Juvenile Crustal Growth Research Articles

Petrogenesis of early Paleozoic syn-collisional granitoids and enclaves in Western Kunlun, Northwest China: Implications for the growth of continental crust

The Western Kunlun Orogenic Belt (WKOB), along the northwestern margin of the Qinghai–Xizang Plateau, was formed by the collision of Gondwana-derived terranes to the south and the Tarim Block to the north and was closely associated with closure of the Proto-Tethys Ocean during the late Neoproterozoic to early Paleozoic. We present a combined zircon UPb geochronology, whole-rock composition, and Sr–Nd–Hf isotopic study of syn-collisional granitoid plutons and mafic microgranular enclaves (MMEs) in the region. Zircon UPb dating yields ages of 443.8 ± 4.4, 451.9 ± 4.2, 462.9 ± 3.5, and 456 ± 4.2 Ma for the Tongayoupuagezi, Shanjie, and Pishigai plutons and MMEs from the Pishigai pluton, respectively. The granitoids are metaluminous to weakly peraluminous (A/CNK = 1.00–1.17) and belong to the high-K calc-alkaline series. They have (87Sr/86Sr)i ratios of 0.7058–0.7154, εNd(t) values of −8.78 to −0.93, and εHf(t) values of −19.72 to +6.87. The MMEs have variable SiO2 contents (45.7–60.2 wt%) and are more mafic than the host granitoids, but have similar Sr–Nd–Hf isotopic compositions to the host granitoids [(87Sr/86Sr)i = 0.7102–0.7110; εNd(t) = −6.57 to −3.56; εHf(t) = −7.17 to −1.81]. The MMEs are fragments of cumulates formed during the early stages of magma evolution. The granitoids were produced by the partial melting of a mélange source. The new data support the view that the Middle–Late Ordovician syn-collisional granitoids with MMEs distributed along the WKOB represent a magmatic response to terrane collision. This suggests that juvenile crustal growth in older orogenic systems, which occurs by arc addition, also involves some vertical addition during the final stage of orogenic collision. Our study suggests that mélange diaper melting is a key mechanism of crustal growth during the syn-collision stage in continental collision zones, associated with slab breakoff.

Insight into intracontinental deformation and crustal reworking of ancient continents implied by crustal structure imaging of the Yangtze Craton

The crustal imprints from multistage tectonic activities in cratons offer valuable insights into continental evolution. Utilizing seismic data from two densely deployed, nearly perpendicular linear arrays, and a newly developed stepwise joint inversion of depth-domain receiver function and surface wave dispersion, we constructed a detailed crustal layering model for the Yangtze Craton. Our analysis revealed elongated double velocity reversal zones as salient features of the crust, which likely record ancient crustal reworking and juvenile crustal growth associated with Neoproterozoic rift-related magmatic processes. The interlayering of low- and high-velocity structures may contribute to the enduring stability of the Yangtze Craton. Additionally, superimposed layers separated by east-dipping interfaces and abrupt changes in crustal thickness in the boundary belts surrounding the Yangtze Craton document the crust's structural responses to intracontinental deformation during continent assembly.

Prolonged arc accretion during growth of juvenile crust in the Arabian Nubian Shield: Insights from the granitoids of northern Ethiopia

The Arabian Nubian Shield (ANS) is one of the largest juvenile continental crust formed after the Archean. To determine the timing and understand the mechanism of its early crustal growth, we conducted geochronological and geochemical studies on granitoids from northern Ethiopia. A plagiogranite, dated at 929.7 ± 2.2 Ma, represents one of the early granitoids from the ANS. It shows low K2O, TiO2, and REE contents, flat REE pattern and absence of Eu anomaly. Moreover, it exhibits (87Sr/86Sr)i, ƐNd(t) and ƐHf(t) values of 0.70341, +5.0 and +8.4, respectively. Its zircon δ18O value is 4.8‰, lower than the mantle value. These characteristics suggest that it was derived from partial melting of altered oceanic crust. The oxygen fugacity obtained from zircon trace elements is ΔFMQ+1.3, consistent with a subduction setting.Other plutons have much younger ages ranging from 860 to 840 Ma and are mainly medium K, I-type granitoids. These granitoids show low MgO, Ni and Cr contents, low Sr/Y, (La/Yb)N, Nb/Ta and Zr/Sm ratios. They exhibit whole-rock (87Sr/86Sr)i values of 0.70236–0.70304, ƐNd(t) values of 4.2–5.2, ƐHf(t) values of 5.8–7.6 and zircon ƐHf(t) values of 5.5–10.4, slightly lower than coeval depleted mantle. Furthermore, they exhibit fractionated REE patterns, enrichments in LILE and depletion in HFSE. These geochemical features suggest that these granitoids were generated by remelting of arc crust. Compiled data show that the associated basaltic rocks are of arc affinity, which confirms the coeval development of arc magmatism in the southern ANS. Our results suggest that the growth of juvenile crust in the southern ANS started from the early Neoproterozoic and peaked at ca. 880 to 800 Ma, much earlier than those in the northern ANS. The prolonged history accounts for the growth of the vast juvenile crust in the ANS.

