Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collision al and often shifts to shoshonitic or alkaline-peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition a re studied on the basis of 308 major element and of 86 unpublished trace el ement (including REE) analyses of the Pan-African granitoids from the Tuare g shield (Adrar des Iforas, Mali and Air, Niger). This database covers a wi de variety of magmas from subduction-related to intraplate-type including a bundant HKCA batholiths. Literature data from geodynamically well-constrain ed cases are also included. In addition to a conventional geochemical appro ach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normaliz ed to the interpolated composition of the reference series that has the sam e SiO2 content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing Lithospheric mantle or a lower ju venile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithosp heric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmati sm, The first source is melted only shortly after its generation when the l ithosphere was still hot, which restricts HKCA magmatism mainly to post-col lisional settings. The second asthenospheric/lowest lithosphere source is b y definition close to its melting temperature and can generate magma ubiqui tously both in space and time. The main melting triggers are lithospheric m ajor structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thu s gives indications about the nature of the source and on geotectonic setti ngs. However, the latter is a second rank information, which is partly mode l-dependant. The post-collisional period differs from other settings by a p ropensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent. (C) 1998 Elsevier Scie nce B.V. All rights reserved.