Recognition of Grenvillian volcanic suite in the Shennongjia region and its tectonic significance for the South China Craton

The Grenvillian (1.3-1.0 Ga) orogenic and subduction related events have been regarded as a critical linkage in Rodinia reconstruction. Here we report the first recognized late Mesoproterozoic arc-volcanic sequence in South China. The volcanic sequence is well-exposed in the Shennongjia region, along the northern central margin of the Yangtze Block, and has previously been mapped as a volcanic sequence interbedded with early Mesoproterozoic strata, but reliable dating data have been lacking. The volcanic sequence comprises alkali-, calc-alkaline basalts and tholeiitic andesitic rocks, and is dated at 1103 +/- 8 Ma by the U-Pb zircon method. The alkali basalts have high TiO2 (2.62-4.48 wt.%) and relatively low Mg# (42-55), and display OIB-like incompatible elemental patterns. They have initial Sr isotopic ratios of 0.7021-0.7056 and epsilon(Nd)(t) values of +2.4 to +4.0. By contrast, the calc-alkaline basalts have high Mg# (57-68), Cr (67.1-211 ppm) and Ni (62.8-121 ppm), and exhibit a pronounced depletion in high field strength elements (Nb, Ta, P and Ti) and positive Pb anomalies. Compared to the alkali basalts, they possess higher initial Sr isotopic ratios of 0.7086-0.7120 and large negative epsilon(Nd)(t) values of -15.3 to -14.2. The andesitic rocks are characterized by negative Eu anomalies (Eu/Eu* = 0.62-0.65) and depleted Nb, Ta, P and Ti anomalies along with small negative epsilon(Nd) values (-2.7 to -2.1) and low Sr isotopic ratios (0.7028-0.7042). It is suggested that the alkali basalts were derived from a depleted asthenospheric mantle source, the calc-alkaline basalts from previously metasomatised subcontinental lithospheric mantle rocks, whereas the andesitic rocks were derived from mafic lower crustal anatexis induced by basaltic underplating and/or asthenospheric ascent. The Shennongjia volcanic sequence is thus proposed to have developed within an island-arc setting. Integrating our new data with documented igneous and metamorphic events during the end Mesoproterozoic to early Neoproterozoic, the northwestern and western segments of the Yangtze Block likely comprised a collage of microcontinents during the Grenvillian period and underwent westward lateral continental growth by subduction accretion and microcontinent growth by subduction-related collision. Given the documented similar to 1.6 Ga peak population of detrital zircon spectra for the lower Shennongjia strata, similar to 1.6 Ga significant juvenile mantle input in the Shennongjia area and similar to 1.1 Ga Grenvillian metamorphic record in the westernmost Yangtze segment, we suggest a western Yangtze-South Australia connection during Rodinia assembly of the Grenvillian period. It thus provides a new insight into the position of the South China Craton in the supercontinent. (C) 2011 Elsevier B.V. All rights reserved.