Geochronology and geochemistry of Ordovician plutons in the Erguna Block (NE China): further insights into the tectonic evolution of the Xing'an-Mongolia Orogenic Belt

摘要

The Ordovician plutons in the Erguna Block, NE China, can be classified into two groups: Early Ordovician diorites with zircon U-Pb ages ranging from 486 to 485 Ma and Middle Ordovician gabbros and granites with zircon U-Pb ages ranging from 466 to 463 Ma. The diorites are calc-alkaline in nature and are characterized by weak to moderate enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE). The gabbros and granites have high total alkali contents, and all samples are enriched in LREE and LILE and depleted in HFSE such as Nb, Ta, and Ti. Isotopically, Early Ordovician diorites display values that are less radiogenic [epsilon(Hf)(t) = + 9.9-+16.8] compared to those of Middle Ordovician gabbros [epsilon(Hf)(t) = - 3.0-+5.0]. Middle Ordovician granites have positive epsilon(Hf)(t) values of +1.4 to +4.3 and two-stage Hf model ages (T-DM2) of 1167 to 1356 Ma. These data indicate that the diorites may have been generated by the partial melting of a recently metasomatized mantle source, whereas the gabbros and granites may have been formed by the partial melting of enriched lithospheric mantle and Mesoproterozoic crust, respectively. Our results, combined with other regional results, suggest that Early Ordovician magmatism was likely associated with the northward subduction of the Heihe-Xilinhot oceanic plate beneath the Erguna-Xing'an Block, whereas the Middle Ordovician gabbros and granites were most likely formed in an extensional setting controlled by the rollback of this subducted oceanic plate.