Biotite Tonalite Research Articles

Regolith-hosted rare earth exploration in the Chilean Coastal Range of the Central Andes

Regolith-hosted rare earth element deposits (RH-REE) are well-known in Asia, especially China, and are becoming increasingly important with the growing global demand for such metals. Ore formation controls of these deposits in Asia, which include sub-tropical to temperate climate, and mainly calc-alkaline granites with variable HREE and LREE bearing-minerals, are used to search for compatible exploration targets. However, the poor understanding of parameters controlling the formation of RH-REE mineralization outside China, e.g., Chile, might be hindering exploration efforts world-wide. This work addresses RH-REE mineralization in the Nahuelbuta Range, the local name of the Chilean Coastal Range between the Biobío and Araucanía regions. The studied prospect is associated with late Carboniferous–early Permian I-type biotite tonalite and amphibole-biotite tonalite. Mineralogical analysis (TIMA-X) indicates that allanite and monazite are the main REE-bearing minerals in the host rocks. The regolith in the study area is up to 60 m deep and contains an exchangeable-REE concentration up to 2000 ppm, as determined by portable X-ray fluorescence (pXRF) and ICP-MS. The mineralization is divided into two types: 1) Preserved regoliths (e.g. drillhole RLS-09) where mineralization is HREE-dominant (56% of total REE, (La/Yb)N ~2) and is covered by a leached upper pedolith; and 2) partially eroded regoliths (e.g. drillhole RLS-07) where mineralization is LREE-dominant (58% of total REE, (La/Yb)N ~4) and exposed. A morphometric analysis (SCM and swath profiles) and two 3D models (geomorphic and geochemical) indicate the current preservation of pediments and ore. Analyses of the exchangeable REE-fraction show that pedogenesis, leading to the deposit, is associated with REE-fractionation processes, most notably: a positive cerium anomaly (Ce/Ce*) in the upper pedolith and preferential adsorption of LREE at shallower depths, revealed by decreasing (La/Yb)N ratio with depth. We propose a mineralization model where the advance of the pedolith/saprolith horizon during progressive weathering and the breakdown of allanite and monazite was responsible for the neoformation of RH-REE clays at the base of the pedolith. Additionally, the landscape evolution of the Nahuelbuta Range controlled the preservation of ore bodies, which are either cropping out or hidden underneath the upper pedolith. A good understanding of the interplay between tonalite regolith development controls and regolith preservation during landscape evolution provide a strategy for REE exploration in the region. Moreover, the search for pediplains on suitable lithologies, mapping of regolith profiles, and their analysis via pXRF in the field by quantifying yttrium, appears to be a suitable strategy for early exploration of RH-REE deposits in the Chilean Coastal Range.

Petrogenesis and Assembly of the Don Manuel Igneous Complex, Miocene–Pliocene Porphyry Copper Belt, Central Chile

The 4·0–3·6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry-style copper mineralization. This study uses petrographic, petrological, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence for progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3·5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas. Mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite, testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.

Geology of the Don Manuel igneous complex, central Chile: Implications for igneous processes in porphyry copper systems

The Don Manuel igneous complex and associated porphyry copper mineralization in the Andean Cordillera of central Chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics. The Don Manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes, which intrude the older units. Mineralization is associated with the biotite tonalite and intermediate porphyries, which also contain the greatest abundance of mafic enclaves. Zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing. New zircon U-Pb ages and whole-rock Ar-Ar ages indicate that the Don Manuel igneous complex was emplaced between ca. 4 and 3.6 Ma. The time scale for the episodic intrusion of the Don Manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes. Observations from the Don Manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement. The observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation.

Structural evolution of the Mokrsko-West, Mokrsko-East and Čelina gold deposits, Bohemian Massif, Czech Republic: Role of fluid overpressure

The studied Mokrsko-West (90–100t Au), Mokrsko-East (30t Au) and Čelina (11t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Čelina). Another difference lies in the style of veining — densely spaced networks of 0.1–5mm thin veins (Q2) within the tonalite, compared to thick (usually 5–20cm; Q1–2) and widely spaced veins within the Neoproterozoic rocks.Five generations of quartz veins, referred to as Q0 through Q4 were distinguished: Q0 veins are the oldest and ore-barren, Q1 veins mark the onset of the Au-ore formation, Q2 veins its culmination and Q3 veins its fading. Late quartz gangue (Q4) is associated with uneconomic Ag–Pb–Zn vein-type ores hosted by calcite–barite–(quartz) veins.Quartz vein thickness (~0.3 to ~300mm), spacing (~3mm to ~500mm), distribution, and related extensional strain (ca. 3–25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors. Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (~2km or ~4km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q0 veins and the deepest for the Q2 veins. Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q0 through the Q2 veins, from 10 to 4MPa or from 26 to 10MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15MPa, respectively).The origin of joints and veins is discussed in terms of the stress orientation and crack-seal and crack-jump mechanisms. Field relationships unambiguously indicate that the veins hosted by Neoproterozoic rocks originated by reopening of the pre-existing extension joints (J1) due to fluid overpressure. The origin of the densely-spaced thin veins (Q2) hosted by the tonalite at the Mokrsko-West deposit is, however, less certain. It is probable that the tonalite was already affected by microfracturing analogous to the J1 joints prior to the formation of quartz veins.The formation of the Q1–2 veins at the Mokrsko-East deposit was constrained by the Re–Os dating of molybdenite to 342.9±1.4Ma. The ore-bearing hydrothermal system is thus ca 12Ma younger than the tonalite that hosts the Mokrsko-West deposit. A similar ca 15–2Ma difference between the age of the host-intrusion and the age of the hydrothermal event was encountered in several other gold deposits in the vicinity of the Central Bohemian Plutonic Complex. Two hypotheses to explain this are discussed in the paper.

Age and hafnium isotopic evolution of the Didesa and Kemashi Domains, western Ethiopia

The Western Ethiopian Shield (WES) forms a vast, underexplored, region of the East African Orogen. Lying towards the west of the orogen and between the dominantly juvenile Arabian-Nubian Shield in the north and the high-grade Mozambique Belt to its south, it holds a key position; vital in understanding the role of the East African Orogen in Gondwana formation. The WES is made up of a range of supra-crustal and plutonic rocks that formed in Tonian volcanic arc environments. The relative timing and duration of arc formation within the East African Orogen is still slowly being unravelled and we present new U-Pb and Hf isotopic data from zircons that help to define the maximum depositional age and provenance of the protoliths to meta-sedimentary units, as well as constrain the age of igneous intrusions located within the WES. Detrital zircons, obtained from a meta-sandstone, yield provenance age peaks at 2.8–2.4Ga, ca. 1.8Ga and 1.15–0.84Ga and a maximum depositional age of 838±13Ma. Hafnium isotopes from the same zircons demonstrate that both the oldest and youngest populations have broadly juvenile Hf isotopic values. However, the ca. 1.8Ga population shows significantly evolved Hf isotopic values. Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb ages from two granites and a felsic granite and hornblende+biotite tonalite in the Nekempt–Ghimbi region of the Western Ethiopian Shield (Didesa and Kemashi Domains) indicate two pulses of magmatism at 850–840Ma and 780–760Ma. Partial melting and deformation in the Didesa Terrane occurred at ca. 660 Ma. Further west, the post-tectonic Ganjii granite yielded a 206Pb/238U age of 584±10Ma, constraining pervasive deformation in the area.Age constraints on orogenesis in the Western Ethiopian Shield (ca. 660Ma) are similar to those in NE Uganda (ca. 690–660Ma), but are older than the Ediacaran peak orogenesis reported from the Southern Ethiopian Shield, Eritrea and northern Ethiopia and from SE Kenya. This suggests that closure of the western Mozambique Ocean involved progressive volcanic-arc accretion to the active margin of Cryogenian–Ediacaran Africa.

Compositional variations in tourmalines from peraluminous rocks of the Dipilto Granitic Batholith, Eastern Chortis Terrane, Nicaragua: tracers of magmatic to hydrothermal evolution

The granitoids of the Dipilto Batholith in northern Nicaragua range in composition from amphibole–biotite tonalite to cordierite–biotite and biotite granite. The petrographic and geochemical features indicate hybrid origin for the metaluminous granodiorite and tonalite (El Paraiso Suite). Dominant peraluminous granites (leucogranite dykes, Yumpali and La Piedra suites) represent a typical product of fractional crystallization of crust-derived melts. Tourmaline occurs as rare mineral in peraluminous granites of the Yumpali Suite (randomly distributed grains and quartz– tourmaline nodules), but is common in the younger leucogranite, pegmatite dykes and hydrothermal veins. Chemical variations (mainly X Fe and F) in tourmaline as well as in biotite are consistent with magmatic differentiation processes in granites and pegmatites. The presence of quartz–tourmaline hydrothermal veins indicates involvement of B-rich post-magmatic hydrothermal fluids at the final stage of the Dipilto Batholith evolution. Tourmaline chemistry evolved from Mg-rich schorl in peraluminous granites, to F-rich schorl–foitite in the hydrothermal veins. This evolutionary trend is typical of F-poor granite–pegmatite systems.

Mapa Geológico de parte da Folha Arroio América - SH-22-Y-A-IV-1 (MI 2995/1) - Nota explicativa

Esta nota explicativa se refere ao mapa geológico de parte da Folha Arroio América (SH.22-Y-A-IV-1 / MI 2995/1) e tem por base os resultados obtidos durante o Projeto Arroio América, executado em 2003 por professores e alunos do Curso de Geologia da Universidade Federal do Rio Grande do Sul. Diversas unidades litoestratigráficas foram reconhecidas, algumas das quais são inéditas e formalmente designadas. A principal estrutura regional reconhecida é denominada Zona de Cisalhamento Arroio América (ZCAA), composta por diversas zonas miloníticas subverticais de direção NE e cinemática transcorrente esquerda dada por lineação de estiramento de baixo caimento. Significativas reativações de antigos lineamentos em condições rúpteis são encontradas, bem como estruturas rúpteis de direção NW que afetam o embasamento e as rochas sedimentares de idade neoproterozoica a permiana. As sequências de embasamento mais antigas são ortognaisses do Complexo Cambaí, e as rochas supracrustais do Complexo Metamórfico Arroio Marmeleiro (CMAM). Os Dioritos Arroio Branquilho formam uma unidade inédita de biotita-hornblenda dioritos e quartzo-dioritos foliados, que compreendem duas subunidades cogenéticas – a fácies equigranular média e a fácies fina. As relações estruturais observadas indicam que seu posicionamento é anterior à ZCAA. O Tonalito Arroio dos Carros (TAC) e os Granitoides Rincão dos Encerrados (GRE) são também unidades inéditas, correlacionadas à Suíte Lagoa da Meia Lua. O TAC compreende hornblenda tonalitos foliados, de granulação média a grossa, e sua foliação magmática grada para uma foliação milonítica devido a seu caráter sintectônico à ZCAA. Os GRE são biotita tonalitos a granodioritos equigranulares finos a médios, tardi-tectônicos em relação à mesma zona de cisalhamento. O Granito Passo da Chácara, igualmente inédito, é a unidade granítica mais jovem, compreendendo biotita leucossienogranitos fracamente foliados a maciços. A Formação Maricá é composta por arenitos arcóseo-líticos com intercalação subordinada de conglomerados e folhelhos. Evidências localizadas de condições anquimetamórficas são constatadas, bem como clivagem ardosiana em composições pelíticas. É subdividida em duas fácies, uma resultante de sistema fluvial anastomosado e outra formada em condições marinhas rasas. O Monzonito Tapera, bem como as formações Hilário, Cerro da Pedra e Acampamento Velho, são subunidades do Grupo Bom Jardim. No Monzonito Tapera são reconhecidas quatro fácies com base em características texturais e composicionais: monzonítica plutônica, leucodiorítica plutônica, diorítica plutônica e hipoabissal monzonítica. Nas rochas encaixantes do CMAM, são descritos efeitos de metamorfismo de contato em condições de fácies hornblenda cornubianito. A Formação Hilário compreende derrames andesíticos e diques de andesitos e lamprófiros. A Formação Cerro da Pedra é definida neste trabalho como um pacote de conglomerados formados em sistemas de leques aluviais. A fácies proximal é reconhecida na forma de para- e ortoconglomerados oligomíticos, enquanto a fácies distal é representada por ortoconglomerados polimíticos. A Formação Acampamento Velho é representada por um domo de lava e diques riolíticos associados. A Formação Rio do Sul, de idade permiana, pertencente ao Grupo Itararé, é restrita ao extremo norte da área mapeada, onde ocorrem camadas de arenito fino intercaladas com siltitos.

Anomalous xenocryst dispersion during tonalite–granodiorite crystal mush hybridization in the mid crust: Mineralogical and geochemical evidence from Variscan appinites (Avila Batholith, Central Iberia)

The Variscan appinites from the San Martin del Pimpollar intrusion (Avila Batholith, Central Iberia) display geochemical and mineralogical evidence for hybridization with crustal-derived, partially molten, granodiorites and for rapid crystallization of residual melts.Quenching, apparently induced by the thermal contrast between mixing magmas, caused crystallization of hollow microphenocrysts of labradorite–bytownite, and of aggregates of fine-grained plagioclase+quartz+biotite±hornblende which trapped abundant acicular grains of apatite. Both inner cores and outer rims of anorthite-rich microphenocrysts were subsequently infilled and overgrown, respectively, by andesine. The sharp change in the anorthite content of plagioclase microcphenocrysts could be due to disequilibrium partitioning during supercooling, although decrease in H2O activity or Al/(Al+Si) ratio of the melt after An-rich plagioclase crystallization cannot be disregarded. Coarse-grained xenoliths and xenocrysts (quartz with reaction rims and embayment, rapakivi K-feldspar with irregular distribution of Ba-rich zones, oscillatory zoned oligoclase–andesine and biotite with inclusions of stubby apatite) were entrained into the appintic magma during hybridization. The xenocrysts were most probably derived from the country-rock granodiorites. The composition of coarse-grained biotite, however, is similar to that of matrix grains, implying total compositional resetting by exchange reaction with the appinitic magma if it was originally xenocrystic. Otherwise, biotite+apatite saturation would have to have been achieved before quenching.The most evolved rock terms of the appinite suite, hybrid granodiorites and hybrid biotite tonalites, have Nd and Sr isotope compositions matching those of the country-rock granodiorites. However, hybrid biotite tonalites are significantly more depleted in K2O, Ba, Zr, Th and REE than would be expected from appinite/country-rock granodiorite hybridization. This anomalous geochemical pattern could be caused by an inefficient dispersion of xenocrysts into the hybrid magma due to the quenching of the residual appinitic melts. Therefore, the ratios of mineral abundances in the country rocks may not have been conserved during xenocryst entrainment into the appinitic magma.

