Granodioritic Rocks Research Articles

Tiny fragments of acidophilous steppic grasslands serve as yet unknown habitats of endangered aeolian sand specialists among Aculeata (Hymenoptera)

Aeolian sand specialists are traditionally recognized among the most threatened species of central European landscapes due to habitat loss. In the present study, we examined the diversity of bees and wasps, and the cover and composition of vegetation in 17 neglected small fragments of Artemisio campestris–Corynephoretum canescentis acidophilous grasslands, which developed on weathered granodiorite rocks in the Czech Republic. The size of the examined fragments ranged from 128 to 14,000 m2, with a total area of only 5.7 ha. At the examined sites, we identified species-rich assemblages of endangered aculeate hymenopterans that were previously thought to be specialized on aeolian sands. We found that despite the small area and relative isolation of the examined sites, they hosted 26 Red-Listed species of bees and wasps and a species new to Bohemia (Megachile pilicrus), respectively. The examined habitats also supported the presence of 18 Red-Listed receding field weeds and steppic grassland specialists. The analyzed bee and wasp assemblages were characterized by their low dominance (1-Simpson index = 0.091) and high alpha diversity (Fisher’s alpha = 48.9). Although the examined fragments of acidophilous steppic grasslands differed in size by two orders of magnitude, the total abundance and species richness of the bees and wasps were not correlated with the size of the fragments. This suggests the long-term stability despite the minute size of some of the residual fragments. Land protection and active management are proposed to conserve these habitats for future generations.

Multistage tectono‐magmatic evolution of the central Urumieh–Dokhtar magmatic arc, south Ardestan, Iran: Insights from zircon geochronology and geochemistry

Combining high‐spatial resolution geochronology and geochemistry of zircon provides constraints on the geodynamic evolution of the Zagros orogen. This study underlines the significance of hitherto unknown magmatic/geological events in the central Urumieh–Dokhtar magmatic arc (UDMA) of Iran at ca. 35–22 and 10 Ma. Recently reported U–Pb zircon ages of 25–24 Ma for the granodioritic rocks from the south Ardestan area were interpreted to constrain the crystallization age. Newly determined U–Pb zircon ages of ca. 35 to 22 Ma from south Ardestan gabbroic to granitic rocks and related dikes indicate that the melt evolution continued from the Oligocene into Early Miocene time. In addition, a ca. 10 Ma U–Pb age for the zircons from a south Ardestan diorite provides evidence for a so far unknown, igneous episode. Representing the youngest magmatic stage recognized in the central part of the UDMA, the 10 Ma age is interpreted to document magmatic activity related to the Arabia–Eurasia collision, as recognized elsewhere in the Zagros orogen. South Ardestan zircons with geochemical signatures indicating continental affinity likely recrystallized in the course of a major event (asthenospheric mantle melt formation) that has affected crustal rocks. The significant lower LREEs of these zircons compared to hydrothermal zircons, moderate Th/U (>0.4), and high (Sm/La)N (>10) ratios associated with low [La] (<1 ppm) suggest that these crystals have an igneous origin and are not related to hydrothermal or metamorphic processes. These findings bear important implications for the geodynamic evolution of the Zagros orogen and support a model suggesting that magmatic activity still affected the UDMA in Late Miocene time.

The genetic relationship between JTA–like magmas and typical adakites: An example from the Late Cretaceous Nuri complex, southern Tibet

Phanerozoic adakites typically represent modern analogues of Middle–Late Archean (3.5–2.5 Ga) tonalite–trondhjemite–granodiorite rocks (TTGs) due to their similarities in geochemical compositions. However, no Phanerozoic analogues have been reported for the Early Archean TTGs (>3.5 Ga), which are characterized by low Sr, Y and HREEs contents with low MgO and compatible elements contents, until recent years. The newfound Phanerozoic analogues have been defined as a new adakite subgroup named as “Jamaican–type adakites” (JTAs), and been regarded as the results of partial melting of subducted oceanic plateau crust. JTA–like magmas also occur in the Nuri complex, southern Tibet. We report whole–rock geochemical, SrNd isotopic data, zircon UPb ages and in-situ Hf isotopic data for adakitic quartz diorites, quartz monzonites and JTA–like quartz porphyries in the Nuri complex. Zircon UPb dating indicates that these rocks were emplaced in the Late Cretaceous at 93.5–92.1 Ma. Combined with the continuous variation trends of major and trace elements, the similarities in terms of whole–rock SrNd and zircon Hf isotopic compositions indicate that the three types of rocks are genetically related, while the quartz diorites should represent the relatively primitive melts. High MgO (5.00–5.41 wt%), Cr (138–159 ppm), Ni (65.9–96.8 ppm) contents and Mg# values (66–69), high εHf(t) values (10.5–11.9), εNd(t) values (3.7), and low (87Sr/86Sr)i values (0.7046–0.7050) indicate that the quartz diorites were derived from partial melting of subducted Neo–Tethyan oceanic slab, and subsequently underwent metasomatic reaction with the mantle wedge during their ascent. Quartz monzonites and JTA–like quartz porphyries were originated from quartz diorites by two stages of fractional crystallization. Low MgO and compatible elements contents of the quartz porphyries were caused by fractionation of hornblende, biotite, with minor magnetite and allanite, while low Sr contents and Sr/Y ratios were mainly controlled by plagioclase fractionation. This means that the JTA–like geochemical features of the quartz porphyries were generated by fractional crystallization from the adakites derived from partial melting of subducted slab, rather than directly derived from partial melting of subducted oceanic plateau. Therefore, various petrogenetic possibilities of the JTA–like magmas should be considered when using them as modern analogues.

