Geochemistry and genesis of the Late Archean Low-Al and High-Al TTGs from the Huai' an terrane, North China Craton

Trondhjemite, Tonalite and Granodiorite (TTG) as dominated lithological assemblages in Archean high-grade metamorphic terranes can provide important clues for the history and mechanism of crustal growth. However, their genesis is still debated. The Huai' an terrane located in the north-central region of the North China Craton, is mainly composed of Archean gray gneisses, which are dominated by TTG rocks. The Huai' an TTG are predominantly tonalite with minor trondhjemite and granodiorite. Previously, we recognized some Low-Al trondhjemites from the Huai' an gray gneisses with pronounced negative Eu anomalies and enriched heavy rare earth elements (REE) as well as high SiO2 contents. Zircon SHRIMP U-Pb dating yielded formation age of 2.53Ga, indicating their coeval magmas with the widespread High-Al TTGs, regionally. This work provides new data from Low-Al and High-Al TTGs to give a detailed investigation on their petrogenesis. As a result, the Low-Al trondhjemites are enriched in SiO2 (76% similar to 79%), with low Al2O3 (11.01% similar to 12.61%), CaO (1.27% similar to 1.59%), MgO (0.74% similar to 0.24%) contents and highly variable Mg-# (18 similar to 53). In contrast, major elements of the High-Al TTGs vary largely, for example, SiO2 is between 63% similar to 77%, Al2O3 is 13.2% similar to 17.77%, CaO is 1.8% similar to 5.78%, MgO is 0.18% similar to 3.84%, and Mg-# is 35 similar to 64. In addition, the Low-Al trondhjemites exhibit pronounced negative Eu anomalies (Eu/Eu* is 0.59 similar to 0.44) and enriched heavy REE ((La/Yb)(N) = 4 similar to 7, (Gd/Yb)(N) = 0.36 similar to 1.27), whereas the High-Al TTGs show slightly negative to pronounced positive Eu anomalies (Eu/Eu* = 0.8 similar to 5.35) and relatively low heavy REE ((La/Yb)(N) = 10 similar to 103, (Gd/Yb)(N) = 1.97 similar to 5.72). In primitive mantle normalized spidergrams, the Low-Al trondhjemites are depleted in Sr and Ba, while the High-Al tonalites are enriched in the elements. Lu/Hf ratios of the Low-Al trondhjemite (0.10 similar to 0.16) are relatively higher than those of the High-Al rocks (0.01 similar to 0.07), indicating that they have different sources and petrogenesis. Based on a combination of zircon U-Pb and Lu-Hf isotopes and whole-rock geochemistry, we consider that the Low-Al trondhjemitic magma was derived by partial melting of juvenile crust leaving a residue composed of pyroxene and plagioclase, without or with minor amphibole, and experienced the crystallization of monazite en route. The High-Al TTGs were generated by partial melting of juvenile crust at relatively high pressure with a residual mineral assemblage of garnet, amphibole, pyroxene and minor plagioclase, and the magma experienced fractional crystallization of hornblende and clinopyroxene with some extent of feldspar accumulation. It is proposed that the Huai' an TTGs were generated by anatexis of lower crust at various depths as a consequence of the Late Archean mantle plume activity or the combined effect of mantle plume and slab subduction.