The role of ridge subduction in determining the geochemistry and Nd–Sr–Pb isotopic evolution of the Kodiak batholith in southern Alaska

The Paleocene Kodiak batholith, part of the Sanak–Baranof belt of Tertiary near-trench intrusive rocks, forms an elongate body (~ 150 km long) that transects Kodiak Island from SW to NE. The batholith consists of three zones (Southern, Central, and Northern) of kyanite-, muscovite-, and garnet-bearing biotite tonalite and granodiorite and less abundant granite that intruded an accretionary prism (Kodiak Formation, and Ghost Rocks Formation). Small and likely coeval bodies (Northern, Western, and Eastern satellite groups) of quartz gabbro, diorite, tonalite, granodiorite, and leucogranite flank the batholith. The batholith is calc-alkalic, has an aluminum saturation index of > 1.1, FeO t/(FeO t + MgO) ~ 0.65 (at SiO 2 = 65 wt.%), and increases in SiO 2 (~ 61 wt.%–73 wt.%) and decreases in TiO 2 (~ 0.9 wt.%–0.3 wt.%) from SW to NE. As a group, the granitic rocks have light REE-enriched chondrite-normalized patterns with small or no negative Eu anomalies, primitive mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. Small to large negative anomalies for Th are also distinctive. The quartz gabbros and diorites are generally characterized by generally flat to light REE chondrite-normalized patterns (no Eu anomalies), and mantle-normalized negative anomalies for Nb, Ti, and P. Pb isotopic compositions ( 206Pb/ 204Pb = 18.850–18.960; 207Pb/ 204Pb = 15.575–15.694; 208Pb/ 204Pb = 38.350–39.039) are intermediate between depleted mantle and average continental crust. The Southern zone and a portion of the Central zone are characterized by negative ε Nd values of − 3.7 to − 0.3 and T DM ages ranging from ~ 838 Ma to 1011 Ma. Other granitic rocks from the Central and Northern zones have higher ε Nd values of − 0.4 to + 4.7 and younger T DM ages of ~ 450 to 797 Ma. Granitic and mafic plutons from the Eastern satellites show a wide range of ε Nd values of − 2.7 to + 6.4, and T DM ages from 204 Ma to 2124 Ma. 87Sr/ 86Sr values of the Southern and Central zones overlap and tend to be slightly more radiogenic ( 87Sr/ 86Sr > 0.70426) than the Northern zone ( 87Sr/ 86Sr < 0.70472). 206Pb/ 204Pb values increase slightly from the Southern and Central zones toward the Northern zone. There is no clear correlation of the major or trace elements with ε Nd, Pb or Sr isotopic values. Kodiak Formation and the Ghost Rocks Formation overlap the isotopic compositions (e.g., 206Pb/ 204Pb = 18.978 to 19.165, 87Sr/ 86Sr of 0.705715 to 0.707118, and ε Nd of − 6.7 to − 1.5 at 59 Ma) and T DM values (959 to 1489 Ma) of the batholith. Production of large volumes of granitic rocks in the Sanak–Baranof belt, and particularly on Kodiak Island, reflects a sequence of processes that includes underplating of mantle-derived mafic (possibly from the mantle wedge) and intermediate rocks under the accretionary flysch, interlayering of mantle-derived and flyschoid rocks, and partial melting of the mixed lithologic assemblages. Limited degrees of fractional crystallization or assimilation and fractional crystallization influenced compositions of the granitic rocks. The contribution of mantle-derived rocks that resided in the accretionary prism for only a short period of time prior to partial melting likely exceeds 40% (up to 80%). The balance (60 to 20%) is from a recently recycled crustal component represented by the Kodiak Formation. This type of progressive intracrustal melting from mixed sources controlled the geochemical character of the batholith and is most consistent with the hypothesis that the granitic rocks are associated with a slab-window produced by collision of a spreading oceanic center and a subduction zone and migration beneath the accretionary prism.

