On the article “Weathered potassic volcanic rocks as protoliths of the hematitic phyllites of the southern Serra do Espinhaço (MinasGerais)”: a discussion

Chaves and Knauer (2020) have presented three new whole-rock chemical analyses of phyllitic hematite, a unique metamorphic rock of the southern Serra do Espinhaço. Based on their three samples and a selection of other three samples from the literature, Chaves and Knauer have proposed that the geochemical uniqueness of the rock – i.e., high contents of K2O, Al2O3 and Fe2O3, and depletion in SiO2 – would represent a weathered, feldspathoid-rich alkaline basalt. This contribution is a discussion of their new data, the trace-element contents of which are at odds with those of a potassic, mantle-derived volcanic protolith for the hematitic phyllite. Its Nb/Th ratios of ~3 and chondrite-normalised La/Yb ratios of ~9–17, for instance, are typical of the continental crust. We also point out aspects that escaped the attention of Chaves and Knauer (2020), one of which is the ubiquitous occurrence of tourmaline in the hematitic phyllite.

Vulcânicas potássicas intemperizadas como protólitos dos filitos hematíticos da Serra do Espinhaço Meridional (Minas Gerais)

The hematitic phyllite is a rock that occurs in the São João da Chapada and Sopa-Brumadinho formations of the southern Espinhaço range. Its origin is widely discussed in papers on Espinhaço, but there is no consensus on its protolith due to certain characteristics of the lithotype, such as its chemical composition and textural features. The pattern of rare earth elements strongly enriched [(La/Yb)N 6.80-17.68], with light rare earth elements [(La/Sm)N 2.54-4.83] richer than heavy ones [(Gd/Yb)N 1.28-3,32], suggests that the protolith was an alkaline volcanic rock formed during the rift that generated the Espinhaço basin. The major elements indicate that the alkaline rock met weathering processes, becoming a regolith. During the Brasiliano metamorphism, it finally became hematitic phyllite. Other characteristics of the lithotype, such as the presence of sericite-bearing rounded parts (possibly formed by alteration and deformation of leucite crystals) and the preservation of igneous layering, suggest a potassic volcanic origin for hematitic phyllite. In diagram that allows identifying altered and metamorphic volcanic rocks, the investigated samples have composition similar to a feldspathoid-rich alkali-basalt, probably a leucite tephrite, a leucitite or even a lamproite, rocks from mantle source.

The occurrence of microdiamonds in Mesoproterozoic Chapada Diamantina intrusive rocks: Bahia / Brazil

The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515 ± 3 Ma, which may correlate with 1710 ± 12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.

ESTRATIGRAFIA DO SUPERGRUPO ESPINHAÇO NA REGIÃO DE PLANALTO DE MINAS, DIAMANTINA – MG.

In the region of Desembargador Otoni – Planalto de Minas (Northeastern Minas Gerais State, Brazil) two stratigraphics units of Precambrian age were recognized: the Espinhaço Supergroup and the Macaúbas Group. The metapelites of the Macaúbas Group (rhythmites with associated diamictites) represent varvites wuth dropstones which cover with a pronounced angular and erosional unconformity units of the Espinhaço Supergroup. This supergroup is divided in lithostratigraphic units. The Planalto de Minas Formation (at the base) is composed by a thick sequence of greenschists (originates from metabasalts) with variable composition and textures, locally with intercalations of thin quartzites and phyllites. The Tapera and Serra do Atalho formations are built up by laminated, micaceous meta-arenites and pure or micaceous meta-arenites, normally with cross bending. Respectively. Intercalations of phyllites and greenschists occur in both units, in addition the Serra do Atalho Formation contains also hematitic phyllite and a thin intercalation of metarhyolite. This volcanic, volcanoclastic, and clastics units were deposited in a basin related to the initial phase (at about 1750 Ma) of crustal extension of the Espinhaço Rift.