Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province (eastern China): Implications for geodynamics and Cu-Au mineralization

Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu-Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al2O3, MgO, Sr, Sr/Y and La /Yb values, and low Y and Yb contents. They have low epsilon(Nd(r)) values (-3.46 to -6.28) and high (Sr-87/Sr-86)(i) ratios (0.7051-0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 +/- 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140-125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower epsilon(Nd(t)) values ranging from -4.65 to -7.03 and relatively higher (Sr-87/Sr-86)(i) ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions (Pb-206/Pb-204=17.90-18.83, Pb-207 /Pb-204=15.45-15.62 and Pb-208/Pb-204=38.07-38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., similar to 1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu-Au mineralization. (C) 2006 Elsevier B.V. All rights reserved.