Origin of Late Triassic lamprophyres in the western Yanshan Tectonic Belt related to destruction of the North China Craton: Geochronological and geochemical constraints

Abstract

Lamprophyres, ultrapotassic igneous rocks generally derived from the melting of enriched subcontinental lithospheric mantle, typically form in extensional tectonic settings and are associated with coeval transcrustal processes. The genesis of lamprophyres provides insights into the generation, storage, and emplacement of lamprophyric magmas, and further constrains the nature of mantle sources. However, the origin of lamprophyres remains enigmatic and requires further investigation. The Late Triassic lamprophyres along the western Yanshan Tectonic Belt of the northern North China Craton (NCC) offer a key opportunity to understand the tectonic evolution of this region, particularly their relationship with the onset of craton destruction. Here we report the results of an integrated geochronological and geochemical study of Late Triassic lamprophyres and quartz monzonite porphyries in Beijing on the western Yanshan Tectonic Belt. U–Pb zircon dating using LA–ICP–MS reveals that the lamprophyres and quartz monzonite porphyries were emplaced contemporaneously during ca. 235–220 Ma, marking a Late Triassic magmatic event in the northern NCC. The Late Triassic lamprophyre and quartz monzonite porphyry sills are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but are depleted in high field strength elements (HFSEs), exhibiting arc-like geochemical characteristics. They also display high MgO and total alkali concentrations with low silica contents. Unradiogenic whole-rock Nd and Hf isotope compositions [(εNd(t) = −11.3 to −10.8) and (εHf(t) = −16.3 to −12.5)] indicate that the magmas of the Triassic suite were likely derived from an enriched lithospheric mantle source, followed by fractional crystallization of ferromagnesian phases and minor crustal assimilation. Geochemical modeling suggests that the parental magma of the lamprophyres was generated through a low degree (<5 %) partial melting of spinel–garnet lherzolites. These results suggest that the investigated rocks originated from a subcontinental lithospheric mantle enriched by fluid and/or melt metasomatism, likely during subduction of the Paleo-Asian oceanic plate beneath the NCC. Radiogenic zircon Hf isotope compositions [e.g., εHf(t) = 6.4 to 8.9] of the Late Triassic lamprophyres point to the involvement of a depleted mantle source. The upwelling of the asthenosphere, triggered by intensive intra-continental extension during the Triassic, likely provided the heat required to melt the enriched subcontinental lithosphere beneath the NCC. This Late Triassic interaction between the asthenosphere and the lithospheric mantle marks the onset of NCC destruction and contributed to its thermo-mechanical erosion.