Paleoproterozoic Magmatism Of Yangtze Craton: Timing And Geological Implications

The Yangtze Craton is one of the largest Precambrian blocks in the eastern China. Recent studies suggest that there existed a Paleoproterozioc collisional orogen event in the Yangtze Craton, and thus it involved in the assembly of the supercontinent Columbia. Previous studies have mostly focused on the nature and timing of the Paleoproterozoic high-grade metamorphism. The subsequent magmatism has, however, not been well constrained, although it may bear important implications for the early evolution of the Yangtze Craton. In this paper, we carried out an integrated study of petrography, major and trace element geochemistry, zircon U-Pb age and Hf isotope composition for an A-type granite and a mafic dyke from the Kongling terrain in the northern part of the Yangtze Craton. The results not only provide significant constraints on the Paleoproterozoic tectonic evolution, but also shade some lights on the cratonization of the Yangtze Craton.Mafic dyke is a sign of regional extension, and thus has important tectonic significance. A great amount of mafic dykes occur in the Kongling terrain of the Yangtze Craton, which have great bearing on the early evolution of the Yangtze Craton. Their ages, however, have not been well constrained. In this paper we report an integrated study of zircon U-Pb age and Hf isotope compositions for a mafic dyke in the Kongling terrain. The zircons yielded a weighted mean 207Pb/206Pb age of 1852±11Ma, which represents its intrusion age. They haveεHf(t) values of-6.3 to 0.5, with a weighted mean of-3.06±0.88, suggesting that the mafic dyke came from metasomatic mantle. The results indicate that the Yangtze Block had transformed into post-collisional extensional regime at ca.1850 Ma. In the same period, the Yangtze Craton shows enough rigidity to produce brittle rupture, and thus has the characteristics of a craton.The Quanyitang pluton is an A-type granitic intrusion with a coarse-grained and porphyritic texture, and is mainly composed of alkali feldspar, quartz, biotite and plagioclase. Zircon U-Pb dating yields 207Pb/206Pb ages of 1.85 Ga, which represent its intrusion age. The Quanyitang pluton has high SiO2 (72:6-74.4 wt%), K2O (5.09-5.56 wt%) and Na2O (3.02-3.36 wt%), and low Fe2O3 (2.56-3.19 wt%) and MgO (0.25-0.37 wt%). It is metaluminous (A/CNK=0.95-0.98), and belongs to high-K calc-alkaline series. On the chondrite-normalized REE diagram, the samples from the Quanyitang pluton invariably show relatively enrichment of light rare earth elements (LREEs), with high (La/Yb)N ratios (10.6-21.7), and significant negative Eu anomalies. Their primitive-mantle normalized spidergrams display negative Ba, Nb, Ta, Sr, P, Eu and Ti anomalies. The calculated zircons saturation temperature of ca.845℃imply that Quanyitang granite formed at high temperatures. These geochemical features show that the Quanyitang pluton is clear an A-type granite. The zircons show significant negative eHf(t) values of-17.6 to-20.9, corresponding to two-stage Hf modal ages of 3.6 to 3.8 Ga, indicating that it was derived from partial melting of Paleoarchean crustal rocks. Based on the geochemical features, it is inferred that the Quanyitang granite was generated by dehydration melting of Archean tonalitic gneiss with a plagioclase-rich residual assemblage in shallow crust. The Quanyitang granite belongs to the A2-type granite, and has intrusion age younger than the ca.2.0 Ga collisional event in the Yangtze Craton. This suggests that it formed in a post-collisionall tectonic setting, and thus a tectonic regime switch of collision to extension might have occurred at ca.1.85 Ga in the Yangtze Craton. The occurrence of K-rich A-type granite provides unambiguous evidence that the cratonization of the Yangtze Craton might had been completed at ca.1.85 Ga, and the cratonization processes might result from the ca.2.0 Ga collisional event due to the assemblage of the supercontinent Columbia.