Adakitic Rocks Research Articles

Petrogenesis and tectonic implications of Eocene-Oligocene potassic felsic porphyries in the Sanjiang Region, southeastern Tibetan Plateau

• Eocene-Oligocene (36–33 Ma) potassic felsic rocks occur in the Sanjiang Region. • Eocene potassic felsic rocks produced by partial melting of mafic rocks in a thickened lower crust . • Eocene magmatic activities caused by post-orogenic delamination of lithospheric mantle and asthenosphere upwelling in the Eocene. This study presents zircon U-Pb dating and Hf isotope, whole-rock geochemistry and Sr-Nd isotopes of the potassic felsic porphyries from western Yunnan in the Sanjiang Region. Zircon U-Pb dating reveals that quartz-monzonite porphyry, granite porphyry, and adakitic granite porphyry were emplaced during 36–33 Ma. Geochemical data show that quartz-monzonite and granite porphyries have enriched LREE ((La/Yb) N = 39–67)and depleted HFSEs (e.g. Nb, Ta, P and Ti), with weak Eu anomalies. Moreover, they display similar 87 Sr/ 86 Sr ratios of 0.7068–0.7070 and negative εNd(t) values of −6.7 to −5.4, with zircon εHf(t) values of −5.65 to 3.06. The adakitic granite porphyries have high Sr (1273–1377 ppm), low Y (9.2–10.7 ppm), and low Yb (0.9–1.0 ppm) contents and high Sr/Y (128–138) and (La/Yb) N (17.7–19.8) values, typical of adakite rocks. In addition, these adakitic granite porphyries possess negative whole-rock εNd(t) values −0.2 to −0.3 and positive zircon εHf(t) values 2.7–6.0, associated with low Mg#, MgO, Ni and Cr contents. These geochemical features suggest that three kinds of potassic felsic porphyries could be produced by partial melting of mafic rocks (e.g., amphibolite) in a thickened lower crust. Our data, combined with previous studies and regional geology, suggest that delamination of lithospheric mantle and asthenosphere upwelling beneath the Sanjiang Region, both contributed to Cenozoic magmatic activities.

Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks

To investigate the behavior of Li isotopes during magmatic differentiation and the petrogenesis of Cu-bearing ore deposits, a suite of post-collisional adakitic rocks from Qulong region, southern Tibet, was studied. Their lithologies range from diorite through granodiorite to granite porphyry with the latter containing giant Cu deposits. Detailed evidence of field observation and geochemical signature suggest that these three sets of rocks were most likely formed by various degrees of partial melting and fractional crystallization from the same source. The dioritic enclave, granodiorite and granite porphyry have δ7Li values ranging from 0.2 to 8.2‰, 3.1 to 6.8‰, and 3.9 to 7.4‰, respectively. Most of these samples are overlapping in Li isotope composition, comparable to other granitoids worldwide, indicating insignificant Li isotope fractionation during partial melting and magma differentiation in adakite-like rocks. By contrast, their Li concentrations are mainly controlled by fractional crystallization as suggested from different modal mineralogy. This process does not lead to further enrichment of Cu although they have initial high concentrations (an average of ∼ 104 ppm). In comparison, granite porphyry has extremely high Cu contents (up to 2000 ppm) and their δ7Li values are positively correlated with Cu content, suggesting the involvement of magmatic fluids that most likely exsolved from deep magma chamber. Such fluids not only modified the Li isotopic compositions of granite porphyries, but also extracted metal elements from the highly evolved magma, eventually resulting in the Cu mineralization. Our work here provides new insight into the formation and evolution of the porphyry Cu-bearing deposit.

