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The stable isotope systems of Li and B are useful tools for the understanding of ocean island basalt genesis. We have applied both systems to a comprehensive set of well-characterised samples across the Azores islands in order to further evaluate the nature of the mantle source. These ocean islands represent the surface expression of a low-buoyancy mantle plume. The variability of Li and B concentrations and their isotopic compositions across the archipelago builds on recent radiogenic isotope studies from the islands to the west of the Mid-Atlantic Ridge (Flores and Corvo). The results for both systems yield some values that are typical for ocean island basalts; however, the variability of both δ7Li and δ11B observed in primitive lavas is most extreme on the western islands of Flores and Corvo (δ7Li = + 3.5 to + 8.2‰ and δ11B = − 3.5 to + 11.8‰). The large spread in isotopic composition is most likely due to contamination of the magmas during ascent through hydrothermally altered oceanic crust. Models of assimilation fractional crystallisation best explain the variability observed on each island and allow for robust estimates of the mantle source. The implication of these observations is that the interpretation of the radiogenic isotopes and trace elements may have to be treated with care if the stable isotopes record contamination by assimilation of hydrothermally altered material and that the use of Li and B as mantle source tracers may be obscured by shallow level processes. Further, it seems unlikely that the large range observed in Sr and Nd isotopes can reflect variable contributions from recycled low-T materials such as sediment or altered MORB, since there is no correlation between the radiogenic and stable isotope data.

The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma), respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma. We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.

We present a new high precision analytical method for the determination of Mg/Ca and Sr/Ca ratios in carbonates using an inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) with a 650-W cold plasma technique and a desolvation introduction system. Signal intensities are detected in pulse-counting mode and Mg/Ca and Sr/Ca ratios are calculated directly from intensity ratios of 24 Mg/ 43 Ca and 86 Sr/ 43 Ca using external matrix-matched standards for every 4–5 samples to correct for instrumental mass discrimination and low-frequency ratio drift. Significant matrix effect of Ca content on Mg/Ca determination (− 0.018 Mg/Ca (mmol/mol)/[Ca] (ppm)), can be overcome by diluting [Ca] to 6–8 ppm in the sample solution or using an empirical correction. The Sr/Ca ratio affects the Mg/Ca determination, with a factor of − 0.32% Mg/Ca per mmol/mol. This is mainly caused by the influence of doubly charged 86 Sr, which biases the intensity measurement of the 43 Ca + ion beam. This effect results in a trivial offset of less than 0.1% on Mg/Ca measurements for Quaternary foraminiferal and coral samples. The internal precision of our method ranges from 0.1 to 0.2%. Replicate measurements made on standards and samples show long-term external uncertainties (2σ) of Mg/Ca = 0.84% and Sr/Ca = 0.49%. The minimum sample size requirement is only 3.5 μg of carbonate. The application of this newly developed technique on the planktonic foraminifera Globigerinoides ruber from a core recovered in the southern South China Sea yields a glacial–interglacial difference in sea surface temperature (SST) of 3 °C. Three-year coral Sr/Ca data suggest that the seasonal SST ranged from 22.6–23.8 °C in winter to 26.9–27.9 °C in summer in Nanwan, south Taiwan, during 2000–2002. The coral-Sr/Ca inferred SSTs in 2002 match well with instrumental records, which demonstrates the validity of this ICP-QMS method.

Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb / 238U age of 127 ± 2 Ma and an 40Ar / 39Ar age on hornblende of 123.5 ± 0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon 206Pb / 238U age of 124 ± 3 Ma and a hornblende 40Ar / 39Ar age of 124.2 ± 0.4 Ma, indicating coeval crystallization of the mafic enclaves and host monzogranite. Whole rock 40Ar / 39Ar dating gives an emplacement age of 111.2 ± 0.1 Ma for the mafic dikes. The monzogranites have low MgO and Cr, high Na2O, and Sr–Nd–Hf isotopic data (87Sr / 86Sr > 0.7085, ɛNd(t) = − 20.5 and ɛHf(t) = − 22.5 to − 56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high ɛNd(t) and ɛHf(t) values indicates magma mixing and involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) and Cr (up to 1233 ppm), low TiO2 (1.11–1.24 wt.%) and total Fe2O3 (8.33–9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) and LREEs, and depletion in HFSE (e.g., Nb, Ta and Ti), together with initial 87Sr / 86Sr ratios of 0.7076–0.7078 and negative ɛNd(t) values (− 17.6 to − 18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang–Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U–Pb ages of ∼122 Ma. These rocks have Sr, Nd and Hf isotopic compositions (initial 87Sr / 86Sr = 0.7083–0.7088, ɛNd(t) = − 16.5 to − 17.7 and ɛHf(t) = − 20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb and HREE, low Sr and Ba and strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene- and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted from mantle–crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the eastern North China Craton during the Early Cretaceous.

The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon 206Pb/238U age of 215±5 Ma was obtained for the quartz syenite and mineral 40Ar/39Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K2O≈4.4–6.4 wt.% and K2O/Na2O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al2O3 (∼0.76) and TiO2 (∼1.12 wt.%). They also have high Sr (87Sr/86Sr∼0.7073), low Nd (ɛNd=∼−16.5) isotopic ratios, enriched LILE (Ba/La=66–74), LREE [(La/Yb)N=28–33] and depleted HFSE (La/Nb=4–6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle.