Anomalous xenocryst dispersion during tonalite-granodiorite crystal mush hybridization in the mid crust: Mineralogical and geochemical evidence from Variscan appinites (Avila Batholith, Central Iberia)

摘要

The Variscan appinites from the San Martin del Pimpollar intrusion (Avila Batholith, Central Iberia) display geochemical and mineralogical evidence for hybridization with crustal-derived, partially molten, granodiorites and for rapid crystallization of residual melts. Quenching, apparently induced by the thermal contrast between mixing magmas, caused crystallization of hollow microphenocrysts of labradorite-bytownite, and of aggregates of fine-grained plagiodase+quartz+biotite± hornblende which trapped abundant acicular grains of apatite. Both inner cores and outer rims of anorthite-rich microphenocrysts were subsequently infilled and overgrown, respectively, by andesine. The sharp change in the anorthite content of plagioclase microcphenocrysts could be due to disequilibrium partitioning during supercooling, although decrease in H2O activity or A1/(Al+Si) ratio of the melt after An-rich plagioclase crystallization cannot be disregarded. Coarse-grained xenoliths and xenocrysts (quartz with reaction rims and embayment, rapakivi K-feldspar with irregular distribution of Ba-rich zones, oscillatory zoned oligodase-andesine and biotite with inclusions of stubby apatite) were entrained into the appintic magma during hybridizatioa The xenocrysts were most probably derived from the country-rock granodiorites. The composition of coarse-grained biotite, however, is similar to that of matrix grains, implying total compositional resetting by exchange reaction with the appinitic magma if it was originally xenocrystic Otherwise, biotite+apatite saturation would have to have been achieved before quenching. The most evolved rock terms of the appinite suite, hybrid granodiorites and hybrid biotite tonalites, have Nd and Sr isotope compositions matching those of the country-rock granodiorites. However, hybrid biotite tonalites are significantly more depleted in K2O, Ba, Zr, Th and REE than would be expected from appinite/country-rock granodiorite hybridization. This anomalous geochemical pattern could be caused by an inefficient dispersion of xenocrysts into the hybrid magma due to the quenching of the residual appinitic melts. Therefore, the ratios of mineral abundances in the country rocks may not have been conserved during xenocryst entrainment into the appinitic magma.