Post-orogenic thermal reset of the Pingtan-Dongshan metamorphic belt (SE China): Insights from zircon fission track and U-Pb double dating

摘要

Zircon double dating technique for U-Pb and fission track (FT) ages is used to reassess the cooling conditions for the Pingtan-Dongshan Metamorphic Belt (PDMB). Three Cretaceous gneissic bodies from the Chinmen area and two dioritic plutons from the Zhangzhou Igneous Complex (ZIC) in two sides of the Changle-Nanao fault are studied. U-Pb dating results define the magma emplacement of gneisses at 144-134 Ma and dioritic rocks at 102-99 Ma, and FT dating results give ages of 96-86 Ma and similar to 87 Ma for corresponding samples. Combining Ar-40/Ar-39 mineral ages reported previously, the fast cooling curve (86 degrees C/Myr; 100-96 Ma) of ZIC plutons matches near-surface intrusives. However, those for deep-seated Chinmen gneisses are problematic as their resultant conditions are similar to ZIC samples. A recent concept favors the Early to Late Cretaceous tectomagmatic evolution in the coastal SE China involving episodes of orogenic magmatism (147-130 Ma) and shallow magmatism under post-orogenic extension (110-85 Ma). As the Pingtan island in northern PDMB is considered as a remnant piece of colliding fragment, the cooling rate known for gneissic bodies in this island (35 degrees C/Myr; 130-120 Ma) can better represent the record of this collision. On the basis of these features, we advocate a magmatic quiescence at 130-120 Ma representing collision between PDMB and a concealed microcontinent which resulted in rapid uplift of this metamorphic belt. Cooling curves obtained for Chinmen gneisses are suggested to have further subjected to thermal reset by high temperature magmatism taking place during the post-orogenic stage.