Zircon δ18O Values Research Articles

Ca. 815 Ma intra-plate granitoids and mafic dykes from Emeishan pluton in the western Yangtze Block, SW China: A record of rifting during the breakup of Rodinia

The western Yangtze Block, South China is characterized by voluminous Neoproterozoic extension-related volcano-sedimentary sequences and intrusions (e.g. Emeishan granitoid intrusion) and spatially associated mafic dykes. The origin and tectonic setting of Emeishan granitoid intrusion and mafic dykes in the Yangtze Block is poorly study. Understanding the origin and geodynamic setting of dykes and granitoid intrusion are significant in deciphering thermal structure and composition of mantle, tectonic evolution as well as crustal growth processes in the Yangtze Block. SIMS Zircon U–Pb dating of Emeishan granitoid intrusion and mafic dykes suggests that the granitoids (ca. 818 Ma) and mafic dykes (ca. 814 Ma) are contemporaneous. Compositionally, the Emeishan granitoids are restricted and characterized by high SiO2 (69.11–70.50 wt%), Na2O + K2O (3.52–4.13%) and Al2O3 (14.16–14.47%), but low CaO (1.78–2.03%), MgO (0.72–0.91%) contents. They feature strong LREE-enrichment (La/YbN = 7.2–12.3) in conjunction with negative Eu anomalies. The mafic dykes have low SiO2 (48.09–48.65 wt%), K2O (1.94–2.12 wt%) and Na2O (1.64–1.75 wt%) contents, and high Mg# (61.67–62.26). On primitive mantle-normalized trace element plot, they are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dykes and granitoids record similar zircon εHf(t) values (+2.2 to +12.1, +1.4 to +6.8), and zircon δ18O values (4.47‰ and 10.41‰, 4.49‰ and 12.05‰, respectively). The rifting-related geochemical signature of the Emeishan pluton and contemporaneous margin and intra-plate plume-related magmatism suggest that they were derived from mantle melting in extensional tectonic setting during mid-Neoproterozoic.

Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.

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.

Identifying Crystal Accumulation in Granitoids through Amphibole Composition and In Situ Zircon O Isotopes in North Qilian Orogen

Abstract Granitoids are the main constituents of the continental crust, and an understanding of their petrogenesis is key to the origin and evolution of continents. Whether crystal fractionation is the dominant way to generate evolved magmas has long been debated, mostly because such processes would produce large volumes of complementary cumulates, which remains elusive. Mafic magmatic enclaves (MMEs) are ubiquitous in granitoids and their presence was initially recognized as cumulates. However, because many MMEs lack obvious evidence of accumulation, such as the classic cumulate textures and modal layering, the cumulate origin of MMEs has been abandoned and the model of magma mixing between mafic and felsic magmas has become popular. In this study, we conduct a combined study of amphibole composition and in situ O isotopes in zircons on three suites of orogenic granitoids with MMEs from the North Qilian Orogenic Belt (NQOB). We find that the MMEs and their host granodiorites show overlapping zircon δ18O values, affirming that they share the same parental magmas. The amphibole compositions indicate that amphiboles from the MMEs are not in equilibrium with a melt whose composition was that of the bulk-rock. These new data, together with the published bulk-rock data, suggest that the MMEs in our study have clear cumulate signatures and are thus of cumulate origin. Our study provides evidence for crystal accumulation in granitoids in the NQOB. This new understanding calls for re-examination on the petrogenesis of some intermediate magmatic rocks (granitoid/andesite) in discussing models of continental crustal growth.

Crustal thickening and continental formation in the Neoarchean: Geochemical records by granitoids from the Taihua Complex in the North China Craton

