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Abstract: In order to characterize the recycled lower continental crust (LCC) in the Dabie orogen, 17 Early Cretaceous low-Mg adakitic (LMA) and 9 normal (non-adakitic) granitoids have been investigated for Srph name="sbnd" />Pb isotopes. Combined with literature data, LMA have low ƐNd(t) (−27.8 to −14.7) and 206Pb/204Pb(i) (15.69–17.16) and low to moderately high 87Sr/86Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield isotope ratios similar to adakitic rocks, except a few with 87Sr/86Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a 208Pb/204Pb(i)– 206Pb/204Pb(i) diagram. For a given 206Pb/204Pb(i), the 208Pb/204Pb(i) or ∆8/4 (152–217) of Dabie LMA are close to the majority of UHP gneisses and the Neoproterozoic mafic rocks from the northern margin of the South China Block (SCB), but significantly higher than adakitic rocks from the North China Block (∆8/4<150). Considering the commonly present Neoproterozoic inherited zircons, we suggest that the LMA in the Dabie orogen are derived from a thickened LCC that could be dominantly composed of ancient SCB lower crust. The Srph name="sbnd" />Pb isotopic composition of LMA are similar to those of Post-Collisional Mafic Igneous rocks (PCMI) from the Dabie orogen, but different to exhumed UHP rocks. The Srph name="sbnd" />Pb isotopic system of the mantle source of the PCMI could be enriched in components dominantly from delaminated LCC of the Dabie orogen instead of deeply subducted continental crust, which is in contrast to O- and C-isotope data by [Zhao, Z.F., Zheng, Y.F., Wei, C.S., Wu, Y.B., Chen, F.K., and Jahn, B.M., 2005. Zircon Ulement and C geochemistry of post-collisional mafic–ultramafic rocks from the Dabie orogen in east-central China. Lithos 83(1–2), 1–28; Dai, L.Q., Zhao, Z.F., Zheng, Y.F., Li, Q.L., Yang, Y.H., and Dai, M.N., 2011. Zircon Hf evidence for crust–mantle interaction during continental deep subduction. Earth and Planetary Science Letters 308, 229–244]. Combining the Srph name="sbnd" />Pbce:glyph name="sbnd" />C isotopic data, two types of commonly present magma components have been identified in the PCMI, reflecting crust–mantle interaction during the Triassic subduction and the Early Cretaceous orogen collapse, respectively. [Copyright &y& Elsevier]

Abstract: We present here a combined study of Early Cretaceous granodiorites, granites and mafic rocks from the Bengbu area on south-eastern margin of the North China Craton (NCC), in order to provide insights into the thinning mechanism of lithosphere on the cratonic margins. Zircon U–Pb and Ar–Ar dating reveals an adakitic–basaltic–granitic magma sequence with the adakitic intrusions (granodiorites; 123–115Ma) formed prior to the granites and mafic dykes (both ca.112Ma). The granodiorites are geochemically similar to low-Mg# adakitic rocks and isotopically have enriched Sr–Nd–Hf compositions, low radiogenic Pb (206Pb/204Pbi =16.524–17.017), and mantle-like δ18Ozircon (5.8±0.5‰). They were originated from the thickened lower continental crust (LCC) of the NCC. In contrast, the granites have low Sr/Y (<20), La/YbN, Dy/YbN, and prominent negative Eu anomalies, indicating their origination at the shallow middle-lower crustal levels. The mafic dykes are enriched in LREE relative to HREE, and exhibit negative anomalies of Nb, Ta, Ti and positive anomalies of Pb. They have low Nb/U, Ce/Pb, high La/Nb, Ba/Nb, enriched Sr–Nd isotopic compositions and low radiogenic Pb (206Pb/204Pbi =16.789 to 17.262), indicating LCC materials involved into the mantle sources. Modeling suggests derivation of the mafic dykes from the depleted upper mantle hybridized by 10–15% recycled LCC. The contrasting chemical characteristics of adakitic rocks and granites imply that recycling of the LCC was likely achieved through rapid foundering at 115–112Ma, which resulted in thinning of the overlying crust and hybridization of the underlying mantle with closely coeval generation of granitic and basaltic magmas. Anatexis of the pre-existing thickened LCC at 123–115Ma that attenuated the LCC itself and left the residues denser as a result of felsic (adakitic) melt extraction is proposed to have resulted in gravitational instability and foundering of the lithosphere. Such a temporal sequence of magma generation and processes of lithospheric thinning also occurred on north margin of the NCC and are quite similar to the mountain-root removal beneath the Dabie Orogen in central China. This manifests that previous melting of the LCC is perhaps an important trigger responsible for lithospheric foundering in orogenic belts and cartonic margins, where the lithosphere underwent pre-thickening by continental collisions and had stabilized over a long time. [Copyright &y& Elsevier]

