Geochronology and petrogenesis of the early Paleozoic I-type granite in the Taishan area, South China: Middle-lower crustal melting during orogenic collapse

Abstract

The early Paleozoic Wuyi–Yunkai orogeny represents the first extensive tectonothermal event in South China since the Neoproterozoic break-up of the Rodinia supercontinent. The early Paleozoic igneous rocks in the South China Block (SCB) are important for understanding the processes responsible for the orogeny. The Taishan Batholith in the inner Cathaysia Block consists dominantly of granodiorite and hornblende-bearing granite. Zircon U–Pb dating of four samples gives a consistent crystallization age of ca. 436Ma, synchronous with widespread late-orogenic S-type granitic intrusions in the Wuyi–Yunkai orogen. The Taishan granitoid rocks are metaluminous or peraluminous (A/CNK=0.94–1.15) and have variable SiO2 (61.5–74.9wt.%), MgO (0.4–2.4wt.%), Fe2O3 (1.8–7.4wt.%) and CaO (1.1–5.5wt.%), with gradually decreasing P2O5 from mafic to felsic components. Thus the Taishan Batholith displays the petrological and geochemical characteristics of I-type granites. Additionally, all samples have strongly negative εNd(t) values (−11 to −9), suggesting an overall ancient source. The early Paleozoic I-type granitoid rocks in the Taishan area were derived from partial melting of ancient middle to lower crust at a high thermal gradient (>35°C/km), and underwent extensive fractional crystallization of hornblende, plagioclase, and biotite. The Wuyi–Yunkai orogenic collapse may have been responsible for middle to lower crustal melting of the Cathaysia Block. The early Paleozoic I- and S-type granitoid magmatism lack significant input of a juvenile component, indicating that the processes accompanying orogenic collapse such as asthenosphere upwelling and basaltic underplating, contributed heat to trigger widespread early Paleozoic felsic magmatism in the SCB. An intraplate orogenic event, instead of continental or arc collision, may be the preferred model for the Wuyi–Yunkai orogeny.