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