Magmatism In Region Research Articles

Two-stage mineralization of the Jinkeng Sn-Cu deposit in Eastern Guangdong, Southeast China: Response to magmatic activities and tectonic transformation

The Jinkeng Sn-Cu polymetallic deposit in South China consists of two different types of orebodies: 1) NE-striking skarn- and vein-type Cu-Pb-Zn-Sn orebodies in volcanic rocks suffering subsequent ductile shear deformation, and 2) NW-striking quartz-cassiterite-sulfide veins filled in the faults at the porphyritic granodiorite outward from the fine-grained granite which does not exhibit deformation. The relationships among Sn-Cu polymetallic mineralization, regional magmatism, and deformation metamorphism are still controversial. To address it, the geochronological, whole-rock and mineral geochemical research along with the detailed field investigation were conducted in this study. Our zircon U-Pb dating results show that the volcanic rocks formed at 158–162 Ma, earlier than the porphyritic granodiorite which yields the emplacement age of 146–147 Ma. In-situ LA-ICP-MS U-Pb dating of cassiterite from the deformed skarn ores indicates the early-stage mineralization occurred at 146 ∼ 148 Ma, which is similar to the emplacement age of the porphyritic granodiorite, but earlier than the formation of quartz-cassiterite-sulfide veins and the fine-grained granite (141∼144 Ma). Further, the whole-rock and biotite geochemistry, and zircon Hf isotope compositions suggest that the porphyritic granodiorite exhibits a lower degree of magma differentiation, higher oxygen fugacity, higher Cl and lower F contents than the fine-grained granite. The porphyritic granodiorite might provide the most Cu-Pb-Zn budget accompanied by minor Sn for early-stage mineralization. Overall, our study suggests that the Jinkeng Sn-Cu polymetallic deposit formed in two scenarios where the early dominated Cu-Pb-Zn and minor Sn mineralization related to the emplacement of the porphyritic granodiorite is superimposed by the late vein-type Sn-dominated mineralization related to the emplacement of the fine-grained granite. The two isolated mineralizing events (∼147 Ma and ∼141 Ma, respectively) in Jinkeng were probably responded to the regional magmatic activities triggered by the tectonic transformation where two extensional tectonic events were separated by a short contractional event (147–144 Ma).

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Heat transport process associated with the 2021 eruption of Aso volcano revealed by thermal and gas monitoring

The thermal activity of a magmatic–hydrothermal system commonly changes at various stages of volcanic activity. Few studies have provided an entire picture of the thermal activity of such a system over an eruptive cycle, which is essential for understanding the subsurface heat transport process that culminates in an eruption. This study quantitatively evaluated a sequence of thermal activity associated with two phreatic eruptions in 2021 at Aso volcano. We estimated plume-laden heat discharge rates and corresponding H2O flux during 2020–2022 by using two simple methods. We then validated the estimated H2O flux by comparison with volcanic gas monitoring results. Our results showed that the heat discharge rate varied substantially throughout the eruptive cycle. During the pre-eruptive quiescent period (June 2020–May 2021), anomalously large heat discharge (300–800 MW) were observed that were likely due to enhanced magma convection degassing. During the run-up period (June–October 2021), there was no evident change in heat discharge (300–500 MW), but this was accompanied by simultaneous pressurization and heating of an underlying hydrothermal system. These signals imply progress of partial sealing of the hydrothermal system. In the co-eruptive period, the subsequent heat supply from a magmatic region resulted in additional pressurization, which led to the first eruption (October 14, 2021). The heat discharge rates peaked (2000–4000 MW) the day before the second eruption (October 19, 2021), which was accompanied by sustained pressurization of the magma chamber that eventually resulted in a more explosive eruption. In the post-eruptive period, enhanced heat discharge (~ 1000 MW) continued for four months, and finally returned to the background level of the quiescent period (< 300 MW) in early March 2022. Despite using simple models, we quantitatively tracked transient thermal activity and revealed the underlying heat transport processes throughout the Aso 2021 eruptive activity.Graphical abstract

Oligocene‐Miocene Exhumation of the Pinaleño Metamorphic Core Complex, Southeastern Arizona: Support for Magmatism and Plate Margin Reorganization as Controls on Regional Exhumation Trends

AbstractThe Pinaleño Mountains of southeastern Arizona is the eastern‐most metamorphic core complex in the southern U.S. and northern Mexican Cordillera. This study investigates the thermal history and exhumation record of the Pinaleño core complex using mica 40Ar/39Ar, apatite and zircon (U‐Th)/He, and apatite fission‐track thermochronometers. The Pinaleño Mountains experienced two periods of rapid cooling during the Cenozoic. The first period, from ca. 27 to 21 Ma, records tectonic exhumation related to the development of the core complex and extensional shear zone. This period was followed by a relatively quiescent interval from 21 to 13.5 Ma that records little to no exhumation. The second period of rapid cooling, from 13.5 to 11 Ma, records tectonic exhumation related to high‐angle normal faulting, characteristic of the Basin and Range province. The exhumation timing of the Pinaleño core complex matches previously recognized spatiotemporal trends in the southern Basin and Range province and indicates that core complex exhumation in this region started in southeastern Arizona (ca. 32–33°N) and migrated both northward and southward. These trends correlate well with the latitude and timing of subduction of the Pacific‐Farallon spreading ridge and the migration of the Mendocino (northward) and Rivera (southward) triple junctions. Spatiotemporal core complex exhumation trends also correlate well with regional magmatism associated with the mid‐Cenozoic flare‐up, including syn‐extensional intrusive rocks found in the footwalls of core complexes.

Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China

Abstract Late Paleozoic plutons are widely distributed in the Alxa region, southernmost Central Asian Orogenic Belt, and provided an important clue in constraint for the closing time of the Paleo-Asian Ocean (PAO). In this article, we present new zircon U-Pb ages and whole-rock geochemical data from the Permian Huoersen and Zongnaishan plutons in the southern Alxa region. The Huoersen gabbro (ca. 285 Ma) is enriched in large-ion lithophile elements and depleted in high-field strength elements, similar to the features of continental marginal arc. They were most likely generated by partial melting of depleted mantle that was modified by subduction metasomatic fluids. The Zongnaishan granites (ca. 267 Ma) show characteristics of I-type granites and were generated in a syn-collision setting. The Huoersen granites (ca. 259 Ma) are peralkaline and have positive Ɛ Nd(t) (+1.2 to +1.5) values, exhibiting A2-subtype granites affinities. They were formed by melting of lower crust in post-collision extension setting. Based on geodynamic mechanism, a three-stage evolution model is delineated: subduction, syn-collision to post-collisional extension for oceanic branch of the PAO during the Permian. In general, the rock assemblages indicate a tectonic switch from subduction to post-collision extension regimes and the final closure of the PAO.

Unveiling attenuation structures in the northern Taiwan volcanic zone.

This cutting-edge study delves into regional magmatism in northern Taiwan through advanced 3-D P- and S-wave frequency-dependent attenuation tomography. Positioned at the dynamic convergence boundary between the Philippine Sea Plate and the Eurasian Plate, Taiwan experiences moderate earthquakes and intriguing volcanic activity, with a focus on the Tatun volcano group. Employing the Formosa seismic array for high-resolution results, our research identifies high-attenuation anomalies (low Q) beneath the northern Taiwan volcanic zone (NTVZ) and offshore submarine volcanoes, indicative of potential hydrothermal activities and magma reservoirs at varying depths. Additionally, we explore low-attenuation anomalies (high Q) in the forearc region of the Ryukyu subduction zone, suggestive of partial saturation linked to serpentinization processes resulting from seawater infiltration or forearc mantle hydration. These findings shed light on the complex geological features and provide essential insights into the crustal properties of northern Taiwan, contributing to a deeper understanding of its magmatic evolution and tectonic processes.

