Petrogenesis of Neoproterozoic high-K calc-alkaline granites forming large batholiths in SE Brazil: Clues from trace-element chemistry of apatite, titanite and hornblende

Abstract

Granites may lose or gain residual melts respectively by compaction and reactive porous flow, and therefore their composition is potentially different from that of the original magmas. Consequently, the chemistry of individual minerals with well-understood textural relationships should be prioritized for inferring magma sources and crystallization conditions. We investigated the major and trace element chemistry of coexisting apatite, titanite and hornblende in thin sections of I-type high-K calc-alkaline (HKCA) granitic rocks forming two voluminous Neoproterozoic batholiths in SE Brazil. Detailed textural studies, major-element analyses by EPMA, and trace-element analyses by LA-ICPMS bring key information on magma genesis and evolution. Titanite is absent or forms thin rims in ferri-ilmenite in granites from Socorro Batholith, resulting from magmatic crystallization at higher temperatures (>800 °C) compared to similar granites from the Agudos Grandes Batholith, where it occurs as large crystals preserving temperature-related zonation, reflected in decreasing LREE and Zr contents. The influence of titanite crystallization is particularly evident in the trace-element chemistry of hornblende and apatite from the Agudos Grandes granites, with one order of magnitude lower REE contents and smaller to suppressed negative Eu anomalies. Systematically higher Sr/Y (and in part also La/Yb) of hornblende, apatite and titanite from Agudos Grandes granites confirms suggestions that they equilibrated at significantly greater depths (and by inference in a thicker crust) as compared to the Socorro granites. Quantitative estimates of Sr/Y in the original melts are hampered by coprecipitation of minerals that accommodate these elements and by potential Sr diffusion, especially in apatite. However, as an early-crystallizing phase in granitic magmas, apatite trace-element composition is a precious source of petrogenetic information; in our study, it reveals that most of the volume of the Agudos Grandes granites crystallized below 800 °C (i.e., after titanite starts to crystallize) and confirms, in agreement with other proxies, that the two batholiths crystallized under oxidizing conditions (e.g., sulfur in apatite up to 800 ppm).