Variscan Sn–W–Mo metallogeny in the gravity picture of the Krušné hory/Erzgebirge granite batholith (Central Europe)

Late Variscan wolframite (± molybdenite) and cassiterite–wolframite greisen, skarn and vein deposits occur in a close spatial association with the granites of the Krušné hory/Erzgebirge batholith (KHEB) in Central Europe. We examined the distribution of the deposits in relation to the gravity field affected by Late Variscan granites using the data from previous gravity and metallogenic studies. Late Variscan granites are differentiated into earlier biotite monzogranites (low-F granites) and later biotite or lithium mica syenogranites (high-F granites) in accordance with the previous classifications. All the outcrops of granites in the KHEB region and their hidden continuation are confined to the Bouguer anomaly contour of − 20 mGal. The Sn–W–Mo (rare metal) deposits and occurrences are within the gravity contour of − 30 mGal with the exception of the Grossschirma stratiform tin deposit in the Freiberg polymetallic ore district. We constructed a geological model based on the gravity data along two profiles across the KHEB showing the position of some rare metal deposits and of outcropping and hidden granite bodies. The models show that the overlapping of earlier and later granites is in the areas of the most intense regional gravity minima. These coincide with the Eastern Volcano-Plutonic Complex (Altenberg minimum), which encloses large volumes of felsic extrusives, microgranite dikes and granites, and the Western Plutonic Complex (Eibenstock minimum), with small volumes of felsic dikes and predominance of earlier and later granites, with no extrusives preserved. There is no distinct relationship between the masses of Late Variscan granites and the distribution and the sizes of associated W ± Mo and Sn–W deposits. We prefer the idea that rare metal mineralization was formed by hydrothermal fluids derived from outside of presently outcropping granites. It originated in two cycles: one connected with the formation of earlier granites producing W ± Mo associations and the other one associated with later granites connected with Sn–W mineralization. Mineralizing fluids were probably generated by mantle–crustal interaction in the crust near the mantle–crust boundary as also indicated by lamprophyric intrusions coeval with the Late Variscan granitic magmatism.     

Earth's surface movements in the hazardous area of Jezeří Castle,Krušné hory Mountains

Abstact The results are summarized of the correlation between the detailed geomorphological analysis and the geodynamic interpretation of the twelve high-precision levelling measurements of the Earth's recent surface movements (1983–1989) in the hazardous area of Jezeí Castle in the Kruné hory Mts. Indicated are the types and régime of these movements (Tab 1, Fig 3) which are climatic, tectonic and anthropogenous in origin. Both slightly and strongly deformed zones in the near-surface part of crystalline rocks massif were formed (Fig 1). The landform patterns (Fig 2), as well as the present-day activity of geological processes are described.     

Composition of coexisting zircon and xenotime in rare-metal granites from the Krušné Hory/Erzgebirge Mts. (Saxothuringian Zone,Bohemian Massif)

Zircon and xenotime, from two mineralogically and chemically contrasting granite suites occurring in the Kru?né Hory/Erzgebirge Mts., display extended compositional variability with respect to abundances of Zr, Hf, REE, Y, P, Th, Ca, Al, Fe and As. According to their geochemical signatures, P-rich (S-type) and P-poor (A-type) granites could be distinguished here. Both granite suites display high Ga/Al ratios (>2.6) and according to FeO_tot./(FeO_tot. + MgO) ratio can be classified as ferrous granites. Consequently, the both ratios cannot be used for discrimination S- and A-type granites. Both minerals are characterized by a variety of complex zircon-xenotime textures. They are usually strong hydrated and enriched in F. Zircon from P-rich granites displays a significant enrichment in P (up 0.24 apfu P), whereas zircon from P-poor granites has lower P and higher Y (up to 0.15 apfu Y). The xenotime-type substitution is the most important mechanism of isomorphic substitution in zircon in both granite suites. Zircon from both granite suites is typically enriched in Hf, especially unaltered zircon from P-rich granites (up to 8.2 wt. % HfO₂). However in altered zircons the Hf/Zr ratio is higher in the P-poor granites. The Hf-rich zircon from unaltered P-rich granite crystallised from low temperature granite melt, whereas altered zircons crystallised during post-magmatic hydrothermal alteration (greisenization). Xenotime from P-poor granites displays a considerable enrichment in HREE (up to 40 mol. % HREEPO₄) compared to xenotime from P-rich granites (up to 20 mol. % HREEPO₄). Xenotime compositions from P-rich granites are influenced by brabantite-type substitution, whereas for xenotime from P-poor granites the huttonite-type substitution is dominant. Unusual enrichments in HREE is significant for xenotime from P-poor granites, especially in Yb (up to 0.17 apfu Yb) and Dy (up to 0.11 apfu).     

