Partial Melting Of Juvenile Crust Research Articles

Eocene rhyolites in the Shanglaxiu-Xialaxiu area of north-eastern Qiangtang Block, Tibet: Partial melting of juvenile crust?

Many granitoids and their eruptive equivalents are substantially derived from the uppermost proportion of continental crust, and so their petrogenesis and source are significant in understanding the evolution and characteristics of continental crust. The Eocene felsic rocks in the Qiangtang block (central Tibet) commonly have enriched Nd isotope compositions and have been considered to be related to the recycling of ancient continental crustal material. In this paper we report new data on early Eocene felsic rocks with positive εNd(t) values in the northeastern Qiangtang block, which can be used to help understand Cenozoic evolution and characteristics of continental crust in the Qiangtang block. These rocks consist of rhyolites and occur in the Shanglaxiu and Xialaxiu areas in the north−eastern Qiangtang block. Zircon U−Pb dating yields ages of 47.8 ± 0.6 and 44.2 ± 0.7 Ma for the Shanglaxiu and Xialaxiu rhyolites, respectively, and so they represent the earliest record of Eocene magmatism in the Qiangtang block. The Shanglaxiu−Xialaxiu rhyolites have relatively low calculated zircon saturation temperatures (719–819 °C), along with high SiO2 (74.8–83.3 wt%) and K2O (5.1–7.8 wt%) contents, and low MgO (0.2–0.3 wt%), Cr and Ni contents. They are enriched in light rare earth elements (LREEs), Rb, K, Th, Zr, and Hf, and are depleted in Nb, Ta, Ti, Sr, and P, with moderately negative Eu anomalies. The Shanglaxiu rhyolites have positive whole−rock εNd(t) (0.3–0.4) and zircon εHf(t) (1.8–10.4), and variable zircon δ18O (6.3–7.3‰). The Xialaxiu rhyolites have slightly different whole−rock εNd(t) (−1.5 to −1.6), but similar zircon εHf(t) (2.2–7.7) and δ18O (6.7–7.2‰). We propose that the Shanglaxiu−Xialaxiu rhyolites were most probably generated by partial melting of the pre−existing juvenile crust with the addition of minor sedimentary material. This partial melting was possibly triggered by the south−dipping continental subduction of the Songpan−Ganzi block. The pre−existing juvenile crust was probably formed during the subduction of Paleo−Tethyan oceanic slab in Paleozoic−Early Mesozoic.

Late Cretaceous high Sr/Y rocks and local crustal thickening in the eastern Gangdese batholith, southern Tibet

The voluminous Late Cretaceous calc-alkaline magmatic rocks in the Gangdese batholith provide an important record on the mechanisms of the thickening and growth of the continental crust in a continental arc environment. Here, we report data from zircon UPb geochronology and geochemical (whole-rock element and SrNd isotopes and zircon Hf isotope) analyses for the granodiorite and granite from Langxian to Lilong in the eastern Gangdese batholith. These data show that 1) the intermediate-felsic rocks formed at 82–70 Ma and are Na-rich peraluminous rocks. 2) The granodiorite commonly developed magmatic epidote with allanite-rich cores. 3) They are characterized by high Sr/Y and (La/Yb)N ratios, exhibiting an adakitic affinity. 4) Most of the samples have positive εHf(t) (+5.2 − +12.4) and bulk εNd(t) (+1.9 − +2.8) values, indicating that they were derived from partial melting of juvenile crust. 5) The crustal thickness in the eastern part of the Gangdese batholith locally thickened to ∼67 km. The data presented in this paper demonstrates that the Late cretaceous granodiorite and granite were generated by partial melting of the juvenile thickened lower crust (∼67 km) with deep seated emplacement, and then they experienced rapid exhumation. The magmatic flare-up (90–75 Ma) and ocean ridge subduction in the eastern Gangdese batholith are responsible for the magma inflation and local crustal thickening.

Late Neoarchean plume-induced crust-mantle interaction and crustal reworking: Evidence from the Jianping Complex, North China Craton

The Archean basement of the North China Craton (NCC) is predominantly composed of 2.6–2.5 Ga lithological assemblages, while the tectonic regime that governed the formation and evolution of these Neoarchean basement rocks has been a controversial issue. Based on field relationships, petrographical features, whole-rock chemistry and zircon Hf isotope data, diverse late Neoarchean granitoids exposed in the Jianping Complex of the NCC are categorized into TTGs, sanukitoids and potassic granites. Their petrogenesis are associated with partial melting of metabasalts, melting of metasomatized mantle, and melting of crustal lithologies including tonalite and metasediments, respectively. Zircon U-Pb results suggest that different types of Neoarchean granitoids in the Jianping Complex are nearly coeval with crystallization ages of ∼ 2.54–2.52 Ga and metamorphic ages of ∼ 2.49–2.48 Ga. New Hf isotopic data reveal that these granitoids show positive εHf(t) values, with the tonalitic gneisses of εHf(t) values closer to the depleted mantle than those of granodioritic gneiss, sanukitoids and potassic granite. This result reflects potentially more enriched source for other granitoids than the tonalitic gneisses, which is linked with increasing level of crustal-mantle interaction and crustal reworking the late Neoarchean. Integrating other evidence from geochemical, structural and metamorphic studies, it is inferred that the plume activities play an important role in the emplacement of late Neoarchean granitoids in the Jianping Complex, with TTGs formed through the partial melting of juvenile crust induced by plume activity, sanukitoids produced from melting of metasomatized mantle resulted from sagduction process, and potassic granites formed by reworking of earlier crustal lithology caused by sustained underplating of mantle material. Overall, we interpreted that the late Neoarchean crustal growth and crustal-mantle interaction in the Jianping Complex can be attributed to upwelling of mantle plume and potential sagduction process in a vertical tectonic regime.

Geochronology and Geochemistry of Early Cretaceous A‐type Granites in Central–Eastern Inner Mongolia, China: Implications for Late Mesozoic Tectonic Evolution of the Southern Great Xing'an Range

AbstractThe southern Great Xing'an Range is the most critical Sn‐polymetallic metallogenic belt in northeast China. However, the tectonic setting of the Early Cretaceous magmatic‐metallogenic “flare‐up” event remains uncertain. This paper presents an integrated study on the occurrence, petrology, zircon U‐Pb ages, whole‐rock geochemistry, and in situ zircon Hf isotopes for Wenduerchagan granites of Xi Ujimqin Banner, central‐eastern Inner Mongolia. These granites consist primarily of granite porphyry (with ages of 137 ± 1 Ma and 138 ± 1 Ma) and (porphyritic) alkali feldspar granite (with an age of 141 ± 2 Ma), corresponding to the early Early Cretaceous. They are A‐type granites characterized by high silicon, alkali, and TFeO/MgO contents while being depleted of Ba, Nb, Ta, Sr, P, and Ti. They show right‐dipping trend rare‐earth element distribution characteristics with negative Eu anomalies (Eu/Eu* = 0.01–0.20) and weak heavy rare‐earth element fractionation ((Gd/Yb)N = 0.77–2.30). They demonstrate homogeneous zircon Hf isotopic compositions (positive εHf(t) values from +5.3 to +7.1 and young two‐stage Hf model ages of 851–742 Ma) and high zircon saturation temperatures (av. 810°C). These geochemical characteristics indicate that Wenduerchagan granites originated from the partial melting of juvenile crust under high‐temperature and low‐pressure conditions. Wenduerchagan granites most likely formed in a post‐collisional compression‐extension transition regime caused by the closure of the Mongol–Okhotsk Ocean, when combined with regional geology. Such a transition regime can probably be attributed to the upwelling of the asthenospheric mantle caused by the break‐off of a subducted Mongol–Okhotsk oceanic slab. Upwelling asthenospheric mantle provided sufficient energy and favorable tectonic conditions for magmatism and mineralization of the Early Cretaceous.

The mechanism responsible for the geochemical diversity of the Putuoshan volcanic-plutonic complex and its implications for the late Mesozoic crustal evolution of the Cathaysia Block, Southeast China

Essential mechanisms for the geochemical diversity observed in a silicic igneous rock association, which is of paramount importance for understanding the differentiation and growth of continental crust, remain highly controversial. Here the issues are explored by comparing the Late Cretaceous intrusions (minor-phase miarolitic alkali-feldspar granite and main-phase alkali-feldspar granite) and coeval volcanic rocks (rhyolite and rhyodacite) within the Putuoshan (PTS) volcanic-plutonic complex in the coastal area of southeast (SE) China. Zircon UPb ages indicate that the rhyolite, rhyodacite, minor-phase granite and the previously studied main-phase granite are nearly contemporaneous (ca. 98–94 Ma), but the coexisting dacites are obviously earlier at ca. 160 Ma. The dacites show adakitic geochemical characteristics and extremely enriched NdHf isotopic compositions with Paleoproterozoic model ages, suggesting their derivation from remelting of Paleoproterozoic basement rocks in a thickened lower crust. In contrast, the coexisting Late Cretaceous volcanic and intrusive rocks have comparable and relatively depleted NdHf isotopic compositions. Our data, combined with previous studies suggest that parental magmas of these Late Cretaceous magmatic rocks were derived from partial melting of juvenile crust with varying degrees of involvement of mantle-derived melts, and the observed compositional variation are mainly controlled by crystal-melt segregation and rejuvenation of crystal mush facilitated by recharge of hot mafic magmas in the shallow magma reservoir. The Jurassic and Late Cretaceous magmatic rocks within the PTS complex were generated in an advancing subduction compressive regime and a retreating subduction extensional regime of the paleo-Pacific plate, respectively. The gradually increasing tectonic extension during the Cretaceous due to slab rollback and the induced underplating of depleted mantle-derived magmas not only gave rise to the continental rejuvenation of SE China but also are the main driving mechanism of this Late Cretaceous long-lived transcrustal magma system.

Melt-fluid interaction in the formation of peralkaline granite: Evidence from the Baiyinwula intrusion, Inner Mongolia, China