Constraints on the Growth and Evolution of Continental Crust in the Northeast Cathaysia Block: Insights from Detrital Zircon U-Pb Age and Hf-O Isotope Analysis in Tingjiang Sediments

Fluvial detrital zircons provide crucial insights into the early Precambrian crustal evolution. In this study, we utilize U-Pb geochronology and Hf-O isotopic compositions of detrital zircons from the Tingjiang River to comprehend the provenance and continental crustal evolution of the northeast Cathaysia Block. Zircon U-Pb ages display seven major populations at 105-177 Ma, 200-262 Ma, 376-520 Ma with a striking peak at ca. 422 Ma, 606-775 Ma, 836-1457 Ma, 1710-2042Ma, and 2300-2661Ma, corresponding to the documented tectonic-thermal events in the South China Block. Episodic magmatism was the response to the formation and breakup of the Columbia and Rodinia supercontinent. The episode of 422 Ma matches with the ages of widely spread early Paleozoic intraplate orogenic events in the South China Block. The isotopic signatures of Hf and O in the detrital zircons indicate that the majority of Precambrian zircons likely originated from the re-melted ancient continental crust, with contributions from juvenile mantle-derived magmas. Phanerozoic zircons, on the other hand, were formed through partial melting of recycled crustal material. Analysis of crustal growth rates, based on the two-stage Hf model ages, suggests a significant presence of juvenile crustal growth during the Paleoproterozoic era. Specifically, it is estimated that approximately 40% and 90% of the present crust within the northeastern Cathaysia Block were formed around 2.3 Ga and 1.6 Ga, respectively.

Spatiotemporal Variation of the Cretaceous‐Eocene Arc Magmatism in Lhasa‐Tengchong Terrane

AbstractIt was recognized that two magmatic belts in the Lhasa‐Tengchong terrane formed due to the Mesozoic‐Cenozoic Tethyan evolution. Still, their spatiotemporal variations of magmatic flare‐ups/lulls are rarely discussed. Here we use the new U‐Pb and Lu‐Hf isotopic data of captured zircons and a comprehensive data set to show that the flare‐up of northern magmatic belt has peak ages of 110 Ma in central and northern Lhasa and 120 Ma in eastern Tengchong, possibly related to the tectonic transition from Meso‐ and Neo‐Tethyan double subduction to Neo‐Tethyan single subduction. For the southern magmatic belt, the flare‐ups at 100–85 Ma and 65–45 Ma in eastern southern Lhasa indicate obvious juvenile crustal growth, while flare‐ups at 75–45 Ma in western southern Lhasa and Tengchong record ancient crustal reworking. Such flare‐up variations in the southern magmatic belt possibly resulted from asynchronous changes in the Neo‐Tethyan slab dip.

Permian Granitic Plutons from the Northern Margin of the North China Craton: Implications for the Tectonic Evolution of the Central Asian Orogenic Belt

As the world’s largest accretionary orogen, the Central Asian Orogenic Belt (CAOB) underwent continuous juvenile crustal growth in the Phanerozoic. The northern margin of the North China Craton (NCC) and its adjacent area form the eastern segment of the CAOB, which is a key area for learning about the geological evolution of the Paleo-Asian Ocean (PAO). In the Permian, the west of the northern margin of the NCC was a post-collision extensional environment, while the east was in a subduction stage. As a connecting area, the Permian evolution of the PAO in the middle of the northern margin of the NCC has not been systematically studied. In order to fill the gap and understand the continuous temporal and spatial evolutionary process of the PAO, this paper focuses on the Permian granitic rocks in the Chifeng area. Zircon U-Pb dating and the geochemical analysis of whole-rock major and trace elements were conducted to build a granite chronological framework, and to discuss the genesis and tectonic background of the granitic rocks, along with tectono-magmatic evolutionary history in the Chifeng area. The respective LA-ICP-MS zircon U-Pb dating results from eight samples are 269 ± 1, 268 ± 3, 260 ± 4, 260 ± 1, 260 ± 1, 255 ± 2, 254 ± 2 and 256 ± 1 Ma, respectively. These results, combined with previous data, revealed that the Permian granitic rocks had undergone three events of magmatism: (1) monzogranitic-syenitic phase (294–284 Ma; Cisuralian); (2) monzogranitic phase (269–260 Ma; Guadalupian) and (3) late monzogranitic-syenitic phase (256–254 Ma; Lopingian). From the Early Permian (294–284 Ma) to the Middle Permian (269–260 Ma), granites with fine-medium-grained locally porphyritic texture and massive structure showed a high-potassium calc-alkaline series formed in a compressional setting, indicating a continuous collision between the Xing’an-Mongolian Orogenic Belt (XMOB) and the NCC. During the Late Permian-Early Triassic (256–248 Ma), granites with massive structure and medium-grained texture in the Chifeng area were magmatism dominated by A- and I-type granites of high-potassium calc-alkaline series, combined with the coeval basic rocks, which constituted a typical “bimodal” rock assemblage. This suggests that the Chifeng area was located in an extensional setting where the subducting slab broke off during the collision between the XMOB and NCC. These granitic plutons from the Permian are believed to have been generated by the subduction-collision of the Paleo-Asian oceanic crust beneath the NCC, according to emplacement time and occurrence location. Our findings provide strong evidence for Permian continuous temporal and spatial tectonic evolution and the characterization of the eventual closure of the PAO in Chifeng area at the northern margin of the NCC.