U-Pb zircon dating, Sr-Nd isotope and petrogenesis of Sarduiyeh granitoid in SE of the UDMA, Iran: implication for the source origin and magmatic evolution

ABSTRACT The Sarduiyeh granitoid (SG) is intruded in the southeastern part of the Dehaj-Sarduiyeh volcano-sedimentary belt in the southeastern end of the Urumieh-Dokhtar Magmatic Arc (UDMA) in Iran. The medium-to-coarse-grained granitoid unit, with granular texture consists mainly of diorite, tonalite, granodiorite and monzogranitic rocks. Mineralogically, these rocks consist mainly of plagioclase, K-feldspar, quartz, biotite and hornblende. The whole rock geochemical analyses indicates that the SG is calc-alkaline, I-type, metaluminous, enriched in large ion lithophile elements (LILE; such as K, Cs, Pb) and depleted in high field strength elements (HFSE; such as Ti, Nb, Ta, Zr). Chondrite normalized plot of SG rare earth elements (REE) show light rare earth element enrichments with (LaN/YbN = 2.44–8.68) and flat heavy rare earth element patterns with (GdN/YbN = 1.02–1.36). The rather high Y (av. 19.35 ppm), low Sr content (av. 293.76 ppm) and low Cr and Ni contents (av. 20.1 and 4.69 ppm, respectively) of the SG demonstrate its normal calc-alkaline and non-adakitic nature, the features of Jebal Barez-type granitoids. The geochemical characteristics and isotopic composition, low ISr (0.7046–0.7049) and positive ɛtNd (+3.4 to +4.03) values, of the SG suggest that its parental magma formed as a result of partial melting from metabasic rocks of lower crust in a subduction-related arc setting. Fractionation of an assemblage dominated by plagioclase, K-feldspar, amphibole and magnetite may have been responsible for the evolution of the SG magma. U-Pb zircon geochronology gives an age of 27.95 ± 0.27 Ma for the SG, suggesting that the final collision between the Arabian plate and Central Iranian microcontinent may have happened in the Late Oligocene.

The use of ASTER data and geochemical analyses for the exploration of gold at Samut area, South Eastern Desert of Egypt

The present study investigates the use of ASTER and geochemical data in the exploration of new gold occurrences in altered granodiorite rocks at Samut area in the South Eastern Desert of Egypt. The mapping approach developed and applied in this study integrates image ASTER and field data sets to produce a map showing the alteration zones within granitoid rocks in the Samut area. Due to the differences in hydrothermal mineral components as well as their intensities and influences on the surrounding rocks, various alteration processes have been occurred within the host granodioritic rocks. Such alteration processes include sericitization, chloritization, silicification, and ferrugination, resulted from the chemical interaction between the host rock minerals and mineral bearing ascending solutions. Each alteration process is characterized by significant minerals. The sericite and altered plagioclase characterize the sericitization process, whereas biotite, muscovite, chlorite, and hornblende minerals are associated with chloritization. However, silicification process is manifested by the quartz veins and fractured quartz grains. The ferrugination process is characterized by the transformation of magnetite to hematite and goethite. The chemical analyses of the unaltered granodiorites yielded Au content between 0.01 and 1.61 ppm, while in the altered granodiorite; Au content is around 0.02 to 14.79 ppm. The presence of gold in the unaltered and altered granodiorite even in a low percentage, but in huge tonnages gives an importance and makes the study area worthy of gold exploration and possible exploitation and hence international propagation. The Samut district is made up of intermediate to acidic metavolcanics intruded by a hornblende granodioritic pluton and cut by acidic dykes and quartz veins and veinlets. The associated hydrothermal activity with the plutonic intrusion led to the occurrence of promising alteration zones for gold in the granodiorite.

Mineralogy, petrography and geochemistry of an early Eocene weathering profile on basement granodiorite of Qaidam basin, northern Tibet: Tectonic and paleoclimatic implications

Weathering, as an important process in the earth surface system, can be significantly influenced by tectonics and climates over long time scales. Here, we use mineralogical, petrographic and geochemical data of a paleoweathering profile developed on basement granodioritic rocks of northern Qaidam basin, northern Tibet, to reconstruct early Eocene weathering conditions and to discuss how paleoclimates and tectonics dominated the weathering process. The results indicate that neoformed mineral phases in weathering products are dominated by smectite, and the profile has overwhelmingly low chemical index of alteration values (ca. 51–59) and significantly decreasing micropetrographic index values (from 25.0 to 0.2) from bottom to top. These findings suggest that the basement rocks experienced mild chemical weathering but relatively intensive physical weathering. We favor that non-steady-state weathering, in which mechanical erosion rates compare favorably with rates of chemical weathering, prevailed in northern Tibet during the early Eocene. The weathering conditions were likely an integrated response to active tectonism and dry climates at that time. Furthermore, chemical element mobility evaluation demonstrates that most of large ion lithophile elements and light rare earth elements (LREEs) of granodioritic rocks are quite active during weathering and can be easily leached even under mild chemical weathering conditions. Significant mass loss of Al and LREEs in upper weathered samples probably reflects acidic weathering conditions, which were likely due to extremely high atmospheric CO2 level during the early Eocene. This study, from the unique perspective of weathering process, suggests that intensive deformation and rapid tectonic erosion occurred in northern Tibet during the early Eocene, as a far-field response to the India-Eurasia collision. It also agrees with warm and relatively dry climates, which were likely attributed to the global greenhouse climates and the Paleogene planetary-wind-dominant climate system in Asia, respectively.

Petrology, geochemistry and stable isotope studies of the Miocene igneous rocks and related sulphide mineralisation of Oued Amizour (NE Algeria)

The Oued Amizour granitoids of North Africa were studied in order to assess the role of magma processes in the formation of the large Zn-deposit in the region. Access to material from a ∼800 m drillhole cutting the entire igneous succession greatly facilitated the investigation. The rocks reveal, from the top to the bottom, two distinct lithologies: volcanic, mainly pyroclastic rocks and sub-volcanic microgranitic – plutonic granodioritic rocks. Both lithologies were subject to hydrothermal alteration. The boundary between the two lithologies is marked by a massive, ∼10 m thick anhydrite layer. The geochemistry of these rocks shows that they are intermediate to felsic in composition and exhibit high-K to shoshonitic calc-alkaline features with I-type signature. The negative Eu, Sr and Nb anomalies and the high LILE and LREE enrichment observed in the whole igneous suite indicate that the various volcanic and plutonic rocks are genetically related. The granitoids are considered to have formed under post-collisional geotectonic regime during the Miocene, similar to those described in the Alpine Belt of northern Africa.The main sulphide mineralisation is dominated by sphalerite with rare marcasite, melnikovite, galena and minor but ubiquitous pyrite. The sphalerite ore body of about 20–30 m thick is hosted by hydrothermally altered andesite breccias and volcanic tuff, between the anhydrite layer at the base and the volcanic pile at the top. Replacement, colloform and open space filling are the main ore texture patterns which indicate hydrothermal processes throughout successive percolation phases.δ34S of sulphides (sphalerite, pyrite and chalcopyrite) vary between −7.2‰ and +4.5‰ (n = 22; mean = −1.3‰), suggesting that magmatic fluids have played a major role in the formation of the Oued Amizour Zn-deposit. However, δ34S of sulphates (anhydrite and gypsum) which are isotopically heavier ranging between +13.2‰ and +20.6‰(n = 10; mean = +16.3‰), likely reflect mixing between Miocene seawater sulphate and magmatic sulphur. Fluid inclusion microthermometric measurements on anhydrite and gangue calcite yield a mean temperature of ∼200 °C and salinity between 22.3 and 26.6% eq. NaCl. Calcite δ18OV-SMOW varies between +11.2 and +20.2‰ (n = 12, mean = +16.7‰) and δ13CV-PDB varies between −3.7 and −11.0‰ (mean = −7.8‰). These results confirm the magmatic origin of the mineralising fluids, with a marked contribution of seawater, for precipitation of the Oued Amizour Zn mineralisation. Low δ13CV-PDB could, however, be attributed to the contribution of carbon from unconsolidated mudstones and/or magmatic carbon. The observed data on the Oued Amizour Zn deposit show that the mineralisation is part of VHMS group with a Kuroko-type signature.