Age and geochemical evolution of granite magmatism in Olkhon region from Caledonian syncollisional ore-free granite to the rare metal granite and pegmatite of Middle Paleozoic intraplate setting

The detailed description of two granite complexes in the Olkhon subterrane is given. The Early Paleozoic Sharanur complex was formed by granitization of gneisses of the Olkhon series. It includes migmatites, granite-gneisses, granites and pegmatites of normal alkalinity; they belong to the type of syncollisional granites. The Middle Paleozoic Aya granite complex includes mother Aya massif of amazonite-bearing granites and several types of rare-metal pegmatites. They have elevated alkalinity, low of Ba, Sr, and high LILE and HFSE elements contents. The Aya pegmatites lie in northwest cracks of stretching and associated with the rise of the territory under the influence of the North Asian plume. These cracks and pegmatites mark the beginning of a new intraplate geodynamic setting. Two geochemical types are distinguished among the pegmatites of this complex. These are amazonite pegmatites of Li-F type with Ta mineralization and complex type pegmatite with Be-Rb-Nb-Ta and Li-F mineralization (the Ilixin vein). The Tashkiney pegmatite vein is similar to Ilixin, but lies in the gneisses of the Olkhon series. It shows high concentrations of Be, Nb, Ta, as well as W, Sn, but lacks Li and F, due to a greater depth and higher temperature of the melt crystallization of this pegmatite.©2024 China Geology Editorial Office.

Structural patterns and ages of Permian-Triassic multistage strike-slip faulting in the Chifeng area, southeastern Inner Mongolia, China: Implications for the final closure of the Paleo-Asian Ocean

The closure of the Paleo-Asian Ocean (PAO) to the north of the North China Craton (NCC) holds significance in comprehending the Phanerozoic continental evolution of East Asia. However, the timing of the final closure of the PAO, tectono-magmatic records of this process in the northern margin of the NCC, and related geodynamics remain to be resolved. Here we undertaken detailed macro- and micro-structural investigations on the E-W-striking and NNE-striking fault system in the Chifeng area, northern edge of the NCC. Combined with new zircon U-Pb and muscovite 40Ar/39Ar dating, we delineated three phases of strike-slip faulting in the research region during the Permian-Triassic, including WNW-ESE-striking and NE-SW-striking ductile sinistral shearing at ∼ 270–258 Ma and 258–246 Ma, respectively, and subsequent ∼ E-W-oriented ductile–brittle dextral shearing at ∼ 230–209 Ma. Based on analysis of paleotectonic stress fields, we propose that the first two-stage deformation shared a regional unified N-NNE contractional setting, while the late ∼ E-W-striking dextral shearing reflected a distinct regional NNW-SSE shortening event. The regional magmatism reveals three age peaks of ∼ 270 Ma, ∼250 Ma and ∼ 230 Ma, corresponding well to the above three-phase deformation. Further, zircon Hf isotopic data imply that the composition of juvenile material in regional deep-seated crust gradually increased from Permian to Triassic. The N-NNE contraction may be controlled by the eastward scissor-like final closure of the PAO during the Middle Permian-Early Triassic. The middle-late Late Triassic NNW-SSE contraction is likely influenced by the remote effect of the low-angle southward subduction of the Mongol-Okhotsk oceanic plate.

Potential geothermal reservoir systems in the Kenyan Great Rift Valley and volcanic region assessed by ambient noise analysis

To identify potential geothermal reservoirs in the Kenyan Great Rift, we analyzed ambient noise data to reveal seismic velocities and map azimuth-based Rayleigh-wave phase velocities. We extracted phase velocity dispersion curves from the ambient noise cross-correlation using the zero-crossing method and constructed S-wave velocity models using 3D surface-wave tomography. S-wave velocities were lower under Silali and Paka volcanoes (i.e., in magmatic regions) and in fractured areas (i.e., geothermal reservoirs) normal to the rift axis. We detected magmas at depths of >3 km beneath the volcanoes. Low-velocity anomalies may be related to fluid migration pathways. Most potential geothermal reservoirs are located above magma bodies and below the rift axis and are associated with intensive fracturing and porous rocks. Our approach used a limited number of seismometers, making it useful for application in areas with restricted access and challenging deployment; therefore, it could be used for the assessment of preliminary results in other magmatically active regions.