Reconstruction of a large runout landslide in the Krušné hory Mts. (Czech Republic)

The aims of this study were to summarize current knowledge of a large runout prehistoric landslide, critically review all of the existing data and, in particular, gather new data in order to estimate the age of the accumulation and reveal the movement mechanism. The reconstruction of a large rockslide-rock avalanche in the NW part of the Czech Republic was supported by the analysis and interpretation of 216 boreholes and by GIS analysis of the original 1950s pre-mining surface using digitized old military topographic maps. For the age estimation, we used the Schmidt hammer test. The total volume of the quaternary deposit was calculated to be between 25.4 and 27.4 mil m³, occupying an area of 778,000 m² and consisting of six to eight generations of colluvial sediments. Three main landslide events were identified based on extensive Schmidt hammer sampling, and the approximate age was established using a regression equation assembled by Engel (2007). All three of the documented events occurred around the time of significant climate change. The oldest event occurred due to the Oldest Dryas warming, the largest event probably occurred at the end of the Younger Dryas (11,700 yBP), and the youngest of the documented events was purely of a Holocene age, with the highest landslide frequency being during the Atlantic temperature fluctuations (approximately 8200 yBP). The slope deformation occurred on a fault slope with a relative height of over 400 m and in tectonically weakened rocks. Sediments in the Most Basin were weakened from meltwater during rapid warming periods, which allowed mobilization of rockslide deposits and runout of up to 1000 m from the mountain foothills.     

Major element geochemistry of recent sediments from the Fláje basin (Krus̆né hory–Erzgebirge,Czech Republic)

The weathering products of the granitic rocks (which are poor in base cations) and their sediments in the Fláje basin, in an environment that is strongly influenced by SO₂ and NO_x emissions have been studied. High deposition of SO₂ and consequent acidification of the environment causes further depletion of base cations, mobilisation of Al and modification of the weathering reactions resulting in changes of the geochemical features of recent sediments. Depletion of the bottom sediments of the Fláje basin in base cations and Al and their enrichment in Si, compared to the fresh stream sediments and eluvium, suggests a mineralogical modification of the weathering product due to prolonged contact with basin water. Depletion of the non-silicate phase of the sediment in base cations which are replaced with Al on the other hand appears to be more effective in eluvium and those sediments which are exposed directly to the effect of acid deposition, i.e. which are not below water level for the major part of the year. The water of the basin appears to be the main factor influencing the composition of the recent sediments of the basin in the acidified catchment.     

Petrology of Volcán Tequila,Jalisco, Mexico: disequilibrium phenocryst assemblages and evolution of the subvolcanic magma system