The Earth’s lithosphere changed dramatically in many aspects during the Neoarchean era. In this period, the crust became more felsic and shows significant diversification in the composition of granitoids. Although this change has been noticed for many years, the mechanism that drove the change remains unclear. Here we report a geochemical study of Neoarchean granitoids from the Taihua Complex in the North China Craton. Zircon U-Pb dating reveals two episodes of granitoid magmatisms at ca. 2.65 Ga and ca. 2.55–2.50 Ga. Sodic TTGs at ca. 2.65 Ga are characterized by low K2O/Na2O ratios of < 0.6, high Sr/Y and (La/Yb)N ratios, positive Eu anomalies, positive zircon ɛHf(t) values of 0.9 to 5.2, and zircon δ18O values of 6.3–6.5‰, suggesting that they were derived from partial melting of the thickened oceanic crust. In contrast, K-rich granitoids at ca. 2.65 Ga have higher K2O/Na2O ratios, low Sr/Y and (La/Yb)N ratios, negative Eu anomalies, mostly positive zircon ɛHf(t) values of −0.4 to 8.0, and high zircon δ18O values of 7.0–8.4‰. The K-rich granitoids at 2.55–2.50 Ga show similarly high Sr/Y and (La/Yb)N ratios to the sodic TTGs at ca. 2.65 Ga. All these geochemical evidences suggest that these Neoarchean K-rich granitoids were derived from partial melting of the low-T seawater-hydrothermally altered oceanic basalt at different depths. Therefore, the sodic TTG and K-rich granitoids are linked to each other through both similarity and difference in their source compositions rather than magmatic processes such as partial melting and crystal fractionation. On the other hand, high-K granites at ca. 2.55–2.50 Ga are highly siliceous, have very high K2O/Na2O ratios of 4.0–7.9, and contain many relict zircons with U-Pb ages of ca. 3.49–2.68 Ga, indicating their formation by reworking of the pre-existing continental crust. Using a filtered global geochemical database, we find that the Neoarchean is the most important period for the growth of both sodic TTGs and K-rich granitoids. The Sr/Y and (La/Yb)N ratios of these granitoids also reach the maximum during this period, suggesting global-scale thickening of the oceanic crust caused by warm plate convergence. Once the orogenic lithospheric mantle was foundered for thinning, the thickened oceanic crust experienced partial melting at different depths. This gave rise to the different types of granitoid for growth of the continental crust along convergent plate boundaries. Therefore, partial melting of the thickened crust at different depths is a viable mechanism for the compositional transition of continental crust in the Neoarchean.

Identification of ca. 520 Ma mid-ocean-ridge–type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite

AbstractAn ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)–like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621–0.12008 and very low 187Re/188Os values of 0.031–0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2–4.1) and relatively high (87Sr/86Sr)i ratios (0.7046–0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09–0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6–1.65 wt%), Cr (0.22–6.26 ppm), and Ni (0.77–4.74 ppm) compositions and low Mg# (22.4–31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰–5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy–type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic.

Early Paleozoic and Late Mesozoic crustal reworking of the South China Block: Insights from Early Silurian biotite granodiorites and Late Jurassic biotite granites in the Guangzhou area of the south-east Wuyi-Yunkai orogeny

The South China Block (SCB) is considered to have undergone extensive reworking of continental crust. However, the processes and mechanisms of this reworking remain uncertain. Here, we investigated Early Silurian (442 Ma) biotite granodiorites and Late Jurassic (164–159 Ma) biotite granites in the Guangzhou area of SE Wuyi-Yunkai orogen (WYO), South China. The biotite granodiorites and biotite granites show major and trace element characteristics similar with crustal-derived melts. They have enriched whole-rock Sr–Nd ((87Sr/86Sr)i = 0.7108–0.7210; εNd(t) = − 11.7 to −7.6) and zircon Hf (εHf(t) = −17.2 to −1.7) isotopic compositions. Early Silurian biotite granodiorites have relatively homogeneous zircon δ18O values (8.6‰–9.3‰) while Late Jurassic biotite granites show a wide range of zircon δ18O values (7.4‰–10.6‰). We suggest that the Early Silurian biotite granodiorites were formed by partial melting of amphibolites and that the Late Jurassic biotite granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate juvenile crustal rocks. Combining our results with those of previous studies in the SCB, we suggest that Early Paleozoic intracontinental reworking of the SCB can be divided into three stages: crust double-thickening, orogenic collapse, and post-orogenic lithospheric extension. During the Late Ordovician to Early Silurian, Guangzhou area of the SE WYO underwent reworking mainly of middle–lower-crustal rocks in response to crust double-thickening. In addition, during the Late Jurassic, the area underwent reworking mainly of middle–lower-crustal metasedimentary rocks, which was induced by heating from mantle-derived mafic magmas.