Petrogenesis of two episodes of Neoarchean TTG gneisses (∼2.72 Ga and 2.56–2.54 Ga) from the Zhongtiao Mountains, North China Craton: Implications for crustal evolution

The petrogenesis of tonalite-trondhjemite-granodiorite (TTG) suites can provide key clues for the formation and evolution of continental crust. Neoarchean TTG gneisses are widely distributed in the Zhongtiao Mountains (ZTM), North China Craton (NCC). In this contribution, to better understand Neoarchean crustal evolution of the NCC, two discrete episodes of TTG gneisses (∼2.72 Ga and ∼2.56–2.54 Ga) from the ZTM are investigated. New results approve that the two episodes of TTG gneisses are all characterized by high SiO2, Na2O contents, (La/Yb)N and Sr/Y values, but low K2O/Na2O ratios. These geochemical features are similar to the Archean TTG suites and Phanerozoic high-SiO2 adakites, suggesting that they were both derived from partial melting of mafic rocks. The characteristics of trace elements (e.g., La/Yb, Sr/Y, Y/Yb, (Ho/Yb)N and Nb/Ta ratios) imply that both of them correspond to MP-TTGs, and garnet-amphibole exist in the residue phase. The ∼2.72 Ga TTGs show high Mg# values (ave. = 51), Cr (4.87–104 ppm) and Ni (6.37–55.7 ppm) contents, and negative Nb, Ta anomalies. Some high Nb/Ta ratios (up to 17.6) imply that rutile might be involved in the residual phase, which should require a higher pressure. Zircons from these rocks yield positive εHf(t) values of +4.0 ∼ +6.8 (ave. = +5.6), with two-stage Hf model ages (TDM2) of ∼2.85–2.70 Ga, indicating a juvenile basaltic crustal source. Hence, we interpret that ∼2.72 Ga TTGs in the ZTM were derived from partial melting of mafic rocks, and more likely related to subduction process. In contrast, the ∼2.56–2.54 Ga TTGs have lower Mg# values (39–54, mostly <45), Cr (6.02–22.8 ppm) and Ni (4.93–15.7 ppm) contents. They exhibit more evolved zircon Hf (average εHf(t) = +3.21) and whole-rock Nd (εNd(t) = +1.87) isotopic compositions, and their TDM2 ages are concentrated at ∼2.85–2.68 Ga. These ∼2.5 Ga TTGs in the ZTM show similar geochemical features with contemporary TTGs in the XQL, DF and EZT of the southern NCC, identical to melts from partial melting of mafic rocks under a thickened lower crust. The ∼2.5 Ga TTGs, A-type granites and mafic rocks in the ZTM jointly indicate a complicated crust-mantle process at different depths related to mantle plume activity during the late Neoarchean. Based on comparison of the spatial-temporal distribution and geochemical features of ∼2.7 Ga and ∼2.5 Ga TTGs in the NCC, we propose that Neoarchean crustal evolution of the ZTM is similar to the whole NCC, which recorded a significant crustal growth event at ∼2.7 Ga, and an important crustal reworking event associated with some juvenile crustal growth at ∼2.5 Ga.

Geochemistry and zircon U-Pb-Hf isotopes of Paleozoic granitoids along the Solonker suture zone in Inner Mongolia, China: Constraints on bidirectional subduction and closure timing of the Paleo-Asian Ocean

