Depletion In Nb Research Articles

A late Neoarchean continental arc system in the northeastern North China Craton: constrain from the Guanghua mélange in the Tonghua area, southern Jilin Province

ABSTRACT The late Neoarchean tonalite – trondhjemite – granodiorite (TTG) and mafic rocks record essential information regarding the early tectonic evolution of the North China Craton, which is crucial for understanding the onset and development of the plate tectonic regime. In this study, we present a comprehensive analysis of petrography, geochronology, and geochemistry of the Guanghua Mélange from the Tonghua area in NE China. The mélange comprises basalt, gabbro, pyroxene diorite, and trondhjemite, and is classified into three groups: basaltic – gabbroic rocks (Group #1), dioritic rocks (Group #2), and trondhjemites (Group #3). Zircon U-Pb analyses indicate that they formed at 2596–2550 Ma, 2531–2515 Ma, and 2502 Ma, respectively, with metamorphism occurring during ~ 2500–2450 Ma. Meanwhile, geochemical data indicate that the calc – alkaline basaltic rocks (Group #1) exhibit slight depletion of Nb or Ta with weakly negative Eu anomalies, suggesting derivation from the lithospheric mantle and formation within a continental arc environment. The pyroxene diorites (Group #2) display similar geochemical characteristics to Group #1, likely formed by partial melting of depleted mantle that underwent sgignificant metasomatism due to variable amounts of slab-derived fluids or melts. The tholeiitic granitoids (Group #3) exhibit strongly fractionated chondrite-normalized REE patterns with high (La/Yb)N and Sr contents, low Y contents, and depletion of Nb, Ta or Ti, resembling adakites generated in arc setting, indicating likely formation through partial melting of subducted oceanic crust. This process was controlled by the initial plate tectonics under a higher geothermal gradient. Consequently, when combined with previous findings, we conclude that an initial subduction of oceanic plate operated in the northeastern North China Craton during the late Neoarchean (>2.55 Ga). Subsequently, it transformed into modern-type plate tectonics during ~ 2.55–2.53 Ga, eventually followed by the continent-continent collision at ~ 2.50–2.45 Ga, resulting in the formation of the Ji – Liao – Ji (Jilin – Liaoning – Hebei) Collision Orogenic Belt.

The Sailajiazitage volcanic and related rocks in the Tiekelik belt imply a Neoproterozoic seamount accreted to the southern Tarim Craton

The process of accretion along the margins of cratons is fundamental to continental growth. Subduction and accretion have been revealed in the northern and central parts of the Tarim Craton, however, the tectonic setting of its southern margin is still controversial, which impedes the identification of its location in the reconstruction of Rodinia Supercontinent. Here, we present our results of field observations, geochronology, geochemistry and Sr-Nd-Pb isotopes for the volcanic and related rocks of the Sailajiazitage Group in the Tiekelik belt of the southern Tarim Craton. Field outcrop exhibits that these rocks were imbricated with pillow basalt and chert. The volcanic and related rocks yield ages of 883.4 ± 4.3 Ma (basalt), 858.6 ± 3.6 Ma (trachyte) and 830.3 ± 3.7 Ma (rhyolite), respectively. The basalt shows OIB character without depletion of Nb and Ta, suggesting a seamount setting. The values of ƐNd(t) (−6.0 to 1.7) and Sr and Pb isotopes show the basalts have a source of the EMI mantle with a mixture of lower continent crust, derived from 7 to 10% partial melting of garnet peridotite. Incorporating regional sedimentary rocks, MORB and adakitic plutons, we propose that the volcanic and related rocks of the Sailajiazitage Group formed in a seamount setting during the early Neoproterozoic and were accreted to the southern Tarim Craton during the late Neoproterozoic to the late Ordovician, changing the previously assigned basement nature of the southern Tarim Craton and challenging the model of reconstruction of the southern Tarim Craton in the Rodinia Supercontinent represented by the Sailajiazitage volcanic and related rocks.

Paleoproterozoic crustal evolution of the Tarim Craton, NW China: Constraints from geochronology and geochemistry of orthogneisses and granitic veins in the Xingdi region of the Quruqtagh Block

The Archean–Paleoproterozoic basement rocks, exposed in the Xingdi region of the Quruqtagh Block, provide a great opportunity to characterize the early crustal evolution of the Tarim Craton and reconstruct the Columbia supercontinent. Here we report zircon U–Pb ages, Lu–Hf isotope compositions, and bulk-rock geochemical data of orthogneisses and granitic veins from the Xingdi region of the Quruqtagh Block in the northeastern margin of the Tarim Craton. The orthogneisses formed in the late Neoarchean to early Paleoproterozoic (2504 ± 33 Ma, 2419 ± 24 Ma and 2450 ± 27 Ma), and were metamorphosed in the late Paleoproterozoic (1789 ± 43 Ma and 1748 ± 35 Ma). The orthogneisses have relatively high Sr (312–710 ppm) contents, and low Y (5.3–13.8 ppm) and Yb (0.46–1.10 ppm) contents, resulting in high Sr/Y (32–118) and La/Yb (17–64) ratios that are characteristics of adakites. The relatively high Mg# (49–55) and depleted zircon Hf isotope characteristics (positive εHf(t), with TDM2 of ca. 2.5–2.8 Ga) indicate that the orthogneisses crystallized from partial melting of garnet-bearing amphibolite in a subduction environment. The granitic veins that intrude the orthogneisses formed during the late Paleoproterozoic (1760 ± 25 Ma), which is roughly contemporaneous with the metamorphic ages of the orthogneisses; the granitic veins are characterized by extremely high SiO2 (>80 %), enrichments in Rb, Ba and U, and depletions in Nb and Ta. It is inferred that the granitic veins formed by in situ anatexis of the orthogneisses during the late Paleoproterozoic. Based on the zircon ages with Hf isotope compositions, it is evident that the continental crust in the Xingdi region of the Tarim Craton largely formed between 2.5 and 2.8 Ga and was reworked at ca. 1.8 Ga; this evolutionary history is similar to that of the Dunhuang Block. Comparison of the Paleoproterozoic tectono-thermal events in the different terranes of the Tarim Craton, it is suggested that the Tarim Craton is composed of discrete terranes that detached from different ancient cratonic nuclei prior to the late Paleoproterozoic, and assembled within the Columbia supercontinent respectively.

Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen, Central China: Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification. The mechanism by which mafic crust is transformed into silicic melts (i.e., maturation of continental crust) is important for understanding the formation of the continental crust. The North Qinling Orogen (NQO) is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system, which is ideal for investigating continental crustal maturation. We obtained zircon and monazite U–Pb age and O isotope data, zircon Lu–Hf isotope data, and whole-rock major- and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO. The granitoids are divided into three groups. Group 1 includes the Taiping tonalite (445 ± 3 Ma), the Manziying syenogranite (445 ± 2 Ma), and the Huoshenmiao granodiorite (436 ± 2 Ma). The Taiping and Huoshenmiao plutons have relatively high SiO2 contents (68.64–71.67 wt.%) and Na2O/K2O ratios (1.15–4.19), with enrichments in Rb, Ba, Th, and U and depletions in Nb, Ta, P, and Ti, and they are geochemically similar to sodic arc magmas. The Manziying syenogranite is a peraluminous potassic granite with high K2O contents (4.59–5.27 wt.%). Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features (εHf[t] = +5.9 to +8.8; δ18O = 3.98‰–5.41‰), indicative of derivation via partial melting of oceanic arc crust, which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation. Group 2 consists of the Wuduoshan monzogranite (418 ± 2 Ma) and the Sikeshu granodiorite (423 ± 3 Ma). These plutons have relatively high SiO2 (65.59–72.06 wt.%), K2O (3.26–4.79 wt.%), and Al2O3 (14.65–16.12 wt.%) contents and Sr/Y (33–87) and (La/Yb)N (23–48) ratios. The Wuduoshan monzogranite has positive zircon εHf(t) (+0.4 to +3.1) and uniform δ18O (6.38‰–8.07‰) values, but the Sikeshu granodiorite has more variable isotopic compositions (εHf[t] = −1.9 to +5.0; δ18O = 6.37‰–10.60‰). The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO. These features indicate that the two plutons formed by partial melting of basement rocks (i.e., subducted into the lower crust) of the North Qinling unit, along with juvenile crustal material. Group 3 is represented by the Xiaguan monzogranite, which formed at 434–430 Ma, and can be subdivided into heavy rare earth element (REE)-depleted and -enriched units. The former has high Sr/Y (56–98) and (La/Yb)N (34–73) ratios and low MgO (0.13–0.24 wt.%), Cr (0.37–1.69 ppm), and Ni (0.32–1.09 ppm) contents, similar to adakites derived from metabasaltic sources. The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid. Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen. The addition of evolved crustal material also facilitated this process; therefore, the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.

