sulfide and Sn-polymetallic veins are characterized by pyrite–pyrrhotite–arsenopyrite assemblage and stoichiometric arsenopyrite; the Ag-polymetallic and quartz molybdenite veins are characterized by pyrite–arsenopyrite assemblage and As/S<1 in arsenopyrite. However, some deposits do not agree with the above-mentioned criteria, which can be explained either by overprinting of several mineralization types in an individual deposit or by metamorphism of its ores. The results demonstrate high importance of re-mobilization of metals due to the later hydrothermal and thermal events.

Petrogenesis and tectonic significance of an early Palaeozoic mafic-intermediate suite of rocks from the Central Tianshan, northwest China

We report petrologic, geochemical, and zircon U−Pb and Hf isotope data from an early Palaeozoic mafic-intermediate suite of rocks in Baluntai, heartland of the Central Tianshan. Analysed major elements of the intermediate rocks show a close affinity to metaluminous I-type granitoids, resembling arc plutons. The mafic rocks display depletions of Nb, Ta, and Ti typical of volcanic arc basalts (VAB). All of the samples fall within the subduction-related field in tectonic discrimination diagrams. Zircons from a granodiorite and a hornblende diorite yield indistinguishable crystallization ages of 475.1 ± 1.7 million years and 473.7 ± 1.6 Ma, indicating an early Palaeozoic magmatic event. Zircons in the mafic rocks yield younger crystallization ages of 427 ± 1 Ma and 426.5 ± 1.4 Ma. Combined with previous published results, our data suggest that the southward subduction of the Palaeo-Tianshan oceanic crust beneath the northern margin of the Tarim block probably began no later than Early Ordovician time and did not end prior to the middle Silurian. Zircons from the granodiorite and hornblende diorite show ϵHf(t) values of −11.19 to −5.98 and −12.85 to −6.01, respectively, suggesting the reworking of ancient crust. Correspondingly, these zircons yield two T DM2 age ranges: 2140–1812 Ma and 2241–1812 Ma, probably representing a significant episode of juvenile addition during the assembly of Columbia. In contrast, zircons from the mafic rocks yield ϵHf(t) values of 3.12–8.91 and 3.19–8.76, corresponding to the T DM1 ages ranging from 911 to 685 Ma and from 905 to 688 Ma, respectively, suggesting crustal growth in the Central Tianshan microcontinent associated with the initial break-up of Rodinia.

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.

Enclaves in granitoids of north of Jonnagiri schist belt, Kurnool district, Andhra Pradesh: Evidence of magma mixing and mingling

Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are fine-grained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.

Lutetian arc-type magmatism along the southern Eurasian margin: New U-Pb LA-ICPMS and whole-rock geochemical data from Marmara Island, NW Turkey

The rocks of Turkey, Greece and Syria preserve evidence for the destruction of Tethys, the construction of much of the continental crust of the region and the formation of the Tauride orogenic belt. These events occurred between the Late Cretaceous and Miocene, but the detailed evolution of the southern Eurasian margin during this period of progressive continental accretion is largely unknown. Marmara Island is a basement high lying at a key location in the Cenozoic Turkish tectonic collage, with a Palaeogene suture zone to the south and a deep Eocene sedimentary basin to the north. North-dipping metamorphic thrust sheets make up the island and are interlayered with a major metagranitoid intrusion. We have dated the intrusion by Laser Ablation ICP-MS analysis of U and Pb isotopes on zircon separates to 47.6 ± 2 Ma. We also performed major- and trace-elemental geochemical analysis of 16 samples of the intrusion that revealed that the intrusion is a calc-alkaline, metaluminous granitoid, marked by Nb depletion relative to LREE and LIL-element enrichment when compared to ocean ridge granite (ORG). We interpret the metagranitoid sill as a member of a mid-Eocene magmatic arc, forming a 30 km wide and more than 200 km long arcuate belt in NW Turkey that post-dates suturing along the Izmir-Ankara-Erzincan Suture zone. The arc magmatism was emplaced at the early stages of mountain building, related to collision of Eurasia with the Menderes-Taurus Platform in early Eocene times. Orogenesis and magmatism loaded the crust to the north creating coeval upward-deepening marine basins partially filled by volcanoclastic sediments.

Genesis of intermediate igneous rocks at the end of the Sveconorwegian (Grenvillian) orogeny (S Norway) and their contribution to intracrustal differentiation

Abundant ferroan, metaluminous granitoids (970–950 Ma) emplaced at the end of the Sveconorwegian collisional orogeny (1130–900 Ma) are dominated by intermediate to silicic compositions with rare mafic facies. Both 73% fractional crystallization of an amphibole-bearing gabbroic cumulate substracted from the parent mafic composition and 30% non-modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition. A better fit is obtained for the partial melting process. Both processes could have occurred simultaneously to produce mafic cumulates and restites. As there is no evidence for large volumes of dense mafic rocks in the Sveconorwegian upper crust, these dense mafic rocks were probably produced in the lower crust. Formation of these granitoids, thus, contributed to the vertical stratification of the Proterozoic continental crust and also to the transfer of water from the lower crust to the surface.