The genesis of peralkaline granites (with molar (Na + K)/Al > 1) remains a conundrum, particularly their emplacement accompanied by contemporaneous calc-alkaline rhyolites rather than peralkaline rocks. In this study we present a petrogenetic comparison of calc-alkaline rhyolites (296–293 Ma) and subsequent peralkaline granites (291 Ma) in Baiyinwula, Inner Mongolia of north China. The calc-alkaline rhyolites display a narrow variation of GdN/YbN (1.0–1.4) with increasing Zr/Sm (29–79) and positive correlation of εNd(t) (+ 1.5 to +5.1) versus 1/Nd (0.02–0.08). Accordingly, their formation is interpreted to have involved partial melting of juvenile crust, followed by feldspar-dominant fractionation and crustal contamination. The peralkaline granites exhibit roughly constant variations of MgO (0.06–0.24 wt%), Na2O (4.05–4.89 wt%), K2O (3.77–4.61 wt%) and increasing peralkalinity (0.97–1.12) and Fe-index (0.90–0.97) with increasing Zr (350–1022 μg/g) and Zr/Sm, precluding their genesis from calc- or per-alkaline parental magmas through high degree of fractionation. Interestingly, the mixing hyperbola (R2 = 0.85) of GdN/YbN (0.5–1.2) and Zr/Sm (50–240) for peralkaline granites indicate interaction between magmas and alkali-rich fluids, but is distinctly different from those derived by partial melting of metasomatized sources which exhibit a trend of partial melting. Simulation based on two-component (calc-alkaline rhyolitic melts and alkali-rich fluids) mixing yields highly consistent inference. Both magmatic albite in peralkaline granites and dissolved morphology of zircons from calc-alkaline rhyolites are indicators for onset of melt and fluid interaction. Combining the modelling results with positive correlation between εNd(t) (+ 5.6 to +6.6) and Zr/Sm, we infer that the potential end-member source of alkali-rich fluids is from mantle alkali-magmas or degassing. Considering the petrogenesis of peralkaline granites and their occurrences in the Uliastai Continental Margin (UCM) distributed along the major strike-slip fault, and temporally marking the final stage of the Late Palaeozoic magmatism, we propose a model for the magmatic transition from calc- to peralkaline magmas in an extensional setting during the culmination of the magmatic event. Subsequently, the calcalkaline melts began to wane, corresponding to the gradual weakening of the geothermal gradient in the lowermost crust. In contrast, the activities of mantle degassed fluids became more intense and migrated through the major fault, metasomatizing crustal rocks or injecting into and interacting with the pre-existing calc-alkaline magmas in the roof of the magma chamber.

The role of long-lived arc volcanism in the formation of the VMS deposits: A case study of the volcanic-sedimentary sequence of Kangbutiebao formation associated with VMS deposits, Altai Mountains

Questions about the role of volcanism in the formation of VMS deposits and the origin of metals in pre-ore volcanic rocks in accretionary orogen generally puzzle scholars. The volcanic-sedimentary sequence of Lower Devonian Kangbutiebao Formation (KBTB Fm) associated with VMS deposits in the Altai Mountains is studied to attempt to solve these issues. The KBTB Fm is predominantly composed of felsic volcanic rocks with subordinate sedimentary rocks. Zircon U-Pb dating of rhyolite and rhyodacite suggests that felsic volcanism occurred in the Early Devonian (400.1 ± 0.8 Ma to 395.3 ± 0.9 Ma). Partial melting of juvenile crust under the environment of lower temperature (662 to 728 °C) and pressure (<∼15 km) resulted in volcanic rocks with arc-like trace elements and depleted Hf isotopes. Both compositional heterogeneity of source rocks and contrasting melting temperatures are responsible for the variety of petrochemistry. Sedimentary rocks exhibit low CIA values (51–66) and varied ICV values (0.76–1.78), implying that the provenance was affected by weak weathering. As suggested by detrital zircon geochronology, detrital materials from sedimentary rocks are dominated by pyroclastic materials that erupted from the volcano, together with minor eroded detrital materials from adjacent intrusions. It is proposed that the KBTB Fm was formed under continental rifted arc or back-arc condition, and local extensional setting was induced by slab-rollback of the Paleo-Asian Ocean, which produced bimodal volcanic rocks and VMS deposits. Large volumes of sequences with low metallic abundance were produced and protracted hydrothermal circulation was sustained under the condition of long-lived arc volcanism, leading to most VMS deposits with low to moderate grade, and rarely, high grade. All these parameters are grouped together (i.e., abundant but low contents of metals, long-lived heat source, and syn-volcanic faults), leading to most VMS deposits hosted by the KBTB Fm in the southern Chinese Altai.

Subduction-related Late Triassic Luerma porphyry copper deposit, western Gangdese, Tibet, China: Evidence from geology, geochemistry, and geochronology

Due to the difference of lithospheric structure between the eastern and western segment of Gangdese metallogenic belt, it is generally believed that porphyry copper deposits are mainly developed in the east. However, the newly discovered of Luerma porphyry copper deposit in the western segment of Gangdese provides an excellent research object for us to supplement this understanding. The molybdenite re-Os isotope dating of two samples from quartz veinlet ores (ca. 213.1 ± 5.2 Ma and ca. 212.0 ± 3.7 Ma), combined with the LA-ICP-MS zircon U-Pb dating of three ore-bearing quartz monzonite porphyry samples (ca. 212.7 ± 0.9 Ma, 213.1 ± 1.1 Ma, and ca. 211.8 ± 1.8 Ma), indicates a new generation of Late Triassic Luerma copper porphyry-type mineralization events in the western Gangdese magmatic belt. The Luerma porphyry is a strongly peraluminous shoshonite series rock, is rich in silicon total alkalis, potassium and aluminum, has low contents of calcium, magnesium, and iron. It is enriched in light rare earth elements and large ion lithophile elements, relatively depleted in heavy rare earth elements and high field strength elements, and has negative Eu anomalies and no Ce anomalies. The porphyry exhibits relatively low (87Sr/86Sr)i values (0.705617 on average), high εNd(t) values (0.63 on average), high εHf(t) values (11.98 on average), and young two-stage Hf model ages. The Luerma ore-related porphyries are most likely generated dominantly from partial melting of juvenile crust that formed by metasomatized mantle during the subduction of the NeoTethys Ocean to the Lhasa terrane and underwent crystal fractionation during the process. The deposit is currently the oldest and westernmost porphyry deposit in the Gangdese porphyry copper belt, providing evidence that porphyry copper mineralization in this belt began in at least the Late Triassic.

