Granitic Rocks Research Articles

Performance of Gradient and Gradient-Free Optimizers in Transient Hydraulic Tomography.

Sub-surface characterization in fractured aquifers is challenging due to the co-existence of contrasting materials namely matrix and fractures. Transient hydraulic tomography (THT) is proved to be an efficient and robust technique to estimate hydraulic (Km, Kf) and storage (Sm, Sf) properties in such complex hydrogeologic settings. However, performance of THT is governed by data quality and optimization technique used in inversion. We assessed the performance of gradient and gradient-free optimizers with THT inversion. Laboratory experiments were performed on a two-dimensional, granite rock (80 cm × 45 cm × 5 cm) with known fracture pattern. Cross-hole pumping experiments were conducted at 10 ports (located on fractures), and time-drawdown responses were monitored at 25 ports (located on matrix and fractures). Pumping ports were ranked based on weighted signal-to-noise ratio (SNR) computed at each observation port. Noise-free, good quality (SNR > 100) datasets were inverted using Levenberg-Marquardt: LM (gradient) and Nelder-Mead: NM (gradient-free) methods. All simulations were performed using a coupled simulation-optimization model. Performance of the two optimizers is evaluated by comparing model predictions with observations made at two validation ports that were not used in simulation. Both LM and NM algorithms have broadly captured the preferential flow paths (fracture network) via K and S tomograms, however LM has outperformed NM during validation ( ). Our results conclude that, while method of optimization has a trivial effect on model predictions, exclusion of low quality (SNR ≤ 100) datasets can significantly improve the model performance.

Statistical analysis of heat-induced rock physics and mineralogical alteration processes of monzogranite samples from Bátaapáti, Hungary

Heat-induced physical changes of rocks, as would happen in fire accidents, have crucial importance in the long-term stability of underground openings such as tunnels or radioactive waste disposal chambers. The thermal behaviour of monzogranite from the Radioactive waste repository of Bátaapáti (Hungary) was studied in that context. Room temperature (22 °C) and a series of high temperature (250 °C, 375 °C, 500 °C, 625 °C, 750 °C) heat-treated samples were tested. P and S- ultrasound pulse velocity, bulk density, Duroskop rebound, uniaxial compressive strength, and modulus of elasticity were measured and compared, detecting temperature-related changes. Descriptive statistics and non-parametric Kruskal–Wallis and Median tests were carried out to analyze the heat-induced rock mechanical and mineralogical alteration processes. Based on these results using the data set of physical parameters, it is possible to predict the heating temperature of granitic rocks when the firing or heat stress conditions are unknown. Additionally, the data could be utilized for design and monitoring systems in underground repositories or tunnel systems constructed in granitic rock settings globally. The study demonstrated that significant alteration occurs above 500 °C; all studied rock-mechanical parameters decreased rapidly. These changes are linked to the thermal expansion, micro-cracking in micro-fabric, and volumetric increase and fragmentation of quartz inversion. Consequently, the propagation velocities of P- and S-waves also decreased significantly. Bulk density and Duroskop surface strength significantly decreased above 375 °C, while uniaxial compressive strength and modulus of elasticity dropped drastically at 750 °C after a linear decrease.

Performance evaluation of granite rock based on the quantitative piezoceramic sensing technique

ABSTRACT Granite is a common engineering material that exhibits complex mechanical properties under external loads. This study conducted experimental research and analysis in conjunction with the active monitoring technology of piezoelectric ceramics. A quantitative analysis method for the mechanical properties of rock materials based on piezoelectric health monitoring was established, and for the first time, the piezoelectric monitoring results of rock were mapped and compared with the uniaxial compression performance indicators of rock. In this study, two sets of cyclic impact experiments were conducted on granite samples using a drop hammer. The piezoelectric signals of the granite samples were detected using piezoelectric ceramic active sensing technology. A piezoelectric ceramic damage monitoring method was proposed, and the damage factor of the granite samples was calculated using the wavelet packet energy method. Subsequently, uniaxial compression experiments were performed on the damaged granite samples to obtain mechanical performance data. Finally, a mathematical relationship model was established between the piezoelectric signal and the uniaxial compressive strength of the rocks. It was found that the damage factor of the piezoelectric monitoring signal of the damaged rock were linearly related to the uniaxial compressive strength of the damaged rock.

