Late Paleozoic to Early Mesozoic mafic–ultramafic complexes from the northern North China Block: Constraints on the composition and evolution of the lithospheric mantle

Abstract

The Late Paleozoic to Early Mesozoic mafic–ultramafic complexes from the northern North China Block (NCB) are used to constrain the isotopic composition and evolution of mantle reservoirs during different tectonic stages prior to the initiation of lithosphere destruction in the Late Mesozoic. Zircon U–Pb ages show that emplacement of the mafic–ultramafic complexes occurred in at least three stages: Middle Devonian (ca. 395 Ma), Late Carboniferous–Early Permian (308–276 Ma) and Triassic (250–220 Ma). Most of the Middle Devonian complexes display low initial 87Sr/ 86Sr ratios of 0.70467–0.70492, weak to moderate negative ε Nd( t) values from − 6.3 to − 1.5 and weakly negative to weakly positive zircon ε Hf( t) values from − 11.5 to 5.5, indicating that the parent magma was derived from slightly enriched lithospheric mantle. The Late Carboniferous–Early Permian complexes are characterized by low initial 87Sr/ 86Sr ratios of 0.70521–0.70604, significant negative ε Nd( t) values from − 14.1 to − 9.3 and zircon ε Hf( t) values from − 17.0 to − 10.5, and were probably derived from metasomatized lithospheric mantle. The Early Triassic complexes exhibit initial 87Sr/ 86Sr ratios of 0.70656–0.70825 and significant negative ε Nd( t) values from − 17.1 to − 8.2, indicating their derivation from enriched lithospheric mantle. However, the Late Triassic mafic–ultramafic complexes display weakly negative to weakly positive ε Nd( t) values of − 4.4 to 0.5 and zircon ε Hf( t) values from − 2.9 to 1.7, indicating that magma sources likely resulted from mixing of depleted asthenospheric mantle and enriched lithospheric mantle. Zircon ages, geochemistry and Sr–Nd–Hf isotopic studies suggest that the lithospheric mantle beneath the northern NCB had a slightly enriched signature during the Middle Devonian, but it became significantly enriched during the Late Carboniferous to Early Permian due to metasomatism of lithospheric mantle during the southward subduction of the Paleo-Asian oceanic plate. After final closure of the Paleo-Asian Ocean and amalgamation of the Mongolian arc terranes with the northern NCB in the latest Permian to earliest Triassic, enriched lithospheric mantle still existed beneath the northern NCB during the Early Triassic. However, in the Late Triassic involvement of asthenospheric mantle became significant, owing to strong upwelling of asthenospheric mantle and post-collisional lithospheric delamination in the Early–Late Triassic. These results suggest that the cratonic lithospheric mantle beneath the northern NCB underwent multiple stages of modification prior to its destruction and thinning. Initiation of lithospheric destruction and thinning in the northern NCB likely occurred during the Early–Late Triassic, which is much earlier than in other parts of the NCB, where it occurred in the Late Mesozoic. It appears likely that lithospheric destruction and thinning of the NCB initially started at the northern and eastern margins of the craton as a result of post-collisional lithospheric delamination, and then spread to the interior of the craton.