Episodic Paleoarchean-Paleoproterozoic (3.3–2.0 Ga) granitoid magmatism in Yangtze Craton, South China: Implications for late Archean tectonics

Abstract

The Archean Kongling Terrane preserves voluminous Paleoarchean-Paleoproterozoic granitoids, recording the early formation and evolution history of the Yangtze Craton, South China. We present petrography, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of twelve gneissic granitoids from this terrane, including trondhjemites, biotite-granites, and two-mica granites. Magmatic zircons in these granitoids yield emplacement ages of 3.31–3.29Ga for trondhjemites, 2.81–2.78 and 2.66Ga for biotite-granites, as well as 2.70–2.64, 2.42, and 2.00Ga for two-mica granites. The 2.8–2.6Ga granites contain minor 3.4–2.9Ga inherited zircons and 2.0Ga metamorphic zircons that formed mainly by recrystallization. Major and trace element compositions of the trondhjemites suggest a garnet amphibolite-facies low-K mafic source, similar to the typical Archean medium-pressure trondhjemite-tonalite-granodiorites (TTGs), whereas those of the biotite- and two-mica granites indicate tonalitic and tonalitic-sedimentary sources, respectively. The Lu–Hf data (chondritic ɛHf(t) values −0.8 to 0 and Eoarchean two-stage Hf model ages 3.75–3.70Ga) for the trondhjemites support partial melting of a long-lived ancient depleted-mantle-derived or a juvenile lower-mantle-derived mafic crust. The Lu–Hf data and inherited zircon ages also suggest that the 2.8Ga granites were derived from the local 3.4–2.9Ga TTGs. Together with previously addressed 3.2 and 3.0–2.9Ga TTG magmatism, a remarkable change is observed at 2.8Ga in the petrology, geochemistry, and petrogenesis of the episodic 3.3–2.0Ga granitoid magmatism from TTG- to granite-dominated in the Kongling Terrane, which may reflect a transition from subduction- to collision-related tectonics. In addition, we note that the 2.70–2.64Ga biotite- and two-mica granites (this study) as well as 2.67–2.62Ga A-type granites (published data) were emplaced shortly after a 2.75–2.72Ga high-grade metamorphic event (published data) in the Kongling Terrane. These granites could be generated as a result of orogenic root collapse and subsequent mantle upwelling, implying a 2.7Ga orogenic event in the Yangtze Craton.