Low MgO Contents Research Articles

Transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean: constraints from the Jurassic adakitic and I-type granites, and calc-alkaline diorites at the northern margin of the North China Craton

ABSTRACT There is a broad consensus that subduction of the Paleo-Pacific oceanic plate has caused extensive magmatism, lithospheric deformation, and metal mineralization in the eastern margin of the Eurasian continent during the Late Mesozoic. However, the onset and extent of subduction of the Paleo-Pacific Ocean, especially the timing of the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific, and extent of the subduction of the Paleo-Pacific Ocean remain poorly constrained. To address these key issues, we present regional geological observations, zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of the Jurassic intrusions at the northern margin of the North China Craton, with the aim of constraining their petrogenesis and the tectono-magmatic events related to subduction of the Paleo-Pacific Ocean. The geochronological and geochemical results indicate that these intrusions were emplaced between 177 and 155 Ma, with distinct geochemical features. The Huajianggou pluton (HJG), Tongjiatun pluton (TJT), and Haoguantun pluton (HGT) are characterized by high Al2O3 and Sr contents, and low MgO, TFe2O3, Y, Yb, and heavy rare earth elements (HREEs) contents, coupled with high Sr/Y values (34.10–94.36), showing adakitic affinities. Based on these geochemical characteristics, we suggest that these adakitic magmas (i.e. the HJG, TJT, and HGT plutons) were produced by partial melting of thickened lower crust in response to earlier subduction of the Paleo-Pacific oceanic plate beneath the northern margin of the North China Craton. In contrast, the Baijiagou pluton (BJG) and Sheshangou pluton (SSG) are high-K calc-alkaline rocks with low Sr/Y ratios (0.80–17.77), and they exhibit petrological and geochemical characteristics of I-type granites, indicating an origin from the lower crust, with variable degrees of fractional crystallization. In addition, the Bakeshu pluton (BKS) is mainly comprised of diorite that displays high SiO2, low MgO, Cr, and Ni contents, with Mg# of 39.8–42.9, indicating a lower crustal origin. Despite the range in ages of these intrusions, all the Jurassic calc-alkaline intrusions show typical geochemical characteristics of active continental margins, such as enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs). Combined with spatial-temporal variations of coeval magmatic rocks along the northern margin of the North China Craton, we suggest that the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean was initiated in the Early Jurassic. These Jurassic intrusions formed in an active continental margin setting, related to westward subduction of the Paleo-Pacific oceanic plate. Importantly, the Early Jurassic subduction-related calc-alkaline magmatic rocks in northeast (NE) China constitute a NE-trending magmatic arc belt, which is best explained as continental magmatism triggered by the westward subduction of the Paleo-Pacific oceanic plate.

Late Paleoproterozoic crustal thickening of the Jiao–Liao–Ji belt, North China Craton: Insights from ca. 1.95–1.88 Ga syn-collisional adakitic granites

The Jiao–Liao–Ji belt (JLJB) of the North China Craton is commonly regarded to have been involved in the global-scale collisional events that led to the amalgamation of the Columbia/Nuna supercontinent during the late Paleoproterozoic. Previous studies have revealed that the JLJB underwent a high-pressure granulite-facies metamorphic event with a clockwise P–T–t path during ca. 1.95–1.90 Ga. However, few studies have examined the contemporaneous magmatism and its genetic relationship to the high-pressure granulite-facies metamorphism. In addition, the petrogenesis of the ca. 1.95–1.90 Ga magmatism and its possible constraints on the crustal-thickening process are poorly understood. Our field investigations have discovered a series of late Paleoproterozoic granitoid plutons in Liaodong Peninsula, including the Fangjiawei, Qingchengzi, Songshugou, and Shizijie plutons. Systematic analyses of geochronology, petrology, and geochemistry were conducted to constrain the formation ages, petrogenesis, and tectonic environment of these granites. Zircon U–Pb dating reveals that the granites formed during ca. 1.95–1.88 Ga, coeval with the timing of the regional prograde or peak metamorphism and collision in the JLJB. They show geochemical characteristics of typical adakites, including high Sr (393–860 ppm) contents, low Y (1.03–3.30 ppm) and Yb (0.09–0.27 ppm) contents, and relatively high Sr/Y (125–832) and LaN/YbN (9.34–33.6) ratios. Low MgO, Cr, and Ni contents and the existence of ca. 2.5–2.0 Ga inherited zircons with ancient TDM2 ages indicate that the parental magmas were generated from partial melting of a thickened lower crust. The positive correlation between Dy/Yb and La/Yb ratios and the relatively low Nb/Ta (5.1–18.9, average of 13.6) ratios all suggest that garnets, rather than amphiboles or rutiles, were the dominant phase in the residual source, indicating a melting depth of 30–50 km. Our new results, combined with previous findings, support a rifting-and-collision model whereby the JLJB underwent a transformation from early intracontinental rifting to subsequent opening and closure of an incipient ocean, with the final collision between the Longgang Block and Nangrim Block occurring at ca. 1.95–1.88 Ga with substantial crustal thickening.

