Granitoid Series in Terms of Magnetic Susceptibility: A Case Study from the Barberton Region,South Africa

Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (∼3450 Ma) have low magnetic susceptibilities, less than 3 × 10⁻³ SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (∼3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 × 10⁻³ SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate.Late-tectonic granodiorite - adamellite batholithic complexes (∼3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.     

Petrogenesis of Mesoproterozoic K‐rich granitoids,southern Mt Angelay igneous complex,Cloncurry district,northwest Queensland

The K‐rich granitoids of the southern Mt Angelay igneous complex belong to the younger phases of the Williams and Naraku Batholiths (<1540 Ma) in the Cloncurry district. Granitoids of the complex form a series of I‐type, K‐rich, metaluminous monzodiorite to subaluminous syenogranite. These intrusions have geochemical affinities akin to ‘A‐type’ granites and contain plagioclase, alkali feldspar, quartz, biotite, hornblende and typically accessory magnetite, titanite, apatite and zircon. With increasing SiO₂ the granitoids vary from alkaline to subalkaline, and exhibit a decrease in TiO₂, Al₂O₃, Fe₂O₃*, MnO, MgO, CaO, P₂O₅, Cu, Sr, Zr, LREE and Eu, with an increase in Na₂O, K₂O, Rb, Pb, Th, U, Y and HREE. This suite of relatively oxidised granitoids (<1.0 log units above NNO) were emplaced after the peak of metamorphism and pre‐ to post‐D₃, a major east‐west horizontal‐shortening event. The synchronous emplacement of high‐temperature mafic (>960°C) and foliated felsic (>900°C) granitoids formed zones of mingled and mixed monzonite and quartz monzonite to monzogranite containing abundant rapakivi K‐feldspar. These intrusions are interpreted to have been derived from source rocks of different compositions, and probably by different degrees of partial melting. The unfoliated felsic granitoids are considered to represent the fractionated equivalents of older foliated felsic granitoids. All granitoids possess a Sr‐depleted and Y‐undepleted signature, which suggests that the source material probably contained plagioclase and no garnet, restricting magma production to <800–1000 MPa (～24–30 km). Underplating of mantle‐derived mafic material into mid‐crustal levels is considered the most viable mechanism to produce these high‐temperature K‐rich granitoids at these pressures. The composition of the felsic granitoids is consistent with derivation from a crustal source with a tonalitic to granodioritic composition. However, the mafic granitoids require a more mafic, possibly gabbroic source, which may have been supplemented with minor mantle‐derived material. These granitoids are also enriched in Th, U, LREE and depleted in Ba, Ti, Nb and Sr and compare closely to the Mesoproterozoic granitoids of the Gawler Craton. The economic significance of these styles of granitoids may also be highlighted by the close spatial relationship of hematitic K‐feldspar, magnetite, fluorite and pyrite‐rich veins, alteration and filled miarolitic cavities with the least‐evolved felsic intrusions. This style of veining has a probable magmatic origin and is similar to the gangue assemblage associated with Ernest Henry‐style Fe‐oxide‐(Cu–Au) mineralisation, which suggests that these granitoids represent prospective sources of fluids associated with Cu–Au mineralisation in the district.     

Sulfur Isotope Variations of Metallic Ore Deposits in the Gyeongsang Basin,South Korea

Many metallic ore deposits of the Late Cretaceous to Early Tertiary periods are distributed in the Gyeongsang Basin. Previous and newly analyzed sulfur isotope data of 309 sulfide samples from 56 ore deposits were reviewed to discuss the genetic characteristics in relation to granitoid rocks. The metallogenic provinces of the Gyeongsang Basin are divided into the Au–Ag(–Cu–Pb–Zn) province in the western basin where the sedimentary rocks of the Shindong and Hayang groups are distributed, Pb–Zn(–Au–Ag–Cu), Cu–Pb–Zn(–Au–Ag), and Fe–W(–Mo) province in the central basin where the volcanic rocks of the Yucheon Group are dominant, and Cu(–Mo–W–Fe) province in the southeastern basin where both sedimentary rocks of the Hayang Group and Tertiary volcanic rocks are present. Average sulfur isotope compositions of the ore deposits show high tendencies ranging from 2.2 to 11.7‰ (average 5.4‰) in the Pb–Zn(–Au–Ag–Cu) province, ?0.7 to 11.5‰ (average 4.6‰) in the Cu–Pb–Zn(–Au–Ag) province, and 3.7 to 11.4‰ (average 7.5‰) in the Fe–W(–Mo) province in relation to magnetite‐series granitoids, whereas they are low in the Au–Ag(–Cu–Pb–Zn) province in relation to ilmenite‐series granitoids, ranging from ?2.9 to 5.7‰ (average 1.7‰). In the Cu(–Mo–W–Fe) province δ³⁴S values are intermediate ranging from 0.3 to 7.7‰ (average 3.6‰) and locally high δ³⁴S values are likely attributable to sulfur derived from the Tertiary volcanic rocks during hydrothermal alteration through faults commonly developed in this region. Magma originated by the partial melting of the ³⁴S‐enriched oceanic plate intruded into the volcanic rocks and formed magnetite‐series granitoids in the central basin, which contributed to high δ³⁴S values of the metallic deposits. Conversely, ilmenite‐series granitoids were formed by assimilation of sedimentary rocks rich in organic sulfur that influenced the low δ³⁴S values of the deposits in the western and southeastern provinces.     

Granitoids and Their Magnetic Susceptibility in South Korea

Abstract: Magnetic susceptibility (MS) measurements were carried out for 1,120 samples in the Middle Proterozoic to Early Tertiary granitoids so far recognized in South Korea, and the lateral and spatial variation of their magnetic susceptibility, i.e., content of magnetite, is studied. The Middle Proterozoic two mica granitoids related to cassiterite (Sn) deposits in northeastern part of the Sobaegsan Massif show very low MS (less than 0.3 A‐ 10^‐3 SI unit), and the Permo‐Triassic tonalitic to granodioritic and monzonitic rocks which are barren in mineralization, distributed in the middle part of South Korea also show low MS (less than 1 A‐ 10^‐3 SI unit). On the contrary the Late Triassic to Jurassic granitoids (= Daebo granitoids) which were evolved from tonalite through granodiorite to granite, and are most widely distributed in South Korea, show a wide variation on MS. Particularly in the Andong, Igsan, Gimcheon and Geochang areas, the granitoids which are barren in mineralization, are characterized by high MS (more than 10 A‐ 10^‐3 SI unit). The Chuncheon, Jecheon, Namyang and Geumsan plutons related to molybdenite (Mo) and/or wolframite or scheelite (W), and fluorite (F) mineralizations show a little high MS (more than 3 A‐ 10^‐3 SI unit). However, more than 60% of the Daebo granitoids show low MS (less than 3 A‐ 10^‐3 SI unit) and the rest show a little high MS (more than 3 A‐ 10^‐3 SI unit). Heterogeneous distribution of magnetite content in the Daebo granitoids is considered to reflect heterogeneity of redox state of the source materials for these granitoids. The Cretaceous to Early Tertiary granitoids (= Bulgugsa granitoids) in the Gyeongsang Basin had been generally evolved in the order of tonalite, diorite, granodiorite, granite and alkali‐feldspar granites, which are closely related to base metal ore deposits, and mostly show higher MS (more than 3 A‐ 10^‐3 SI unit) than other granitoids mentioned above, although some exceptions are recognized in highly evolved alkali‐feldspar granites (SiO₂ > 76%). In contrast, as most of the highly oxidized or evolved Cretaceous granitoids distributed in areas other than the Gyeongsang Basin show lower MS than those of the Gyeongsang Basin, and appear to be magnetite free, ilmenite‐series granites, but they might be hematite bearing magnetite‐series granitoids. Highly oxidized nature of the Bulgugsa granitoids may be due to high Fe₂O₃/FeO ratio of the source materials and also high level intrusion style of the granitic magma activities. Most of the granitic rocks of the Middle Proterozoic, Permo‐Triassic and more than 60% of the Late Triassic to Jurassic (Daebo granitoids) belong to ilmenite–series, however less than 40% of the Daebo granitoids and most Cretaceous ones are magnetite–series. Thus, the granitic magma intruded in Korean Peninsula became oxidized while the intrusive ages become younger.     

矾山碱性岩体磷铁矿床金的地球化学

河北矾山钾质碱性岩体,由３期侵入岩和脉岩正长岩组成。第１期岩石为层状超镁铁质岩系,具韵律层结构。层状岩系中赋存巨大磁铁磷灰石矿床。碱性岩体全岩２２５个样品平均含金为７．８×１０－９。矾山岩体金的丰度为６．１×１０－９,是地壳金丰度（３．５×１０－９）的１．７４倍。第１期岩石平均金含量为８．８×１０－９,第２期岩石为５．１×１０－９,第３期岩石为７．４×１０－９,正长岩为４．２×１０－９。第１期侵入岩中辉石岩平均金含量为９．３１×１０－９,黑云辉石岩平均为７．７８×１０－９,伟晶正长黑云辉石岩为７．４０×１０－９,间隙状正长辉石岩为８．００×１０－９,磁铁磷灰石岩为１３．７８×１０－９,磷灰石岩和黑云磷灰石岩为１１．８０×１０－９,黑云母岩为１８．６３×１０－９。在垂直层状岩系的剖面上,岩石金含量呈韵律性变化。在东矿区,岩石金含量由西向东趋于降低。在岩浆液相分离过程中,金倾向富集在含铁、镁、钙和磷的熔体相中,而在岩浆结晶分异过程中,金可能富集在流体相中。     

The origin and evolution of skarn-forming fluids from the Phu Lon deposit,northern Loei Fold Belt,Thailand: Evidence from fluid inclusion and sulfur isotope studies

The Phu Lon skarn Cu–Au deposit is located in the northern Loei Fold Belt (LFB), Thailand. It is hosted by Devonian volcano-sedimentary sequences intercalated with limestone and marble units, intruded by diorite and quartz monzonite porphyries. Phu Lon is a calcic skarn with both endoskarn and exoskarn facies. In both skarn facies, andradite and diopside comprise the main prograde skarn minerals, whereas epidote, chlorite, tremolite, actinolite and calcite are the principal retrograde skarn minerals.Four types of fluid inclusions in garnet were distinguished: (1) liquid-rich inclusions; (2) daughter mineral-bearing inclusions; (3) salt-saturated inclusions; and (4) vapor-rich inclusions. Epidote contains only one type of fluid inclusion: liquid-rich inclusions. Fluid inclusions associated with garnet (prograde skarn stage) display high homogenization temperatures and moderate salinities (421.6–468.5 °C; 17.4–23.1 wt% NaCl equiv.). By contrast, fluid inclusions associated with epidote (retrograde skarn stage) record lower homogenization temperatures and salinities (350.9–399.8 °C; 0.5–8 wt% NaCl equiv.). These data suggest a possible mixing of saline magmatic fluids with external, dilute fluid sources (e.g., meteoric fluids), as the system cooled. Some fluid inclusions in garnet contain hematite daughters, suggesting an oxidizing magmatic environment. Sulfur isotope determinations on sulfide minerals from both the prograde and retrograde stages show a uniform and narrow range of δ³⁴S values (?2.6 to ?1.1 ‰ δ³⁴S), suggesting that the ore-forming fluid contained sulfur of orthomagmatic origin. Overall, the Phu Lon deposit is interpreted as an oxidized Cu–Au skarn based on the mineralogy and fluid inclusion characteristics.     

Preconcentration/Separation of Gold and Palladium by a Microcolumn Packed with Azadirachta Indica Leaf Powder and Their Determination in Geological Samples by ICP‐OES

A new method using a microcolumn (20 mm length × 2.0 mm i.d.) packed with Azadirachta Indica leaf powder (Neem leaf) as an adsorbent for the preconcentration/separation of Au and Pd prior to their determination by ICP‐OES in geological samples is presented. Various factors affecting the separation and preconcentration of the target analytes such as pH, sample flow rate and volume, eluent concentration and volume and interfering ions were studied and the optimal experimental conditions were established. The adsorption capacity of Azadirachta Indica leaf for Au and Pd was found to be 39.2 and 9.8 mg g⁻¹, respectively. The detection limits (3s) of this method for Au and Pd with an enrichment factor of 50 were 47 ng l⁻¹ and 59 ng l⁻¹ and the relative standard deviations were 4.8% and 5.7% (n = 7, adsorption capacity C = 5 ng ml⁻¹), respectively. In order to validate the proposed method, the certified reference material, GBW07293, was analysed, and a good agreement was obtained between the certified and determined values.     

Variations in rates of nucleation and growth of biotite porphyroblasts

Differences in rates of nucleation and diffusion‐limited growth for biotite porphyroblasts in adjacent centimetre‐scale layers of a garnet‐biotite schist from the Picuris Mountains of New Mexico are revealed by variations in crystal size and abundance between two layers with strong compositional similarity. Relationships between fabrics recorded by inclusion patterns in biotite and garnet porphyroblasts are interpreted to reflect garnet growth following biotite growth, without substantial alteration of the biotite sizes. Sizes and locations of biotite crystals, obtained via high‐resolution X‐ray computed tomography, document that of the two adjacent layers, one has a larger mean crystal volume (9.5 × 10^?4v. 2.4 × 10^?4 cm³), fewer biotite crystals per unit volume (232 v. 576 crystals cm^?3), and a higher volume fraction of biotite (23%v. 14%). The two layers have similar mineral assemblages and mineral chemistry. Both layers show evidence for diffusional control of nucleation and growth. Pseudosection analysis suggests that the large‐biotite layer began to crystallize biotite at a temperature ～67 °C greater than the small‐biotite layer. Diffusion rates differed between layers, because of their different temperature ranges of crystallization, but this effect can be quantified. The bulk compositional difference between the layers, manifested in different modal amounts of biotite, has an effect on the biotite sizes that is also quantifiable and insufficient to account for the difference in biotite size. After these other possible causes of variation in crystal sizes have been eliminated, variability in nucleation and diffusion rates remain as the dominant factors responsible for the difference in porphyroblastic textures. Numerical simulations suggest that relative to the small‐biotite layer, the large‐biotite layer experienced elevated diffusion rates because of the higher crystallization temperature, as well as increased nucleation rates in order to achieve the observed size and number density of crystals. The simulations can replicate the observed textures only by invoking unreasonably large values for the thermal dependence of nucleation rates (activation energies), strongly suggesting that the observed textural differences arise from variations between layers in the abundance and energetics of potential nucleation sites.     

Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

Oxygen isotopic composition was determined on representative samples of the Cape Ashizuri plutonic rocks, in order to estimate the genetic background of the biotite granite and alkaline granitoids. The biotite granite (70.1–76.1% SiO₂) ranged from 8.61 to 9.30‰ δ¹⁸O and averaged as 8.9‰ δ¹⁸O (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1‰) and Takakumayama (11.6‰) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14–8.70‰, with an average of 8.0‰ (n = 3), and monzonite and gabbro vary from 6.14 to 7.86‰, with an average of 7.0‰ (n = 3). These alkaline granitoids may be lower crustal in origin. The gabbroids containing 12.5% MgO at the maximum with low Sr initial ratio, are considered to be derived from the upper mantle through the fore‐arc tectonic break‐up on the subducting slab.     

