Rocks Of Age Research Articles

Geochemical and zircon 18O–Hf–U/Pb isotopic study of volcanic rocks from Lutao Island in the northern Luzon magmatic arc

AbstractLutao is an islet off the eastern coast of the main island of Taiwan and is composed of pyroclastic agglomerates, volcanic breccias, lava flows, and dykes. There have been no reports of SIMS zircon oxygen isotopic data from Lutao Island so far, which has significantly hampered further understanding of its magmatic processes, such as crystallization, evolution, and generation of magma. In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, origin, and magmatic processes of the volcanic rocks from Lutao Island. Integration of our new U–Pb and Hf–O isotope data with available published data allows us to better assess the timing and magmatic processes of Lutao Island in the context of the northern Luzon arc system. This work is the first report of SIMS zircon oxygen isotopic data of volcanic rocks from Lutao island. This study shows that the volcanic rocks from Lutao Island were generated by mantle-derived magma associated with high-T° hydrothermal alteration by seawater-derived fluids.

Method of estimating sea-surface paleotemperatures through biotic proxies: A case study in Upper Paleozoic paleoclimatic, paleogeographic and paleotectonic reconstructions of Siberia.

This study introduces a novel approach for quantitatively assessing sea-surface paleotemperatures examined in the Upper Paleozoic of Siberia, utilizing the obtained in the region data as a case study of the use of this method. The method relies on evaluating the taxonomic composition and ecological proxies of biota. It utilizes a comprehensive dataset encompassing the geographic distribution and ecology of various biotic groups in Siberia and adjacent regions, leveraging the newly developed by the authors large PaleoSib database and partially the Paleobiology Database (paleobiology.org) The taxonomy has been used according to the database of Global Biodiversity Information Facility (gbif.org). Fossils collected from individual locations often exhibit a wide spectrum of paleotemperatures. To address this variability, we developed an algorithm for calculating average biotic paleotemperatures in each locality/time slice. Our computations of the available data have unveiled a coherent pattern of paleoclimate dynamics, particularly Sea Surface Temperature, across Siberian basins and surrounding areas during the Late Paleozoic era. These findings significantly contribute to a refined comprehension of paleoclimate and paleotectonic dynamics in the region during that specific time. To enhance paleotemperature analyses, we have integrated lithological indices with biotic ones, fortifying the overall methodology and furnishing a more robust framework for interpreting paleoclimate data. The method could be a helpful tool in regional and interregional studies, regardless of the utilized rock's age and fossil groups.

Processes controlling magma fertility at buenavista del cobre porphyry copper deposit (cananea, méxico): A new petrogenetic model based on zircon u-pb dating and apatite geochemistry

The Buenavista del Cobre is a world-class porphyry Cu-Mo deposit located in the Cananea Mining District, northern Sonora, México. Using zircon U-Pb dating, we show that the Proterozoic Cananea Granite, unconformably overlain regionally by Paleozoic limestones hosting skarn deposits underlies the present-day pit. We also present new crystallization ages for host rocks of copper mineralization in the District, dating for the first time a volcanic rock of the Henrietta Formation at 186.8 ± 1.1/3.0 Ma and the El Torre Syenite at 176.3 ± 1.1/2.9 Ma. Zircon U-Pb dating of the different porphyritic bodies reveals that the magmatic activity at Buenavista del Cobre lasted at least 4 Myr, from 59.7 ± 0.5/1.1 Ma to 56.1 ± 0.2/0.9 Ma. The deposit is composed by several porphyry intrusions referred to as “ore-rich” and “ore-poor” based on their individual metal contributions, which provides the opportunity to study the origin and processes enhancing magma fertility in an individual deposit. Combining our new geochronological dataset with geochemistry of apatite from the different porphyries allows us to propose a new petrogenetic model for the Buenavista del Cobre deposit. As the apatite Eu and Ce anomalies overlap with no clear difference between the ore-rich and ore-poor intrusions, we propose that the magmatic oxidation states of the magmas were similar. However, differences in apatite REE signatures, as well as variations in apatite Sr compositions between the two groups suggest that fractional crystallization processes in the parental magma influence the fertility of the porphyries. Additionally, apatite Cl contents of ore-rich porphyry intrusions are higher (>0.4 wt%) than the ore-poor intrusions (<0.2 wt%), suggesting an important role of the initial Cl content of the magmas in the mineralization process. These observations give new insights on the petrogenetic processes at origin of porphyry magma fertility. We propose that the evolution of the parental melt by fractional crystallization of hydrous minerals (hornblende) at upper crustal levels induced low H2O content of the residual magma, resulting in the formation of ore-poor porphyries. In contrast, we suggest that fractionation of anhydrous minerals (plagioclase) increased the H2O content in the residual melt, leading to the formation of ore-rich porphyries. Our new data allow us to propose an original genetic model for the Buenavista del Cobre deposit, which involves two cycles of supply, cooling and partial crystallization. This contribution shows that petrogenetic processes controlling porphyry copper magmas fertility are recorded in the composition of apatite at the deposit scale and highlights the importance of considering apatite geochemistry as an exploration tool.

