Hybridisation of mafic microgranular enclaves in the Lavras Granite Complex, southern Brazil

Twenty-five mafic microgranular enclaves of the Lavras Granite Complex in s outhern Brazil were studied petrographically and geochemically to establish their origin and to investigate the processes involved in their differenti ation. Mesoscopic and microscopic textures indicate that they are products of magma mingling bt tween a basic end member of probable mildly alkaline a ffinity and host shoshonitic and alkaline granitic rocks. The hybridisation process involved at least the following mechanisms: (i) chemical diffusion of volatiles and very mobile elements such as K to the less polymerised li quids, leading to the crystallisation of hydrated mafic minerals; (ii) chem ical diffusion of Ti and P to the less polymerised liquids, leading to tita nite and apatite crystallisation; (iii) mechanical accretion in the basic m agma of early crystallised host granite phases that promoted enrichment of their major constituents acid of trace elements with high partition coeffic ients in these phases; (iv) chemical diffusion of elements such as Rb, Nb, Y, and Yb with high KJ. in the major enclave phases, from host magma into t he basic enclaves. These processes occurred simultaneously, probably before the dispersion of basic batch magma forming the mafic microgranular enclav es, and caused hybridisation and complex geochemical patterns. The patterns are very different from the linear trends predicted for near-equilibrium s ystems such as those of magma mixing or fractional crystallisation. (C) 200 0 Elsevier Science Ltd. All rights reserved.