Origins of two types of Archean potassic granite constrained by Mg isotopes and statistical geochemistry: Implications for continental crustal evolution

Abstract The chemical composition of the upper continental crust (UCC) dramatically changed from predominantly sodic to more potassic during the late Archean (3000–2500 Ma), although how this change occurred remains poorly constrained. The origins of Neoarchean potassic granites may provide clues for this crucial change, as their formation dominated the compositional transition of the UCC. In this study, we conducted high-precision Mg isotopic analyses of Neoarchean potassic granites (2558–2520 Ma) and tonalite–trondhjemite–granodiorites (TTGs; 2595–2574 Ma) from the North China Craton and statistical geochemistry in K2O/Na2O ratios of global Archean granitoids. Results show that these TTGs, with K2O/Na2O ratios of ~0.61 and δ26Mg values of −0.39‰ to −0.22‰, were generated from heterogeneous basaltic sources. In contrast, potassic granites yield two distinctive groups of δ26Mg values and can be subdivided into I- and S-types. The I-type granite has mantle-like δ26Mg values of −0.28‰ to −0.22‰ and weak enrichment in K2O (K2O/Na2O = 1.0–1.5), and was likely derived by partial melting of igneous sources. The S-type is characterized by much higher δ26Mg values of +0.60‰ to +0.91‰ and strong K enrichment (K2O/Na2O = 1.57–23.26), with geochemical characteristics suggesting derivation by partial melting of sediments. Based on the geochemical characteristics of the S-type potassic granites in this study, we sort the S-type granites out of 4066 Archean felsic igneous rocks worldwide. Their temporal distribution suggests that S-type potassic granites have appeared in the middle Archean (3500–3000 Ma) and became widespread in the late Archean (3000–2500 Ma), but were scarce in the early Archean (4000–3500 Ma). The onset of S-type granites indicates reworking of supracrustal rocks may have started since 3500 Ma. And widespread S-type potassic granites during 3000–2500 Ma play an important role for the high K2O/Na2O ratio of UCC at the end of Archean. A mass-balance calculation shows that 25.88 wt% I-type, 4.12 wt% S-type potassic granite and 70 wt% TTGs could increase the K2O/Na2O ratio of the UCC to ~0.83 which is the global average value for felsic rocks at the end-Archean. To increase K2O/Na2O of the Archean UCC, contributions made by I- and S-type potassic granites could probably reach 3:2, respectively. Consequently, the evolved crustal composition from predominantly sodic to potassic results from the synergy of re-working of igneous (unaltered) and sedimentary (weathered) rocks, by which the former contributed more quantitatively, while the latter contributed more efficiently.