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Abstract: The late Mesozoic high-Mg adakitic rocks from Eastern and Central China provide important insight into the foundering mechanism of the over-thickened lower continental crust. The Chituling high-Mg adakites (131±3 Ma, SHRIMP zircon U–Pb age) from the eastern margin of the Southern Dabie ultrahigh pressure metamorphic zone, adjacent to the Tan–Lu fault, have high Al2O3 and Sr contents, high Sr/Y, La/Yb, and Mg# (44–63), but low Y and Yb contents. The samples also have moderately enriched 87Sr/86Sr i (0.70691–0.70775), very low ε Nd(T) (−20.7 to −24.9), unradiogenic Pb isotopes, enrichment of large ion lithophile elements, and depletion of high field strength elements. These geochemical features indicate that they did not result from melting of young oceanic crust, assimilation and fractional crystallization, or magma mixing. Instead, they were derived from partial melting of delaminated lower continental crust, with subsequent reaction with surrounding mantle peridotites during ascent to crustal depths. The reactions between the adakitic melt and peridotites also generated the enriched mantle source of the post-collisional basaltic rocks in the Dabie orogen. Distribution of the late Mesozoic high-Mg adakites in eastern and central China generally forms a high-Mg adakite belt along the southern Tan–Lu fault. Therefore, we propose that the large strike-slip motion of the Tan–Lu fault in eastern and central China due to the western subduction of the Pacific plate in the early Cretaceous might trigger the foundering of some fragments of the over-thickened lithosphere near the Tan–Lu fault, which caused mantle upwelling and partial melting of the thickened lower crust in the Dabie orogen and eastern boundary of the North China Craton. This further weakened the gravitationally instable lithosphere, consequently resulting in delamination and foundering of the mountain root underneath the Dabie orogen, which could be an important foundering mechanism of lower continental crust. [Copyright &y& Elsevier]

Reports on the decoupling of surface and subsurface sutures in the Dabie orogen and a continent-collisional lithospheric-wedging model. Differences of neodymium (Nd) and lead (Pb) isotopic compositions between Mesozoic mafic igneous rocks from the North China Block (NCB) and the South China Block (SCB); Subsurface suture between the NCB and SCB; Southward movement of subsurface suture in the Dabie-Sulu orogen.

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 K 2 O/Na 2 O ratios of global Archean granitoids. Results show that these TTGs, with K 2 O/Na 2 O 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 K 2 O (K 2 O/Na 2 O = 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 2 O enrichment (K 2 O/Na 2 O = 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 K 2 O/Na 2 O 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 K 2 O/Na 2 O ratio of the UCC to ~0.83 which is the global average value for felsic rocks at the end-Archean. To increase K 2 O/Na 2 O 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. • Archean potassic granites hold the key to crustal change from sodic to potassic. • Mg isotopes identify Archean S-type potassic granites in the North China Craton. • The S-type potassic granites begin in the middle and spread in the late Archean. • Remelting of igneous and sedimentary rocks changes upper crustal composition. [ABSTRACT FROM AUTHOR]