Basement differences control granitoid compositions: insights from zircon Hf isotopic mapping of Paleozoic granitoids in the northern Beishan orogenic collage, NW China

ABSTRACT The source of granitoids in Phanerozoic accretionary orogens is debatable. Paleozoic granitoids are widespread in the southern Central Asian Orogenic Belt (CAOB) and are considered the most eloquent witness to continental crustal growth during the Phanerozoic. This study presents a comprehensive analysis of zircon U-Pb and Lu-Hf isotopic and geochemical data from six Paleozoic granitic plutons in the northern Beishan Orogenic Collage (NBOC). Zircon U-Pb data reveal that these plutons recorded a long-lasting magmatism from the Late Silurian to the Late Carboniferous. The elemental and isotopic data show a major contribution from the Precambrian crustal basement as the source of the Late Silurian to Carboniferous granitoids in the Mingshui – Hanshan massif (MHM). In contrast, the source compositions of the Carboniferous granitoids in the Heiyingshan arc (HYA) indicate a dominant role of juvenile materials. Combined with published data, three magmatic pulses from the Ordovician to Carboniferous were identified in the NBOC: Stage I magmatism (Early Ordovician to Late Silurian) formed in a subduction-related compressional setting, Stage II magmatism (Late Silurian to Early Devonian) occurred in an extensional setting, and Stage III (Carboniferous) formed in a compressional setting. For the MHM, the zircon Hf isotopic mapping results show that the eastern segment was predominantly composed of a Paleoproterozoic basement, whereas the western segment mainly consisted of a young Mesoproterozoic – Neoproterozoic basement. The Heiyingshan magmatic rocks were probably formed as a continental arc during the Mesoproterozoic – Neoproterozoic and accreted to the MHM during the closure of the Paleo-Asian Ocean. Overall, these data indicate that the NBOC had remnants of the ancient Precambrian basement during the Paleozoic. Accordingly, numerous Phanerozoic granitic rocks in the NBOC have mostly resulted from the recycling of ancient crustal materials along with the addition of minor new juvenile materials. Our study has highlighted that systematic basement differences can exert a major influence on the nature of granitoids.

Low-temperature thermochronology of the Longmala - Mengya’a Pb-Zn deposits (southern Tibet): Implications for ore exhumation and preservation

The large Gangdese metallogenic belt, located in the southern part of the Tibetan Plateau, hosts a variety of magmatic-hydrothermal ore deposits. Previous works in the area primarily concerned the ore genesis, while the exhumation history of these ore deposit has received less attention. In this contribution we explore the multi-stage thermo-tectonic evolution of the Longmala and Mengya’a Pb-Zn deposits, two large magmatic-hydrothermal ore deposits in the central Lhasa sub-terrane. In more detail, the thermal history of granitoids and deformed Paleozoic sediments inside and on the periphery of the ore districts have been constrained by low-temperature thermochronological methods, including the zircon (U-Th)/He and apatite fission track thermochronology. Additionally, zircon U-Pb geochronology was conducted on granitic host rocks to determine their emplacement ages. Our thermochronological data and inverse thermal history modeling results reflect two phases of accelerated basement cooling during the Cenozoic (i.e., ∼46–41 Ma and ∼20–5 Ma), interpreted as exhumation phases. The ore body was exhumed to the near surface during the Miocene, which provides essential information for ore exploration in the region. We also explore the degrees of basement rock exhumation along the Gangdese metallogenic belt and their spatio-temporal relations to major ore deposit distributions. It is suggested that complex interactions between tectonic-structural activities and surface erosion occurred during the late Oligocene-Miocene (∼25–5 Ma), which resulting in widespread exhumation along the Gangdese batholith. During this period, the ore-bearing rocks were gradually (with medium to high cooling rates) exhumed to the near-surface from ∼3–4 km crustal depth, making these ore deposits feasible for commercial mining.