Permian ridge subduction-related magmatism in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Permian large-scale transcurrent tectonics and syn -kinematic ultramafic–felsic magmatism are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval Ni Cu mineralized mafic-ultramafic intrusions and felsic intrusions, and the geodynamics of the region remains equivocal. To assess these topics and improve our understanding of this geologically complex and economically significant region, zircon Hf isotope and bulk-rock geochemical data from Permian granitoids along the Kanggur–Huangshan Shear Zone (KHSZ) in the Eastern Tianshan are determined and interpreted in the context of previous geochronological–geochemical data. The Kungurian Longdong (ca. 278 Ma) and Hongshi (ca. 273 Ma) granitoids are high-K, calc-alkaline, metaluminous to weakly peraluminous I-type granitoids, whereas the Capitanian Shuangchagoubei (ca. 262 Ma) granitoids are calc-alkaline, peraluminous, adakitic I-type granitoids. All of the granitoid samples are enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, U, and K), and depleted in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, indicating that subduction-related magmatism was still active in the late Permian. Taking into account the regional geology, and the presence of Permian high-temperature metamorphism, adakites, I-type granites, Alaskan-type Ni Cu sulfide and orogenic Au mineralization, and EW-trending N-MORB type gabbro and mélange along the KHSZ, we suggest that subduction of a spreading center during the Permian generated a slab window, allowing the upwelling of asthenospheric mantle. This permitted the generation and migration of mantle-derived magmas to shallower crustal levels along translithospheric faults (e.g., Kanggur and Yamansu faults), forming the mafic–ultramafic intrusions and associated Ni Cu sulfide deposits. The continuous migration of magma provided the necessary heat to induce partial melting of juvenile crust, generating the Permian high-K to calc-alkaline I-type granitoids. Ridge subduction-induced juvenile crustal reworking likely played a central role in the formation of the granitoids during the late Paleozoic accretionary orogenesis of the southern Altaids. • Subduction-related magmatism was active in the Eastern Tianshan in the late Permian. • Crustal growth and reworking processes must have occurred simultaneously in the Eastern Tianshan during the Permian. • Ridge subduction-induced juvenile crustal reworking likely played an important role in the formation of the granitoids in the southern Altaids.

Early Miocene (21–23 Ma) high Sr/Y volcanic field in the Gangdese batholith, southern Tibet

A detailed petrographic, geochemical and geochronological study reveals a suite of Miocene (~21–23 Ma) high Sr/Y and La/Yb volcanic rocks of dacite compositions and adjacent Late Cretaceous tonalite and granodiorite (~78–80 Ma) in the Qiewa of the southern Gangdese batholith. The analyses results show that (1) the dacites are characterized by Na-rich, peraluminous, high Sr/Y and La/Yb ratios, showing adakitic affinity, the granitic wall-rocks (tonalite and granodiorite) and enclave have the Na-rich and metaluminous features; and (2) the dacites have relatively radiogenic Sr ( 86 Sr/ 87 Sr(t) = 0.7049–0.7061), unradiogenic Nd (ε Nd (t) = −3.8 - +0.1), and positive Hf (ε Hf (t) = +3.0 − +9.5) isotope compositions, while its associated plutonic rocks have relatively lower Sr ( 86 Sr/ 87 Sr(t) = 0.7035–0.7039), but higher Nd (ε Nd (t) = +3.3 − +4.9) and Hf (ε Hf (t) = +11.1 − +14.7) isotope ratios. Data presented in this study demonstrates that Early Miocene dacites are generated by partial melting of thickened (~70 km) juvenile lower crust, whereas the Late Cretaceous granitic wall-rocks (tonalite and granodiorite) and enclave are derived from partial melting of juvenile crust with a normal crustal thickness (~40–44 km). Deposition of ~23 Ma dacitic volcanic rocks directly upon the ~80 Ma plutonic rocks suggests that the Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma, possibly triggered by the delamination of thickened continental lithosphere. • The Qiewa dacites and the underlying plutonic rocks are formed at ~21–23 Ma and ~ 78–80 Ma, respectively. • The high Sr/Y dacites are derived from partial melting of a thickened (~70 km) enriched lower crust. • The plutonic rocks result from partial melting of a normal thickness (~40–44 km) of juvenile curst. • The Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma.

Zircon U–Pb geochronology and Sr–Nd–Hf–O isotopic constraints on the relationship between Mo and Pb–Zn mineralization in the Haisugou pluton in the southern Great Xing’an Range, northeast China