Volcán Tequila is an extinct stratovolcano in the western Mexican Volcanic Belt that has erupted lavas ranging from andesite to rhyolite during the last 0.9 Ma. Following an early period of rhyolitic volcanism, the main edifice of the volcano was constructed by central vent eruptions that produced 25 km³ of pyroxene-andesite. At about 0.2 Ma central activity ceased and numerous flows of hornblende-bearing andesite, dacite, and rhyodacite erupted from vents located around the flanks of the volcano. Bimodal plagioclase phenocryst rim compositions in lavas from both the main edifice and the flanks indicate that magma mixing commonly occurred shortly prior to or during eruption. Compositions of endmember magmas involved in mixing, as constrained by whole-rock major and trace element abundances, phenocryst compositions, and mineral-melt exchange equilibria, are similar to those of some lavas erupted from the central vent and on the flanks of the volcano. Estimated pre-eruptive temperatures for hornblende-bearing lavas (970°–830°C) are systematically lower than for lavas that lack hornblende (1045°–970°C), whereas magmatic H₂O contents are systematically higher for hornblende-bearing lavas. In addition to stabilizing hornblende, high magmatic water contents promoted crystallization of calcic plagioclase (An_70–82). Frequent injections of magma into the base of the subvolcanic plumbing system followed by eruption of mixed magma probably prevented formation of large volumes of silicic magma, which have caused paroxysmal, caldera-forming eruptions at other stratovolcanoes in western Mexico. The later stages of volcanic activity, represented by the flank lavas, indicate a change from a large magma storage reservoir to numerous small ones that developed along a NW-trending zone parallel to regional fault trends. Sr and Nd isotopic data for lavas from the Tequila region and other volcanoes in western Mexico demonstrate that differentiated calc-alkaline magmas are formed primarily through crystal fractionation of mantle-derived calc-alkaline basalt coupled with assimilation of crustal material. Present Address:Department of the Geophysical Sciences The University of Chicago, Chicago IL, 60637, USA     

Geological, mineralogical and geochemical characteristics of the Radzimowice Au–As–Cu deposit from the Kaczawa Mountains (Western Sudetes, Poland): an example of the transition of porphyry and epithermal style

The sheeted quartz–sulfide veins of the Radzimowice Au–As–Cu deposit in the Kaczawa Mountains are related to Upper Carboniferous post-collisional potassic magmatism of the composite Zelezniak porphyry intrusion. Multiple intrusive activity ranges from early calc-alkaline to sub-alkaline and alkaline rocks and is followed by multiple hydrothermal events. Early crustally derived dacitic magma has low mg# (<63) and very low concentrations of mantle-compatible trace elements, high large-ion lithophile elements (LILE), moderate light rare-earth elements (LREE), and low high-field-strength elements (HFSE). Later phases of more alkaline rocks have higher mg# (60–70), and LILE, LREE, and HFSE characteristics that indicate mafic magma contributions in a felsic magma chamber. The last episode of the magmatic evolution is represented by lamprophyre dikes which pre-date ore mineralization and are spatially related to quartz–sulfide–carbonate veins. The dikes consist of kersantite and spessartite of calc-alkaline affinity with K₂O/Na₂O ratios of 1.1–1.9, mg# of 77–79, and high abundances of mantle-compatible trace elements such as Cr, Ni, and V. They have high LILE, low LREE, and low HFSE contents suggesting a subduction-related post-collisional arc-setting. The mineralization started with arsenopyrite that was strongly brecciated and overprinted by multiple quartz–carbonate phases associated with base-metal sulfides and Au–Ag–Bi–Te–Pb±S minerals. The sulfur isotope composition of sulfides ranges from –1.1 to 2.8 ³⁴S and suggests a magmatic source. At least two generations of gold deposition are recognized: (1) early refractory, and (2) subsequent non-refractory gold mineralization of epithermal style. Co-rich arsenopyrite with refractory gold and pyrite are the most abundant minerals of the early stage of sulfide precipitation. Early arsenopyrite formed at 535–345°C along the arsenopyrite–pyrrhotite–loellingite buffer and late arsenopyrite crystallized below 370°C along the arsenopyrite–pyrite buffer. Non-refractory gold associated with base-metal sulfides and with Bi–Te–Ag–Pb–S mineral assemblages has an average fineness of about 685, and is represented by electrum of two generations, and minor maldonite (Au₂Bi). Fluid inclusions from various quartz generations co-genetic with base-metal sulfides and associated with carbonates, tellurides and non-refractory gold indicate fluids with moderate salinity (9–15 wt% NaCl equiv.) and a temperature and pressure drop from 350 to 190°C and 1.2 to 0.8 kbar, respectively. According to the result of the sulfur isotope fractionation geothermometer the temperature of base-metal crystallization was in the range from 322 to 289°C. Preliminary results of oxygen isotope studies of quartz from veins indicate a gradual increase in the proportion of meteoric water in the epithermal stage. The gold to silver ratio in ore samples with >3 ppm Au is about 1:5 (geometric mean). Hydrothermal alteration started with sericitization, pyritization, and kaolinitization in vein selvages followed by alkaline hydrothermal alteration of propylitic character (illitization and chloritization), albitization and carbonatization. The mineralization of the Radzimowice deposit is considered as related to alkaline magmatism and is characterized by the superposition of low-sulfidation epithermal mineralization on higher-temperature and deeper-seated mesothermal/porphyry style.Editorial handling: B. Lehmann     