Late Mesozoic–early Cenozoic intermediate–acid intrusive rocks from the Gerlache Strait area, Antarctic Peninsula: Zircon U–Pb geochronology, petrogenesis and tectonic implications

This paper reports zircon U–Pb geochronology, whole-rock chemistry and Sr–Nd–Hf isotopic data of intermediate-acid intrusive rocks from the Gerlache Strait area, Antarctic Peninsula. These intrusive rocks can be divided into three groups. Group #1 has an emplacement age of ca. 117 Ma, and is further subdivided according to K2O contents into High-K (HKS) and Low-K (LKS) Subgroups. The HKS monzogranites are weakly peraluminous, and yield εHf(t) and εNd(t) values of +0.9 to +2.3 and +0.1 to +2.3, respectively, with corresponding model Nd ages (TDMNd) of 766–471 Ma, indicating that they were derived from the partial melting of dominantly early Neoproterozoic to early Paleozoic juvenile basaltic materials with minor involvement of sediments. In contrast, the LKS granodioritic magma was derived from partial melting of early Paleozoic juvenile basaltic materials with εHf(t) and εNd(t) values of +10.3 to +12.5 and +5.4 to +5.5, respectively, and corresponding TDMNd ages of 548–500 Ma. Group #2 consists of granite porphyry that formed at ca. 82 Ma and is characterized by positive values of εHf(t) (+5.1 to +8.1) and εNd(t) (+1.9 to +3.1) with weakly peraluminous chemistry and corresponding TDMNd ages of 712–548 Ma, suggesting that it was generated from partial melting of dominantly Neoproterozoic juvenile basaltic materials with minor involvement of sediments. Group #3 comprises dioritic–granodioritic rocks that were emplaced at ca. 62–54 Ma. These rocks have Mg# values of 39–46, and εHf(t) and εNd(t) values of +6.7 to +12.2 and +3.6 to +7.2, respectively, with corresponding TDMNd ages of 529–307 Ma, indicating that they were derived from partial melting of Paleozoic juvenile basaltic materials with minor injection of mantle materials. Spatially, the lithology of these plutons roughly changes from acid to intermediate from east to west, concomitant with increasing εHf(t) and εNd(t) values and decreasing initial 87Sr/86Sr ratios and TDMNd ages. This trend reflects changes in source characteristics from a Gondwana affinity to a juvenile crust affinity. Overall, the southeast-to-northwest migration of magmatism in the northern Antarctic Peninsula since the Mesozoic may have been related to retreating subduction of the paleo-Pacific plate.

Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U–Pb dating, geochemistry and Sr–Nd–Hf isotopes was carried out for three types of intrusive rocks from the Hannan region, including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the Tiechuanshan and Huangguan A-type granites. LA-ICP-MS zircon U–Pb dating results show that the studied intrusions have crystallization ages ranging from ca. 880 to 770Ma. The ca. 880Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low high field strength elements (HFSE) concentrations that indicate a cumulate origin. These gabbros display subduction-related geochemical signatures with whole-rock initial 87Sr/86Sr (ISr)=0.704–0.705, εNd(t)=−0.4 to +0.6 and zircon εHf(t)=+6.9 to +8.4, suggesting that they were derived from the partial melting of an enriched lithospheric mantle that had been modified by slab-derived materials. The ca. 860Ma Tianpinghe I-type granites have high ISr (> 0.704) and negative εNd(t) (−5.6 to −3.5) values and zircon εHf(t)=−1.9 to +1.6, indicating their sources were mixtures of Neoproterozoic juvenile mafic crust and minor Paleoproterozoic crustal components, whereas the ca. 770Ma Taojiaba I-type granites have relatively low ISr (<0.704) and positive εNd(t) (+1.5 to +2.4) with zircon εHf(t)=+6.2 to +9.5, implying that they were dominantly sourced from the Neoproterozoic juvenile crustal materials. The geochemical and Sr–Nd–Hf isotopic data imply that the ca. 780Ma Tiechuanshan A-type granites were crystallized from a reduced and peralkaline magma, which was generated by partial melting of a heterogeneous source involving Neoproterozoic juvenile and ancient mafic crust under dry and reducing conditions. In contrast, the parental magma of the ca. 774Ma Huangguan A-type granites was oxidized, aluminous and calc-alkalic and resulted from the dehydration melting of tonalitic to granodioritic rocks under water-subsaturated and oxidizing environments. In combination with previous work, the ca. 880–860Ma Beiba gabbros and Tianpinghe I-type granites are interpreted to be formed during the stage of initial back-arc extension that was caused by subduction-induced convection in the mantle wedge. Subsequently, the ca. 780–770Ma Tiechuanshan and Huangguan A-type granites and Taojiaba granodiorites were formed in a continental back-arc rift setting related to the rollback of subducted oceanic slab. Our new results support that an arc-back-arc system developed in the northwestern Yangtze Block during the Neoproterozoic.

