Comment on “A preserved early Ediacaran magmatic arc at the northernmost part of the transversal zone - central domain of the Borborema Province, Northeast of South America”, by B. B. de Brito Neves et al. (2016)

ABSTRACT:

The recent proposition for a long-lived (635-580 Ma) magmatic arc in the central portion of the Borborema Province is critically evaluated. Evidence favoring subduction include: low-T, high-P metamorphism; local occurrence of retroeclogites; and the probable participation of juvenile material in the genesis of 630-620 Ma-old plutons. However, several factors argue against the existence of a large ocean separating the Northern and Central subprovinces. First, early Tonian detrital zircons (1.0-0.9 Ga) in the Northern Subprovince indicate derivation from the Central Subprovince, and thus a connection between them until the time of deposition (ca. 660 Ma). Second, the age of peak metamorphism in the Northern Subprovince implies concomitant deformation in the upper and lower plates at 630-610 Ma. Third, 630-620 Ma-old granites have characteristics (e.g., high δO ¹⁸ values, low magnetic susceptibility) unlike plutons emplaced in continental active margins. Fourth, 590-580 Ma-old granites are clearly ­syn-transcurrent, i.e., post-collisional. Finally, the Patos shear zone, proposed to be a boundary transform between the two plates, is younger than 570 Ma, and thus cannot represent a suture zone. Therefore, if a magmatic arc existed, it had short duration and was developed at the end of the Cryogenian.

KEYWORDS:
subduction; collision; detrital zircons; granitic magmatism; orogenic metamorphism