Granitoid Complex Research Articles

Magnetotelluric array in the central Finnish Lapland II: 3-D inversion and tectonic implications

The northern part of the Fennoscandian shield records several Palaeoproterozoic phases of rifting, crustal shortening, metamorphism and structural reactivation. The geologic history has left behind a complex crustal geoelectric structure as evidenced by magnetotelluric (MT) data from the central Finnish Lapland acquired in the context of the MaSca-project in 2014. The data are characterized by strong 3-D effects such as high phase tensor skew values and anomalous induction vectors. Interestingly, however, at the same time a dominant E-W principal direction from the phase tensor data can be inferred. 3-D conductivity models derived using the ModEM code display high crustal conductance (> 10,000 S) in the vicinity of the Central Lapland Greenstone Belt, the Peräpohja Belt and the Kuusamo Belt. Conductors in the northern and southern part of the study area are separated by a resistor coinciding with the Central Lapland Granitoid Complex. A remarkable feature is an arc-shaped conductor inside the northern part of the Central Lapland Granitoid Complex, which continues into the Central Lapland Greenstone Belt in the north. The conductor is associated with a major induction vector anomaly and also coincides with extreme responses (negative phase tensor principal values), which are discussed in an accompanying paper. The conductive structures in the models are interpreted as deeply buried graphite and sulphide bearing metasedimentary rocks or as reactivated Archaean shear zones. The revealed geoelectric structures also partly correlate with seismic reflection and other geophysical data from the area. A possible explanation for the observed pervasive E-W principal direction of the phase tensor data could be the aulacogen (failed rift) suggested in recent tectonic evolution models.

Paleoproterozoic granitoids of the Don terrane, East-Sarmatian Orogen: age, magma source and tectonic implications

The Pavlovsk complex granitoid plutons and associated gabbro-diorites are confined to the East Sarmatian Orogen (ESO) that compose the western part of the junction between the Sarmatian and Volgo-Uralia segments of the East-European Craton. The plutons form the largest batholiths with an area of up to 2000 km2 and a total length of more than 330 km. The conditions for their emplacement and crystallization, sources of melts and tectonic position remain debatable. Here we present new results on geochronological, geochemical and isotope- data on the Pavlovsk complex granitoids and associated gabbro-diorites and discuss these in conjunction with previously published data. The geochemical characteristics, such as elevated concentrations of Mg, K, Sr and Ba (usually > 500 ppm), ratios (La/Yb)N (25–73) and Sr/Y (23–223) in the absence of Eu anomalies of the Pavlovsk-type plutons are similar to high-Mg Archean granites. Zircon U-Pb data indicate the formation of the granitoids and associated gabbro-diorites during a short span at 2060–2075 Ma. The magma emplacement occurred in a transitional environment (from late-collisional to post-collisional) after the collision of two crustal segments (Sarmatia and Volgo-Uralia). We identify a significant ancient Archean continental crust in the magma source for the northern plutons and leucogranites including the presence of Archean zircon cores and Archean model TNd (DM) ages. All the plutons are dominated by a mixed magma source generated through the interaction of juvenile lower crustal and enriched mantle components. The Nd isotopic composition of the studied plutons is significantly more radiogenic as compared with the host Archean rocks (εNd (T) from +0.3 to −4.2). Our geochemical data suggest that that the mantle source of the gabbro-diorites and granitoids were modified (metasomatized) by subduction-related processes. The contrasting geochemical and isotopic features of the granitoids in the ESO as compared to the Pavlovsk granitoids suggest different sources for the parent melts.

Full-Scale Crustal Interpretation of Kokkola\u2013Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.

Geochemical signatures of manganese ores around Barbil, Noamundi-Koira basin, Singhbhum Craton, Eastern India