Fluid evolution of Au-Cu zones in Um Balad area, North Eastern Desert of Egypt: Implications from mineral chemistry and fluid inclusions

Scanning electron microscope (SEM), Electron microprobe (EMPA) and fluid inclusion studies of the ore body, as well as geochemical analyses of country rocks were performed to determine the nature and characteristics of the mineralizing fluid responsible for Au-Cu deposits in Um Balad area, Northern Eastern Desert of Egypt. The Um Balad Au-Cu deposits are confined to well developed-quartz veins and veinlets cutting through the hosting country rocks. Petrographic and geochemical investigations of the hosting rocks distinguished between two main rock units; 1) metagabbro-diorite rocks with tholeiitic nature derived in island arc/continental margin tectonic regime, and 2) granodiorite rocks formed from calc-alkaline magma in continental margin regime. Wallrock alterations are represented by propylitic and argillic types. The mineralized quartz veins are striking in NE-SW direction and dipping between (35°–45°) in SE direction, other mineralized mafic dykes enriched with auriferous quartz veinlets are trending NE-SW and dipping 70°/SE. The main ore minerals are represented by gold, chalcopyrite, pyrite, sphalerite, malachite, covellite and goethite. While, geffroyite, cuprite, chrysocolla, pseudomalachite, britholite, wolframite, scheelite, hematite and rutile are detected as minor constituents. Fluid inclusions microthermometry and isochore calculations combined with chlorite geothermometry revealed that the Um Balad deposits were formed at temperature ranging from 305 °C to 325 °C and pressure between (100–500 bar). The mineralization had been developed in the shallow levels, beneath the water table at depth of 350–1760 m, rather than common mesothermal vein-type deposits in Egypt. Magmatic water have been suggested as the main source for the mineralized fluid. The transportation of the gold metal seems to be happen as bisulfide complexes in moderately acidic environment. The deposition was resulted from combination of changes in physico-chemical parameters, temperature and pressure plus the instability of the reduced sulfur complexes. A contamination with metamorphic and/or meteoric water was also proposed that has strong influence during the depositional process.

Geochemical signatures of the groundwaters from Ourense thermal springs, Galicia, Spain

Different hot springs and boreholes in the city of Ourense, Galicia, Spain, have been studied to determine the mineral equilibrium conditions of the discharged groundwaters and the reservoir temperatures predicted by the equilibrium conditions. Ourense is located in the Mino River’s valley. The area is characterized by two fault systems, which determine groundwater circulation. A NW trending fault system is the permeable system that transfers groundwater and heat to springs in the Mino River valley as it is evident from the location of the springs in the region. Groundwaters traveling and discharging from granitic and schistose rocks are mainly bicarbonate waters. In comparison, groundwaters traveling and discharging from granodiorite rocks can be bicarbonate, sulfate or chloride waters. Different equilibrium activity diagrams for the dominant cations in groundwater (Na+, K+, Ca2+, Mg2+) have been constructed to correlate water equilibrium conditions with the mineral assemblage, K-feldspar, clinochlore, muscovite, quartz, and calcite. Granites and schists are the rocks within which groundwater circulation approaches mineral equilibrium with equilibrium temperatures around 140–160 °C. Groundwaters circulating throughout granodiorite seem a little bit high in Mg2+ to reach equilibrium conditions. Mino River’s tectonic valley presents strong morphological contrasts in terms of faulting and rock types to the north of the river that allow increased longitude and depth of groundwater circulation. These conditions allow close achievement of water–mineral equilibrium conditions. More research is needed to know the extension of this energy resource and optimize its use.

Provenance and Palaeoweathering Attributes of Proterozoic Rock Formations from the Son Valley, Central India

The characteristics of source rocks and weathering of Palaeoproterozoic phyllitic rock of Mahakoshal Group and Mesoproterozoic shales and siliciclastics of Vindhyan Supergroup exposed in Son Valley, Central India have been investigated by analyzing their chemical compositions. The investigations pertaining to the relationship between major-elements were carried out along Parshoi, Chitrangi, and Markundi areas of Son valley, Central, India. The studied rock strata have been classified into three categories namely phyllitic rocks, shales and sandstone. The A-CN-K ternary diagram, CIW, CIA, MIA, and ICV values indicate about the similar provenance or source rocks subjected to severe chemical weathering, under dry and hot-humid climates in a basic and acidic environment with changing lower to higher PCO2 of continental flora. Various geochemical discriminantts diagrams, elemental ratios suggest that rocks are derived from post-Archaean-Proterozoic igneous source. The igneous source was mainly granite with a minor contribution of granodioritic rock, in a passive margin setting. The sediments responsible for formation of shale and sandstones were most likely deposited in the interglacial period in between the Mesoproterozoic and Neoproterozoic glacial times. Compositionally the sandstones is distinctive of cratonic environments with their passive continental margin setting. However, the phyllities of Mahakoshal Group suggests their formation under lower weathering conditions in dry climatic conditions which were operating on more intermediate to basic igneous rocks with abundance of mafic minerals.