Rutile U-Pb dating reveals the Oligocene age of the Dashuigou Te-Bi deposit and a contemporaneity with regional alkaline magmatism in Sanjiang region

The Dashuigou deposit, located in the transitional zone between the Yangtze craton to the east and the Songpan-Ganze orogen to the west, is the world’s only known deposit where tellurium and bismuth are exploited as primary products. The Te-Bi orebodies mainly occur as the dolomite-tetradymite and Te-bearing pyrrhotite veins in Triassic low-grade metamorphic rocks. The ore minerals mainly consist of tetradymite, tsumoite, tellurobismuthite, pyrrhotite, and chalcopyrite. While mineralogy and geochemistry of the ores in the Dashuigou deposit have been well documented in the past decades, the age of the Te-Bi mineralization in the Dashuigou district is not well defined due to lack of both suitable dating mineral and chronometer, and thereby impedes further understanding on its ore genesis.Hydrothermal rutile coeval with tetradymite has been identified in this study from a high-grade dolomite-tetradymite vein in the Dashuigou deposit. This rutile is closely associated with tetradymite, tsumoite, Te-bearing pyrrhotite, and contains many tiny tetradymite inclusions. Here, we present the first in situ trace element compositions and U-Pb age of these rutile crystals using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The rutile is enriched in V, Nb, Ta, Sn, and Cr, but has low contents of As, Sb, and W, indicating a magmatic origin. U-Pb dating on rutile demonstrates a well-constrained age of 32.4 ± 3 Ma, which is interpreted as the timing of Te-Bi mineralization for the Dashuigou deposit. This new age provides the first evidence suggesting that the Dashuigou deposit is contemporary with regional Eocene-Oligocene alkaline magmatism and related metallogenic event in Sanjiang region. Compared with the Beiya Te-Bi-bearing porphyry Cu-Au and Yao’an Au deposits in the alkaline magmatic belt, we propose that the Dashuigou Te-Bi deposit was likely product of a concealed alkaline magmatic-hydrothermal system. This study also demonstrates the rutile U-Pb geochronology being a powerful tool for dating unusual ore systems.

Three episodes of Triassic volcanism in the Eastern Kunlun Orogen, NW China: constraints for evolution of the Palaeo-Tethys Ocean

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB), located north of the Qinghai-Tibetan Plateau, recorded voluminous late Palaeozoic – early Mesozoic intermediate-acid volcanic rocks. The tectonic evolution of the Palaeo-Tethys Ocean from initial subduction and final closure and post-collision extension has been hotly debated. This study examines the petrogenesis of Palaeozoic to Early Mesozoic volcanic rocks at six locations: Xiahe, Elashankou, Jirimai, Harizha, Nagengkangqieer, and Yazigounan. The investigation involves detailed field observations, petrology, zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock geochemical analysis. The rocks analysed represent three episodic volcanic events. The ca. 257–245 Ma volcanic rocks were derived from magma mixing response to the subduction. The ca. 227–220 Ma highly fractionated I-type volcanics were derived from partial melting of the ancient lower crust, with varying contribution proportions of mantle materials. The ca. 216 Ma A-type volcanic rocks derived from partial melting of the juvenile crust. In this study, using existing regional geological data and spatiotemporal distribution of the magmatic region, combined with our new data, we attempt to propose more rational models to reveal the tectonic evolution of the Palaeo-Tethys Ocean, and we recognize three significant time nodes: (1) Ca. 240 Ma marks the final closure time of the Palaeo-Tethys Ocean. (2) Ca. 230 Ma indicates the vital tectonic regime transition from compression to extension response to slab break-off and continued lithospheric delamination. (3) Ca. 216 Ma mark the beginning of the final stage of orogeny in the EKOB, and mild magmatism lasts to ca. 200 Ma in response to the mountain collapse.

