In order to contribute to the understanding of the metamorphic processes of the metamorphic
rocks of the Araguaia Belt (CA), this thesis presents field data and metamorphic modeling,
used as tools to obtain the maximum conditions of P-T reached by the units located in the
Xambioá-Araguanã (TO) region of the rocks studied are five samples of Staurolite-garnetbiotite-
muscovite-schist with Kyanite and a sample of Garnet amphibolite, belonging to the
Xambioá Formation and Xambica Suite, respectively. The study of the metamorphism
occurred through petrographic, chemical analyzes of rocks and minerals of the main
associations of pelitic rocks, semipelitic and mafic rocks that comprise the zone of greater
metamorphic degree of CA, for later analysis by metamorphic modeling in the NCKFMASH
system and optimized geothermobarometry, In the avPT mode, in the THERMOCALC
program, as well as the Hb-Pl software, which made it possible to identify PT metamorphic
peak conditions. In addition to defining an approximate age of metamorphism using the Ar-Ar
method in biotite and amphibole. The petrographic-mineralogical study identified the main
mineral parageneses in the studied micaxists, such as St + Grt + Bt + Ms + Qtz ± Pl (An12-31)
+ Ky, and in the amphibolites Hb + Grt + Bt + Pl (An12-25). The mineral chemistry results
showed that the pomegranate composition of the six samples is dominated by the almandide
molecule, which is followed by pyropo, thickenstock, and andradite, increasing Fe2 + and Mg
from the nuclei to the edges, with a concomitant decrease of Mn and Ca. The higher Fe and
Mg contents towards the edges indicate an increase in the temperature conditions during
mineral development. The composition of the biotite is in the transition of the biotite with
phlogopite, becoming more ferromagnesian mica. Staurolite from the core to the border is
enriched in Fe2 + and depleted in Mg, which may suggest reactions in contact with the matrix
and / or with micas or grenades. Metamorphic modeling in the NCKFMASH system resulted
in pseudosections with similar topologies for samples BP002, BP149 and BP299, suggesting
that the maximum metamorphic conditions they were subjected to are similar, being situated
within a PT window with pressure approximately between 7 and 9 kbar and temperature
between 630 and 665 ° C. The models of composite isopleths of the minerals calculated in the
pseudosections indicate a higher participation of Mg and a lower participation of Ca in the
composition of the main minerals according to the increase of the metamorphic P-T
conditions in the region, compatible with the mineral chemistry results of the studied rocks. In
addition to the plagioclase and the pomegranate of the sample BP002 presenting as good
indicators of metamorphism conditions, with values of 8 kbar and 660 °C, in which the coreedge
variation of Ca and Na in plagioclase crystals and the variation Core-edge in the iron content of the grenades show a progressive barometric-type metamorphic trajectory,
represented by a small slope curve, coming from the trivariant field Chl + Grt + Bt + Ms. PT
estimates obtained in the avPT mode of THERMOCALC in all the selected samples (BP002,
BP005, BP149, BP149, BP299, BP006), as well as in the Hb-Plag software in the
amphibolites (BP006), presented enough Consistent and consistent with calculations of
metamorphic peak conditions via THERMOCALC for rocks in the Xambioá-Araguanã
region, although there are discrepancies in the calculated results. All calculated P-T results are
compatible with the field of the amphibolite facies of the mean P-T series, typical of
continental orogenic belt environments and therefore characteristic of continental collisions.
Metamorphic conditions are reinforced with the mineral chemistry results of calcium
amphiboles suggesting the same average pressure conditions of the Dalradian terrain of
Scotland. The air-air ages in minerals obtained in the metapelites and amphibolites in this
thesis were similar, around 504 Ma, and they show that the peak of the metamorphism of the
amphibolite facies in the region under study has an older age and they suggest that it was next
to the Neoproterozoic-Paleozoic interface, because these Ar-Ar ages obtained are younger
ages than the K-Ar ages of 520-560 Ma, interpreted as relating the metamorphism of the CA
with the Brasilian thermo-tectonic event, and because it is within the error of the U-Th-Pb
monazites chemical age in feldspathic schist biotite from an area near the city of Presidente
Kennedy (TO), 513 ± 14 Ma interpreted only at a younger age than the mean age evaluated
for the metamorphism of the Araguaia Belt (550 - 530 Ma).
|
