Paleoproterozoic crustal-derived carbonatites in Trans-North China Orogen

Fengzhen Paleoproterozoic (1681 Ma) carbonatites, located in the Trans-North China Orogen (TNCO), are featured by an assemblage of calcite, phlogopite, clinopyroxene, olivine, apatite and barite, lacking pyrochlore or fenitization. Geochemically, they exhibit negative Eu anomalies in rare earth element patterns, sediment-like C-O isotopic signatures, and enriched initial Sr-Nd isotopic compositions, distinct from those of typical mantle-derived carbonatites. Phlogopite, the predominant Li-bearing phase in the carbonatites, shows heavier Li isotopic compositions (δ⁷Li = 4.87–10.87 ‰, mean value = 7.97 ± 1.53 ‰) than those in mantle-derived carbonatites worldwide. These δ⁷Li values align more closely with those of contemporary (meta)sedimentary carbonate rocks. These features collectively suggest a crustal origin of the Fengzhen carbonatites. Isotopic modeling indicates that parental magma of the Fengzhen carbonatites likely originated from melting of (meta)sedimentary carbonate rocks, with contributions from granites and pyroxenites during the Paleoproterozoic collision between Eastern and Western Blocks of the North China Craton. The crustal-like Li isotopic signatures of the Fengzhen carbonatites, along with their spatial proximity to the Daqingshan crustal-derived carbonatites, imply that additional Paleoproterozoic crustal-derived carbonatites may be present within the TNCO. This study supports the hypothesis that analogous crustal processes may generate carbonatite within orogenic belts and provides novel insights into crustal contributions to carbonatites.