Isotopic and geochemical evidence for differentiation and crustal contamination from granitoids of the Berens river subprovince, Superior province, Canada

Nd isotope and geochemical data are presented for biotite tonalite, hornblende tonalite, hornblende granodiorite, biotite granite and peraluminous granite from the Archean Berens River subprovince of the Superior province in northwestern Ontario, Canada. The older tonalitic suites were synchronous with greenstone belt volcanism and were followed by granodioritic and granitic intrusions. The suites vary from 57 to 74% SiO 2, are light rare-earth element (REE) enriched, with variable Eu and Nb anomalies and heavy REE depletion among the tonalites. The Nd isotope compositions for the granitoids vary from depleted values of ɛ Nd = +1.5 (tonalites) to enriched values of −1.5 (peraluminous granite). The range of isotopic composition supports a model where the granitoids were emplaced into, contaminated by, and locally melted an older pre-existing Archean crust (ca. 3.0 Ga). The Nd isotope compositions of the granites and the tonalites decrease towards the north (increased crustal contamination) away from the greenstone belts that form the southern margin. This implies a thickening of the crust towards the north and is consistent with an Andean-style convergent margin in which oceanic crust subducted northwards beneath an older Archean crustal plate. Crustal contamination increased heavy REE contents in the tonalites and decreased the Sr/Y ratios for all granitoids. Failure to account for this crustal contamination can lead to errors in estimating the degree of partial melting based on the Sr/Y ratio. Compositional differences between the biotite tonalites and hornblende tonalites likely reflect different degrees of partial melting of a garnet-bearing amphibolite source.

The genesis of I- and S-type granitoid rocks of the Early Ordovician Oledo pluton, Central Iberian Zone (central Portugal)

The Early Ordovician Oledo pluton consists of four distinct granodioritic to granitic phase (G1–G4), which intruded a Cambrian schist-metagraywacke complex, but were themselves intruded by a Late Carboniferous pluton. ID-TIMS U–Pb ages for zircon and monazite from these granitic rocks indicate emplacements within a short period of time at 479–480 Ma. Granodiorite G1 is the most deformed rock with shear zones and deformation at the border. G1 and G3 contain fine-grained biotite tonalite and biotite granodiorite microgranular enclaves, which are darker and richer in mafic minerals than the host granodiorites. The geological, mineralogical, geochemical and Sr, Nd and O isotopic data show that tonalitic and granodioritic enclaves and host G1 are of I-type and were related predominantly by a fractional crystallization process. Least-square analysis of major elements and modelling of trace elements indicate that granodioritic enclaves and host G1 could be derived from the tonalitic enclave magma by fractional crystallization of plagioclase, grunerite, biotite and ilmenite. Granodiorite G2 is of hybrid origin. Most variation diagrams for granodioritic enclaves and host G3 granodiorite and their biotites show linear trends. Modelling of major and trace elements of granodioritic enclaves indicate that they result from mixing of relatively primitive granodiorite magma with magma derived from crustal melting. Tonalitic enclaves correspond to globules of a more mafic relatively primitive magma. Granite G4 has the most pronounced crustal signature and is of S-type.

Archaean TTGs as sources of younger granitic magmas: melting of sodic metatonalites at 0.6–1.2 GPa

Two natural, low K2O/Na2O, TTG tonalitic gneisses (one hornblende-bearing and the other biotite-bearing) were partially melted at 0.8–1.2 GPa (fluid-absent). The chief melting reactions involve the breakdown of the biotite and hornblende. The hornblende tonalite is slightly less fertile than the biotite tonalite, but melt volumes reach around 30% at 1,000°C. This contrasts with results of most previous work on more potassic TTGs, which generally showed much lower fertility, though commonly producing more potassic melts. Garnet is formed in biotite-bearing tonalitic protoliths at P > 0.8 GPa and at > 1.0 GPa in hornblende-bearing tonalitic protoliths. All fluid-absent experiments produced peraluminous granitic to granodioritic melts, typically with SiO2 > 70 wt.%. For the biotite tonalite, increasing T formed progressively more melt with progressively lower K2O/Na2O. However, the compositions of melts from the hornblende tonalite do not vary significantly with T. With increasing P, melts from the biotite tonalite become less potassic, due to the increasing thermal stability of biotite. For the hornblende tonalite, again there is no consistent trend. Fluid-absent melting of sodic TTGs produces melts with insufficient K2O to model the magmas that formed the voluminous, late, potassic granites that are common in Archaean terranes. Reconnaissance fluid-present experiments at 0.6 GPa imply that H2O-saturated partial melting of TTGs is also not a viable process for producing magmas that formed these granites. The protoliths for these must have been more potassic and less silicic. Nevertheless, at granulite-facies conditions, sodic TTGs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths. Upward abstraction of this melt would result in some LILE depletion of the terrane. Younger K-rich magmatism is unlikely to represent recycling of TTG crust on its own, and it seems most likely that evolved crustal rocks and/or highly enriched mantle must be involved.

