Petrogenesis of the Huashanguan A-type granite complex and its implications for the early evolution of the Yangtze Block

An integrated study of petrography, zircon U-Pb ages, Hf-O isotope compositions, as well as whole rock geochemistry was carried out for a Paleoproterozoic granite complex in the northern Yangtze Block, to investigate its petrogenesis and geological significances. Petrographically, the Huashanguan complex is a granite complex comprised of monzogranite in the marginal facies and K-feldspar granite in the central facies. U-Pb dating of magmatic zircons from the monzogranite and K-feldspar granite yielded ages of 1816 ± 50 Ma and 1843 ± 9 Ma respectively. The more precise age of 1843 ± 9 Ma is suggested to represent the formation age of the Huashanguan complex. The granites have uniform negative zircon εHf(t) values of −20.2 to −15.8, with corresponding TDM2 ages of ca. 3.6-3.5 Ga. The δ18O values average at 6.80 ± 0.09‰. These indicate that the granites were generated by partly melting of Archean rocks, implying the widespread of Archean basement in the Yangtze Block. The monzogranites display low FeOT and MgO contents and evolved trace element features, suggesting their formation by more fractional crystallization from similar parent magma of the K-feldspar granites. The occurrence of the Huashanguan granite complex indicates that the Yangtze Block has experienced a tectonic transition from tectonic convergence to extension at ca. 1.85 Ga, possibly due to the collapse of the ca. 2.0 Ga orogeny.