A multidisciplinary approach combining geochemical, gravity and structural data: implications for pluton emplacement and zonation

Abstract

Granitic pluton emplacement and zonation are controlled, among others, by regional deformation and the rate of magma supply. The latter has consequences for the disposition of successively emplaced, more chemically evolved, batches of magma. Our general interpretation is based on a multidisciplinary approach combining field observations, gravity data, internal structures and geochemical variations. Magma feeders are identified in the plutons as the deepest zones, inferred from gravity measurements, when they also correspond to vertical lineations. Correlation of the root zone location with compositional zoning indicate how the magma evolved during emplacement. Two case studies of Hercynian granite plutons illustrate the interpretations: the normally zoned Cabeza de Araya pluton (Spain), and the multiphase Fichtelgebirge pluton (Germany) which displays both normal and reverse zoning. It is proposed that reverse zoning reflects discontinuous magma injection due to a tectonic rate slower than the rate of magma supply. Conversely, normal zoning can occur when magma injection is continuous in time, with successive magma batches entering within not yet crystallized magma. The two case studies illustrate how the understanding of compositional zoning and emplacement of granitic plutons can be improved by multidisciplinary approaches combining classical and modern techniques. Rheological transitions during partial melting and crystallization to felsic magma segregation and transfer.

Textures and melt-crystal-gas interactions in granites

“…between evolving magmas, and cooling may result in apparent mineralogical zoning of the whole massif. Such an argument has been suggested to explain normal and reverse zoning (Hecht and Vigneresse, 1999). A fast intrusion rate does not leave time to the initial magma to reach high crystallinity.…”

“…Maps of foliation planes, with their intrinsic lineations have brought new insights of internal magmatic structures (Bouchez, 1997). Such data have been coupled with gravity inferred depths (Vigneresse and Bouchez, 1997) or chemistry surveying (Hecht and Vigneresse, 1999;Tartèse et al, 2011), informing about magma feeders and evolution. Other quantitative informations obtained from AMS have poorly been exploited.…”

“…The next magma batch can hardly fracture the former quasi solid one, and intrudes on its side. An inverse petrographic zoning results, indicated by crosscutting fabrics (Hecht and Vigneresse, 1999).…”

Unusually dumbbell-shaped Guposhan–Huashan twin granite plutons in Nanling Range of south China: Discussion on their incremental emplacement and growth mechanism

“…Specific factors that affect the way of ''outward building'' and ''inward building'' may include amount and rate of magma supply, size of earlier host intrusion, incremental time interval, rheology of country rocks, and intensity of regional deformation (Hecht and Vigneresse, 1999;Glazner et al, 2004). In general, with a relatively dynamic regional deformation regime, the large volume and high rate of magma supply at a shorter time interval would prefer ''inward building'' emplacement within a large volume of earlier intrusion.…”

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…It is now recognized that many intrusions were emplaced incrementally via distinct magma pulses (Richey, 1927;Harry and Richey, 1963;Cook and Weir, 1980;Pitcher, 1997;Hecht and Vigneresse, 1999;Kryza et al, 2014;Anderson et al, 2016). Much of the compositional varia tion observed in such intrusions will not have been produced in situ and instead relates to pro cesses operating within a source region or inter mediate staging chamber (Pitcher, 1997;Cole man et al, 2004;Lipman, 2007;Miller, 2008;Farina et al, 2012;Anderson et al, 2016).…”