ABSTRACT Palaeoproterozoic Noamundi – Koira Basin (NKB), on the western margin of the >3.0 Ga old Singhbhum Granitoid Complex, forms a horseshoe-shaped synclinorium. Volcanic rocks belonging to Bonai volcanic suite (BVS) occur outside and all-around the NKB that consists of three distinct but conformable stratigraphic units viz; Lower Shale Formation, Middle Banded Hematite Quartzite (BHQ) and/or Banded Hematite Jasper (BHJ) and Upper Shale Formation. The layered, lensoid, lateritoid, and vein type Mn-ore bodies are associated with the Upper Shale Formation. Compositions of the Mn ores widely variable values, SiO2 (1.00%- 16.40%), Al2O3 (1.62%-17.20%), Fe2O3 (10.55%-63.28%), MnO (31.75% −75.72%), Ni (25–260 ppm), Co (78–350 ppm), Zn (82–509 ppm) and Cr (13–187 ppm) clearly reflect their submarine hydrothermal source. Litho-stratigraphic considerations also lead to suggest that BHJ/BHQ and Mn-ore deposits of NKB are genetically related to the BVS. It is suggested that manganese and iron ores were deposited in a (? Forearc) basin associated with the mature island arc or active continental margin setting. FeO and MnO were added to the basin by hydrothermal solutes produced during the Bonai volcanism. BHQ/BHJ precipitation took place under a low redox potential while Mn-ore deposition is a consequence of the high redox potential.

Geochemical characteristics and metalogeny of Herzin granitoid complexes (Eastern Kazakhstan)

Geochemical characteristics and metalogeny of Herzin granitoid complexes (Eastern Kazakhstan)

Petrography, geochemistry, and geochronology of the Sc-enriched Kiviniemi ferrodiorite intrusion, eastern Finland

The Kiviniemi mafic intrusion, near the eastern margin of the Paleoproterozoic Central Finland Granitoid Complex, is both spatially and temporally associated with post-kinematic Fe-Ti-P-enriched Svecofennian orogenic mafic magmatism. The main rock types in this small (~ 15 ha) intrusion are garnet-bearing fayalite ferrodiorite, leucoferrodiorite, ferromonzodiorite, and pyroxene diorite. The garnet-bearing fayalite ferrodiorite and leucoferrodiorite contain 50–281 ppm Sc, 275–5600 ppm Zr, and 58–189 ppm Y (n = 42), delineating a mineralized deposit some 2.5 ha in extent. Overall, these rocks show an evolved (iron-enriched) tholeiitic character; low values of Ni (< 20–40 ppm), Cr (< 20 ppm), and Cu (< 20–80 ppm); and high contents of Zn (213–700 ppm). The rock-forming minerals in the ferrodioritic rocks are (ferro)hedenbergite, plagioclase (~ An40), ferropargasite and ferroedenite, almandine garnet, and fayalite (Fo1–4). Accessory minerals include zircon, ilmenite, fluorapatite, biotite, pyrite, pyrrhotite, potassium feldspar, grunerite, and clinoferrosilite. Some relict cumulate textures have been preserved, but primary magmatic features have largely been overprinted by strong recrystallization and corona formation. The main carriers of Sc are amphibole, clinopyroxene, and apatite. The remarkably strong enrichment of Sc in ferromagnesian silicates and apatite, rather than in specific Sc-minerals, implies magmatic enrichment. Post-kinematic mafic intrusions in central Finland constitute a bimodal association with co-existing granitoid counterparts. The Kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts, indicative of contemporaneity of the two magmas. This conclusion is in accord with the coincident U-Pb zircon ages for the ferrodiorite, at 1857 ± 2 Ma (multigrain ID-TIMS) and the megacrystic granite, at 1860 ± 7 Ma (single-crystal LA-MC-ICP-MS). The initial εNd value of the ferrodiorite and the granite are + 0.1 and − 2.5, respectively. These Nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some Archaean crustal material into the granite in the course of magmatic evolution. The resource estimation calculated for Kiviniemi intrusion by using 40 g/t Sc cut off value is 13.4 Mt of rock with an average grade of 162.7 g/t scandium, 1726 g/t zirconium, and 81 g/t yttrium.

Petrochemical and isotopic evidences for multiple sources involving in the generation of post-collisional granitoid complex: A case study of the Mangling complex from the eastern Qinling orogen, China

Granitoids widely occur in collisional orogens, which are a critical target for probing the nature of deep crust and mantle-crust interaction. In this study, we present an integrated study of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock geochemistry, for the Mangling complex in the eastern Qinling orogen. The Mangling complex is composed mainly of monzogranites with some mafic microgranular enclaves (MMEs) and subordinate diorites. Zircon U–Pb dating yields identical crystallization ages of 141–143 Ma for the diorites and monzogranites, suggesting that they are formed simultaneously and belong to post-collisional magmatism. The Mangling diorites have relatively low SiO2 but high MgO, Cr and Ni contents and Mg# values. They are characterized by enrichment of large ion lithophile elements (LILEs) but depletion of high field-strength elements (HFSEs). They show initial Sr isotope (ISr) of 0.706491 to0.706622, εNd(t) of −10.82 to −10.38, and zircon εHf(t) of −10.8 to −7.9 and δ18O values of 6.48–7.18‰. These geochemical features indicate that the diorites were derived from an enriched lithospheric mantle source. In contrast, the Mangling monzogranites have high silica and low Mg# values. They exhibit more variable isotopic compositions: Isr = 0.707191–0.708310, εNd(t) = −13.74 to −8.10, and zircon εHf(t) = −17.2 to −7.0 and δ18O = 5.83–7.08‰. Some inherited zircon cores have been observed in the monzogranite samples with U–Pb ages of 1773–1791 Ma. These inherited cores have δ18O values of 5.42–5.69‰ and εHf(t) values of −7.8 to −5.9. Their U–Pb ages and Hf–O isotopic compositions are similar to those of the Xiong'er Group in the southern margin of the North China Craton (NCC), which have underthrusted the juvenile (in terms of isotopes) Kuanping unit where the Mangling complex has been emplaced. Isotopic modeling shows that the monzogranites were likely produced by fusion of the Kuanping unit and Xiong'er Group, with the involvement of enriched mantle-derived materials. The crustal rocks as well as the enriched sub-lithospheric mantle were both contributed to the Mangling complex, and some of the crustal components show more juvenile isotopic fingerprints. Therefore, it is not a mandatory to involve a depleted juvenile mantle-derived component to recognize mantle-crust interaction during granitoid generation, especially in which the preceded juvenile arcs were superimposed on ancient crust and/or enriched mantle. The ‘inverse’ isotopic compositions in crust and mantle components imply that a detailed petrochemical and isotopic study should be conducted to address the issue about crust-mantle interaction during granitoid generation.

U–Pb (zircon) geochronologic constraint on tectono-magmatic evolution of Chaur granitoid complex (CGC) of Himachal Himalaya, NW India: implications for the Neoproterozoic magmatism related to Grenvillian orogeny and assembly of the Rodinia supercontinent

Many elongated, lenticular intrusive granitoids of various ages are scattered within the Lesser Himalayan metamorphic belt, all along the ~ 2500 km length of the Himalaya. The Neoproterozoic Chaur granitoid complex (CGC) of Chaur area is characterized by foliated and non-foliated peraluminous granites occurring as an isolated granitoid body within the Jutogh group. In this work, we present the whole-rock geochemical data of six samples and U–Pb (zircon) geochronology of two different granites of the CGC and one granitic gneiss sample of Jutogh group from Himachal Pradesh of NW Himalaya. Our newly obtained results of U–Pb (zircon) geochronological age populations from all granitoid sample yield age between 766 and 1080 Ma with few younger phases and older inherited ages. We obtained U–Pb (zircon) ages from two sample of the CGC, out of which one gives the two prominent age spectra for 206Pb/238U with weighted mean age of 826 ± 4.97/9.74 Ma, MSWD = 0.65, n = 8) and 868 ± 6.21/12.17 Ma, MSWD = 1.28, n = 7). Similarly, another granite of CGC gives the weight mean age of 929 ± 6.48/12.70 Ma (MSWD = 1.28, n = 11). The granitic gneiss of the Jutogh group also gives two prominent age spectra for 206Pb/238U, with weighted mean age of 861 ± 8.27/16.21 Ma (MSWD = 0.31, n = 10) and 932 ± 10.0/19.6 Ma (MSWD = 1.57, n = 8). The whole-rock geochemical data show calc-alkaline composition of all six samples and suggest a subduction-related accretion setup. The depletion in the Nb, Sr, P and Ti in CGC indicates a magmatic arc type magma. U–Pb (zircon) ages of all three samples have a similar phase of crystallization and we defined as ~ 930 Ma age of crystallization of CGC. The whole-rock geochemical data suggest that all the three samples possibly came from the same magma source during the Neoproterozoic magmatic events in the northern marginal part of the Indian plate. It is envisaged that the unknown microcontinents present in the northern margin collide with the Indian plate and the subduction process coincides with the onset of the Grenvillian orogeny during the Neoproterozoic. The extension of these minor collision orogen may have been connected with Lhasa, Trim as well as Greater India blocks. In this collisional process, the crustal melt was generated and intruded in the form of CGC within the pre-existing Paleoproterozoic crust of Indian plate. The whole process indicates that the subduction of unknown microcontinent under the Indian plate may be correlated with the Grenvillian orogeny and formation of the Rodinia supercontinent during Neoproterozoic.

