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 K2O 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.

中文翻译：

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受镁同位素和统计地球化学约束的两种太古代钾质花岗岩的成因：对大陆地壳演化的启示

摘要 在太古代晚期（3000-2500 Ma），上大陆地壳 (UCC) 的化学成分从主要是钠元素变为更多的钾元素，尽管这种变化是如何发生的仍然缺乏约束。新太古代钾质花岗岩的起源可能为这一关键变化提供线索，因为它们的形成主导了 UCC 的成分转变。在这项研究中，我们对华北克拉通的新太古代钾质花岗岩（2558-2520 Ma）和方长岩-长闪长岩-花岗闪长岩（TTGs；2595-2574 Ma）进行了高精度镁同位素分析，并对 K2O/Na2O 比率的统计地球化学进行了分析。全球太古代花岗岩。结果表明，这些 TTG 的 K2O/Na2O 比值为 ~0.61，δ26Mg 值为 -0.39‰ 至 -0.22‰，来自非均质玄武岩来源。相比之下，钾质花岗岩产生两组不同的 δ26Mg 值，可以细分为 I ​​型和 S 型。I型花岗岩的δ26Mg值为-0.28‰～-0.22‰，K2O富集较弱（K2O/Na2O=1.0-1.5），可能来源于火成岩源的部分熔融。S型的特征是δ26Mg值高得多，为+0.60‰至+0.91‰，K2O富集强（K2O/Na2O=1.57-23.26），地球化学特征表明沉积物部分熔融。根据本研究中S型钾质花岗岩的地球化学特征，我们从全球4066块太古代长英质火成岩中分选出S型花岗岩。它们的时间分布表明 S 型钾质花岗岩出现在太古代中期（3500-3000 Ma）并在太古代晚期（3000-2500 Ma）广泛分布，但在太古代早期（4000-3500 Ma）稀少。S 型花岗岩的出现表明上地壳岩石的改造可能从 3500 Ma 开始。3000-2500 Ma 期间广泛分布的 S 型钾质花岗岩对太古代末期 UCC 的高 K2O/Na2O 比起重要作用。质量平衡计算表明，25.88 重量％的 I 型、4.12 重量％的 S 型钾花岗岩和 70 重量％的 TTG 可以将 UCC 的 K2O/Na2O 比率增加到 ~0.83，这是长英质岩石的全球平均值太古代末期。为了增加太古代UCC的K2O/Na2O，I型和S型钾质花岗岩的贡献可能分别达到3：2。因此，火成岩（未改变）和沉积（风化）岩再加工的协同作用导致地壳成分从主要钠盐到钾盐的演化，