Geochemical, mineralogy, and Sr–Nd–Pb isotopic compositions of the gold-related lamprophyre in the Bengbu–Wuhe Area, southeastern North China Craton: Implications for gold mineralization

Besides granite porphyries, many mafic dikes also have a close temporal and spatial relationship with orebodies in the BWGB. However, compared with the granite porphyries, no detailed attempt has been made to determine the petrogenesis of these mafic dikes or to elucidate their relationships with the gold mineralization. Here, we present comprehensive bulk-rock geochemical data; pyrite, apatite, and amphibole compositions; Sr-Nd-Pb isotopes, and U-Pb zircon data for the lamprophyres to provide constrains on their petrogenesis, magmatic properties, geodynamic setting, and metallogenic implications. We obtained zircon U-Pb ages, which reveal that the lamprophyres were emplaced ~ 130 Ma. The lamprophyres are characterized by pronounced light rare earth element (LREE) enrichment, heavy rare earth element (HREE) depletions ((La/Yb)N > 10). They also have low SiO2 (<51 wt%) and high MgO (>4.1 wt%) contents; low Rb/Sr (0.01–0.04), Th/La (0.1–0.12), and Lu/Yb (0.13–0.16) ratios; high Ba/Rb ratios (>18), low Sr-Nd values (87Sr/86Sr(t) = 0.7060–0.7076; εNd(t) = –8.8 to –12.5); and a wide range of radiogenic Pb isotopic ratios (206Pb/204Pb(t) = 16.37–17.35), indicating an enriched mantle source metasomatized by the upper crust of the subducted South China Block (SCB). Their low apatite MnO contents and high apatite SO3 contents, high apatite molar OH/F ratios, and high amphibole Mg* and VIAl* values suggest that the mafic magmas were more highly oxidized and hydrated. Taken together, the lithospheric mantle source was initially metasomatized by the SCB and was further hydrated by the subducted Paleo-Pacific Plate, resulting in a mafic melt with a high H2O content and oxygen fugacity value, which are favorable conditions for gold transportation.

Middle Triassic lower crust‐derived adakitic magmatism: Thickening of the Dananhu intra‐oceanic arc and its implications for arc–arc amalgamation in the Eastern Tianshan (NW China)

The timing and mechanism of the closure of the Palaeo‐Asian Ocean are problematic and controversial. To help resolve these problems, we report geochronological, geochemical, and isotopic data from mid‐Triassic adakitic intrusions in the Eastern Tianshan, NW China. U–Pb dating shows that the adakitic intrusions formed at 243–234 Ma. These mid‐Triassic adakitic intrusions are characterized by high Sr/Y and La/Yb ratios, low Yb and Y, and positive εNd (t) (+3.12 − +4.71) and low (87Sr/86Sr)i (0.703956–0.704487) values. However, they have high K and low A/NK values (1.16–1.71), relatively low MgO (0.43–1.81 wt%) contents, and Mg# (44–61), low abundances of compatible elements (Cr = 4.09–16.69 ppm, Ni = 2.02–8.03 ppm), which are different from the typical slab‐melting adakites. These features indicate that they were derived from the partial melting of the relatively depleted thickened lower crust. Our new geochronological, geochemical, and isotopic data integrated with the established amalgamation of nearby arcs lead us to conclude that the adakitic intrusions were most likely generated by partial melting of the tectonically thickened (&gt;40 km) juvenile Dananhu intra‐oceanic arc and, therefore, the Kanguer branch of the Palaeo‐Asian Ocean closed since ca. 234 Ma.

Construction of a highly silicic upper crust in southeastern China: Insights from the Cretaceous intermediate-to-felsic rocks in eastern Zhejiang