Geology and metamorphic zonation of the Ryoke Metamorphic Belt on Kasado-jima Island, SW Japan

The Cretaceous low-pressure/high-temperature Ryoke Metamorphic Belt contains abundant Ryoke and San-yo Granitoids that intruded to various crustal depths during or after formation of the belt. To clarify the tectono-metamorphic history of the belt, it is necessary to evaluate the thermal effects of the intrusion of the different types of granitoids. The geology of Kasado-jima Island, SW Japan, consists of metasedimentary rocks, massive biotite granite, and migmatitic biotite tonalite. The granite and tonalite occur in the central and southern areas of the island, respectively. Within the central area, granite intrudes metasedimentary rocks and cuts across bedding-parallel schistosity within the metapelites; this indicates that intrusion of the granite postdates Ryoke metamorphism. In contrast, tonalite in the southern area was emplaced parallel to the schistosity. On the basis of the field occurrences and petrographic characteristics of these granitoids, we correlate the biotite tonalite and biotite granite with the Older Ryoke and San-yo Granitoids, respectively. Based on the paragenesis of schistosity-forming minerals within metapelites, we identify two mineral zones in the island: the biotite and cordierite zones. The biotite zone is characterized by a mineral assemblage of muscovite + biotite ± garnet, and the cordierite zone is characterized by the assemblage muscovite + biotite + K-feldspar + cordierite or garnet. On the basis of mineral chemistry, we estimate peak metamorphic conditions for the biotite and cordierite zones to be ∼ 470 °C (reference pressure: 200 MPa) and ∼ 550 °C at 250 MPa, respectively. In the central part of the island, a contact aureole adjacent to the biotite granite is identified by the occurrence of randomly oriented muscovite and very large dendritic cordierite porphyroblasts. These randomly oriented porphyroblasts may have formed under a high degree of supersaturation associated with the thermal perturbation of the shallow intrusion of the biotite granite into cooler crust. In the southern area, most metamorphic minerals are of similar size, and their long axes are aligned to form a schistosity; this indicates a small degree of thermal perturbation and/or a high strain rate. These observations indicate that the biotite tonalite on Kasado-jima Island, as a deep-seated granitoid, was related to regional low-pressure/high-temperature metamorphism.

Geology and U–Pb geochronology of the Kipawa Syenite Complex — a thrust related alkaline pluton — and adjacent rocks in the Grenville Province of western Quebec

The Kipawa Syenite Complex, a thin, folded sheet of amphibole syenite, quartz syenite and minor nepheline syenite, lies along a west-vergent thrust separating a lower slice comprising the Kikwissi granodiorite and biotite tonalite dated at 2717 +15–11 Ma, and unconformably overlying metasedimentary rocks from an overlying slice containing the Red Pine Chute orthogneiss, an alkali granite gneiss, and the Mattawa Quartzite. The syenite complex, dated at 1033 ± 3 Ma, lies within the lower slice but has metasomatically altered the overlying slice. Texturally guided U–Pb spot analyses on partially metasomatised zircons from the alkali granite gneiss yield a cluster of 207Pb/206Pb ages at 1389 ± 8 Ma, interpreted as the time of igneous crystallization and four ages overlapping the time of syenite emplacement, interpreted as in situ, metasomatic growth. The highest structural slice comprises garnet amphibolite separated from lower slices by the Allochthon Boundary Thrust. Metamorphic grade increases upward from greenschist grade in the biotite tonalite to amphibolite grade (690 °C, 9 kbar (1 kbar = 100 MPa)) at the lower boundary of the alkali granite. Emplacement of the Kipawa Syenite Complex took place after assembly of the thrust stack had begun and after emplacement of the allochthon or hot slab responsible for the inverted metamorphic gradient. Origin of the syenite is tentatively ascribed to anatexis of material metasomatized by flow of alkaline solutions along a major shear surface. Crystallization of new zircon in the margins of the syenite shows that metasomatism continued from ca. 1035 to 990 Ma, redistributing alkalies, fluorine, rare-earth elements and zirconium.