Geochronology, geochemistry, and isotopic composition of the early Neoproterozoic granitoids in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China: Petrogenesis and tectonic implications

The early Neoproterozoic granitoids exposed in the Bikou Terrane preserve vital information for understanding the Tonian tectonic evolution along the northwestern margin of the Yangtze Block, South China. However, their accurate forming age, petrogenesis, and tectonic attributes remain ambiguous. This study reports new results on zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Baiquesi and Bahaihe quartz monzonite, Shilingou monzogranite, and Maliupu granodiorite from the Bikou Terrane. Detailed zircon U–Pb dating results reveal that the quartz monzonite and monzogranite were formed at ca. 860 Ma, whereas the granodiorite was emplaced at ca. 825 Ma. The quartz monzonite and monzogranite samples exhibit certain linear correlations in the Harker diagram, congruent distribution patterns of rare earth elements, and consistent isotopic compositions, indicating that they shared the same magma source, whereas the monzogranite was formed by differentiation crystallization of primitive quartz monzonite melt. The quartz monzonite samples are calc-alkaline in composition and have high ratios of Sr/Y and (La/Yb)N, coinciding well with the adakitic rocks. The relatively high contents of MgO, Ni, Cr, and Sr, low contents of Yb and Y, low ratios of La/Ce, Rb/Sr, and Th/La, and positive values of εHf(t) (+4.8–+6.7) and εNd(t) (+1.7–+2.1) indicate that they were formed by the interaction between slab-derived melt and mantle wedge. This mechanism may be triggered by a slab rollback process in an arc-related setting. The Maliupu granodiorite samples are classified as calc-alkaline I-type granitoids, and have low concentrations of MgO, Cr, Ni, and Y, and low ratios of Sr/Y and (La/Yb)N. They are featured by highly evolved εHf(t) values from −15.0 to −10.9 and εNd(t) values from −11.8 to −11.9, with Hf and Nd model ages ranging between 2.63 and 2.20 Ga, implying they were derived from partial melting of the ancient lower mafic crust and were formed in a subduction-related setting. Therefore, the new data, combined with those from previous studies in the Bikou Terrane, provide an essential link in the evidence chain for the hypothesis that the northwestern periphery of the Yangtze Block was in a persistent subduction-related tectonic regime in the early Neoproterozoic.

Petrogenesis of Eocene Wangdui adakitic pluton in the western Gangdese belt, southern Tibet: implications for crustal thickening

AbstractLower-crust-derived adakitic rocks in the Gangdese belt provide important constraints on the timing of Tibetan crustal thickening and on the relative contributions of magmatic and tectonic processes. Here we present geochronological and geochemical data for the Wangdui porphyritic monzogranites in the western Gangdese belt. Zircon U–Pb dating yields emplacement ages of 46–44 Ma. All samples have high Sr (321–599 ppm), low Yb (0.76–1.33 ppm) and Y (10.6–18.3 ppm) contents, with high La/Yb (51.1–72.3) and Sr/Y (21.0–51.4) ratios, indicating adakitic affinities. The low MgO (0.97–1.76 wt %), Cr (7.49–53.6 ppm) and Ni (4.75–29.1 ppm) contents, as well as high 87Sr/86Sr(i) (0.7143–0.7145), low ϵNd(t) (−10.4 to −9.8) and zircon ϵHf(t) (−17.7 to 0.4) values, suggest that the Wangdui pluton most likely originated from partial melting of the thickened ancient lower crust. In combination with previously published data, despite the east–west-trending heterogeneity of crustal composition in the Gangdese belt, the La/Yb ratios of magmatic rocks reveal that both western and eastern segments experienced remarkable crustal thickening in the Eocene. However, in contrast to the thickened juvenile lower crust in the eastern segment formed by the underplating of mantle-derived magmas, tectonic shortening plays a more crucial role in thickening of the ancient basement in western Gangdese. In fact, such Eocene-thickened ancient lower-crust-derived adakitic rocks are widely distributed in the central Himalayan–Tibetan orogen. This, together with the extensive development of fold–thrust belts, suggests that tectonic shortening might be the main mechanism accounting for the crustal thickening associated with the India–Asia collision.

Permian–Triassic adakitic igneous activity at Northern Mongolia: Implication for Permian–Triassic subduction system at the Siberian continental margin

The geochemistry of the Permian–Triassic large-scale igneous rock body of northern Mongolia is a key factor in understanding the subduction-related magmatism at the margin of “Siberian continent.” Several studies have been done in the Permian–Triassic igneous body; however, its detailed magmagenesis and tectonic significance remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen Formation) and the Late Permian–Middle Triassic plutonic rocks (Selenge plutonic rock complex) of the Permian–Triassic igneous body, and intermediate dike intruding into them, and discusses the Late Permian–Middle Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low K2O/Na2O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen Formation is calculated to be 37.1° N based on thermal remanent magnetization. Therefore, the Late Permian–Middle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic rock complex, indicating that adakitic igneous activity continued after the Early Triassic.

Geochemical characteristics and indication of graphite deposits in Xinrong Region, Shanxi, China

The NE–SW-trending graphite belt found in the Xinrong Region, Datong City is one the richest graphite reserves in China, extending for more than 22 km. The ore-bearing layer of the Huangtuyao Formation is a graphite-bearing gneiss. Based on major and trace element analysis data, we determined from three diagrams that the graphite-bearing gneiss is a parametamorphic rock, and this gneiss was formed by the regional metamorphism of carbonaceous argillite. The detritus of this ore deposit originated from an arc region as seen from the K 2 O/Na 2 O–SiO 2 diagram. According to the Ta/Yb, Sr/Yb and K 2 O/Na 2 O ratios as well as the composition of Gehuyao gneiss, we concluded that this gneiss has typical low-potassium adakitic rock characteristics. Combined with the MgO/SiO 2 diagram and the characteristics of low K 2 O and high Al 2 O 3 , the genetic model of Gehuyao gneiss is melted subducted oceanic crust. Based on regional geological data and comparison of samples, we found an inherent relationship in the main composition content between the graphite-bearing gneiss and the Gehuyao gneiss. This indicates that the ore deposit accumulated the weathering products of Gehuyao gneiss during the sedimentary period. This research provides evidence for ore body evolution and makes it possible to establish the sedimentary–metamorphic model of the graphite ore in this region. In light of the range of the ore body, we predict that the potential area for graphite mineralization in the North China Craton was 2 ± 1.5 km away from the boundary. Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: https://www.lyellcollection.org/cc/applications-of-innovations-in-geochemical-data-analysis

Origin of Tibetan post-collisional high-K adakitic granites: Anatexis of intermediate to felsic arc rocks

Abstract Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30−22 Ma), and inherited zircons (66−48 Ma) with the same εHf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66−48 Ma magmatic zircon cores recording the anatectic event at 29−25 Ma. Phase equilibrium modeling yields an anatectic temperature of ~740 °C and pressure (P) of ~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle–derived shoshonitic magmas results in the hybridized post-collisional Oligocene–Miocene mafic microgranular enclave–bearing potassic adakitic granitic rocks in southern Tibet.

Petrogenesis and Geodynamic Implications of Late Triassic Mogetong Adakitic Pluton in East Kunlun Orogen, Northern Tibet: Constraints from Zircon U–Pb–Hf Isotopes and Whole-Rock Geochemistry

Adakites or adakitic rocks usually show special geochemical signatures and are petrological probes to reveal the tectono–magmatic evolutionary history of paleo–orogenic belts. Here, we present a comprehensive study on the zircon U–Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Mogetong adakitic pluton in East Kunlun orogen, Northern Tibetan Plateau, to constrain its petrogenesis and tectonic setting, and thus to reveal its implications for the Paleo–Tethyan orogeny. The studied pluton comprises of quartz monzonite porphyry with zircon U–Pb crystallization age of ca. 215 Ma, which is coeval to their diorite enclaves (ca. 212 Ma). The quartz monzonite porphyries have intermediate SiO2 (63.31–65.74 wt%), relatively high Al2O3 (15.52–16.02 wt%), K2O (2.83–3.34 wt%), and Sr (462–729 ppm), but low Y (9.14–15.7 ppm) and Yb (0.73–1.39 ppm) with high Mg# (47–55), Sr/Y (30–57) and La/Yb ratios resembling typical high–K calc-alkaline and high Mg# adakitic rocks. Zircon Lu–Hf isotopes show that the studied samples have weakly juvenile zircon Lu-Hf isotopes (εHf(t) = 1.80–4.03) with older model age (1.00–1.14 Ga). The relative low content of Cr (14–59 ppm) and Ni (8–30 ppm), as well as the petrological, geochemical, and Lu-Hf isotopic data, indicates that the Mogetong adakitic rocks were generated by partial melting of thickened lower crust with a certain contribution of the underplated mantle-derived magma in slab break-off setting. This study shows that the Late Triassic adakitic magmatism in East Kunlun orogen may be the response of tectonic transition from oceanic subduction to post–subduction extension, and the reworking of ancient continental crust with subsequent variable crust-mantle magma mixing is the major mechanism of continental crust evolution in the Paleo–Tethyan orogenic belt.

Reworking of continental crust on northeastern North China Craton: Evidence from geochronology and geochemistry of Early Cretaceous granitic rocks

This paper reports new whole–rock major– and trace–element and zircon UPb and HfO isotopic data for Early Cretaceous granitic rocks on Liaodong Peninsula, NE China, with the aim of constraining continental reworking in the northeastern North China Craton. On the basis of SiO2, MgO, and Sr contents and Sr/Y values, the rocks can be divided into three groups. The low–Mg adakitic rocks show zircon UPb ages of (134–120 Ma), and high SiO2 (67.11–71.14 wt%) and low MgO (0.53–0.62 wt%) contents, and high Sr (531–1108 ppm) contents, Sr/Y (29.3–132) ratios, with enriched Sr–Nd–Hf–O isotopic compositions, indicating an origin of thickened lower mafic crust. The high–Mg adakitic rocks show ages of (126–114 Ma) and are characterized by low SiO2 (53.11–63.88 wt%) and high MgO (2.07–8.93 wt%) contents, and high Sr (391–1041 ppm) contents, Sr/Y (23.9–64.1) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting that they were derived from partial melting of delaminated lower mafic crust. The normal granitic rocks show ages of (127–121 Ma) and high SiO2 (71.65–77.22 wt%) and low MgO (0.10–0.51 wt%) and Sr (12.1–309 ppm) contents, Sr/Y (0.89–24.5) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting an origin from lower mafic crust with normal crustal thickness. These constraints, combined with the regional occurrence of A–type granites, bimodal igneous rocks, and metamorphic core complexes, as well as the regional distribution of Mesozoic granitic rocks displaying a younging trend from southeast to northwest for the Jurassic rocks but a reverse trend for the Cretaceous rocks, recorded the roll–back of the Paleo–Pacific plate towards the Eurasian continental margin.

Early Paleozoic arc-back-arc system evolution in the junction of the Qinling and Qilian Orogens: Geochemical constraints from ca. 445–430 Ma magmatic rocks in the Tianshui area

ABSTRACT The Tianshui area is located at the junction of the Qinling and Qilian Orogens, the evolution of which is crucial in understanding the Early Palaeozoic tectonic evolution of the Central China Orogenic System. We present here geochronological, geochemical, and Sr-Nd isotopic data for four representative Silurian magmatic rocks, termed the Dangchuan, Huamiao, Yanjiadian, and Hongtupu, along a cross section across the main rock units of the Tianhui area. The 432 Ma Dangchuan syenogranites are peraluminous with negative εNd(t) values, high initial 87Sr/86Sr ratios, and low Rb/Sr and high CaO/Na2O ratios. We interpret the syenogranites to be generated by partial melting of metagreywackes during a Silurian orogenic event. The 436 Ma Huamiao diorites are high-Mg diorites with high MgO contents and Mg# (63–65) and enriched Sr-Nd isotopic compositions. The 445 Ma Yanjiadian diorites are high-Mg adakitic rocks with high Sr and Mg# and low Y and Yb contents. They were both generated by the interaction between sediment/slab-derived fluids/melts and mantle peridotite in the subduction-related setting. The 445 Ma Hongtupu basalts include E-MORB and IAB with depleted Sr-Nd isotopic compositions. They were derived from enriched mantle and sediment-modified mantle, respectively, and formed in a back-arc basin setting. Synthesizing our new and published data, we propose a tectonic model involving generation of Baihua-Huluhe arc-back-arc system, subduction of the Huluhe back-arc oceanic basin, and collision between the North Qinling arc terrane and North China Block, which is closely associated with the closure of the Proto-Tethys Ocean.

