Post on 06-Sep-2020
Tonian oceanic crust processes decoded with multi-techniques from metamorphosed, hydrothermal, coetaneous zircon-tourmaline, southern Brasiliano Orogen
Mariana Werle¹, Léo A. Hartmann¹, Cassiana R.L. Michelin¹, Cristiano Lana², Glaúcia Queiroga², Karine R. Arena¹
¹Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Brazil²Departamento de Geologia, Universidade Federal de Ouro Preto, Brazil
marianawerle96@gmail.com
Tourmaline is a resilient, much-informative mineral, and thus a proxy for identification of diversified geotectonic environments;
Metamorphosed, hydrothermal, coetaneous zircon-tourmaline assemblages document the dynamic evolution of Tonian oceanic crust, southern Brasiliano Orogen;
We describe massive tourmalinites from Ibaré and Bossoroca ophiolites which obducted onto an oceanic island arc during low amphibolite facies metamorphism. Multi techniques, grain internal structure and composition relates the minerals to evolution of Rodinia and Gondwana Supercontinents
This work aims to characterize tourmaline and its zircon inclusions in Bossoroca and Ibaré ophiolites, southern Brasiliano Orogen, Brazil.
In the field, Bossoroca tourmalinite is massive and enclosed in metaserpentinite I. (olivine + talc + chromite, jackstraw texture, low amphibolite facies) and close (1-50 m) to amphibolite and chromite talc magnesite fels (fig 2 a). Ibaré tourmalinite is close to chloritite and serpentinite (fig 2 b).
Under optical microscope, Bossoroca tourmaline only displays zoning in a narrow light rim. Intense II. cataclasis fractured and broke the crystals, generating vugs. Chlorite (Chl) is present in two generations. Chl 1 is in apparent equilibrium with homogeneous, core tourmaline, whereas Chl 2 is in equilibrium with tourmaline rim (fig. 3 a, b). Zircon occurs in tourmaline mostly in strings of small (5-10 µm) crystals but reaching 50 µm (fig. 3 c).
Three zones of different gray tones are observed in Bossoroca tourmaline in backscattered electron III. images (BSE) – Tur 1, Tur 2, Tur 3 (fig. 4 a). These zones are also displayed in EPMA compositions because elemental variations in Tur 1, Tur 2 and Tur 3 zones are observed in BSE images and compositional X-ray maps (fig. 4 b, c). Ibaré tourmaline is homogeneous (fig. 4 d, e, f).
granite-related veins
cont. crustMORB/mantle
bulkoceanic crust
serpentinite
modernseawater
11δ B (‰)
-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 40
-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 40
2
0
4
6
8
11δ B (‰)
Num
ber Ibaré
tourmaline
terrigenous marine sediments
Bossoroca tourmaline
Tur 1
Tur 2Tur 3
Continental,Brazilian Shield
Oceanic crust,Brasiliano Orogen
0
4
7
12
18
24
30
Fe
Conc.
0
4
7
14
18
24
30Conc.
Mg
Chl 1
Chl 2
Chl 2
500 mm
BSE
2 e.g., Tur 2
200 μm
BSE
Chl
Tur
Fe
920
345
0 Mg
993
372
0
112
2
23
2
33
3
Chl 1
a b c
d e f
11 Based on isotopic determinations, Ibaré tourmaline has δ B = +5, whereas Bossoroca tourmaline Tur IV. 11 11 111 has δ B = +1 to +2.2 (peak at +1.8), Tur 2 has δ B = –1 to +0.4 (peak at 0), Tur 3 has δ B = –8.2 to –9.2, peak
at –8.5) (fig. 5).
U-Pb zircon ages from tourmalinite indicate hydrothermal crystallization near 900 Ma V.and partial recrystallization near 700 Ma.
Results indicate that Bossoroca tourmalinite formed in oceanic crust (zircon εHf = +12) environment at 900 Ma, mid-ocean ridge environment (initial fragmentation of Rodinia), later accreted to Gondwana near 700 Ma. The most significant result is the boron isotopes showing oceanic crystallization of Ibaré tur and Tur 1 and Tur 2 from Bossoroca, contrasting with partial recrystallization (Tur 3) in shear zone in the continent. Comparative resilience of zrn-tur indicates that cores of both minerals formed at 900 Ma from oceanic water, dated rims of zircon and tur 3 at 700 Ma, but zrn shows no evidence of recrystallization during tur 3. Intra-grain characteristics of zrn-tur from massive tourmalinite helped elucidate processes at the grain to plate scale.
