Metamorphic record of collision and collapse in the Ediacaran‐Cambrian Araçuaí orogen, SE‐Brazil: Insights from P–T pseudosections and monazite dating

Abstract

The Araçuaí orogen is the Brazilian counterpart of the Araçuaí‐West Congo orogenic system (AWCO), a component of the Ediacaran‐Cambrian orogenic network formed during the amalgamation of West Gondwana. The northwestern portion of the Araçuaí orogen is dominated by a succession of metasedimentary rocks made up of Meso‐ to Neoproterozoic rift, passive margin and syn‐orogenic sequences, locally intruded by post‐collisional granites. These sequences are involved in three distinct tectonic units, which from west to east are: the southern Espinhaço fold‐thrust system (SE‐thrust system), the normal‐sense Chapada Acauã shear zone (CASZ) and the Salinas synclinorium. Three deformation phases were documented in the region. The first two phases (D1 and D2) are characterized by contractional structures and represent the collisional development stage of the orogen. The third phase (D3) is extensional and currently viewed as a manifestation of orogenic collapse of the system. The distribution of the metamorphic mineral assemblages in the region characterizes two metamorphic domains. The M‐Domain I on the west, encompassing the SE‐thrust system and the CASZ, is marked by a syn‐collisional (syn‐D1) Barrovian‐type metamorphism with P–T conditions increasing eastwards and reaching ~8.5 kbar at ~650°C between 575 and 565 Ma. The M‐Domain II comprises the Salinas synclinorium in the hangingwall of the CASZ, and besides the greenschist facies syn‐collisional metamorphism, records mainly a Buchan‐type metamorphic event, which took place under 3–5.5 kbar and up to 640°C at c. 530 Ma. The northwestern Araçuaí orogen exhibits, thus, a paired metamorphic pattern, in which the Barrovian and Buchan‐type metamorphic domains are juxtaposed by a normal‐sense shear zone. Lithospheric thinning during the extensional collapse of the orogen promoted ascent of the geotherms and melt generation. A large volume of granites was emplaced in the high grade and anatectic core of the orogen during this stage, and heat advected from these intrusions caused the development of Buchan facies series over a relatively large area. Renewed granite plutonism, hydrothermal activities followed by progressive cooling affected the system between 530 and 490 Ma.

The Rio Pardo salient, northern Araçuaí orogen: an example of a complex basin-controlled fold-thrust belt curve

“…The metasedimentary rocks involved in the western limb record two metamorphic events. A Barrovian metamorphism assisted the D 1 deformation phase with P-T conditions increasing eastwards along the SER-thrust system, and reaching ~8.5 kbar at ~650ºC (Peixoto et al 2018). Age determinations on the monazite indicated that this event took place between 575 and 565 Ma.…”

“…As the dominant fabric elements of the external Araçuaí orogen, the D 1 /D 2 structures record the Brasiliano event in the region, which is in turn viewed as a product of a general E-W contraction (Pedrosa-Soares et al 2001. The Barrovian metamorphism syn-kinematic to D 1 was recently dated in this region between 575 and 565 Ma (Peixoto et al 2018), the age interval of the collisional stage of the Araçuaí orogen (Pedrosa-Soares et al 2001, 2011b.…”

“…A second low pressure metamorphic event (Buchan-type) is recorded in the eastern half of the western limb. It took place under 3 to 5.5 kbar and up to 640ºC at around 530 Ma (Peixoto et al 2018).…”

“…In this part of the salient, only structures from the D 1 , D 2 and D 4 phases were observed. They occur in association with three regional and roughly NS-trending structures, namely, the eastern half of the southern Espinhaço range fold-thrust system (SER-thrust system), the Chapada Acauã normal shear zone (CASZ), and the Salinas synclinorium , 2017, Santos et al 2009, Peixoto et al 2018). In addition to the Archean/Paleoproterozoic gneisses and granitoids, the metasedimentary units affected by these structures include quartzites and phyllites of the Espinhaço Supergroup, a thick succession of the Macaúbas Group, and the metaturbidites of the Salinas Formation, locally cut by granites of the G4 Supersuite (Dussin & Dussin 1995, Pedrosa-Soares et al 2001, 2011a, Lima et al 2002, De Campos et al 2004, Santos et al 2009, Kuchenbecker et al 2015 (Figs.…”

“…Occurrences of the Serra da Inhaúma Group are restricted to the southwestern end of the hinge zone and a narrow strip along its leading edge. The salient nucleus is occupied by a large volume of granitic intrusions of the G2 (syn-collisional), G4 and G5 (post-collisional) Supersuites (Pedrosa-Soares et al 2001, 2011b, De Campos et al 2004, Peixoto et al 2018.…”