The Salave Mine, a Variscan intrusion-related gold deposit (IRGD) in the NW of Spain: Geological context, hydrothermal alterations and ore features

The Salave Gold deposit is located at the northern end of the Oscos Gold Belt, which forms part of the Asturian-Leonese Zone of the Variscan Massif. The Gold Belt and the Salave deposit are genetically related to the emplacement of post-kinematic granitoids, which were largely controlled by Variscan fracture systems. In the Salave area, three main igneous complexes, Porcia, Salave and Represas, outcrop in an approximately E-W direction. Gold deposits are only found in the intense hydrothermal alterations affecting the western part of the Salave outcrop, just where the Mondoñedo thrust reaches the Cantabrian Sea. The geometry of the mineralized body is approximately tabular and is roughly parallel to the Mondoñedo thrust. A network of fractures controlled the gold ore. It consisted of sub-horizontal and sub-vertical fractures, the latter acting as feeders. Along the fracture network an intense albitization replaced the granodiorite and partially filled the open fractures. Farther from the fracture system the granodiorite rock is affected by sericitic-propylitic alteration halo. The most external alteration halo is the chloritic alteration. The transition from one alteration to another is gradual and irregular. The albitized areas and some areas slightly beyond them are then intensely sericitized (advanced sericitic alteration), sulfurized and carbonatized along the network of fractures, and becomes the richest areas in gold. The sulfurized areas are rich in arsenopyrite, pyrite and stibnite, always over 10% and reaching as much as 40% in volume. All the altered rocks mentioned have associated late carbonates as veins or micro-veins, disseminated or filling open spaces.The isocon diagrams (using Zr and Nb as immobile elements) and representative analytical data show that in chloritic alteration there is a small gain of mass, in propylitic alteration the gain is negligible, in albitic there is no gain or loss, in less intense advanced sericitic there is a small loss of mass and in the intense advanced sericitic the loss of mass is significant. As far as the metallic elements, sulfur, arsenic and antimony, are concerned, significant amounts of S, Au, As, Sb and Bi were consistently added to the original rock, mainly during advanced sericitic alterations. Ag is added during albitization to advanced sericitic alteration. Pb, Cu, W, Mo and U are more erratic but appear to have been added from the beginning, together with some Bi, S and As, and to have remained quite constant during all the alterations as if they had been introduced previously to gold mineralization.Four mineral associations have been described at Salave, Cu-Fe-Zn-S, Mo-Bi-S-(W), S-As-Fe-(Au) and S-Sb-Au-(Pb-Bi-Ag-Cu-Zn). The two gold related ones, are post granodiorite consolidation. Visible gold (or electrum) is scarce. Invisible gold at Salave was detected using microprobe analyses. It accounts for most of the Salave gold. The invisible gold in arsenopyrite is in solid solution, while in pyrite a small amount appears in solid solution, but most is probably in the form of microinclusions. Invisible gold is higher in arsenopyrite than in pyrite. The Salave gold deposit is a fracture-controlled deposit whose characteristics, such as stable isotope data, fluid inclusions and geochemistry, indicate a strong igneous influence, albeit partially influenced by meteoric waters and host-rocks. Thus, the deposit should be considered as a reduced intrusion-related gold deposit.

Mantle contribution and tectonic transition in the Aqishan-Yamansu Belt, Eastern Tianshan, NW China: Insights from geochronology and geochemistry of Early Carboniferous to Early Permian felsic intrusions

Late Paleozoic is a key period for the accretion and collision of the southern Central Asian Orogenic Belt (CAOB). Here, we present new zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotopic compositions for four Late Paleozoic felsic plutons in Eastern Tianshan (or Tienshan in some literatures) in order to constrain the tectonic evolution of the southern CAOB. The granodioritic pluton and its dioritic enclaves were synchronously formed in the Early Carboniferous (336 ± 3 Ma and 335 ± 2 Ma, respectively). These rocks are depleted in Nb, Ta and Ti, and enriched in Rb, Ba, Th and U related to the primitive mantle, which show typical features of arc rocks. They both have similar Sr–Nd isotopic ratios to those granitic rocks from the eastern Central Tianshan Block and have the latest Mesoproterozoic two stage Nd model ages (TDM2) (1111–1195 Ma for the granodioritic pluton and 1104–1108 Ma for the enclaves, respectively), indicating that their source magmas may have been derived from the Mesoproterozoic crust. The albitophyric pluton was also emplaced in the Early Carboniferous (333 ± 3 Ma). Rocks of this pluton have similar εNd(t) values (−0.69 to −0.37) and TDM2 ages (1135–1161 Ma) to those of the granodioritic rocks, suggest similar crustal source for both types of rocks. In contrast, the K-feldspar granitic and monzonitic plutons were emplaced in the Early Permian (292 ± 3 Ma and 281 ± 2 Ma, respectively). Samples of the K-feldspar granitic pluton have high K2O + Na2O, FeO/MgO, Ga/Al, HFSE (e.g., Zr and Hf) and low CaO, Sr and Ba, exhibiting characteristics of A2-type granites, which probably emplaced in a post-collisional extension environment. They have higher εNd(t) values (+2.77 to +3.27) and more juvenile TDM2 ages (799–841 Ma) than the Early Carboniferous plutons, suggesting that they were derived from relatively younger crustal sources. The monzonitic granites are metaluminous to weakly peraluminous with A/CNK ranging from 0.93 to 1.05, and have very low P2O5, indicating characteristics of I-type granites. They also have positive εNd(t) values (+2.22 to +2.34) and juvenile TDM2 ages (868–878 Ma), suggesting this pluton was also produced by partial melting of relatively young crustal source. Based on an isotopic mixing simulation, significant mantle contributions were added to the magma source of both the Early Carboniferous and the Early Permian felsic rocks. The mantle contribution changes from ~60% in the Early Carboniferous to ~75% in the Early Permian. The remarkably increasing of mantle materials in the magma source of the felsic rocks in the Aqishan-Yamansu belt was most likely induced by the tectonic transition from an Early Carboniferous continental arc to an Early Permian post-collisional extension environment.