Magnetic susceptibility and geochemistry of Variscan West Carpathian granites: implications for tectonic setting

Granites usually display a bimodal distribution of their magnetic susceptibility. One mode corresponds to susceptibilities of the order of 10 −5–10 −4 and the other one to those of the order of 10 −3–10 −2 [SI]. The former mode, which corresponds to ilmenite-bearing granites, is often equated with the S (sedimental) type, while the latter, which corresponds to magnetite-bearing granite, is equated with the I (igneous) type. In the West Carpathians, isotope geochemistry discriminates two groups of Variscan granites: (1) the older group (350–330 Ma) is peraluminous, from two-mica granites to granodiorites, carrying monazite and ilmenite; it resembles to common S-type and/or Ilmenite Series granite, (2) the younger group (310–300 Ma) is metaluminous to subaluminous, corresponding to biotite tonalites to granodiorites, carrying allanite and magnetite; it can be compared to the I-type and/or Magnetite Series granites. Magnetic susceptibilities of the West Carpathian granites are in general rather low, in the order of 10 −4, hence corresponding to the typical values of the S type granites. A minority of specimens have susceptibilities in the order of 10 −5 and a few specimens have susceptibilities higher, in the order of 10 −3. This is in apparent contradiction with the granite origin as revealed geochemically. Unusual redox conditions controlled by the tectonic setting are possible explanations of this discrepancy.

U–Pb Geochronology of VMS mineralization in the Iberian Pyrite Belt

A geochronology study using U–Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76±0.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded 207Pb/206Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26±0.81 Ma, slightly younger and outside of the 2σ error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with 207Pb/206Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U–Pb zircon age of 352.9±1.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with 207Pb/206Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21±0.73 and 356.4±0.8 Ma, for footwall and hanging wall samples, respectively. The hanging wall sample has two zircon analyses with 207Pb/206Pb ages of 464 and 466 Ma, indicating inheritance from an Ordovician or older source. The age for an altered dacite tuff sample from the hanging wall of the Las Cruces deposit is 353.97±0.69 Ma. One zircon analysis from the Las Cruces sample has a 207Pb/206Pb age of 1048 Ma, indicating inheritance from a Neoproterozoic source. These U–Pb ages refine the IPB geochronology provided by previous studies, and they suggest that either volcanism progressed toward the center of the IPB, or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation. The zircon inheritance provides direct evidence for Proterozoic to Ordovician sources, reflecting either basement rocks beneath the Phyllite–Quartzite Group during VMS formation in late Tournaisian times, or a Proterozoic to Ordovician detrital component in Phyllite–Quartzite Group source rocks. The presence of an older crustal component is consistent with VMS formation during rift development at a continental margin.

Mineralogical constraints оn the petrogenesis of the Pangeon granitoids (Central Macedonia, northern Greece)

The Pangeon granitoids intrude the metamorphic rocks of the Lower Tectonic Unit of the Rhodope Massif and are discriminated into two petrographic types: A medium- to coarse-grained, porphyritic, hornblende+biotite tonalite to granodiorite (PTG) and a fine- to medium-grained, biotite±muscovite, granodiorite to granite (MGG). Microgranular mafic enclaves (MME) of monzodioritic composition are present in the PTG rocks. The hornblendes are characterized as hastingsite, magnesiohastingsite and edenite. The biotites show calc-alkaline rock series characteristics (MME and PTG) or calc-alkaline to peraluminous characteristics (MGG). Muscovites are distinguished in primary, late magmatic and secondary (sericite) on the basis of their textural and chemical criteria. Textural evidence and compositional variations of the minerals support mainly a fractional crystallization rather than a restite unmixing or magma mixing process for the evolution within each one rock type of the Pangeon granitoids while they rule out a genetic relation between MME, PTG and MGG through fractional crystallization. Crystallization pressure values obtained by AI-in-hornblende for the PTG along with the presence of magmatic epidote suggest a magma emplacement at mid-crustal depths. Textural and mineralogical data of MME suggest that they were incorporated into their host as solids during the emplacement of the granitoids.

Geology and Geochronology of the Suomujärvi Complex: a new Archaean gneiss region in the NE Baltic Shield, Finland

Recent mapping in the eastern Central Lapland Granitoid Complex and dating of zircons at the NORDSIM facility was performed in order to highlight the existence of an 892-km 2 area of Archaean gneisses not previously identified in southeast Lapland, Finland. This Archaean enclave is composed of northeast-trending, km-scale bands of orthogneisses with varying mafic content and is named the Suomujärvi Complex. Along its southeastern boundary lies the Aholanvaara volcano-sedimentary package. The evolution of the Suomujärvi Complex began at ca. 2810 Ma with intrusion of the homogeneous Jumisko biotite tonalite–granodiorite in the west. This lithology becomes increasingly heterogeneous to the east where muscovite and an inherited ca. 2870 Ma old rock component represented by zircon cores are present. These orthogneisses were later intruded by the 2755±18 Ma (1 σ) protolith of the Miehinkävaara amphibole and biotite bearing diorite–tonalite gneisses producing diffuse contacts characterized by gradual increases in mafic minerals. All of the orthogneisses were then migmatized/metamorphosed. After ca. 2730 Ma the Aholanvaara volcano-sedimentary package was deposited. Its main quartzite unit was dominantly derived from an unknown 2731±17 Ma (1 σ) source but it also contains a ca. 3.4 Ga detrital component. The absence of 2800–2820 Ma detrital material in these sediments indicates they were not derived locally from the Suomujärvi Complex. Instead, they share affinities with supracrustal packages further south in Posio, east in the Belomorian Province and north in the Central Lapland Greenstone belt. Widespread, voluminous intrusion of granites occurred in the area surrounding the Suomujärvi gneisses and left a marked metamorphic overprint on the gneisses during the ca. 1.8 extensional collapse of the Svecofennian and Lapland-Kola orogens. Multi-scale folding produced subhorizontal, southwest-plunging, tight to isoclinal, upright folds and the dominant axial planer, northeast-trending fabric in the area. This fabric is cut by a northwest-trending fabric in the southwestern part of the area. Geological characteristics of the Suomujärvi Complex and ages of the orthogneisses within it indicate that it most likely represents the southwestern margin of the Archaean Belomorian block.

