Neoarchaean DTTGs from the Dunhuang Block, Tarim Craton: insights into petrogenesis and crust-mantle interactions

Earth's first continental crust is formed by Archaean and mainly consisted of tonalite-trondhjemite-granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust-mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U-Pb dating and Lu-Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14-71.70 wt%), low MgO contents (0.65-1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [& sigma;Hf (t) = -5.48-3.19 and & sigma;Nd (t) = -5.77-0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80-3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [& sigma;Hf (t) = -5.48-3.19 and & sigma;Nd (t)= -3.99-3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72-59.11 wt%) but high MgO (3.51-4.52 wt%) contents with positive Nd-Hf isotopes [& sigma;Hf (t) = -0.16-4.17 and & sigma;Nd (t) = 2.00-4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust-mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction.