Geochronological and geochemical constraints on the petrogenesis of the. 2.6-2.5 Ga amphibolites, low- and high-Al TTGs in the Wangwushan area, southern North China Craton: Implications for the Neoarchean crustal evolution

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 epsilon(Nd)(t) values of -2.8 + 4.0 (T-DM1 = 3.7-2.6 Ga), and zircon epsilon(Hf)(t) values of +1.2-+7.2 (T-DM1 = 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 epsilon(Nd)(t) values (-0.6- + 5.2) (T-DM2 = 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 epsilon(Hf)(t) (+ 2.2- + 8.8 and + 1.3- + 6.9) and whole-rock epsilon(Nd)(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 wit