Partial Melting Of Crustal Materials Research Articles

Pan-African and Early Paleozoic Orogenic Events in Southern Tibet: Evidence from Geochronology and Geochemistry of the Kangbuzhenri Gneissic Granite in the Zhegu Area

The Zhegu area in southern Tibet is situated in the central and eastern part of the Tethys Himalayan tectonic belt, with the Kangbuzhenri area being abundant in gneissic granites. This study examines the petrology, chronology, and geochemistry of the Kangbuzhenri gneissic granite, providing insights into its Pan-African and Early Paleozoic geological evolution. The zircon U-Pb chronology indicates an upper intercept age of ~539 Ma, reflecting Pan-African orogenic events in the eastern part of the Tethys Himalayan tectonic belt, and a lower intercept age of ~144 Ma, representing a late tectonic–thermal event. Geochemically, the gneissic granites are calc-alkaline peraluminous rocks with high SiO2 and Al2O3 contents and low TiO2, P2O5, MgO, and FeOT contents. The gneissic granites are enriched in LREE and LILEs (Rb, Pb, Th, U, etc.), but relatively depleted in HREE and HFSEs (Nb, Ti, P, etc.). Most of them show a weak negative δEu anomaly, except for two samples which show a significant negative δEu anomaly due to the crystallization of plagioclase. Based on the above study, most of the gneissic granites exhibited the characteristics of an I-type granite, while two of the samples were a highly differentiated I-type granite with S-type affinities. All the above characteristics indicate that the gneissic granite likely originated from the partial melting of crustal materials and sediments with a minor involvement of mantle-derived materials. Combined with the previous chronological studies, the Kangbuzhenri gneissic granites were formed in an extensional tectonic environment during post-collision orogeny and then they were influenced by the Kerguelen mantle plume tectonic–thermal event around ~144 Ma and the subsequent Southern Tibet Detachment System (STDS).

Genesis of Andesitic Magma Erupted at Yufu Volcano, Kyushu Island, Southwest Japan Arc: Evidence from the Chemical Compositions of Amphibole Phenocrysts

Abstract The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the trend of whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt %) and felsic (SiO2 ≈ 68 wt %) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks.

O–Hf isotope ratios of Alvand S-type granite, western Iran, reveal crustal melting in an extensional regime

There are a few intrusive bodies in the central Sanandaj-Sirjan Zone (C-SaSZ), which is parallel to the Zagros suture zone, in western Iran. The Alvand batholith in the Hamedan area is one of these bodies and consists of rock types with large variation, in particular, two-mica garnet-bearing granite with an S-type signature. Zircon UPb dating gives crystallization ages of 170–163 Ma for the S-type granite. The 176Hf/177Hf ratios in the zircon grains vary from 0.282001 to 0.282780 (rim), with low 176Lu/177Hf ratios (0.002 to 0.018). The εHf(t) values (rim) are clustered into two main groups: a negative εHf(t) group (−6.9 to 0.0, n = 39), with some highly negative εHf(t) values (−25.8 to −21.9, n = 3), and slightly positive εHf(t) values (+0.3 to +2.9, n = 14) of inherited grains. The δ18O values for the zircon grains are positive and range from +10.4‰ to +11.8‰. The high δ18O (positive) and mostly negative εHf(t) values are more consistent with continental crustal sources for these rocks, and the Hf model ages (TDM2) show a wide range from 2.7 to 0.8 Ga, suggesting the heterogeneity of the crustal components. The long incubation time (2089 to 544 Ma) indicates a long history of crustal residence and crustal reworking. The incubation age of the zircon grains, >300 Ma, confirms the recycling of crustal material by partial melting. The simultaneous occurrence of S-type granites with mafic bodies, such as gabbro and diorite members with tholeiitic signatures, suggests that the injection of mafic magma increased the geothermal gradient from the bottom and sides along the major faults, which was the trajectory for the injection of hot mafic melts to shallow depths. These processes abnormally increased the geothermal gradients locally, and the partial melting of crustal and supracrustal materials produced S-type granites and migmatization among the hot blade-shaped mafic bodies. Therefore, the partial melting of crustal material in an extensional tectonic regime and/or in a thin continental crust occurred in a continental rift and/or in the Neo-Tethys passive margin. The present work concludes that the passive margin can be suggested as the tectonic setting for the generation of S-type granites, without the collisional regime that has been widely considered for the sources of this type of granite in the world.

Boron isotope constraints on the migration and accumulation of rare alkali metals in the geyserite cesium deposits in Southern Tibet

Geothermal waters in the Shiquanhe-Yarlun Zangbo geothermal field of southern Tibet are highly enriched in rare alkali metals (RAMs), which form a globally unique geyserite Cs-deposit. Boron isotopes, together with the geochemistry of a local leucogranite, geothermal springs and geyserite were investigated to constrain the migration and accumulation of RAMs in the geyserite cesium deposit. The δ11B values of −9.7‰ to −8.4‰ in tourmalines from the Himalayan leucogranite have shown typical S-type granites. A mass balance calculation indicates that the magma of the S-type granite contained < 10 % dehydrated fluid, which resulted in an enrichment of RAMs by 4–5 times during the partial melting of crustal materials, compared to that in the UCC. The linear relationships of ΣRAMs vs. B and Na + K vs. soluble Si, indicate that further enrichment of RAMs in the geothermal fluids was promoted by intense water–rock interaction at temperatures of 210 to 250 °C in the geothermal system reservoir. A strong adsorption process controlled the hyper enrichment of RAMs (especially Cs) during silica precipitation from the upwelling geothermal water, with the sharp decreases in temperature and pressure leading to the enrichment of Cs in fresh geyserites. Despite loss of RAMs during dehydration of the geyserite during phase transformation from opal-A, opal-CT, and chalcedony to quartz, freshly deposited geyserites have great potential for exploration of RAMs resources in this area.

Apatite as a record of ore-forming processes: Magmatic-hydrothermal evolution of the Hutouya Cu–Fe–Pb–Zn ore district in the Qiman Tagh Metallogenic Belt, NW China

Apatite is a common accessory phase in magmatic rocks and associated hydrothermal deposits that holds information about the evolution from magma to ore-forming fluid. In this study, we systematically studied the geochronological and geochemical characteristics of apatites hosted in the Fe skarn ores and associated intrusions of syenogranite and monzogranite in the Hutouya polymetallic Fe–Cu–Pb–Zn ore district, Qiman Tagh Metallogenic Belt (QTMB), NW China, in order to gain insight into the metallogenic processes from magmatic crystallization to hydrothermal fluid exsolution. The apatite U–Pb dating results of skarns and related intrusions are 229–228 Ma, and the monzogranite emplacement is slightly later than the syenogranite emplacement and skarn formation. The REE concentrations of the apatites from the skarn-related intrusions are characterized by LREE-enriched patterns and weak negative Eu anomalies. However, the apatites from the skarn ores have high HREE contents, and are characterized by flat chondrite-normalized patterns and strong negative Eu anomalies. These data, combined with variations of element concentrations and ratios in the apatites (eg., Sr, SO3, F, Cl, REE, (La/Yb)N, (Sm/Nd)N, (La/Sm)N, (Eu/Eu*)N, (Ce/Ce*)N), record the transition of ore-forming processes from the magmatic crystallization to hydrothermal exsolution, and indicate that the skarn-related magmas with moderately reduced signatures may have been derived from partial melting of crustal materials potentially affected by the limited mantle input under an extensional environment.

STRUCTURAL ANALYSIS, PETROGRAPHIC STUDY AND GEOCHEMICAL ASSESSMENT OF PAN-AFRICAN GRANITOID, GUSAU SHEET 54SE NORTHWEST NIGERIA