Abstract: Post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone show evidence for derivation from a mantle source that was overprinted by partial melts derived from subducted crust of the South China Block. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and depletion of high field strength elements (HFSE, e.g., Nb, Ti). The Zhujiapu intrusion developed in the North Dabie zone has moderately enriched 87Sr/86Sr i (0.70605–0.70827), negative ε Nd(T) values (−2.3 to −18.8), and unradiogenic Pb isotopes. Such typical “continental” geochemical features do not result from crustal contamination or magma mixing, but reflect the properties of the mantle source. Because the trace element and Sr–Nd isotopic compositions of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone are similar to those of the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block, it is unclear whether the “continental” features are due to the contribution of lithospheric mantle and lower crust from the North China Block or deeply subducted materials from the South China Block. However, for a given 206Pb/204Pb i or Δ7/4, the 208Pb/204Pb i or Δ8/4 (159–180) of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone is close to the Mesozoic granitoids, eclogites, and gray gneisses from the Dabie–Sulu orogen, but significantly higher than the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block (Δ8/4<160). This indicates that the subducted slabs of the South China Block experienced a higher time-integrated Th/U but similar U/Pb as the deep lithosphere of the North China Block. We therefore propose that the mantle source of the post-collisional mafic–ultramafic intrusive rocks from the Dabie–Sulu orogen result from the depleted upper mantle metasomatised by melts from recycled deeply subducted crust. Due to the continent–continent collision between the South China Block and North China Block in the late Triassic, the lithosphere of the Dabie orogen was over-thickened resulting in eclogitic metamorphism in the mafic lower crust, producing a gravitational instability. The subducted eclogitic lower crust foundered and was recycled into the upper mantle beneath the orogen, resulting in mountain root removal, less mafic composition of unusually evolved continental crust, and upper mantle heterogeneity. [Copyright &y& Elsevier]

Abstract: The petrogenesis of Early Cretaceous adakitic intrusions in the Lower Yangtze River belt (LYRB), central-eastern China, and their genetic association with Cu–Au mineralization have recently been debated. This study presented integrated in-situ zircon U–Pb–Hf–O isotopic and trace elemental data for the LYRB adakites, and a comparison with ore-barren adakites from the south Tan-Lu fault (STLF) adjacent to the LYRB. Magmatic zircons from these two series of intrusions have U–Pb ages of 145–132Ma and 136–132Ma respectively. The STLF zircons have δ18O ranging from 5.6 to 6.7‰ and εHf(t) from −28.8 to −16.4, plotted within the range of global lower crustal metabasaltic xenoliths, consistent with low-radiogenic Pb of the host adakitic rocks. In contrast, both Hf and O isotopic compositions of zircons from the LYRB are greatly variable with heavier δ18O (4.7 to 9.6‰) and higher εHf(t) values (−25.5 to +2.0) compared with the STLF series. The co-variations of Hf–O isotopes in the LYRB series reflect source heterogeneity as a result of mixing of basaltic oceanic crust with sediments (10–20%), consistent with high-radiogenic Pb and enriched Sr–Nd isotopic compositions of the host adakites. The high La, U and low Ti concentrations in the LYRB zircons also imply a volatile (perhaps, CO3 2−-rich, carbonatite-like) source. Combined with whole-rock geochemical data, the new results further suggest contrasting origins of the LYRB and STLF adakites from subducted oceanic crust and foundering lower continental crust, respectively. The LYRB zircons have much higher ratios of Ce4+/Ce3+ (avg.417) and Eu/Eu* (avg. 0.67) than the STLF zircons (avg. 84 and 0.44). This difference confirms that the ore-bearing adakitic magmas are more oxidized relative to the ore-barren ones. There is roughly a positive correlation between zircon Ce4+/Ce3+ and δ18O in the LYRB series, probably indicating that the elevated fO2 was related to components enriched in heavy oxygen isotopes. A possible candidate is sediments, carried by subducting slabs. The involvement of sediments may significantly promote oxidation of the resulting adakitic melts, a key factor for generation of Cu–Au mineralization, e.g., in the LYRB. This study also indicates that combined in-situ analysis of zircon REEs and δ18O could be a powerful tool to decipher the intrinsic links of fO2 with sediment components in subduction zones. [Copyright &y& Elsevier]