The Cretaceous granitoids in eastern Zhejiang were examined to establish a model for the construction of the highly silicic upper crust in southeastern China. The studied rocks include quartz monzodiorite, quartz monzonite, alkali feldspar granite, and their microgranular enclaves (ME). Zircon UPb dating suggests that the granitic rocks and MEs were emplaced at approximately 102–114 Ma. Geochemical features and isotopic compositions of the quartz monzodiorite, quartz monzonite, and alkali feldspar granite indicate that they are mainly produced by the partial melting of Neoproterozoic to Mesoproterozoic crustal rocks. The quartz monzodiorites, quartz monzonites, and alkali feldspar granite appeared to derive from a similar silicic magma reservoir because they have comparable crystallization age, zircon trace element variation trend, Sr-, Nd-, and Hf-isotopic compositions, and complementary geochemical compositions. The studied quartz monzodiorites, quartz monzonites, and low silica alkali feldspar granites (SiO2 < 75 wt%) are deemed a residual cumulate according to their geochemical compositions and textural features (e.g., aggregates of coarse-grained euhedral plagioclase crystals in “glomeroporphyritic aggregates”), whereas the high silica alkali feldspar granites (SiO2 > 75 wt%) resemble frozen remnants of high silica melts segregated from a crystal mush because they are highly enriched in SiO2 and depleted in Ba, Sr, and Eu. The MEs hosted in the quartz monzodiorites and quartz monzonites have low SiO2 and high MgO contents, high initial 87Sr/86Sr ratio and negative εNd(t) and εHf(t) values, and variable zircon δ18O and εHf(t) values. Petrographic observations and geochemical features suggest that they are formed from solidified mafic magmas and generated by the crystal fractionation of mantle-derived hydrous mafic magma with subsequent crustal contamination. Compared with these mafic MEs, the MEs hosted by the alkali feldspar granites have relatively high SiO2 and low K2O contents, indicating that the felsic MEs are generated by crystal fractionation of the parental magma of the mafic MEs. Since abundant MEs occurred in the quartz monzodiorites, quartz monzonites, and alkali feldspar granites, magma recharge may have promoted the extraction and upward percolation of high silicic melts from the compacting crystal-rich mush in the shallow crust. Thus, the chemical and lithological diversity of the granitic plutons in eastern Zhejiang is generated in the upper crust via crystal-melt segregation. Specifically, the quartz monzonites and quartz monzodiorites are formed as residual cumulate, whereas the high silica alkali feldspar granites are generated as frozen remnants of high silicic melts segregated from a compacting crystal mush. More widely, our work suggests that the formation of the highly silicic upper crust in southeastern China is controlled by crystal-melt segregation processes in the shallow crust.

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18 Ma for tonalites; 754 ± 3.9 Ma for trondhjemite; 738 ± 3.8 Ma for granodiorite; and 717 ± 3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6–+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745 ± 31 °C for tonalite to 810 ± 25 °C for trondhjemite; 840 ± 10 °C for granodiorite; and 868 ± 10 °C for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N = 0.9–1.1), but >3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc–continent collisions) in the late Cryogenian -Ediacaran times (650–542 Ma).

Paleoproterozoic (1.96–1.86 Ga) granites in Xinyang record zoned deep crustal structure and multi-stage reworking beneath the southern North China Craton

As one of the oldest cratons on Earth, the North China Craton (NCC) is a frequent area for geological research due to its complex crustal evolution. Regarding the Paleoproterozoic crustal evolution of the craton, most studies have focused on continental amalgamation (i.e. horizontal crustal evolution), but few on the vertical crustal evolution. Here we report whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotope geochemistry for the Xinyang granites to constraint the vertical crustal structure and evolution of the southern NCC. The studied granites comprise two-mica K-feldspar granite and garnet-bearing K-feldspar granite. All the rocks have high SiO2 and total alkali, but low CaO, MgO and Fe2O3T contents. Their A/CNK values (1.05–1.28) suggest that they are peraluminous rocks. These granites are enriched in LILE (Rb, Th and K), depleted in Sr, P and HFSE (Nb and Ti). The two-mica K-feldspar granites show obviously fractionated REE patterns ((La/Yb)N = 18.3–387) with negative or positive Eu anomalies (δEu = 0.47–1.03, 4.85–6.72), while the garnet-bearing K-feldspar granites have flat REE patterns ((La/Yb)N = 1.69–4.89) with extraordinary negative Eu anomalies (δEu = 0.22–0.42). The magmatic zircons of the two-mica K-feldspar granites yield crystallization ages ranging from ~ 1.86 Ga to ~ 1.90 Ga, and those of the garnet-bearing K-feldspar granites give crystallization ages of ~ 1.90 Ga. Previous work also reported a biotite K-feldspar granite pluton formed at ~ 1.96 Ga in the Xinyang area. Taken together, three major episodes of Paleoproterozoic granitoid events (~1.86 Ga, ~1.90 Ga and ~ 1.96 Ga) are recognized in the area. Combined with regional geochronological investigations, the peraluminous granitic magmatic activities occurring during 1.96–1.86 Ga in the Xinyang area could be linked to the collision between the Eastern and Western Blocks of the NCC. Alternatively, zircon Hf isotopic compositions suggest that these rocks derived from partial melting of pre-existing crust with heterogeneous compositions. Zircons from the ~ 1.86 Ga two-mica K-feldspar granites yield εHf(t) values of −5.2–1.7 with Tcrust of 2.40–2.86 Ga, compared to zircons from the ~ 1.90 Ga two-mica K-feldspar granites which yield wide range of εHf(t) values of −13.5–1.0 with Tcrust of 2.50–3.42 Ga. In contrast, the ~ 1.90 Ga garnet-bearing K-feldspar granites have homogeneous negative zircon εHf(t) values ranging from −11.8 to −10.4, with Tcrust ranging from 3.23 to 3.30 Ga. Moreover, zircons from the biotite K-feldspar granites have εHf(t) values of −0.6–2.6, corresponding to Tcrust of 2.44–2.77 Ga. Based on abovementioned geochemical signals, we propose that the Paleoproterozoic crust in the southern NCC likely consisted of two parts in vertical, the upper Middle-Late Archean crust and the lower early Paleoproterozoic crust with Eo-Paleoarchean crustal remnants. During the Paleoproterozoic, the southern NCC experienced multi-stage crustal reworking, indicating that a stable continental crust likely had been formed by the end of Archean.

