Distribution and geochemical features of Hercynian-Indosinian granitoids in southeastern China |
| |
| Authors: | Wang Dezi Liu ChanGsm |
| |
| Institution: | (1) Department of Geology, Nanjing University, 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. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