We present high-precision Ca isotopic data for 21 Cenozoic basaltic lavas from eastern China to evaluate the potential of Ca isotopes as a tool to trace the recycling of surface carbonates. These samples were derived from a variety of peridotite to pyroxenite mantle sources hybridized by different slab-derived agents. The δ44/42Ca of eastern China Cenozoic basaltic lavas ranges from 0.24 ± 0.03‰ (2SE, if not specified) to 0.39 ± 0.03‰, with an average of 0.31 ± 0.09‰ (2SD, N = 21), systematically lower than mid-ocean ridge basalts (MORB, 0.39 ± 0.07‰, 2SD, N = 30). No convincing correlation has been observed with δ26Mg and δ66Zn, indicating that metasomatism by recycled carbonates may not always yield Ca isotopically distinct mantle sources. δ44/42Ca shows correlations with parameters reflecting degree of partial melting (e.g., K 2 O, Nb, LREE, U, Th, Rb, and Sr) and residual garnet signature (i.e., (Dy/Yb) N). Nevertheless, modeling results suggest that Ca isotope fractionation during mantle and/or slab melting with the presence of garnet can partially account for the observed variable δ44/42Ca. Recycled carbonates with low δ44/42Ca were likely involved in the origins of the lightest lavas. Particularly, the nephelinitic lightest lavas also have the lowest δ26Mg, Ti/Ti⁎ and Hf/Hf⁎ among all the studied samples, reflecting a role of carbonatitic metasomatism in their sources. This study illustrates that a strongly light Ca isotope anomaly (e.g., δ44/42Ca ∼ 0.24‰) observed in basaltic lavas is likely associated with the recycling of carbonates into the mantle. • Cenozoic basaltic lavas from eastern China have δ44/42Ca variable from a MORB-like value (∼0.39‰) to ∼0.24‰. • The observed δ44/42Ca variation can be partially attributed to isotope fractionation during melting processes. • Involvement of light carbonates in the sources appears to be rather likely for the lightest lavas. [ABSTRACT FROM AUTHOR]

The chemical composition of the upper continental crust (UCC) dramatically changed from predominantly sodic to more potassic during the late Archean (3000–2500 Ma), although how this change occurred remains poorly constrained. The origins of Neoarchean potassic granites may provide clues for this crucial change, as their formation dominated the compositional transition of the UCC. In this study, we conducted high-precision Mg isotopic analyses of Neoarchean potassic granites (2558–2520 Ma) and tonalite–trondhjemite–granodiorites (TTGs; 2595–2574 Ma) from the North China Craton and statistical geochemistry in K 2 O/Na 2 O ratios of global Archean granitoids. Results show that these TTGs, with K 2 O/Na 2 O ratios of ~0.61 and δ26Mg values of −0.39‰ to −0.22‰, were generated from heterogeneous basaltic sources. In contrast, potassic granites yield two distinctive groups of δ26Mg values and can be subdivided into I- and S-types. The I-type granite has mantle-like δ26Mg values of −0.28‰ to −0.22‰ and weak enrichment in K 2 O (K 2 O/Na 2 O = 1.0–1.5), and was likely derived by partial melting of igneous sources. The S-type is characterized by much higher δ26Mg values of +0.60‰ to +0.91‰ and strong K 2 O enrichment (K 2 O/Na 2 O = 1.57–23.26), with geochemical characteristics suggesting derivation by partial melting of sediments. Based on the geochemical characteristics of the S-type potassic granites in this study, we sort the S-type granites out of 4066 Archean felsic igneous rocks worldwide. Their temporal distribution suggests that S-type potassic granites have appeared in the middle Archean (3500–3000 Ma) and became widespread in the late Archean (3000–2500 Ma), but were scarce in the early Archean (4000–3500 Ma). The onset of S-type granites indicates reworking of supracrustal rocks may have started since 3500 Ma. And widespread S-type potassic granites during 3000–2500 Ma play an important role for the high K 2 O/Na 2 O ratio of UCC at the end of Archean. A mass-balance calculation shows that 25.88 wt% I-type, 4.12 wt% S-type potassic granite and 70 wt% TTGs could increase the K 2 O/Na 2 O ratio of the UCC to ~0.83 which is the global average value for felsic rocks at the end-Archean. To increase K 2 O/Na 2 O of the Archean UCC, contributions made by I- and S-type potassic granites could probably reach 3:2, respectively. Consequently, the evolved crustal composition from predominantly sodic to potassic results from the synergy of re-working of igneous (unaltered) and sedimentary (weathered) rocks, by which the former contributed more quantitatively, while the latter contributed more efficiently. • Archean potassic granites hold the key to crustal change from sodic to potassic. • Mg isotopes identify Archean S-type potassic granites in the North China Craton. • The S-type potassic granites begin in the middle and spread in the late Archean. • Remelting of igneous and sedimentary rocks changes upper crustal composition. [ABSTRACT FROM AUTHOR]

