Origin of fayalite granitoids: New insights from the Cobquecura Pluton, Chile, and its metapelitic xenoliths

Abstract

Fayalite granitoids of the Cobquecura Pluton, Central Chile, occur in an active continental margin setting, which is unusual for such rocks. They are closely associated with gabbros, and both lithologies contain hercynite-rich, Fe-rich metapelitic xenoliths. The primitive magmas of the Cobquecura Pluton are derived from a mafic source, similar to the regional subarc subcontinental mantle. Sr-Nd-Pb isotope data demonstrate that crustal contributions were important in the petrogenesis of the fayalite granitoids. The crustal contribution, however, does not consist of rocks similar to the regionally exposed Paleozoic basement rocks, but rather resembles partial melts derived from metapelitic rocks at depth. The corresponding restites are the hercynite-rich xenoliths. A relatively high content of rare earth elements and high field strength elements in the fayalite granitoids is linked to selective assimilation of crust. This is evident from the abundant xenoliths that are entrained in the fayalite granitoids, and whose leucosomes are rich in accessory minerals carrying these elements. Such Fe-rich rocks similar to the hercynite-rich xenoliths could have acted as a chemical buffer keeping the oxygen fugacity low, which is crucial for the generation of fayalite bearing granitoids.

Lithium and boron in late-orogenic granites – Isotopic fingerprints for the source of crustal melts?

“…As these phases have the highest Rb/Sr in sedimentary rocks and their metamorphic correspondents, they also have evolved the most radiogenic Sr-isotopic composition, and their melting would result in a residue that has a markedly less radiogenic Sr-isotopic composition than the original bulk rock (cf. Zeng et al, 2005;Vásquez et al, 2009) Rudnick and Gao, 2003).…”

Generation and emplacement of shear-related highly mobile crustal melts: the synkinematic leucogranites from the Variscan Tormes Dome, Western Spain

“…(3) The isotopic signature may be strongly influenced by partial melting of the source rock (e.g., Bickle et al 1988;Villaseca et al 1998;Jung 2005;Zeng et al 2005;Vásquez et al 2009), which may mobilize melts with isotopic and geochemical fingerprints different from the restitic material and, thus, from the bulk rock composition of locally exposed protoliths.…”

“…The preferential melting of minerals with steeper REE pattern than the bulk rock (relative LREE enrichment) will yield melts that have lower eNd t values than the residue (e.g., Zeng et al 2005;Vásquez et al 2009). Major minerals control the chemical composition of the melt and the amount of melt produced.…”

Multiple, isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

“…Anatexis preferentially yields melts enriched in certain low-melting constituents and its chemical and isotopic fingerprint may vary in relation to both the degree of partial melting and the previous history of melt extraction. Thus, end-members dominated by anatectic melts may show varied chemical and isotopic composition due to progressive melting and repeated melt extraction (Zeng et al, 2005;Vasquez et al, 2009). Furthermore, the timing of magma contamination is important, as crystallization of plagioclase retracts Sr from the melt and, thus, for a given amount of contamination, the Sr-isotopic composition of plagioclase from a melt with low Sr content will be more strongly shifted towards the composition of the contaminant than plagioclase from a melt with high Sr content.…”