The study area is underlain by migmatite-gneiss quartzite rock suits with infolding mica schist which is intruded by batholitic Pan-African granitoids. Petrographic studies revealed quartz, orthoclase, hornblende and biotite as major minerals which are common to all members of rock units. The geochemical result of gneissic rock shows SiO2 content that range between 77.04%.wt to 80%.wt while mica schist depicts SiO2 content of 74.42 to 77.42%.wt. this implies silicious protolith for the both rock units. On the discrimination diagram gneiss and mica schist plot mainly in High-K calc-alkaline to calc-alkaline fields. Whereas, bivariate diagram of TiO2 versus SiO2 revealed that both gneissic rock and mica schist have sedimentary progenitors. High Rb, La and Th and low Nb, Sr, and Ti values shown by both gneiss and mica schist are compatible with typical crustal melts and suggest evolution from partial melting of crustal materials. The Pan-African granitoids has similar geochemical relationships. Geochemical studies shows that the rock units have limited variation in their major element compositions and give a decrease in CaO wt. %, and increasing Ba/Sr, Zr/Y ratios, K2O wt. % with increasing silica. The rocks are distinctly peraluminous, calc-alkaline as they plot in the High-K calc-alkaline and calc-alkaline, syn-collision fields on various discrimination diagrams. The characteristics suggest that the Basement rocks of the study area were formed from the fractional crystallization of a calc-alkaline magma in a syn- collision tectonic terrain.

Zircon U–Pb and titanite U–Th–Pb ages of the Ghorveh mixed granitoid pluton: Implications for the Late Jurassic supra‐subduction extension of the Sanandaj–Sirjan Zone, Iran

The Ghorveh batholith, located in the central part of the Sanandaj–Sirjan Zone (SSZ), Iran, preserves several sequences of Mesozoic plutonic and volcanic rocks. This study presents systematic analyses of the calc‐alkaline granites and monzodiorites from the southern part of the Ghorveh pluton. The granites are metaluminous to slightly peraluminous with A/CNK ratios of 0.97–1.02, indicating an A‐type granitic affinity. High Rb, Th, and K and low Sr, P, and Ti concentrations indicate an involvement of crustal materials. Whole‐rock negative εNd(t) values and relatively high TDM1 model ages support a continental crustal source. Thus, the granites are of fractionated A‐type in composition produced by partial melting of crustal materials. The monzodiorites have incompatible trace element patterns with enrichment in light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs) relative to heavy rare earth elements and high‐field strength elements. This combined with the high Mg# values and the positive εNd and εHf(t) values, indicate a mantle‐derived origin. Dating results show that the granites and monzodiorites have uniform crystallization ages of ~150 Ma. Previous results and our new data suggest that the Jurassic subduction of Neo‐Tethyan Oceanic crust under the Eurasian continent was followed by slab rollback that induced back‐arc extension. Heat transfer from arc magmas increased the degree of decompressional melting in the thinned lithosphere. Melting of the presumably homogeneous crust was concurrent with emplacement of the intermediate composition magmas. Consequently, we propose that the Ghorveh area of the central SSZ developed in a supra‐subduction extensional phase during the Late Jurassic.

Tectonic transition from Paleo- to Neo-Tethyan Ocean in Tangjia-Sumdo area, Southern Tibet: Constraints from Early Jurassic magmatism

The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 ± 1.1 Ma), monzogranites (194.3 ± 1.1 Ma and 200.5 ± 1.2 Ma), and granites (184.5 ± 2.1 Ma, 185.5 ± 4.0 Ma and 185.6 ± 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated εHf(t) values of the diorite are − 16.7 to − 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block.

Petrogenesis of the Early Paleozoic S-type granites in southern Alxa, Northwest China and its implications on fluid flourishing in the middle and lower crust

Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO2 (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO2 (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO2 = 72.50–76.04 wt%) are different from the two-mica granites in having higher Na2O (3.38–4.58 wt%) and lower K2O (3.70–4.90 wt%) with higher Na2O/K2O ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.

Petrogenesis and tectonic implication of the lower Silurian high-Sr/Y subvolcanic rocks from the South Qilian suture zone in the Qilian Orogen, NW China