High-pressure/ultrahigh-pressure eclogites from the Hong’an Block,East-Central China: geochemical characterization,isotope disequilibrium and geochronological controversy

The Hongan Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist–greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The available age data are inconclusive that hinder our understanding of the tectonic evolution of the Block. The metamorphic temperatures in the Hongan Block (T_meta 700 to 500°C) are lower by 50–150°C than that of the Dabie and Sulu terranes. In this work, we undertook new trace element and Sr–Nd–O isotopic analyses on minerals in order to gain more insight into the geochronological problems. The results are as follows: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g., epidote) in garnet and omphacite has contributed to isotope disequilibrium. (2) Sm–Nd isotope analyses yielded no isochron ages for the Hongan eclogites. (3) Rb–Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established, yielding ages from 210 Ma to 225 Ma. The pattern of Rb–Sr isotope disequilibrium appears to be independent of the petrological and O-isotope temperatures. (4) In contrast to the unequilibrated Sm–Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature due to mass balance constraints. (5) Whole-rock ¹⁸O values show a large variation from +10 to –8, suggesting that their protoliths have undergone very different processes of water–rock interaction. In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hongan Block took place in the Triassic at about 220–230 Ma, as observed in the Dabie and Sulu terranes. The significance of published Paleozoic dates (450–300 Ma) for the Xiongdian eclogite is not clear. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data.Electronic Supplementary Material Supplementary material is available for this article at     

Crystallization conditions of magma from Longmen potassic pluton of the Trans-North China Orogen,North China Craton: Constraints from mineral chemistry and zircon trace elementSCIEI北大核心CSCD

华北克拉通中部造山带发育的早白垩世钾质岩是研究华北克拉通中部幔源岩浆成因的天然样品,然而其成岩物理化学条件仍不明确。本文选取了涞源县龙门岩体中闪长岩和二长岩开展了矿物原位地球化学和锆石微量元素分析,以探讨矿物形成的温压条件及氧逸度与矿化的关系。分析结果显示,二长岩和闪长岩的锆石Th/U比值较高(>0.1),轻稀土亏损、重稀土富集,具明显的Ce正异常和Eu负异常特征,并具有明显的震荡环带,为典型岩浆成因锆石。电子探针结果显示,龙门岩体中斜长石主要为中性斜长石,黑云母为原生镁质黑云母,角闪石主要为钙铁闪石。闪长岩中角闪石结晶温度为793~842℃,压力为0.50~0.75GPa,深度范围为19~28km,具有较低氧逸度(log f O 2为-12.21~-11.38)和高的含水量(8.5%~10.75%);二长岩中黑云母结晶温度为764~788℃,具有更低的氧逸度(log f O 2=-15.14~-14.54);闪长岩和二长岩中锆石结晶温度为770~978℃,具有较低的氧逸度(ΔFMQ=-0.37~+1.70)。矿物化学分析表明,角闪石和黑云母虽都在地壳开始结晶,但角闪石形成于比黑云母更深的岩浆房。龙门岩体中角闪石、黑云母和锆石的结晶主要受岩浆分异控制,其地球化学动力学机制主要为岩浆上侵时减压熔融,进而引起岩浆氧逸度随岩浆演化而变化。随着岩浆上涌,闪长岩中角闪石随着温度降低和压力减少逐渐结晶,地幔交代作用导致二长岩中的黑云母具有较高的X Mg值,进而形成镁质黑云母。结合区域地质背景,古太平洋板块(伊佐奈琦板块)俯冲-后撤释放的流体交代上覆岩石圈地幔对中部造山带钾质岩(~141Ma)的形成有一定贡献,该过程导致了岩浆具有较高的含水量,但龙门岩体低的氧逸度和区域上较厚的岩石圈制约了成矿金属物质(Fe-Au等)的迁移和富集,因而不利于亲硫、亲铁元素聚集以及铁、金矿化的形成。     

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins,Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.     

