Unveiling the hidden evidences of magma mixing processes via combination of in situ Sr isotope and trace elements analyses on plagioclase crystals

Abstract:

The 580 Ma Salto Rapakivi Granite is part of the Itu Batholith, which is the most extensive volume of granitic rocks with A-type affinity in the post-orogenic Itu Granite Province, Brazil. The pluton can be divided into three lithologic units: rapakivi granite, the porphyry granite and the hololeucocratic granite. The rapakivi granites contain abundant felsic microgranular enclaves (FMEs) that can reach up to a meter in diameter. The FMEs show field and petrographic features indicative of interaction in the plastic state. Conversely, mafic microgranular enclaves (MMEs) are scarce and occur as small ellipsoids included in the rapakivi granite, porphyry granite and in the FME.

Ratio x ratio diagrams define parabolic mixing trends indicative that 20-25% of a silica-poor component was added to a typical rapakivi granite in order to produce the FMEs.

In situ Sr isotopes obtained in plagioclase xenocrysts hosted in different granite facies reveal that the melt from which the host granites crystallized was more radiogenic (plagioclase ⁸⁷Sr/⁸⁶Sr ~0.7068) than the melts that gave origin to the porphyry and FMEs (xenocrysts captured by porphyry and FMEs exhibit ⁸⁷Sr/⁸⁶Sr ratios varying from 0.7060 to 0.7063). MME show much more primitive signatures (⁸⁷Sr/⁸⁶Sr 0.7055 to 0.7058).

Trace element compositions determined adjacently to the spots analyzed for Sr isotopes reveal a general core-to-rim decrease in Sr, Ba, and LREE, which is consistent with crystallization in a system fractionating plagioclase, alkali feldspar and apatite.

When plotted against ⁸⁷Sr/⁸⁶Sr ratios, Sr contents of the melts calculated to be in equilibrium with plagioclase also define parabolic trends indicative that the FMEs would be generated via addition of ~20-30% of a primitive component to a silica-rich endmember.

Mixing vectors require that a large proportion of primitive melt would have to be mixed with an evolved endmember in order to produce the compositional spectrum of the FMEs. An obvious obstacle to that assumption is the fact that mafic rocks are only preserved in the form of scarce, cm-sized enclaves within Salto pluton. Thus, results suggest the existence of deeper magmatic chambers in sites where the two components might have coexisted in high temperature in order to mix and produce hybrid magmas represented by FMEs and the porphyry granites.