The closure of the Paleo-Asian Ocean resulted in formation of the Central Asian Orogenic Belt (CAOB) accompanied by considerable Phanerozoic juvenile crustal growth. As an international frontier topic, whether the Solonker suture zone is the site of final closure of the Paleo-Asian Ocean and records the terminal evolution of the CAOB in Inner Mongolia is still in controversial. This study presents the geochemistry and zircon U-Pb-Hf isotopes of granitoids along the Solonker suture zone to constrain the subduction and closure processes of the Paleo-Asian Ocean. Granitoids from the Xilinhot area in the northern accretionary zone comprise diorites and granodiorites with LA-ICPMS zircon U-Pb ages of 316 ± 2 Ma, 302 ± 2 Ma, 285 ± 2 Ma and 243 ± 4 Ma, positive εHf(t) values of +2.39 to +13.31 and TDM2 model ages of 481–1191 Ma. The ca. 316–285 Ma granitoids were formed by fractional crystallization of regional coeval calc-alkaline basaltic magma in an arc-related setting and the ca. 243 Ma granitoid was generated mainly from partial melting of dominant basaltic source rocks and minor metagraywackes in the northward subduction-related setting. The granitoids from the Linxi area in the southern accretionary zone include muscovite monzogranites and monzogranites with LA-ICPMS zircon U-Pb ages of 525 ± 6 Ma, 274 ± 2 Ma and 261 ± 4 Ma. The ca. 525 Ma muscovite monzogranites have negative zircon εHf(t) values of −19.36 to −13.61 with TDM2 model ages of 2348–2711 Ma. They were generated from partial melting of Neoarchean to Paleoproterozoic supracrustal materials. The ca. 274–261 Ma monzogranites show negative zircon εHf(t) values ranging from −10.48 to −0.66 with TDM2 model ages of 1335–1942 Ma. They were derived from remelting of Paleoproterozoic to Mesoproterozoic supracrustal materials in the southward subduction zone along the northern active margin of the North China Craton. Combining our results with existing data, we infer that the early Paleozoic southward subduction of the Paleo-Asian Ocean started before ca. 525 Ma, and the northward subduction zone lasted to ca. 243 Ma, which implies the bidirectional subduction of the ocean lasted at least until ca. 243 Ma. Our work suggests that along the Solonker suture zone, the regional amalgamation may have occurred after ca. 243 Ma, which closed the Paleo-Asian Ocean.

Tracking the formation and evolution of the Ordos Block basement, North China Craton: U-Pb age and Lu-Hf isotope record of detrital zircons from the early Mesoproterozoic sandstones

The Ordos Block (OB) is generally considered to be an important micro-block concerning the early geological evolution of the North China Craton (NCC). Previous geophysical and limited geological researches for the OB basement have provided a preliminary understanding of the lithological assemblages and geological structure of the basement, but little was discussed about its formation time and early Precambrian evolution. In this contribution, to better understand the formation and evolution of the OB basement, the new LA-ICP-MS U-Pb ages, Lu-Hf isotope, and trace element composition have been carried for the detrital zircons from the sandstones of the Changcheng System overlying the OB basement. The youngest concordant U-Pb age constrains the Changcheng System to be deposited after ∼1.7 Ga, consistent with the Changcheng System in the other regions of the NCC. The 3.6–3.3 Ga detrital zircons preserved in the sandstones from the Changcheng System, associated with the 3.4 Ga inherited zircon in basement rocks, indicate the potential presence of Paleoarchean crustal materials in the OB. The 2.8–2.6 Ga zircons with positive εHf(t) values (+0.6 to +6.4) have the two-stage model ages (TDMC = 3.23–2.75 Ga) close to their formation ages, combined with the crustal thickening during this period, suggesting a major crustal growth in the early Neoarchean. In addition, the main age clusters of the 2.5–2.4 Ga, 2.2–2.0 Ga, and 1.95–1.8 Ga yield εHf(t) values of −6.9 to +7.7 with TDMC of 3.39–2.47 Ga, −13.4 to +7.5 with TDMC of 3.49–2.19 Ga, and −14.0 to +2.2 with TDMC of 3.41–2.40 Ga, respectively, indicative of three main stages of crustal reworking associated with two stage of minor juvenile crustal growth around 2.5–2.4 Ga and 2.2–2.0 Ga. They were corresponding to partial melting of the continental crust in the late Neoarchean, magmatism resulting from the plate subduction in the mid-Paleoproterozoic, and the collision and amalgamation between eastern and western blocks in the late Paleoproterozoic, respectively. All demonstrate that the OB was an ancient block formed mainly in the early Neoarchean and then subjected to three strong tectonothermal events of 2.5–2.4 Ga, 2.2–2.0 Ga, and 1.95–1.8 Ga, which occurred in the peripheral orogenic belts.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Age, genesis, and geological significance of Permian–Triassic boundary volcanic tuffs in the northeastern margin of South China