The Haisugou pluton and its contact zones with Middle Permian strata contain Cu–Mo and Pb–Zn ore deposits, providing an excellent example to study the relationship between Pb–Zn–Ag and Mo mineralization in the southern Great Xing’an Range (SGXR). Here we present U–Pb zircon ages and Sr–Nd–Hf–O isotope data for biotite monzogranite in the Xiaojingzi Pb–Zn deposit and Mo-bearing biotite monzogranite in the Xiaojingzi Cu–Mo deposit. These rocks yield zircon U–Pb ages of 136.2 ± 1.1 and 135.5 ± 1.3 Ma, respectively. The biotite monzogranites are enriched in SiO2 (average = 68.9 wt%) and Na2O + K2O (average = 7.7 wt%), contain 14.3–15.1 wt% Al2O3 (average = 14.7 wt%), and have A/CNK values of 0.97–1.05, indicating the biotite monzogranites are high-K calc-alkaline granites, and peraluminous. The biotite monzogranites are also characterized by negative Eu anomalies (Eu/Eu* = 0.78–0.45), enrichments in Rb, Th, U, Nd, and Hf, and depletions in Ba, Sr, P, Ti, and Nb. The biotite monzogranites have (87Sr/86Sr)i ratios of 0.70519–0.70457, εNd(t) values of + 0.37 to –0.39, and TDM2 ages of 978–915 Ma. They also have zircon εHf(t) values of 3.6–9.1, TDM2 model ages of 959–608 Ma, and δ18O values of 6.68‰–7.42‰. The Mo and Pb–Zn mineralization around the Haisugou pluton were genetically related to the granitic pluton and occurred simultaneously, forming a porphyry system. Geochemical and Sr–Nd–Hf–O isotope data indicate the Haisugou pluton was probably formed by partial melting of juvenile crust and mixing with mantle-derived components. In the Early Cretaceous, the SGXR was in an extensional setting, which was related to asthenospheric upwelling after the closure of the Mongol–Okhotsk Ocean.

Magmatic and post-magmatic evolution of post-collisional rare-metal bearing granite: The Neoproterozoic Homrit Akarem Granitic Intrusion, south Eastern Desert of Egypt, Arabian-Nubian Shield

The Homrit Akarem granitic intrusion (HAGI) outcrops near the western edge of the south Eastern Desert basement exposure in Egypt. It is a composite of two cogenetic intrusive bodies: an early albite granite phase shallowly emplaced at the apex of a magmatic cupola, and a later subjacent pink granite phase with a marginal zone of muscovite granite and better preservation of magmatic features. Mineral chemistry of primary biotite and garnet, together with whole-rock chemistry, identify the HAGI as a highly fractionated A-type peraluminous intrusion. The chemistry of F-dominant, Li-bearing, Fe3+-rich primary magmatic mica in the pink granite resembles that typically found in highly evolved Nb-Y-F pegmatites. The HAGI is the evolved product of a primary magma generated by partial melting of juvenile crust of the Arabian-Nubian Shield (ANS), emplaced along a regional strike-slip fault system that promoted its ascent. The main emplacement mechanism and evolutionary sequence of the HAGI was magmatic, although secondary minerals and textures resulting from hydrothermal fluid interactions are observed, especially at its margins. Primary columbite-(Mn) crystallized from melt and was partly replaced by secondary fluorcalciomicrolite. The high fluorine content of magmatic fluids exsolved from the intrusion is indicated by quartz-fluorite veins, greisenization, albitization, and F-bearing secondary oxide minerals. The magmatic derivation of this fluid is demonstrated by the F-dominant primary mica, a siderophyllite-polylithionite solid solution commonly known as zinnwaldite. The chemistry of zinnwaldite constrains the F/OH activity ratio and oxygen fugacity of its parental melt and thereby resolves the ambiguity between pressure and the effects of F in controlling the normative quartz content of rare-metal granites. The HAGI is less mineralized than the post-collisional rare-metal granites found further east in the south Eastern Desert, replicating a trend observed previously in the central Eastern Desert and suggesting that east-west zoning in rare metal enrichment is a persistent feature across latitudes at the western edge of the ANS.

Mesoproterozoic (∼1.4 Ga) magmatism in the Liaoyuan Accretionary Belt, NE China: New implications for tectonic affinity and crustal evolution of microcontinents along the southern Central Asian Orogenic Belt

The Mesoproterozoic granitic magmatism has been identified in the middle and western part of the southern margin of Central Asian Orogenic Belt (CAOB), but rarely reported from the eastern part. In this study, Mesoproterozoic granitic rocks were identified in the Liaoyuan Accretionary Belt (LAB), an area located in the northeast China, within the eastern part of the southern CAOB. Information obtained from these samples is likely to elucidate the current understanding of the Precambrian evolution in this area. The five granitic rocks yielded zircon U–Pb weighted mean ages between 1321 ± 43 Ma and 1388 ± 31 Ma. These samples are characterized by high SiO2 (69.30–77.51 wt%), MgO (1.96–2.34 wt%), Fe2O3 (3.48–3.82 wt%), and Cr (40–80 ppm) contents, moderate Al2O3 contents (11.23–15.80 wt%), and high Mg# values (45–56). Furthermore, they are rich in light rare earth elements (LREEs), Zr, and Hf, but depleted in heavy rare earth elements (HREEs), Nb, Ta, and Ti. Moreover, they also exhibit significant negative Eu anomalies (Eu/Eu* = 0.25–0.84). In addition, the samples have high Zr + Ce + Y + Nb contents (300–497 ppm) indicating A-type granite affinities. The zircons depicted positive εHf(t) values (3.31 to 11.1, average value of 7.5) with two stage model ages (TDM2) varying from 1480 Ma to 2128 Ma. Based on the above data, it was concluded that the Mesoproterozoic granitic rocks in this area were derived from partial melting of juvenile crust with addition of mantle-derived materials. The granites were emplaced in an extensional setting caused by roll back or break off of subducted oceanic slab. Juvenile ∼ 1.4 Ga granitic rocks, like these in the LAB, were also preserved in other microcontinents along the southern CAOB. Their geochemical characteristics are comparable to rocks derived from contemporaneous igneous activity in Laurentia and Baltica, but not those in the North China, Tarim or the Siberian Cratons. Based on the present and previous studies, the microcontinents that preserved the juvenile ∼ 1.4 Ga rocks might belong to a continental terrane in the periphery of the Columbia supercontinent and show tectonic affinity with the combined Laurentia–Baltica landmass.

