No abstract

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).

A set of granite porphyry (grnt‐porphyry) and sub‐rhyolite porphyry (rhy‐porphyry) with specific blue quartz phenocrysts was disintegrated in the northern Kongling terrane, China. Geochemical and isotopic analyses were performed on samples from Kongziqiao (KZQ) and Baizhuping (BZP). In terms of major elements, both are rich in silicon and alkalis, but depleted in calcium and magnesium. Their trace element contents are also similar, exhibiting negative anomalies for large‐ion lithophile elements (LILEs) such as K, Rb and Sr but positive anomalies for high‐field‐strength elements (HFSEs) such as Hf, Ce and Lu. Other elements such as Nb, Ti, P, Sm, Ho, Y and Yb show negative anomalies; however, the KZQ grnt‐porphyry is characterized by positive Ba and negative Zr anomalies; whereas the BZP sub rhy‐porphyry has negative Ba, U and Ta anomalies. The overall geochemistry is characteristic of A‐type granite formed in an extensional environment (A2‐type). Yield ages of the KZQ grnt‐porphyry are (2,239 Ma) and that of the BZP (1,895 Ma) sub rhy‐porphyry are Paleoproterozoic. Analysis of Hf isotopes shows that both have negative ε Hf (t) values, but the εHf (t) value of the KZQ grnt‐porphyry is slightly more positive (minimum −12.4; mean − 8.8) and its depleted mantle model age (TDM1 = 2.85–3.06 Ga, mean 2.92 Ga) is slightly older than the BZP sub rhy‐porphyry (minimum ε Hf (t) = −17.1, mean − 14.9; TDM1 = 2.79–2.92 Ga, mean 2.85 Ga). This suggests that they are derived from partial melting of different generations of ancient crustal material, which implies significant stratification in the Archean crust. The ε Nd (t) values of two samples of each rock type also indicate that both were derived from Ca. 2.9 Ga ancient crust (εNd (t) KZQ = −5.58, −4.74; εNd (t)BZP = −8.55, −8.22). The petrogenesis of both rock types is related to deep crustal extensional collapse caused by post‐orogenic extension before 2.2 Ga and after 1.9 Ga. The formation of two porphyries represents the changes in tectonic system of the Yangtze Block, and this tectonic evolution may have contributed to the reconstruction of the Columbia supercontinent.

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units in the Yangtze Craton, China, and the metabasite xenoliths they host, remain controversial, and resolving these issues is important if we are to understand the geodynamics of the early Yangtze Craton. We have discovered many biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of a Mesoarchean granite–greenstone belt in the Kongling area. The dikes of alkali granite might also be related to this oceanic plate subduction and the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).