Distinct mafic magmatism of the northeastern Longgang Block: Evidence for coexisting mantle plume and subduction during the Neoarchean North China Craton

Abstract

The question of which specific tectonic regimes played an essential role in shaping the Neoarchean evolution of the North China Craton (NCC) has been a contentious and controversial topic. Mafic rocks, containing valuable geochemical information from the mantle and deep crust, serve as an important source of data to provide essential constraints on above issue. This study presents bulk-rock geochemistry, zircon U-Pb geochronology and Hf-Nd isotopes for the Neoarchean mafic rocks in the core area of the Longgang Block of the NCC. Petrographically, these mafic rocks are composed of the amphibolite and metadiabase. Zircon U-Pb dating results revealed that they were synchronously emplaced at ca. 2.5 Ga. Geochemically, the amphibolites belong to subalkaline tholeiite series, exhibiting enrichment in light rare earth elements (LREEs), intensely negative Nb, Ta and Ti anomalies, and slightly enriched Nd and relatively variable zircon Hf isotopic compositions. These geochemical and isotopic features show large affinity to arc-like magmatism, implying that they were derived from subduction-related metasomatized lithospheric mantle. In contrast, the metadiabases exhibit high-Ti alkaline basalt affinities, OIB-like REE, and trace element patterns. They also show clearly positive Nb, Ta and Ti anomalies, indicating that they originated from a mantle plume-related tectonic environment. The new geochemical and isotopic data reveal that these Neoarchean mafic rocks could have resulted from coexisting mantle plume and subduction processes. Combined with available regional structural, geochemical, and metamorphic data, it is likely that during the Neoarchean, both a mantle plume and subduction jointly controlled the crustal growth and tectonic evolution of the Longgang Block in the eastern NCC.