Precambrian tectonic attribution and evolution of the Songliao terrane revealed by zircon xenocrysts from Cenozoic alkali basalts, Xilinhot region, NE China

摘要

U-Pb dating and Hf isotopic analyses were carried out on xenocrystic zircons from the Cenozoic alkali basalts in the Xilinhot region, NE China, to constrain the nature and evolution of the underlying Songliao terrane, and its role in the assembly of the Central Asian Orogenic Belt. Our data show a wide range of U-Pb age distributions, with Precambrian clusters of ca. 2500 Ma, 2400-2200 Ma, 2000-1700 Ma, 1500 Ma, 1000-800 Ma, 700-500 Ma, and Phanerozoic peaks at 494 Ma, 411 Ma, 312 Ma, 263 Ma, 214 Ma, 140 Ma and 44 Ma. The Phanerozoic age peaks are well-recognized in supracrustal rocks in the Songliao terrane or near the sample location. Although most Phanerozoic zircons have positive ε_(Hf)(t) values, which is consistent with many locations throughout the Central Asian Orogenic Belt, some Phanerozoic ones have negative εHf(t) values and Tcrust up to ca. 2.5 Ga, indicating the involvement of old crustal components during the Phanerozoic thermal events. More importantly, zircons with207 Pb/206Pb ages between ca. 2500 Ma and 1700 Ma have Tcrust of 2.5-3.5 Ga, revealing the possible occurrence of unexposed Archean crustal remnants beneath the Songliao terrane, where no known supracrustal rocks are older than Neoproterozoic. Integrating the U-Pb and Hf isotopic results, we interpret the Songliao terrane as ancient in nature and with a complex crustal evolution history, including the addition of juvenile materials at ?2.5 Ga, reworking of pre-existing crustal components at 2.4-2.2 Ga, 2.0-1.8 Ga, 1.8-1.7 Ga, 1.0 Ga, 0.7-0.6 Ga, and massive crustal growth during the Phanerozoic. Comparing these data with the Precambrian zircon age distributions of other cratons or micro-continents, we suggest the Songliao terrane shows an affinity to both Monglia and NE Gondwana.