The missing link between granites and granitic pegmatites

Abstract

In this contribution we provide evidence for the extraction of volatile and incompatible element enriched melts from common granites. This provides a mechanism showing that at least a large proportion of granitic pegmatites could be genetically directly connected to a main granite body. In granites there are often two principal types of melt inclusions: (i) those that represent the bulk chemistry of the granite and (ii) those with very different compositions. In the Variscan Erzgebirge granites, the second type is characterized by the abundance of fluorine. However, in other geodynamic settings inclusions in granites can contain high concentrations of other elements which may take over the function of fluorine. From textural relationships the second inclusion type represents intergranular melts enriched in all elements incompatible with the ideal haplogranite system. Due to the high volatile content of such melts, the viscosity can be several orders of magnitude lower than the quasi-solid bulk system and can therefore move rapidly through the partially or totally crystallized host, and flow together into a separate system forming pegmatite bodies inside or outside the granite body. Another important effect of the high volatile content is the phase separation resulting from the speciation changes OH -→ H 2 O or CO 3 2-→ CO 2 due to temperature and/or pressure changes at different locations within the granite-intergranular melt system. Since melt inclusions provide a means of conserving original un-degassed compositions, they yield important evidence for closing the gap between granites and granitic pegmatites. The paper is dedicated to two Czech colleagues -Petr Černý and Milan Novák -who have devoted their lives to the study of granitic pegmatites.

The mechanics of shallow magma reservoir outgassing

“…As plutons have a residual volatile content typically <1 wt % [ Caricchi and Blundy , ; Whitney , ], the remaining MVP must ultimately escape the cooling reservoirs by alternative processes such as gas‐filter pressing [ Pistone et al ., ; Sisson and Bacon , ] and capillary fracturing [ Holtzman et al ., ; Shin and Santamarina , ] that are not taken into account into our pore‐scale parametrization. These processes likely enhance the extraction of the melt‐MVP mixture and lead to the generation of either eruptible high‐SiO 2 melt pockets [ Bachmann and Bergantz , ] or aplite and pegmatite veins [ London and Morgan , ; Thomas and Davidson , ].…”

“…Geochemistry, Geophysics, Geosystems 10.1002/2017GC006912 parametrization. These processes likely enhance the extraction of the melt-MVP mixture and lead to the generation of either eruptible high-SiO 2 melt pockets [Bachmann and Bergantz, 2003] or aplite and pegmatite veins [London and Morgan, 2012;Thomas and Davidson, 2013]. Figure 9.…”

Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the magmatic-hydrothermal transition (Cerro de Pasco district, Peru)

“…In contrast with the porphyry environment, where, as mentioned above, they have been rarely recognized, heterogeneous melt inclusions consisting of coexisting silicate phase and water-rich phase have been well documented in pegmatites and aplites associated with S-type granites (e.g., Davidson, 2013, 2016;Veksler et al, 2002). In pegmatites, HSMIs are interpreted to result from entrapment of -supercritical melt‖ containing up to 33 wt % H 2 O (Thomas and Davidson, 2013). Such heterogeneous melt inclusions are strongly enriched, up to several weight percent, in B, F, Cs, Rb and P (Thomas and Davidson 2016, and references therein); elements acting as fluxing agents in the melt and thus increasing the miscibility gap between silicate melt and water, allowing the formation of such water-rich melts at the magmatic-hydrothermal transition (Veksler, 2004).…”

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

“…Nevertheless, the consensus is that the main Ta mineralization stage appears late in the pegmatite-forming process. Most researchers agree that fluid exsolution occurs during pegmatite formation, which is documented by miarolitic cavities, fluid inclusions, as well as stable isotope (H, B, O) fractionation (Thomas et al, 2012;Thomas and Davidson, 2013;London, 2014;Siegel et al, 2016;Thomas and Davidson, 2016). However, the timing of fluid release is still debated.…”