Gabbroides of Peterman island (West Antarctica). First information on geochemistry

Peterman Island is located in the archipelago of the Wilhelm Islands on the west coast of the Antarctic Peninsula (Graham Land). It is composed of gabbroids and granitoids of the Andean complex, which formed almost 100 million years later than the volcanic group of the Antarctic Peninsula. To clarify their genesis and geodynamic conditions of formation, gabbroids of the Andean complex are of particular interest, since the petrological models of their formation are well developed. Gabbroid intrusions comprise small bodies that are widespread along the Antarctic Peninsula. Among them stand out olivine gabbros, normal gabbros, norites and hornblende gabbros. Also are found small bodies of melanogabbro-pegmatites and intramagmatic dykes, that are associated with the manifestations of ore mineralization of magnetite, ilmenite and sulfides. For this reason, they are of interest for both the minerals search and for solving the question of their genesis. To this end, we performed geochemical studies of Peterman Island gabbroids. Gabbroids of Peterman Island are represented by amphibolized medium-grained gabbro with hypidiomorphic texture. Among them, xenoliths of thinly stratified gabbroids 3 × 8 m in size were found, which are characteristic of stratified intrusions, for example, Stillwater, Bushveld, etc. Gabbroids of Peterman Island have low content of silica and potassium and according to the petrochemical characteristics correspond to peridotite gabbro. They have low contents of Cr, Ni, V and high strength lithophilic Y and Nb elements. Gabbroids have been crystallized from basic magma, differentiated in the intermediate crustal magma chambers. Positive anomalies of Sr, Eu, and Ti in the multielement diagrams and positive anomalies of europium Eu/Eu* suggest the accumulation of plagioclase and apparently, ilmenite in the magmatic chamber. The primary magma source for gabbroids was probably the primitive mantle (PM). Gabbroids are contaminated with crustal matter. This contamination is probably due to their regressive metamorphism, caused by the introduction of later intrusions of Andean complex granitoid. Finely layered xenolithic gabbroids do not differ from other homogeneous gabbros of Peterman Island in terms of chemical composition.This xenolith most likely represents a part (fragment) of the wall of the magma chamber in which the differentiation of the initial main magma took place. According to the obtained geochemical data, a wide range of compositions of the Andean complex gabbroids formed as a result of crystallization differentiation of magma melted from rocks of the composition of the primitive mantle (PM) in crustal magma chambers, which also resulted in the accumulation of ore elements — V, Co, and Cu in the residual magmatic melts.

Physico-chemical conditions of crystallization and ore-bearing of granitoids of the border zone of the Southern and Middle Urals

The geological structure of the border zone of the Southern and Middle Urals (Kunashak area), as well as the composition and conditions for the formation of granitoid complexes developed here, whose age varies from Devonian to Triassic, is briefly considered. Thermobarometric studies of quartz of medium-grained biotite granites showed that in the age range ― Poletaev (D3), Shalkar (P1), Sultayev (P1), and Yugokonev (P1T2) complexes ― water pressure during the formation of melt inclusions was 2.22.7, 2.53.0, 3.94.4 and 3.64.5 kbar, and the crystallization temperature of rocks ― 800840, 900940, 840900 and 940980С. This indicates a consistent increase in the crystallization depth of the granite massifs in this series from the hypabyssal-subsurface to the hypabyssal-abyssal facies. At the same time, in the melt-fluid inclusions, a decrease in fluid volume is noted ― 14.216.8, 8.114.3, 6.27.9 and 4.36.8 vol.%, Water concentrations ― 3.74.8, 2.13.9, 1.82.3 and 1.31.9 wt.% as well as chlorine ― 0.040.08, 0.030.07, 0.030.06 and 0.020.05 wt.%.&#x0D; It is shown that the granitoids of the Sultayev and Yugokonev complexes are very promising for tantalum, niobium, tungsten, molybdenum and beryllium. Of the greatest interest in rare-metal mineralization are the leucocratic aplitic and greisen granites of the final phases of the formation of intrusions. The level of erosional slice of various structural and formation zones of the Kunashak area relative to each other has been established.

