Gneiss Research Articles

Ascent and emplacement controls of mafic magmas in the mid crust − evidence from 3D modelling of basic bodies of the Koperberg Suite, Namaqualand

Abstract The 3D modelling of basic bodies of the Koperberg Suite (1060 to 1030 Ma) and their wall rocks from Narrap Mine illustrates the distribution, geometries and, by implication, processes that determined the ascent and emplacement of the mantle-derived mafic magmas into the partially-molten, mid-crustal granite-gneiss sequence of the Okiep Copper District in Namaqualand. The lens-like, discontinuous geometry of basic bodies suggests the transfer of the mafic magmas as self-contained, buoyancy-driven hydrofractures. The presence of both shallowly-dipping, foliation-parallel sills and subvertical lenses in zones of steep foliation development, so-called steep structures, indicates an emplacement of the mafic magmas under low deviatoric stresses and irrespective of the regional stress field. Instead, the emplacement of the mafic magmas parallel to pre-existing anisotropies (tectonic fabrics or lithological contacts) highlights those differences in tensile strength and fracture toughness parallel to or across anisotropies determined the propagation of the magmas. This also accounts for the common occurrence of basic bodies in steep structures in which the vertical gneissosity promoted the buoyancy-driven ascent of the mafic magmas. On a regional scale, the mechanical stratification of the subhorizontal, sheet-like granite gneisses and interlayered metasediments exerted important controls on the ascent of Koperberg Suite magmas. The preferential emplacement of basic bodies in schist and gneiss units suggests that the lower rigidity of the ductile wall rocks facilitated magma emplacement through a combination of viscous wall-rock deformation and fracture blunting that led to the arrest of the magma-filled hydrofractures. Multiple intrusive relationships of successively emplaced magma batches suggest that later magmas reutilised earlier established magma pathways, particularly in steep structures. High-rigidity lithologies, such as the massive Springbok Quartzite, in contrast, only allowed for smaller fracture apertures and limited dilation, resulting in the pinching of basic bodies and rather stringer-like geometries. It is conceivable that the higher fracture toughness of the quartzite may also have prevented propagation of the mafic magmas through the Springbok Quartzite and, instead, led to the ponding of basic bodies below the metasediments. The geometry and structural and lithological controls of basic bodies at Narrap Mine are similar to Koperberg Suite intrusions documented from many of the other mine workings in the Okiep Copper District. This suggests similar underlying emplacement controls of the cupriferous rocks, which can be extrapolated on a regional scale and that may guide exploration.

Unraveling the early Paleozoic tectonic history of the South Qinling Belt: Evidence from geochronology, geochemistry, and Sm-Nd isotopes of meta-sedimentary rocks

The determination of the petrogenesis and age of meta-sedimentary rocks plays a crucial role in enhancing our understanding of regional tectonic evolution. In this study, we conducted comprehensive geochemical, zircon U-Pb, and Sm-Nd isotope dating analyses of the biotite gneiss of the Changjiaoba group to investigate the tectonic evolution of the South Qinling belt. The age range of detrital zircons from the biotite gneiss was estimated to be 446–2798 Ma, with prominent peaks observed at approximately 454, 660, 790, 1998, and 2770 Ma. The earliest data indicate that the final sedimentary age of the Changjiaoba group corresponds to the late Ordovician. The protoliths of the biotite gneiss were argillaceous rocks, classified as parametamorphic rock. The chondrite-normalized rare earth element (REE) diagram exhibited right-inclined patterns with negative Eu anomalies. The upper continental crust (UCC)-normalized trace element patterns exhibited insignificant enrichment of Ba, Sr, and K; negative anomalies of Hf, Th, U, and Ti; and depletion of Cr and Ni. Geochemical indicators suggest that the precursor sediments are felsic rocks that have undergone moderate weathering. The εNd (t) values of the biotite gneiss (−10.2 to −3.4) suggest a mixture of a few juvenile crust components and abundant ancient material, indicating an arc-realted setting. Collectively, our findings suggest that the primary material sources of the Changjiaoba group were predominantly the North Qinling belt during the Paleozoic and the Yangtze block during the Neoproterozoic. The tectonic setting discrimination diagrams imply that the biotite gneiss were formed in an active continental margin environment, supporting the notion of the northward oceanic subduction during the early Paleozoic. In conclusion, our study provides valuable insights into the composition and deposition history of the Changjiaoba group in the South Qinling belt, contributing to our understanding of the regional geology.