Permo‐Carboniferous crustal accretion in the northern Beishan Orogen, Inner Mongolia: Constraints from Sr–Nd–Hf isotopes and zircon U–Pb geochronology of diorites and granitoids

To research the Late Palaeozoic accretionary orogeny and vertical crustal accretion in the northern Beishan Orogen, the zircon U–Pb age, zircon Hf, and whole‐rock Sr–Nd isotopic composition from the Permo‐Carboniferous granitoids and diorites at Hazhu were recorded. Zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating shows that the diorite (323.9 ± 1.4 Ma) and quartz monzodiorite (322.6 ± 1.4 Ma) were emplaced during the Early Carboniferous; the tonalite (317.8 ± 1.4 Ma), granodiorite (318.4 ± 1.3 Ma), and monzogranite (316.0 ± 1.3 Ma) were emplaced during the Late Carboniferous; and the monzogranite (257.8 ± 1.1 Ma) was emplaced during the Late Permian. Compared with the Permian granitoids in this area, the mafic microgranular enclave contents in the Carboniferous granitoids are lower, with relatively high (87Sr/86Sr)i values (0.700812–0.706410), low εNd(T) values (0.95–3.72), and old Nd isotopic model age TDM (730–1,228 Ma). However, these Carboniferous granitoids still exhibit partial melting of juvenile crust. The εHf(T) values of Carboniferous granitoids are high (4.3–14.0) but lower than those of the Permian monzogranite (8.1–12.3). The Hf model age (392–781 Ma) is older than that of the Permian monzogranite (408–585 Ma). Recent regional geological data suggested that the Permo‐Carboniferous granitoids at Hazhu are all of juvenile crustal origin, exhibiting considerable mantle source components. Carboniferous granitoids and diorites have obvious arc magmatic properties, which are formed of juvenile crust during subduction accretion orogeny; Permian granitoids show the addition of more mantle‐derived components, which are formed by mantle‐derived magma underplating in an extensional environment and demonstrate the vertical growth of the crust. Our results provide new evidence for the time limit of the Late Palaeozoic accretion and orogeny in the northern Beishan Orogen and even for the vertical accretion of Phanerozoic crust along the southern margin of the Central Asian Orogenic Belt.

Early Permian back‐arc extension in the southeastern Central Asian Orogenic Belt: Evidence from the gabbro–granite complex, Xilinhot, Inner Mongolia, China

Gabbro–granite complexes provide vital constraints on magmatism mechanisms, crust–mantle interaction processes, and regional tectonics. The petrogenesis and tectonic setting of Early Permian igneous rocks in the southeastern Central Asian Orogenic Belt have long been controversial. In this study, the Early Permian Wulajitu gabbro–granite complex in Xilinhot, Inner Mongolia was selected as a research object and an integrated study was conducted using field occurrence, petrology, zircon U–Pb ages, whole‐rock geochemistry, and in situ zircon Hf isotopes. The Wulajitu gabbro–granite complex consists of gabbro and syenogranite with crystallization ages of 277 ± 1 Ma and 274 ± 2 Ma, respectively, belonging to the Early Permian. Their SiO2 contents vary in the range from 43.07–51.60 wt% and 70.30–75.52 wt%, respectively, and exhibit a gap of 51.60–70.30 wt%, displaying a typical bimodal geochemical distribution. The gabbros are low‐K tholeiitic–calc‐alkaline series rocks that are enriched in large‐ion lithophile elements (e.g., Ba, K, and Sr), depleted in high‐field‐strength elements (e.g., Nb and Ta), and have positive εHf (t) values from +9.3 to +13.2, indicating that they likely originated from partial melting of depleted mantle modified by subduction slab fluids. The syenogranites are aluminous A‐type granites characterized by high SiO2 and K2O + Na2O; low Al2O3, CaO, and MgO; depletion in Ba, Nb, Ta, Sr, P, and Ti; enrichment in Rb, Th, U, K, Zr, and Hf; as well as a seagull‐type rare‐earth element distribution pattern with strongly negative Eu anomalies (Eu/Eu* = 0.30–0.52). They show homogeneous zircon Hf isotopic compositions (positive εHf (t) values from +11.4 to +13.7). All these geochemical signatures suggest that the syenogranites were generated from partial melting of juvenile crust under high temperature and low pressure. The Wulajitu gabbro–granite complex most likely formed in a back‐arc spreading basin. Such an extensional regime was closely related to asthenospheric mantle upwelling caused by the rollback of the subducted Palaeo‐Asian Ocean slab in the Early Permian.