Chemical Composition of Rock-Forming Minerals in Copper–Gold-Bearing Tonalite Porphyries at the Batu Hijau Deposit, Sumbawa Island, Indonesia: Implications for Crystallization Conditions and Fluorine–Chlorine Fugacity

Copper–gold mineralization at the world‐class Batu Hijau porphyry deposit, Sumbawa Island, Indonesia, is closely related to the emplacement of multiple stages of tonalite porphyries. Petrographic examination indicates that at least two texturally distinct types of tonalite porphyries are currently recognized in the deposit, which are designated as “intermediate tonalite” and “young tonalite”. They are mineralogically identical, consisting of phenocrysts of plagioclase, hornblende, quartz, biotite and magnetite ± ilmenite, which are set in a medium‐coarse grained groundmass of plagioclase and quartz. The chemical composition of the rock‐forming minerals, including plagioclase, hornblende, biotite, magnetite and ilmenite in the tonalite porphyries was systematically analyzed by electron microprobe. The chemical data of these minerals were used to constrain the crystallization conditions and fluorine–chlorine fugacity of the corresponding tonalitic magma during its emplacement and crystallization. The crystallization conditions, including temperature (T), pressure (P) and oxygen fugacity (fO₂), were calculated by applying the hornblende–plagioclase and magnetite–ilmenite thermometers and the Al‐in‐hornblende barometer. The thermobarometric data indicate that the tonalite porphyries were emplaced at 764 ± 22°C and 1.5 ± 0.3 × 10⁵ kPa. If the pressure is assumed to be lithostatic, it is interpreted that the rim of hornblende and plagioclase phenocrysts crystallized at depths of approximately 5.5 km. As estimated from magnetite–ilmenite thermometry, the subsolidus conditions of the tonalite intrusion occurred at temperatures of 540–590°C and log fO₂ ranging from ?20 to ?15 (between Ni‐NiO and hematite–magnetite buffers). This occurred at relatively high fO₂ (oxidizing) condition. The fluorine–chlorine fugacity in the magma during crystallization was determined on the basis of the chemical composition of magmatic biotite. The calculation indicates that the fluorine–chlorine fugacity, represented by log (fH₂O)/(fHF) and (fH₂O)/(fHCl) in the corresponding tonalitic magma range from 4.31 to 4.63 and 3.62 to 3.79, respectively. The chlorine fugacity (HCl) to water (H₂O) is relatively higher than the fluorine fugacity (HF to water), reflecting a high activity of chlorine in the tonalitic magma during crystallization. The relatively higher activity of chlorine (rather than fluorine) may indicate the significant role of chloride complexes (CuCl₂^? and AuCl₂^?) in transporting and precipitating copper and gold at the Batu Hijau deposit.     

Paleoproterozoic Granitoids on Liaodong Peninsula,North China Craton

Paleoproterozoic granitoids are an important constituent of the Jiao–Liao–Ji Belt(JLJB). The spatial-temporal distribution and types of Paleoproterozoic granitoids are closely related to the evolution of the JLJB. In this paper, we review the field occurrence, petrography, geochronology, and geochemistry of Paleoproterozoic granitoids on Liaodong Peninsula, northeast China. The Paleoproterozoic granitoids can be divided into pre-tectonic(～2.15 Ga; peak age=2.18 Ga) and post-tectonic(～1.85 Ga) granitoids. The pre-tectonic granitoids are magnetite and hornblende–biotite monzogranites and granodiorites. Pre-tectonic monzogranites are widespread in the JLJB and have A_2-type affinities. In contrast, pretectonic granodiorites are only present in the Simenzi area and have adakitic affinities. The post-tectonic granitoids consist of porphyritic monzogranite, syenite, diorite, granodiorite, quartz monzonite, monzogranite, and granitic pegmatite, which are adakitic rocks and I-, S-, and A_2-type granitoids. The assemblage of pre-tectonic A_2-type granitoids and adakitic rocks indicates the initial tectonic setting of the JLJB was a continental back-arc basin. The assemblage of post-tectonic adakitic rocks and I-, S-, and A_2-type granitoids indicates a post-collisional setting. The 2.20–2.15 Ga A_2-type granitoids and adakitic rocks were associated with the initial stage of back-arc extension, and the peak of back-arc extension is inferred from the subsequent(2.15–2.10 Ga) mafic intrusive activity. The ～1.90 Ga adakitic rocks mark the beginning of the postcollisional stage, which was followed by the intrusion of low-temperature S-and I-type granitoids. High-to low-pressure granitoids(S-type) were generated during the peak of post-collisional lithospheric delamination and asthenospheric upwelling. The emplacement of later granitic pegmatites occurred during the waning of the orogeny.     

