Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan-Lu Fault Belt (Northeastern Yangtze Block)

- Adakitic rocks along South Tan-Lu fault belt formed between 124-129 Ma.
- Origin of the adakitic rocks is partial melting of delaminated lower crust.
- Crustal materials dominated processing of these igneous rocks.

The Outang intrusion was newly found near the South Tan-Lu fault belt (STLF) of the northeastern margin of Yangtze block due to the excavating construction of the Beijing-Shanghai high-speed railway. Observations show that lithology of the Outang intrusion is intermediate rock series, mainly composed of monzodiorite and quartz monzonite. Major and trace elements, zircon U-Pb dating and Hf isotope were analyzed. Two ages of 124.6 ± 2.9 Ma (MSWD = 0.60, n = 29) and 129.2 ± 4.1 Ma (MSWD = 1.5, n = 21) are obtained, showing that the regional igneous activity was in early Cretaceous, being consistent with the massive Yanshanian magmatic events in Eastern China. The quartz monzonite was identified as a high-Mg adakitic rocks, showing geochemical features of high Si, high K, and low Sr, which indicates that the igneous genesis may be results of partial melting of the delaminated lower crust. The negative εHf(t) values with much older tDM2 ages indicate that the original magma may derive from remelting of ancient continental crust. A tectonic model has been proposed to account for the massive subduction of the Pacific Plate beneath eastern China in Jurassic with effect of delamination and thinning, from which we conjecture that the Archean materials could compose the basement of the STLF caused by massive subduction of the Pacific Plate beneath the eastern China continent.