Hydrothermal Alteration Mineralogy Associated with Gold Mineralization in Buladu Area, Gorontalo, Northern Sulawesi, Indonesia

Geologically, the Buladu area is part of the North Sulawesi magmatic arc, located in the morphology of the mountains along the coastline of North Gorontalo district, Indonesia. Multiple subductions of North Sulawesi in the northern arm of Sulawesi and east Sangihe subduction in the east has triggered an active magmatism and volcanism. Formation and occurrences deposits of precious- and base metals have been found in the North Sulawesi Volcanic Arc. This paper describes a recent study on hydrothermal alteration mineralogy associated with one of the prospects in the region, namely Buladu gold prospect that for years has been operated by local artisanal miners. The study is focused on hand specimen and microscopic observation as well as XRD analysis. Lithologically, the study area is arranged by sandstone are members of Dolokapa volcano-sedimentary formation, above unconformity andesitic-basaltic volcanic breccia are members of the Pliocene Wobudu Formation and is intruded by granodiorite rocks. The petrographic study reveals that host rock of the mineralization is flow-volcanic breccia which shows porphyritic and flows structures. Resulting assemblages of secondary minerals are mineral associations characteristic of the magmatic-hydrothermal system the following imperative alteration zones could be recognized an outer zone of chloritic alteration and the inner zone of argillic alteration. The alteration showed characteristics of pervasive to selective alteration, where clay and quartz totally replaced groundmass, whereas chlorite and clay minerals were selectively altered plagioclase phenocrysts. Clay mineral species identified by XRD include chlorite and illite. These mineralogical features indicate that the hydrothermal alteration is an argillic type, which is spatially distal to the mineralization. Mafic mineral in andesite lava partly altered into mineral assemblage chlorite-epidote-quartz-carbonate show propylitic type, which is the outermost zone of mineralization.

Strength degradation of sandstone and granodiorite under uniaxial cyclic loading

Change in mechanical properties of rocks under static loading has been widely studied and documented. However, the response of rocks to cyclic loads is still a much-debated topic. Fatigue is the phenomenon when rocks under cyclic loading fail at much lower strength as compared to those subjected to the monotonic loading conditions. A few selected cored granodiorite and sandstone specimens have been subjected to uniaxial cyclic compression tests to obtain the unconfined fatigue strength and life. This study seeks to examine the effects of cyclic loading conditions, loading amplitude and applied stress level on the fatigue life of sandstone, as a soft rock, and granodiorite, as a hard rock, under uniaxial compression test. One aim of this study is to determine which of the loading conditions has a stronger effect on rock fatigue response. The fatigue response of hard rocks and soft rocks is also compared. It is shown that the loading amplitude is the most important factor affecting the cyclic response of the tested rocks. The more the loading amplitude, the shorter the fatigue life, and the greater the strength degradation. The granodiorite specimens showed more strength degradation compared to the sandstone specimens when subjected to cyclic loading. It is shown that failure modes of specimens under cyclic loadings are different from those under static loadings. More local cracks were observed under cyclic loadings especially for granodiorite rock specimens.

Geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes of the Early Jurassic granodiorite from the Sankuanggou intrusion, Heilongjiang Province, Northeastern China: Petrogenesis and geodynamic implications

Mesozoic granitoid rocks represent a volumetrically component of the Northeastern (NE) China and preserve useful information about the tectonomagmatic history of this region. The Sankuanggou intrusion associated with skarn Fe-Cu deposit in the Duobaoshan ore field within NE China primarily consists of granodiorite with minor alkali-feldspar granite and diorite, which intrudes the Ordovician Duobaoshan Formation in the region. Zircon LA-ICP-MS U–Pb geochronology and whole-rock geochemistry, and Sr-Nd-Pb-Hf isotope analysis were performed on the Sankuanggou intrusion to investigate the petrogenesis and geodynamic implications. Zircon U–Pb dating of magmatic zircons from the granodiorite rock suggests that the intrusion was emplaced in the Early Jurassic (177±1 Ma). Geochemically, it belongs to the metaluminous to slightly peraluminous high-K calc-alkaline I-type granitoids with a narrow range of SiO2 concentration (65.73–67.33 wt.%), high Ba, Sr, LREE and LILE contents and low abundance of Rb, Y, HREE and HFSE. All of these studied samples have homogeneous initial isotope traits with (87Sr/86Sr)i ranging from 0.70415 to 0.70423, εNd(t) of +3.6 to +4.0, (206Pb/204Pb)i=17.933–18.458, (207Pb/204Pb)i=15.520–15.587 and (208Pb/204Pb)i=37.523–38.087, and zircon εHf(t) values varying from +4.8 to +9.9. These results, combined with the previous data, demonstrate that the Sankuanggou granitoids were formed by partial melting of the pre-existing juvenile crust in an extensional regime related to the post-collisional setting following the closure of the CAOB rather than previously proposed continental arc setting related to Paleo-Pacific or the Mongol–Okhotsk subduction, although their potential influence should not be dismissed.

