The sequence of geochemical changes leading to granitization

Abstract

Summary The available chemical analyses relating to the reaction effects of granitic "magma" on various types of country rocks have been collected together in order to determine whether there is any systematic sequence of change when rocks are granitized. A pictorial view of the major changes is presented by plotting the analyses for individual areas on von Wolff diagrams. As a result of the investigation it is found that, just as skarn develops in limestones at granite contacts as a result of the introduction and fixation of iron, magnesium, alkalis etc., so the initial change in rocks of all types includes enrichment in mafic constituents and alkalis. Only subsequently are the rocks granitized in the strict sense of the term. When pelitic and semi-pelitic rocks are metasomatically altered in contact with granite "magma" they undergo changes in two distinct stages. During the first stage of alteration they become molecularly desilicated relatively to the bases present, sometimes to such an extent as to become chemically undersaturated. The products of this desilication change, which may attain the composition of syenite or of basic or ultrabasic igneous rocks, are characterized by a higher percentage of alkalis and/or cafemic constituents than either the parent sediment or the adjoining granite. For such an increase of any constituent beyond the amounts present in the parent rock and the neighbouring granite the term geochemical culmination is proposed. When the desilication change is wholly or largely one of basification (introduction of Fe, Mg, Ca) it is characterized by increase, commonly attaining geochemical culmination, of one or more of the minor constituents TiO 2 , P 2 O 5 , and MnO; when it is essentially one of felspathization, however, TiO 2 P 2 0 5 , and MnO may all decrease. During the second stage of alteration the desilicated rock is granitized , i.e. its composition approaches that of the associated granite. Silica and one of the alkalis are added, whilst alumina, cafemic constituents, and the minor constituents TiO 2 , P 2 ,0 5 , and MnO decrease. The early stage of alteration of psammitic rocks is found to be essentially one of felspathization, and to give rise to granitic or syenitic rock which commonly differs from the adjoining granite in the relative abundance of its alkalis. This alkali enrichment is sometimes accompanied by geochemical culmination of one or more of the cafemic constituents, and by one or more of the minor constituents Ti0 2 , P 2 0 5 , and MnO. Chemical analyses relating to a further stage of alteration appear to be lacking. When basic igneous rocks are metasomatically altered by granite "magma" two stages of change are again recognizable. In the first stage the basic igneous rock is desilicated and /or basified by addition and geochemical culmination of cafemic constituents and one or more of the minor constituents Ti0 2 , P 2 0 5 , and MnO. In the second stage of alteration the basified rock is granitized, Si0 2 , alkalis, and sometimes A1 2 0 3 being added, whilst all other constituents decrease. The application of the findings to the identification of the parentage of xenoliths in granitic rocks is discussed. There are two schools of thought relating to the source of the Fe and Mg added to rocks adjoining granite bodies : one believes the mafic oxides to be of magma tic origin and derived from granite magma, either as a pneumatolitic or hydrothermal phase, or as a residual solution; the other regards the Fe and Mg as constituents driven from country rocks which have been granitized. The evidence on which these views are based is discussed.