Physico-Chemical Conditions of Crystallization and Ore Potential of Granitoids from the Boundary Zone of the Southern and Middle Urals

Abstract—The paper briefly considers the geological structure of the boundary zone between the Southern and Middle Urals (Kunashak area), as well as the composition and conditions of formation of Devonian–Triassic granitoid complexes developed here. Thermobarometric studies of quartz from medium-grained biotite granites showed that the Poletaev (D3), Shalkar (P1), Sultayev (P1), and Yugokonev (P1–T2) complexes were formed at water pressure of 2.2–2.7, 2.5 –3.0, 3.9–4.4 and 3.6–4.5 kbar and crystallization temperature of 800–840, 900–940, 840–900 and 940–980°С, respectively. This indicates a systematic increase in the crystallization depth of the granite massifs in this series from the hypabyssal–subsurface to the hypabyssal-abyssal facies. At the same time, the melt–fluid inclusion study revealed a decrease in fluid volume (–14.2–16.8, 8.1–14.3, 6.2–7.9 and 4.3–6.8 vol %), and concentrations of water (–3.7–4.8, 2.1–3.9, 1.8–2.3 and 1.3–1.9 wt %) and chlorine (–0.04–0.08, 0.03–0.07, 0.03–0.06 and 0.02–0.05 wt %) in this series. It has been shown that the granitoids of the Sultayev and Yugokonev complexes are very promising for tantalum, niobium, tungsten, molybdenum, and beryllium. The leucocratic aplitic and greisen granites of the final phases of the intrusions are most promising for the rare-metal mineralization. The erosion level of different structural and formation zones of the Kunashak area relative to each other has been established.

The Mesoarchean Tonalite-Trondhjemite-Granodiorite Associations of Eastern Sarmatia: Age and Geological Setting

Two episodes of Mesoarchean magmatism (ca. 2.95 and 3.03 Ga) have been recognized within the Kursk domain of Eastern Sarmatia. They led to the formation of the Saltyki granitoid complex, which is similar in composition to the granitoids of the typical tonalite–trondhjemite–granodiorite (TTG) associations of Archean cratons. These episodes correlate well with the previously determined emplacement start and completion dates of the Mesoarchean TTG associations in the Middle Dnieper and Azov domains of the Ukrainian shield. This confirms the assumption that the Kursk domain is a part of the joint Mesoarchean nucleus of Eastern Sarmatia. The geochemistry and Sm–Nd isotope characteristics of the Mesoarchean Saltyki granitoids suggest that their protoliths were the more ancient TTG-gneisses.

Mineral compositions of magmatic dikes cutting across Khe Phen granites (Huong Tra, Thua Thien Hue, Central Vietnam)

The Khe Phen granite quarry located in Huong Tra district (Thua Thien Hue province) has been confirmed as a part of the Ba Na granitoid complex (G/K2bn), mostly composed of two-mica granite and porphyritic granite. Field survey data show that the granites here are cut across by five distinct narrow dikes (about 50–70 cm wide), including granite pegmatite, granite aplite, aplite, granodiorite, and lamprophyre. The mineral compositions of the granite pegmatite and aplite dikes are similar to those of the host granite, which is mainly comprised of quartz (27–35%), orthoclase (45–58%), plagioclase (4–15%), biotite (1–3%), and a few opaque minerals. Meanwhile, the granodiorite and lamprophyre dikes are melanocratic and compositionally much more mafic, particularly lamprophyre, evidenced by a presence of hornblende (50–55%), plagioclase (33–40%), quartz (3–15%), calcite (5–17%), etc. The origin and emplacement age of the latter dikes have not been reported so far and thus, are needed for further studies based on geochemical and isotopic data.

Timing of Lithospheric Extension in Northeastern China: Evidence from the Late Mesozoic Nianzishan A-Type Granitoid Complex

New zircon U-Pb dates obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), whole-rock geochemical data and Sm-Nd and Rb-Sr isotopic data are presented for miarolitic alkaline granites, porphyritic syenite and rhyolites of the Nianzishan A-type granitoid complex (NAGC) in the Great Xing'an Range-Songliao Basin in Northeast (NE) China. New crystallization ages of 112.95±0.93 and 114.1±1.71 Ma for granite and 118.6±0.51 Ma for porphyritic syenite were determined by high-precision LA-ICP-MS. The eNd(t) of the rocks range from +1.85 to +2.06, with Nd model ages (TDM1) from 671 to 821 Ma, indicating that the NAGC originated from juvenile source rocks and exhibits geochemical characteristics of A1-and AA-type granite which formed in an extensional setting. We attribute the magmatism to regional extension and lithospheric thinning caused by the subduction of the western Pacific Plate about 120 to 100 Ma.