SHRIMP zircon U–Pb dating and Hf isotope analyses of the Muniushan Monzogranite, Guocheng, Jiaobei Terrane, China: Implications for the tectonic evolution of the Jiao–Liao–Ji Belt, North China Craton

The Paleoproterozoic granitoid rocks in the Jiaobei Terrane, which lies in the southern part of the Jiao–Liao–Ji Belt (JLJB) in China, provide valuable information on the tectonic and evolutionary history of the North China Craton (NCC). SHRIMP U–Pb dating of zircons from the Muniushan Monzogranite in the Guocheng gold deposit area, as well as in situ Hf isotope analyses of the zircons, were used to constrain the ages of emplacement and metamorphism, and to understand the petrogenesis of the granite. The geochronological data show emplacement ages of ca. 2.1Ga and metamorphic ages of ca. 1.85Ga. The magmatic cores of the zircons have εHf(t) values of −0.51 to 4.00 (average 1.98) and depleted mantle two-stage model ages (TDM2) of 2444–2703Ma (average 2563Ma). The rocks show the same trends of crustal evolution as ca. 2.5Ga tonalite–trondhjemite–granodiorite (TTG) rocks in the Jiaobei Terrane, indicating that they were formed by the remelting of those TTG rocks. The Muniushan Monzogranite is part of a linear belt of ca. 2.2–2.0Ga Paleoproterozoic granitoids within the JLJB. The granitoids are A-types, and are accompanied by coeval mafic intrusions and bimodal volcanic rocks, thus indicating widespread extension along the JLJB in the period 2.2–2.0Ga. The ca. 1.85Ga metamorphism was related to the collision of the Longgang and Nangrim blocks. The Paleoproterozoic evolution of the NCC constrains the crustal architecture and the source of Mesozoic granitoids in the Jiaobei–Sulu region. Inherited zircons of Paleoproterozoic age are widespread in the Mesozoic granitoids. In addition, Paleoproterozoic ages have been reported from lower crustal granulite xenoliths in the Sulu Orogen, and the southeastern margin of the NCC mirrors the evolution of the JLJB. It is probable, therefore, that NCC lower crust underlies the Sulu Orogen, and that the NCC was the main source for the Mesozoic granitoids in the Jiaobei–Sulu region. This view is consistent with the appearance of inherited zircons of Archean and Paleoproterozoic age on both sides of the orogen, normal δ18O values of magmatic zircons, and Paleoproterozoic to Archean two-stage Hf model ages for the granitoids.

Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians)

AbstractU–Pb Sensitive High‐Resolution Ion MicroProbe (SHRIMP) dating of zircon in combination with (U–Th)/He dating of zircon and apatite is applied to constrain the emplacement and exhumation history of the youngest granitic rocks in the Western Carpathians collected in the Central Slovakian Neovolcanic Field. Two samples of diorite from the locality Banky, and granodiorite from Banská Hodruša yield the U–Pb zircon concordia ages of 15.21 ±0.19 Ma and 12.92 ±0.27 Ma, respectively, recording the time of zircon crystallization and the intrusions’ emplacement. Zircon (U–Th)/He ages of 14.70 ±0.94 (Banky) and 12.65 ±0.61 Ma (Banská Hodruša), and apatite (U–Th)/He ages of 14.45 ±0.70 Ma (diorite) and 12.26 ±0.77 Ma (granodiorite) are less than 1 Myr younger than the corresponding zircon U–Pb ages. For both diorite and granodiorite rocks their chronological data thus document a simple cooling process from magmatic crystallization/solidification temperatures to near‐surface temperatures in the Middle Miocene, without subsequent reheating. Geospeedometry data suggest for rapid cooling at an average rate of 678 ±158 °C/Myr, and the exhumation rate of 5 mm/year corresponding to active tectonic‐forced exhumation. The quick cooling is interpreted to record the exhumation of the studied granitic rocks complex that closely followed its emplacement, and was likely accompanied by a drop in the paleo‐geothermal gradient due to cessation of volcanic activity in the area.

Corrigendum: Earth's first stable continents did not form by subduction.

This corrects the article DOI: 10.1038/nature21383.