Genesis and magmatic-hydrothermal evolution of the Shapoling Mo deposit, East Qinling, China: Insights from geochronology, petrogenesis and fluid evolution

The Shapoling deposit, located in Xiong'ershan area, East Qinling, China, is characterized by a magmatic-hydrothermal Mo vein system. We present whole-rock geochemical, zircon U-Pb-Hf-O isotopic, quartz H-O isotopic and fluid inclusion data for the Shapoling deposit. We use these data to investigate the petrogenesis of granitoids associated with Mo mineralization and the processes of magmatic-hydrothermal evolution that formed the Mo deposit. The granitoids from the Shapoling deposit are metaluminous, alkalic to calc-alkalic, shoshonitic to high-K series, and belong to highly fractionated I-type granitoids. These granitoids are strongly enriched in light rare earths (LREEs) and large ion lithophile elements (LILE), but depleted in heavy rare earths (HREEs) with negative europium anomalies and high field-strength elements (HFSE), mostly plotting in the range of the Taihua Group rocks. Most of the Shapoling granitoids show relatively lower Sr/Y and (La/Yb)N than the typical adakitic rocks, showing that the parental magmas were generated under lower pressure. Therefore, the Shapoling granitoids were generated at thin crustal levels by lithospheric thinning process linked to the tectonic transition from compression to extension. Zircon U-Pb ages of the Shapoling granitoids range from 132.9 ± 1.3 Ma to 126.5 ± 0.7 Ma. The εHf(t) values and δ18Ozircon values show a variation trend from −19.1 to −8.0 and 5.33‰ to 6.30‰, which are consistent with Hf-O isotopic features of mixing of the partially melted Taihua Group with mantle-derived components. FIluid inclusion microthermometry results suggest that the hydrothermal fluids forming the Shapoling Mo deposit changed from early moderate- to low-salinity CO2-rich fluids in the H2O-NaCl-CO2 system, to late H2O-NaCl low-salinity fluids. The corresponding quartzs have δD values varying from −93.3‰ to −78.2‰ and δ18O values from −5.14‰ to 0.92‰, indicating a dominant magmatic source but were diluted by meteoric water in the late stage. Fluid boiling is the key factor that controlling the precipitation of molybdenite. Our study provides insights into the formation of Mo system associated with a prolonged and active magmatic system, upwelling of hot asthenosphere and intense mantle-crust interaction. The prolonged magmatic system is steadily replenished by mantle-derived materials in the magma chamber, which feed the hydrothermal systems and govern the formation of the Shapoling Mo deposit.

Origin of the Miocene Adakitic Rocks and Implication for Tectonic Transition in the Himalayan Orogen: Constraints from Kuday Granitoid Porphyry in Southern Tibet

Abstract Postcollisional adakitic magmatism in the Himalayan Orogen provides a probe into the evolution of the collisional orogen. During the Miocene, the Himalayan Orogen underwent a tectonic transition, which was characterized by a series of tectonic events, including the activity of the North-South Trending Rift, exhumation of eclogite, rapid uplift of the orogen, and the extensive adakitic rocks. In this study, we reported the geochemistry and geochronological data of the Kuday dikes intruding into the Tethys Himalayan Sequence near the Sakya Dome of southern Tibet. The Kuday dikes are granitoid porphyries with zircon U-Pb ages of ca. 11 Ma. The Kuday granitoid porphyry dikes have high SiO2 (63.01–68.41 wt.%) and Al2O3 (17.31–19.87 wt.%) but low Mg (0.88–1.41 wt.%), Mg# (36–50), Ni (2.8–19.3 ppm), and Cr (2.9–26.4 ppm), indicating no input of mantle material. They have high Sr (934–1881 ppm), (La/Yb)N (18.84–113.13), and Sr/Y ratios (89.25–305.85) but low K2O/Na2O ratios (0.17–0.79), indicating that they are adakitic affinity. They display initial 87Sr/86Sr ratios of 0.707–0.711 and εNdt values of -3.7–-6.7. These geochemical signatures indicate that the Kuday granitoid porphyrite dikes were derived from the partial melting of the thickened lower crust of the Himalayan Orogen. Partial melting of the thickened lower crust requires additional heat, so the delamination model with lithospheric mantle thinning and asthenospheric upwelling is proposed to explain the formation of the Kuday adakitic rock. The delamination model can also provide a reasonable explanation for the tectonic events during the Miocene in the Himalayan Orogen.

