Zircon Age and Hf Isotope Compositions of Amphibolite Enclaves from the Kongling Complex

The Kongling complex, an important part of the basememt block for the Yangtze craton, is exposed in the northern part of the Huangling anticline in western Hubei province. It mainly consists of TTG gneisses, amphibolites, leptynites and khondalite series. So far, the oldest basement rocks recognized are the TTG gneisses with ages around 2.9 Ga in the Yangtze craton, but the amphibolites occur as enclaves in the TTG gneisses, implying the amphibolites are older than TTG gneisses. Zircon U-Pb ages and Lu-Hf isotopic compositions of amphibolites from the Kongling complex were analyzed by LA-ICP-MS. Accorinding to CL images and Th/U ratios, all zircons from amphibolite enclaves can be divided into three types: igneous, metamorphic alteration and metamorphic growth zircon(.1)The protolith ignous zircons show a core-mantle-rim structure. The cores have well-developed magmatic oscillatory zoning, and the mantles are thin and bright. The U-Pb spot analyses were done on cores, and the results form a discordia with the upper intercept at(3000±24)Ma, MSWD=2.4. This age is interpreted as the eruption time of tholeiitic basalt, which was the protolith of amphibolite enclaves. Therefore, the amphibolite enclaves are the oldest basement rocks in the Yangtze craton found to date.(2)The metamorphic alteration zircon grains still keep the same shape as their protolith igneous zircons, and show weak oscillatory zoning, sector zoning and patched zoning, which reveals that the metamorphic zircons were transformed from the igneous zircons. These zircon U-Pb analysis results form a discordia with the upper intercept at(2715± 9)Ma, MSWD=1.4, indicating the ignous zircons were transformed into metamorphic zircons at this time. Therefore, the first metamorphic event occurred at(2.75~2.7)Ga, and the TTG granites and tholeiitic basaltic rock enclaves were transformed into TTG gneisses and amphibolite enclaves during the first metamorphic event.(3)The metamorphic growth zircons have such internal structure as cloudy zoning, sector zoning, fire-tree zoning, planar zoning, indicating they were formed in amphibolite-facies. These zircon U-Pb analysis results form a discordia with the upper intercept at(2558±40)Ma, MSWD=0.93. This age is interpreted as the timing of tectono-thermal event, related to the Shuiyuesi movement, and caused unconformable contact between Archean and Proterozoic. In conclusion, the granite-greenstone terrain was transformed to Neoarchean stable landmass by the first and second metamorphic events, and then the stage of cratonization began to last for 500 Ma.(4)The zircons have the negative eHf (t)average values of -11.59~-3.89, and the Hf model age of 3.4 Ga, iudicateing a new demonstration for the existence of 3.4 Ga crustal relicts, which is older than the Kongling Group(3.2 Ga).