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

摘要

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, Al_2O_3, 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 sNd (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 sNd (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 epsilon Nd (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.