Nitrogen and oxygen isotopic profiles and redox structure across the Chinese Altay reveal the source rocks of granites, core of the Central Asian Orogenic Belt

Abstract

Two rock series in the Chinese Altay (CA) were identified. The protoliths of the overlying series are fine sandstone and siltstone that consist of quartz and clay but contain little feldspar. The underlying series includes biotite–plagioclase gneiss and amphibolite entrained by the CA granite, migmatitic granite and the Early-Devonian Kangbutiebao Fm. as the uncovered, latest-formed southwestern margin. The CA granites include oligoclase microcline granite and biotite plagioclase granite that exhibit extensive K-Na-metasomatism of plagioclase porphyroclast. The two series both have high positive εNd-εHf values but are NO isotopically distinguishable. This study employed NO isotopes to trace source rocks of the CA granites and to explore devolatilization from metamorphism, migmatization to partial melting.The overlying series yielded much higher N content (20.2–484 ppm; average: 87.3 ± 99.0 ppm; 1σ; n = 36), δ15N (−3–19‰; average: 5.8 ± 4.3‰; n = 36) and δ18O (6.9–15.6‰; average: 13.6 ± 3.05‰; n = 29) than the underlying series that yielded N (9.9–71.1 ppm; average: 32 ± 17.1 ppm; n = 22), δ15N (−7.3–8.1‰; average: 1.1 ± 4.7‰; n = 22) and δ18O (4.5–12.1‰; average: 8.8 ± 1.9‰; n = 22). The protoliths of the overlying series were accumulated on oxic shallow sea in the CA and, hence, recorded high δ15N, whereas the protoliths of the underlying series extended to anoxic deep-ocean and experienced high-temperature seawater reaction, which resulted in low δ18O and δ15N values of the protoliths of the underlying series. The CA granites yielded much lower N content, δ15N and δ18O than the overlying series but have values overlapping those of the underlying series with a marginal increase in δ18O. The δ15N values tend to increase with decreasing element normalized-N contents from the underlying series to the CA granites, suggesting that migmatization and partial melting of the underlying series have caused 15N/14N fractionation. Quantitative modeling demonstrates that NH3 devolatilization of the underlying series at 550 to 750 °C can fit the underlying series and the CA granites, whereas 15N/14N fractionation trajectories of the overlying series miss the CA granites. These results suggest that the CA granites have been derived from the underlying series. Most data of the underlying series and the CA granites can be modeled by Rayleigh distillation suggesting escape of degassed NH3 from essentially solid-state protolith/source rock from metamorphism to migmatization. Partial conversion to N2 or N2O caused 15N-enrichment in the two series at the leading edge of the overlying series, the eastern CA. The magma derived from the underlying series mixed with magma stemming from the metasomatized mantle with contribution from country rock-derived fluid result in high δ15N of the Qinggeli granodiorite and microdioritic enclaves.