Accessory Fe-Ti oxides in the West-Carpathian I-type granitoids: witnesses of the granite mixing and late oxidation processes

Fe-Ti-oxides may reach hundreds ppm in I-type granitoids and close to microgranular mafic enclaves (MME) up to several thousands ppm. Western Carpathian I-type granitoids have magnetic susceptibility above 3?×?10^?4 SI units, whereas S-type granites are lower. Associated MMEs reach up to 160?×?10^?4 SI. The measurement of magnetic susceptibility in field appears a useful tool for regional mapping of I-type granites and searching enclaves. The increased contents of Fe-oxides around MME within host I-type granitoids are interpreted as result of hybridization with mafic magma. The hybridisation is manifested by occurrence of two Fe-Ti-oxide generations: (1) orthomagmatic titanomagnetite from pre-mixing stage, (2) late-magmatic magnetite of post-mixing stage. The titanomagnetites show composite textures with exsolved ilmenite. The oxybarometry (Sauerzapf et al. 2008; Ghiorso Evans 2009) yields temperatures 700?C750°C at fO₂ about NNO, and 650?C700°C below FMQ, respectively. Post-mixing pure magnetites originated from early titanomagnetite, annite and anorthite associated with titanite and apatite. The late oxidation seems to be responsible for high magnetic susceptibility of metaluminous I-type tonalites. Both post- and pre- mixing Fe-Ti oxides are locally converted to hematite.     

An experimental study of effect of pre‐existing fabric on deformation of foliated mylonite at high temperature and pressure

We performed deformation experiments on a foliated mylonite under high temperature and pressure conditions in this study. To investigate the effect of pre‐existing fabric on the rheology of rocks, our samples were drilled from natural mylonite with the cylinder axis parallel to the foliation (PAR) and perpendicular to the foliation (PER). We performed 25 tests on seven PAR samples and 21 tests on seven PER samples at temperatures ranging from 600 to 890 °C, confining pressures ranging from 800 to 1400 MPa, and steady‐state strain rates of 1 × 10⁻⁴, 1 × 10⁻⁵ and 2.5 × 10⁻⁶ s⁻¹. In the temperatures of 600–700 °C, the deformation is accommodated by semi‐brittle flow, with the average stress exponent being 6–7 assuming power law flow; in the temperature range of 800–890 °C, deformation is mainly by plastic flow, with an average stress exponent of n = 3 and activation energies of Q = 354 ± 52 kJ/mol (PER and PAR samples). The experimental results show that the strengths of PER samples are higher than those of PAR samples. Deformation microstructures have been studied by optical and electron microscopy. The original foliation of PER samples is destroyed by deformation and replaced by a new foliation, but the deformation of PAR samples followed the original foliation. Electron backscatter diffraction (EBSD) measurements show a strong lattice preferred orientation (LPO) of the quartz c axis fabrics of the starting samples and deformed PER and PAR samples. However, the c axis fabric of quartz in experimentally deformed PER and PAR samples varied with temperature and strain rate is different from that seen in the starting mylonite sample. The initial quartz c axis fabric of the starting mylonite sample has been transformed into a new fabric during experimental deformation. Dehydration melting of biotite and hornblende occurred in both PER and PAR samples at temperatures of 800–890 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Petrogenesis and Dating of Two Types of Granite from North Qilian, China

1 IntroductionThree E-W-trending lithologic belts of the Paleozoic ageoccur in the eastern part of north Qilian. From north tosouth they are (a) the Huangyanghe-Gulang-Dajing-Beidengzi belt (the northern lithologic belt), (b) theLeigongshan-Maomaoshan-Laohushan belt (the middlelithologic belt) and (c) the Mayaxieshan-Shihuigou-Beiyinbelt (the southern lithologic belt) (Fig. 1). The rocks inthese belts are mainly Paleozoic volcanics and sandysiltstone. There are some Mesozoic and Cenozo…     

Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region,southern New England Orogen,New South Wales

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean ²⁰⁶Pb/²³⁸U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occurred after complete crystallization of the pluton and prior to emplacement of dykes and I‐type intrusives. K‐Ar biotite and hornblende ages show broadly synchronous intrusion of I‐type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of I‐type melts. Chemical similarities between the most mafic Moonbi Suite members and calc‐alkaline lamprophyres may also indicate a direct mantle contribution to some I‐type magmas.     

