Neoproterozoic tectonic evolution of the northwestern Yangtze craton, South China: implications for amalgamation and break-up of the Rodinia Supercontinent

Abstract

Abstract Three suites of Neoproterozoic igneous rocks (the Xixiang arc volcanic rocks, the Tiechuanshan bimodal basalt–rhyolite succession and the Wangjiangshan Gabbro) along the northwestern margin of the Yangtze craton in the Hannan area have been investigated. Single-grain zircon U–Pb TIMS dating of dacite and rhyolite indicates that the lower and upper units of the Xixiang Group erupted at 950±4 and 895±3 Ma, respectively, whereas the Tiechuanshan Formation was extruded at 817±5 Ma. A set of radiometric constraints suggest that the Wangjiangshan Gabbro was formed at ca. 820 Ma. The lower Xixiang unit is dominated by high-Mg andesite and low-Ti tholeiite. These rocks are characterized by high Mg numbers (69–73), Cr (209–621 ppm) and Ni (98–171 ppm), low TiO2 (0.47–0.69 wt.%), depletion in LREE (LaN/YbN=0.4–1.1) and variable negative anomalies of high strength field elements HFSE (Th, Nb, Ta Zr, Hf, P, and Ti). Together with high positive eNd (950 Ma) values (+6.0 to +8.8), these features show an affinity of boninitic associations, which are usually found in fore-arc settings. The upper Xixiang unit ranges in lithology from basalt through andesite to dacite–rhyolite with ascending order. The rocks are characterized by enrichment in LREE (LaN/YbN=1.70–7.75) and large ion lithophile elements and having pronounced negative Nb, Ta, Ti and Eu anomalies. Their eNd (897 Ma) values range from +2.0 to +6.8. These features characterize modern arc volcanic suites. Rhyolite from the top of the Xixiang Group has a back-arc affinity as indicated by their high Na2O and K2O contents and LILE/HFSE ratio. The Tiechuanshan Formation consists of bimodal basalt and rhyolite–dacite with a silica gap between 49.8 and 66.3 wt.%. The basalt shows variable negative Nb and Ta anomalies, and can be divided into tholeiitic and alkaline members with eNd (817 Ma) being +4.6 to +5.3 and +0.2 to +3.8, respectively. The tholeiites exhibit primitive mantle-normalized incompatible elemental patterns resembling that of E-MORB along with high Cr (190–291 ppm), Ni (65–93 ppm) and Mg numbers (69–54). The alkaline basalts, on the other hand, display oceanic island basalt (OIB)-like incompatible elemental distributions characterized by enrichment in LILE and TiO2 (2.59–1.91 wt.%) together with lower Mg numbers (46–17). The Tiechuanshan rhyolites and dacites are characterized by remarkable negative Eu ( Eu / Eu ∗ =0.37–0.73 ), Nb, Ta and Ti anomalies, high LaN/YbN (10.9–16.7), negative eNd (t) values (−2.5 to −4.9) and high initial 87 Sr / 86 Sr (t) ratios (0.7054–0.7106). These characteristics indicate significant contributions of crustal components. The Tiechuanshan assemblage is similar to volcanic rocks developed in continental rifts and continental flood basalt (CFB) provinces, which are considered to be plume-related. The Wangjiangshan Gabbro is tholeiitic in composition and has low Rb, U, Th, Nb, Ta, Zr and Hf concentrations along with positive eNd (820 Ma), which implies derivation from a previously depleted mantle source. The less fractionated samples show high MgO (9.34–8.96 wt.%), Cr (368–312 ppm) and Ni (201–185 ppm) and positive eNd (t) values (+5.5 to +3.8). They suggest high degree partial melting of the source mantle under a relatively high geotherm compared to the coeval southeastern Australian Gairdner Dyke. The ca. 80 Ma age difference, contrasting rock associations and distinct geochemical and isotopic compositions of felsic rocks between the upper Xixiang Group and the Tiechuanshan Formation, suggest their diverse tectonic settings of formation. It is thus inferred that the lower and upper units of the Xixiang Group were formed in fore-arc and back-arc settings, respectively, whereas the Tiechuanshan volcanic suite and the Wangjiangshan Gabbro were developed in a plume-related intra-continental rift setting. Therefore, the Xixiang and Tiechuanshan volcanic suites and the Wangjiangshan Gabbro reflect a tectonic evolution from arc to intra-continental rift in the Neoproterozoic along the northwestern margin of the Yangtze craton. The transition occurred at ca. 820 Ma. The Tiechuanshan mafic rocks are geochronologically and geochemically indistinguishable from the Gairdner Dyke and its volcanic equivalents from southeastern Australia. Coeval mafic-ultramafic and granitic plutons in southern South China have also been documented, and are interpreted to be plume-related as well. Accordingly, the Neoproterozoic tectonic evolution in the Hannan area and elsewhere in South China supports a Rodinia Supercontinent reconstruction model, in which an ascending superplume centered beneath South China at ∼820 Ma triggered the rifting and break-up of the Rodinia Supercontinent, separating South China from southeastern Australia.