Metasomatized mantle source of nascent oceanic crust in the Guaymas Basin, Gulf of California

The geochemical characteristics of nascent oceanic crust play a crucial role in unraveling the mantle dynamics of the initial opening of an ocean. The Gulf of California, located at the East Pacific margin, is one of the best examples of an active and obliquely rifted continental margin. However, the mantle source composition of the nascent oceanic crust in the central part of the Gulf has not yet been comprehensively investigated. Here, we report in situ major and trace element contents as well as BSr isotope compositions for basaltic glass samples from off-axis sills drilled by the International Ocean Discovery Program (IODP) Expedition 385 at Sites U1547 and U1548 in the Guaymas Basin in the central part of the Gulf of California. These glassy samples represent tholeiites and predominantly show trace element patterns akin to those of enriched mid-ocean ridge basalts (E-MORBs) but with distinctive enrichments in Ba and K and depletions in Nb, Ta, and Ti. In addition, these samples have high B contents (3.07–3.67 ppm) with enriched Sr isotopes (87Sr/86Sr = 0.7032–0.7037) and heavy B isotopes (δ11B = −5.52‰ to 1.20‰). The mixing model based on B/Nb values and SrB isotopes shows that the nascent oceanic crust in the Guaymas Basin might be generated through partial melting of a depleted MORB mantle (DMM) source metasomatized by melts from subducted slab materials referring to partially dehydrated sediment and altered oceanic crust components. The magmas in the Gulf of California show a systematic decline in their enrichment in fluid-mobile elements (Ba) and depletion in fluid-immobile elements (Nb, Ta, and Ti) from the northern (e.g., Isla San Luis volcanic center) to the central part (Guaymas Basin) and southward to the mouth (e.g., Alarcón Basin) of the Gulf. This suggests that the enriched (recycled) components in their mantle source were gradually extracted and exhausted during Gulf opening and oceanic crustal accretion that advanced in a northward direction. Our results indicate that the Guaymas Basin magmas were derived from a mantle that was fertilized by subduction components. The subduction signature corroborates the Gulf opening as a process that started in response to long-term oblique convergence at the eastern Pacific plate margin without any influence from a mantle plume. This evolution is different from nascent ocean basins that evolved from intraplate rifting, such as the Red Sea, leading to vastly different mantle source characteristics.

Geochemical Evidence for Genesis of Nb–Ta–Be Rare Metal Mineralization in Highly Fractionated Leucogranites at the Lalong Dome, Tethyan Himalaya, China

Leucogranites in the Lalong Dome are composed of two-mica granite, muscovite granite, albite granite, and pegmatite from core to rim. Albite granite-type Be–Nb–Ta rare metal ore bodies are hosted by albite granite and pegmatite. Based on field and petrographic observations and whole-rock geochemical data, highly differentiated leucogranites have been identified in the Lalong Dome. Two-mica granites, albite granites, and pegmatites yielded monazite ages of 23.6 Ma, 21.9 Ma, and 20.6 Ma, respectively. The timing of rare metal mineralization is 20.9 Ma using U–Pb columbite dating. Leucogranites have the following characteristics: high SiO2 content (>73 wt.%); peraluminosity with high Al2O3 content (13.6–15.2 wt.%) and A/CNK (mostly > 1.1); low TiO2, CaO, and MgO content; enrichment of Rb, Th, and U; depletion of Ba, Nb, Zr, Sr, and Ti; strong negative Eu anomalies; low εNd(t) values ranging from −12.7 to −9.77. These features show that the leucogranites are crust-derived high-potassium calc-alkaline and peraluminous S-type granites derived from muscovite dehydration melting under the water-absent condition, which possibly resulted from structural decompression responding to the activity of the South Tibetan detachment system (STDS). Geochemical data imply a continuous magma fractional crystallization process from two-mica granites through muscovite granites to albite granites and pegmatites. The differentiation index (Di) gradually strengthens from two-mica granite, muscovite granite, and albite granite to pegmatite, in which albite granite and pegmatite are highest (Di = 94). The Nb/Ta and Zr/Hf ratios of albite granite and pegmatite were less than 5 and 18, respectively, which suggests that albite granite and pegmatite belong to rare metal granites and have excellent potential for rare metal mineralization.

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

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

Timing of Izanagi-Pacific ridge subduction beneath Eurasia: Constraints from metamorphic soles in Hokkaido, Japan

The subduction of the Pacific Plate beneath Eurasia controls not only Cenozoic intraplate volcanism, but also deep-focus earthquakes along the continental margin of eastern Asia. However, the timing of subduction of the Paleo-Pacific (Izanagi)−Pacific ridge and the initial subduction of the Pacific Plate beneath Eurasia remain subjects of debate. Metamorphic soles provide key evidence for ridge subduction, and together with coeval igneous activity, they are widely used to constrain the timing of ridge subduction. Here, we present the results of a study of amphibolites from the Hidaka metamorphic belt, Hokkaido, northern Japan, which are interpreted to be metamorphic soles. Our integrated study involved secondary ion mass spectrometry (SIMS) U-Pb zircon dating, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic analyses. SIMS U-Pb zircon dating indicates that the amphibolite-facies sole metamorphism occurred in the late Eocene. Phase equilibria modeling suggests that the peak P−T conditions of metamorphism were 8.0−9.5 kbar/700−730 °C. The amphibolites belong to the tholeiitic series, and they are relatively enriched in heavy rare earth elements relative to light rare earth elements, with depletions in Nb, Ta, P, and Ti. They have (87Sr/86Sr)i = 0.704207−0.704998, εNd(t) = +11.65 to +11.96, εHf(t) = +14.28 to +16.32, (206Pb/204Pb)i = 18.240−18.255, and (207Pb/204Pb)i = 15.522−15.525. The geochemistry of these amphibolites reveals that their protoliths had normal mid-oceanic-ridge basalt affinities, and their Pb isotopic signatures indicate Indian-type mantle rather than Pacific-type mantle. Considering the geochemistry of coeval igneous rocks, we conclude that the late Eocene metamorphism in the Hidaka metamorphic belt records the intraoceanic subduction that followed heat transfer from the incipient mantle wedge toward the top of the subducting plate, and it provides a key constraint on the timing of subduction of the Izanagi-Pacific ridge.

Middle Permian granitoids in the western section of the northern Qaidam Block, NW China: Petrogenesis and tectonic implications

Origin and tectonic setting of late Late Paleozoic to Early Mesozoic igneous rocks in the northern Qaidam Block (NQ) have long been a matter of debate. In this paper, we report new zircon U-Pb ages and Hf isotopic compositions and bulk rock major and trace elemental compositions for granitoids of the Yanchangbeishan area in the NQ. The granodiorite, tonalite, granodiorite, and syenogranite yielded zircon U-Pb ages of 271.9 Ma, 262.0 Ma, 272.4 Ma and 261.3 Ma, respectively, corresponding to εHf(t) values are 13.04–14.32, 8.14–10.77, 8.66–11.88, and 8.22–10.43, respectively. These granitoids are compositionally I-type granites and exhibit varying degrees of enrichment in Rb, Th, U, and K, and depletion in Nb, Ta, and Ti. According to field relationships, petrology, geochronology, and geochemistry, the granitoid types can be divided into three groups: (1) ca. 272 Ma granodiorites, which are formed through partial melting of metagreywackes and amphibolites derived from depleted mantle; (2) ca. 262 Ma tonalite, which represents a typical high-silica adakite derived from partial melting of subducted oceanic crust during the evolution of depleted mantle; and (3) ca. 261 Ma syenogranite, which is formed by the partial melting and high-degree fractional crystallization of mixed crustal materials. Considering our findings and the regional geological background, we propose a five-stage tectonic evolution model of subduction slab roll-back and subduction zone retreat for the late Late Paleozoic to Early Mesozoic of NQ.

Geochemistry and U–Pb zircon geochronology of S-type granites in the Karagwe Ankolean Belt, northwestern Tanzania: Implications for rare-metals mineralization

The Mesoproterozoic Karagwe Ankolean Belt in the Central Eastern Africa region hosts suites of granites that intruded supracrustal rocks and are associated with Sn–W mineralization. In northwestern Tanzania, the granites are S-type, peraluminous and calc-alkalic to alkalic-calcic in composition. Geochemical analyses of the granites reveal the following concentrations: SiO2 (71–77 wt%), Al2O3 (11–15 wt%), K2O (3–7 wt%), MgO (0.06–0.86 wt%), TiO2 (0.07–0.36 wt%) and Fe2O3T (0.71–2.40 wt%). The SiO2 contents correlate negatively with MgO, CaO and K2O and positively correlate with Nb, Ta, Zr, Sr, Ba, Hf, Th and the REE (Eu, Ce and Sm). Primitive mantle normalized spider plots show elevated LILE against depletions in Ba, Nb, Ti and La, suggesting inherited arc-magmatic signatures from the protolith. Chondrite-normalized REE spider plots show elevated LREE and relatively flat MREE and HREE as indicated by (La/Yb)cn = 2–9 and negative Eu* anomalies (0.05–0.24). Modelling of nearby Uha group sedimentary rocks partial melts, supports fractional crystallization and upper crustal contamination for generation of the KAB granites. Trace elemental ratios of Y/Nb, Nb/Ta, Zr/Hf and P205 suggest the granites are weakly mineralized to barren in Sn–W.The U–Pb zircon ages point to two successive and overlapping periods: from 1418 ± 15 Ma to 1395 ± 6 Ma (mean = 1406 Ma) and from 1385 ± 8 Ma to 1376 ± 11 Ma (mean = 1381 Ma). Inheritance U–Pb dates of xenocryst zircons, comprise early to late Paleoproterozoic ages ranging from 2376 ± 30 Ma, 2000 ± 27 Ma to 1901 ± 39 Ma, and from 1795 ± 7 Ma to 1738 ± 7 Ma, implying the participation of the Paleoproterozoic sedimentary rocks in the petrogenesis of the granites. A zircon rim overgrowth age of 1015 ± 26 Ma is interpreted to date a metamorphic/anatexis event that coincided with the c.1000 Ma amalgamation of the supercontinent Rodinia and the emplacement of Sn–W enriched granites. The ages of S-type granite magmatism (1418–1376 Ma) obtained from this study are restricted to the Mesoproterozoic within the widespread bimodal magmatism event c.1375 Ma.