The Permian–Triassic boundary (PTB) volcanic tuffs are widely distributed in the East Palaeo‐Tethys region, but their age and origin have been controversial. The Permian–Triassic deep‐water sediments in the northeastern margin of South China are favourable research objects, and we report new results from the PTB deep basin sediments in the Susong area, Anhui Province, in this paper. Two volcanic tuffs are found in the Zuoshan PTB in the Zuoshan section, and Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U–Pb zircon dating indicates that the upper and lower tuffs are 251.3 ± 1.7 Ma and 252.3 ± 2.1 Ma, respectively. The PTB age is 251.6 Ma according to a numerical interpolation method of the average deposition rate of the PTB. Fusulinids and non‐fusulinid foraminifers in the limestone overlying the lower tuff define the rock as end‐Permian. Zircon trace elements show that both tuffs are sourced from felsic volcanism in continental arc background, and their source magmas were derived from the continental crust. Zircons of PTB age from the upper tuff on the bottom of the Triassic have negative εHf(t) value from −8.02 to −3.75, and the Palaeo‐Mesoproterozoic two‐stage model ages of 1502–1780 Ma, showing that their source rocks were only derived from the remelting of the Palaeo‐Mesoproterozoic continental crust. The zircons from the lower tuff on the top of the Permian show two types, including negative εHf(t) values from −6.48 to −1.25 and Palaeo‐Mesoproterozoic two‐stage model ages of 1350–1679 Ma, and positive εHf(t) values from +3.98 to +10.15, and Neoproterozoic two‐stage model ages of 629–1023 Ma, indicating that their sources were derived from the reworking of the Palaeo‐Mesoproterozoic continental crust and early Neoproterozoic depleted mantle joined in juvenile crust growth in the northern margin of the South China Block at the beginning of the collision between the South China and North China blocks. The initial collision started no later than 252.3 Ma and developed significantly from east to west.

Genesis of Mesozoic high-Mg dioritic rocks from the eastern North China Craton: Implications for the evolution of continental lithosphere

Abstract Pre-Cenozoic high-Mg andesites (HMAs) are mostly present in continental interiors, but their genetic relationship with continental lithosphere evolution remains unclear because of uncertainties of their mantle source, magmatic processes, and physicochemical conditions of formation. Early Cretaceous high-Mg dioritic rocks (HMDs, analogs of HMAs) of the Jinling complex in the Luxi area are typical intra-plate intrusions of the eastern North China Craton (NCC) and can be subdivided into two groups (Group-I and -II) on the basis of their petrographic and geochemical features. Group-I HMDs show low SiO2 contents (52.47–56.10 wt%) and Sr/Y (34.5–39.6) and (La/Yb)N (10.3–13.6) ratios but high contents of MgO (7.86–9.13 wt%), Y (18.3–20.3 ppm), Yb (1.43–1.47 ppm), and compatible elements (Cr = 407–585 ppm; Ni = 117–216 ppm), classifying as sanukitic rocks. Group-II HMDs are characterized by high SiO2 contents (63.81–64.87 wt%) and Sr/Y (47.1–63.4) and (La/Yb)N (16.1–17.5) ratios with low MgO (2.90–3.08 wt%), Y (0.88–1.04 ppm), Yb (0.88–1.04 ppm), and compatible elements (Cr = 201–213 ppm; Ni = 55–57 ppm) contents, belonging to adakitic rocks. Group-I and Group-II HMDs of the Jinling complex are closely related in spatial and temporal distribution, and all have enriched Sr-Nd isotopic compositions and arc-like trace element patterns with abundant hydrous minerals. Therefore, the Jinling HMDs should share a common source of ancient sub-continental lithospheric mantle that was metasomatized by aqueous fluids derived from the subducted Paleo-Pacific slab. The Jinling HMDs were not formed from interaction between slab-derived melts and mantle-wedge peridotites but were instead derived from partial melting of hydrous mantle peridotites in the continental interior of the eastern NCC. The distinctly different petrography, geochemistry, and mineralogy of the two groups of rocks resulted mainly from differing magmatic processes at crustal depths. Thus, Pre-Cenozoic intra-plate HMAs/HMDs are genetically distinct from Cenozoic HMAs that were mostly present in arc settings and generally represent juvenile crust growth. In a way, Archean tonalitic-trondhjemiticgranodioritic rocks (TTG) and sanukitoids, geochemically similar to HMAs/HMDs, could also be derived from interaction between slab-derived melts and mantle-wedge peridotites in arc settings or partial melting of hydrous mantle peridotites in continental interiors, and thus might not always be related with continental crustal growth and the onset of plate subduction.

Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

The Mewongo area of the Nyong Complex is located in the northwestern part of the Congo craton in southwestern Cameroon. The metavolcanosedimentary succession in this area comprises banded iron formations (BIFs), interlayered with amphibolites and gneisses and intruded by metamorphosed dolerite dykes. Within the framework of detailed field investigation, we report whole-rock geochemical data for the BIFs, metadolerites, and gneisses to constrain their provenance and tectonic setting, and present LA-ICP-MS U-Pb-Hf isotopic data for zircons from amphibolite and gneiss to assess the depositional age of BIFs from the Nyong Complex and understand the crustal evolution at the northwestern margin of the Congo craton. The geochemical composition of the Mewongo BIFs indicates a contribution of hydrothermal fluids, derived via submarine alteration of volcanics, and seawater. Geochemical and LA-ICP-MS U-Pb detrital zircon data for the gneisses reveal felsic protolith with ages between 2.97 and 2.65 Ga. Our zircon U-Pb-Hf isotope data suggest that a significant growth of juvenile crust occurred at ca. 2.7–2.2 Ga, followed by a reworking event at ca. 2.1 Ga with only minor juvenile crust addition. The metadolerites, cutting the metavolcanosedimentary succession, range in composition between tholeiitic and calc-alkaline basalts. They can be subdivided into two groups based on their trace element signature. The group I metadolerites have OIB-like geochemical composition (e.g., enriched LILE, HFSE, and LREE), while the group II metadolerites exhibit MORB-like geochemical signature (e.g., flat HFSE and HREE patterns). Metadolerites belong to two different so far undated magmatic events on the Congo craton that were previously recognized on the adjacent Ntem Complex.LA-ICP-MS U-Pb zircon dating of amphibolites, combined with chemical composition of metadolerites and gneisses, indicate deposition of the metavolcanosedimentary sequence at ca. 2.1 Ga in a pro-foreland basin sandwiched between the West and East Gabonian blocks, at the time when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny. The Nyong Complex BIFs were therefore deposited in a deep, open-marine setting during the ca. 2.1–2.0 Ga time interval.

