Early Neoproterozoic (∼840 Ma) slab window in South China: Key magmatic records in the Chencai Complex

Abstract

It has been generally accepted that the Neoproterozoic amalgamation of the Yangtze and Cathaysia Blocks along the Jiangnan orogenic zone created the united South China Block (SCB). However, its final oceanic consumption time and subsequent assemblage process remain controversial. This paper presents a set of new geochronological, geochemical and Sr-Nd-Hf-O isotopic data for the early Neoproterozoic mafic to felsic igneous rocks in the previously-mapped Chencai Group and the associated equivalents. Our data show that the compositions of these rocks plot into the fields of the subalkaline and alkaline series, and they can be geochemically subdivided into five groups. Group 1 has SiO2 = 56.68–63.97 wt.% and MgO = 2.31–8.90 wt.% with Mg-number of 42–69, resembling the high-Mg intermediate rocks. Their 87Sr/86Sr(t) ratios range from 0.7022 to 0.7043, εNd(t) from +3.0 to +5.5, εHf(t) from +4.9 to +7.7 and δ18O from 4.78‰ to 5.57‰, most likely showing their petrogenetic relevance to the interaction of young slab-derived fluids with the overlying refractory mantle. Group 2 with SiO2 of 63.68–74.89 wt.% shows high Sr, Ba, Sr/Y and (La/Yb)cn, but low Y and Yb, resembling high-silica adakites. Its arc-like elemental signatures and highly positive εNd(t) -εHf(t) values, along with δ18O vaules of 4.95–5.39‰, suggest its derivation from the recently-ponded mafic lower crust with a mantle wedge source similar to Group 1. Group 3 is characterized by the arc-like basaltic and andesitic rocks with SiO2 = 45.06–63.53 wt.%, MgO = 3.38–12.99 wt.%, 87Sr/86Sr(t) = 0.7032–0.7047, εNd(t) = +2.0 ∼ + 5.9, εHf(t) = +3.2∼+7.8, originating from a MORB-like source newly modified by slab-derived fluids. Group 4 has SiO2 = 44.55–52.46 wt.%, MgO = 3.34–8.07 wt.%, Nb = 18.2–55.0 ppm and Nb/La = 1.01–1.76. These rocks can be classified as the high-Nb OIB-like alkaline mafic rocks, and interpreted as the product of the enriched component-modified asthenosphere. Group 5 has similar elemental signatures to the Nb-enriched basalt, with SiO2 = 44.52–48.80 wt.%, Nb/La = 0.38–0.81 and Nb = 8.2–17.4 ppm, and 87Sr/86Sr(t) = 0.7032–0.7044 and εNd(t) = +2.7∼+5.5, suggestive of an origin from a mantle wedge source recently modified by both slab-derived and OIB-like components. Our zircon U-Pb geochronological results for the representative samples of five groups indicate that all the five groups formed at 830–859 Ma with a cluster age of ∼830–845 Ma in the early Neoproterozoic, identical to the deposited time of the Lengjiaxi Group and its equivalents. The coexistence of the 830–859 Ma high-Mg intermediate (Group 1) and high-Si adakitic (Group 2) rocks, arc-like basaltic and andesitic (Group 3) rocks, and high-Nb (Group 4) and Nb-enriched (Group 5) mafic rocks in the Chencai Complex suggests an early Neoproterozoic arc-trench setting and the ongoing of subduction until ∼830 Ma. The initial assemblage of the Yangtze with Cathaysia Blocks along the whole Jiangnan Orogen might be roughly synchronous (∼820–830 Ma). An early Neoproterozoic slab window model at subduction termination (∼830 Ma) is proposed, representing an improved understanding of the petrogenesis of the early Neoproterozoic Chencai igneous rocks.