Petrogenesis and tectonic setting of the Early Permian gabbro–granite complex in the southeastern Central Asian Orogenic Belt, central Inner Mongolia, China

Gabbro–granite complexes formed in active continental margins provide critical constraints on magmatic processes, crust–mantle interactions, and tectonic evolution. The petrogenesis and tectonic setting of the Early Permian igneous rocks in the southeastern Central Asian Orogenic Belt have been a subject of dispute for a long time. This contribution presents an integrated study on the field occurrence, petrology, petrography, zircon U–Pb ages, in situ zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Early Permian Yiheshaorong gabbro–granite complex, Xi Ujimqin Banner, central Inner Mongolia. Lithologically, the Yiheshaorong complex consists predominantly of gabbro, pyroxene diorite, and alkali feldspar granite. Results of LA–ICP–MS zircon U–Pb dating suggest that crystallization ages of the gabbro, pyroxene diorite, and alkali feldspar granite are 281–280, 278, and 281–280 Ma, respectively. Silica (SiO2) content varies from 41.26 to 79.64 wt% and exhibits a gap between 56.84 and 75.45 wt%, displaying a good bimodal geochemical distribution. Geochemically, the gabbros and pyroxene diorites are characterized by enrichment of large ion lithophile elements (LILEs; e.g., Ba and K) and depletion of high field strength elements (HFSEs; e.g., Nb, Ti, and P). These rocks show similar homogeneous zircon Hf isotopic (positive εHf (t) values from +7.8 to +10.1) and whole-rock Sr–Nd isotopic (low initial 87Sr/86Sr ratios from 0.7044 to 0.7051, positive εNd (t) values from +3.7 to +4.7) compositions. The gabbros and pyroxene diorites originated from partial melting of depleted asthenospheric mantle metasomatized by subduction slab fluids; and they experienced various degrees of crystallization differentiation during ascension. The alkali feldspar granites are ferroan, alkali-calcic–calc-alkalic series rocks marked by high SiO2, K2O + Na2O, and FeOT/MgO; and low Al2O3, CaO, and MgO contents. Their trace elements are marked by enrichment of LILEs (such as K and Rb); depletion of HFSEs (such as Nb and Ti), Ba, Sr, and P; and strongly negative Eu anomalies (Eu/Eu* = 0.17–0.57), with coincident distribution patterns forming a seagull-type distribution form, showing an A-type granite affinity. The alkali feldspar granites are characterized by positive εHf (t) values (+8.6 to +10.3, young TDM2 ages, 645–758 Ma) and εNd (t) values (+3.5 to +3.8 as well as young TDM2 ages, 667–687 Ma). All these geochemical characteristics indicate that the alkali feldspar granites originated from the partial melting of juvenile crust under high temperature and low pressure condition. The Yiheshaorong complex was most likely formed in a back-arc spreading basin. Such an extensional regime can likely be attributed to the upwelling of asthenospheric mantle caused by the rollback and break-off of a subducted Paleo-Asian Ocean slab in the Early Permian, which induced large-scale extension and magmatism in the study area.

Petrogenesis of early Eocene granites and associated mafic enclaves in the Gangdese batholith, Tibet: Implications for net crustal growth in collision zones

Granitoids and related mafic microgranular enclaves (MMEs) in continental collision zones may provide crucial insights into the formation and evolution of continental crust. In this paper, we report an investigation into the mineralogy, zircon U–Pb geochronology, whole-rock major and trace elements, and Sr–Nd–Hf isotopic geochemistry of granodiorites and hosted MMEs, as well as monzogranites, from the Zhaxiding intrusive complex in the Gangdese batholith, southern Tibet. Zircon U–Pb dating suggests that these rocks were emplaced during the early Eocene (49 to 47 Ma). Field investigation and petrological observations indicate that the MMEs represent globules of a mafic magma that was injected into the host felsic magma. On the basis of depleted zircon Hf (εHf(t) values of +6.5 to +9.2) and whole-rock Sr–Nd (εNd(t) values of +1.6 to +2.6) isotopic compositions, along with their whole-rock compositions, the MMEs are interpreted as having resulted from mixing of mantle-derived magma with juvenile crust-derived melts. The granodiorites and monzogranites are metaluminous and have positive zircon εHf(t) (+6.3 to +8.9 for granodiorites and +1.7 to +10.2 for monzogranites) and whole-rock εNd(t) (+1.6 to +1.9 for granodiorites and +0.5 to +1.1 for monzogranites) values. The granodiorites further exhibit continental crust-like chemical compositions (e.g., enrichment in Rb, K, and Pb and depletion in Nb and Ta) and high Mg# values, suggesting that they originated from partial melting of juvenile crust with the involvement of mantle-derived melts. However, the monzogranites have lower P, Ti, and total rare earth element contents, similar Rb, Ba, and Sr contents, and uniform Eu anomalies compared with the granodiorites, and were likely generated by fractional crystallization of amphibole and Fe–Ti oxides from juvenile crust-derived magma. We propose that underplating of mantle-derived magma likely triggered coeval crustal magmatism and that mixing of the mantle- and crust-derived magmas and subsequent crystallization differentiation formed the intermediate to felsic Zhaxiding intrusive complex in the Gangdese batholith. The continental crust-like bulk compositions of the intermediate to felsic magmatic rocks represent a net addition of juvenile material to continental crust. Our study thus indicates that magma mixing and differentiation play a significant role in the formation of andesitic to dacitic continental crust in collision zones.

