矿床地球化学国家重点实验室知识库

The formation of massif-type anorthosites and the associated mangerite, charnockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the “Earth’s Middle Age” (~1.70–0.75 Ga), and the magma sources of massif-type anorthosites are highly debated between the depleted mantle and the mafic lower crust. The ~1.7 Ga Damiao massif-type anorthosite in the North China Craton consists mainly of anorthosite and leuconorite with minor melanorite, mangerite, Fe-Ti-(P) ores and abundant ferrodioritic or gabbroic dykes. In-situ zircon Hf-O isotopic data from anorthosite reveal decoupling of mantle-like δ¹⁸O values (5.1–7.4) and ancient crustal-like ε_Hf(T) values (−4.0 to −8.9). Since the significant ¹⁷⁷Lu/¹⁷⁶Hf reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative ε_Hf(T) values after long period of time (e.g., 0.5–1.0 Ga). In contrast, if no supracrustal materials were added, these mafic crustal materials would retain the mantle δ¹⁸O features. Thus, the decoupling of mantle-like δ¹⁸O and ancient crustal-like ε_Hf(T) values suggest a lower crustal origin of the Damiao massif-type anorthosite. Compilation of Sr-Nd-O isotopic data of massif-type anorthosites reveal decoupling of ε_Hf(T), initial ⁸⁷Sr/⁸⁶Sr and δ¹⁸O values, which signifies important lower crustal input in magmas sources of most massif-type anorthosites. We proposed that zircon U-Pb age of the Damiao massif-type anorthosite (~1.70 Ga) and regional age correlation suggest that the anorthosite was emplaced in an intraplate, anorogenic extensional setting after the Paleoproterozoic orogeny had ended.