Morphology,trace elements,and geochronology of zircons from monzogranite in the Northeast Xing’an Block,northeastern China: constraints on the genesis of the host magma

Mineralogy and Petrology - The morphology, trace-element composition and geochronology of 43 zircon grains from two monzogranite samples from the Northeast Xing’an Block, northeastern China,...     

Interaction of weathering solutions with oxygen and U–Pb isotopic systems of radiation-damaged zircon from an Archean granite, Darling Range Batholith, Western Australia

Zircons from Archean granites from the Darling Range Batholith in the Yilgarn Craton of Western Australia have been shown to have complexly discordant U–Pb systems with a strong component of zero age disturbance. The only geological event that has affected the granites in recent times is the pervasive regional weathering. Our aim in this study was to investigate the effects of weathering on the U–Pb and oxygen systems of the zircons, and to this end, we report secondary ion mass spectrometry (SIMS) results of OH, oxygen and U–Pb isotope systems of six typical zircons from a sample of the granite. These results confirmed the presence of OH in highly radiation-damaged parts of the zircons, demonstrating fluid interaction within grains. The presence of OH was accompanied by significant changes in the ¹⁸O/¹⁶O ratios. The data suggest trends where δ¹⁸O values in individual grains both increase and decrease with increasing OH. SIMS measurements showed the U–Pb systems are variably and unsystematically discordant in radiation-damaged parts of the zircons, particularly those with elevated OH contents. The complex U–Pb systems are interpreted in terms of multiple disturbance events between 450 and 0 Ma involving low-temperature fluid-induced movement of radiogenic Pb, decoupled from parent U and Th, within the radiation-damaged zircons, together with some Pb loss.     

Post-orogenic shoshonitic magmas of the Yzerfontein pluton,South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

The post-orogenic Yzerfontein pluton, in the Saldania Belt of South Africa was constructed through numerous injections of shoshonitic magmas. Most magma compositions are adequately modelled as products of fractionation, but the monzogranites and syenogranites may have a separate origin. A separate high-Mg mafic series has a less radiogenic mantle source. Fine-grained magmatic enclaves in the intermediate shoshonitic rocks are autoliths. The pluton was emplaced between 533 ± 3 and 537 ± 3 Ma (LA-SF-ICP-MS U–Pb zircon), essentially synchronously with many granitic magmas of the Cape Granite Suite (CGS). Yzerfontein may represent a high-level expression of the mantle heat source that initiated partial melting of the local crust and produced the CGS granitic magmas, late in the Saldanian Orogeny. However, magma mixing is not evident at emplacement level and there are no magmatic kinships with the I-type granitic rocks of the CGS. The mantle wedge is inferred to have been enriched during subduction along the active continental margin. In the late- to post-orogenic phase, the enriched mantle partially melted to produce heterogeneous magma batches, exemplified by those that formed the Yzerfontein pluton, which was further hybridised through minor assimilation of crustal materials. Like Yzerfontein, the small volumes of mafic rocks associated with many batholiths, worldwide, are probably also low-volume, high-level expressions of crustal growth through the emplacement of major amounts of mafic magma into the deep crust.     

Synchronous granite intrusion and E–W extension in the Cuonadong dome,southern Tibet,China: evidence from field observations and thermochronologic results

International Journal of Earth Sciences - The Tethys Himalaya sedimentary sequence (THS) is characterized by N–S trending extensional rifts (NSTR) and the North Himalayan gneiss domes (NHGD)....     

Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.     

Granites of the intracontinental termination of a magmatic arc: an example from the Ediacaran Araçuaí orogen,southeastern Brazil

The Araçuaí orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Araçuaí–West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the São Francisco–Congo Craton, this portion of the Adamastor was only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma. The Rio Doce magmatic arc terminates in the northern portion of the Araçuaí orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to study magmatism at arc terminations, and to understand the interplay between calc-alkaline magma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb–Ta, Sr and Ti anomalies, typically associated with subduction-related magmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. Zircon U–Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590 Ma, experienced a migmatization event at ca. 585 Ma. The younger granitoids, emplaced between 570 and 590 Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydration-melting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done.