Petrogenesis and tectonic setting of granitic plutons in the Guaizihu region, North Alxa Block, China: constraints from whole-rock geochemistry, zircon U–Pb ages and Hf isotope compositions

To better constrain the petrogenesis and tectonic setting of the granitic plutons in the Guaizihu region, NW China, this study presents a new analysis of zircon U–Pb ages, whole-rock geochemistry and Hf isotopes of syenogranites. The syenogranites are medium- to coarse-grained, with moderate to high SiO2 (72.55–74.27 wt%) and Al2O3 (13.02–13.75 wt%) contents and moderate total alkalis (Na2O + K2O, 7.58–7.98 wt%), which indicate that the syenogranites exhibit calc-alkaline and peraluminous characteristics. The low Ce, 10 000 Ga/Al, and A/CNK values and the negative correlation of SiO2 and P2O5 confirm that the syenogranites are of I-type origin. The whole-rock geochemistry demonstrates that the magma was generated by partial melting and experienced fractional crystallisation. Specifically, the correlations of Rb with Ba and Sr and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallisation. In addition, Eu anomalies and La/Sm–La relations show that both fractional crystallisation and partial melting contributed to the magmatic process. The positive εHf(t) values (0.11–9.78); enrichments in Rb, Th and other LILEs; depletions in Nb, Sr and other HFSEs; and the ratios of geochemical elements indicate that the syenogranites were generated from the partial melting of juvenile crust. The granites are inferred to have formed in a post-collisional extensional environment. Additionally, the U–Pb zircon ages (284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma) of syenogranites suggest that final closure of the Paleo-Asian Ocean occurred before 285 Ma. KEY POINTS Granites in the Guaizihu region are dated at 284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma, and the Paleo-Asian Ocean, where the Engger Us Ophiolite Belt is located, closed before 285 Ma. Granites are medium- to coarse-grained and highly fractionated I-type syenogranites. Granites have been derived from magma generated by the partial melting of juvenile crust followed by fractional crystallisation. Granites were formed in an intraplate extensional environment related to a post-collisional event.

Geochemistry and petrogenesis of Jurassic granites in Nyemo area, South Lhasa terrane, Tibet

冈底斯岩基是新特提斯洋北向俯冲和印度-欧亚大陆碰撞的重要岩浆记录，尼木地区位于冈底斯岩基中部。本文报道了尼木地区寄主二长花岗岩和辉长质包体的锆石U-Pb年代学、元素地球化学和锆石Hf同位素。锆石U-Pb定年结果表明，寄主花岗岩（197~190Ma）和包体（195Ma）同期侵位，形成于早侏罗纪。寄主二长花岗岩为弱过铝质高钾钙碱性系列岩石，富集轻稀土和K、U、Sr等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素，锆石ε_Hf（t）值为+13.9~+16.0，其可能来源于初生地壳的部分熔融；辉长质包体为准铝质钙碱性系列岩石，具有与寄主岩相似的稀土和微量元素分配特征，锆石ε_Hf（t）值为+13.8~+16.0，其可能代表了侏罗纪形成的新生下地壳，即寄主岩石的岩浆源区；结合前人研究，本文认为尼木地区的寄主花岗岩和暗色包体可能与新特提斯洋北向俯冲引起的地幔和初生地壳的部分熔融有关。;The Gangdese batholith is an important magmatic record of the northward subduction of the Neo-Tethys Ocean and the Indian-Eurasian collision, and Nyemo is located in the middle of the Gangdese batholith. In this paper, we report the zircon U-Pb geochronology, element geochemistry and zircon Hf isotopes of the host granites and gabbroic enclaves from Nyemo area. Zircon U-Pb dating results show that both the host granites (197~190Ma) and enclaves (195Ma) were formed in the Early Jurassic, and they should be emplaced at the same time. The host rocks are slightly peraluminous and high K calc-alkaline granites, which enriched light rare earth elements such as K, U and Sr, and depleted high field strength elements such as Nb, Ta and Ti. Their zircon ε_Hf(t) values varied within the range of +13.9~+16.0, indicating they may have been derived from partial melting of the juvenile crust. The gabbroic enclaves are metaluminous calc-alkaline rocks with rare earth elements and trace elements distribution characteristics similar to the host rocks, and their zircon ε_Hf(t) values are within +13.8~+16.0, showing that they may represent the juvenile lower crust formed in the Jurassic, which is the magmatic source of the host rocks. Based on previous studies, we suggest that the host granites and enclaves in Nyemo may be related to partial melting of mantle and juvenile crust caused by the northward subduction of the Neo-Tethys Ocean.

Zircon U–Pb ages, geochemistry and Sr–Nd isotopes of the Golshekanan granitoid, Urumieh–Dokhtar magmatic arc, Iran: evidence for partial melting of juvenile crust

AbstractThe Golshekanan granitoid body is situated in the central part of the Urumieh–Dokhtar magmatic arc (UDMA) in central Iran, and includes granite and granodiorite with minor monzonite and diorite. Zircon U–Pb dating yields a late Eocene (Priabonian) crystallization age of 37.6 ± 0.2 Ma. The body is calc-alkaline and metaluminous to weakly peraluminous (A/CNK ≤ 1.10) with SiO2ranging from 61.1 to 71.5 wt% and MgO from 0.8 to 3.3 wt%, with Na2O + K2O of 4.0–8.5 wt%. Primitive mantle-normalized trace-element patterns display enrichments in the large-ion lithophile elements (LILE), such as Rb, Cs, Ba and K, and depletion from the high-field-strength elements (HFSEs), such as Nb, Ti, Ta and P. The rocks are enriched in LREEs relative to HREEs (average (La/Yb)CN= 4.3) and exhibit weak negative Eu anomalies (average Eu/Eu* = 0.75), revealing typical active continental margin arc affinity. The low initial87Sr/86Sr ratios (0.70440–0.70504) and notable positive ϵNd(t) values (+4.0 to +5.2) indicate an origin by partial melting of juvenile rocks in the lower crust, possibly with some involvement of sub-continental lithospheric mantle beneath Central Iran. These processes probably occurred due to the Neo-Tethys oceanic slab retreat and (or) rollback during late Eocene time.