Recovering the composition of melt and the fluid regime at the onset of crustal anatexis and S-type granite formation

Abstract

Using a metatexite from the Spanish Betic Cordillera as an example, we show that in situ and otherwise impossible to retrieve compositional information on natural anatectic melts can be reliably gained from experimentally rehomogenized melt inclusions in peritectic garnets. Experiments were conducted on single garnet crystals in a piston cylinder apparatus until the complete homogenization of crystal-bearing melt inclusions at the conditions inferred for the anatexis. The compositions of quenched glasses, representative of the early anatectic melts, are leucogranitic and peraluminous, and differ from those of leucosomes in the host rock. The H2O contents in the glasses suggest that melts formed at low temperature (~700 °C) may not be as hydrous and mobile as thought. Providing for the fi rst time the precise melt composition (including the volatile components) in the specifi c anatectic rock under study, our approach improves our understanding of crustal melting and generation of S-type granites

The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

“…Conversely, MI are commonly affected by late fractures crosscutting the entire Grt. Some small rounded cavities can be interpreted as micro-to nano-porosity; Raman micromapping conducted in similar microstructures from other MI study (Bartoli et al 2013a) strongly suggest that they were filled with fluid dissolved in the former hydrous melt that exsolved upon crystallization. Some fluid inclusions, of possible primary origin, are spatially associated with MI in clusters within Grt.…”

“…Among all the previously reported peak P for these rocks, including classical thermobarometic and phase equilibria modeling calculations (Loomis 1972;Torres-Roldán 1981;Balanyá et al 1997;Argles et al 1999;Platt et al 2003;Barich et al 2014;Massonne 2014), we have chosen the upper value of 1.5 GPa provided by most of these studies, expecting that this imposed external P on the Grt chips would prevent decrepitation of included MI due to increase in their internal P upon heating (e.g. Bartoli et al 2013a). Following most of the previous petrologic studies, this P would correspond to that of generation of Grt cores (Loomis 1972;Torres-Roldán 1981;Argles et al 1999;Platt et al 2003;Barich et al 2014).…”

“…The above evidence, together with the distribution of leucogranitic MI at the cores of large Grt and granodioritic-to-tonalitic MI at the rims of large Grt or within small Grt (Table 2 and Significance for melt inclusion studies, and crustal melting and differentiation Cesare et al (1997) and Acosta-Vigil et al (2007 hence to obtain precise information on the fluid regime during crustal anatexis, is the detailed study of MI Bartoli et al 2013aBartoli et al , 2014 show Grt crystals and schlierens, respectively; the coin is 25 mm across; modified after The bulk rock compositions of the studied mylonitic gneiss and the thick leucocratic band shown in Fig. 2b (former leucosome) are shown in blue and red symbols, respectively.…”

“…Sawyer 1996Sawyer , 2008; and references therein), (ii) glass analyses from the relatively few reported studies of glassy MI in metasedimentary/metagranitic anatectic enclaves and rehomogenized nanogranitoids in migmatites and granulites (Cesare et al 1997;2009;Acosta-Vigil et al 2007;Ferrero et al 2012Ferrero et al , 2014Ferrero et al , 2015Bartoli et al 2013aBartoli et al , 2013b press), and (iii) Helz 1976;Rutter and Wyllie 1988;Hacker 1990;Rushmer 1991;Rapp et al 1991;Wolf and Wyllie 1994;Rapp and Watson 1995;Skjerlie and Patiño Douce 2002;Schmidt et al 2004;Laurie and Stevens 2012;Quian and Hermann 2013).…”

“…However, detailed microstructural and, particularly, geochemical studies on these small data repositories in slowly cooled crystalline rocks, have only developed after the recent discovery of MI in low-to-medium P anatectic terranes (Cesare et al 2009), building up on the previous studies of glassy MI of anatectic enclaves (Cesare et al 1997(Cesare et al , 2003Cesare 2008;Acosta-Vigil et al 2010, 2012a. Thus, detailed studies of MI in anatectic terranes have shown the great potential of this new approach, providing precise information on the primary compositions of melt from a particular terrane and lithology, including volatile concentrations and clues on the fluid regime during partial melting (Cesare et al 2009Bartoli et al 2013aBartoli et al , 2014. Most of the MI in crystalline rocks, however, appear today crystallized to a granitic assemblage due to slow cooling at depth, with grain size commonly <1 µm; Cesare et al (2009) named them as nanogranites.…”