AbstractAs the remnant of the South Qilian Ocean, the South Qilian suture zone recorded abundant information on the Cambrian–Ordovician subduction history of the southern branch of the Proto-Tethyan Ocean. However, the closure timing of the South Qilian Ocean and subsequent collision are poorly constrained. In this study, we report early Silurian (433–435 Ma) U–Pb ages of felsic subvolcanic rocks from Lianhuashan, Ayishan and Shihuiyao of the Lajishan district within the South Qilian suture zone. They intruded the Late Ordovician – Silurian sedimentary or Late Ordovician volcanic rocks and have high SiO2(61.43–73.06 wt%), Sr/Y ratios with significant different rare earth elements (REEs) and trace-element spider diagrams, and Sr–Nd isotopic compositions, probably implying that they were formed through distinctly different generation mechanisms. Geochemistry of the Lianhuashan dacites reveals compositions typical of adakitic rocks derived from partial melting of lower crust in a thickened setting. The Ayishan dacites were derived from partial melting of crustal materials with the involvement of minor peridotite mantle, and the Shihuiyao rhyolites were derived from partial melting of felsic crust. The similar geochemical characteristics of coeval post-collisional igneous rocks in the Central Qilian and South Qilian blocks indicates that the lower Silurian subvolcanic rocks were generated in a thickened crust of post-collisional setting. Considering their intrusive contacts with Late Ordovician – Silurian retro-foreland basin and Late Ordovician collisional volcanic rocks, we propose that the South Qilian suture zone was at a transitional stage from collisional to post-collisional during the early Silurian Period.

Identification of the Early Jurassic mylonitic granitic pluton and tectonic implications in Namling area, southern Tibet

A number of studies revealed that the Gangdese magmatic belt of southern Tibet was closely related to the northward subduction of the Neo-Tethys oceanic lithosphere and Indo-Asian collision. However, pre-Cretaceous magmatism is still poorly constrained in the Gangdese magmatic belt, southern Tibet. Here, we conducted systematically geochronology and geochemistry studies on a newly-identified granitic pluton in the middle Gangdese magmatic belt (Namling area), southern Tibet. Zircon SHRIMP II U–Pb dating for one representative sample gives a weighted age of 184.2 ​± ​1.8 ​Ma (MSWD ​= ​1.11), corresponding to emplacement and crystallization age of the granitic pluton in the Early Jurassic (Pliensbachian). High SiO2 (68.9–72.1 ​wt.%) contents and intermediate Mg# values (35–38) together suggest that the newly-identified granitic pluton was probably formed by partial melting of crustal material with minor injection of mantle-derived magma, precluding an origin from melting of meta-sedimentary rocks that are characterized by low Mg# and high zircon δ18O values (>8‰). Geochemically, the newly-identified granitic pluton belongs to typical I-type granitic affinity, whereas this is inconsistent with aluminium saturation index (ASI ​= ​A/CNK ratios) and geochemical signatures. This suggests that zircon oxygen isotopes (4.30‰–5.28‰) and mineral features (lacking Al-rich minerals) are reliable indicators for discriminating granitic origin. Significantly depleted whole-rock Sr-Nd-Hf isotopic compositions and zircon εHf(t) values indicate that the granitic pluton was derived from partial melting of depleted arc-type lavas. In addition, the granitic pluton shows zircon δ18O values ranging from 4.30‰ to 5.28‰ (with a mean value of 4.77‰) that are consistent with mantle-derived zircon values (5.3‰±0.6‰) within the uncertainties, indicating that the granitic pluton might have experienced weak short-living high-temperature hydrous fluid-rock interaction. Combined with the Sr-Nd-Hf-O isotopes and geochemical signatures, we propose that the newly-identified granitic pluton was originated from partial melting of depleted mafic lower crust, and experienced only negligible wall-rock contamination during ascent. Integrated with published data, we also propose that the initial subduction of the Neo-Tethys oceanic lithosphere occurred no later than the Pliensbachian of the Early Jurassic.

Origin and evolution of the Neoproterozoic Dengganping Granitic Complex in the western margin of the Yangtze Block, SW China: Implications for breakup of Rodina Supercontinent

A better understanding of the origin and evolution of widespread Neoproterozoic granites along the western margin of the Yangtze Block is important in locating the position of South China within the Rodinia Supercontinent, and the mechanism by which the supercontinent was fragmented. Published and our new detailed geochronological and geochemical data from the Dengganping Complex show that it formed during an extended magmatic event during the Neoproterozoic. This was during the emplacement of a fractionated ca. 815 Ma syenogranite, which was followed by partial melting during ca. 740 Ma and the emplacement of a biotite monzogranite. The geochemistry of the syenogranite indicates it is: (1) a peraluminous high-K type of granite; (2) calc-alkaline; (3) fractionated; (4) enriched in Th, U, Zr, Hf and LREE (LREE/HREE values of 5.3–9); (5) depleted in Nb and Sr; (6) has a negative Eu anomaly (δEu = 0.3–0.5); (7) has whole rock εNd(t) value of −0.7 to +0.5; (8) has zircon εHf(t) value of −1.7 to +5.2; and (9) has zircon 18O values ranging from 5.13 to 7.38‰. These characteristics are indicative of a source derived from the partial melting of crustal material. The biotite monzogranite is: (i) peraluminous calc-alkaline to high-K calc-alkaline; (ii) enriched in LREE (LREE/HREE = 9.9–14.6), Rb, Ba, Th and U; (iii) depleted in Nb and Ta; (iv) has weak negative δEu and δCe anomalies; (v) has whole rock εNd(t) value of between −3.2 and −1.8; has relatively high positive zircon εHf(t) values of +4.1 to +13; and has zircon 18O values ranging from 3.44 to 6.75‰. These features are indicative of partial melting of a juvenile mafic crust that might have experienced a high-temperature water-rock interaction. By comparing the geochronology of Neoproterozoic magmatic rocks along the faulted western margin of the Yangtze Block, it is suggested that the region was part of the Rodinia Supercontinent when it started to breakup during widespread rifting between ca. 825 and 710 Ma. We believe the fault zone formed in response to an opening ocean was induced by the presence of a mantle plume during this period.

A new concept for the genesis of felsic magma: the separation of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth’s surface, and that this process may have contributed to the growth of the continental crust.

Magma plumbing system for the Millennium Eruption at Changbaishan volcano, China: Constraints from whole-rock U–Th disequilibrium

A caldera-forming eruption in 946 CE, known as the Millennium Eruption (ME), at Changbaishan (or Baekdusan) volcano, located on the border between China and North Korea, was one of the largest explosive eruptions on Earth in the last 2000 years. In this study, we obtained new whole-rock Sr, Nd, and Pb isotopic data and 238U–230Th disequilibrium data of representative volcanic products of the ME, as well as those of a pre-ME (4–5 ka) eruption, to understand the evolution of the magma plumbing system related to the caldera-forming eruption. The volcanic products of the ME have SiO2 contents ranging from ~54 to ~75 wt%, and the whole-rock (230Th/232Th) ratios tend to decrease systematically from basaltic trachy-andesite through trachyte to comendite. The relatively high (230Th/232Th) ratios of the felsic (trachytic and comenditic) volcanic products suggest that they were formed primarily from fractional crystallization of mafic magmas, rather than from partial melting of crustal materials. The basaltic trachy-andesite sample has a distinctly high (230Th/232Th) ratio and the least radiogenic Pb isotopic ratios among ME volcanic products, suggesting that the mafic magma was recently introduced into the main felsic magma system and is hypothesized to have triggered the eruption. The lower (230Th/232Th) ratios of the comendite samples than those of the trachyte samples, as well as published U-series disequilibrium data, suggest that the comendite magmas were produced by mixing old (> ~ 23 ka) and young (<~6 ka) comenditic magmas. The old comendite magma was the main component of ME volcanic products. Pre-ME pantelleritic magmas with distinctly low (230Th/232Th) and high 87Sr/86Sr ratios from those of ME magmas may have formed a short-lived magma chamber until ~4–5 ka, which was isolated from the long-lived, main comendite magma chamber.

Zircon U–Pb ages, geochemistry, Sr–Nd–Hf isotopic compositions, and geological implications of the Shaziling pluton in the Nanling Orogenic Belt, southern China

Laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb ages indicate that the Shaziling pluton in the Jiuyishan area, Ningyuan County, Hunan Province crystallised at 151.9 ± 1.1 to 154.1 ± 1.2 Ma, indicating that it was emplaced during the Late Jurassic. The Shaziling pluton is metaluminous to peraluminous (A/CNK = 0.93–1.09), and has high silica and alkali contents, low Ca and Mg contents, and K2O/Na2O and FeO*/MgO ratios of 1.37–2.65 (mean = 1.80) and 5.43–15.33 (mean = 7.14), respectively. The total rare earth element (REE) content varies from 186.75 to 413.17 ppm, and the pluton is enriched in light REEs and shows negative Eu anomalies (0.095–0.224). It is enriched in the large‐ion lithophile elements Rb and Th, and depleted in Ni, Cr, Eu, Ti, V, P, and Sr. The pluton yields Ga/Al ratios of 245–582 ppm (mean = 350 ppm) and Zr + Nb + Ce + Y contents of 256.8–630.7 ppm (mean = 441.95 ppm), similar to A‐type granites. The Shaziling pluton has a high initial 87Sr/86Sr ratio (Sri = 0.71603–0.71817), and low εNd(t) (−6.8 to −7.4) and εHf(t) (4.8 to −14.2) values. The granitic magmas were produced by partial melting of crustal material, along with a mantle contribution. The Nd and Hf model ages are 1546–1498 and 1756–1061 Ma, respectively, suggesting that the source rocks separated from the mantle reservoir during the Mesoproterozoic. The Shaziling pluton was formed in an intracontinental extensional environment related to subduction.

Origin of late Neoarchean granitoid diversity in the Western Shandong province, North China Craton

Diverse late Neoarchean granitoids are widely exposed in the Western Shandong province (WSP). According to geological relationships, lithological features, magmatic zircon U–Pb–Lu–Hf isotopes and whole-rock chemical characteristics, eight main lithological assemblages are identified, and their petrogenesis is determined. (1) The ~2.54 Ga and ~2.51 Ga potassic granites show the highest SiO2 contents (67.06–75.84 wt%) and were sourced from partial melting of metamorphic greywackes. (2) The ~2.53 Ga quartz syenites exhibit the highest alkali contents (10.32–11.23 wt%) and were sourced from mantle-derived basaltic rocks and recycled sediments. (3) The ~2.53–2.52 Ga quartz diorites (less-differentiated sanukitoids) formed from partial melting of metasomatized mantle. (4) The ~2.53 Ga hornblendites have crystallization ages and Lu–Hf isotopes similar to those of the quartz diorites, implying that they are cogenetic cumulates from the quartz dioritic magmas. (5) The ~2.56–2.52 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses were formed by melting of thickened crust. (6) The ~2.53 Ga high-Mg granodiorites (differentiated sanukitoids) were formed by mixing of crust-mantle melts. (7) The ~2.51–2.49 Ga gabbros originated from partial melting of fluid-metasomatized mantle. (8) The ~2.54–2.53 Ga high-Mg andesites (boninites) originated from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in LILEs and LREEs by slab-derived melts.Integrating the petrogenesis of these late Neoarchean diversified granitoids and associated volcanic rocks, we consider that the late Neoarchean was a crucial period of crust-mantle interaction in the WSP, and partial melting of the metasomatized mantle produced intermediate-mafic melts, then underplating of the mantle-derived magmas caused the partial melting of crustal materials, further led to the mixing of crust-mantle magmas. The above complex crust-mantle interactions triggered the transfer of energy and materials from the Earth’s interior to the surface, forming modern-like continental crust thickness and geothermal gradient. Moreover, combining late Neoarchean intense granitic magmatism and ~2.49 Ga mafic dike swarms implies that the North China Craton experienced cratonization processes at the end of the Neoarchean.

Geochemical characterization of Granitoids in Katchuan Irruan area: further evidence for peraluminous and shoshonitic compositions and post-collisional setting of granitic rocks in the Precambrian Basement Complex of Nigeria

Petrographic studies on Granitoids from Katchuan Irruan and adjoining areas, southeastern Nigeria, has shown that they are garnetiferous biotite granite, aplitic granite, porphyritic hornblende biotite granite, porphyritic muscovite biotite granite, weakly foliated leucogranodiorite and simple pegmatite. They are closely associated with the Precambrian Basement Complex rocks which they intruded. Modal analysis shows that the rocks consist of quartz (10%–25%), oligoclase (10%–30%), K-feldspar (15%–35%), biotite (3%–25%), with occasional garnet, hornblende, muscovite, and accessory chlorite, haematite and magnetite. Geochemical data indicates that the rocks are generally shoshonitic, alkali-calcic to calcic, ferroan and peraluminous. They are enriched in large ion lithophile elements as well as high field strength elements (Hf, Ta, Yb, Sm, Zr and Y). Their trace elements and REE patterns are similar, indicating that they are co-genetic. They are characterized by high fractionation factor (La/Yb)N (3.04–228.44) and pronounced negative Eu anomalies (Eu/Eu*) (0.23–0.71). Their overall geochemical features indicate that they were most likely derived from partial melting of crustal materials in an orogenic (post-collisional) tectonic setting. They are therefore related to the Pan-African granites, otherwise known as the Older Granites which were emplaced during the Pan African orogenic event.