INTEGRATED GEOLOGY AND ENGINEERING GEOLOGICAL PROPERTIES OF LATERITIC SOIL AT ITA – ONIYAN AREA, SOUTHWESTERN NIGERIA

This study investigates the geology and the engineering geological properties of lateritic soil of Ita – Oniyan area of Ondo state, Southwestern, Nigeria. Twenty soil samples were collected from twenty test pits at depth of about 1.0m and were air–dried for two weeks and, thereafter were subjected to laboratory analyses which include natural moisture content, particle size analysis, consistency limits, linear shrinkage, specific gravity, standard compaction, shear strength, and unsoaked California Bearing Ratio (CBR) tests. Results showed that the natural moisture content of the soils ranged from 7% to 28%, the liquid limit ranged from 30.2% to 69.1%, the plastic limit ranged from 17.5% to 43.4%, while the plasticity index ranged from 8.6% to 36.9%. The soils were grouped into CL (low plasticity), CI (medium plasticity), and CH (high plasticity). Linear shrinkage ranged from 3.7% to 11.9%. Grain size analysis gave coarse contents varying from 30.4% to 98.1% and fine contents from 27.4% to 86.9%. The unsoaked CBR ranged from 18% to 72%, for compaction, MDD ranged from 1482kg/m3 to 1981kg/m3, and OMC from 13.7% to 32.5%. Geologic mapping showed that the major rock types in the study area were biotite gneiss, charnockite, quartzite and granite parent rocks from which the soil was formed. The soil samples are made up of lateritic soil with high clay content mainly illite, since the soil samples in the study area have poor engineering properties therefore the soil needed to be stabilized to improve their engineering properties.

Impact of different land use types on groundwater quality in Ibadan, Nigeria

Studies of comparative analysis of groundwater quality under different land use types are scarce, and such studies remain critical in achieving sustainable water resource management. This study used suitable multi-statistical techniques alongside the Water Quality Index (WQI) to assess and compare the effects of residential, industrial, and agricultural land use types on groundwater quality in a region of Ibadan, Nigeria, primarily underlain by banded gneisses. The study also established the appropriateness of the water samples for human consumption. Thirty water samples, consisting of 10 samples each from three different land use types, were randomly collected and subjected to established methods of analysis for nine physicochemical parameters. The results from the Kruskal-Wallis test and Dunn's test showed that average concentration levels of pH, total hardness, calcium, magnesium, chloride, bicarbonate, nitrate, and sulfate in the tested water samples were significantly different (p-value ≤ 0.05) among the three land use types investigated. The results also suggested that where poor environmental conditions are common, residential land use type could pose a greater risk to groundwater quality than either agricultural land use or industrial land use. At least, a three-factor model was suggested and explained 63.4 %, 70.2 %, and 91.6 % of the total groundwater quality variation under the residential, agricultural, and industrial land uses respectively using Kaiser's varimax rotation method. In addition, the WQI results showed that 100 % of the water samples from industrial and agricultural land uses were appropriate for drinking but only 20 % of the water samples from residential land use were. The findings indicated that differing land use types affect groundwater quality in different ways. It was thus recommended that an improved sanitation system should be provided by the authorities to help reduce groundwater pollution in the residential area. This can help toward achieving water security mentioned in the African Union's Agenda 2063.

Mesoarchean synchronous emplacement of TTG gneisses and potassic granitoids in the Nyabessane granite-greenstone terranes, NW Congo Craton (southern Cameroon): Zircon U[sbnd]Pb geochronology, petrogenesis and tectonic implications

Archean granitoids played key roles in the generation and differentiation of the Archean continental crust and provide clues to understand crustal processes in the early Earth. Abundant Mesoarchean granitoids were emplaced in the Nyabessane granite-greenstone terrane (NGB), part of the Ntem Complex of the Northwest Congo Craton. They include charnockites, tonalite-trondhjemite-granodiorite (TTG), granitic and monzogranitic gneisses. Here, we present a geochemical and geochronological (zircon LA-ICP-SF-MS UPb) study of these granitoids to determine their petrogenesis and to better constrain the crustal evolution of the Ntem Complex. Field and petrographic observations indicate that most of these granitoids underwent extensive metamorphism and deformations, associated with anatexis. Zircon UPb dating results suggest that the charnockite, TTGs and granitic gneisses, and monzogranites have emplacement ages of 2910 ± 11 Ma, 2870–2865 Ma and 2852 ± 31 Ma, respectively. The charnockites have low SiO2 (55–58 wt%) and high Al2O3 (16–18 wt%), CaO (7–8 wt%) and MgO (∼ 4.5 wt%) content with Mg# ∼ 54, and exhibit magnesian, metaluminous characteristics of the Cordilleran granitoid-type formed in magmatic arc. The 2.87–2.86 Ga TTG gneisses are silica-rich (55–58 wt% SiO2), sodic (3–5 wt% Na2O, Na2O/K2O = 1–3), with HREE-depleted, and display the typical Archean medium- to low pressure TTG geochemical features groups. Their chemical compositions are characteristics of TTG-like melts derived from the partial melting of hydrated low- to high-K metabasic/thickened lower crust at various depths followed by magmatic differentiation during ascent. When compared to the TTG gneisses in the NGB, the ∼2.87 Ga granitic gneisses are K-rich, and high Gd/Yb, Th/Yb, Th/Nb, Sr/Y and La/Yb ratios but lower MgO, Yb, V, Y, Cr, Ni and Sr contents, matching typical Archean hybrid and potassic granitic rocks. We propose that the granitic gneisses were derived from the intra-crustal melting of a pre-existing felsic crust. The granitic and the TTG gneisses were generated contemporaneously through the same magmatic event at ∼2.87–2.86 Ga. The ∼2852 Ma monzogranitic gneisses are ferroan and metaluminous rocks, and show LREE enrichment with strongly fractionated REE patterns and positive Eu anomalies. Geochemical features, together with the presence of ∼2.9 Ga-old inherited zircon grains are consistent with the remelting of Mesoarchean granitoids. Considering the petrogenetic, regional geological and geochronological data, the Mesoarchean granitoid magmatism of the Ntem Complex was likely generated via complex transitional geodynamic regimes involving subduction and accretion processes.

Study on correlation between TBM tailings gradation and granitic gneiss surrounding rock stability

In the process of double-shield TBM excavation, it is difficult to directly observe and test the characteristics of the surrounding rock. In this paper, the screening test of the different type tailings in the wet state was carried out to obtain the gradation curve and curve evaluation index. Combining with the excavation parameters and the surrounding rock characteristics of the tailings, a comprehensive analysis was carried out to establish the evaluation system among with the tailings gradation characteristics, lithology characteristics and excavation parameters. The results showed that: Sparsely fissured rock: the tailings are extensional fractures, the tailings gradation curve is inverse “S” type and gentle, and the evaluation index value: (1.50 > lg(Cu) > 1.35), (1.90 > Cc > 1.10). Broken surrounding rock: the curve is “L” type and steep, the content of coarse particles is much more than that of fine particles and (1.10 > lg(Cu) > 1.00), (2.60 > Cc > 2.40). Fractured rock: the curve is “Step” type, the tailings particles lack the middle particle size, the minerals are mostly weathered, (2.15 > lg(Cu )> 1.95), (0.09 > Cc > 0.07). The research results have good applicability to the surrounding rock stability evaluation of the example tunnel, which verifies the feasibility of the method.

Petrology and Petrochemistry of Basement Rocks in Ila Orangun Area, Southwestern Nigeria