Theoretical Developments for the Interpretation of Cavity Flow Permeability Tests Conducted on Intact Lac du Bonnet Granite

The permeability of intact geologic media features prominently in many geo-environmental endeavours. The laboratory estimation of permeability is an important adjunct to the field estimation of bulk permeability values, which involves a great deal of supplementary in situ investigations to correctly interpret field data. Laboratory permeability estimation is also a viable method if core samples are recovered from in situ geological mapping of the region under study. The basic methodologies for permeability estimation rely on either steady-state or transient tests of the geologic material depending on the anticipated permeability value. This paper presents a steady flow test conducted on a partially drilled cavity located on the axis of a cylindrical specimen. Certain compact theoretical relationships are proposed for the estimation of steady flow from a cavity of finite dimensions located along the axis of a cylindrical specimen. The relationships are used to estimate the permeability of a cylinder of Lac du Bonnet granite obtained from the western flank of the Canadian Shield. The results from the cavity flow permeability experiments are compared with other estimates for the permeability of granitic rocks reported in the literature.

Water Deficit Caused by Land Use Changes and Its Implications on the Ecological Protection of the Endorheic Dalinor Lake Watershed in Inner Mongolia, China

Dalinor Lake, the second-largest endorheic salt lake in Inner Mongolia, has shown a shrinking trend given the lack of a significant decrease in precipitation (PRE). Based on high-spatial-resolution datasets, we employed a linear regression model, Theil–Sen median trend analysis, the Mann–Kendall test, and a land use transfer matrix to identify the spatio-temporal distribution and trends of PRE and actual evapotranspiration (AET) at the watershed scale during 2001–2019; then, the water deficit (WD) caused by land use changes in different surface lithology zones was analyzed. The results showed that the annual PRE and WD of the Dalinor Lake watershed showed insignificant upward trends, while the annual AET showed a significant upward trend. Spatially, about 89% of the watershed showed a significant upward trend for AET, while 12% showed a weak significant upward trend for PRE. The WDs of the aeolian sand zone and the sand, gravel, and silt accumulation zone were most heavily affected by the new increased land use from 2001 to 2019, accounting for 43.14% and 25.56% of the total WD of the watershed, respectively. Specifically, the WD of the aeolian sand zone caused by the new increased grassland and farmland in 2019 accounted for 41.92% and 18.52% of the total WD of the zone, respectively. The WD of the sand, gravel, and silt accumulation zone caused by the new increased grassland and farmland in 2019 accounted for 37.07% and 35.59% of the total WD of the zone, respectively. The WD caused by the new increased land use was increased by 7.78 million m3 in 2019 compared with the corresponding land use type in 2001, which would decrease the water yield. It is necessary to strengthen the protection of regional forest ecosystems in the granite and terrigenous clastic rock zone; standardize pasture management and reduce farmland reclamation in the sand, gravel, and silt accumulation zone, the aeolian sand zone, and the basalt platform zone; and reduce unnecessary impervious land construction in the aeolian sand zone.

Impact of reactive mineral facies distributions on radionuclide sorption properties in multiscale heterogeneous granite rocks

Impact of reactive mineral facies distributions on radionuclide sorption properties in multiscale heterogeneous granite rocks

Arc building in the Abukuma Mountains of NE Japan: Insights from petrology, geochemistry and zircon U-Pb-Hf isotope data

Subduction zones along convergent plate margins are the major regions of continental crust building on Earth. The petrological and geochemical features of arc crust are therefore critical in understanding the formation and evolution of continental crust. Here we investigate the magmatic suites in the Abukuma Mountains of NE Japan, which document episodic accretionary events during the Cretaceous. The rock suite in the Abukuma Mountains mainly comprises hornblende (Hbl)-gabbro, diorite, granodiorite and monzogranite. Zircon UPb dating suggests that dioritic to granitic rocks in the Abukuma Mountains were formed during 110 to 115 Ma, followed by gabbroic intrusions (100 to 110 Ma) and large-scale high-T/low-P metamorphism. But combined with literature geochronological results, the gabbroic rocks in Abukuma are coeval with the formation of intermediate to felsic rocks with a peak age of ∼108 Ma. Major and trace elements characteristics of these rocks including their calc-alkaline nature and negative Nb, Ta anomalies suggest their arc-related origin linked with the subduction of the Paleo Pacific (Izanagi) plate. About 200 km north of the Abukuma Mountains, the Kitakami magmatic rocks were also formed during the subduction of Izanagi plate, but previous studies show that parts of the Kitakami intermediate-felsic rocks have distinct adakitic features and more juvenile Hf signatures (ƐHf(t) = 7.9 to 10.6) than those in the Abukuma suite (ƐHf(t) = 1.5 to 8.8, average at 6.1 ± 1.8). The geochemical features of the magmatic rocks in NE Japan are consistent with slab-break off, mantle upwelling and partial melting resulting in the formation of adakitic rocks in the Kitakami region. However, subsequent to this event, the slab subduction was reestablished, leading to the formation of normal island arc rocks in the Abukuma area. Our study provides new insights into the geodynamic evolution of NE Japan.