Tectonic Position of the Neoproterozoic Gabbro-Ultrabasite and Gabbroid Complexes of the Bayan Nuur Block of the Songino Ledge, Central Asian Orogenic Belt

The Early Caledonian folded region of the southern framing of the Siberian platform, between the Zavkhan and Tuva-Mongolian terranes, contains blocks of the Songino ledge crystalline rocks. In the Bayan Nuur block (southern part of the Songino ledge), the Neoproterozoic (890–780 Ma) Bayan Nuur gneiss-migmatitic and Kholbo Nuur metaterrigenous-volcanogenic complexes are distinguished. The zone of their junction is traced by thrust structures, which are reflected in all structural-material complexes of the Bayan Nuur block. There are tectonic slabs in the junction zone. Their peculiarity is the presence of rocks formed before the formation of thrust gabbro-ultrabasite complex and post-thrust gabbroids and gabbro-diorites. The upper limit of the thrust formation interval is defined by Bayan Nuur pluton granitoids having the age of 790 ± 3 Ma and by gabbroids and anorthosites of Onzula tectonic plate giving the ages of 785 ± 3 and 784 ± 3 Ma. The lower limit of the thrust formation interval is determined by the age of the Bayan Nuur ultrametamorphic granitoid complex (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbros and rocks of gabbro-ultrabasite complex possessing clearly manifested thrust-associated structures are recognized in the tectonic slab of the ridge reaching the area of Mount Sant Uul. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID-TIMS). The data indicate that the post-thrust and pre-thrust units cannot be parts of a united layered complex. In turn, pre-thrust units can be considered as paleooceanic fragments in the accretionary structure of the Bayan Nuur block, Songino ledge.

U-Pb zircon age of the pegmatoid of the Muong Lat complex

Muong Lat granitoid complex located in Thanh Hoa province. It is composed of two mica granite, aplite and pegmatoid. Rock forming minerals include mainly K-feldspar, quartz biotite and muscovite. Crystallization age of pegmatite in Muong Lat body is determined by U-Pb zircon LA-ICP-MS method. Most of the analytical results 206Pb age / 238U ranged from 217–238 Ma, average 230 ± 7 Ma. Age results indicate, the existence magma activity in the study area. Three inherited zircon core gaves age of ~2.8Ga, ~2.4Ga, ~1.8Ga, ~400Ma and ~750 Ma, suggesting that the continental crust of the study area including material composition of the Neoarchean, Paleoproterozoic, Paleozoic and Neoproterozoic

New Data on Granitoids of the Kara Gold Ore Cluster (Eastern Transbaikalia): Age, Genesis, and Sources of Material

Abstract —Study of granitoids spatially and genetically associated with gold mineralization within the Kara gold ore cluster has provided a new insight into their genesis, association with ore mineralization, and the sources of their ore material. The regional granitoids associated with gold mineralization are part of two individual complexes. One of them, earlier assigned to the Amanan complex, has an isotopic age of 182.9 ± 2.6 Ma and must be related to the subduction zone that existed on the southern margin of the Siberian continent in that period. Its granitoids differ in age and composition from the granitoids of the Amanan complex and must be separated as an independent taxonomic unit after an additional geological study. The second, Amudzhikan–Sretensk, complex has an isotopic age of 151.7 ± 1.9 Ma and might be related to the collision of the Siberian and Mongol–Chinese continents after the closure of the Mongol–Okhotsk ocean. In geochemistry the granitoids of the Amanan(?) complex correspond to adakites and must be considered melting products of the basaltic layer of the oceanic lithosphere. The granitoids of the Amudzhikan-Sretensk complex are similar in geochemistry to sanukitoids, melting products of subcontinental sources contaminated with continental-crust material. The granitoids of both complexes have high contents of gold and must be considered gold-bearing. In the Amanan(?) complex, adakites are the gold-richest rocks (as estimated from the slab melt composition), which indicates the primary nature of this gold. In the Amudzhikan–Sretensk complex, the highest contents of gold are specific to primitive sanukitoids, melting products of a mantle source with gold signatures. This suggests the primary nature of gold, whose content is determined by the portion of slab melt in the source of the rock material. The presence of adakites and primitive sanukitoids in the regional granitoid complexes indicates the existence of a subcontinental mantle source with gold signatures during the magma generation. The source formed in the subduction zone that existed on the southern margin of the Siberian continent in the Early Jurassic and was remobilized under collision of the Siberian and Mongol–Chinese continents in the Late Jurassic. This source might have controlled both granitoid magmatism and ore mineralization.