U–Pb–Hf–O–Nd isotopic and geochemical constraints on the origin of Archean TTG gneisses from the North China Craton: Implications for crustal growth

Tonalite–trondhjemite–granodiorite (TTG), an Archean dominant lithological assemblage within the cratonic provinces, is believed to have been formed by partial melting of the hydrous basalts in the garnet stability field. The origin of TTG is a window to decipher the evolution of Archean continental crust, whereas the petrogenesis is not well constrained. To solve this issue, we have carried out field observations, zircon U–Pb–Hf–O isotope, as well as whole-rock Nd–O isotope and geochemical analyses on the TTG and associated dioritic gneisses from the North China Craton. U–Pb isotopic dating using the LA–ICP–MS on zircons reveals that the granitoid gneisses in northern Liaoning were contemporaneously generated during ca. 2.56–2.51 Ga and immediately experienced a regional metamorphic event at 2.48–2.45 Ga. The dioritic gneisses occur as quenched inclusions with rounded to ellipsoidal globules in shape rather than lithospheric relics. Such occurrence, together with isotopic and geochemical data, suggests that the mafic enclaves and host TTG represent two co-existing, compositionally distinct magmas. The TTG gneisses are featured by elevated SiO2 (64.72–86.46 wt%) contents, (La/Yb)N (68 on average) and Sr/Y (119 on average) ratios, and low MgO (0.35–2.58 wt%) contents and Mg# values, with radiogenic Hf and Nd isotopes (εHf(t) = +3.5 to +9.0 and εNd(t) = +0.8 to +5.4), indicating the derivation from partial melting of the juvenile basaltic sources at lower crustal levels. Compared with the TTG gneisses, the dioritic gneisses have lower SiO2 contents, higher MgO contents and Mg# values, with similar Hf–Nd isotopes (εHf(t) = +3.6 to +9.5 and εNd(t) = +3.3 to +5.1), implying the depleted mantle source. A model is proposed whereby partial melts from juvenile basaltic sources evolved via mixing with mafic enclaves (viz. dioritic magma) and fractional crystallization, and finally were contaminated by the sedimentary rocks, forming the TTG magma. The granitoid gneisses in northern Liaoning witnessed the major vertical growth of continental crust at the end of the Neoarchean, which is supported by Hf–Nd–O isotopic data and zircon Hf–O isotopic modeling. Our results show that the geochemistry of TTG (such as elevated La/Yb and Sr/Y) might be affected by complex petrogenetic processes, i.e., partial melting of individual sources, magma mixing, fractional crystallization, and crustal contamination, rather than solely controlled by the P–T conditions of crustal anataxis as previously thought, which might have been also overlooked in other Archean cratons.

Crustal melting in a protracted hot setting in the Altai Orogen (NW China): Evidence from Permian leucogranite dykes in the metamorphic belt

Numerous leucogranite dykes, and spatially associated high-grade metamorphic rocks, are well-preserved in the Chinese Altai orogenic belt. Integrated petrological, geochronological, and geochemical results of the leucogranite dykes are presented to decipher their petrogenesis and constrain the tectono-thermal setting of the Altai orogen. The leucogranite dykes have high SiO2 (71.9–81.2 wt%) and Al2O3 contents (11.1–15.7 wt%), and low CaO, MgO and TiO2 contents, with a total alkali content (K2O + Na2O) of 4.55–9.88 wt%. All leucogranites show weakly to strongly peraluminous S-type granite affinities with A/CNK ratios of 1.0–2.2. Trace element concentrations indicate a characteristic high Rb content, low Ba and Sr contents, and Rb/Sr ratios of 0.43–35.8. They exhibit nearly flat REE patterns with strong negative Eu anomalies on chondrite-normalized REE patterns. Considering their low zircon saturation temperatures (682–758 °C) and low εNd(t) values (˗12.4 to ˗2.7), as well as the presence of Al-rich minerals (muscovite and garnet), we infer that the leucogranites were low-temperature granites and generated mainly by fluid-absent muscovite dehydration melting of metasediments. LA-ICPMS zircon and monazite U-Pb dating suggests crystallization ages of 279–267 Ma, which places these leucogranite dykes in temporal association with the ultrahigh- to high-grade metamorphic rocks and implies a prolonged heat budget in the region. Together with contemporary magmatic activities and metamorphism in the Chinese Altai orogen, the generation of the leucogranite dykes may have been attributed to long-lasting high heat flow in the early Permian, most likely related to the assembly of the Junggar plate and the Chinese Altai.

