Sample location and description
Syenogranite gneiss, sample1. This sample was collected at the Caseca quarry, 3 km south of the bridge on Br-101 over the Camboriú River and represents the voluminous G2 syn-tangential phase of the gneiss, which contains the G1 Archaean TTG xenoliths (not studied here) and amphibolite xenoliths (sample 2). The G2 blocks vary from a few square meters to a few hundred square meters and are cut by dikes of a Neoproterozoic G3 Brasiliano granite. The major structural feature of the G2 phase is a low-angle, thrust-related banding with most of the original magmatic fabric largely preserved, although solid-state overprinting is observed locally. Feldspars show both a grain-shape fabric and biotite, a pervasive foliation. These features indicate syn-tectonic injection of granites during a major thrusting episode. Country-rock remnants of G1 tonalitic gneisses and amphibolites exhibit strong assimilation. The analyzed sample was collected from a large xenolith with 100 m2 of exposed area in the quarry face, surrounded by a G3 Brasiliano intrusive phase. A low-strain site was selected in order to avoid fine-scale lithological and possibly chronological mixing of G2 with older G1 remnants so that purer end-member ages could be obtained. For the same reason, the sample was collected about 1 m from the sharp contact with the enclosing G3 dike-like Brasiliano granite.
Under the microscope, the G2 sample has a syenogranitic
composition with a granoblastic, coarse-grained, locally porphyritic
texture and mildly developed solid-state, protomylonitic structure,
with some feldspar comminution and quartz recovery. It has quartz
(34 vol%), microcline (42 vol%) oligoclase (20 vol%) as its main
felsic components. Mafic accessories are brown biotite (3 vol%)
and hornblende (1 vol%). Hornblende may become the only mafic
accessory close to remnants of assimilated amphibolites in schlieren
domains. Minor accessories in this sample are magnetite, allanite,
titanite, apatite, and zircon. Chemically, the sample is a silicic
orthogneiss (71.9 wt% SiO2); in comparison with Australian
felsic I-type granites (Whalen et al. 1987), the sample has a
normal potassium content (K2O = 3.21 wt%), higher aluminum
(Al2O3 = 15.18 wt%), sodium (Na2O
= 4.29 wt%), calcium (CaO = 2.29 wt%), and Sr (397 ppm) contents,
and lower Rb (99 ppm) and Y (15 ppm) contents. The rare-earth
element (REE) profile is fractionated, with moderate LREE enrichment,
and features no Eu anomaly. The absence of a Eu anomaly precludes
significant crustal differentiation
and contamination for the precursor magma. The REE profile is
compatible with granites derived from the lower crustal, produced
by high-degree partial melting of depleted TTG precursors. Post-magmatic
rock alteration is severe and pervasive. The alteration is caused
by crystallization of muscovite from plagioclase and microcline,
epidote from plagioclase, chlorite, epidote and iron oxides from
biotite (greenschist facies). Zircon is mainly included in the
biotite and shows also alteration effects such as yellowish cloudy
portions. This alteration is interpreted as resulting from overprinting
during the waning stages of the Neoproterozoic Brasiliano Cycle.
Amphibolite xenolith, sample 2. The AMPH1 sample was collected from a 10 m long and 2 m wide xenolith in the G2 granitic gneiss at the Caseca quarry, close to the G2 (sample 1) monzogranitic gneiss sampling site. Like the enclosing G3 gneiss, it was also injected by the post-tectonic (G3) Guabiruba granite. The contact with the G2 gneiss is diffuse and reactive, featuring an outer border of amphibolite assimilation by the G2 phase, indicative of deep crustal level of injection (migmatite). The contact with the G3 syenogranite is sharp, non-reactive, characteristic of shallow crustal levels.
Microscopically, the sample is a fine grained amphibolite, composed
of plagioclase/andesine (40 vol%) hornblende (43 vol%), titanomagnetite
(3 vol%) and minor quartz, apatite, and titanomagnetite. The texture
is nematoblastic, made up of well-oriented microbands of hornblende
and plagioclase. There are some domains with euhedral plagioclase.
Chemically it is a basic rock (SiO2 = 49.98 wt%), featuring
high-K contents (K2O = 1.81 wt%) and Rb (93 ppm). The
REE profile shows an unfractionated tholeiitic pattern, with LREE
enrichment, and a weak negative Eu anomaly. No pre-metamorphic
textures are preserved, but the paragenesis and the chemical signature
point to a mafic precursor such as a volcanic tholeiite. The K2O
and Rb enrichment are interpreted as due to post-magmatic alteration.