From field studies, six (6) lithological units were identified to be common around the study area which includes quartzites, granites, granite gneiss, porphyritic granites, amphibolite and pegmatites. The results of petrographical analyses show the mineral assemblages of the various rock types in the area which include quartzites as containing quartz, biotite, muscovite and myrmekite. Granite gneiss contains quartz, biotite, orthoclase, myrmekite, microcline, nepheline, hornblende and plagioclase. Pegmatite contains quartz, myrmekite, plagioclase, hornblende, microcline and muscovites. Porphyritic granites were observed to contain quartz, biotite, microcline, orthoclase and tourmaline. Granite contains quartz, biotite, hornblende, plagioclase and orthoclase while Amphibolite schist contains quartz, biotite, hornblende and muscovites. For the purpose of this study twenty (20) pulverized rock samples were taken to the laboratory for geochemical analysis with their results used in the classification as well as suggest the geochemical attributes of the rocks. Geochemical results obtained and interpreted using various geochemical plots or discrimination plots all classified the rocks in the area as belonging to both the peralkaline metaluminous and peraluminous types. Results for the major oxides ratios produced for Na2O/K2O, Al2O3/Na2O + CaO + K2O and Na2O + CaO + K2O/Al2O3 show the excess of alumina, Al2O3 over the alkaline Na2O +CaO +K2O thus suggesting peraluminous rocks. While the excess of the alkali over the alumina suggests the peralkaline metaluminous rock type. The results of correlation coefficient show a perfect strong positive correlation which shows that they are of same geogenic sources while negative correlation coefficient values indicate a perfect weak negative correlation suggesting that they are of heterogeneous geogenic sources. The result of the factor analysis show the elemental association of the trace elements and heavy metals represented as variables in the statistical analysis. Hence, establishing the elemental association of Component 1 as consisting of Ag-As-Ba-Cd-Co, Component 2 as Cr-Cu-Ni, Component 3 as Pb-Rb, Component 4 as Zn-Fe-V and Component 5 as U-Mn. Component 1 suggest the possibilities of sulphide and gold mineralization. Component 2 suggesting pathfinder elements while Components 3, 4 and 5 suggest weathered or dispersed elements. Hence, the occurrence of heavy metallic minerals in the rocks will serve huge economic significance with focus on the pegmatites in the area.

A Long-Lived Accretionary Process during the Amalgamation of the North China Craton: Insights from Neoarchean–Paleoproterozoic Polyphase Magmatism in the Lüliang Complex

Abstract There has been a long debate regarding the timing of the final amalgamation of the North China Craton, which is considered to have occurred either during the Neoarchean or Paleoproterozoic era. One major point of contention is whether there existed a long-lived subduction lasting through the Neoarchean to Paleoproterozoic. The Lüliang Complex contains multiphases of magmatism and thus represents the most viable region to address this controversy. In this study, we carried geochronological and geochemical analysis on the representative granitoids. Secondary ion mass spectrometry U–Pb dating revealed four distinct granitoid groups emplaced at 2531 ± 4, 2189–2173, 2027 ± 25, and 1852 ± 41 Ma, respectively. Notably, the 2531 Ma granitic gneiss was identified for the first time in this region. Based on the geochemical characteristics, the granitoids can be divided into two types. The 2531 and 2027 Ma groups display I-type features, while the 2189–2173 and 1852 Ma groups exhibit A-type geochemical affinities. Both I-type groups exhibit enrichment in Rb, depletion in Nb, Ta, and Ti, moderate fractionated REE patterns, substantial negative Eu anomalies, low Sr/Y ratios, and positive εHf(t) (+3.51 to +5.53 and +5.59 to +7.32, respectively), indicating that they were generated from partial melting of the juvenile mafic crust. In contrast, the 2189–2173 Ma granitoids belong to A2-type and were most likely generated by the partial melting of felsic rocks in the back-arc region, while the 1852 Ma granitoids belong to A1-type and were most possibly the result of partial melting of mafic-intermediate rocks during the post-collisional stage. Based on the records of A-type granitic magmatism and the ~1950 Ma peak metamorphism throughout the Trans-North China Orogen, we propose that a long-lived subduction process (2531–1950 Ma) can mostly explain the existing geological phenomena. It is likely that the subduction between the Eastern and Western Blocks should have commenced at ~2531 Ma, followed by a long-lived subduction. The two blocks ultimately collided with each other to form the North China Craton at ~1950 Ma, which triggered post-collisional exhumation and partial melting at ~1852 Ma.

Study on Experimental and Constitutive Model of Granite Gneiss under Hydro-Mechanical Properties

Permeability, as a critical element in making sure the underground facilities are secure, is a vital consideration in analyzing the rock material seepage. Selecting granite gneiss from underground oil storage as the research sample in this study. Triaxial mechanical property tests under different hydro-mechanical properties were carried out under the rock full-automatic triaxial servo system that controls the application of axial pressure, confining pressure, and seepage pressure. Through experiments carried out, we obtained the rock samples’ mechanical properties and permeability in three stages of the stress–strain process. The study shows that the seepage pressure considerably affects granite gneiss strength and deformation parameters under hydro-mechanical properties. On the basis of the same confining pressure, in pace with the growth in seepage pressure, elastic modulus, the deformation modulus, and the peak strength present a prominent decreasing inclination. The derived mechanical parameters are bound up with the stages we divided. This study analyzes and discusses the relationship among the strains and permeability, establishing the granite gneiss hydro-mechanical coupling constitutive model. Verification shows the results in numerical and experimental matches well, indicating that the rock hydro-mechanical properties could be effectively represented by the constitutive model.

