Magma mixing in the genesis of the Kalatongke dioritic intrusion: Implications for the tectonic switch from subduction to post-collision, Chinese Altay, NW China

Abstract

The Kalatongke complex, located in south Chinese Altay of the Central Asian Orogenic Belt (CAOB), consists of the ~308Ma dioritic suite intruded by the 287Ma noritic suite. The noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization. The dioritic suite is composed of ferrodiorite, diorite and quartz diorite.Ferrodiorites have 43.6 to 51.8wt.% SiO2, 8.2 to 12.1wt.% Fe2O3T, 2.87 to 4.86wt.% MgO, with Mg# ranging from 42 to 53. They have highly variable Al2O3 (17.4–20.7wt.%), Na2O (2.78–4.50%), K2O (0.58–3.70%), Sr (280–1300ppm) and Ba (219–3100ppm). Diorites and quartz diorites have 15.2–19.6wt.% Al2O3, 239–890ppm Sr and170–1100ppm Ba, 11.2–25.5ppm Y and1.2–2.2ppm Yb. All rocks have LREE enriched REE patterns and do not display Eu anomalies. They have primitive mantle-normalized trace element patterns enriched in large ion lithosphile elements (LILE) (e.g. Ba and Sr) and depleted in Ti, Nb and Ta.Both ferrodiorites and diorites have relatively constant initial 87Sr/86Sr ratios of 0.7037 to 0.7040 but highly variable εNd(t) values of +4.5 to +11.6, whereas quartz diorites have higher initial 87Sr/86Sr ratios (0.7056) and lower εNd(t) values (+0.5). Plagioclase from ferrodiorites, diorites and quartz diorites has similar initial 87Sr/86Sr ratios of 0.7015 to 0.7055.The dioritic suite formed from a mixed magma of an evolved mantle-derived basaltic magma by partial melting of subduction-modified mantle source in response to slab breakoff and a crustal-derived granitic magma at ~308Ma. Before mixing with granitic magmas, basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite. The mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene, amphibole and plagioclase. The noritic suite formed from a mantle-derived high-Mg magma in a post-collisional environment. Both the dioritic and noritic suites in Kalatongke suggest the switch from subduction to extension and the final collision between the Junggar Block and Chinese Altay at ~308Ma.