Geochronology and petrogenesis of Neoarchean potassic meta-granites from Huai'an Complex: Implications for the evolution of the North China Craton

Abstract

Neoarchean potassic meta-granites occurring in the Huai'an Complex within the north central part of the North China Craton (NCC) comprise leucocratic syenogranites, biotite monzogranites, and garnet-bearing syenogranites. High-pressure garnet–granulites occur as lenses, sheets or dismembered dikes with the granitoids. Here we present SHRIMP zircon U–Pb data from the leucocratic syenogranites which show an age of 2493 ± 6 Ma (MSWD = 1), and LA-ICP-MS U–Pb data from the biotite-monzogranites and garnet–syenogranites, revealing their formation ages of 2437 ± 10 Ma (MSWD = 2.0) and 1977–2003 Ma, respectively. We also report metamorphic ages of ca. 1800 Ma from zircons in the garnet–syenogranites. Geochemical studies show low MgO (0.5–0.06 wt.%), Cr (6.19–0.53 ppm) and Ni (27.66–2.01 ppm) with varying SiO 2 (62.3 to 76.8 wt.%) contents, indicating their derivation from a crustal source. The leucosyenogranites exhibit elevated positive Eu anomalies with decreasing Sr, Th, U, and bulk REE concentrations, and Th/U ratio, whereas Rb and Ba values increase with decreasing Th/U ratio, indicating fluid–melt interaction. The leucosyenogranites have high Sr and Ba contents and Sr/Y and (La/Yb) N ratios and low Y and HREEs, with zircon εHf(t) values ranging from + 1.7 to + 5.8 and Hf model ages of 2.60–2.73 Ga, reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition, or an eclogitic mineral assemblage in the residue. Geochemical compositions of the biotite–monzogranites, with positive εNd(t) values (+ 2.3 to + 2.5) and model ages < 2.60 Ga, indicate that they are evolved partial melts from a thickened juvenile crust of garnet amphibolite composition, or with garnet and amphiboles in the residue, followed by fractionation dominated by plagioclase precipitation. The garnet–syenogranites display variable contents of SiO 2 (62.3–72.3 wt.%), and relatively high Zr (491–1096 ppm), Ga (23.5–31.2 ppm), Zn (38–124 ppm), Y (34.1–85.6 ppm) and Nb (9.4–90.0 ppm) abundances with 10 4 × Ga/Al ratios of 3.1–4.2. They also possess low Sr (50–161 ppm) concentrations and strong negative Eu anomalies. These features, integrated with Nd model ages of 2.46–2.50 Ga and relatively constant negative εNd(t) values (− 2.5 to − 2.9) with increasing SiO 2 contents, suggest A-type granitic affinities of intraplate extensional or rift setting with evolution of the magma from partial melts derived from Neoarchean tonalitic rocks. The distinct geochemical features and zircon saturation temperatures of the granitoids are indicative of high melting temperatures (900–1000 °C) at relatively low pressures (6–10 kbar). Based on previous works and our new data, we link the ca. 2500–2450 Ma and 2000 Ma magmatic events to a Neoarchean amalgamation of microblocks and Paleoproterozoic mobile belts. The metamorphic ages of 1800–1820 Ma might reflect an amphibolite facies retrograde event.