Tectonic switch of the north Yangtze Craton at ca. 2.0 Ga: Implications for its position in Columbia supercontinent

Two episodes of magmatism with zircon U-Pb ages of 2.46 Ga and 2.0 Ga were studied in the Jinpan Complex in the north Yangtze Craton of South China. The ca. 2.46 Ga gneissic granites with various chemical compositions were formed by remelting of different crustal materials in a collisional event. The occurrence of both negative and positive whole-rock εNd(t) and εHf(t) values in these rocks indicates the growth of juvenile crust and reworking of Archean crust during early Paleoproterozoic. The ca. 2.0 Ga trondhjemites have high (La/Yb)N and Sr/Y ratios as well as negative whole-rock εNd(t), εHf(t) and zircon εHf(t) values, suggesting that they were formed by remelting of thickened lower crust in a collisional orogen. The ca. 2.0 Ga A-type granites have high Zr, Nb, Y contents and Ga/Al, Y/Nb ratios, belonging to A2 type granite. Their negative whole-rock εNd(t), εHf(t) and zircon εHf(t) values are similar to those of the ca. 2.0 Ga trondhjemites. These evidences suggest that they were produced by remelting of the residue after extraction of earlier granitic magma in an extensional tectonic setting. The 2.0 Ga mafic dykes show significant enrichment in LILEs but weak negative Nb, Ta anomalies and various Nb/U ratios. This is consistent with a parent magma similar to continental flood basalts and experienced different degrees of crust contamination. The co-occurrence of 2.0 Ga trondhjemites, A-type granites and mafic dykes implies a tectonic switch from convergence to extension at ca. 2.0 Ga in the north Yangtze Craton. Based on the similarities of the ca. 2.0 Ga magmatic rock association and metamorphic records in the north Yangtze Craton and south Siberia, the position of the Yangtze Craton in Columbia supercontinent is proposed to be close to south Siberia.

Permian arc magmatism in southern Tibet: Implications for the subduction and accretion of the Zhikong–Sumdo Paleo-Tethys Ocean

The tectonic evolution of the Zhikong–Sumdo Paleo-Tethys Ocean (Hereafter referred to as Zhikong–Sumdo Ocean) in southern Tibet is controversial due to the limited records of arc magmas. Here, the newly discovered Zhangda complex in the Zhikong area of Tibet is selected as the research object. Zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry and Sr-Nd isotope analyses performed based on field geological surveys show that the Zhangda complex is composed of Early Permian hornblende gabbro (278.3 ± 1.1 Ma), cumulate gabbro (277.4 ± 1.7 Ma), tonalite dike (281.1 ± 1.9 Ma), Middle–Late Permian gabbro diorite (267.8 ± 2.9 Ma, 269.0 ± 3.1 Ma), and tonalite (258.0 ± 2.7 Ma). The Zhangda mafic rocks show weak fractionation of light and heavy rare earth elements, and they are enriched in large ion lithophile elements and depleted in high field strength elements. Meanwhile, the Zhangda tonalites show the characteristics of metaluminous calc-alkaline I-type granite. They are enriched in light rare earth and large ion lithophile elements and are depleted in heavy rare earth and high field strength elements. The ɛHf(t), ɛNd(t) and the initial 87Sr/86Sr values of the mafic rocks are + 9.3–+14.4, +4.2–+5.9 and 0.702803–0.704264, respectively, while the ɛHf(t), ɛNd(t) and initial 87Sr/86Sr values of the tonalites are + 9.8–+16.3, +5.5–+6.1 and 0.703105–0.704071, respectively. Combined with the regional geological survey results, the Permian Zhangda complex was found to be formed in a tectonic setting of subduction north of the Zhikong–Sumdo Ocean. The Zhangda mafic rocks were formed by partial melting of the depleted mantle modified by subducted metasomatic fluids, while the Zhangda tonalites were the products of partial melting of the juvenile crust. During the Zhikong–Sumdo Ocean subduction, mantle-derived magma underplating led to the growth of juvenile crust in southern Tibet.

Subduction Erosion Revealed by Late Mesozoic Magmatism in the Gangdese Arc, South Tibet

AbstractSubduction of oceanic lithosphere can cause crustal growth and destruction, but whether this balancing act is common in a single fossil convergent system has been unclear. Here we report geochronologic‐geochemical‐isotopic data on Mesozoic igneous rocks in the central‐eastern Gangdese arc. Our results reveal: (a) A coupling of magmatic arc migration toward the continent interior with a marked incompatible enrichment of sub‐arc mantle magma sources; (b) numerous Late Cretaceous adakites were generated via combined partial melting of both eroded forearc crust debris and subducted oceanic crust; (c) the truncation of forearc crust and extremely short arc‐suture distance. Our work demonstrates that although there was significant vertical growth of juvenile crust throughout the Gangdese arc during the Late Mesozoic, a large amount of continental crust was returned back into the mantle through lateral subduction erosion at the same time. This study provides important insights into long‐term material recycling on Earth.

Ridge subduction and episodes of crustal growth in accretionary belts: Evidence from late Paleozoic felsic igneous rocks in the southeastern Central Asian Orogenic Belt, Inner Mongolia, China

Abstract The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and preserves evidence that more than 50% of this orogen represents juvenile crustal growth over an extended period of some 750 million years from ca. 1000 Ma to ca. 250 Ma. However, the mechanism of crustal growth is controversial, as implied by a variety of proposed models ranging from contributions of mantle-derived basaltic underplating in a post-collisional extensional setting to subduction-related processes in an island arc setting. To distinguish among these models, we report petrological, geochemical, whole-rock Sr-Nd isotope and zircon U-Pb and Hf isotope analyses of late Paleozoic felsic igneous rocks from the northern Inner Mongolia region, southeastern CAOB. New zircon U-Pb analyses of three plutonic and extrusive magmatic suites yield Late Carboniferous to Early Permian ages of 319–279 Ma. The Xi Ujimqin granodiorites have low-K tholeiitic to calc-alkaline, metaluminous to weakly peraluminous compositions, and are magnesian I-type granitoids. These granitoids are also characterized by relatively high MgO and Sr contents, high Mg# values, mid-ocean ridge basalt (MORB)-like Nd-Hf isotope compositions and young Nd-Hf model ages of 600–298 Ma. These features indicate that the parental magmas of the Xi Ujimqin I-type granitoids originated from a depleted lithospheric mantle that had been metasomatized by fluids released from a subducting slab. By contrast, the Xilinhot alkali-feldspar granites and Dashizhai rhyolites display geochemical signatures of ferroan granites. These, together with their moderately depleted Nd-Hf isotope compositions and young Nd-Hf model ages of 809–277 Ma, suggest that they were produced by re-melting of juvenile lower crust via underplating of mantle-derived magmas. The coexistence of an association of boninite-adakite-high-Mg andesite-Nb-enriched basalt, and MORB to ocean island basalt-type mafic rocks in the northern Inner Mongolia region implies a dramatic change in composition from fluid-related calc-alkaline arc magmatism to melt-related and mantle-derived magmatism initiated by upwelling asthenosphere. We infer that ridge subduction may have occurred in this region during Late Carboniferous to Early Permian. Combined with previous studies, there are two ridge subduction events during the early and late Paleozoic in Inner Mongolia within the southeastern CAOB, which coincide with two large-volume magmatic flare-ups (300 ± 20 and 450 ± 20 Ma). Furthermore, our model calculations suggest that most of the juvenile crust in the southeastern CAOB was generated in a relatively short interval (~20–40 m.y.) during each long-lived tectonic cycle (&amp;gt;140 m.y. spanning oceanic rifting, subduction, and collision) of magmatic activity. Ridge subduction may have played a significant role in the rhythmic growth of juvenile crust in the CAOB during the Phanerozoic. Repeated cycles of punctuated, rapid growth of juvenile crust associated with long-lived subduction systems represent a pattern that may be generalized to other Phanerozoic accretionary belts on Earth.

Melting of mafic slab and mantle peridotite during ridge subduction of the Proto-Tethys Ocean (Qilian Orogen, NW China)