Geochronology and stable isotope signature of alteration related to hydrothermal magnetite ores in Central Anatolia,Turkey

Hydrothermal iron ores at Divri?i, east Central Anatolia, are contained in two orebodies, the magnetite-rich A-kafa and the limonitic B-kafa (resources of 133.8 Mt with 56% Fe and 0.5% Cu). The magnetite ores are hosted in serpentinites of the Divri?i ophiolite at the contact with plutons of the Murmano complex. Hydrothermal biotite from the Divri?i A-kafa yield identical weighted mean plateau ages of 73.75?±?0.62 and 74.34?±?0.83 Ma (2σ). This biotite represents a late alteration phase, and its age is a minimum age for the magnetite ore. Similar magnetite ores occur at Hasançelebi and Karakuz, south of Divri?i. There, the iron ores are hosted in volcanic or subvolcanic rocks, respectively, and are associated with a voluminous scapolite ± amphibole ± biotite alteration. At Hasançelebi, biotite is intergrown with parts of the magnetite, and both minerals formed coevally. The weighted mean plateau ages of hydrothermal biotite of 73.43?±?0.41 and 74.92?±?0.39 Ma (2σ), therefore, represent mineralization ages. Hydrothermal biotite from a vein cutting the scapolitized host rocks south of the Hasançelebi prospect has a weighted mean plateau age of 73.12?±?0.75 Ma (2σ). This age, together with the two biotite ages from the Hasançelebi ores, constrains the minimum age of the volcanic host rocks, syenitic porphyry dikes therein, and the scapolite alteration affecting both rock types. Pyrite and calcite also represent late hydrothermal stages in all of these magnetite deposits. The sulfur isotope composition of pyrite between 11.5 and 17.4‰ δ³⁴S_(VCDT) points towards a non-magmatic sulfur source of probably evaporitic origin. Calcite from the Divri?i deposit has δ¹⁸O_(VSMOV) values between +15.1 and +26.5‰ and δ¹³C_(VPDB) values between ?2.5 and +2.0‰, which are compatible with an involvement of modified marine evaporitic fluids during the late hydrothermal stages, assuming calcite formation temperatures of about 300°C. The presence of evaporite-derived brines also during the early stages is corroborated by the pre-magnetite scapolite alteration at Divri?i, and Hasançelebi-Karakuz, and with paleogeographic and paleoclimatic reconstructions. The data are compatible with a previously proposed genetic model for the Divri?i deposit in which hydrothermal fluids leach and redistribute iron from ophiolitic rocks concomitant with the cooling of the nearby plutons.     

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex,Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial ⁸⁷Sr/⁸⁶Sr ratios of up to 0.7134 and εNd₁₅₀ values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7073 and 0.7089, and εNd₁₅₀ values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7048 and 0.7061, and εNd₁₀₀ values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.     

40Ar/39Ar Ages of the Da Lien Granite Related to the Nui Phao W Mineralization in Northern Vietnam

⁴⁰Ar/³⁹Ar dating was conducted on the Da Lien granite related to greisen‐skarn type polymetallic (W‐CaF₂‐Cu‐Bi‐Au) mineralization in Nui Phao, northern part of Vietnam in the South China Plate. Biotite and muscovite separates from the biotite‐muscovite granite and greisenized granite indicate four plateau ages: 82.2 ± 0.4 Ma, 82.8 ± 0.3 Ma, 81.5 ± 0.3 Ma and 82.5 ± 0.4 Ma. The plateau ages were not significantly influenced by excess ⁴⁰Ar in dated minerals or by loss of radiogenic ⁴⁰Ar due to hydrothermal activities. The results indicate that solidification of granite related to the polymetallic mineralization occurred in the Late Cretaceous between 82.8 Ma and 81.5 Ma.     

