Geochemical and Sr–Pb–Nd isotopic characteristics of the Shakhtama porphyry Mo–Cu system (Eastern Transbaikalia, Russia)

• Published in: Journal of Asian earth sciences Vol. 79; pp. 655 - 665
• Main Authors: Berzina, A.P., Berzina, A.N., Gimon, V.O.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 05.01.2014
• Subjects: Cu–Mo porphyry deposits; Eastern Transbaikalia; K-adakite; Magma sources; Metal sources; Pb isotopes; Shakhtama; Sm–Nd isotopes; K-adakite; Cu–Mo porphyry deposits; Shakhtama; Eastern Transbaikalia; Pb isotopes; Metal sources; Sm–Nd isotopes; Magma sources
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2013.07.028

•K-adakitic magmas were generated by partial melting of thickened lower crust.•Magmas lacking adakitic signatures were generated at shallower levels of lower crust.•The derivation of melts involved Precambrian and juvenile Phanerozoic lower crust.•Stronger contribution of juvenile material to the porphyry magmas is suggested. The Shakhtama Mo–Cu porphyry deposit is located within the eastern segment of the Central Asian Orogenic Belt, bordering the southern margin of the Mongol–Okhotsk suture zone. The deposit includes rocks of two magmatic complexes: the precursor plutonic (J2) and ore-bearing porphyry (J3) complexes. The plutonic complex was emplaced at the final stages of the collisional regime in the region; the formation of the porphyry complex may have overlapped with a transition to extension. The Shakhtama rocks are predominantly metaluminous, I-type high K calc-alkaline to shoshonitic in composition, with relatively high Mg#, Ni, Cr and V. They are characterized by crustal-like ISr (0.70741–0.70782), relatively radiogenic Pb isotopic compositions, εNd(T) values close to CHUR (−2.7 to +2.1) and Nd model ages from 0.8 to 1.2Ga. Both complexes are composed of rocks with K-adakitic features and rocks without adakite trace element signatures. The regional geological setting together with geochemical and isotopic data indicate that both juvenile and old continental crust contributed to their origin. High-Mg# K-adakitic Shakhtama magmas were most likely generated by partial melting of thickened lower crust during delamination and interaction with mantle material, while magmas lacking adakite-like signatures were probably generated at shallower levels of lower crust. The derivation of melts, related to the formation of plutonic and porphyry complexes involved variable amounts of old Precambrian lower crust and juvenile Phanerozoic crust. Isotopic data imply stronger contribution of juvenile mantle-derived material to the fertile magmas of the porphyry complex. Juvenile crust is proposed as an important source of fluids and metals for the Shakhtama ore-magmatic system.