Genesis of Late Cretaceous granite and its related Nb–Ta–W mineralization in Shangbao, Nanling Range: Insights from geochemistry of whole-rock and Nb–Ta minerals

The Shangbao mining district, located in Nanling Range, South China, includes a series of middle-large scale granite-related W–Sn–Nb–Ta deposits. The genesis for W-Sn-Nb–Ta mineralization in this region is in fact ambiguous. In addition, the research of the latest mineralized granites in Nanling Range may be contributed to further understand the mineralization mechanism. In this study, we report the geochronological, geochemical and isotopic compositions of the Shangbao granite, together with the texture and element of Nb–Ta bearing minerals. LA-MC-ICP-MS U–Pb dating of zircon and columbite in granite from Shangbao Nb–Ta deposit yields weighted mean 206Pb/238U age of 87.3 ± 0.9 Ma and 86.83 ± 0.8 Ma, respectively, revealing the emplacement of the granites and their mineralization during Late Cretaceous. The major elements of these granites show high contents of K2O + Na2O, Fe2O3, SiO2 and F, low contents of P2O5, CaO, TiO2 and MgO, suggesting high-K calc-alkaline series with slightly peraluminous signature. Besides, they exhibit flat REE pattern and high content of Zr + Nb + Ce + Y, revealing the Shangbao granite affinity of highly fractionated A-type granite. Lu–Hf and Sr–Nd isotopes indicate that the Nb–Ta bearing magma was formed by the mixing crust-mantle materials. Combined with the previous studies, emplacement of Shangbao granite is linked to an expansional geodynamic setting, likely caused by the roll-back of the paleo-Pacific plate subduction.The principal economic Nb–Ta oxides of Shangbao granite are columbite group minerals (CGM). Three major groups are identified given textural and chemical characteristics of CGM, i.e., (i) Group I displays systematic core-rim evolution from columbite to tantalite, and eventually to columbite; (ii) Group II shows that the columbite and tantalite have undergone metasomatism through the wolframoixiolite, and numerous REE-mineral were contained; (iii) Group III is anhedral columbite crystallized together with rutile. As revealed from geochemical characteristics of these three groups, the formation of Nb–Ta oxides within Shangbao granite display three generating stages, supporting the magmatic-hydrothermal origin. The high-degree fractional crystallization of Shangbao granitic magma is the main factor leading to the enrichment of Nb to allow saturation of columbite.

Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield

At Hadhb't Ad-Dayheen, in the central Arabian Shield, a post-collisional igneous complex called the Ad-Dayheen intrusion is exposed. It was emplaced in the Early Ediacaran (613–625 Ma), during the final tectono-magmatic stage of Arabian Shield development. Despite limited and discontinuous ring-shaped outcrops due to alluvial cover and later faulting, three pulses of intrusion can be recognized in the field: an early pulse of monzogranite; a second pulse of syenogranite and alkali feldspar granite; and a final pulse of alkaline and peralkaline granite, mineralized microgranite, and pegmatite. Samples show distinctively low contents of CaO, MgO, and Sr in contrast to elevated concentrations of alkalis, Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (REE); these are common characteristics of post-collisional rare-metal-bearing A-type granites. The suite displays positive Nb–Ta anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.11–0.35). The alkaline/peralkaline granites and microgranite of the Ad-Dayheen intrusion feature disseminated mineralization, whereas mineralization is localized in the pegmatite. The primary magma feeding the Ad-Dayheen intrusion was mostly generated by partial melting of the juvenile crust of the Arabian Shield, with a minor mantle contribution. We argue that an episode of lithospheric delamination led to crustal uplift, erosional decompression, and generation of mantle melts that supplied heat to drive crustal melting. The anatectic deep crustal melts assimilated a F-bearing component that also added rare metals to the magma. Each pulse can be described by a fractional crystallization model, but the parental liquid of each subsequent pulse was first modified by further addition of fluorine and rare metals and loss of CaO, Sr, Ba, and Eu due to fluorite fractionation. Texture and morphology of the ore minerals indicate that mineralization (U, Th, Zr, Nb, Ta, Y, Hf and REE) took place in two stages: a magmatic stage coinciding with emplacement of the intrusion, followed by a hydrothermal stage. The magmatic process enriched the residual melt in high field strength elements (HFSE) and REE. The later hydrothermal stage further localized these elements and increased their concentrations to economic grades. The pegmatite is highly mineralized and contains high concentrations of U (81–179 μg/g), Th (244–600 μg/g), Zr (2397–14,927 μg/g), Nb (1352–2047 μg/g), Ta (96–156 μg/g), Y (828–2238 μg/g), Hf (131–377 μg/g) and ∑REE (1969–4761 μg/g).

