Paleoproterozoic Granites Found in Cathaysia Geological Block, Suggests Archean Basement

Origin and geological significance of Paleoproterozoic granites in the northeastern Cathaysia Block, South China

Authors:

Liu et al

Abstract:

New whole-rock major- and trace-element analyses, zircon U–Pb dating and Hf-isotope analyses on five granite bodies in southwestern Zhejiang Province define a significant Paleoproterozoic magmatic episode in the eastern Cathaysia Block.

Based on geochemical data from twenty-seven Paleoproterozoic granite samples, S-, I- and A-type granites have been recognized. A-type granites have low SiO2 (62.5–70.5%), and high contents of Fe2O3t (3.82–9.95%), CaO, P2O5, Ba, Ga, Zn, Y, high-field-strength elements (HFSE, e.g. Zr, Hf, Nb and Ta) and rare-earth elements (REE), with high FeOt/(MgO + FeOt) (0.78–0.89) and Ga/Al (>3.1) ratios. In contrast, the S-type granites share slightly high SiO2 (65.9–74.1%) contents and low Fe2O3t, HFSE and REE contents with high A/CNK (1.04–1.37) and low Al2O3/TiO2 ratios. I-type granites were recognized for the first time in this study area. They are characterized by low A/CNK (0.94–1.07, a mean of 1.01), Rb/Ba (0.17–0.33) and Rb/Sr (0.48–1.14, a mean of 0.87) ratios, low contents of TiO2, Ga, Zn, HFSE, high Na/K ratios and high CaO contents.

LA-ICPMS U–Pb dating of zircons shows a progression from I-type granites (Chimushan and Xiaocu plutons) formed at ∼1912 Ma and ∼1887 Ma, to S-type granites (Chimushan, Xiaocu and Wengkeng plutons) at ∼1884 Ma, ∼1882 Ma and ∼1876 Ma, and then to A-type granites (Huaqiao, Xiaocu and Chimushan plutons) at ∼1878 Ma, ∼1869 Ma and ∼1859 Ma. These Paleoproterozoic granites suffered at least two episodes of metamorphism in early Mesozoic (250–230 Ma) and late Mesozoic (110–90 Ma) time. The integration of previously available ages and geochemical data with this study indicates that Paleoproterozoic I-type and S-type granites (1912–1875 Ma) were generated before the A-type granites (1878–1832 Ma). The S- and I-type granites have low formation temperatures of 725–840 °C and were probably formed in an early post-collisional tectonic setting, whereas the A-type granites have high crystallization temperatures (840–920 °C) and formed in the later post-collisional extensional setting. Since 1782–1766 Ma mafic rocks also occur in the area, it is suggested that a complete late Paleoproterozoic orogeny from syn-collisional to post-collisional and final anorogenic tectonic settings occurred in the northeastern Cathaysia Block from >1912 Ma to 1766 Ma, lasting about 150 Myr. This in turn suggests that the Cathaysia Block was once a part of the Paleoproterozoic Columbia supercontinent.

The S- and A-type granites share similar zircon Hf-isotope compositions with an average ɛ Hf(t ) value of ca. −4.19 and two-stage Hf-isotope model ages (View the MathML sourceTDMC) of 2.78 Ga, suggesting sources similar to those of the Badu Complex. Geochemical and isotopic compositions suggest that the Xiaocu I-type granites originated from meta-felsic igneous rocks, whereas the Chimushan I-type granites crystallized from mixed magmas derived from both old meta-sedimentary and juvenile meta-mafic rocks. The complex types and compositions of the Paleoproterozoic granites in the study area suggest that the lower crust in the northeastern Cathaysia Block was quite heterogeneous. The discovery of Paleoproterozoic I-type granites, coupled with the ∼2.8 Ga model ages and inherited zircons in these granites, demonstrates the existence of Archean basement rocks in the Cathaysia Block.