Late Eocene Two-Pyroxene Trachydacites from the Southern Qiangtang Terrane, Central Tibetan Plateau: High-Temperature Melting of Overthickened and Dehydrated Lower Crust

AbstractOrthopyroxene-bearing granitic rock (e.g. charnockite) is relatively rare but provides an excellent opportunity to probe the thermal and tectonic evolution of deep orogenic crust because of its distinct mineral assemblage. Here we present petrological, mineralogical, elemental, and Sr–Nd–Hf–O isotopic data for late Eocene (ca. 36 Ma; zircon U–Pb ages) volcanic rocks exposed in the Ejiu region in the southern Qiangtang Terrane to investigate how the central Tibetan crust evolved to its modern thickness and thermal state. The Ejiu volcanic rocks (EVRs) are trachydacites with anhydrous mineral assemblages (i.e. two pyroxenes, sanidine, plagioclase, and ilmenite, without amphibole and biotite) and geochemical characteristics (e.g. high P2O5 and TiO2) that resemble those of charnockite-type magmatic rocks. Mineral and whole-rock thermometry and hygrometry suggests that the parent magma crystallized under hot (~1000°C) and dry (H2O &amp;lt; 2 wt.%) condition. Besides, the EVRs display adakitic affinities according to their high SiO2 and Al2O3 contents, high Sr/Y, La/Yb, and Gd/Yb ratios, and low Y and Yb contents, without marked negative Eu anomalies. The calculated melts in equilibrium with pyroxenes also display adakitic compositions (e.g. high Sr/Y and La/Yb ratios), indicating that the adakitic compositions of the EVRs did not result from late-stage magmatic evolution. In addition, the melts of the EVRs were saturated in TiO2, as inferred from the high TiO2 contents of these rocks and the presence of ilmenite. An integrated analysis of the geochemical, petrological, and mineralogical data suggests that the EVRs were neither evolutional products nor partial melts of hydrous mafic materials at normal crustal pressures, but were formed by fusion of an eclogitized mafic protolith with residue containing garnet and rutile but lacking amphibole and plagioclase. The whole-rock Sr–Nd and zircon Hf isotope compositions of the EVRs [(87Sr/86Sr)i = 0.7053 to 0.7066; εNd(t) = −1.40 to −0.99; zircon εHf(t) = +1.08 to +5.31] indicate that the parental protolith was relatively juvenile in nature, but also contained some supracrustal materials given the high zircon δ18O values [zircon δ18O = +8.21‰ to +11.00‰]. The above arguments lead us to propose that of partial melting of a previously dehydrated—but chemically undepleted—mafic lower continental crust at high pressure (&amp;gt;1.5 GPa) and high temperature (&amp;gt;1000°C) generated the EVRs. Based on a synthesis of independent geological and geophysical data, we further suggest that the southern Qiangtang Terrane crust of the central Tibetan Plateau was thick, dry, and elevated during the Late Cretaceous to early Eocene time, and that it became abnormally hot owing to the ascending asthenosphere after lithospheric foundering during the middle Eocene.

Oligocene Leucogranites of the Gangdese Batholith, Southern Tibet: Fractional Crystallization of Felsic Melts from Juvenile Lower Crust

