The Paleozoic tectonic evolution and metallogenesis of the northern margin of East Junggar, Central Asia Orogenic Belt: Geochronological and geochemical constraints from igneous rocks of the Qiaoxiahala Fe-Cu deposit

Abstract

The East Junggar terrane (NW China) is an important constituent of the Central Asian Orogenic Belt (CAOB). From the Devonian to Permian, regional magmatism evolved from mainly calc-alkaline (I-type) to alkaline (A-type). The Qiaoxiahala Fe-Cu deposit, located in the Late Paleozoic Dulate island arc (northern margin of the East Junggar), is hosted in the volcanic rocks of the Middle Devonian Beitashan Formation. Two magmatic stages were identified in the deposit, the Qiaoxiahala diorite porphyry (380±4.0Ma) and a younger aplite (331±3.1Ma).The (high-K) calc-alkaline Beitashan Formation basaltic rocks are characterized by LILE and LREE enrichments and HFSE depletions, pointing to a subduction-related affinity. The high Mg# (42–75), elevated Ce/Th and Ba/Th, depleted Nb, positive εNd(t) (6.6), low (87Sr/86Sr)i (0.7037) and MORB-like Pb isotope characters all suggest an origin involving partial melting of a MORB-like depleted mantle wedge (metasomatized by slab-derived fluids) with little evidence of crustal contamination.The calc-alkaline (I-type) diorite porphyry, characterized by LILE and LREE enrichments and HFSE depletions, may have formed from fractional crystallization of the basaltic rocks, with its parental magma derived from the same depleted mantle wedge. The negative εHf(t) (−8.26), Hf model age (TDMC) of 1406Ma and the presence of inherited zircons (ca. 470 and 506Ma) indicate that the diorite has assimilated older crustal material.The alkaline, metaluminous (A-type) aplite is characterized by HFSE enrichment and depletions in Sr, P and Ti, distinct from the basaltic rocks and diorite porphyry at Qiaoxiahala. The low Mg# (35–38), positive Zr and Hf, positive εHf(t) (4.77–9.75) and εNd(t) (6.85–6.86) and low T2DM (538–520Ma) suggest a juvenile lower crustal source due to partial melting of basaltic lower crust as a result of underplating of mantle-derived melts and accompanied by magma mixing.The tectonic evolution of the Paleozoic East Junggar was associated with the initiation of subduction of the Paleo-Asian Ocean, with the formation of an island arc during the Early to Middle Devonian and generation of the coeval arc-related Fe-Cu (-Au) and porphyry Cu mineralization. Collision between the Chinese Altay and East Junggar happened in the Late Devonian with Early Carboniferous post-collisional tectonics likely explaining the synchronous Cu-Mo mineralization, while the late-Early to Late Carboniferous post-orogenic event ended without related mineralization in Qiaoxiahala.