ON THE GENETIC SERIES OF GRANITES,AS EXEMPLIFIED BY THE MESOZOIC GRANITES OF SOUTH CHINA
Xu Keqin;Hu Shouxi;Sun Mingzhi;Zhang Jingrong;Ye Jun Department of Geology, Nanjing University
Based upon the sources of matter from which, the granites may have been derived, theirmodes of formation, their tectonic positions, and their petrologic characteristics, the authorsconsider that, the granites in South China may be divided into three genetic series, namely,(1) the continental crust-transformation series, (2) the syntexis series formed in a transi-tional crust, and (3) mantle-derived series. Because of the limited distribution of the mantle-derived granites, this paper will con-fine its discussion to the first two genetic series of granites. The continental crust-transformation series of granites is distributed in the typical con-tinental crust, such as the Caledonian folded region of South China, which forms the centralpart of the Mesozoic continental plate of southeastern China. In this region, generally occurthe multiplecyclic granites, and the Mesozoic granites of the continental crust-transforma-tion series are commonly exposed extensively. In the central part of this region, the graniteswere formed earlier, and become comparatively younger in ages toward the marginal parts.The granites of the syntexis series occur in the active continental margin and in some deepfault zones and faulted depressions, where the crust is usually thinner and the Moho sur-face is higher. In such zones, the granites of the syntexis series are generally linear in distri-butions. The granites of the continental crust-transformation series ade commonly normal gran-ites, over-saturated with alumina and poor in lime; the potash-feldspar is marked with a gre-ater triclinicity; biotites being commonly low in magnesia and with lower coefficient ofoxidation; the accessory minerals are poor in magnetite and belong to calcium-poor associ-ations. And, in the multi-staged composite intrusive bodies, the rocks evolve commonly frompotash-rich to soda-rich varieties toward the latest stages. The granites of the syntexis series are composed of many varieties of rocks (e. g., fromdiorite→quartz diorite→ granodiorite→ quartz monzonite→ potash granite), of which,the average composition approximates granodiorites. They are comparatively low in alu-mina and rich in lime; the potash feldspar are marked with a small triclinicity; biotites be-ing rich in magnesia and with a higher coefficient of oxidation; the accessories are rich inmagnetite and consist of calcium-rich associations; and in the multi-staged intrusive bodies,the rocks generally evolve from soda-rich toward potash-rich varieties. In the granites of the continental crust-transformation series, comparatively rich traceelements are: W, Sn, Ta, Rb, Li, F, and comparatively poor trace elements are: Cr, Ni, Co,V, Au, Cu, Pb, Zn, Sr, Cl, whereas in the granites of syntexis series, comparatively richtrace elements are: Cr, Ni, Co, V, Au, Cu, Pb, Zn, Sr, Cl, and comparatively poor in W,Sn, Ta, Rb, Li, F, etc. As regards the composition of the rare-earth elements and the initial ratios of Sr~(87)/Sr~(86),the rocks of the two genetic series are clearly distinct from each other. In the granites ofthe continental crust-transformation series, the ratios of ∑Ce/∑Y are low, showing char-acteristic negative Eu anomalies, whereas the rocks of the syntexis series commonly havelarger ∑Ce/∑Y ratios, without negative Eu anomalies. The (Sr~(87)/Sr~(86))_0 ratios of the con-tinental crust-transformation series are generally greater than 0.710, while in the rocks ofthe other series, such ratios are generally between 0.705 and 0.710. The above-mentioned characteristics obviously show that, the granites of the continen-tal crust-transformation series are derived mainly from the materials of the continental crust,and are less affected by the upper mantle, while rocks of the syntexis series clearly deriveparts of their materials from the upper mantle. A discussion on the relations between the formation of the genetic series of granitesand plate tectonics is given.