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 [_ƐHf (t) = -5.48–3.19 and _ƐNd (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 [_ƐHf (t) = -5.48–3.19 and _ƐNd (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 [_ƐHf (t) = -0.16–4.17 and _ƐNd (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.地球的首批大陆地壳形成于太古宙（Archaean），主要由英云闪长岩-奥长花岗岩-花岗闪长岩及少量闪长岩构成，此类岩石简称DTTGs，其对于探究早期壳幔动力学体制、理解大陆地壳的形成机制具有至关重要的意义。本文对敦煌地块（Dunhuang Block）内的DTTGs岩石开展了锆石U-Pb定年、Lu-Hf同位素测试、全岩地球化学分析与岩石学研究。区内DTTGs的侵位时代可划分为三期，分别约为2.67 Ga、2.60 Ga与2.50 Ga。约2.67 Ga的TTG岩石具有高二氧化硅（SiO₂）含量（68.14~71.70 wt%）、低氧化镁（MgO）含量（0.65~1.31 wt%），以及高球粒陨石标准化La/Yb比值[(La/Yb)N，平均为146]，同时表现出富集的Nd-Hf同位素组成[εHf(t) = -5.48~3.19，εNd(t) = -5.77~0.53]，指示其起源于加厚下地壳中角闪岩的部分熔融。与之相对，约2.60 Ga的过渡型TTG岩石具有相对较高的MgO含量（2.80~3.39 wt%），平坦的稀土元素（REE，Rare earth element）配分模式，以及中等的(La/Yb)N比值（平均为20.49），其Nd-Hf同位素组成较为分散[εHf(t) = -5.48~3.19，εNd(t) = -3.99~3.08]。据此认为，该期过渡型TTG的熔体源于幔源岩浆上涌诱发的浅部地壳部分熔融。约2.50 Ga的闪长岩具有较低的SiO₂含量（55.72~59.11 wt%）但较高的MgO含量（3.51~4.52 wt%），同时表现为正的Nd-Hf同位素组成[εHf(t) = -0.16~4.17，εNd(t) = 2.00~4.45]，表明其起源于受俯冲板片流体交代的地幔楔的部分熔融。结合详细的岩石成因研究与地壳厚度变化特征，本文认为复杂的壳幔相互作用可能是敦煌地块新太古代（Neoarchaean）DTTGs多样性形成的核心原因；该地块在新太古代之前经历了长期的弧增生过程，随后在2.67~2.60 Ga期间发生地壳拆沉作用，之后过渡到俯冲动力学环境。