Structural Architecture and Mineralogical Relationship in Chitradurga Schist Belt, Dharwar Cration, South India

Abstract: The succession in the late Archaean supracrustal belts in the Chitradurga region begins with basal quartz-pebble conglomerates and quartzites which were deposited on a basement of tonalitic--granitic gneisses, ca. 3000 Ma, containing tracts and enclaves of older supracrustal rocks. Thebasal beds are overlain by basic metalavas and amphibolites interbeddedwith crossbedded quartzites and siliceous phyllites. This series passes up into siliceous phyllites, locally containing a large lenticular mass of polymict conglomerate (Talya Conglomerate), overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates. Greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks, the latter being more abundant at higher levels. With progressive deepening of the whole basin, finegrained, graded greywackes and local polymict conglomerates (Aimangala Conglomerate) were deposited ; the turbidite sequence forms the youngest sedimentary rocks exposed.Estimated total stratigraphic thickness is at least 10 km indeeper parts of the basin. Deformation led to a series of upright anticlines and synclines and relatedLS fabrics with extreme variations in plunge of the coaxial fold axes and Lfabrics within axial surfaces which maintain a steep orientation. Low-grademetamorphism outlasted the deformation. Sedimentation and volcanism in the Chitradurga supracrustal belt is linkedto variable subsidence of the basement foundation, probably as a result of rifting. The structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history. Variable displacement of the basement during theselate Archaean events in the Karnataka craton took place on shear zones and related fractures that acted as channels for water and CO: asindicated by vein minerals and alteration products. Uplift of basement segments and their distension by granitic diapirs led to deformation of thesupracrustal cover made ductile by its high water content

Evaluation of Groundwater Potential Using Aquifer Characteristics in Urambo District, Tabora Region, Tanzania

In developing countries like Tanzania, groundwater studies are essential for water resource planning, development, and management. Limited hydrogeological information on groundwater occurrence, availability, and distribution in Urambo District is termed a key factor that hinders groundwater development. This research was aimed at the evaluation of groundwater potential zones in a granitic gneiss aquifer in Urambo District by integrating six indicators (transmissivity, specific capacity, static water level, yield, total dissolved solids, and geology) that were developed and applied in the study area. The indicators were further combined, and a groundwater potential index map (GWPIM) was prepared using relative weights derived from the analytical hierarchy process (AHP). The results show that 67% and 27% of the study area are categorized as moderate and high groundwater potential zones, respectively. Groundwater is controlled by both Quaternary sediments (sands and gravels) and weathered to fractured granitic gneiss. Quaternary sediments host the major shallow aquifers (<35 m) with relatively high transmissivity, specific capacity, and yield (1.5 m2/day, 16.36 m2/day, and 108 m3/day, respectively). Granitic gneiss is not strongly fractured/weathered and forms an aquifer with a relatively low yield of about 10.08 m3/day. The findings were validated using three boreholes, and the results are consistent with the developed GWPIM. Such findings are of great importance in groundwater development as the techniques applied can be extended to other areas in Tanzania as well as other countries that experience similar geological environments.

Enrichment of cobalt in the Xiarihamu Ni-Co sulfide deposit in the East Kunlun orogenic belt, Western China: Constraints from distribution of cobalt in sulfides and Fe and S isotopes of sulfides

