Geochemical evidence for reworking of the juvenile crust in the Neoarchean for felsic magmatism in the Yunzhongshan area, the North China Craton

The North China Craton experienced two phases of tectonothermal events at ca. 2.8-2.7 Ga and ca. 2.6-2.5 Ga, respectively. Although 2.6-2.5 Ga granitic gneisses and metavolcano-sedimentary sequences are widespread in the craton, the major stage of crustal growth has been determined to occur at 2.8-2.7 Ga in terms of whole-rock Sm-Nd and zircon Lu-Hf isotope studies. It has been intriguing how the vast 2.8-2.7 Ga juvenile mafic crust would have differentiated into the 2.6-2.5 Ga felsic crust. This issue is addressed in the present study by investigating zircon U-Pb ages and Lu-Hf isotopes as well as whole-rock major-trace elements and Sm-Nd isotopes in Neoarchean felsic igneous rocks from the Yunzhongshan area in the central part of the North China Craton. Zircon U-Pb dating of felsic volcanics, TTG gneisses and high-K granites yields three groups of crystallization ages at 2562-2508 Ma, 2537-2504 Ma and 2518-2508 Ma, respectively, consistent with ages for mafic volcanics in the target area. Zircon Lu-Hf and whole-rock Sm-Nd isotope data indicate that all these rocks were ultimately originated from the ca. 2.8-2.7 Ga juvenile mafic crust. The felsic volcanics are subdivided into low-Si and highSi suites. The low-Si rocks have higher MgO and FeO contents and contain abundant 2.8-2.7 Ga relict zircons with an age peak of similar to 2.72 Ga, and were produced by partial melting of similar to 2.72 Ga TTG at similar to 730 to 750 degrees C. The high-Si volcanics were derived from partial melting of mafic restites after extraction of the similar to 2.72 Ga TTG, with significant amounts of garnet and amphibole, but no or rare plagioclase as residual phases, corresponding to a melting pressure at similar to 15 kbar. The TTG gneisses were produced by partial melting of similar to 2.8 to 2.7 Ga garnet amphibolites at similar to 10 to 15 kbar with variable amounts of garnet, amphibole and plagioclase in the residue. The high-K granites were produced by partial melting of similar to 2.8 to 2.7 Ga medium-high K mafic rocks at pressures of < 10 to 12 kbar with a considerable amount of plagioclase in the residue. The TTG rocks and high-Si volcanics require a subduction origin to account for their enrichment in LILE, so that they were mostly likely derived from partial melting of the older arc crust with moderate enrichment in LILE and LREE. By contrast, the ultimate sources of the high-K granites would be the similar to 2.8 to 2.7 Ga medium-high K basalts, which have lower Zr/Sm ratios than the TTG gneisses and felsic volcanics, and even lower than the bulk continental crust, but consistent with modern island arc basalts. It is possible that the 2.8-2.7 Ga magmatism was subduction-related, with a compositional change of the mafic arc crust from relatively sodic to more potassic at around 2.7 Ga. This may be caused by accretion of the ancient oceanic arc onto the ancient continental margin. It was followed by both growth of the juvenile crust and reworking of the older arc crust in the late Neoarchean. Such series of magmatic processes can account for the voluminous generation of the 2.56-2.50 Ga felsic and mafic igneous rocks in the central part of the North China Craton. Taken together, both growth and reworking of the juvenile crust were realized through accretionary and rifting orogeneses, marking the operation of plate tectonics in the Neoarchean.