This paper presents the petrographic and microstructural data of the Carreira Comprida
Anorthosite, into the Porto Nacional - Brejinho de Nazaré region (TO), on the northern portion
of the Tocantins Province, into the Goiás Massif, between the Araguaia and Brasilia belts. This
paper aim is to identify the metamorphism effects into this anortositic body, and to describe the
mineralogical, textural and microstructural transformations in the pluton, aiming to identify the
metamorphic conditions and the structural/microstructural transformations due to the
deformation’s effects caused by the shearing thought the Transbrasiliano Lineament. At first, it
was made a survey of all cartographic and bibliographic content concerning the geological
knowledge of the Tocantins Province, the Porto Nacional high-grade metamorphic terrain and
the Carreira Comprida Anorthosite. Then, the fieldwork phase had been done to collect
representative samples of the unit for petrographic studies. From the petrographic and
microstructural data obtained, it was possible to divide the anorthosite into three distinct
petrographic types according their paragenesis and dominant microstructural features in each
domain: a) Anorthosites with preserved magmatic features (Plg + Cpx + Ilm); b) meta anorthosites (Plg + Cpx + Hbl + Bt + Grt + Ttn); and c) mylonitic anorthosites (Plg + Hbl + Bt
+ Grt + Ttn). The presence of a metamorphic mineral assemblage defines the Carreira Comprida
Anorthosite as a meta-anorthosite. The obtained mineral associations reveal amphibolite facies
metamorphism, with temperatures ranging between 600 and 700ºC at medium to high pressure
conditions. The petrographic domain marked by “a)” has well preserved magmatic features,
without hornblende and biotite in its composition, demonstrating lower metamorphic conditions
for this petrographic type. The microstructural analysis pointed the presence of microtextures
caused by deformation under ductile conditions. The obtained data interpretation data indicates
the variation into the meta-anorthosite’s metamorphic conditions and deformation grades, with
more intense deformation occurring in the mylonitic anorthosites located in the eastern portion
of the pluton, affected by the Porto Nacional Shear Zone.
|
