Longming Li

Two late Mesozoic volcanic events in Fujian Province: constraints on the tectonic evolution of southeastern China

Two representative geologic sections of the late Mesozoic ‘Nanyuan Formation’ volcanic rocks in eastern Fujian Province have been dated by SHRIMP zircon U–Pb techniques. The results identified two active volcanic stages at 162–150 Ma and 143–130 Ma. The rock association of the earlier period is composed of andesite‐dacitic crystal tuff‐rhyolitic ignimbrite. The rocks are enriched in alkali elements, Al2O3, large ion lithophile element (LILE) and light rare earth element (LREE), depleted in MgO, high field strength element (HFSE) and siderophile elements, and have similar εNd (t) values. These features indicate that the rocks were derived from a mantle source metasomatized by subduction‐related fluids, and their chemical variations indicate fractional crystalization during magmatic evolution. In contrast, the rock association of the later period consists of tholeiitic basalt‐rhyolitic crystal ignimbrites. The tholeiitic basalts have negative εNd (t) values (−3.4 to −2.6) and exhibit fractionated REE pattern with weak negative Eu anomalies. These rocks are enriched in LREE and depleted in HFSE with ratios of Ce/Pb = 6.9–11.1, Th/U = 2.7–4.0, La/Nb = 2.2–3.0, suggesting that they were also subduction‐related and experienced proportional fractionation of olivine and clinoproxene along with significant crustal contamination. The rhyolitic crystal ignimbrites are generally characterized by rather high K, Rb, Th and relatively low Nb, Sr, Ti, P, with relatively low εNd (t) values (−5.3 to −6.0), impling that they were derived from a different source from the tholeiitic basalt. Judging by extensive overthrust structures, the occurrence of the earlier rock association is considered to have been generated in a compressional tectonic environment during the early stage (>150 Ma) of Pacific plate subduction, In contrast, the later bimodal association was generated in an extensional tectonic setting during a later stage (<143 Ma ) of subduction.

First Direct Evidence of Pan-African Orogeny Associated with Gondwana Assembly in the Cathaysia Block of Southern China

Metamorphic zircon from a hornblendite in the South China Block (SCB) yield U-Pb age of 533u2009±u20097u2009Ma and Hf model ages from 900 to 1200u2009Ma. Geochemical and isotopic characteristics indicate that primary magma of the hornblendites was probably derived from an enriched asthenospheric mantle source. This Late Neoproterozoic–Cambrian (Pan-African) metamorphic event provides the first direct evidence that the SCB was an integral part of the Gondwana assembly. Combined with available geological data which show that the SCB has great affinity with India or Australia, the Pan-African metamorphic event most likely belongs to the eastern Kuunga orogeny. We propose that the SCB was located at the nexus between India, Antarctica and Australia along the northern margin of East Gondwana, with the Cathaysia Block connecting western Australia whereas the Yangtze Block facing northern India at ca. 533u2009Ma.

Petrogenesis and mantle source characteristics of Cenozoic alkaline diabase, Jiangxi Province, southeastern China

Major element, trace element, and Sr–Nd–Pb isotopic compositions of Cenozoic diabase in southeastern China provide insights into the nature of their mantle sources and processes. The diabases are alkaline in lithochemistry (Na2O + K2O = 4.37–5.19 wt.%) and have overall oceanic island basalt-like trace element patterns, without negative Nb–Ta anomalies. In addition, they are characterized by lower La/Nb (<1.5) and La/Ta (<22), and higher Ce/Pb (>15) and Nb/U (>30) ratios, indicating an origin in the asthenospheric mantle. The relatively lower 143Nd/144Nd (0.512632–0.512648) and 206Pb/204Pb (18.20–18.22), but intermediate 87Sr/86Sr (0.7061–0.7063) ratios of the diabases are similar to enriched mantle type 1, suggesting crustal contamination or mixing with metasomatized lithsopheric mantle. However, the low Th and U contents and lack of correlations of Nd isotope compositions and MgO preclude significant crustal contamination. Alternatively, the moderate TiO2 contents (2.01–2.09 wt.%) and high Cr concentrations (>240 ppm) suggest interaction between asthenosphere-derived melts and metasomatized lithospheric mantle. Petrological modelling suggests that the diabases were generated from a low degree (~3–5%) of partial melting of lherzolite with ~2–3% garnet. Jiangxi diabase was generated in a within-plate extensional regime, probably related to the far effect of the Himalaya–Tibetan orogen.