To explore the mantle source properties and the magmatic evolution process for intraplate basaltic lavas, as well as to investigate the potential of Cr isotopes to trace recycled crustal materials, we present Cr isotope compositions for 72 well-characterized samples of Cenozoic intraplate basaltic lavas from eastern China. These basalts display a resolvable δ53Cr range of −0.24‰ to −0.03‰ (n = 72), comparable to MORB and OIB. With respect to the evolved lavas, their δ53Cr scatter can be interpreted as a result of fractional crystallization of predominant olivine (or/and clinopyroxene) with 1% to 3% spinel from heterogeneous magma sources under different oxygen fugacity conditions. Combined with Sr-Nd-Zn-Fe isotope data, the least evolved basalts with MgO of ∼10% display a large δ53Cr range, indicating the presence of different magma component end-members, including one low-Si and two high-Si components. Two high-Si enriched components have δ53Cr values lower than those of primitive N-MORB and OIB magmas, consistent with their derivations most likely from relatively enriched mantle pyroxenites. Notably, the low-Si nephelinitic component could not be a simple end-member as proposed before. Correlations of δ53Cr with ε Nd(t) -87Sr/86Sr (i) -Fe/Mn-Ti/Ti*-Hf/Hf*-δ66Zn-δ56Fe [Ti/Ti* = Ti N /(Nd N -0.055 × Sm N 0.333 × Gd N 0.722), Hf/Hf* = Hf N /(Sm N × Nd N)0.5. N represents the primitive mantle-normalized values] for nephelinites from both the North China Block and the South China Block suggest mixing between partial melts of carbonated peridotite and pyroxenite sources. The carbonated peridotite derived melt is characterized by depleted Sr Nd isotopes, low Fe/Mn, Ti/Ti* and Hf/Hf* ratios, and heavier than BSE's Cr-Fe-Zn isotope compositions. The highly oxidized environment induced by carbonatization effectively reduces Cr isotope fractionation between melts and residues during partial melting of the carbonated peridotite, resulting in the heavy Cr isotope feature of melts being inherited from the carbonated mantle source, consistent with the heavy Cr isotope compositions of carbonated mantle xenoliths obtained previously. Thus, this work highlights that Cr isotopes might act as a potential tracer for subducted sedimentary carbonates. • This is the first Cr isotope investigation of intraplate basaltic lavas. • Chromium isotopes document limited chromite crystallization. • Chromium isotopes might act as a potential tracer for mantle carbonatization. [ABSTRACT FROM AUTHOR]

Iron (Fe) isotope fractionation could occur during crustal melting, which may contribute to the isotopic variations of granites. Our current knowledge on Fe isotope fractionation during crustal melting remains rare. Thirteen leucosomes, 9 melanosomes and 1 amphibolitic schollen in 11 migmatites from the Dabie orogen, Central China were measured to investigate Fe isotope fractionation during crustal anatexis. The melanosomes and amphibolitic schollen yield δ 56 Fe values from 0.018 ± 0.031‰ to 0.152 ± 0.027‰ with an average of 0.106‰. The leucosomes have δ 56 Fe values from 0.107 ± 0.035‰ to 0.512 ± 0.028‰, variably higher than their coexisting melanosomes by 0.00 ~ 0.36‰. The δ 56 Fe of leucosomes and melanosomes, as well as apparent isotope fractionation (Δ 56 Fe L-M ) between them, do not correlate with the abundance of crystalline carbonate, loss on ignition, and Th/U, indicating the insignificant effect of fluid components. High δ 56 Fe values of three leucosomes (≥ 0.34‰), giving large Δ 56 Fe L-M from 0.22 to 0.36‰, were likely produced by feldspar accumulation, evidenced by their high Eu*, low FeOt/Al 2 O 3 and high plagioclase abundance up to 50 vol%. Capture of peritectic amphiboles in another leucosome may explain its low Δ 56 Fe L-M ~ 0‰. After screening out these samples, 7 leucosome – melanosome pairs yield identical Δ 56 Fe L-M averaging 0.093 ± 0.056‰ (2SD, N = 7) within analytical uncertainties. No correlation between Δ 56 Fe L-M and Mg#, FeOt/Al 2 O 3 or TiO 2 /FeOt rules out the possibility that the fractionation observed here is produced by fractional crystallization and/or sub-solidus isotope re-equilibrium between leucosomes and melanosomes. Therefore, we suggest these consistent Δ 56 Fe L-M should record equilibrium Fe isotope fractionation during crustal anatexis producing these migmatites. The leucosomes and melanosomes have comparable Fe 3 + /ΣFe ~ 0.40, and Δ 56 Fe L-M approximates 0.07‰ at Fe 3 + / ΣFe leucosome Fe 3 + / ΣFe menalosome = 1. Fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of Fe 3 + and Fe 2 + , but by the difference in coordination number of iron between granitic melts and residual mafic minerals. The observation here argues that Fe isotope fractionation during crustal partial melting should also contribute to the previously revealed δ 56 Fe variation in high silica granitic rocks. [ABSTRACT FROM AUTHOR]