Early Cretaceous granitic dykes in the western Dabie Orogen, China: Geochronology, petrogenesis, and tectonic implications

Whole‐rock major‐trace elemental, whole‐rock Sr–Nd, and zircon U–Pb–Hf isotopic analyses have been carried out on two suites of Early Cretaceous granitic dykes in the western Dabie Orogen (Central China) with the aim of addressing their petrogenesis and geodynamic significance. LA‐ICPMS zircon U–Pb dating reveals that the E‐W‐trending granitic dykes have emplacement ages of 133.9 ± 1.6 and 132.8 ± 1.0 Ma, while the NW‐trending granitic dykes have emplacement ages of 121.4 ± 0.9 and 120.5 ± 0.7 Ma. These rocks have high concentrations of SiO2 and K2O, belonging to the high‐K calc‐alkaline and shoshonitic series. They are metaluminous to peraluminous and are mainly fractionated I‐type granite. The E‐W‐trending granites have moderate initial 87Sr/86Sr ratios (0.70614 to 0.70878) and negative εNd(t) values varying from −16.2 to −17.2 with two‐stage Nd model ages of 2.20 to 2.43 Ga. They exhibit a narrow range of zircon εHf(t) values (−22.34 to −23.70) with two‐stage Hf model ages of 2.31 to 2.36 Ga. These isotopic signatures suggest a homogeneous crustal source without significant magma contamination or mixing. The NW‐trending granites have initial 87Sr/86Sr ratios ranging from 0.70774 to 0.71034 and negative εNd(t) values varying from −10.9 to −9.5 with two‐stage Nd model ages of 1.69 to 1.80 Ga. They yield variable zircon εHf(t) values of −17.06 to −0.39 with two‐stage Hf model ages of 1.07 to 1.99 Ga, implying heterogeneous crustal signatures with minor mantle materials involved. Combining the geological, geochemical, and isotopic evidences, the E‐W‐trending granitic dykes are considered to be derived from partial melting of crustal materials under normal thickness during a transitional period following the tectonic collapse, while the NW‐trending granitic dykes resulted from crust melting in an extensional setting, most likely induced by the asthenospheric upwelling. Our results support that the western Dabie Orogen could have experienced S‐N‐directed compression (pre. 133 Ma), S‐N‐directed extension (ca. 133 Ma), and general E‐W‐directed extension (ca. 120 Ma). We propose that the Dabie Orogen has experienced tectonic collapse of the thickened lower crust and also has been affected by the subduction of the Izanagi (or Palaeo‐Pacific) Plate during the Early Cretaceous.

High Nb\u2013Ta rhyolites of the Kyihe region, north Daxingan Mountains, China: geochemistry, petrogenesis, and zircon U\u2013Pb geochronology

Rhyolites with high Nb–Ta contents were recently discovered in the north Daxingan Mountains, China. We determined the geochemical characteristics and zircon U–Pb ages of these rhyolites to elucidate their tectonic setting of formation and petrogenesis. Zircons from the high Nb–Ta rhyolites are idiomorphic or hypidiomorphic, short prismatic crystals with oscillatory zoning; the zircon trace element has a higher Th/U ratio (> 0.4); zircon rare earth element (REE) content is high (average is 1729 × 10−6) and indicates heavy REE (HREE) enrichment (average is 1561 × 10−6) and shows positive Ce (Ce/Ce* = 2.1–103.4) and negative Eu (Eu/Eu* = 0.18–0.64) anomalies typical of crustal magmatic zircons. Their weighted-mean LA–ICP–MS U–Pb age of 155 ± 1 Ma indicates that they formed in the Upper Jurassic. The rhyolites are characterized by high SiO2 and alkali contents and low Fe, Ca, Mg, and Mn contents, and are weakly peraluminous, indicating that they are high-K calc-alkaline rocks. Trace element compositions are characterized by enrichments in Nb, Ta, Zr, Hf, Ce, and Rb and depletions in Sr, Eu, Ba, P, Ti, Co, and Ni, with significant positive Ce (Ce/Ce* = 2.4–2.7) and negative Eu (Eu/Eu* = 0.06) anomalies. Niobium and Ta are hosted in the zircons. In (Na2O+K2O+FeOT+MgO+TiO2) vs (Na2O+K2O)/(FeOT+MgO+TiO2) and (Al2O3+FeOT+MgO+TiO2) vs Al2O3/(FeOT+MgO+TiO2) mineral characterization diagrams, data for the samples plot in the metamorphic greywacke or basic argillaceous rock fields, indicating that the magma originated from partial melting of crustal material. εSr(t) values cover a wide range (− 18.2 to + 102.9), whereas εNd(t) values have a narrow range (1.9–2.0) with T2DM model ages of 789–785 Ma, indicating that the source was the Neoproterozoic Xinghua Ferry Group crustal basement. This crustal rock suite comprises a volcanic–sedimentary formation of metamorphosed mafic volcanic and terrigenous clastic rocks derived from a mixture of mantle and crustal materials. Residual phases in the source region include Ca-rich plagioclase, amphibole, orthopyroxene, and zircon + garnet. Together with the positive Ce anomalies and low-Sr/high-Yb characteristics of the rhyolites, this indicates that the source rocks melted at relatively shallow depths (< 30 km), low pressures (< 0.8 GPa), and high O2 fugacity. Ga × 104/Al > 2.6, Ta vs Yb, and (Rb/30) vs Hf vs (3Ta) discrimination diagrams, data for the samples plot in the A-type rhyolite and intraplate granite fields, whereas in the Nb vs Y vs Ce diagram, the data plot in the A1-type field. It is concluded that an extensional tectonic setting, resulting from closure of the Mongolia–Okhotsk Ocean at the end-Triassic, or northward subduction of oceanic lithosphere under the Siberian Plate, caused underplating of mantle-derived basaltic magma and partial melting of metamorphic crustal rocks.

Rhyolites in the Emeishan large igneous province (SW China) with implications for plume-related felsic magmatism

In the Emeishan Large Igneous Province (ELIP), western Yangtze Block, SW China, minor amounts of silicic volcanic rocks (∼5%) are spatially and temporally associated with flood basalts. The distribution and origin of these silicic rocks are keys to understanding the processes associated with mantle plume activities. In the Binchuan and Panzhihua areas, southwestern ELIP, there are Nb-Ta-poor and Nb-Ta-rich rhyolites. Nb-Ta-poor rhyolites interlayered with basalts in the Binchuan area occur in the lower part of the Emeishan volcanic sequence and have a crustal origin. They are characterized by negative Nb-Ta anomalies, low εNd(t) and high 87Sr/86Sr(i) values, and zircon saturation temperatures (836–852 °C). Nb-Ta-rich rhyolites are associated with the main phase of the Emeishan volcanic sequence in the Binchuan and Panzhihua areas and are characterized by positive Nb-Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low 87Sr/86Sr(i) ratios, positive εNd(t) and εHf(t) values, and high A/CNK ratios and Fe∗s. In addition, they have high Ga/Al ratios, high field strength elements (HFSEs), and high zircon saturation temperatures (>920 °C), typical of aluminous A-type silicic rocks, similar to the spatially distributed syenitic intrusions of the ELIP. These Nb-Ta-rich rhyolites are best interpreted as hybrid products of crystal fractionation of high-Ti basaltic magmas, coupled with minor crustal assimilation (6–14%), on the basis of Rhyolite-MELTS and EC-AFC simulations. The temporal and spatial distributions of these two types of rhyolite reflect various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head reached the base of the western Yangtze Block, enriched components in the lithospheric mantle were melted and the melts partially ponded near the crust-mantle boundary or in the lower crust where they induced partial melting of crustal materials to form Nb-Ta-poor rhyolites. During the main phase of volcanism, large magma chambers and extensive plumbing systems developed due to decompressional melting of the mantle plume. In this stage, abundant mafic-ultramafic and felsic intrusions were formed, and Nb-Ta-rich rhyolites and A-type granites were emplaced.