Oxygen isotope composition of carbonates,silicates, and oxides in selected carbonatites: constraints on crystallization temperatures of carbonatite magmas

Oxygen isotope compositions and fractionations between calcite (Cc) and magnetite (Mt), diopside-rich clinopyroxene (Di), monticellite (Mnt), kimzeyite-rich garnet (Gt), and biotite (Bt) were measured for carbonatites from Oka (Canada), Magnet Cove (USA), Jacupiranga (Brazil), and Essonville (Canada), to obtain crystallization temperatures and explore the crystallization history of carbonatites. The highest isotopic temperatures are obtained from Cc–Mt fractionations from Oka (745–770 °C) and Cc–Mnt fractionations from Magnet Cove (700 and 760 °C). Cc–Mt temperatures for very coarse-grained, euhedral magnetite phenocrysts and calcite from Jacupiranga are 700 °C. In samples that contain diopside and magnetite, the Cc–Mt temperatures are always higher than Cc–Di temperatures. This difference is consistent with crystallization of magnetite before diopside, minor retrograde resetting of magnetite isotopic compositions, and the order of crystallization inferred from inclusions of Mt in Di. Cc–Mt, Cc–Di, and Cc–Mnt fractionations are thus interpreted to represent those established during crystallization at rapid cooling rates (10³–10⁴ °C/my). Diffusion model calculations indicate that at slower post-crystallization cooling rates (10–10² °C/my), magnetite compositions should experience significant isotopic resetting by diffusional exchange with Cc, Bt, and apatite, and yield lower temperatures than Cc–Di. Cc–Bt fractionations correspond to the lowest temperatures (440–560 °C). Although some of these are relatively high isotopic temperatures for biotite, they most likely represent those established during subsolidus retrograde exchange between biotite and calcite during rapid subsolidus cooling.     

Redox Condition,Nature and Tectono-magmatic Environment of Granitoids and Granite gneisses from the Karbi Anglong Hills,Northeast India: Constraints from Magnetic Susceptibility and Biotite Geochemistry

The Karbi Anglong hills (erstwhile Mikir hills) in northeast India are detached and separated from the Meghalaya plateau by a NW-SE trending Kopili rift. The Karbi Anglong hills granitoids (KAHG) and its granite gneissic variants belong to Cambrian plutons formed during Pan-African orogenic cycle, which commonly intrude the basement granite gneisses and Shillong Group metasediments. The KAHG can be broadly classified into three major granitoid facies viz., coarse grained porphyritic granitoid, medium grained massive non-porphyritic granitoid, and granite gneiss, which share a common mineral assemblage of plagioclase-K-feldspar-quartz-biotite±hornblende-apatite-titanite-zircon-magnetite but differ greatly in mineral proportion and texture. Modal mineralogy of KAHG, granite gneiss and basement granite gneiss largely represents monzogranite and syenogranite. The magnetic susceptibility (MS) of the KAHG, granite gneiss and basement granite gneiss varies widely between 0.11×10^-3 and 43.144×10^-3 SI units, corresponding to ilmenite series (<3×10^-3 SI; reduced type) and magnetite series (>3×10^-3 SI; oxidized type) of granitoids respectively. The observed MS variations are most likely intrinsic to heterogeneous source regions, modal variations of orthomagnetic and ferromagnetic minerals, and tectonothermal and deformational processes that acted upon these rocks. The primary and re-equilibrated compositions of biotites from the KAHG, granite gneiss and basement granite gneiss suggest calcalkaline, metaluminous (I-type) nature of felsic host magma formed in a subduction or post-collisional to peraluminous (S-type) host magma originated in syn-collisional tectonic settings, which were evolved and stabilized between FMQ and NNO buffers typically corresponding to reducing and oxidising magma environments respectively.