Unmixing the hydrothermal and detrital components from a metalliferous sediment core from the eastern Southwest Indian Ridge – Insights from mineral magnetism

Mineral magnetic and geochemical investigations were carried out on a short sediment core collected from the eastern Southwest Indian Ridge (ESWIR) to study the variations in the source components. The magnetic concentration parameter χarm (representing the concentration of stable single domain (SSD) sized grains), magnetic grain size ratios χarm/χlf, χarm/SIRM, geochemical ratios Fe/Ti, Co/Zn, and Zn/Fe and Ba content serve as hydrothermal indicators, all of which show a distinct top - bottom variability. The magnetomineralogical parameters S-ratio and coercivity (Hc) also follow these variations, suggesting an increased contribution of hydrothermal components in the top 15 cm of the core. Significant enrichments of Fe, Mn, and Mg in the samples, along with depletions of detrital elements Al, Si, Rb, Zr, Hf, and Nb in the element/Upper Continental Crust (UCC) values, show that the majority of the detrital component is locally sourced. Thermomagnetic measurements of samples indicate that the major remanence carrier is magnetite in the core, along with minor contributions from titanomagnetite, hematite, and iron sulfides.

Ti-Zr-Nb based BCC alloy containing Mo prepared by laser directed energy deposition—ω phase and cellular structure

This study focuses on the fabrication of Ti-Zr-Nb β-Ti alloys via laser direct energy deposition (DED) technique using pre-alloyed powder with and without having Mo traces. Microstructural analysis revealed the presence of cellular structures within the grains, resembling dendritic solidification, although no discernible dendritic segregation was observed. Nano-scale solute element clustering, particularly the formation of Nb depletion pockets, led to the development of the ω phase within the β matrix. Strengthening effects resulting from the presence of the ω phase were confirmed through TEM analysis. The DED-fabricated β-Ti alloy exhibited enhanced strength (∼1 GPa), significant elongation (∼15%), and a low elastic modulus (∼65 GPa), positioning it as a promising candidate for load-bearing biomedical implants.

Zircon U–Pb geochronology, Nd isotopes and geochemistry of mafic granulites from the Central Indian Tectonic Zone: isotopic constraints on Proterozoic crustal evolution

Abstract The Central Indian Tectonic Zone (CITZ) comprises northern and southern Indian cratonic blocks and is a tectonic window that is suitable for investigating the Proterozoic crustal evolution because of the presence of a wide variety of lithologies. Geochemical and geochronological data on mafic granulites by previous workers do not ascertain the possibility of mafic protoliths and their coeval link to other CITZ units. Thus, determining the precise timing of the formation of mafic granulites may indicate a connection between metamorphism and fragmentation of the Columbian supercontinent. This study presents zircon U–Pb ages, Nd isotopes and the geochemistry of mafic granulites to evaluate their genesis and timing of metamorphism. The results show the tholeiitic affinity and primary magmatic differentiation of the parental melt. Depletion of Nb, P, Zr and Ti and positive enrichment of Ba, U and Pb indicate the derivation of mafic granulites from a variably enriched subcontinental lithospheric mantle (SCLM) source. The zircon U–Pb ages (1564 ± 8 to 1598 ± 9 Ma) are interpreted as a period of granulite-facies metamorphism. The T DM (depleted-mantle) model ages (2.9–3.4 Ga) of mafic granulites indicate the timing of mafic protolith extraction. The mineral isochron age c. 1.0 Ga indicates that these rocks underwent some events during an early Neoproterozoic period. Protolith of mafic granulites could be related to the evolution of melts derived from metasomatized SCLM through fractional crystallization processes.

