Geochemistry and δ11 B evolution of tourmaline from tourmalinite as a record of oceanic crust in the Tonian Ibaré ophiolite, southern Brasiliano Orogen.

Karine Da Rosa Arena,Marco Paulo De Castro,Gláucia Nascimento Queiroga,Léo Afraneo Hartmann,Cristiano De Carvalho Lana

doi:10.1590/0001-3765202020180193

Abstract

The isotopic and geochemical evolution of tourmaline constrain the processes of paleo-oceanic lithosphere in ophiolites. The Brasiliano Orogen is a major structure of South America and requires characterization for the understanding of Gondwana supercontinent evolution. We made a pioneering investigation of tourmaline from a tourmalinite in the Ibaré ophiolite by integrating field work with chemical analyses of tourmaline by electron microprobe (EPMA) and δ11B determinations via laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Remarkably massive tourmalinite (>90 vol.% tourmaline, some chlorite) enclosed in serpentinite has homogeneous dravite in chemical and isotopic composition (δ11B = +3.5 to +5.2‰). These results indicate a geotectonic environment in the altered oceanic crust for the origin of the tourmalinite. This first δ11B characterization of tourmaline from tourmalinite sets limits to the evolution of the Neoproterozoic to Cambrian Brasiliano Orogen and Gondwana evolution.

Highlights

Tourmaline is a most useful mineral because it is robust and can retain a record of geological processes
Tourmaline was described in pegmatites (Borborema Province, NE Brazil – Trumbull et al 2013) and as platiniferous gold– tourmaline aggregates (Gold–palladium belt of Minas Gerais, Brazil – Cabral et al 2017)
The results indicate a remarkably homogeneous dravite, including boron isotopes (δ11B = +3.5 to +5.2‰)

Summary

Introduction

Tourmaline is a most useful mineral because it is robust and can retain a record of geological processes (van Hinsberg et al 2011). The mineral is helpful for understanding both continental (Chaussidon & Albarède 1992, Trumbull et al 2008, Cabral et al 2017) and oceanic (Smith et al 1995, Farber et al 2015) settings. Fewer studies concentrated on oceanic lithosphere, including crust and mantle or on volatile transfer processes from the subduction setting to the mantle wedge and arc magmatism (Palmer 1991, Rosner et al 2003, Savov et al 2005, Boschi et al 2008, Yamaoka et al 2012). Tourmaline was described in pegmatites (Borborema Province, NE Brazil – Trumbull et al 2013) and as platiniferous gold– tourmaline aggregates (Gold–palladium belt of Minas Gerais, Brazil – Cabral et al 2017)

Methods

Results

Conclusion