The Xiarihamu Ni-Co sulfide deposit in the East Kunlun orogenic belt (EKOB), western China, is the largest magmatic Ni-Cu-(Co) sulfide deposit found in the orogenic belts worldwide. The geochemical behavior of Co during the evolution of sulfide melt is key to understand the enrichment process of Co in the Xiarihamu deposit, however remains unclear. In this study, the Co contents of sulfide inclusions enclosed within olivine and pyroxene and those of the sulfides interstitial to olivine and pyroxene in the Xiarihamu deposit were examinated by the Scanning Electron Microscopy (SEM) -Energy Dispersive Spectroscopy (EDS) and Electron Probe Microanalyzer (EPMA). The Fe and S isotopic compositions of the sulfides were measured in situ by LA-MC-ICPMS. The results show that Co contents increase from silicate to sulfide and arsenide minerals, and from chalcopyrite to pyrrhotite and pentlandite in the sulfides. High Co contents in pentlandite of pyroxenite (av. 2.79 %) and gabbro (av. 7.34 %) indicate that Co is enriched in the pentlandite of pyroxenite in different stage of magma. The pyrrhotite and pentlandite show positive correlation of Co with Fe and Ni, and suggested isomorphic substitution of Co for Fe and Ni. The δ56Fe values increase from pyrrhotite to pentlandite due to kinetic fractionation of Fe isotopes of Fe valent and bond length/strength, because pyrrhotite has longer bond and weaker bond strength of Fe-S than pentlandite, and display lighter Fe isotopic compositions. The δ56Fe values of pentlandite increase substantially from host peridotite to pyroxenite, which could be related to subsolidus fractional crystallization processes of monosulfide solid solution (MSS). The sulfide segregated and crystallized early from the MSS is richen in light Fe isotope. The sulfides in gabbro have high δ34S values, similar to δ34S of the sulfides in the country rocks of Proterozoic granitic gneiss, indicating a continuous addition of external sulfur into magma chamber. The δ34S and δ56Fe values of the sulfides approximately increase with increasing Co contents. Higher δ56Fe and δ34S values and Co contents of pentlandite in the pyroxenite suggest that the Co enrichment is mainly associated with sulfide saturation of magmas by crustal contamination.

Study on the complex magmatic and hydrothermal metallogenic processes in the giant Huayangchuan uranium (U)-polymetallic ore district: Constraints from geochemical and geochronological evidences

The supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the west part of Lesser Qinling Orogen, Central China, which is renowned for its unique metallogenic type and complex mineralization processes. This newly-discovered deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs. The genesis of this deposit remains hugely controversial and the related magmatic and metallogenic framework have not been well established. In our study, we present newly obtained zircon U-Pb age data, whole-rock trace elements, detailed SEM observation, carbon and oxygen isotope determination and titanite U-Pb age result to provide geochemical and geochronological constraints on the magmatic and hydrothermal metallogenic processes in Huayangchuan ore district and give insight into the REE, Nb and U mineralization and remobilization mechanism. Carbonatite in Huayangchuan deposit is rich in Ba, Sr, Nb, U and REE and the δ18OSMOW and δ13CPDB (‰) values of carbonatite vary between + 7.30 ‰ to + 9.80 ‰ and − 6.00 ‰ to − 7.20 ‰, respectively, which is the unique characteristic of igneous carbonatite. The total REE contents of carbonatite range from 1319.5 to 2535.8 ppm, significantly higher than that of the granite pegmatite dykes and Taihua Group granitic gneiss. Two zircon grains from the Huayangchuan carbonatite yield the 206 Pb / 238 U age of 223 ± 5.6 Ma and 217 ± 8.6 Ma. Uranium and niobium are mainly hosted in the primary mineral of betafite, minorly in uraninite and fergusonite. REE are remained in various REE carbonates, phosphates and silicates, e.g. allanite, parite, monazite, apatite, yttrialite and xenotime. Betafite contains numerous mineral inclusions of calcite, allanite and apatite, which displays as vermicular relicts with metasomatic corrosion texture. The intimate relationship between betafite, calcite, apatite, allanite and REE carbonates implies that U and Nb mineralization occurred during carbonatite-related magmatic stage. Late Triassic carbonatite magmatism (∼220 Ma) was responsible for the world-class U-Nb-Mo-REE mineralization in the Qinling Orogen. Whereas, LA-ICP-MS U-Pb data of titanite collected from Huayangchuan carbonatite reveal two sets of ages in the Huayangchuan deposit: during the Late Triassic and the Early Cretaceous. LA-ICP-MS data of the Triassic titanite and Cretaceous titanite in Huayangchuan deposit yield the lower intercept age of 223.1 ± 9.4 Ma and 141.9 ± 4.7 Ma respectively in the Tera-Wasserburg (238U/206Pb - 207Pb/206Pb) diagram. Cretaceous hydrothermal titanites show enrichment in Y, HREEs and depletion in TiO2 and LREEs relative to the Triassic magmatic titanites. Primary betafite and titanite were subjected to later hydrothermal alteration. The close genetic relation between altered titanite and Ti-Fe-Nb-Pb-U mineral inclusions demonstrates that Early Cretaceous hydrothermal fluids contributed to the Nb, U and Ti remobilization. Early Cretaceous regional granitic magmatic activity brought hydrothermal fluids to alter the early carbonatite magmatic pyrochlore and titanite. U, Nb, Ti, Y, Pb and HREEs were transferred and subsequently re-precipitated into uraninite, and hydrothermal titanite.