20 µm
712 Ma
20 µm
926 Ma
Chl 1
Tur
Tur 2
Tur 3
Chl 2
C
Tur
Chl 2
TurZrn
a b c Intercept age = 920.4 ±9.8 MaMSWD = 1.9
0.14
0.16
0.12
0.10
0.08
0.8 1.0 1.2 1.4 1.6 1.8207 235
Pb/ U
206
238
Pb
/U
650
750
850
950
0.18
ZirconBossoroca tourmalinite
28712 Ma
31a805 Ma
51926 Ma
U-Pb isotopes (20 µm)
Lu-Hf isotopes (50 µm)
Trace elements (25 µm)
20 µm
926 Ma
20 µm
712 Ma
Introduction
Amphibolite
Meta-serpentinite
Chr Tlc Mgsfels
Serpentinite
Chloritite
a bIbarétourmalinite
Bossoroca tourmalinite
Objectives and Methods
Fieldwork
Backscattered electron imaging
EPMA analyses and characteristic X-ray maps
LA-ICP-MS trace elements and U-Pb-Hf isotopes
B isotopes
Zircon
Tourmaline
Tourmaline
Zircon
Tourmaline
Results
.Figure 1: (a) Geological map of Brazil showing the distribution of Neoproterozoic ophiolitic rocks in the Brasiliano Orogen (Suita et al. 2004). ; (b) geological map of the Precambrian-Cambrian Sul-Riograndense Shield (Dom Feliciano Belt and basement) southern Brazil. (Extracted from Arena et al. 2017).
Figure 2: Field photographs of Bossoroca and Ibaré tourmalinites. (a) Field view, showing Bossoroca tourmalinite location and outcrop. (b) Field view, showing Ibaré tourmalinite location and outcrop.
Figure 3: Optical photomicrography of Bossoroca tourmalinite. (a) Optical photomicrography (crossed nicols) showing Tur 1, Tur 2 (dark core) and Tur 3 (light rim). Deformed Chl 1 is in apparent equilibrium with tourmaline and Chl 2 seems in equilibrium with light tourmaline rim; (b) Chl 2 in masses of small crystals, partly filling fractures; (c) Zircon included in tourmaline in strings of small (5 - 10 mm) crystals.
Figure 4: BSE and characteristic X-ray distribution of elements in tourmaline and chlorite. (a), (b), (c) Bossoroca tourmalinite; (d), (e), (f) Ibaré tourmalinite.
Rodinia
Rodinia
Passinho arc
São Gabriel arc
Granites, e.g., 585 Ma Ibaré and Palma ophiolites, 722 Ma
Passinho intra-oceanic
arc, 880-750 Ma
Rio de La Plata
Craton
Cerro Mantiqueiras ophiolite, 923 Ma and 786 Ma
a
b
c
Ibaré and Palma ophiolites, 880 Ma
920 Ma
880 Ma
722 Ma
Rodinia
Rio de la Plata
Craton
Bossoroca ophiolite, 920 Ma
Bossorocatourmalinite
Kalahari
CratonSão Gabriel oceanic to
continental arc, 750-700 Ma
Rodinia
Conclusions
References Arena, K.R., Hartmann, L.A., Lana, C., 2017. U–Pb–Hf isotopes and trace elements of metasomatic zircon delimit the evolution of neoproterozoic Capané ophiolite in the Southern Brasiliano Orogen. International Geology Review.
Farber, K., Dziggel, A., Trumbull, R.B. et al (2015). Tourmaline B-isotopes as tracers of fluid sources in silicified Palaeoarchaean oceanic crust of the Mendon Formation, Barberton greenstone belt, South Africa. Chemical Geology 417:134–147. doi.org/10.1016/j.chemgeo.2015.10.009
Suita, M.T.F., Pedrosa-Soares, A.C., Leite, C.A.S., Nilson, A.A., Prichard, H.M., 2004. Complexos ofiolíticos do Brasil e a metalogenia comparada das faixas araçuaí e brasília. In: Pereira, E.S., Castroviejo, R., Ortiz, F. (Eds.), Complejos ofiolíticos em Ibero América: Edita Proyecto XIII.1, Madrid-España. 379, pp. 101–132.
11Figure 5: Histogram showing δ B. (a) Different geological settings of boron sources (Farber et al. 2015); (b) Frequency histogram for Bossoroca (Tur 1, Tur 2, Tur 3) and Ibaré tourmalinites.
a
b
Figure 6: Concordia diagram displaying zircon isotopic determinations and selected BSE images of zircon, displaying location of spots analyzed for U-Pb-Hf isotopes and trace elements.
Figure 9: Evolutionary model of oceanic crust from initial rupturing of Rodinia. Bossoroca tourmalinite formed during initial oceanic crust formation, in a continent-influenced environment of narrow-rift.
Conselho Nacional do Desenvolvimento Científico e Tecnológico (Government of Brazil) supported systematically investigations by the authors, including undergraduate scholarship to Mariana Werle. EPMA analyses were obtained from ‘Laboratório de Microanálises do DEGEO/EM e Laboratório integrante da Rede de Microscopia e Microanálises de Minas Gerais e FAPEMIG’.
Acknowledgements
a b
2019