The geochronology of pegmatites in the Jiajika lithium deposit, western Sichuan, China: Implications for multi-stage magmatic-hydrothermal events in the Songpan-Ganze rare metal metallogenic belt

The Songpan-Ganze orogenic belt is a large-scale rare metal metallogenic belt in western China. Several large-scale granitic pegmatite-type lithium deposits are located in the Songpan-Ganze orogenic belt, which include the Jiajika Li deposit, one of the largest pegmatite-type Li deposits in China. The geochronological constraints of pegmatites and granite in Jiajika are still inadequate for revealing the relationships between mineralization and tectonothermal events. In this study, we performed columbite-group minerals (CGM) and cassiterite U–Pb dating on seven different pegmatite veins with different occurrences and deformation styles in the Jiajika deposit. These pegmatites yield CGM U–Pb ages of 213 to 208 Ma and cassiterite U–Pb ages of 199 to 192 Ma, regardless of their occurrences and deformation. Columbite-group minerals exhibit low Ta# (=Ta/(Ta + Nb)) cores and high Ta# rims, which were formed during the magmatic stage and the hydrothermal stage, respectively. The appearance of cassitertite is closely related with greisenization. Therefore, the CGM U–Pb ages, which were obtained from the low Ta# cores of CGM, are interpreted as magmatic crystallization age, while the cassiterite ages reflect another younger stage of magmatic-hydrothermal event. According to field observation, we consider that the occurrences and deformation of pegmatite veins are controlled by the development of various fracture systems formed during the ascent of the granitic pluton. Our results suggest long-lasting multi-stage magmatic-hydrothermal events in the Jiajika region. Geochronological data of other rare metal deposits located in the Songpan-Ganze orogenic belt and the Western Kunlun orogenic belt are comparable with those of the Jiajika Li ore deposit, indicating that the entire Songpan-Ganze rare metal metallogenic belt experienced long-lasting and multiple magmatic and fluid activities. The overlaps of the geochronological results of granites and pegmatites in the Songpan-Ganze rare metal metallogenic belt suggest that regional magmatism is closely related to Li mineralization. The enrichment of Li in the Songpan-Ganze rare metal metallogenic belt is possibly related to its unique geodynamic setting and the long duration of magmatic-hydrothermal activities.

Unraveling 800 Ma of pegmatite formation in the western Grenville Front tectonic Zone near Sudbury, Canada

The origin of pegmatites has been at the core of petrological research for decades, but despite much effort their origin remains controversial – protracted fractionation of a causative felsic magma versus anatexis of an appropriate protolith. To address this controversy, we present the results of U-Pb analyses (zircon (n = 774), monazite (n = 230), xenotime (n = 295)) for 32 well characterized pegmatites from an extensive swarm of felsic dike rocks outcropping, along a 12 km transect ca. 12 km south of Sudbury (Ontario, Canada) in the westernmost part of the Grenville Front Tectonic Zone (GFTZ). Collectively these rocks exhibit a remarkably large morphological, textural and mineralogical variation related to the crystallization of three distinct pegmatite-forming events that span 800 Ma. The derived concordant U-Pb ages of the dated phases coincide with the late Paleoproterozoic (ca. 1740 Ma) Yavapai orogeny, the Mesoproterozoic (ca. 1450–1430 Ma) Chieflakian event, and the early Neoproterozoic (ca. 1000–970 Ma) Grenville orogeny. Field observations and related geochemical data suggest the two older events are genetically related to regional magmatism (i.e., fractionation of a progenitor), whereas the Neoproterozoic pegmatites were generated via partial melting of an appropriate protolith at depth. The data therefore support a duality of models for pegmatite genesis. We emphasize the important role of imaging (CL and BSE) as an integral part of our protocol, as this revealed widespread coupled dissolution-precipitation (CDP) replacement textures and metamictization in both zircon and xenotime, the latter due to high U contents (i.e., >1000 pm). Interestingly, the Neoproterozoic pegmatites show similar U(±Th), Nb(±Ta) and REE enrichments like their contemporaneous Sveconorwegian counterparts. Many zircons record U-Pb system resetting at ca. 220–200 Ma which is possibly related to the far-field effects of the onset of the Central Atlantic Magmatic Province (CAMP) related to the opening of the Atlantic Ocean. Finally, this study shows that anatexis is a viable process of rare-element pegmatite formation, even in areas rich in potential parental granites, which suggests that anatectic rare-element pegmatites might be globally under-recognized.

