Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

Abstract

The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb)N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K2O/Na2O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO2 and Na2O contents, but lower contents of K2O, MgO, TiO2, P2O5 and most trace elements (e.g., Rb, Ba, LREE) with lower K2O/Na2O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and εHf(t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher εNd(t) values than Group I ones and the granitic gneisses by 3–5 εNd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions.