Zircon isotope evidence for ≥3.5 Ga continental crust in the Yangtze craton of China

The presence or absence of Paleoarchean crust in different continents provides an important basis for a comparison of their growth histories. Since the finding of ca. 3.8 Ga detrital zircon in the North China craton by means of SHRIMP U–Pb dating, it has been a question whether the Yangtze craton, one of the two major blocks constituting the basement of China continent, also contain Paleoarchean crustal relicts. It is generally accepted that the basement of the Yangtze craton mainly formed in the Paleoproterozoic, with only minor Archean rocks in ages of 2.5–3.3 Ga. A combined study of zircon U–Pb dating and Hf isotope analyses for the Kongling migmatite and gneiss from this craton reveals the existence of ca. 3.5 Ga rocks. The zircon U–Pb ages are divided into three groups: ∼3.2 Ga in cores, ∼2.9 Ga in most grains and ∼2.0 Ga in rims. Both ∼3.2 Ga cores and ∼2.9 Ga grains have negative ɛHf(t) values of −1.6 and −4.0, respectively. Mantle-like δ18O values of 5.6–5.8‰ were obtained for the zircons, precluding a significant input by sedimentary source component for the magmas that are responsible for protolith of the Kongling metamorphic rocks. From the Hf model ages of these zircons, about 3.5 Ga crustal relicts are speculated to occur in the Kongling rocks. This provides the first demonstration for the existence of Paleoarchean rocks in the Yangtze craton. Thus the early evolutionary history of the Yangtze craton may resemble the North China craton that has crustal relicts with the minimum ages of 3.5 Ga. These Paleoarchean rocks resided in the crust of the Yangtze craton for at least several hundreds of million years before remelting, probably leaving some clues to be traced and verified. Paleoproterozoic U–Pb ages of 1.8–2.0 Ga are identified in magmatic and metamorphic rocks in the Yangtze craton, providing a likely connection of this craton with the North China craton with respect to assembly and breakup of the supercontinent Columbia during the late Paleoproterozoic.