Abstract The formation of high-silica leucogranites and related detailed evolution of granitic crystal mush in southern Tibet bear significant information on the tectonic and magmatic evolution of the Asian–Indian continent–continent collisional zone. Here, we first report an integrated investigation of the Oligocene (ca. 30 Ma) leucogranites and main body granitoids exposed within the Gangdese Batholith in Gyaca County, southern Tibet. The Gyaca leucogranites can be divided into two groups in terms of field observation (gradational contacts vs. dykes), petrography and geochemistry (plagioclase fractionation vs. accumulation trends), and are characterized by their formation through different stages of evolution from (early) fractionation to (later) accumulation (up to 30%) of plagioclase for Group I and II leucogranites, respectively. Overall, the two groups of leucogranites are both characterized by high SiO2 (71.4–75.7 wt.%), Na2O/K2O (&amp;gt;1.0) and Sr/Y (58–629), and low Rb/Sr (0.02–0.27). The Gyaca main body granitoids resemble the published Gangdese granitoids and most of them also have high Na2O/K2O and they generally show varied SiO2 (64.4–76.1 wt.%) and other major and trace elements. The Gyaca leucogranites and main body granitoids have very similar Sr–Nd–Hf–O isotopic compositions, with initial 87Sr/86Sr ratios from 0.7054 to 0.7064, ɛNd(t) values from −3.40 to +0.65, zircon ɛHf(t) values from −3.0 to +5.2, and zircon δ18O values from 5.59‰ to 6.84‰. These leucogranites and main body granitoids are interpreted to have a same magma source and can be formed by water-present melting of garnet amphibolites from juvenile lower crust plus minor materials from felsic ancient crust beneath the southern Lhasa Terrane. The genetic association of the Oligocene Gyaca leucogranites and main body granitoids and their geochemical diversity reveal an evolved magmatic system. The two types of leucogranites are probably formed by crystal-melt fractionation and plagioclase accumulation at different stages during the solidification of the magma chamber. The discovery of ca. 30 Ma leucogranites in the Gangdese Batholith, in combination with the Oligocene–Miocene high Sr/Y Gangdese granitoids and coeval Himalayan leucogranites (HLGs), indicate the coexistence of diverse granitic rocks in southern Tibet may principally result from partial melting of local deep crustal materials. A new petrogenetic model which illustrates the evolution and multiple emplacements of crystal mush in a granitic magma chamber is proposed for the formation and magmatic evolution of leucogranites in melts from juvenile lower crust in Tibet.

Felsic dyke swarms from central Inner Mongolian: Implications for the Triassic tectonic setting in the southeast Central Asian Orogenic Belt

This work presents the whole-rock elemental geochemistry and Sr − Nd isotopes and zircon Hf − O isotopes for dikes from Tagaolemiao and Tuilamamiao, the southern margin of the southeast Central Asian Orogenic Belt (SE CAOB), to explore their origin and dynamic setting. Zircon U − Pb dating reveals that the Tagaolemiao dikes emplaced at ca. 242 Ma and the Tuilamamiao dikes emplaced at ca. 250 Ma, which postdated the Late Permian magmatic lull and convergent events in the SE CAOB. Dikes from the two areas are I-type rhyolitic rocks and exhibit high SiO2, low MgO, and low Cr, Co, and Ni contents. The Tagaolemiao dikes have depleted zircon Hf (εHf(t) = +3.75 to +9.82), whole-rock Sr − Nd (ISr(t) = 0.704340–0.704855; εNd(t) = +1.3 to +2.1) isotopic composition and variable zircon δ18O values (4.88–7.29‰), and were derived from the mixing melts of the juvenile crust and depleted mantle. In contrast, the dikes from Tuilamamiao show negative zircon εHf(t) (−2.33 to −10.85) and whole-rock εNd(t) (−6.7 to −15.5) values, low ISr(t) (0.706640–0.708208) and high δ18O (6.38–7.61‰) values, which originated from the partial melting of the old continental lower crust. The Triassic dyke swarms from this work indicate the SE CAOB switched into an intra-continental extension tectonic setting after the Paleozoic long-term accretion-convergence.

Late Mesozoic diorites of the middle Gangdese magmatic belt of southern Tibet: New insights from SHRIMP U-Pb dating and Sr-Nd-Hf-O isotopes

