Mesozoic felsic arc magmatism and continental olivine tholeiites in Zhejiang Province and their relationship with the tectonic activity in southeastern China

Abstract

The Mesozoic geology of Zhejiang Province in southeastern China is characterized by two distinct geodynamic settings separated by a major tectonic event. During the Late Jurassic-Early Cretaceous, a continental magmatic arc developed with a westward subduction of the Kula or Izanagi plate under the South China Block. During the development of this magmatic arc, microsyenite-porphyries and quartz-bearing porphyries were emplaced in western Zhejiang about 124 Ma ago. These porphyritic rocks intruded felsic tuffs that rest unconformably on Carboniferous and Permian folded strata. Geochemically, they are enriched in LIL and light rare earth elements (LREE) which are features of acid shoshonitic suites. Isotopically, they are characterized by negative ϵ Nd( T = 124) (−5.3 to −4.7) values and high initial 87 Sr 86 Sr ratios (0.7079–0.7084). Later, in eastern Zhejiang, quartz-phyric ignimbrites and subordinate acid andesites were emplaced and intruded by nearly contemporaneous microsyenitic and rhyolitic domes during the Cretaceous, about 97 Ma ago. All these rocks belong to the high-K calc-alkaline suites. They show geochemical features of volcanic arc granites emplaced in an active continental margin and are characterized by negative ϵ Nd( T = 97) (−8.8 to −7.1) and high initial Sr isotopic ratios (0.7081–0.7089). The negative ϵ Nd( T = 97) values of all the Cretaceous calc-alkaline and shoshonitic rocks of intermediate to acid composition suggest that these rocks were probably derived from a mantle wedge source, contaminated by variable amounts of old crustal components, likely represented by the Proterozoic basement rocks of the South China Block. The contemporaneous igneous rocks in Vietnam, as well as in the Fujian and Guangdong Provinces (China) represent the lateral counterparts of the Zhejiang calc-alkaline plutonic and volcanic rocks. In the late Early Cretaceous, an important tectonic event occurred which provoked the ending of the Inagazi or Kula plate subduction and affected the continental magmatic arc. This tectonic event is likely related to the collision of the China-Indochina margin with the West Philippine Block. After this tectonic event, in the beginning of the Late Cretaceous, continental stretching and crustal attenuation affected the South China Block leading to the development of NE-SW-faulted basins in which continental olivine-tholeiites and minor alkaline rhyolites were erupted, both characterized by low ϵ Nd( T = 97) ratios (ca. −4). Meanwhile, continental conglomerates and sandstones were also deposited. This rifting process began likely before the India—Asia indentation and lasted at least until the Late Tertiary.