The Trans-Baikal Parametric Borehole: First Evidence of the Deep Structure of the Borshchovochnyi Metamorphic Core Complex

The Trans-Baikal parametric borehole drilled out a metamorphic core complex in the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol–Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological, and laboratory–analytical studies made it possible to identify three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (slightly metamorphosed rocks) is represented by a serpentinite mélange separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate as a part of the crystalline basement complex comprises rocks of granite–schist and granite–plagiogneiss subcomplexes. The granite–schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section as layered veins with synkinematic signs. According to the preliminary estimates, the age of biotite granite is about 130 Ma. Schist is characterized by a mylonite structure at intervals of interbedding with biotite granite veins. In accordance with the geophysical survey data on the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors in the seismic RSS–CDP section, which can be traced to a depth of at least 10 km. The granite–plagiogneiss subcomplex is represented by plagiogneiss, leucogranite, and gneissic granite. In the Trans-Baikal parametric borehole, a detailed section of the Borshchovochnyi metamorphic core complex, fully characterized by core samples, was obtained. It should be considered as a reference for studying these structures. The investigation of this section will make it possible later to answer many questions about the formation and evolution of the Central Asian fold belt.

Lattice preferred orientation of quartz in granitic gneisses from Tso Morari Crystalline Complex, Eastern Ladakh, trans-Himalaya: evaluating effect of Dauphiné twin in dynamic recrystallization during exhumation

Abstract The Tso Morari Crystalline complex (TMCC) of eastern Ladakh, India, is part of the north Indian continental margin and is characterized by eclogitic enclaves embedded within ortho- and paragneisses known as the Puga Gneiss. Two fault zones bound the TMCC: the Karzok fault to the southwest and the Zildat fault to the northeast. In the present study, we carried out Electron Backscatter Diffraction study of quartz of 10 samples collected from the Puga Gneiss. The relict and recrystallized quartz grains were treated separately to understand the deformation conditions of the Puga Gneiss during early and late deformation stages related to UHP metamorphism and final stage of exhumation during retrogression, respectively. Microstructural observations suggest dynamic recrystallization in quartz and plagioclase at different temperature ranges. Misorientation analysis of both relict and recrystallized quartz grains reveals presence of Dauphiné Twins. Lattice preferred Orientation (LPO) of <c> axis of relict quartz grains generally shows more than one point maxima indicating that the relict grains preserve LPO developed during different stages of metamorphism/deformation. On the other hand, LPO of <c> axis of recrystallized grains from Karzok and Zildat fault zones shows asymmetric single girdle either normal or at an angle to the foliation plane, which suggests simple shear. We conclude that grain size reduction and recrystallization of the Puga Gneiss was greatly influenced by Dauphiné Twin and the final exhumation of the TMCC took place in a simple shear environment aided by activity along its two binding fault zones.

Emphasizing the Creep Damage Constitutive Model of Hydro-Mechanical Properties of Rocks: A Case Study of Granite Gneiss

The constitutive model of rock materials can describe the mechanical behavior of rocks in creep tests. Also, it is one of the important means to study the deformation and strength characteristics of rocks in complex stress environments. This paper is based on the analysis of the porosity variation characteristics of the internal structure under the coupling effect of rock hydro-mechanical properties. The concept of the hydro-mechanical properties variable is proposed, and the relationship between the coupling variable, damage and plastic deformation is established. By introducing the coupling variable, instantaneous damage variable and time-dependent damage variable into the yield surface equation, as well as the plastic potential energy equation and the stiffness matrix of the elastic–plastic creep constitutive equation, a hydro-mechanical properties creep damage coupling model was established to simulate the creep mechanical properties of rock under coupling. Based on the triaxial creep test results of granite gneiss, the model parameters are determined. By comparing the test results with numerical results, it was revealed that the model can better describe the creep mechanical properties of rocks under the coupling effect of hydromechanical properties.