The Gangdese magmatic belt, which formed by Mesozoic northward subduction of Neo-Tethyan oceanic lithosphere and Early Mesozoic India-Asia collision, is an excellent natural laboratory for study of subducting slabs, crustal growth, the reworking of convergent continental margins and interaction of crust-mantle in general. Although many studies have examined this belt, the petrogenesis of arc magmas and the geodynamic architecture of the subducting slab are still contentious. Discoveries of igneous rock exposures are important in reconstructing the tectonic regime and evolution of southern Tibet, and provide new constraints on unresolved issues. Here we report two newly identified intermediate rocks, shedding new light on both the formation of arc-type magmas, and the tectonic evolution of the Neo-Tethys during the Late Mesozoic. Zircon SHRIMP II U-Pb dating indicates that a Late Cretaceous gabbro diorite pluton formed at 90.3 ± 1.4 Ma, and a Late Jurassic diorite porphyry pluton crystallized at 157.9 ± 2.3 Ma. The former has positive zircon εHf(t) values of 8.03–10.86 and whole-rock εNd(t) values of 4.11–4.37, with δ18Ozircon values of 5.84‰–6.70‰, slightly above the values of mantle zircon (5.3‰ ± 0.6‰). The latter also has positive zircon εHf(t) (10.29–12.77) and whole-rock εNd(t) (5.54–5.78) values, with normal mantle-like zircon δ18O values of 5.19‰ to 6.00‰. All samples of both plutons display arc-type signatures characterized by enrichment in light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti and P). Based on multiple isotopic (Sr–Nd–Hf–O) and whole-rock geochemical characteristics, we conclude that the Late Cretaceous gabbro diorite was derived from partial melting of depleted mantle that had been metasomatized by subducted-sediment-derived melts (~5 vol%). The Late Jurassic diorite porphyry is inferred to be a product of fractional crystallization of primary andesitic magmas with limited contents of Fe-Ti oxide and amphibole, and minor plagioclase accumulation. Pelagic sediments and enriched materials were a negligible component of the precursor magmas of the diorite porphyry, relative to the gabbro diorite pluton. Considering the temporospatial distribution of igneous rocks over the whole Gangdese magmatic belt, we suggest that Late Jurassic magmatism was related to normal angle subduction of the Yarlung Tsangpo Neo-Tethyan oceanic lithosphere, and early Late Cretaceous magmatism was triggered by rollback of the Neo-Tethyan oceanic slab.

Early hydrosphere-rock interactions and intra-crustal recycling recorded by remarkably high-δ18O Mesoarchean granitoids in the Sulu orogenic belt, eastern China

When, why and how global plate tectonics were initiated, as well as secular changes of plate tectonics over time are fundamental issues in the earth sciences. In this study, we present a combined U-Pb, Hf and O isotope dataset for complex zircons from the high-grade granitic (TTG) gneisses of the Haiyangsuo complex in the Sulu orogenic belt, eastern China. This combined dataset reveals that the granitoids formed by at least ca. 3.22, 2.81, 2.70 and 2.58 Ga and recorded multiple metamorphic thermal events that occurred at ca. 3.05, 2.60–2.50 and 1.93–1.85 Ga. The 3.22 and 2.58 Ga granitoids have negative εHf(t) values (averages of −2.08 and −10.81, respectively) and were derived from remelting of evolved Eoarchean crust, in combination of 3.47 Ga inherited zircon, reflecting occurrence of matured felsic continent crust in the Paleoarchean, whereas the 2.81 and 2.70 Ga granitoids show positive εHf(t) values (averages of + 4.32 and + 2.23, respectively) and formed by remelting of juvenile crust derived from depleted mantle at ca. 3.0 Ga. The 3.22, 2.70 and 2.58 Ga granitoids have mantle-like or slightly enriched zircon δ18O values (averages of 5.53 ± 0.15, 5.41 ± 0.18 and 6.10 ± 0.22‰, respectively). However, the 2.81 Ga granitoid displays remarkably high zircon δ18O values ranging from 9.65 to 7.02‰ with an average of 8.48 ± 0.29‰ (1 SD), this demonstrates occurrences of early hydrosphere-rock interactions and intra-crustal recycling in the Mesoarchean. Synthetically, here we propose the following tectonic model for the formation of the high-δ18O Mesoarchean granitoids: (1) Generation of voluminous high-density lower crust along with the formation of early felsic continent dominated by TTGs; (2) gravity-driven subduction of denser oceanic crust relative to continental crust induced by delamination of high-density lower crust together with lithospheric mantle; and (3) subsequently partial melting of altered high-δ18O hydrated oceanic crust caused by delamination-induced hot asthenospheric mantle upwelling to form the high-δ18O Mesoarchean granitoid. Furthermore, we argue that gravity-driven subduction occurred in the Archean, and subduction may be responsible for the trend towards increasing δ18O values within zircons since the Mesoarchean.

The largest plagiogranite on Earth formed by re-melting of juvenile proto-continental crust

The growth of continental crust through melt extraction from the mantle is a critical component of the chemical evolution of the Earth and the development of plate tectonics. However, the mechanisms involved remain debated. Here, we conduct petrological and geochemical analyses on a large (up to 5000 km2) granitoid body in the Arabian-Nubian shield near El-Shadli, Egypt. We identify these rocks as the largest known plagiogranitic complex on Earth, which shares characteristics such as low potassium, high sodium and flat rare earth element chondrite-normalized patterns with spatially associated gabbroic rocks. The hafnium isotopic compositions of zircon indicate a juvenile source for the magma. However, low zircon δ18O values suggest interaction with hydrothermal fluids. We propose that the El-Shadli plagiogranites were produced by extensive partial melting of juvenile, previously accreted oceanic crust and that this previously overlooked mechanism for the formation of plagiogranite is also responsible for the transformation of juvenile crust into a chemically stratified continental crust.

