Distribution and geochemical features of Hercynian-Indosinian granitoids in southeastern China

Abstract

The tectonic patterns of the Hercynian-Indosinian cycle in southeastern China can be divided into two types. The first type is the Hercynian-Indosinian eugeosynclinal system, which is situated on the outer side of the Caledonian uplift belts. The second type is the Hercynian-Indosinian fault depression zone or superimposed basin system, which is developed on the basement formed by the Caledonian orogenic movement. There are two genetically different series of granitoids corresponding to two tectonic settings in the Hercynian-Indosinian tectonic regime of southeastern China.

Among the transformation-series batholiths one of the largest in the Qinzhou Gulf geosyncline is known as the Darongshan granite (NE-extending) with an outcrop area of about 7000 km². The age of emplacement determined by whole-rock Rb-Sr isochron method ranges from 223 to 263 m.y., corresponding to late Permian, with higher Sr inital ratios (0.7179–0.7274). The granite is chemically rich in K₂O but poor in Na₂O with the atomic Na/K ratio<1.0 and the mole ratio Al₂O₃/(Na₂O + K₂O + CaO) > 1.0. K-feldsparg are usually predominated by medium to maximum microclines with a higher degree of Al-Si ordering characterized by the concentration of Al in T₁ sites of the feldspars (t₁ (o) = 0.57–0.99). Biotites are often rich in Fe but poor in Mg, belonging to Fe-biotite or siderophyllite, with a lower MF value (Mg/(Mg + Fe⁺³ +Fe²⁺ +Mn)), normally <0.4, and a lower Fe₃₊/Fe²⁺ ratio of 0.03-0.02 in their octahedron layers. This fact indicates that the oxidation degree of iron is relatively low in the transformation-series granites as compared with the syntexis-series granitoids.

Granitic batholiths of the syntexis series distributed along the fault belts in the Hercynian depressions are usually more mafic, meta-aluminous granodiorites and smaller in size than those of the transformation series, with an outcrop area of 1–200 km₂. The Taiping granodiorite, for instance, is situated on the southern side of the Lower Yangtze River depression. It was emplaced 223 m.y. ago as dated by whole-rock Rb-Sr isochron method, with an initial Sr ratio of 0.706. Petrochemically, it is characterized by Na/K > 1.0 (1.31), mole Al₂O₃/(Na₂O + K₂O + CaO) < 1.0(0.95) and high Fe₂O₃/(Fe₂O₃ + FeO)(0.47), evidently differing from the transformation-series granites. K-feldspars are predominated by orthoclase with a lower degree of Si-Al ordering (t₁ (o) = t₁(m) = 0.30). Biotites are commonly rich in Mg in their octahedron layers (MF = 0.42–0.50) with higher Fe³⁺/Fe²⁺ ratios (0.36-0.51), indicating their formation under higher oxygen fugacity conditions. All this goes to prove that the syntexis-series granitoids were probably derived from mafic igneous or metamorphic source rocks in the lower crust.