The early Late Triassic Yumen mafic rocks in the eastern Tethyan Himalaya: A record of the progressive opening of the Yarlung Zangbo Neo-Tethyan Ocean

The timing of the opening of the Yarlung Zangbo Neo–Tethyan Ocean (YZNTO) is debated. In this contribution, geochemical and geochronological analyses of diabase, pillow basalt and layered basalt from the Upper Triassic Yumen tectonic mélange (T3Y) of the eastern Tethyan Himalaya in South Tibet are used to elucidate the timing and the magmatic evolution preceding the development of the YZNTO. Zircon U-Pb geochronology indicates that the Yumen mafic rocks crystallized in the Late Triassic (diabase, c. 225 Ma). These rocks display alkaline basalt affinities and have variable enrichment in Ba, U, Pb, Sr and Ti and depletion in Rb, Th and Nb. The diabase and pillow basalt samples have compositions transitional between enriched mid–ocean ridge basalt and ocean island basalt (OIB); layered basalt is more compatible with an OIB affinity. Whole–rock εNd (t) ratios of the mafic rocks range from +5.5 to +11.5 and zircons from a diabase have εHf(t) values varying from – 1.1 to +10.0. Whole–rock trace element concentrations indicate that the parental magmas of the Yumen mafic rocks were derived from the mantle at variable depths extending from the garnet to spinel stability fields. The Yumen mafic rocks originated from partial melting of a heterogeneous magma source composed of dominant depleted asthenospheric magmas and the addition of some components of an ancient and enriched lithospheric mantle; negligible crustal contamination occurred. Combined with previous studies, we conclude that the Yumen mafic rocks were probably produced in an intercontinental rift setting with extremely thin continental lithosphere in the early Late Triassic prior to the opening of the eastern YZNTO. The YZNTO progressively opened along the northern edge of the Indian plate from west to east in the Phanerozoic.

Primitive high-Mg andesites from the Western Alps, Italy: Products of interaction of sediment diapir derived melts with mantle-wedge peridotite in a continental collision zone

Primitive (high-Cr, Ni), high-Mg andesites (HMA) at convergent plate margins are considered primary, mantle-derived melts, and are not part of the classical tholeiitic or so-called calc-alkaline igneous differentiation series. Their genesis is often considered the product of melting of lower crustal sequences or the subducting slab, though their genesis is far from being understood. Alternatively, they may represent differentiated rocks from boninitic precursors. Here we present isotope and geochemical data for a rare suite of high-Mg andesites that were preserved as decimetre-sized boulders in molasse-type sedimentary rocks of the Oligocene-Miocene Gonfolite Lombarda (Como Formation; Italy), as part of the post-orogenic alpine sedimentary succession. The HMA have moderate to high MgO (1.9–6.4 wt%), high Al2O3 (16.3–18.8 wt%), along with high Cr (30–306 ppm) and Ni (13–92 ppm), and moderate Yb (1.2–2.5 ppm) and Y (12–26 ppm) abundances, matching those of primitive HMA. These HMA are LILE- and LREE-enriched and most samples have negligible negative Eu anomalies. Primitive mantle-normalized compositions show depletions in Nb, Ta, P, Ti and Y. Most samples have unradiogenic Nd (εNd: −6.2 to −7.8) and radiogenic 87Sr/86Sr isotope compositions (0.708 to 0.710), elevated δ18O values (+7.6 ‰ to +9.5 ‰) and radiogenic Pb isotope compositions, with a pronounced variation in 207Pb/204Pb and 208Pb/204Pb at rather constant 206Pb/204Pb. Initial εHf isotope values range from −0.4 to −2.3, some of which deviate from the HfNd crust-mantle isotope array. One sample is unevolved (87Sr/86Sr: 0.708; εNd: −3.3, εHf: +1.8, δ18O: 8.5 ‰) whereas another sample is strongly evolved (87Sr/86Sr: 0.720; εNd: −9.3, εHf: −5.8, δ18O: 9.3 ‰); the latter probably representing a crustal melt. The remaining samples have intermediate compositions (87Sr/86Sr: 0.709–0.713; εNd: −6.2 to −8.5, εHf: −0.4 to −4.3, δ18O: 7.6–10.6 ‰) that either result from limited AFC processes or represent source heterogeneities. The sum of these features cannot be the result of a simple slab or mantle wedge melting process. To account for the observed geochemical signatures of the high-Mg andesites, a three-stage model is suggested that invokes melting of slab-derived sediment diapirs, followed by melt-rock-reaction in the mantle wedge and subsequent crustal assimilation-fractional crystallization (AFC). The diapirs that rise into the mantle wedge are composed of subducted wet, Alpine sediments. Hydrous dacitic (siliceous) partial melts derived from them react with the ambient mantle wedge and cause the formation of boninitic melts with elevated MgO, Ni and Cr; the archetype features of primitive high-Mg andesites. Subsequent interaction with the overlying arc crust is evidenced by crustal xenoliths, which may account for some isotope features of the rock suite. We consider this scenario most plausible because it is relatively simple, and explain most if not all geochemical features, based on modelling and experimental evidence.