Origin of syn-collisional granitoids in the Gangdese orogen: Reworking of the juvenile arc crust and the ancient continental crust

AbstractGranitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantle-derived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive εNd(t) values indicate juvenile crust whereas negative εNd(t) values indicate ancient crust. Positive εNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82–64.41 wt%, and it was emplaced at 50–48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93–68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, εHf(t) values of 6.5−10.5, zircon εHf(t) values of 1.6−12.1, and zircon δ18O values of 5.28–6.26‰. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070−0.7085, negative εNd(t) values of −5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, and relatively lower zircon εHf(t) values of −2.8–8.0 and slightly higher zircon δ18O values of 6.25–6.68‰. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone.

Crustal architecture studies in the Iranian Cadomian arc: Insights into source, timing and metallogeny

The Jalal Abad magmatic rocks, situated at the southern edge of the Saghand-Bafgh-Zarand district, include a thick pile of Cadomian extrusive and pyroclastic units intruded by younger granitoid stocks. New zircon U–Pb ages show eruptions at ∼552 Ma, followed by emplacement of granodiorite at ∼537 Ma. The Jalal Abad magmatic rocks have typical high-K and shoshonitic signatures, and are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSE). Zircon ɛHf(t) from the Jalal Abad magmatic rocks ranges from +3.9 to −3.9 for volcanic rocks and −1.2 to +8.1 for granodiorite. Zircon δ18O values for the Jalal Abad are variable from +5.1 to +8.8‰, progressively higher than those of mantle-derived melts. The whole-rock ɛNd(t) values range between −7.7 to −7.4 for granodiorite, −4.6 to −3 for volcanic rocks and −6.2 to −8.2 for ignimbrites/tuff. The whole-rock Nd and zircon Hf crustal model ages (TDMC) for the Jalal Abad magmatic rocks range between 0.8 and 2.3 Ga. All of the Jalal Abad magmatic rocks have quite similar trace element patterns, and slightly different whole-rock Nd and zircon Hf isotopic composition, indicating the involvement of the thick continental crust during the formation of these rocks. Modeling of zircon Hf–O data, bulk-rock trace elements, and Sr–Nd isotopes suggest the magmas were generated by interaction of mantle–derived melts with thick continental crust through assimilation/fractional crystallization (AFC) processes. However, crustal architecture studies in the Iranian Cadomian arcs show that AFC processes were more important during the Mesoarchean–Early Neoproterozoic (3000–1000 Ma), whereas juvenile magmas became increasingly important to the Cadomian (600–500 Ma) magmatism. Early Cambrian intrusive magmas seemingly intruded sedimentary sequences in the study region and provided magmatic constituents and a heat source for hydrothermal processes and mineralization.

Zircon Hf[sbnd]O isotope and magma oxidation state evidence for the origin of Early Cretaceous granitoids and porphyry Mo mineralization in the Tongbai-Hong'an-Dabie orogens, Eastern China