Ocean-ridge subduction and the induced slab windows are important to understand the generation of arc magmatism and growth of juvenile crust both in present and ancient subduction zones. The physical conditions of oceanic ridges control the compositions of magmas during ridge subduction, and therefore magmatic composition can act as a marker of ridge subduction in ancient orogenic belts. Here we present the petrological, geochemical and geochronological studies for sodic rhyodacites and a concurrent ophiolite complex (basalt/dolerite-gabbro and harzburgite) within the North Qilian orogen, NW China. The sodic rhyodacites are low Sr/Y tonalite-trondhjemite with depleted SrNd isotopic compositions, suggesting derivation from melting of normal mid-ocean-ridge basalts (N-MORB) at 449 ± 2.9 Ma. Phase equilibrium and geochemical modeling confirm that the sodic rhyodacites formed via the partial melting of N-MORB sources at 3–5 kbar/780–840 °C. The basalts/dolerites exhibit similar geochemical characteristics to typical N-MORB rocks, and the gabbro gives zircon UPb age of 450 ± 2.5 Ma. The petrogenesis of the basalts/dolerites is similar to the “blow-torch” effect often seen during ocean-ridge subduction. The harzburgites are similar to abyssal peridotites in mineral compositions and have equilibrium temperatures of 1230–1460 °C, suggesting the obduction of a hot asthenospheric mantle on the continental margin. Our results show the integrated melting processes of the upwelling mantle peridotite and the subducted oceanic crust within the slab window during ridge subduction of the Proto-Tethys Ocean. These results are also significant in understanding the petrogenesis of low Sr/Y tonalite-trondhjemites, which are widely preserved in Archean to modern terranes.

The Pre-Grenvillian assembly of the southeastern Laurentian margin through the U–Pb–Hf detrital zircon record of Mesoproterozoic supracrustal sequences (Central Grenville Province, Quebec, Canada)

AbstractThe detrital zircon perspective on the pre-collisional crustal evolution of the Grenville Province remains poorly explored. In this study, we conducted in situ laser ablation U–Pb–Hf isotopic microanalysis on detrital zircon grains from three pre-orogenic (&gt;1 Ga) supracrustal sequences that crop out in the Central Grenville Province (Lac Saint-Jean region, QC, CA). Detrital zircon grains from vestiges of these sequences record three dominant age peaks at c. 1.46 Ga, 1.62 Ga, 1.85 Ga, and a subordinate peak at 2.7 Ga. The 1.46 Ga and 1.62 Ga age peaks are recorded in detrital zircon grains from a quartzite associated with a metavolcanic sequence (i.e. Montauban Group) with a maximum depositional age of c. 1.44 Ga. In contrast, the c.1.85 Ga age peak is observed from recycled zircon grains in metasediments with maximum depositional ages between 1.2 and 1.3 Ga. The suprachondritic Hf isotope composition in detrital zircon grains of the 1.46 Ga and 1.62 Ga age populations records juvenile crustal growth during peri-Laurentian accretionary orogenesis related to the Pinwarian (1.4–1.5 Ga) and Mazatzalian–Labradorian (1.6–1.7 Ga) events. The detrital zircon grains associated with Penokean–Makkovikian (1.8–1.9 Ga) source rocks record reworking of c. 2.7 Ga continental crust derived from a near-chondritic mantle reservoir. Overall, crust-forming and basement reworking events associated with accretionary orogenesis in southeastern Laurentia are retained in the detrital zircon load of Precambrian basins even after the terminal Grenvillian collision and assembly of Rodinia.

The Mesozoic magmatic, metamorphic, and tectonic evolution of the eastern Gangdese magmatic arc, southern Tibet

ABSTRACTMagmatic arcs are natural laboratories for studying the growth of continental crusts. The Gangdese arc, southern Tibet, is an archetypal continental magmatic arc that formed due to Mesozoic subduction of the Neo-Tethyan oceanic lithosphere; however, its formation and evolution remain controversial. In this contribution, we combine newly reported and previously published geochemical and geochronological data for Mesozoic magmatic rocks in the eastern Gangdese arc to reveal its magmatic and metamorphic histories and review its growth, thickening, and fractionation and mineralization processes. Our results show that: (1) the Gangdese arc consists of multiple Mesozoic arc-type magmatic rocks and records voluminous juvenile crustal growth. (2) The Mesozoic magmatic rocks experienced Late Cretaceous granulite-facies metamorphism and partial melting, thus producing hydrous and metallogenic element-rich migmatites that form a major component of the lower arc crust and are a potential source for the Miocene ore-hosting porphyries. (3) The Gangdese arc witnessed crustal thickening and reworking during the Middle to Late Jurassic and Late Cretaceous. (4) Crystallization-fractionation of mantle-derived magmas and partial melting of thickened juvenile lower crust induced intracrustal chemical differentiation during subduction. We suggest that the Gangdese arc underwent the following main tectonic, magmatic, and metamorphic evolution processes: normal subduction and associated mantle-derived magmatism during the Late Triassic to Jurassic; shallow subduction during the Early Cretaceous and an associated magmatic lull; and mid-oceanic ridge subduction, high-temperature metamorphism and an associated magmatic flare-up during the early Late Cretaceous, and flat subduction, high-temperature and high-pressure metamorphism, partial melting, and associated crust-derived magmatism during the late Late Cretaceous. Key issues for further research include the temporal and spatial distributions of Mesozoic magmatic rocks, the evolution of the components and compositions of arc crust over time, and the metallogenic processes that occur in such environments during subduction.