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

The Grenvillian (1.3–1.0Ga) 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±8Ma by the U–Pb zircon method. The alkali basalts have high TiO2 (2.62–4.48wt.%) 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 ɛ Nd(t) values of +2.4 to +4.0. By contrast, the calc-alkaline basalts have high Mg# (57–68), Cr (67.1–211ppm) and Ni (62.8–121ppm), 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 ɛ 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 ɛ 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 ∼1.6Ga peak population of detrital zircon spectra for the lower Shennongjia strata, ∼1.6Ga significant juvenile mantle input in the Shennongjia area and ∼1.1Ga 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.