Volcanic rocks from Amensif-Tnirt district in the Western High Atlas (Morocco): Geochemistry, magma features and new age dating

The High Atlas Mountains include mafic, intermediate and felsic volcanic-subvolcanic intrusive rocks, representing a result of magmatism related to “Cambrian rift” during the Lower Cambrian.The investigated magmatic rocks in the Amensif-Tnirt area are divided into three main groups as the mafic (SiO2 48.31 to 53.06 wt%), intermediate (SiO2 59.16 to 67.38 wt%), and felsic (SiO2 76.39 to 79.65 wt%) rocks. The mafic rocks correspond to microgabbros with high TiO2 contents (1.98–2.12 wt%) and FeOt/MgO ratios (1.65–1.90), La/Yb (9.09–11.60) and Nb/Zr (0.03–0.04) ratios, providing good evidence of extensive intracontinental tholeiite. The Nb/Yb and Zr/Y ratios ranging between 0.36 to 0.52 and 12.76 to 13.69, respectively suggest their derivation from an enriched mantle source during the opening of the Rheic ocean along the NW edge of Gondwana. The mafic rocks may be interpreted as the mixtures of melts derived from two contrasting mantle sources: (1) a garnet-bearing clinopyroxenitic ultramafic source and (2) an ultramafic source composed of spinel and peridotite.The intermediate and felsic rocks correspond to andesite-dacite and rhyolite with TiO2 contents and FeOt/MgO ratios ranging from 0.09 to 1.36 wt% and 1.32 to 4.69, respectively, suggesting their calc-alkaline affinities setting up during a compressive context. The youngest 206Pb/238U ages for intermediate rocks are 290.29 ± 4.68 and 291.27 ± 4.31 Ma, while the youngest felsic rocks yield a concordia age at 272.83 ± 0.73 Ma (MSWD = 0.25). Petrographic and geochemical data indicate that mafic magmatism in the Amensif-Tnirt region may be the equivalent of the first Cambrian volcanic episode in the High Atlas. Intermediate rocks are chronologically correlated to the Permian volcanic activities recorded in Central Morocco (Tiddas, Khenifra, Chougrane …) whereas felsic activity is correlated to the Permian Azegour granite from the High Atlas.

NEW GEOCHRONOLOGICAL AND ISOTOPE DATA ON MESOZOIC GRANITOIDS OF EASTERN TRANSBAIKALIA

Eastern Transbaikalia is a unique rare-metal province of Russia. Most of the studied deposits are associated with granitoid intrusions. Despite the high degree of knowledge of regional magmatism, a number of questions remain related to both the belonging of specific massifs to magmatic complexes and the correlation of certain complexes within various structural-formation zones. This paper compares the material composition and age of several petrotypical and parapetrotypical massifs classified as Late Mesozoic complexes distributed within the Aginskaya, Prishilkinskaya, and Argunskaya structural-formational zones of Eastern Transbaikalia. Based on the results of U-Pb dating, two stages of granitoid magmatism have been established: 1) the end of the Middle – the beginning of the Late Jurassic (163–156 million years ago) and 2) the beginning of the Early Cretaceous (142 million years ago). It is shown that the closely aged rocks of the Borshchovochny, Amudzhikano-Sretensky and Shakhtama complexes have similar material characteristics, while the rocks of the Kukulbey complex (from three massifs) vary in composition. Elevated δ18O values (from +10.8‰ to +11.9 ‰) for the rocks of the Jurassic complexes indicate their formation mainly due to the melting of crustal material, and lower δ18O values (from +9.8 ‰ up to +10.3‰) for the Early Cretaceous granitoids suggest the contribution of a mantle source to their genesis.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

Abstract Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.

Modern-style subduction during the late Neoarchean to early Paleoproterozoic? Geochemical evidence from ca. 2.45 Ga arc-type magmatism in the Feidong Complex, northeastern Yangtze Craton, South China