Petrogenesis of isotopically enriched Quaternary magma with adakitic affinity associated with subduction of old lithosphere beneath central Myanmar

We present a model for the petrogenesis of magma with adakitic affinity in an old subduction zone, which does not involve slab melting and is constrained by new geochronological and geochemical data for Mt. Popa, the largest of three Quaternary volcanoes in central Myanmar (Popa, Monywa and Singu). The edifice is composed of Popa Plateau (0.8–0.6 Ma) with high-K rocks and a stratovolcano (< 0.33 Ma) predominantly composed of medium-K rocks with adakitic affinity (Mg# 45–63, Sr/Y > 40). The distinct K contents indicate that the adakitic magmas cannot be derived from Popa high-K rocks, but they share trace-element signatures and Sr–Nd isotope ratios with medium-K basalts from Monywa volcano. Our estimation of water contents in Popa magma reveals that primary magma for medium-K basalts was generated by partial melting of wedge mantle with normal potential temperature (TP 1330–1340 °C) under wet conditions (H2O 0.25–0.54 wt%). Its melting was probably induced by asthenospheric upwelling that is recognized by tomographic images. Mafic adakitic magma (Mg# ~ 63, Sr/Y ~ 64) was derived from the medium-K basaltic magma in fractional crystallization of a garnet-bearing assemblage at high pressure, and felsic adakitic rocks (Mg# ~ 45, Sr/Y ~ 50) were produced by assimilation-fractional crystallization processes at mid-crustal depths.

Petrogenesis of Early Cretaceous High-Mg Adakitic Pluton in the North Lhasa Block, Tibet Plateau: Implications for the Tethyan Orogeny

Late Mesozoic igneous rocks from the north Lhasa block record the Neo-Tethyan orogeny in the southern Tibet Plateau. This study presents geochronological and geochemical data of Bieruozecuo quartz diorite pluton in the northern margin of the Lhasa block to constrain its petrogenesis and tectonic implications. The LA–ICP–MS zircon U–Pb geochronology of quartz diorites shows that the emplacement occurs at ca. 114–116 Ma, belonging to the products of Early Cretaceous magmatic activities. The high concentrations of TiO2, MgO, and MnO, together with SiO2 vs. K2O and A/NK vs. A/CNK diagrams, all suggest that the Bieruozecuo quartz diorites are meta-aluminous and high-potassium calc-alkaline rocks. Their high Sr and low Y and Yb contents, as well as high Sr/Y and La/Yb ratios, are consistent with the typical adakitic rocks. The REE patterns show a large distribution of compositions, which have LREE, while HREE are buffered, along with large ratios of (La/Yb)N, as well as high values of K2O/Na2O, Mg#, Cr, and Ni, all of which imply the partial melting of a delaminated lower crust, without obvious fractional crystallization during the magma ascending and emplacement. This study suggests that, with the closure of the Bangonghu–Nujiang Tethys Ocean Basin, the post-collisional extension of the north Lhasa block will have started no later than ca. 114–116 Ma. Combined with the previous studies, our new data demonstrates that the partial melting of the delaminated lower crust, in a post-collisional setting, may be the main mechanism responsible for the ca. 116–82 Ma adakitic magmatism in the north Lhasa block.

Mesozoic tectonic transition of the northeastern North China Craton: Evidence from adakitic rocks in southeastern Jilin Province, China

ABSTRACT Chronological and geochemical studies of adakites and adakitic rocks are important in understanding the tectonic evolution and geodynamic processes. We present new zircon U–Pb ages, Hf isotope, and geochemical analyses of adakitic rocks exposed in the Baishan area, southeastern Jilin Province, China. These new ages, together with existing age data, indicate that adakitic magmatism in southeastern Jilin Province can be subdivided into four stages: Early–Middle Triassic (251–235 Ma), Late Triassic (221–219 Ma), late Early–early Late Jurassic (176–156 Ma), and Early Cretaceous (ca. 130 Ma). Early–Middle Triassic adakitic rocks occur in a nearly E–W-trending belt within the northern margin of the northeastern North China Craton, indicating a compressional tectonic setting caused by the scissor-like closure of the Paleo-Asian Ocean from west to east. Late Triassic adakitic rocks occur in the Tonghua area and formed in an extensional setting caused by delamination after subduction and collision between the Yangtze Craton and NCC. Late Early–early Late Jurassic adakitic rocks occur in the Baishan and Kaiyuan areas and originated from partial melting of thickened lower crust in a compressional setting related to subduction of the Paleo-Pacific Plate. Early Cretaceous adakitic rocks occur in the Baishan area and were derived from partial melting of thickened lower crust above the subduction zone and delaminated lower crust, indicating that the subducting Paleo-Pacific Plate retreated to the eastern part of southern Jilin Province during the Early Cretaceous (ca. 130 Ma) and that the tectonic setting of the northeastern part of the NCC changed from compression to extension, starting in the east and progressing westward. In summary, the northeastern part of the NCC has been affected by a series of tectonic events during the Mesozoic, such as the closure of the Paleo-Asian Ocean, the collision between the NCC and YC, and the subduction of the Paleo-Pacific Plate.

Paleoproterozoic tectonic evolution of the Khondalite Belt in the North China Craton: Constraints from the geochronology and geochemistry of 1.9–2.3 Ga felsic and basic intrusive rocks in the Jining area

Although the Paleoproterozoic Khondalite Belt is one of the most important delineation boundaries in the North China Craton, its tectonic evolution remains controversial. Here, we present new geochronological and geochemical data for Paleoproterozoic granodiorite gneiss, amphibolite, and gabbro and monzogranite dykes in the Jining area in order to constrain the tectonic evolution of the Khondalite Belt. Zircon U–Pb dating indicates that the granodiorite gneiss, amphibolite and monzogranite were emplaced at ca 2.3 Ga, 2.0 Ga and 1.9 Ga, respectively. The 2.3 Ga granodiorite gneisses have positive εNd(t) values (+0.6 to +1.3) and high zircon saturation temperatures (864–873 °C), and show geochemical characteristics of A2-type granites, probably indicating extension background related to subduction. The 2.0 Ga amphibolites have high Nb contents and their protoliths display geochemical features of Nb-enriched basalts. The low MgO, Cr, Ni, LREE-enriched patterns, and negative εNd(t) values (−5.5 to −6.3) with relatively old Nd model ages suggest that the protoliths were derived from a fertile mantle source metasomatized by slab-derived melts. The distinct geochemical characteristics suggest that the formation of the amphibolites were attributed to the upwelling asthenosphere triggered by the opening of slab window. Based on the U-Pb ages of zircons and the contact relationship in the outcrops, the intrusive age of the gabbros was constrained to be 2.0–1.9 Ga. These gabbro dykes have high Mg#, Cr, and Ni contents, and show E-MORB-like REE patterns with high contents of large ion lithophile elements and negative Nb-Ta anomalies, indicating a subducting tectonic setting. Their high Ba/Th and Rb/Y ratios, low Th/Ta and Nb/Y ratios imply a mantle source metasomatized by slab-derived fluids. The 1.9 Ga monzogranite dykes have high Sr and Sr/Y ratios, low Yb and HREE, similar to those of typical adakitic rocks. Their low MgO, Cr, Ni, Mg# and negative εNd(t) suggest that the dykes were generated by partial melting of thickened lower continental crust. These different rocks, therefore, indicate a convergent margin occurred along the Khondalite Belt before the collision between the Yinshan Block and the Ordos Block, followed by extension event at ca. 2.3 Ga and closed with a collision at ca. 2.0–1.9 Ga, and then changed to post-collisional tectonic setting with the adakitic monzogranite dykes formed at ca. 1.9 Ga.

Temporal changes in subduction- to collision-related magmatism in the Neotethyan orogen: The Southeast Iran example

