科研之友
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摘要
The crustal evolutionary processes of the North China Craton (NCC) during the late Neoarchean (ca. 2.6–2.5 Ga) are still under large debate. The systematic investigations of widespread ca. 2.6–2.5 Ga meta-volcanic basalts and TTG assemblages can provide a better understanding of tectonic evolution at this period. The Wangwushan Neoarchean amphibolites, high- and low-Al TTGs in southern NCC were formed at 2.57–2.52 Ga. The amphibolites are tholeiitic in composition. They have whole-rock εNd(t) values of −2.8–+4.0 (TDM1 = 3.7–2.6 Ga), and zircon εHf(t) values of +1.2–+7.2 (TDM1 = 2.81–2.57 Ga), indicative of a depleted mantle source with involvement of older crustal materials. Their compositional spectrum accords with those in island-arc field, or straddles the boundary between MORB and island-arc fields, suggesting significant contributions from subduction zone-derived fluids during magma formation. The high-Al TTGs contain high Mg#, Cr and Ni contents, and (La/Yb)N and Sr/Y ratios, together with positive whole-rock εNd(t) values (−0.6–+5.2) (TDM2 = 2.95–2.48 Ga), indicating that they derived from partial melting of a subducted oceanic slab with older crustal materials. Their geochemical features are corresponding to ‘slab-melt’ identification criterion in an arc setting, and illustrate a deep source (>45 km) with garnet amphibolite in the residue without rutile. The low-Al TTGs show low Mg# and (La/Yb)N ratios, negative Eu anomalies, relatively flat HREE (and Y) patterns. Their zircon εHf(t) (+2.2–+8.8 and +1.3–+6.9) and whole-rock εNd(t) values (ca. +2.23) are positive. All the evidences support that the low-Al TTGs derived from a low degree partial melting of basaltic crust with the residue lack of garnet, consistent with a shallower depth (about <30 km). It is likely that the underplated basaltic or high-Al magmas gave rise to re-melting newly formed lower crust to form the low-Al TTGs. Combined with ca. 2.52 Ga diorites and high-K granites in this area, which were formed in a subduction process and post-collisional setting, respectively, we propose that all of these igneous rocks record multi-stage processes in a convergent plate margin.