In-situ zircon and cassiterite LA-ICP-MS geochronology and implications for granite-hosted Sn deposit models and exploration: Insights from the Cameroon Line

Abstract Tin mineralization of significant economic importance occurs across the continental portion of the Cameroon Line (CL). Tin deposits therein occur as both primary and secondary (residual and alluvial) ore. Though the temporal and, by inference, the genetic link between Sn mineralization and the host granite had long been modeled and widely accepted worldwide, in the CL, however, the age of the granite hosting cassiterite is poorly constrained, preventing a robust assessment of the temporal and genetic relationship between the Sn mineralization and its host rock. Here, we present in-situ zircon and cassiterite laser ablation inductively coupled mass spectrometry (LA-ICP-MS) U-Pb data in order not only to constrain the age of the granitic rock hosting the primary Sn ore but also to bracket the time frame of Sn mineralization, with respect to the magmatic-hydrothermal evolution of the parental magma of the host granite. Zircon from two greisen-altered, cassiterite-bearing granite samples yield overlapping and concordant ages of 64.21 ± 0.59 Ma and 65.46 ± 0.95 Ma, respectively, which are also overlapping with regional granite magmatism in the CL (ca. 65–30 Ma). On the other hand, cassiterite, which is spatially associated with the Paleocene zircon, yields Lower Eocene ages of 54.99 ± 0.35 Ma and 56.08 ± 0.46 Ma. The ca. 10 Myr time gap between zircon and cassiterite suggests that the granite is a passive host not genetically related to the Sn mineralization, which may be linked to a younger, concealed intrusion of ca. 55 Ma. This finding contrasts with the most widely accepted petrogenetic model of tin granite, according to which Sn mineralization and the host granite are cogenetic.

Paleoproterozoic sediment-derived magmas reveal a late orogenic stage in the southern São Francisco Craton, Brazil: Evidence from petrogenesis of the 2.0 Ga Cupim Pluton leucogranites

Sediment-derived granites (S-type) can provide detailed information about continental crust reworking processes and tectonic regimes through time. Their compositions reflect the nature of sedimentary rocks that melted to form them. Inherited accessory minerals in these granites can provide information regarding the age and nature of the crustal rocks that contributed to the formation of their sedimentary source rock. In addition, S-type granites typically form during collisional stages in modern orogens, and their Precambrian records can provide clues about ancient tectonics. In this contribution, we present a detailed petrogenesis of the Cupim Pluton, located in the Paleoproterozoic Mineiro Belt (Brazil). The Cupim pluton is composed of two-mica and garnet-bearing leucogranites that are strongly peraluminous (ASI > 1.1), high-K (K2O/Na2O > 1), low MgO + FeO + TiO2 (<2 wt%). Its composition is similar to that of melts experimentally produced by muscovite/biotite dehydration melting reactions. Rb/Sr and Rb/Ba ratios suggest that these leucogranites are formed by the melting of heterogeneous sedimentary source(s). Zircon crystals from the Cupim Pluton unveil a protracted crustal melting period (2.01–1.98 Ga) hitherto unknown in the Mineiro Belt, which itself discloses a late collisional orogenic stage in southern São Francisco Craton. Igneous zircon crystals exhibit εHf(t) between −16.4 and − 0.6, and δ30Si between −1.16 and − 0.26 ‰, which is typical for zircon from sediment-derived granites. Hafnium isotope variations reflect mixing between at least two crustal sources, which is in line with inherited zircon cores that evidence major Archean and minor Paleoproterozoic sediment contributions. The emplacement of the Cupim Pluton leucogranites ends a subduction-collision cycle, following a long period of TTG-sanukitoid magma production, and promoted the stabilization of the southern São Francisco Craton.