The late Neoarchean to early Paleoproterozoic tectonic evolution of the Yangtze Craton remains poorly constrained as a result of the poor exposure of contemporaneous basement rocks. This study presents new zircon U–Pb–Hf isotopic data and whole–rock geochemical and Sr–Nd isotopic data for orthogneisses from the Feidong Complex of the northeastern Yangtze Craton. Two orthogneiss samples yielded weighted mean ages of 2416 ± 21 and 2436 ± 21 Ma, respectively, indicating the timing of protolith formation. These samples contain relatively high concentrations of SiO2 (54.04–63.25 wt%), but low concentrations of Na2O (3.29–4.12 wt%) and K2O (1.87–2.84 wt%), indicating they are calc–alkaline intermediate rocks. They are enriched in the light ion lithophile elements (LILE) and the light rare earth elements (LREE) but are depleted in Nb–Ta, Ti, the heavy REE (HREE), and Y. They also plot within the arc affinity fields of (La/Yb)N vs. (Yb)N and Sr/Y vs. Y diagrams, consistent with their relatively high Th/Yb and Ta/Yb and low Ce/Pb and Nb/U values, all of which are indicative of rocks formed by arc–type magmatism. The two orthogneiss samples have zircon εHf(t) values that vary between –6.83 and +2.54 with two–stage model ages (TDM2) of 3.35–2.79 (peak at 3.2 Ga) and with similar Nd isotopic compositions. The high total REE contents and Nb–Ta depletions within the orthogneiss samples may reflect derivation from magmas sourced from regions containing fluids released from a subducting slab. These data indicate that the calc–alkaline protoliths of these orthogneisses formed from magmas generated by the partial melting of mantle wedge material driven by the addition of fluid released from a subducted slab. This process induced the partial melting of ca. 3.2 Ga lower crustal material in a manner similar to that associated with modern arc–type magmatism. Taken collectively, the regional magmatism recorded by the ca. 2.50–2.47 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses of the Douling Complex, the ca. 2.45 Ga calc–alkaline arc-type magmatic rocks in the Feidong Complex, and contemporaneous metamorphism within the north margin of Yangtze Craton all provide evidence that modern–style subduction occurred within the northeastern Yangtze Craton between the late Neoarchean and the early Paleoproterozoic.

Late Jurassic syn-rift deposition in the Flemish Pass basin, offshore Newfoundland: Evidence for Tithonian magmatism and Appalachian-Variscan sediment sources from quantitative mineral and detrital zircon U–Pb-Hf isotope studies of Mizzen discovery strata

Quantitative mineral and detrital zircon U–Pb-Hf isotope studies of syn-rift sandstone units in the Mizzen F-09 delineation well, northern Flemish Pass basin, were conducted to test competing models for Grand Banks paleogeography and Late Jurassic development of the Newfoundland-Iberia magma-poor rift system. Bodhrán formation (informal nomenclature) strata yield ca. 145–155 Ma detrital zircon grain populations that indicate a late Tithonian (146 ± 1 Ma) maximum depositional age for syn-rift fluvial to tide-modulated fluvial strata in Mizzen F-09 and evidence for Grand Banks rift magmatism to have occurred throughout the Late Jurassic. Late Tithonian tectonic subsidence and magmatism in the northern Flemish Pass basin region were coincident with the onset of necking processes and extreme crustal thinning within the Newfoundland-Iberia rift system. The chondritic to superchondritic Hf isotope compositions of Late Jurassic detrital zircon grains indicate repeated partial melting of lithospheric mantle during rift development, perhaps along master detachment faults or transfer zone systems. Bodhrán formation strata have chemically unstable minerals (plagioclase, potassium feldspar, garnet, muscovite, rutile, staurolite) and late Paleozoic to Paleoarchean detrital zircon age populations that together show provenance from Appalachian-Variscan basement and cover assemblages. A positive correlation between the abundance of metamorphic minerals and unique-aged, mid-Permian (ca. 271–280 Ma) detrital zircon grains from post-Variscan igneous rocks indicate Iberian provenance contributions to Bodhrán formation strata. The new results from Mizzen F-09 are consistent with models for WNW-directed fluvial discharge into the northern Flemish Pass basin and sediment derivation from upper Paleozoic foreland basin or cover assemblages located west of Variscan front, some which may have been stripped off Beothuk Knoll, Flemish Cap, or adjacent highlands.

Differentiation of continent crust by cumulate remelting during continental slab tearing: Evidence from Miocene high-silica potassic rocks in southern Tibet

The role played by felsic igneous magmatism in crustal differentiation is of importance for the evolution and habitability of Earth system. However, the petrogenesis of felsic rocks and their genetic links to mafic magmas remain controversial, hindering our understanding of crustal differentiation processes. Cenozoic post-collisional felsic potassic rocks occur widely in the well-known Himalaya-Tibet orogen and are intimately associated, in space and time, with mantle-derived ultrapotassic rocks, which thus provide a clear case study to investigate the genesis of felsic rocks and their genetic relationship with mafic rocks. Here we report zircon UPb ages and HfO isotopes, mineral major elements, and whole-rock elemental and Sr-Nd-Pb isotopic compositions for the TangraYumco high-silica potassic rocks (trachytes) in central Lhasa block of southern Tibet. The results, combined with literature data of the TangraYumco ultrapotassic rocks, can help better illuminate their genesis and the role of ultrapotassic rocks in their generation. The trachytes erupted in the Miocene (~13 Ma), coeval with ultrapotassic lavas in this area. They have very enriched Sr-Nd-Pb-Hf-O isotope compositions similar to those of ultrapotassic rocks. However, they display more fractionated LREEs (light rare earth elements) than ultrapotassic rocks. The trachytes also show nearly constant Y and Yb contents with increasing SiO2. This indicates that they were not derived by fractional crystallization from ultrapotassic magmas or by mixing between ultrapotassic and ancient crust-derived felsic magmas. Instead, given their lower LREE contents than ultrapotassic rocks and occurrence of clinopyroxene antecrysts, we suggest that the TangraYumco trachytes were likely produced by remelting of newly-formed, Sr-Nd-Pb isotopically enriched, and K-rich cumulates that crystallized from ultrapotassic magmas. Moreover, taking into account regional magmatism and tectonics, we propose that the TangraYumco-Xuruco post-collisional coeval magmatism and rift system represent a response to tearing of subducted Indian continental slab. Finally, in combination with the nature of post-collisional magmatism in the Lhasa block, we suggest that in addition to reworking of juvenile and ancient crust, underplating of post-collisional mafic magmas and remelting of their cumulates could be an important way to differentiate continental crust in collisional orogeny zones.

Petrogenesis of Permian to Triassic granitoids from the East Kunlun orogenic belt: implications for crustal evolution during oceanic subduction and continental collision

ABSTRACT The study of Permian-Triassic granitoids, which widely occur within the East Kunlun Orogenic Belt (EKOB), is relevant for a better understanding of the evolution of the Paleo-Tethys Ocean and regional crustal evolution. This study focuses on the Permian-Triassic granitoids from the Nagengkangqieer region, east segment of EKOB. The Late Permian granodiorite (ca. 252 Ma) shows high-K calc-alkaline and metaluminous features with moderate SiO2 (63.31–67.04 wt.%), relatively high Mg# (48–52), and low Sr/Y ratios. The granodiorite is enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) and shows enriched isotopic compositions (εNd (t) = −5.53 to −5.14, εHf (t) = −5.6 to −1.1). These geochemical characteristics imply that the Late Permian granodiorite was derived from a mixing source of mafic lower crust and mantle materials (<30%). The Middle Triassic porphyritic monzogranite (ca. 239 Ma) and monzogranite (ca. 239 Ma) are high-K calc-alkaline to shoshonitic and weak peraluminous with high SiO2 (70.58–76.54 wt.%) contents and low Mg# (19–36). They display relatively higher FeOT/MgO ratios, lower P2O5 contents, and more enriched isotopic compositions (εNd (t) = −7.87 to −6.71, εHf (t) = −13.7 to −1.2) than those of granodiorites. These findings indicate that the Triassic granites belong to fractionated granites derived from partial melting of the lower continental crust. In combination with the results of this study and the spatio-temporal distribution of regional magmatic, metamorphic, and sedimentary events, we conclude that Late Permian granodiorite formed in a subduction setting, whereas the Middle Triassic granites were formed in a syn-collision setting. Closing of the Paleo-Tethys Ocean occurred in the Middle Triassic (ca. 240 Ma). Additionally, only a small proportion of juvenile materials (<30%) was involved in magmatism and contributed to continental crustal growth during Late Permian subduction, whereas crustal reworking dominated regional magmatism and resulted in a higher crustal maturity.