Petrology of iron-rich magmatic segregations associated with strongly peraluminous trondhjemite in the Cornucopia stock, northeastern Oregon

The Middle Cretaceous Cornucopia stock in the Blue Mountains of northeastern Oregon is a small composite intrusion consisting of hornblende biotite tonalite, biotite trondhjemite, and three cordierite two mica trondhjemite units. Unusual magnetite + biotite-rich tonalitic rocks are associated with the Crater Lake cordierite trondhjemite, the youngest of the intrusions. Oxide-rich tonalites are characterized by high Fe (~47–68 wt% total Fe as FeO), low SiO2 (<36 wt%), and enrichments in HFSE and REE (La(N)=361–903). Oxide-rich tonalites appear in a variety of forms, including composite dikes and sheets, in which they are associated with leucocratic tonalite. Leucotonalite is lower in SiO2 (60–72 wt%) than Crater Lake trondhjemite, and generally has ΣREE contents and Eu anomalies intermediate between the oxide-rich tonalite and Crater Lake compositions. Oxide-rich tonalites crosscut, and are crosscut by, shear zones in the host trondhjemite, indicating their emplacement late in the pluton's crystallization history. Granitic dikes crosscut the composite dikes in all localities. Geochemical considerations and sedimentary-like structures, such as load casts and bedding of magnetite-rich assemblages in the composite dikes and sheets, are suggestive of crystal settling from an Fe-rich parental magma. The Fe-rich liquid parental to the oxide-rich tonalite–leucotonalite pairs formed by extensive, in-situ, plagioclase + quartz-dominated crystallization of strongly peraluminous trondhjemite. Early magnetite saturation in the trondhjemite was suppressed, either because the parental trondhjemitic magma had a lower initial total Fe content or because it had a lower ferric–ferrous ratio, possibly reflecting a lower oxygen fugacity. Accumulation of magnetite from Fe-rich residual magma is a viable mechanism for the concentration of iron, and the subsequent formation of Fe-rich rocks, in calcic siliceous intrusions. Apparently, Fe-enrichment can occur locally in calcic magmas, and is not restricted to rocks of mafic tholeiitic or anorthositic affinity.

Geochronological constraints on the evolution of the Embu Complex, São Paulo, Brazil

Petrologic and geochronological work was carried out on a roadside outcrop of amphibolite facies orthogneisses near São Lourenço da Serra, about 50 km southwest of São Paulo City. These orthogneisses belong to the Embu Complex, within the Neoproterozoic Brasiliano Orogenic Cycle mobile belts of SE Brazil. The outcrop consists of predominantly foliated biotite tonalites and granodiorites, which were cut by granitic veins and pegmatites prior to final deformation. SHRIMP U/Pb measurements on zircons from one granodioritic–tonalitic gneiss indicate magmatic crystallization of the protolith at 811±13 Ma (MSWD=1.0). Zircons with dates of ca. 2000 and ca. 1000 Ma in this rock are interpreted as inherited from older crust. One zircon analyzed from the gneiss and three zircons from a discordant pegmatitic vein indicate an event at 650–700 Ma, perhaps related to the intrusion of the pegmatites. A regression of Rb–Sr whole rock data for four biotite gneisses yielded an imperfect isochron, giving an apparent age of 821±68 Ma and an elevated initial 87Sr/ 86Sr ratio of 0.719±0.005. The elevated initial 87Sr/ 86Sr ratio and the inherited zircons indicate involvement of older crust in the genesis of the gneisses. Rb–Sr feldspar and whole rock pairs yield ca. 560 Ma tielines, giving the time of final cooling below 300–350 °C, and the cessation of medium-grade metamorphism and ductile deformation. These results document a series of tectono-thermal events spanning 250 million years during the Brasiliano Orogenic Cycle. They relate to ca. 800 Ma magmatic arc activity and later allochthonous terrane assembly during closure of the Adamastor Ocean, resulting in the accretion of Western Gondwana.