Unexposed Archean components and complex post-Archean accretion/reworking processes beneath the southern Yangtze Block revealed by zircon xenocrysts from the Paleozoic lamproites, South China

Abstract

Integrated U–Pb dating and Hf-isotope analysis of zircon xenocrysts from the Baifen lamproites of the Zhenyuan area in the southeastern Guizhou Province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the Yangtze Block, South China. A total of 148 zircons with 90–110% concordance yield U–Pb ages varying from 3458 ± 9 Ma to 252 ± 2 Ma. They generally have high Th/U ratios (>0.3), indicating a magmatic origin. The two youngest magmatic zircons give a weighted mean 206Pb/238U age of 253.0 ± 2.8 Ma, which is interpreted as the emplacement age of the host Baifen lamproites. Most zircons with pre-eruption ages are xenocrysts derived from the deeper crust of the southern Yangtze Block. They define two major Paleoproterozoic age populations at 2.17 Ga and 1.8 Ga, with similar Hf model ages (Tcrust) of 2.4–2.8 Ga (peak at 2.64 Ga). The oldest zircon has a 207Pb/206Pb age of 3458 ± 9 Ma with εHf (t) = +1.47 and Tcrust = 3.7 Ga. This demonstrates the presence of Archean crustal components (or remnants) as old as 3.7 Ga beneath the southern Yangtze Block. Magmatic events involving significant addition of juvenile mantle-derived materials mainly occurred at 2.2–2.1 Ga and 2.0–1.8 Ga, but the latter event also involved reworking of the Neoarchean crust and might represent the cratonization of the Yangtze continent during the middle Paleoproterozoic, which could be correlated to the assembly of the Columbia supercontinent. This segment of the Yangtze Block became tectonically quiescent until 1.2 Ga, implying that the crustal stabilization lasted for ca 600 million years. A few late Mesoproterozoic zircon xenocrysts within the age range 1.3–1.0 Ga suggest the occurrence of Grenville-aged events. These Neoproterozoic zircon xenocrysts (992–547 Ma) have a wide range of εHf (t) values (+8.47 to −22.07), consistent with the zircon Hf isotopic compositions of widespread coeval igneous rocks in the Yangtze Block. The data show that the Yangtze Block has undergone important crustal growth and reworking during the Neoproterozoic in response to the breakup of the Rodinia supercontinent. The early–middle Paleozoic zircon xenocrysts (513–357 Ma) have negative εHf (t) values (−2.97 to −12.69) and Tcrust Hf model ages of 1.59–2.27 Ga, indicating reworking of the Precambrian crustal basement without juvenile contributions during the Caledonian orogeny. The positive εHf (t) values (+13.79 to +0.22) of twelve late Paleozoic zircons (284–264 Ma), with a major age peak at ca 275 Ma, suggest that the Precambrian basement beneath the southern Yangtze Block experienced an episode of strong rejuvenation during the Permian. We propose a possible linkage between the Baifen lamproite magmatism (284–252 Ma) and the Emeishan flood-basalt volcanism.