Sorption behavior of Sr(II) in granitic rock: the effect of MX-80 bentonite colloids

MX-80 bentonite colloids’ effect on radionuclide sorption was studied by batch method and transport in flow using a granitic drill core column and Sr(II) as a radionuclide. Batch-type sorption and desorption experiments were conducted to determine Sr(II)’s distribution coefficients on bentonite colloids and metatexitic gneiss. Molecular modeling assessed the radionuclide’s sorption affinity and justified the differences in Sr(II)’s sorption behavior on biotite versus montmorillonite. The distribution coefficients were found to be a hundred times greater for colloids than for granitic rock. Strontium’s breakthrough from the column with bentonite colloids was successful without notable retardation on rock minerals.

Characteristics of early Paleozoic granite-pegmatite and associated lithium-beryllium mineralization in the Tugeman area, Altun orogenic system, northwestern China

The Altun Orogenic Belt (AOB) is located on the northern margin of the Tibetan Plateau. We report the petrography, zircon U-Pb ages, Hf isotopic compositions, and whole-rock geochemistry of the Tugeman pegmatitic and granitic rocks from the AOB to constrain their petrogenesis and tectonic setting. Zircon U-Pb dating of the pegmatitic and granitic rocks yielded ages of 500–440 Ma, which we interpret as the emplacement ages of the granites and formation age of pegmatite. The granites are weakly peraluminous to peraluminous (A/CNK = 1.01–1.26), and they have high SiO2 (63.9–74.2 wt%) and Al2O3 (13.7–17.6 wt%) and low CaO (0.72–3.80 wt%) and P2O5 (0.06–0.44 wt%) contents. They show marked depletion in HREEs, enrichment in LREEs, and pronounced negative Eu anomalies, and they are enriched in Th, U, K, Pb, Nd, and Sm and depleted in Ba, Nb, Ce, Sr, P, and Ti. Zircon εHf(t) values vary from − 12.0 to + 3.3, with two-stage Hf model ages of 2.2–1.2 Ga, implying that the granites were derived mainly from ancient crust with a minor contribution from juvenile crust. Weakly peraluminous pegmatitic rocks have geochemistry characters of SiO2 (74.4–79.9 wt%) and Al2O3 (12.3–14.9 wt%) and low CaO (0.08–0.69 wt%) and P2O5 (0.05–0.33 wt%), depletion in HREEs, enrichment in LREEs, depletion in Ba, La, Ce and Ti, enriched in U, K, Pb and P. Granitic pegmatite can be formed at different stages of crystallization differentiation of granitic magma, but the pegmatite associated with highly differentiated granites is favorable to the Li and/or Be mineralization, while the pegmatite associated with low differentiated granites (such as biotite granites) is unfavorable to the Li and/or Be mineralization. Based on our findings, we propose that the Tugeman granitic magmas originated from partial melting of crustal metagreywacke and metabasalt. Furthermore, we suggest that the early Paleozoic magmatism in the Central Altun Block (CAB) can be categorized into three groups, which correspond to different tectonic events. These events include the northward subduction of the Central Altun Ocean (CAO) beneath the CAB around 520 Ma, the closure of CAO at 490–480 Ma, and post-collisional extension at 480–440 Ma induced by slab breakoff and delamination of the subducted CAO.

Petrogenesis of the Triassic Dongjiangkou pluton in the South Qinling orogenic belt, Central China: Insufficient magma mixing of mafic and felsic magmas

