Lower crustal contribution to the magma formation of the Damiao massif type anorthosite, North China Craton: Evidence from zircon Hf-O isotopes

摘要

The formation of massif-type anorthosites and the associated mangerite, chamockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the "Earth's Middle Age" (similar to 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 similar to 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 delta O-18 values (5.1-7.4) and ancient crustal-like epsilon(Hf)(T) values (-4.0 to -8.9). Since the significant Lu-177/Hf-176 reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative epsilon(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 delta O-18 features. Thus, the decoupling of mantle-like delta O-18 and ancient crustal-like epsilon(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 epsilon(Hf)(T), initial Sr-87/Sr-86 and 8180 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 (similar to 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.

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