The Tongbai-Hong'an-Dabie orogens constitute the east segment of the Qinling-Dabie orogenic belt, which host several large molybdenum deposits, with a proven reserve of over 8 million tons metal Mo. In this study, we present an integrated study of in situ zircon UPb dating and HfO isotopes, in combination with magma oxidation state estimation for the Early Cretaceous granitoids in the Tongbai-Hong'an-Dabie regions. These granitoids can be divided into three groups on the basis of zircon UPb ages, including early-stage ore-related granitoids from Mushan, Xiaofan and Wangwan deposits (143–136 Ma), ore-barren granites (136–130 Ma), and late-stage ore-related granitoids from Qian'echong, Bao'anzhai, Tangjiaping and Shapinggou deposits (130–110 Ma). Early-stage ore-barren granitoids have the lowest average εHf(t) and δ18O values of −27.1 ± 1.6 and 4.25 ± 0.63‰ as well as the oldest two-stage Hf model ages (TDM2) of ~2.8 Ga, which are roughly comparable with the inferred Dabie lower crust, indicating that they were mainly derived from the partial melting of the lower crust. Ore-forming granitoids in two stages show similar two-stage Hf model ages from 2.2 to 2.3 Ga which differ from any inferred layer of crusts, while they exhibit different zircon HfO isotope and oxygen fugacity evolutionary trends. Granitoids display decreasing average εHf(t) and δ18O values from −17.1 ± 1.9 to −27.1 ± 1.6, and 5.71 ± 0.50‰ to 4.25 ± 0.63‰, and increasing average logƒO2 values from −17.4 ± 4.5 to −6.7 ± 3.8 with a decrease in age from 143 to 130 Ma. In addition, relict zircon cores with Paleoproterozic and Archean ages were observed with εHf(t) and δ18O values analogous to those of the Kongling Complex. These characteristics suggest that the early-stage granitoids were possibly produced by the remelting of the subducted lower crust mixed with the Kongling Complex, with the involvement of reducing Mo-rich black shales in the Yangtze Block that seemingly account for the lower oxygen fugacity of ore-forming granitoids with younger two-stage Hf model ages and lead to the formation of those early-stage Mo deposits. As the increasing contributions of the lower crust in the magma source, the metallogenic gap developed from 136 to 130 Ma in the Tongbai-Hong'an-Dabie orogens. Zircon HfO isotopic compositions of ore-related granitoids increase simultaneously up to −14.2 ± 4.3 and 5.82 ± 0.33‰ respectively at a lower slope after 130 Ma. Part of zircon δ18O values is slightly higher than 6.5‰, implying the involvement of supercrustal materials. Meanwhile, magma oxygen fugacity exhibits an abrupt drop at ca. 130 Ma in the Qian'echong deposit with the lowest average logƒO2 value (−19.3 ± 4.5) and Ce4+/Ce3+ ratios (1–138), then rises quickly up to a maximum average logƒO2 value of −7.3 ± 2.3. As such, these granitoids were likely generated by mixing of a blended crustal magma, derived from the lower crust and supercrustal materials, and the mantle-derived component which also contributes to the late-stage Mo mineralization. We propose that not only the highly oxidized magma but also the continuous change of oxygen fugacity in the deep magma chamber may have positive effects on the formation of porphyry Mo deposits. Given the significant geochemical change at ca. 130 Ma instead of 125 Ma, we agree that the intensive magmatism and mineralization in the Tongbai-Hong'an-Dabie orogens are closely associated with the delamination of the thickened lower crust of the Yangtze Block, which caused the asthenosphere upwelling that provided the heat source and led to partial melting of overlaying lower crust.

Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.

Geochronology and geochemistry of bimodal volcanic rocks from the western Jiangnan Orogenic Belt: Petrogenesis, source nature and tectonic implication

This study presents zircon SIMS U-Pb ages, whole-rock geochemistry and Nd-Hf-O isotope data of Neoproterozoic meta-volcanic rocks discovered from the Longsheng area of the western Jiangnan Orogenic Belt. Zircon SIMS U-Pb dating reveals that these rocks were emplaced during 820–806 Ma. The basaltic rocks are enriched in light rare earth element (LREE; [La/Yb]N = 3.38–8.40), slight negative Nb and Ta anomalies, and positive εNd(t) (+4.39 to +4.75) values. In addition, they possess relatively high zircon δ18O values of 7.24–10.78‰. These features, together with the SiO2 contents (41.26–51.76 wt%), and high Mg# of 54–68, Ti/V and Ti/Y ratios of 49–68 and 415–570, suggest that they could derived from a depleted mantle source metasomatized by subduction-related melts. The dacitic rocks are high-K calc-alkaline and exhibit the characteristics of A1-type granite: typical enrichment in alkaline element, LREE ([La/Yb]N = 5.74–15.16) and large ion lithophile element (LILE), and depletion in Nb, Ta, Sr and Ti. Coupled with their different and negative εNd(t) (−8.75 to −9.16) compared with the basaltic rocks. This indicates that the dacitic rocks were probably generated by partial melting of crust as a result of magma underplating. The geochemical and Nd-Hf-O isotopic characteristics, together with regional geology, imply that the formation of the Longsheng bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. These new data, combined with previous results, indicate a preliminary scenario for the tectonic evolution of the Jiangnan Orogenic Belt and the southeastern margin of the Yangtze Block, which can provide significant insight into the understanding the evolution of South China.