Águas Mornas granodiorite gneiss, sample 3. The zircons from sample 3 were extracted from a coarse grained, centimeter-scale banded biotite-granodioritic gneiss, with centimeter- to decameter-scale veins and pockets of quartz-feldspar pegmatoids. The sample was collected at a road cut on the BR-282 Highway close to the town of Águas Mornas. Field relationships resemble those at the Camboriú gneiss sample collection site, except that the post-tectonic G3 granitoid intrusive phase is not present in the Águas Mornas outcrop.
The sample is a biotite-monzogranitic orthogneiss with granoblastic, coarse-grained, locally porphyritic texture. It has well developed compositional banding and foliation and, locally, a protomylonitic structure. Quartz (40 vol%), microcline (35 vol%), and oligoclase (20 vol%) are the major components whereas biotite (5 vol%) is the main accessory mineral. Zircon, allanite, titanite, apatite, and opaque minerals are also minor accessory minerals. Like the Camboriú G2 gneiss, this sample has a high silica content (SiO2 = 68.83 wt%), high potassium (K2O = 4.36 wt%), Ba (995 ppm), Rb (170 ppm), Sr (372 ppm), and Zr (179 ppm).
The following synthesis is based on a review of the Águas
Mornas orthogneisses by Silva (1995, unpublished). On the multicationic
R1-R2 diagram (Batchelor and Bowen 1985), the gneisses plot between
the fields of the syn-collisional and volcanic arc granites. In
the Debon and LeFort (1983) Q-P diagram, they plot in field 2
of adamelites. In the Debon and LeFort diagram, they occupy field
III of the discretely peraluminous granites. The enrichment in
Rb and Y is compatible with volcanic-arc granites (Pearce et al.
1984). The REE profile is characterized by LREE enrichment and
by a strong negative Eu anomaly. The overall chemical signature
is compatible with the felsic I-Australian, arc-derived, crustally
evolved granites. A balance between the REE profiles from
sample 1 (syenogranite) and sample 3 (monzogranite) gneisses outlines
the very distinctive petrological nature of both magmas. The REE
contents of the two magmas have a strong contrast despite a similar
degree of differentiation; this is not compatible with a common
source and similar path of crustal evolution. These petrologic
distinctions are reflected in the measured isotopic ages. Similar
to that observed for sample 1, a greenschist alteration paragenesis
including saussuritization of the plagioclase, sericitization
of the microcline, and chloritization of the biotite are also
observed, and the intensity of alteration is much more pronounced
in the Águas Mornas gneiss. Zircon crystals contained in
the biotite are also affected by the hydrothermal alteration.
Presidente Nereu tonalite, sample 4. The tonalitic pluton is exposed along the Naufrágio river as a small (4 km2) basement inlier, exposed within the sedimentary rocks of the Paraná Basin, 100 km northwest from the northern limit of the Camboriú-complex exposures. The sample was collected by a waterfall 4 km southeast of the town of Presidente Nereu (Fig. 2). The analyzed rock is an undeformed clinopyroxene-hornblende tonalite. The sampling site is located at a deformational pod where the tonalite shows no post-magmatic deformation and preserves its magmatic texture. Regionally, however, the tonalite has low-angle thrust-related fabrics and middle-amphibolite-facies recrystallization. Locally, it also has a NE-SW late-Brasiliano transcurrent overprint.
The major components of the tonalite are clinopyroxene (2 vol%), hornblende (8 vol%), quartz (25 vol%), and oligoclase (65 vol%). Zircon, allanite, apatite, titanite, and opaque minerals are minor accessory minerals. The texture is coarse-grained granoblastic with discrete alignment of the feldspars and mafic phases.
Chemically, the tonalite is a felsic tonalite (SiO2 =
67 wt%) with a LILE depleted signature as is seen from the low
K2O (2 wt%), Zr (187 ppm), Rb (35 ppm), Y (19 ppm),
and also high Na2O (3 wt%) and CaO (4 wt%). The REE
pattern is moderately fractionated without a negative Eu anomaly,
and with discrete depletion in the HREE. All these are characteristic
of the calcalkaline depleted TTG series (sensu Barker 1979; Park
and Tarney 1987). The mafic tholeiitic rocks found in the adjacent
high-grade TTG Archaean basement could be compositionally equivalent
to the material which has been partially melted to produce the
tonalites 2,200 Ma ago.