Magma mixing or mingling between mafic and felsic magmas is a common phenomenon in granitoid magmatism, and chemical and mechanical interactions between the magmas are frequently observed in granitic rocks. The Dongjiangkou pluton is an example of magma mixing among the numerous early Mesozoic granitoid plutons that are exposed in eastern South Qinling. This pluton is predominantly composed of granodiorite and contains abundant mafic/intermediate microgranular enclaves (MMEs) and is sporadically intruded by mafic dykes. Both the granodiorite and MMEs were crystallized at 224–214 Ma and have inherited zircons of similar age ranges. The granodioritic rocks had metaluminous to weak peraluminous features and adakitic signatures and belong to the high-K calc-alkaline series, whereas the MMEs and dioritic dykes had low SiO2 contents and metaluminous features and belong to the high-K calc-alkaline to shoshonite series. All of the rocks had high Mg# values and similar geochemical characteristics regarding trace elements and Sr isotopes, but they had diverse whole-rock Nd-Pb and zircon Hf isotopic compositions. The Dongjiangkou granodiorite was produced by partial melting of old basement rocks and Paleozoic juvenile crust beneath South Qinling triggered by asthenospheric upwelling. Magma(s) of the Dongjiangkou MMEs may have been derived from deep mantle and injected into and mixed with the magma(s) of the granodiorite, but chemical and isotopic homogenization was not achieved during the magma mixing process. Clastic rocks may have played a major role in the formation of crust-derived magma(s), causing strong Nd-Hf isotopic decoupling of several granitoid plutons in South Qinling.

Natural Radioactivity Levels (238U, 232Th, 40K) and Their Geochemical Characteristics of Granitic Rocks from Nuweibi Area, Egyptian Nubian Shield

Natural Radioactivity Levels (238U, 232Th, 40K) and Their Geochemical Characteristics of Granitic Rocks from Nuweibi Area, Egyptian Nubian Shield

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

AbstractThe in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault‐associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture‐induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr‐Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi‐quantitative estimation of the in‐situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications.

The Kaolin and Bentonite Deposit of Tamame De Sayago (Zamora, Spain): Mineralogy, Geochemistry, and Genesis

A geological, mineralogical, and geochemical characterization of the Tamame de Sayago (Zamora, Spain) deposit was carried out with the aim of knowing the conditions that facilitated the genesis in the same deposit of kaolinite and smectites. The alteration processes affecting a Variscan granite were deduced throughout the study of a very wide group of representative samples by X-ray diffraction (XRD), scanning electron microscopy (SEM), chemical analyses of major, minor, and trace elements, as well as δ18O, δ34S stable isotope and K/Ar dating analyses. In addition, 2D and 2.5D graphs of the kaolinite and smectite isoconcentrations were obtained from core data. According to the color and texture, two different clayey rock types were identified and named as homogeneous alteration zones (ZAHO) and heterogeneous alteration zones (ZAHE). The ZAHO are regoliths in which the granite texture is preserved, and the feldspars are almost completely kaolinized. In the ZAHE, the original texture of the granitic rock is lost, and the main clay mineral is smectite. The mineralogical composition is similar, with kaolinite, smectite, mica, quartz, scarce feldspar, and occasionally natroalunite and APS (aluminum-phosphate-sulphate); however, the mineral concentration varies considerably because ZAHO are rich in kaolin areas whereas ZAHE are bentonitic areas. Both rock types contain numerous veins and nodules. The weathering of the Paleozoic granite alongside the absence of sedimentation during the Mesozoic led to the formation of kaolinite that is preserved in ZAHO materials. Nonetheless, during the Cretaceous–Tertiary transit, the conditions of tectonic stability varied. Late Variscan faults reactivated which allowed the percolation of Mg- and Ca-rich hydrothermal fluids through the already kaolinized granite, increasing the alteration of the primary silicates and leading to the formation of smectites in ZAHE materials. The amount of smectites is greater closer to the faults. The stable isotopes indicate the meteoric nature of the low-temperature hydrothermal fluids. The K/Ar data obtained from the natroalunite of veins indicate that those hydrothermal fluids circulated in different pulses from 66.4 ± 1.7 to 58.8 ± 1.5 Ma, as a minimum. Those ages are coincident with the first formation stages of the Duero Basin.

Jurassic Uranium-Thorium Deposit of Peralkaline Granitic Rocks, Bokan Mountain, Prince of Wales Island, Southeastern Alaska

The Lower Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska hosts a high-grade uranium-thorium deposit with past production. The complex is a circular body (~3 km in diameter) which intruded Paleozoic granitoids as well as metasedimentary and metavolcanic rocks of the Alexander Terrane of the North American Cordillera. This shallow seated intrusion, which is composed of fractionated peralkaline granites, is a zoned body with a dominant core of arfvedsonite granite and a rim of aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. Both arfvedsonite and aegirine granites have overlapping compositions. The rocks have high contents of rare earth elements, Th, U and high field strength elements (Zr, Nb, Hf and Ta) and low contents of Sr, Ba, Eu and Ti, typical of peralkaline granites. The complex hosts structurally controlled rare metal mineralization, which is associated with the late-stage of magma evolution and hydrothermal fluids. Fluorine complexing played a role during the transportation of rare metals in hydrothermal fluids. The U-Th deposit, which occurs at the margin of the aegirine granite zone, includes structurally controlled shear zone-hosted lenses and irregularly shaped pipe-like orebodies. U-Th mineralization is associated with desilicified and albitized granitic rocks and includes mainly uranothorite and uraninite. These minerals mostly form small ovoids in veinlets typically 0.1 to 1 mm wide. The mine produced about 77,000 t of ore at a grade of ~0.76% U3O8 and 3% of ThO2. The parent magma of the pluton was likely derived from a metasomatized lithospheric source (mantle or lower crust), which was enriched by subduction related processes during Paleozoic time.

Multi-Scale Potential Field Data Integration Using Fuzzy C-Means Clustering for Automated Geological Mapping of North Singhbhum Mobile Belt, Eastern Indian Craton

Fuzzy C-Means (FCM) clustering is an unsupervised machine learning algorithm that helps to integrate multiple geophysical datasets and provides automated objective-oriented information. This study analyzed ground-based Bouguer gravity and aeromagnetic datasets using the FCM clustering algorithm to classify lithological units in the western part of the North Singhbhum Mobile Belt, a mineralized belt in the Eastern Indian Craton. The potential field signatures of clusters obtained using FCM correlate remarkably well with the existing surface geology on a broad scale. The cluster associated with the highest gravity signatures corresponds to the metabasic rocks, and the cluster with the highest magnetic response represents the mica schist rocks. The cluster characterized by the lowest gravity and magnetic responses reflects the granite gneiss rocks. However, few geological formations are represented by two or more clusters, probably due to the close association of similar rock types. The fuzzy membership scores for most of the data points in each cluster show above 0.8, indicating a consistent relationship between geophysical signatures and the existing lithological units. Further, the study reveals that integrating multi-scale geophysical data helps to disclose bedrock information and litho-units under the sediment cover.

Apatite chemistry of Late Triassic granitic rocks from Yidun Terrane: Implications for petrogenesis and mineralization

Apatite chemistry of Late Triassic granitic rocks from Yidun Terrane: Implications for petrogenesis and mineralization

Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al2O3/(MgO+FeOT) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

Andean-type, bivergent crustal shortening in the Rinkian orogen: New constraints on the tectonic evolution of Laurentia–West Greenland in the Paleoproterozoic

Abstract The “Rinkian belt” of West Greenland is a metamorphic terrain of Paleoproterozoic age comprising: (1) the north Rinkian fold-thrust belt (FTB)—a pro- or fore-arc domain, highly deformed and metamorphosed with widespread anatexis; (2) the Prøven Igneous Complex—a magmatic arc characterized by hypersthene granitic rocks (“charnockites”); (3) the south Rinkian FTB—an inverted back-arc basin; and 4) a continental margin or foreland. Recognition of this tectonic architecture demonstrates that the “Rinkian” is a bona fide orogenic belt—the Rinkian orogen— and not simply the imbricated lower plate of the Nagssugtoqidian orogen. Arc plutons of the Prøven Igneous Complex were emplaced into the Karrat Group at ca. 1.90–1.85 Ga, dividing a back-arc basin into pro- and retro-arc domains. In the former—the north Rinkian FTB—WSW-directed thrusting (deformation events D1-D2) and high-grade metamorphism were taking place by ca. 1.875 Ga and were continuous through ca. 1.850 Ga with a peak temperature at ca. 1.830 Ma accompanied by anatexis in the Karrat Group and lower units of the Prøven Igneous Complex. In the retro-arc domain—the south Rinkian FTB—thrusting to the E (D1) began at ca. 1.870 Ma followed by thrusting to the W (D2) at ca. 1.830–1.820 Ga with displacement focused into a major high-temperature ductile shear zone which carried the Prøven Igneous Complex in the hanging wall of an Andean-type, crustal-scale, “pop-up” structure. High-temperature deformation continued during D3 when the pro-arc, arc, and retro-arc domains were shortened by bivergent detachment folding and thrusting at ca. 1.820–1.810 Ga.