Geochemistry, geochronology, and zircon Hf isotopes of Late Jurassic–Early Cretaceous granitoids in the Xing'an Massif, NE China: Implication for the Late Mesozoic tectonic evolution and crustal growth

AbstractThis paper presents new zircon U–Pb geochronological, Hf isotopic and whole‐rock geochemical data for the granitic plutons in the Xing'an Massif, Northeast China, to constrain the Late Mesozoic tectonic evolution of the Mongol‐Okhotsk Ocean and the Paleo‐Pacific Ocean. The zircon U–Pb ages indicate that the granitoids emplaced during the Late Jurassic–Early Cretaceous. The granodiorites show an adakitic affinity with high Sr/Y ratios and low Yb (&lt; 1.30 μg/g) contents. The monzogranites exhibit high SiO2, low MgO contents, enrichment in LILEs (Rb, K, and Th), and depletion in HSFEs (Ta, Nb, Zr, P, and Ti). Petrological and geochemical features of these monzogranites suggest that they are highly fractionated I‐type granitoids. In addition, the zircon εHf(t) values and two‐stage model ages (TDM2) are in the range of +2.6 to +8.1 and 669–1011 Ma, respectively, indicating that primary magma was generated by partial melting of juvenile lower‐crustal materials, and there was a significant crustal growth in the Phanerozoic in the Northeast China. Combined with the coeval granitoids widely exposed in the Xing'an Massif, we conclude that the Late Jurassic magma in Northeast China was generated in an extensional setting related to the closure of the Mongol‐Okhotsk Ocean, but the Early Cretaceous magma was related to the subduction of the Paleo‐Pacific Plate.

Tracking a continental deep subduction and exhumation from granulitized kyanite eclogites in the South Altyn Tagh, northern Qinghai-Tibet Plateau, China

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint.Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al2O3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens.

Late Permian–Triassic tectonic nature of the eastern Central Asian Orogenic Belt: Constraints from the geochronology and geochemistry of igneous rocks in the Bureya Massif

The late Paleozoic–early Mesozoic tectonic nature of the eastern Central Asian Orogenic Belt (CAOB) has been subject to debate, and the late Paleozoic–early Mesozoic magmatism in the Bureya Massif provides an opportunity to address this issue. We report new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data for late Paleozoic–early Mesozoic igneous rocks from the south of the Bureya Massif. These data provide insight into the petrogenesis of the igneous rocks, and constrain the late Paleozoic–early Mesozoic tectonic nature of the eastern CAOB. Zircon UPb ages indicate that the late Permian (ca. 255 Ma) and Middle–Late Triassic (ca. 216 Ma) magmatic events are identified in the south of the Bureya Massif. The late Permian igneous rocks consist mainly of bimodal rock suites, which include quartz monzodiorites, gneissic monzogranites, and A-type gneissic syenogranites. The Middle–Late Triassic igneous rocks are dominated by porphyritic syenogranites, and A-type granite porphyries, syenogranites, and gneissic macroporphyritic quartz syenites. The quartz monzodiorites have low SiO2 contents (53.9–56.1 wt%), high Mg# (56–57), and Cr (160–175 ppm), Co (25.2–27.8 ppm), and Ni (61.2–72.3 ppm) contents, and are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; e.g., K, Rb, Sr, and Ba). These characteristics, along with enriched zircon Hf compositions [εHf(t) = −8.31 to −0.44], suggest that their primary magma was derived from the partial melting of lithospheric mantle that had been metasomatized by fluids released from a fossil subducted slab. The granitoids have high SiO2, and low MgO contents, and are enriched in LILE, depleted in high field strength elements (HFSE), and yield εHf(t) values of −11.5 to +2.87 and two-stage model ages (TDM2) ages of 1968–1024 Ma. These characteristics suggest that they were derived from the partial melting of Paleoproterozoic–Mesoproterozoic accreted crustal material; however, these granitoids have distinct, heterogeneous geochemical compositions, precluding simple fractional crystallization of a single parental melt, and strongly suggesting that they were originated from distinct petrogenetic processes. The late Permian–Triassic bimodal igneous rock suites and A-type granites imply an extensional environment in the eastern CAOB.

Mantle cooling and cratonization of Archean lithosphere by continuous plate subduction: Constraints from TTGs, sanukitoids, and high-K granites, eastern North China Craton

The evolution of plate subduction styles on the Earth is coupled with its cooling history. Continuous subduction was initiated during the Neoarchean, yet its influence on mantle cooling and the cratonization of Archean lithosphere remains unclear. Successive subduction-related magmatism occurred during the late Neoarchean evolution of the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton (NCC) and preserved crucial clues relating to processes and roles of regional continuous subduction. In this study, petrology, whole rock geochemistry, and zircon U–Pb–Hf isotopes are reported for late Neoarchean TTG gneisses, quartz monzodiorites, and monzogranites from the WSP. Zircon U–Pb dating reveals that the TTG gneisses, quartz monzodiorites, and monzogranites were formed at ~2561–2517 Ma, 2540–2533 Ma, and 2541–2513 Ma, respectively. The TTG gneisses with high (La/Yb)N ratios, low K2O/Na2O ratios, and low MgO contents were mainly derived from partial melting of oceanic slab material. In contrast, the quartz monzodiorites show high K2O and MgO content and high (La/Yb)N ratios, consistent with the characteristics of Archean sanukitoids, suggesting that they were the products of partial melting of mantle source metasomatized by slab-derived melts. The monzogranites with systematically high K2O, SiO2, and low MgO contents were formed by crustal reworking of regional TTGs and sedimentary rocks. Rock assemblages, geochemical features, regional field observation, and structure geology indicate prolonged (~2.56–2.49 Ga) continuous magmatism in the WSP across three northwest-trending belts (Belts A, B, and C) related to continuous subduction and slab rollback. These processes led to a tectonic transition from early compression to late extension. The final stabilization of the continental crust in the late Neoarchean in the eastern NCC was a consequence of successive mantle cooling due to regional continuous subduction recorded by subduction-related rock assemblages (i.e., adakites, sanukitoids, high-Mg basalts-andesites, and Nb-enriched basalts), the paired type of metamorphism, and the rise of alkali basalts. Secular geochemical changes of alkaline basalts outline the role of continuous subduction in dramatic mantle cooling and the final stabilization and cratonization of the eastern NCC.