Age, Mineral Chemistry, and Geochemistry of Metamorphic Basement Rocks from the Southern Yap Arc, Western Pacific

Studying the basement characteristics and tectonic evolution of the proto-Yap Izu–Bonin–Mariana arc system can provide essential clues for understanding the subduction process in the Western Pacific Ocean. The Yap arc is a part of the Izu–Bonin–Mariana (IBM) arc, but the origin and formation time of the metamorphic basement rocks of the Yap arc still need to be determined. In this paper, we present the first systematic and detailed chronological, mineralogical, whole-rock major element and trace element research on metamorphic basement rocks from four stations in the Southern Yap arc and discuss the origin of the metamorphic basement rocks in the Southern Yap arc. The results show that the single mineral U-Pb ages of titanites and 40Ar/39Ar ages of amphiboles in the Southern Yap arc are 19.3 and 19.4 Ma, respectively, which are consistent within the error ranges, and these ages represent the metamorphic age of the basement rocks. The amphibolite is mainly produced via the metamorphism of back-arc basin basalt (BABB) and forearc basalt (FAB). The temperature and pressure conditions of this metamorphism are 446.5–641.6 °C and 0.24–0.73 GPa, the facies series of which is the medium P/T series. This study provides important data for understanding the Yap subduction system and the early evolution of the proto-IBM Arc.

Geochronology and petrogenesis of Early Mesozoic Rb-mineralized granitic rocks from the Liuzhangshan Rb, West Qinling Orogen Belt, China

ABSTRACT The newly discovered Liuzhangshan (LZS) Rb deposit is a large granite-type deposit in the West Qinling Orogenic Belt (WQOB), spatially associated with biotite monzogranite. Previous research has focused on the geological characteristics and metallogenic conditions of the LZS Rb deposit; however, its geochronology and petrogenesis remain unclear. In this study, monazite U-Pb dating of biotite monzogranite in the LZS deposit yielded a U-Pb age of 200.8 ± 0.9 Ma via laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Combined with the age of magmatic rocks in the WQOB, we propose a potential mineralization event related to Rb around 200 Ma in the WQOB. The biotite monzogranite belongs to weakly peraluminous, high-K calc-alkaline serious, containing high SiO2, Na2O, K2O, and low FeO, MgO, CaO, and P2O5. Compared with primitive mantle and relatively unfractionated granodiorites found in the WQOB, the biotite monzogranite enriched in Rb, Th, U, and Pb, deleted in Ba, Sr, and Ti, and showed negative Eu anomalies, a high Rb/Sr ratio, and a low K/Rb ratio, an ∑REE of 31.3 ppm, an LREE/HREE ratio of 6.48, a LaN/YbN ratio of 2.59, an Mg# of 4.15, and a crystallization temperature of 760°C, the biotite monzogranite also had a high differentiation index (DI = 98.2), apparent tetrad effect (TE 1,3 = 1.12 ~ 1.15), characteristic of highly fractionated I-type granite. Monzoite from the biotite monzogranite in the LZS deposit has negative εNd(t) values ranging from −12.45 to −11.17, and TDM1 of 1569 ~ 2018 Ma, indicating that the LZS biotite monzogranite originated from the partial melting of a Mesoproterozoic ancient basement material, with a minor contribution from mantle-derived melts. Highly differentiated, low Sr and Yb contents, Sr/Yb ratios, and low crystallization temperature magma facilitated the enrichment of Rb.

New results of the U/Pb dating of cretaceous igneous rocks of the Komsomolsk ore district (Middle Amur River area)

The Komsomolsk tin ore district is located at the Miaochan Ridge in the Middle Amur River area (Khabarovsk territory) within the Badzhal accretionary terrane of the Sikhote-Alin orogenic belt. The paper presents new results of U-Pb zircon dating of tonalite of the Miaochan complex of the Silinka granitic pluton (99.7±1.1 Ma) and two rhyolites of the lower part of the Kholdami Formation (98.4±1.0 and 98.7±1.2 Ma) from the north-eastern part of the Komsomolsk district. The geochemical characteristics of rocks were also studied, which show that granitoids of the Miaochan complex are I-type granites, and coeval rhyolites of the Kholdami Formation are very peraluminous S-type rocks. New data on the geochemistry and early Cenomanian age of the igneous rocks allows us to consider the ore-magmatic system of the Komsomolsk ore district as part of the previously identified Albian-Cenomanian igneous province of Pacific Asia.

Timing of the subduction initiation and ophiolite emplacement of the inner Tauride Ocean: Insight from the Pınarbaşı ophiolite in Central Türkiye

Late Cretaceous ophiolites are widespread in southern Türkiye, including the Tauride Belt Ophiolites and the Central Anatolian Ophiolites. Here, we report new geological, geochemical, geochronological, and zircon isotopic data from ophiolitic rocks of the Pınarbaşı ophiolite (Eastern Tauride Belt). The Pınarbaşı ophiolite forms incomplete sequences, with its mantle section dominated by serpentinized harzburgite and chromitites, while the crustal section comprises ultramafic and mafic cumulates. Whole-rock and mineral chemistry data suggest a supra-subduction zone origin. LA-ICP-MS zircon U-Pb age of 88.2 ± 0.4 Ma (MSWD = 0.84) was obtained from a plagiogranite dyke cutting serpentinized peridotite of the Pınarbaşı ophiolite. Another, U-Pb zircon crystallization age of 96.8 ± 0.6 Ma (MSWD = 1.53) was obtained from a plagiogranite dyke within the mélange of the Pınarbaşı ophiolite. Hafnium isotope compositions of these zircons (εHf(t) = +26.4 to +3.1) suggest a depleted to slightly enriched mantle source. The formation ages of the crustal rocks (gabbro, plagiogranite, and dolerite) of the Cretaceous ophiolites in the Tauride Belt from this and previous ophiolite occurrences, suggest that supra-subduction zone ophiolites in the Inner Tauride Ocean formed during the period ∼101–87 Ma. On the other hand, the peak metamorphism of the metamorphic sole rocks is reported to have occurred around ∼104 Ma. According to all available petrogenetic and geochronological data, it is considered that the supra-subduction zone crust developed shortly (∼ 3–4 Myr) after the initiation of intra-oceanic subduction and this lasted ∼10–15 Myr.

The Holleford structure: Insights into the pre- and post-impact geology of the Frontenac Terrane, Canada

A multi-chronometric investigation has been conducted on the ∼2.0-km-diameter Holleford impact structure of Ontario, Canada. In situ triple quadrupole laser ablation−inductively coupled−mass spectrometry U-Pb and Rb-Sr geochronology, along with (U-Th)/He thermochronology, have been applied to multiple mineral phases from impact melt-bearing breccias and overlying sedimentary units. Impact-induced resetting is absent in all three isotopic systems. While Holleford’s small size resulted in insufficient heating to reset geochronometers during impact, our investigation has defined a more detailed pre- and post-impact geologic setting within the Precambrian basement and overlying Paleozoic lithologies. Target rock age determinations include those associated with the Proterozoic Frontenac Terrane. These are manifest as the Rigolet (1004 ± 5 Ma, U-Pb apatite dating) and Ottawan (1074 ± 12 Ma, Rb-Sr biotite dating) phases of the Grenville Orogeny. Pre-Grenville ages associated with the Penokean (1806 ± 46 Ma, apatite dating) and the Shawinigan (1184 ± 8 Ma, U-Pb titanite dating; 1134 ± 21 Ma, U-Pb zircon dating) orogenies are also retained. A Paleoproterozoic age records Superior Province tectonometamorphism (2421 ± 97 Ma, U-Pb zircon dating). Dated post-impact events include dolomitization associated with Silurian marine transgression (430 ± 14 Ma, U-Pb dolomite dating) and regional K-metasomatism of Mississippian age (346 ± 5 Ma, U-Pb titanite dating). Extremely young (U-Th)/He ages (27−3 Ma) were obtained from planar fractured and recrystallized apatite, which we interpret to be the result of impact-induced diffusion domain reduction. This, combined with low-temperature heating due to burial, significantly limited He retention. Stratigraphic constraints place the Holleford impact event at ca. 458 Ma, close to the Darriwilian−Sandbian boundary of the Ordovician. This age indicates that Holleford may be part of the Late Ordovician group of terrestrial impact craters related to L-chondrite parent breakup.

Late Paleozoic tectonic switching and metallogenic evolution of the southern Yili arc terrane, NW China

Recognition of tectonic switching is of great significance for understanding the formation and evolution of ancient accretionary orogenic belts and can provide insights into metallogenic evolution. The western Tianshan orogenic belt, which lies within the southern Central Asian Orogenic Belt (CAOB), experienced a prolonged tectono-magmatic evolution during the late Paleozoic in response to evolution of the Paleo-Asian Ocean. The aim of this contribution is to constrain the subduction process and timing of tectonic switching from subduction to collision in southern parts of the western Tianshan. To address this, we present the results of an integrated investigation of whole-rock major and trace element and Sr-Nd-Hf isotope data, together with zircon U-Pb ages for late Paleozoic volcanic rocks from the southern Yili arc terrane of the southern western Tianshan. Based on the temporal-spatial distribution of different compositions of volcanic rocks, three distinct episodes of magmatism have been recognized, including: 1) ∼ 380 – ∼340 Ma, this episode of magmatism has high Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and show large variations in εHf(t) (−9.6 to + 15.6) and εNd(t) (−5.2 to + 5.4) values, which is interpreted to be originated from a mantle source with variable input of slab-derived components incorporated in a trench advance setting; 2) ∼ 340 – ∼320 Ma, this episode of magmatism shows low Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and relatively higher εHf(t) (−3.8 to + 14.6) and εNd(t) (−1.5 to + 5) values, reflecting crust-mantle interactions and likely formation in a back-arc basin environment; 3) ∼ 320 – ∼300 Ma, this episode of magmatism has Ce/Y ratios of basalts and Ho/Yb ratios of felsic rocks, and mostly positive εHf(t) (−1.1 to + 14.5) and εNd(t) (+1.6 to + 8.4) values, suggesting continent–continent collision between the northern Tarim Craton and the southern limb of the Kazakhstan composite continent (i.e., Yili and Chinese Central Tianshan Block). Consistently, the two tectonic switching events (ca. 340 Ma and ca. 320 Ma) were temporally associated with skarn Zn-Pb and orogenic Au mineralization in southern Yili arc terrane, respectively. This study presents an example of multiple tectonic switching and metallogenic evolution in hot accretionary orogens.

Oil and Gas Content of the South Caspian Basin

Abstract —We have compiled maps of areal isotope composition changes and constructed graphs of gas HC distribution depending on the stratigraphic age of the host rocks and graphs of isotope composition changes depending on the depth of sampling to study the hydrocarbon potential of the deep-seated oil and gas pools of the South Caspian Basin. The genesis of gas was studied using the techniques elaborated by M. Schoell and A. James. Two stages of hydrocarbon formation have been established by study of various kinds of gas manifestation. The first stage (Miocene–Eocene) began in the deposits underlying the Productive Series and continued up to the deposits of the latter. This stage was characterized by a frequent change in the directions of both downward and upward movements of gases. The second stage of hydrocarbon formation (Anthropogene–Pliocene) began in the deposits of the Productive Series and was characterized by a change in the regional geodynamic setting. Avalanche sedimentation and the predominance of downward movements over upward ones favored the accumulation of thick sediments at the time when the Productive Series formed. The sedimentation and tectonic processes (downwarping) in the deep-water zone of the basin led to harsh thermobaric conditions in the sedimentary strata. A detailed analysis of the results of gas survey in the deep-water zone of the South Caspian Basin has shown gas generation with a predominance of two components, methane and ethane. Study of the trends of temporal and areal hydrocarbon migration and of the areas of oil and gas generation makes it possible to reveal structures with evidence for large and giant hydrocarbon reserves. We have established that gas hydrocarbons in the bottom and upper-section sediments of the southern Caspian Sea are intimately related to the sources of hydrocarbons, their migration, and other processes running in the deep-seated sediments and in the upper section.

40Ar/39Ar age of phengite from sandstone of the ophiolite-derived clastic sequence of the basin of the Rassokha River, collision belt of the Chersky Range

The Early Permian (275.3 ± 3.1 Ma) 40Ar/39Ar plateau age of detrital mica (Cr-phengite) from clasts in listvenite sandstones of the ophiolite-derived clastic sequence of the Rassokha Terrane of the Chersky Range probably corresponds (or is close) to the time of the formation of listvenites of the provenance (ophiolitic massifs of the range) and restricts the maximum deposition age of clastic rocks. A partial loss of Ar by mica as a result of deformations and postsedimentation transformation of rocks of the ophiolite-derived clastic sequence in the Early Permian is possible but it is less probable.

Position of the Precambrian-Cambrian boundary in the sections of Western Mongolia according to the malacological data

The Early Permian (275.3 ± 3.1 Ma) 40Ar/39Ar plateau age of detrital mica (Cr-phengite) from clasts in listvenite sandstones of the ophiolite-derived clastic sequence of the Rassokha Terrane of the Chersky Range probably corresponds (or is close) to the time of the formation of listvenites of the provenance (ophiolitic massifs of the range) and restricts the maximum deposition age of clastic rocks. A partial loss of Ar by mica as a result of deformations and postsedimentation transformation of rocks of the ophiolite-derived clastic sequence in the Early Permian is possible but it is less probable.

Age and Sources of Rocks of the Kodar Group of the Udokan Complex (Aldan Shield): Results of Geochemical, U–Th–Pb (LA-ICP-MS) Geochronological and Nd–Hf Isotopic Studies

Age and Sources of Rocks of the Kodar Group of the Udokan Complex (Aldan Shield): Results of Geochemical, U–Th–Pb (LA-ICP-MS) Geochronological and Nd–Hf Isotopic Studies

Integration of new zircon U–Pb ages with biostratigraphy to establish a high-precision age model of the Miocene Nakayama Formation on Sado Island in Central Japan

The most common age constraint for the diatomaceous sediments is biostratigraphy of siliceous microfossils. Although biostratigraphy is a powerful tool to establish stratigraphy and correlate with sedimentary sequences in other sites, biostratigraphy generally includes uncertainties difficult to evaluate. In this study, we measured zircon U–Pb ages of eight tuff beds intercalated with diatomaceous mudstone of the Nakayama Formation on Sado Island in Central Japan and integrated the U–Pb ages with diatom and radiolarian biostratigraphy, whose ages and errors were re-evaluated by this study, to establish an age model precisely representing the sedimentary age. Two tuff beds in the upper and middle part of the formation offered zircon U–Pb ages of 6.7 ± 0.2 Ma and 10.87 ± 0.07 Ma, which are consistent with biostratigraphy, and provided a good example of effective integration of zircon U–Pb ages with the biostratigraphy. On the other hand, zircon U–Pb ages of the other six tuff beds in the lower part are around 12 Ma and not distinguishable from each other. In addition, older zircon grains in the 6 tuff beds are assembled in the interval from 30 to 20 Ma, which is consistent with the age of the volcanic basement rocks forming most part of Sado Island. Similarities in chemical compositions of glass shards and age distributions of zircon grains indicate that the volcaniclastic components in the tuff beds should originate from single or associated magmatic activities.