Continental-arc igneous rock compositions change in response to the transition from subduction to collision and these changes can reveal how the crust, lithosphere and magma sources evolved. Neotethys-related Late Cretaceous to Pleistocene subduction- and collision-related magmatic rocks from the ~350 km long southeast Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran provide an excellent natural laboratory to better understand these changes. These igneous rocks are well-exposed and moderately eroded to reveal a nearly complete record since subduction initiation at ~95 Ma. We analyzed new samples for major and trace elements (83 samples), SrNd isotopic compositions (47 samples), and UPb zircon ages (26 samples) and compiled geochemical and geochronological data on the southeast segment of the UDMB. The geochronological data reveal two magmatic pulses at ~80–70 Ma and ~50–0 Ma. Important changes in magmatic compositions reflect initial collision with Arabia at ~32 Ma, changing from normal calc-alkaline to increasingly adakitic immediately after collision began. Five stages can be identified: 1) normal continental-arc magmatism during the Late Cretaceous; 2) arc quiescence in Paleocene and Early Eocene time; 3) Middle-Late Eocene extensional arc magmatism related to slab rollback; 4) early collision and crustal thickening during the Early Oligocene; and 5) slab breakoff, asthenospheric upwelling, and associated adakitic magmatism from Middle Miocene onward. Temporal changes in UDMB magmas reflect the response of the overriding plate to changes in the geometry of the subducting Neotethyan lithosphere and to collision between Arabia and Iran. Crustal thickening and arc narrowing during Miocene to Pleistocene post-collisional magmatism caused adakitic magmatism and associated Cu mineralization. Zircon OHf and apatite O isotopes as well as bulk-rock Nd isotopes of Cu-bearing adakitic rocks are similar to other barren rocks, but nearly all fertile rocks have higher Hf/Y, Eu/Eu⁎(n) in zircon and higher Sr/Y, V/Y, Eu/Eu⁎(n) in apatite than barren rocks.

High- and Low-Mg Adakitic Rocks in Southern Tibet: Implication for the Crustal Growth and Geodynamic Process in the Late Cretaceous

High- and Low-Mg Adakitic Rocks in Southern Tibet: Implication for the Crustal Growth and Geodynamic Process in the Late Cretaceous

Tectonomagmatic reconstruction of the Upper Mesozoic-Cenozoic Neotethyan arcs in the Lut block, East Iran: a review and synthesis

The Lut Block in east Iran with a widespread record of arc-type Mesozoic-Cenozoic magmatic rocks is bordered by the Birjand (Sistan) ophiolitic assemblage to the east and the Sabzevar ophiolitic assemblage to the north. The nature and tectonomagmatic setting of these sutures and associated arcs are controversial. The geochemical attributes of the Upper Cretaceous-Paleogene intrusive and extrusive rocks in the central (CLB) and the northern Lut block (NLB) and in the Sabzevar ophiolite zone (SOZ), Late Cretaceous calc-alkaline and adakitic rocks to the east of the Sistan suture (CAS), and the Bazman intrusives to the west are consistent with a subduction-related volcanic arc setting. The Najmabad granitoids in the NLB compositionally overlap with the Eocene Sabzevar calcalkaline and adakitic lava flows (ESA), implying a common magmatic source and a similar trend of evolution. The island arc basalt-like geochemical patterns and the initial Sr and Nd isotope compositions for the CLB intrusive bodies suggest that the parent magmas formed by partial melting in a suprasubduction mantle wedge. The initial 87Sr/861Sr and εNd(t) values for the CAS, the Bazman intrusives, the ESA, and the Najmabad granitoids are similar and overlap those of the Sabzevar ophiolitic rocks. Here, we review the previously proposed one-sided subduction models for the Lut block and the Sabzevar suture zone, and discuss in favour of a two-sided asymmetric subduction as an alternative geodynamic model.

Generation of Nb-enriched mafic rocks and associated adakitic rocks from the southeastern Central Asian Orogenic Belt: Evidence of crust-mantle interaction

Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated. Here we focus on the Early Mesozoic mafic rocks and granite porphyry, which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean. The granite porphyry samples are calc-alkaline and distinguished by high Sr contents, strong depletion of heavy rare earth elements, resulting in high (La/Yb)N and Sr/Y ratios, and negligible Eu anomalies. Based on their high Na2O and MgO, low K2O contents, positive εHf(t) and εNd(t) and low (87Sr/86Sr)i values, we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust. The Nb-enriched mafic rocks are enriched in Rb, Th, U, Pb and K, and depleted in Nb, Ta, Ba, P and Ti, corroborating a subduction-related origin. Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts. Trace element and isotope modeling results and elevated zircon δ18O values suggest variable subducting sediments input into the mantle wedge, dominated by terrigenous sediments. Synthesizing the widely-developed bimodal rock associations, conjugated dikes, thermal metamorphism, tectonic characteristics, paleomagnetic constraints, and paleogeographical evidence along the Solonke-Changchun suture zone, we identify a slab window triggered by slab break-off, which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.