Jurassic post‐collisional extension in the Songpan–Ganze Terrane, eastern Tibetan Plateau: Evidence from weakly peraluminous A‐type granites within the Zheduo–Gongga Massif

The Mesozoic–Cenozoic Zheduo–Gongga Massif, eastern Songpan–Ganze Terrane, can provide important clues for the crustal evolution in response to syn‐ and post‐orogeny. Here, we document petrological and geochemical characteristics of Early Jurassic granites (181.0 ± 2.2 Ma) within the massif along the Xianshuihe Fault in the eastern Tibetan Plateau. The granites have high contents of SiO2, Na2O + K2O, FeOT/MgO, 10,000 × Ga/Al, REE (except Eu) and HFSE (e.g., Nb, Ta, Zr, Hf), low CaO, TiO2, P2O5 and negative Eu, Sr, and Ba anomalies. Zircon REE shows a similar pattern with the whole‐rock REE. Our new data demonstrate weakly peraluminous A‐type affinity with A/CNK &gt;1.0 and alkali‐calcic, ferroan features. Zircon saturation temperatures and geochemical features (e.g., high SiO2 and low MgO contents, high Rb/Ba and Rb/Sr ratios) indicate that the granites resulted from high‐temperature (&gt;800°C) partial melting of Triassic flysch rocks in relatively shallow crust (&lt;500 MPa or &lt;20 km). Zircon εHf(t) values range from 0.55 to 2.91 and the corresponding two‐stage TDMC are older than the neighbouring Kangding complex and Yangtze margin, implying possible contribution of mantle materials to the parent magma. Considering the tectonic events and the regional geology, the magma was generated in a post‐collisional setting, which was possibly driven by the upwelling of hot mid–lower crustal materials due to the break‐off of subducted Palaeo‐Tethys Ocean slab. Under this situation, the magma may move upwardly into the pre‐existing Xianshuihe Fault during the post‐collisional extension of the thickened crust. Thus, slab break‐off played a key role in the genesis of the Zheduo–Gongga Massif studied herein.

Geochemical and Sr–Nd–Hf–O isotopic constraints on the source and petrogenesis of the Xiangshuigou silicic alkaline igneous complex from the northern margin of the North China Craton

We present eudialyte and zircon U–Pb ages, whole-rock geochemical, and Sr–Nd–Hf and zircon Hf–O isotope data for the Xiangshuigou silicic alkaline igneous complex from the northern margin of the North China Craton (NCC). We use these data to investigate the source and petrogenesis of these alkaline igneous rocks and the lithospheric mantle evolution of the NCC. The Xiangshuigou complex consists of nepheline and porphyritic syenites without associated mafic or intermediate rocks. Zircon and eudialyte U–Pb dating yielded consistent ages of 227–219 Ma for these rocks. All the alkaline igneous rocks from the Xiangshuigou complex have high Na2O, Al2O3, Rb, Th, and Zr, and low MgO, CaO, P2O5, and Ba contents, and are enriched in large-ion lithophile and light rare earth elements. These rocks are not depleted in high-field-strength elements. Whole-rock Nd–Hf isotopic compositions are slightly enriched, with εNd(t) = −3.7 to −3.0 and εHf(t) = −3.1 to +0.6. Zircon Hf–O isotope analyses yielded εHf(t) and δ18O values of −0.6 to −6.0 and +5.1‰ to +6.0‰, respectively. These geochemical and isotopic features suggest that the silicic alkaline igneous rocks were derived by partial melting of an enriched lithospheric mantle source before experiencing protracted two-stage fractional crystallization. The lithospheric mantle source of the Xiangshuigou complex was distinct from the Paleozoic cratonic lithospheric mantle and experienced mantle metasomatism by depleted mantle-derived carbonate-bearing materials prior to the alkaline magmatism. This indicates that, in addition to subducted crustal melts/fluids, asthenosphere-derived materials were also involved in modifying the lithospheric mantle of the northern NCC. As such, our work not only approves the effectivity of using whole-rock geochemistry and Sr–Nd–Hf–O isotopic data to reveal origin of the silicic alkaline igneous rocks, but also argues for a multiple modification for the northern NCC during the late Paleozoic to early Mesozoic.