Late Paleozoic Tectonic Evolution of the Northern Great Xing’an Range, Northeast China: Constraints from Carboniferous Magmatic Rocks in the Wunuer Area

Northeast China composes the main part of the Central Asian Orogenic Belt. Traditionally, Northeast China has been considered a collage of several microcontinental blocks. However, the tectonic evolution of these blocks remains uncertain. Igneous rocks can be used to infer the magmatic histories of the blocks and thus help reconstruct their evolution. In this study, we present new zircon U–Pb and whole-rock geochemical data for Carboniferous igneous rocks from the Wunuer area, northern Great Xing’an Range, Northeast China, to constrain the Carboniferous amalgamation of the united Xing’an–Erguna and Songnen–Zhangguangcai Range massifs. On the basis of zircon U–Pb dating results, we identify two main stages of magmatism, i.e., early Carboniferous (332–329 Ma) and late Carboniferous (312–310 Ma). The early Carboniferous igneous rocks include diorites and granodiorites, with the former being classified as calc-alkaline to tholeiitic and the latter as tholeiitic. Both rock types are enriched in Th and U and depleted in Nb and Ti. The rocks display slightly fractionated rare earth element (REE) patterns, with an enrichment in light REEs and a depletion in heavy (H)REEs. The geochemical characteristics of the early Carboniferous rocks indicate that they formed in a subduction-related continental-arc setting. The late Carboniferous igneous rocks include monzogranites and syenogranites, both of which are classified as high-K calc-alkaline rocks and show enrichment in Th, U, and Rb and depletion in Nb and Ti. The rocks display strongly fractionated REE patterns, with an enrichment in light REEs and a depletion in HREEs. The geochemical characteristics of the late Carboniferous rocks indicate that they formed in a syn-collisional tectonic setting. Combining the new geochronological and geochemical results and inferred tectonic settings with regional magmatic data, we propose a new three-stage model to interpret the late Paleozoic tectonic evolution of the united Xing’an–Erguna and Songnen–Zhangguangcai Range massifs of Northeast China: (1) early Carboniferous (360–340 Ma) subduction of the Paleo-Asian oceanic plate beneath the united Xing’an–Erguna Massif and formation of the Wunuer oceanic basin in the Yakeshi area; (2) early to late Carboniferous (340–310 Ma) sustained subduction of the Paleo-Asian oceanic plate beneath the united Xing’an–Erguna Massif and initiation of subduction of the Wunuer oceanic basin; and (3) late Carboniferous–early Permian (310–275 Ma) syn-collisional to post-collisional tectonic transition between the united Xing’an–Erguna Massif and the Songnen–Zhangguangcai Range Massif.

Cadomian tectonic evolution of Iran: records of an unusually hot and broad extensional convergent margin on the northern margin of Gondwana

ABSTRACT The continental crust of Iran is dominated by abundant calc-alkaline and alkaline plutonic and volcanic rocks and by rifted basins filled with mostly terrigenous sedimentary rocks that formed at a Late Ediacaran to Cambrian extensional convergent plate margin along the northern margin of Gondwana. Here we present new zircon U–Pb age, geochemical, and isotopic data from plutonic (granite-granodiorite) and metamorphic (gneiss) rocks in the Kariznou region of NE Iran to provide insights into the nature of the Cadomian convergent margin of Iran. Geochemical data indicate calc-alkaline signatures, characterized by strong depletions in Nb, Ta, P, and Ti and arc-like trace element patterns. New zircon U–Pb ages show that calc-alkaline granitoids and granitic gneiss formed at ~564 to 537 Ma and 538 Ma, respectively. Bulk rock Sr-Nd isotopic data of calc-alkaline rocks have εNd(t) = –5.42 to −5.53 and −6.93 to −7.43 for granite and gneiss, respectively. The gneisses show stronger interaction with and/or re-melting of older continental crust than do granitic rocks. We interpret Kariznou magmatic rocks as forming in association with strong extension accompanied by crustal assimilation. Extension initiated ~570 Ma with the deposition of Late Ediacaran sediments, and magmatism began at 545–535 Ma, generating calc-alkaline magmas. The tectonomagmatic evolution of the Kariznou region encapsulates the prolonged transition of Cadomian Iran from a strongly extensional convergent margin, possibly as a result of oblique oceanic subduction and slab roll-back of the subducting Proto-Tethys oceanic lithosphere, culminating in the formation of an Early Palaeozoic passive margin on the northern side of Gondwana.

