THE ARCHEAN CRUSTAL LAYERING BENEATH THE YANGTZE CRATON REVEALED BY SUCCESSIVE EMPLACEMENT OF PALEOPROTEROZOIC A-TYPE GRANITES

The formation of granite from crustal remelting is one of the process of cratonization. Garnet-bearing and garnet-free granites were recognized in the Kongling Complex, which is regarded as the rare Precambrian basement of the Yangtze Craton. The former, formed in ca. 1900 Ma, outcropped as granite porphyry dykes with K-feldspar (15%), quartz (5%), plagioclase (3%) and minor garnet phenocrysts within a major groundmass of micro-grained feldspar, quartz and biotite; the other, with emplacement age of ca. 1850 Ma, shows coarse-grained texture and contains K-feldspar (70%), quartz (20%), small amount of biotite and plagioclase. They are characteristic alkalic-calcic, metaluminous A-type granites, possessing 69.2–71.5% (n=12) and 72.6–74.5% (n=18) SiO₂ separately. The porphyries exhibit greater trace element concentration and higher zircon saturation temperature (average=920℃, n=12) than coarse-grained granites (average=841℃, n=14). The εHf(t) values from the porphyries and the coarse-grained granites are -16.3 ~ -10.2 and -20.2 ~ -14.0. Their two-stage Hf model ages are 3.0~3.3 Ga and 3.2~3.5 Ga accordingly. It falls into the evolvement curve of the 2.8~2.9 Ga Kongling TTGs and indicating all the granites are formed by the reworking of the Archean Complex of the Yangtze Craton. Besides, a reliable tonalitic source resembling natural tonalite in the Kongling Complex can be found for both granites through trace element modeling. On account of the intrusive contact relationships, it is suggested that the crust structure beneath the Mesoarchean Yangtze Craton is varying from deeper tonalites (ca. 2937 Ma) to shallower granitic gneisses (ca. 2866 Ma). Hopefully, the vertical layering of Archean continental crust will be of great significance for deciphering the early Yangtze cratonization.