Petrogenesis of the Late Triassic Mengsong strongly peraluminous granites in the southeastern Tibetan Plateau: highly fractionated from crystal mush

ABSTRACT The Triassic Lincang granites make up the largest peraluminous granitic batholith in the Changning-Menglian Paleo-Tethys orogenic belt, southeastern Tibetan Plateau. It is undoubtedly very important to investigate the role of crustal magmatism in the evolution of the Changning-Menglian Paleo-Tethys orogenic belt. The Late Triassic Mengsong pluton is composed of strongly peraluminous two-mica monzogranites, and is located in the southernmost part of the Lincang batholith. Despite all being peraluminous granites, they differ markedly in dimensions, petrographic and geochemical characteristics. Mengsong two-mica monzogranites have low contents of TiO2, FeOtot, MgO, and CaO, but high values of SiO2 and Na2O, compared to biotite monzogranites of the Lincang batholith. The Lincang biotite monzogranites show an enrichment in light rare earth elements (LREE) and weak Eu anomalies (δEu = 0.3–0.8). However, the Mengsong two-mica monzogranites show a clear tetrad effect and contain pronounced negative Eu anomalies (δEu = 0.1–0.3). Geochemical compositions and mineral textures of two-mica monzogranites indicate that they are of highly fractionated type affinity. The two-mica monzogranites are characterized by negative εHf (t) values (−9.3 to −1.7) and εNd (t) values (−9.6 to −7.9), while the biotite monzogranites have lower εHf (t) values (−15.5 to −6.1) and εNd (t) values (−14.5 to −11.5). Isotopic compositions suggest that they were cogenetic and derived from partial melting of the lower crust. Our petrogenetic and zircon U–Pb age data support models that relate the Lincang and Mengsong granites to post-collisional magmatism in the Changning-Menglian Paleo-Tethys orogenic belt during the Middle to Late Triassic. We interpret the Mengsong pluton to represent highly fractionated melts from crystal mush reservoirs and the Lincang granites to be complementary residual.

Duobagou Permian–Triassic granites from the Dunhuang orogenic belt, NW China: implications for the tectonic evolution of the southernmost Central Asian Orogenic Belt

AbstractThe Duobagou Permian–Triassic granites of the Dunhuang orogenic belt are of great importance in understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt. LA-ICP-MS U–Pb zircon ages indicate that Permian–Triassic granitic intrusions from the Duobagou area formed at 276–274 Ma and 246 ± 1 Ma. These granites have high SiO2, Na2O and K2O, but low Al2O3, CaO and MgO contents and belong mainly to the high-K calc-alkaline I-type granite series. Based on whole-rock geochemistry and Sr–Nd and zircon Hf isotopes, the Duobagou Permian–Triassic granites were dominantly derived from the partial melting of lower continental crust formed during late Palaeoproterozoic to Mesoproterozoic times in a post-collisional extensional setting. Permian granites with zircon ϵHf(t) values of −5.4 to +3.1 and Hf model ages of TDM2 = 1.14–1.70 Ga indicate the involvement of a mantle component in their petrogenesis. Triassic granites with higher zircon ϵHf(t) values (+0.5 to +3.8) and TDM2 = 1.08–1.31 Ga suggest more juvenile sources caused by a greater contribution of mantle-derived melts, indicating a significant crustal growth. Regional extension from lithospheric delamination and heating from asthenospheric upwelling were proposed to have triggered the partial melting of lower crust, resulting in the generation of the Permian–Triassic magmatism. This may have been the mechanism for the significant crustal growth during Permian and Triassic times in the southernmost Central Asian Orogenic Belt.

Petrogenesis and tectonic implications of the Maladob ring complex in the South Eastern Desert, Egypt: new insights from mineral chemistry and whole-rock geochemistry

Alkaline ring complexes are widely distributed in the south Eastern Desert (SED) of Egypt, but their petrogenesis and geodynamic settings are a matter of many discussions. In this contribution, mineral chemistry, and whole-rock geochemical data of the Maladob intrusive rocks were used to investigate their petrogenesis. The Maladob ring complex is a small alkaline intrusion in the SED of Egypt, composed of oversaturated syenite, quartz syenites and peralkaline granite. The rocks consist of K-feldspar (Or94–97) and albite (An0-1), alkali amphiboles (arfvedsonite and ferro-katophorite), sodic pyroxene (aegirine), with accessory aenigmatite, zircon, apatite, ilmenite, magnetite and titanite. Geochemically, the rocks are peralkaline, show distinctive geochemical characteristics such as low contents of CaO, MgO, Sr and high contents of alkalis, Rb, Nb, Y, and REE, typical of A-type granites. The rocks are weakly to moderately fractionated with a general enrichment in LREE [(La/Sm)N = 2.96–3.64]. Typically, the Maladob rocks are classified as within plate granites and exhibit A1 subtype characteristics. We suggest that the Maladob rocks were formed from a similar OIB-like mafic magma by fractional crystallization. The fractionation of feldspars and mafic minerals play a significant role during magma evolution. During the Mesozoic intraplate magmatism, the upwelling of asthenosphere causes underplating of lithospheric OIB-like magma which was subjected to prolonged fractional crystallization to produce the Maladob syenites and peralkaline granite.