The magnetic response of hydrothermal alteration in iron-oxide basement complex, NW Nigeria

Aeromagnetic data was processed in order to delineate structural trends which may be indicative of potential gold mineral traps in the Watari hill gold prospect located between longitude 5°8''and 5°16'', and latitude 11°24'' and 11°30'' in Garin Hawal district, northwest Nigeria. Landsat-8 OLI data was processed to produce colour composite, band ratio and minimum noise fraction images that mapped the hydrothermal alteration zone in the study area. The multispectral images which were displayed in Red-Green-Blue colours were subsequently pan-sharpened to display a greater image resolution of the area. The application of enhancement algorithms such as an analytic signal to the aeromagnetic data over the alteration zone helped in mapping the mafic metavolcanics, felsic metasediments and intruding granitoid complexes. Further reductions using the second vertical derivative and tilt angle derivative filters aided in delineating fracture/faults and the contact zones of the formations, especially the metasediments-metavolcanic boundary that delineates the shear zone. This study showed that the low-angle geologic structure between the metasediments and metavolcanics where hydrothermal alteration is present is likely to accommodate veins of gold mineralization in the study area.

The structure of the Western and Central parts of the Ukrainian schield. Controversial issues

Some controversial issues of composing the Scheme of the fault megablock tectonics of the western half of the Ukrainian shield in connection with the decision of the Interdepartmental Tectonic Committee of Ukraine on drawing up the Tectonic map of the Ukrainian shield at a scale of 1:500 000 have been considered. Geophysical data must be more fully taken into account and used in detail and objectively reflecting both the surface and the deep structure of the Earth’s crust of the shield while drawing up such a Scheme. The latest maps of anomalous magnetic and gravitational fields, a map of the topography of the Moho surface and other materials are given, the analysis of which gives grounds for a number of conclusions on specific structures of the western and central parts of the region. It has been established that the formation of a typical megablock structure of the Ukrainian shield began not earlier than in the Neoarchean. It is difficult unambiguously to establish and trace the investigated Archean fault zones in the part of the shield at the present time, and sometimes practically it is almost impossible, since the Early Proterozoic processes of regional granitization and dynamometamorphism radically changed the composition and structure of the earth’s crust. Geophysical data, supported by geological and tectonophysical observations, clearly record the Proterozoic fault zones, which began to form after 2.5 billion years ago. They cross and deform both the Archean and the Early Proterozoic granitoid complexes, accompanied by intensive diffusion, metasomatism and ore formation. It is proved that the Archean-Early Proterozoic structural-formation complexes (SFC), mapped within the shield, cannot and should not be artificially linked with the Proterozoic fault-megablock structure, which was formed at the end of the Early Proterozoic and superimposed on the SFC. It seems like the dubious and outdated thesis about the subhorizontal-floor structure of the Early Precambrian crust of the Ukrainian shield, according to which the structural floors of the crust are formed one above another by stratometamorphic complexes characterized by a directly proportional relationship of age and the degree of regional metamorphism of the rocks. This concept does not take into account the significant role of horizontal motions of lithospheric blocks, which cause the redistribution and localization of PT conditions in the earth’s crust.

Eocene Granitoids of the Okhotsk Granodiorite Complex (South Sakhalin)

As a result of U–Pb dating by LA-ICP-MS, the age of zircons from granitoids was determined to be 42.1–44.3 Ma for the rocks of first phase and 42.4–42.7 Ma for rhyolites from dikes of the second phase. On the basis of the geochemical studies of rocks, it was found for the first time that granodiorites of the Okhotsk massif (first phase) refer to A2-type granitoids. The most likely explanation of petrogenesis for A2-type granitoids of the Okhotsk complex is the gradual cessation of subduction as a result of deceleration of the Pacific Plate and the transition of the Ozerskii terrane to transform northward motion along the Merei suture zone with a certain role of syn-shear extension. Thus, the data obtained on the age and geochemistry of the Okhotsk complex granitoids suggest the possible time of geodynamic regime change in this area as about 43 Ma ago.