Some carbonatites related to Proterozoic rift activities developed on the northern margin of the North China Craton and have a close genetic relationship with certain types of mineralization. We obtained new fission-track and (U-Th)/He geochronological data from fragments of pegmatitic apatite that coexists with Proterozoic igneous carbonatite along the border of three provinces (Hebei, Shanxi, and Inner Mongolia). Twenty fragments yielded a fission-track central age of 124.1 ± 6.5 Ma (1σ). Thermal-history inversion and track-length distribution suggest rapid Early Cretaceous cooling through the apatite partial-annealing zone. Sixteen single-fragment apatite (U-Th)/He ages range between 82.2 ± 1.8 and 109.6 ± 2.3 Ma, with the age variation much larger than the analytical uncertainty. Several reasons could account for this dispersion. While alpha-particle redistribution resulted from U and Th inhomogeneties, chemical variations and radiation damage are the most probable causes. We calculated the mean age, central age, and isochron age of all the analyzed fragments and got a consistent statistical age of ∼100 Ma, which can be used as the best estimation of the cooling age for the pegmatitic apatite. This result also suggests that the Proterozoic carbonatite was exhumed to a subsurface level during the late Early Cretaceous and was not buried deeper than 2–3 km since then, a situation that provided favorable conditions for the preservation of some ore deposits. The denudation of the Proterozoic rocks was consistent with widespread crustal extension, represented by formation of metamorphic core complexes and rift basins, reactivation of normal faults, and rapid exhumation of granites on the eastern Asian continent. These new low-temperature geological data shed some light on the denudation and preservation of related mineral deposits.
Carbonatites are characterized by the highest concentration of rare earth elements (REEs) of any igneous rock and are therefore good targets for REE exploration. Supergene, hydrothermal, and magmatic REE deposits associated with carbonatites have been widely studied. REE enrichment related to fluorapatite metasomatism in Fengzhen carbonatites in the North China block is reported in this study. REE minerals (monazite, britholite, and Ca-REE-fluorocarbonates) associated with barite and quartz formed as inclusions within the fluorapatite and externally on its surface. Monazite, allanite, barite, and quartz occur as rim grains on the edges of the fluorapatite. Zoned fluorapatite was observed and showed varying chemical composition. Based on back-scattered electron imaging, the dark domains with mineral inclusions contain lower Si (0.3–0.6 wt.% SiO2) and light REE (LREE) [0.36–1.54 wt.% (Y+LREE)2O3] contents than inclusion-poor areas [0.7–1 wt.% SiO2; 2.16–4.51 wt.% (Y + LREE)2O3]. This indicates a dissolution–re-precipitation texture. Different types of monazites were distinguished by their chemical compositions. Monazite inclusions have lower La2O3contents (~13 wt.%) and La/Ndcn (~3) ratios than those (18–26 wt.% and 10–14 for La2O3 and La/Ndcn, respectively) growing externally on the fluorapatite. REE enrichment in the metasomatic fluorapatites is related to different stages of carbonatitic liquids. The early carbonatite-exsolved fluids metasomatized the fluorapatites to form REE mineral inclusions. The late carbonatitic fluids from carbonatite magmas that underwent strong fractional crystallization were enriched in REEs, Al, and Fe and metasomatized the fluorapatites to produce allanite and monazite rim grains.
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