Discovery of ca. 2.33–2.22 Ga magmatic activities in the Longshan Complex bordering the southwestern margin of the North China Craton: Implications for tectonic transformation

Abstract

The 2.45–2.20 Ga in the early Paleoproterozoic, known as the tectono-magmatic lull, was marked by a decline in global continental magmatism and orogenic activity. This study is the first to identify ca. 2.33–2.30 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses (2325 ± 23 Ma, 2296 ± 22 Ma) and ca. 2.25–2.22 Ga TTG gneisses (2246 ± 19 Ma, 2242 ± 23 Ma, 2217 ± 24 Ma) in the Zhangjiachuan area of the Longshan Complex, located along the southwestern margin of the North China Craton (NCC). All TTG gneisses exhibit high SiO₂ and Na₂O concentrations but relatively low K₂O and MgO concentrations. The ca. 2.33–2.30 Ga TTG gneisses have medium to high Yb (0.78–2.94 ppm) and Y (11.16–28.01 ppm) concentrations, along with flat heavy rare-earth element patterns, suggesting that amphibole and/or garnet were the residual phases during partial melting under medium-pressure conditions. Their moderate Sr (225–437 ppm) concentrations and predominantly negative Eu anomalies (δEu = 0.61–0.93) further imply the presence of plagioclase in the residue. Alternatively, the ca. 2.25–2.22 Ga TTG gneisses exhibit low Yb (0.27–1.06 ppm) and Y (3.84–12.31 ppm) concentrations, positive or negligible Eu anomalies (δEu = 0.96–1.55), and higher Sr (333–1095 ppm) concentrations, indicating garnet and amphibole as the residual phases under high-pressure conditions. The lower Mg# (28–44), Cr (3.04–34.70 ppm), and Ni (2.83–5.45 ppm) concentrations suggest that the ca. 2.33–2.30 Ga TTG gneisses formed through the partial melting of the lower continental crust. Conversely, the higher Mg# (46–52) but lower Cr (6.03–16.67 ppm) and Ni (6.22–15.97 ppm) concentrations in the ca. 2.25–2.22 Ga TTG gneisses may indicate interaction with mafic melts from the mantle. The ca. 2.33–2.22 Ga TTG gneisses display ε_Hf(t) values ranging from −6.6 to +4.7 and T_DM2 ages of 3.29 to 2.55 Ga, closely resembling those of the ca. 2.30 Ga TTG gneisses found in the southern NCC, suggesting that the ca. 2.33–2.22 Ga TTG gneisses originally formed in the southern NCC. Crust thickness calculations indicate a gradual thickening from 2.35 to 2.30 Ga, followed by a gradual decrease after 2.30/2.26 Ga in the southern NCC. This region experienced mafic–granitic magmatism during the early Paleoproterozoic, likely triggered by magmatic underplating—a process that facilitated mafic material injection into the lower crust, leading to progressive crustal thickening. Concurrently, the migration of intermediate-felsic magmas may have contributed to crustal densification. The combined effects of crustal thickening and densification could have induced lower crustal delamination, potentially explaining the observed post 2.30/2.26 Ga crustal thinning. The ca. 2.25–2.22 Ga TTG gneisses also exhibit geochemical characteristics indicative of interaction with mafic melts from the mantle. Thus, the southern NCC experienced a tectonic transition during the early Paleoproterozoic, evolving from crustal thickening induced by magmatic underplating to extensional tectonics driven by delamination.