The Tongling area is located in the central part of the lower Yangtze depression belonging to the northern margin of the Yangtze plate. The Jiangnan uplift of the Yangtze plate adjoins the southern part of the lower Yangtze depression. The Dabie orogenic belt and the north China plate are all situated on the northwestern margin of this area, bounded by the NNE striking, left-lateral strike-slip Tan-Lu fault belt.
The Dabie orogenic belt is result of multiple collisions between the Yangtze plate and the north China plate from Proterozoic to the early Mesozoic. The tectonic evolution of the lower Yangtze depression experienced four main stages. The first stage is the formation of Precambrian basement, corresponding to Proterozoic metamorphic basement stratigraphy.
The second stage is the formation of cover sequence from the Paleozoic to Middle Triassic, which contains the Lower Silurian to Lower Carboniferous regressive bathyal to littoral clastics and the Middle Carboniferous to Middle Triassic littoral to neritic carbonates interbedded with bathyal and alternative marine¨Ccontinental clastic sand. The third stage is the collision between the Yangtze plate and the north China plate from Middle Triassic to early Jurassic, representing the latest movement in the Dabie orogenic belt.
The fourth stage is post-collision intraplate movement from the Middle Jurassic to Cretaceous, forming continental clastics, volcanics and intermediate-acid intrusive rocks, during deformation. This paper has determined the crystallization age of Xiaotongguanshan quartz monzodiorite, Fenghuangshan granodiorite, Shatanjiao quartz monzonite porphyry, Xinqiao monzonite and Dongguashan pyroxene monzonite from Tongling area using zircon SHRIMP.
The crystallization age of Xiaotongguanshan quartz monzodiorite is 142.8±1.8Ma, the crystallization age of Fenghuangshan granodiorite is 144.2±2.3Ma, the crystallization age of Shatanjiao quartz monzonite porphyry is 151.8±2.6Ma, the crystallization age of Xinqiao monzonite is 146.4±4.3Ma, and the crystallization age of Dongguashan pyroxene monzonite is 148.2±3.1Ma.
These data indicate that they are formed at the end of late Jurassic (142.8¡«151.8Ma). Generally, the intrusive sequence of magma is generally from quartz monzonite through quartz monzodiorite to pyroxene monzodiorite. The geochronological characteristics of the intrusive rocks can be used to interpret that the delamination of lithospheric upper mantle and lower crust occurred at least before medium and later period of the late Jurassic in the middle and lower Yangtz River, causing sudden thinning of lithosphere.
The old zircon inherited cores crystallized from magma (747Ma¡«823Ma) correspond to the breakup of Rondia supercontinent, this suggests that Neoproterozoic igneous source rocks were involved in the generation of magma. Old zircon cores of 2485¡«2515Ma supports that Archean basement exists beneath the middle-lower reaches of Yangtze River.