Triassic magmatism and tectonic setting of the eastern Tianshan, NW China: Constraints from the Weiya intrusive complex

The Weiya intrusive complex, located in the eastern part of the middle Tianshan terrane can provide crucial constraints on the tectonic evolution of the eastern Tianshan during the Triassic period. Here, we perform detailed zircon U–Pb chronological and Hf–O isotopic as well as whole-rock geochemical and Sr–Nd isotopic analyses on the hornblende gabbro, syenogranite, and granodiorite of the Weiya intrusive complex. The hornblende gabbro with a zircon U–Pb age of 241.6 ± 1.0 Ma is an alkaline gabbro and characterized by enrichments in large iron lithophile elements with depletions of Nb and Ta. It has uniform initial 87Sr/86Sr ratios (0.707173–0.707211) and positive εNd(t) values (+0.58 to +0.61) with zircon δ18O and εHf(t) values ranging from 6.0‰ to 7.6‰ and from +6.1 to +10.5, respectively. Geochemical modelling indicates that the mantle source of the hornblende gabbro was metasomatized by ~10% oceanic sediments associated with previous subduction events. The syenogranite (238.1 ± 2.0 Ma), emplaced nearly simultaneously with the hornblende gabbro, is an A-type granite with slightly metaluminous to peraluminous characteristics (A/CNK = 0.99–1.02), and it has high initial 87Sr/86Sr ratios (0.711740–0.713603) and negative εNd(t) values (−0.94 to −0.76). By contrast, the granodiorite (230.8 ± 0.9 Ma), which contains minor late Carboniferous dioritic enclaves (314.4 ± 3.3 Ma) belongs to the high-K calc-alkaline I-type granite with metaluminous characteristics (A/CNK = 0.95–0.98), and it has relatively low initial 87Sr/86Sr ratios (0.706731–0.707081) and negative εNd(t) values (−1.11 to −1.33). The δ18O and εHf(t) values of zircons from the syenogranite vary from 6.1‰ to 7.5‰ and from +2.8 to +5.3, respectively, while those of zircons from the granodiorite vary from 7.6‰ to 9.0‰ and from −8.1 to −3.4, respectively. These geochemical and isotopic data imply that the syenogranite formed by partial melting of Neoproterozoic calc-alkaline granitoids at high temperatures, whereas the granodiorite was derived from partial melts of (meta)-igneous rocks with additions of supracrustal sedimentary materials. Taking account into all available geological data, we suggest that the eastern Tianshan was in an intra-continental setting during the Triassic period, and the Weiya intrusive complex was generated related to lithospheric extension and thinning.

Effect of water on δ18O in zircon

The mineral zircon is stable across a wide range of temperature and pressure conditions, and the most widely used geochronometer in felsic rocks. The oxygen isotope ratio recorded by zircon crystals is a sensitive tracer for crustal recycling in magmatic rocks and is a commonly used tool to address zircon petrogenetic questions (e.g. in studies on the evolution of geodynamics and continents). However, secondary processes (i.e. water uptake facilitated by radiation damage) can alter the oxygen isotopic composition of zircon. We present in-situ oxygen isotopic and 16O1H/16O data of natural zircon samples in combination with zircon trace element concentrations, and parameters that quantify crystal damage (i.e. Raman spectral parameters and alpha dose). In agreement with previous studies, our results demonstrate that the uptake of water into the zircon crystal structure strongly influences its recorded oxygen isotope ratio. Two distinct secondary processes are recorded which shift zircon δ18O values towards isotopically heavier and lighter values, respectively. Lower δ18O values associated with broadening of the zircon Raman bands and increasing 16O1H/16O are the result of a local charge-balance process during the interaction of radiation-damaged zircon with meteoric water. In contrast, an equilibrium fractionation process between zircon and percolating water leads to higher δ18O values with increasing 16O1H/16O. The disturbance of the oxygen isotopic system in zircon can be assessed through monitoring zircon 16O1H/16O; a parameter that is highly sensitive to the influx of water into radiation-damaged areas of the zircon grain, providing a powerful tool to access the validity of primary δ18O signatures.