Triassic “adakitic” rocks in an extensional setting (North China): Melts from the cratonic lower crust

Abstract

Adakite was originally defined as a specific type of magmatic rock derived from melting of subducted oceanic plates (Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347 (6294), 662–665), producing unique chemical signatures with high Sr/Y and La/Yb. However, widespread occurrences of igneous rocks that are geochemically similar to the adakites, but from diverse tectonic settings, suggest that “adakitic” rocks may have a variety of origins. Late Triassic high Sr/Y lavas, mainly trachytes with minor pyroxene andesite and rhyolite, are found at Shuiquangou, in the Yanshanian fold-and-thrust belt on the northern margin of the North China Craton. Data on mineral chemistry, major and trace elements and Sr–Nd isotopes of whole rocks, and in situ U–Pb age and Hf-isotope analyses of zircons are reported here. The Shuiquangou volcanic rocks with high Sr/Y (>72) and (La/Yb)N (>24) also show enrichment in light rare‐earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They have low Ce/Pb (<4.3) and Nb/U (<4.8) and moderate (Gd/Yb)N (2.8–3.9). U–Pb dating of zircons yields concordant and lower-intercept ages of ~220Ma, indicating that they erupted during the late Triassic. Concordant grains and an upper intercept age of ~2.50Ga suggest that Neoarchean materials may have been involved in their petrogenesis. The relatively low initial 87Sr/86Sr (0.70529 to 0.70540) and negative εNd(t) (−3.9 to −9.9) of the these rocks, and the negative εHf(t) (−8.6 to −1.1) of their zircons, suggest that the magmas were derived by partial melting of the cratonic lower crust, induced by continuous magmatic underplating under an extensional regime following the southward subduction of the Paleo-Asian Ocean. Their high Sr/Y is inherited from their source, and does not necessarily imply melting at great depths (e.g., garnet-bearing lower crust). We suggest that partial melting of the ancient lower crust may be important for the petrogenesis of “adakitic” magmas in a continental extensional setting.