The contact metamorphism represents the best example of thermal transfer
emanated by granitic plutons at shallow crustal levels, with mineral and textural
changes in rocks, resulting in an extremely compact and solid rock, called a hornfels.
In the northwestern region of Ceará State, in the Médio Coreaú Domain,
northwestern sector of the Borborema Province, this type of metamorphism has been
reported around the granitic batholiths, which arise mainly near shear zones, such as
Granite Meruoca. This work aims to characterize the contact metamorphism that
occurs in host rocks of the western portion of the Granite Meruoca, represented by
the sandstones of the Pacujá Formation (Jaibaras Group) through petrographic
analysis. The studied profile is located between the cities of Alcântara and Coreaú
(CE), following the road CE-241 toward the Meruoca Mountains. We studied 5
outcrops (2008-TEC-01 to 05), with collect 14 samples, which were made 15 thin
sections. The petrographic study revealed detrital sedimentary rocks, ranging from
sandstone to conglomeratic sandstones and compositionally, impure arkose to lithic
arkose, containing portions of pelitic, carbonate and basic composition. The first
metamorphic stage (M0) is represented by the rocks outside from the aureole, which
show changes generated by burial and hydrothermal metamorphism in conditions of
low greenschist facies, chlorite zone (T=300-350°C; P=2-3 kbar). The aureole starts
at approximately 500 m from the Meruoca Granite (stage M1), with crystallization of
biotite, which does not occur in stage M0, creating a dark and very compact rock in
conditions of facies albite-epidote-hornfels, the biotite isograd (350-400°C). On
contact with granite, can be viewed at full transformations, displaying rocks totally
recrystallized, with structure nodulose or maculose, typical of hornfels. The
paragenesis is composed of microcline + quartz + plagioclase + clinopyroxene
(diopside) + hornblende in conditions of hornblende hornfels facies, garnet isograd (T
= 600°C; P=2 kbar) in the third metamorphic stage (M3). This rock was classified as
diopside-hornblende-microcline hornfels.
|