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

Building and reworking of Archean continental crust in the nucleus of the Yangtze Craton: Constraint from 3.4 Ga and 2.94 Ga granitoid gneisses in the Southern Kongling Complex

The TTG gneisses, a dominant lithological assemblage within Archean cratonic provinces, provide a window into the early evolution of the continental crust. The Kongling complex in South China preserves voluminous Archean granitoids, recording the early formation and evolution history of the Yangtze Craton. We present results of a comprehensive study involving field investigation, petrographic characterization, zircon U–Pb geochronology and Lu-Hf isotope tracing, and whole-rock geochemistry for the Archean granitoid gneisses from the Southern Kongling Complex. Igneous zircons from two granitic gneisses yielded weighted mean 207Pb/206Pb ages of 3399 ± 15 Ma (MSWD = 0.99, n = 24) and 3389 ± 12 Ma (MSWD = 1.02, n = 20). Zircons from the 3.4 Ga granitic gneiss also record a ca.3.0 Ga anatexis event which is contemporaneous with the widespread granitoid gneisses in the Kongling Complex. Geochemical and zircon Hf isotope data (εHf(t) values of − 3.41 to + 0.77 and two-stage Hf model ages of 3.65–3.80 Ga) for the granitic gneisses support a derivation from pre-existing Eoarchean (TDM2 = 3.73 Ga) crust. LA-ICP-MS zircon U-Pb dating yielded emplacement ages of 2948 ± 17 Ma for a migmatized trondhjemitic gneiss, and 2928 ± 14 Ma for a quartz monzonite gneiss. These two gneisses have similar zircon Hf model ages (TDM2 = 3.45 Ga) and both were emplaced during a major phase of mafic magma diapiric process that significantly reworked the old crust. The occurrences of 2.94 Ga trondhjemite and associated mafic blocks suggest intra-crustal differentiation by partial melting of low-K mafic rocks. We suggest that intra-crustal remelting and differentiation played a key role in the formation, evolution, and maturation of the micro-continents in Kongling during the Paleo- to Meso-Archean. The whole-rock geochemical and zircon Hf isotopic compositions of the different generations of granitoid gneisses from the Southern Kongling Complex are similar to those of the coeval gneisses (3.45 Ga and 2.95 Ga) from the northern part of the Kongling Complex, indicating a common tectono-magmatic history shared by both segments tracing back to as early as 3.4 Ga. The Archean granitoid gneisses from the Kongling Complex record remelting and differentiation of Eo- to Paleo-Archean crusts during the Paleo- to Meso-Archean, implying that Eoarchean crust is likely to have been preserved within the Kongling complex.

Zircon U–Pb ages and geochemistry of deep-seated crustal xenoliths from Yangyuan: Implications for the evolution of lower crust beneath the Trans North China Orogen

Deep-seated crustal xenoliths captured by volcanic rocks play a rare opportunity in understanding the composition and evolution of the lower continental crust. Here, we present mineral chemistry, whole-rock major and trace elements and Sr–Nd isotopes, and in-situ zircon U–Pb ages and Hf isotope data for a suite of granulite and biotite gneiss xenoliths entrained in the Oligocene basalts from the Yangyuan county in the central part of the Trans North China Orogen (TNCO) with aims to decipher the composition and evolutionary history of the lower crust beneath this area. Calculated equilibrium temperatures (841–933 °C) and pressures (9.7–14.3 kbar) suggest an origin of the Yangyuan granulites from the lower crust. The dominantly Archean–Proterozoic zircon ages and low Mg# values (29.0–68.4) of the Yangyuan crustal xenoliths indicate that they represent the ancient crust and the fractional crystallization plays a key role during the protolith formation. Furthermore, the zircon U–Pb–Hf isotopes data reveal multiple thermal events, including two main peaks of crustal growth at ∼2.7 Ga and ∼2.5 Ga, ∼1.85–1.80 Ga and the Phanerozoic accretion and reworking of the lower crust beneath Yangyuan. In addition, a comparison study of crustal xenoliths from Yangyuan, Nangaoya and Hannuoba areas demonstrates that the lower crust beneath the central–northern part of the TNCO experienced as a whole the Neoarchean crustal growth, Paleoproterozoic metamorphism and Phanerozoic magmatic underplating. More importantly, the absence of U–Pb age peak of ∼1.95 Ga and Hf model age peaks of ∼3.9–3.8 Ga, ∼2.9–2.8 Ga, ∼2.2–2.1 Ga and ∼700–600 Ma in Yangyuan shows a heterogeneous evolution of the lower crust beneath the TNCO, which could pave the way for a more comprehensive picture of the crustal evolutional history beneath the North China Craton.