The evolution of two petrogenesis-mineralization series of granites in Southern China

Abstract

In this paper, the authors have proposed two distinct series of petrogenesis and mineralization: Series I and Series II. Series I (the Nanling Series) has a sequence of petrogenesis and mineralization: monzonitic granite or granodiorite → biotite granite → leuco-granite → granoporphyry or quartz porphyry → intermediate-basic dikes; REE → Nb, Ta (Li, Rb, Cs), Be, Sn, W, Mo, Bi, As → Cu, Zn, Pb → Sb, Hg, U. Series II (the Yangtze Series): Pyroxene diorite (or gabbro) → diorite or quartz diorite → granodiorite (quartz monzonite or monzonitic granite → granite → K-feldspar granite → granoporphyry or quartz porphyry (syenitic porphyry or quartz syenitic porphyry) → intermediate-basic dikes; Fe → Cu(Au) → Mo(W) → Zn, Pb → Pb (Ag).

Obvious differences have been recognized between Series I and II. As for Series II, the bulk composition is close to that of andesite. Relatively high temperatures of formation (980°–1,140°C) petrochemical enrichment in Mg and Ca, the composition of biotite characterized by high magnesium, assemblages of accessory minerals (magnetite-sphene-apatite or magnetite-ilmenite-REE-rich zireon), trace elements predominated by Cl and Sr with ΣCe>ΣY in the rocks and accessory minerals, relatively high σEu(0.74–0.99) without depletion, δ¹⁸O<10‰ δ³⁴S close to that of meteorite sulfur, relatively high content of platinum metals (>10 times) characteristic of mantle origin, vory low⁸⁷Sr/⁸⁶Sr (0.7036–0.7085) and the occurrence of intermediate-basic dikes (similar to basaltic composition) in the final evolutionary stage are sufficiently enough to show that Series II has a material source in the lower crust or in the upper mantle. The fact that⁸⁷Sr/⁸⁶Sr>0.7037 in most cases implies a partial incorporation of crust materials. As for Series I, the bulk composition is consistent with that of normal granites. Relatively low temperatures of formation (600°–680°C), chemical composition noted for high SiO₂ and K₂O, complicated accessory mineralogy and assemblages (magnetiteilmenite-zircon, or monazite-xenotime-zircon), biotite with high content of iron, trace elements predominated by F, Li, Rb (Cs) and Be, relatively low δEu with distinct depletion, δ¹⁸O>10‰ greatly varying δ³⁴S, low content of platinum metals typical of mantle derivation, high⁸⁷Sr/⁸⁶Sr (0.7112–0.7360), in conjunction with the consideration of mineralizations of REE, Nb, Ta, Sn, Be, W, Bi, etc. of crust origin, indicate that the continental crust is the main material source for Series I. The possibility, however, cannot be ruled out that a small amount of upper mantle-derived material took part in this process, as has been indicated by lower⁸⁷Sr/⁸⁶Sr (≈0.7100), part of δ³⁴S close to that of meteorite sulfur and the occurrence of intermediate-basic dikes (similar to basalts in composition) as the end products of magmatic evolution.

It is of interest to note that granitoid rocks of these two series in southeast China show considerable dissimilarities with those in Japan and Australia in⁸⁷Sr/⁸⁶Sr, δ¹⁸O and δ³⁴S.