Petrology and geochemistry of Li-bearing pegmatites and related granitic rocks in southern Thailand: implications for petrogenesis and lithium potential in Thailand

Lithium (Li) can be found in many minerals, including lepidolite. Lepidolite is found in pegmatite-related tin deposits in the Phang Nga area in southern Thailand. According to their field occurrence, petrography, mineral chemistry, and whole-rock geochemistry, the Li-bearing pegmatites and the granitic rocks in the study area can be linked to tin deposits in southern Thailand as part of the SE Asian tin belt. The Li-bearing pegmatites are characterized by an abundance of lepidolite, K-feldspar, plagioclase, and quartz with some accessory minerals of fluorite, cassiterite, apatite, monazite, and beryl. The granitic rocks show various compositions, including porphyritic biotite–muscovite granite, biotite granite, and muscovite—tourmaline granite with different proportions of K-feldspar, plagioclase, quartz, biotite, muscovite, and tourmaline. Whole-rock geochemistry indicates that both the Li-bearing pegmatites and granitic rocks have a close relationship rooted in their peraluminous S-type granite affinity. The Li-bearing pegmatites evolved from highly fractionated S-type granitic rocks comparable with the Western Belt Granite of Thailand. The enrichment of large-ion lithophile elements (e.g., Rb and K) and the depletion of Ba, Nb, and Ti together with similar rare Earth element patterns reflect the collisional setting indicating the Sibumasu–West Burma and West Burma—Indo-Burma collisions during the Cretaceous to the Eocene. The crystallization pressure—temperature conditions of these rocks were 3.49–4.25 kbar and 622°C–675°C, respectively, with an emplacement depth of 13–15 km. The Li-bearing pegmatites had a relatively high average Li grade compared with those of other Li-bearing pegmatites in the world.

Heterogeneous Subcontinental Lithospheric Mantle below the South Margin of the Siberian Craton: Evidence from Composition of Paleoproterozoic Mafic Associations

Abstract —Paleoproterozoic mafic associations of the Irkut block from the Sharyzhalgai uplift are gabbro-dolerite dikes and small gabbronorite and monzodiorite massifs, which formed at 1.87–1.84 Ga and were coeval with granitoids and basite intrusions of the South Siberian magmatic belt (SSB). All the Paleoproterozoic mafic associations of the Irkut block are characterized by the presence of biotite and alkali feldspar, enrichment in K2O, LILE, Th, and light REE, highly fractionated multielement spectra with sharp Nb and Ti depletion, and extremely low εNd(T) from –5.1 to –10.1. In these compositional features, they are similar to mafic complexes in the central and eastern parts of the SSB (the Baikal uplift and the western Aldan shield). Their geochemical and isotopic characteristics did not result from crustal contamination but point to derivation from the subcontinental lithospheric mantle (SCLM) enriched by reaction with felsic subduction-related and OIB-like mafic melts formed at a low degree of melting. The geochemically contrasting Paleoproterozoic gabbronorites in the Onot block of the Sharyzhalgai uplift are marked by depletion in K2O, Ba, LILE, Th, and light REE, weak depletion in Nb, and higher εNd(T) from –0.3 to –1.4. The gabbronorites indicate not only an increase in the contribution of a depleted source to their genesis but also the heterogeneity of the subcontinental lithospheric mantle below the south margin of the Siberian Craton. The formation of enriched SCLM domains throughout the South Siberian belt was mainly the result of Archean subduction-related metasomatic processes. The wide distribution of Paleoproterozoic mafic complexes with subduction geochemical signatures and negative εNd(T) on most early Precambrian cratons is due to global change in the composition and an increase in the heterogeneity of the subcontinental lithospheric mantle toward the end of the Archean.

Petrogenesis of the late Ediacaran A-type granites of the western part of the Moroccan Anti-Atlas belt: Insights from U–Pb dating, geochemistry and Sr–Nd isotopes of the Sidi El Houssein, Tafraout and Taourgha intrusions

The U–Pb zircon age carried out on three distinct late Ediacaran granites through the Anti-Atlas yielded 207Pb/235U ages of 584 ± 2.29/579 ± 3.83 Ma for Sidi El Houssein granite (Zenaga Inlier), 581 ± 2.05 Ma for Tafraout granites (Kerdous inlier) and 580 ± 3.02 Ma for the Taourgha intrusion (Bas-Drâa inlier). These pink granites display high SiO2 contents that range from 69.19 to 77.3 wt %. They are mainly peraluminous to weak metaluminous and subalkaline and ferron granites with LILE (Rb, K, Th) and HFSE (Ta, Zr, Hf) enrichment, with depletions in Ba, Nb, Sr, and Ti. These rocks have relatively high zircon saturation temperatures (700–900 °C), high Zr + Nb + Ce + Y contents >350 ppm), and 10,000 × Ga/Al ratios> 2.6 that are typical of A-type granites. Isotopic compositions show that the granitoids have variable (87Sr/86Sr)i ratios (0.67–0.73), εNd(t) values (−2.9 to-7.63) and TDM model ages of 1.49–2.53 Ga Suggesting that they were derived from the partial melting of the continental crust with a moderate contribution of the mantle. The late Neoproterozoic granitoids of the Anti-Atlas belt were probably generated during a transtensional regime due to an asthenosphere upwelling that followed the lithospheric delamination related to the collapse of the Panafrican belts.