Reply to the comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024)

Reply to the comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024)

Comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024): Geologos 30, 1 (2024): 1–16

Comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024): Geologos 30, 1 (2024): 1–16

The Genesis of Ultramafic Rock Mass on the Northern Slope of Lüliang Mountain in North Qaidam, China

The ultramafic rock located on the northern slope of Lüliang Mountain in the northwestern region of North Qaidam Orogen is altered to serpentinite. The occurrence of disseminated chromite within the serpentinite holds significant implications for understanding the petrogenesis of the protolith. This work provides strong evidence of a distinct zonal texture in the chromite found in the ultramafic rock, using petrographic microstructure and electron probe composition analysis. The core of the chromite is characterized by high contents of Cr#, with enrichment in Fe3+# (Fe3+/(Cr + Al + Fe3+)) and depletion in Al2O3 and TiO2. The Cr2O3 content ranges from 51.64% to 53.72%, while the Cr# values range from 0.80 to 0.84. The FeO content varies from 24.9% to 27.8%, while the Fe2O3 content ranges from 5.19% to 8.74%. The Al2O3 content ranges from 6.70% to 9.20%, and the TiO2 content is below the detection limit (&lt;0.1%). Furthermore, the rocks exhibit Mg# values ranging from 0.13 to 0.25 and Fe3+# values ranging from 0.07 to 0.12. The mineral chemistry of the chromite core in the ultramafic rock suggests it to be from an ophiolite. This ophiolite originated from the fore-arc deficit asthenosphere in a supra-subduction zone. The estimated average crystallization temperature and pressure of the chromite are 1306.02 °C and 3.41 GPa, respectively. These values suggest that the chromite formed at a depth of approximately 110 km, which is comparable to that of the asthenosphere. The chromite grains are surrounded by thick rims composed of Cr-rich magnetite characterized by enrichment in Fe3+# contents and depletions in Cr2O3, Al2O3, TiO2, and Cr#. The FeO content ranges from 28.25% to 31.15%, while the Fe2O3 content ranges from 44.94% to 68.92%. The Cr2O3 content ranges from 0.18% to 23.59%, and the Al2O3 and TiO2 contents are below the detection limit (&lt;0.1%). Moreover, the rim of the Cr-rich magnetite exhibits Cr# values ranging from 0.90 to 1.00, Mg# values ranging from 0.01 to 0.06, and Fe3+# values ranging from 0.64 to 1.00, indicating late-stage alteration processes. The LA-ICP-MS zircon U-Pb dating of the ultramafic rock yielded an age of 480.6 ± 2.4 Ma (MSWD = 0.46, n = 18), representing the crystallization age of the ultramafic rock. This evidence suggests that the host rock of chromite is an ultramafic cumulate, which is part of the ophiolite suite. It originated from the fore-arc deficit asthenosphere in a supra-subduction zone during the northward subduction of the North Qaidam Ocean in the Ordovician period. Furthermore, clear evidence of Fe-hydrothermal alteration during the post-uplift-denudation stage is observed.

KIBARAN GRANITOID MAGMATISM IN THE REPUBLIC OF BURUNDI: GEOLOGY, COMPOSITIONAL FEATURES, TIME AND GEODYNAMIC CONDITIONS OF OCCURRENCE

The paper examines the specific granitoid magmatism of the northern sector of the Kibaran orogenic belt in the general context of geodynamic typification of granitoids. It presents parameters and main features of the structure with an emphasis on the sectorial nature of the belt typical of intraplate settings and shows the geological features of the eastern and western blocks of the northern sector. The paper provides general information on S- and A-type granitoids and a description of the geological position and macrofeatures of "reference" massifs. The petrochemical characteristics of S- and A-granitoids are given using various diagrams. Analysis of geochronological database available to date (including Rb-Sr isochron age of rocks and SHRIMP U-Pb age of zircons) shows the multi-pulse nature of the manifestation of intraplate granite formation. Along with the main stages (~1375, ~1205, ~986 Ma), there are five others. Based on the total dataset, S-type granitoids in Burundi are characterized by the crust-mantle interaction signs which are somewhat contrary to the generally accepted opinion that they are purely crustal. At the same time, some varieties of A-type granitoids are characterized by corundonormativity which makes them more similar to the S-type. Thus, signs of convergence between S- and A-granites are characteristic of some intraplate environments such as "hot spots", primarily of those longlived and multi-pulse, with the relevant protolith represented by a complex of sediments with high exogenous differentiation. One of the version of the granite formation model is illustrated by the corresponding figure.