A ∼2.5 Ga magmatic event at the northern margin of the Yangtze craton: Evidence from U-Pb dating and Hf isotope analysis of zircons from the Douling Complex in the South Qinling orogen

Abstract

The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt. Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating, which yielded protolith emplacement ages of 2469 ± 22 Ma, 2479 ± 12 Ma, 2497 ± 21 Ma, 2501 ± 17 Ma and 2509 ± 14 Ma, respectively. An important peak age of ∼2.48 Ga was also obtained for a metasedimentary rock in the same region. These discoveries suggest the occurrence of magmatic activity of 2.51–2.47 Ga at the northern margin of the Yangtze craton. The age-corrected ɛHf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between −5.5 and +0.3, and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga. Considering two important periods of ∼3.3–3.2 Ga and ∼2.95–2.90 Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo- to Mesoarchean crust at the northern margin of the Yangtze craton at ∼2.5 Ga. In addition, metamorphic ages of 837 ± 8 Ma and 818 ± 10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock, indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic, although its geological meaning remains ambiguous.