Petrochronology of UHT garnet-free granulites: Linking zircon geochemistry to metamorphic reactions

PETROCHRONICSPETROCHRONOLOGY+GEODYNAMICS

1 2 3 1Mahyra Tedeschi , Daniel Peters , Renato Moraes and Antônio Pedrosa-Soares 1 Programa de Pós-Graduação em Geologia, Universidade Federal de Minas Gerais, IGC-CPMTC, Belo Horizonte, Brazil (mtedeschi@ufmg.br)

2Institut de Recherche en Astrophysique et Planétologie (IRAP), Observatoire Midi-Pyrénées (OMP), CNRS, Toulouse, France3Departamento de Mineralogia e Geotectônica, Instituto de Geociências, Universidade de São Paulo, São Paulo, Brazil

Geological Context

Petrography and mineral chemistry Introduction

Petrochronology

Aknowledgments References

Trace elements in Opx, Cpx and Amp

Partial melting plays an important role in the redistribution of elements within the lower continental crust in space and time. Ultimately, these processes affect the crust's chemical and mechanical properties, transport and enrich metals, and locally result in (hazardous) volcanism.

Petrochronological investigations (i.e., linking time and duration to specific rock-forming processes and their physical conditions; Engi et al., 2017) may provide insights into crustal reworking throughout the evolution of the Earth. The high temperatures to which migmatites are subjected to often overprint prograde mineral assemblages and produce protracted zircon geochronological records, rendering pressure-temperature-time reconstructions a challenging task. In addition, garnet-free rocks lack the possibility of linking petrological information recorded in garnet with geochronological constraints obtained from zircon or monazite, the most wide-spread petrochronogical mineral pairs.

Can trace element compositions of amphibole, orthopyroxene and clinopyroxene combined to zircon be used to link timing data to petrological processes?

Granites

Carrancas nappe systemand Lima Duarte nappe

Andrelândia nappe andCarmo da Cachoeira nappe

Liberdade nappe Migmatites

Pouso Alto nappe and associated klippen

Syn- to Post-Orogenic granites

Metatexite unit

Diatexite unit

Granulite unitAmparo complex

Itapira complex

21º

23º

Ediacaran

Andrelândia nappe system

Cryogenian-Ediacaran

Socorro-Guaxupé nappe

Paleoproterozoic

Pouso Alegre complex

ArcheanPassos nappe(Araxá Group)

47º 44º

Southern Brasília orogenic units

Pouso Alegre

50 kmN

Tonian

São Vicente complex

Bandeira do Sul

Alfenas

Heliodora-Minduri and Mantiqueira complexes

SÃO FRANCISCO CRATON

Para

ná

Basin

AFRICA

SOUTH AMERICA

CC

KC

AC

WAC

SF

B

Brasília orogenic system (B)

Study region

Brasiliano - Pan-African cratonic blocks (SF, São Francisco)

1000 km

A

PB

RPC

B

Basement

Socorro nappe

Guaxupé nappe

Investigated sample

Fig. 1: (A) Distribution of cratonic blocks and orogens in Western Gondwana (red rectangle: location of the studied region illustrated in B): WAC, West African craton; AC, Amazonian craton; SF, São Francisco craton; CC, Congo craton; PB, Paranapanema block; KC, Kalahari craton; RPC, Rio de la Plata craton. (B) Sketch map of the southernmost Brasília orogen (modified from Campos Neto et al., 2011; Cioffi et al., 2016; Westin et al., 2016).

Southern Brasília orogen (SE-Brazil) A complex framework of east-verging nappe

systems developed during the collisional stage in the Neoproterozoic (670-590 Ma);

The Socorro-Guaxupé nappe: metamorphic remains of a pre-collisional magmatic arc formed at 790-640 Ma and

metamorphosed at 660-600 Ma;

Migmatites record ultra-high temperature metamorphism:

T = 800-1050 °C and P = 8-14 kbar

Fig. 2: Sketch cross-section showing the field relationships of the studied rocks of the granulitic – Elói Mendes unit: (1) mafic granulite (residue), (2) banded granulite, (3) opdalite, (4) light-green charnockitic to enderbitic leucosome, and (5) pink hornblende-biotite-bearing granitic leucosome, and (6) mafic granulite enclave. Mesoscopic aspects of each sample: (A) Banded granulite with concordant light green charnockitic leucosome in stromatic structure (C-838-A); (B) mafic schollen (residue) in charnockitic leucosome with peritectic orthopyroxene and clinopyroxene (C-838-2); (C) schlieren of biotite and schollen of stromatic metatexite in the pink hornblende-biotitebearing granitic leucosome (C-838-B); (D) banded granulite (C-833-A); (E) Agmatic structure formed by the mafic granulite enclaves (sample C-716-B) in the diatexite; and (F) euhedral peritectic hornblende in the nebulitic diatexite. Detailed samples on this study correspond to the black stars.

4

6

EB

4

1

GF 4

C-833-A

C-838-2

C-838-A C-838-BC-716-B

2

DC-833-A

2A

C-838-A

N-NW S-SE

C-716-B

4

Pink Hbl-Bt granitic leucosome

São João da Mata unit

Sample

Amphibolitic layer

Opdalite

Light-green charnockitic to enderbitic leucosome

Banded granulite

Mafic granulite residue

5

4

3

2

1

Elói Mendes unit

Mafic granulite enclaves 6

5

C

C-838-B

Field relationshipFrom north to south transitional migmatitic granulite with stromatic metatexites to schollen diatexites passes to schollen to nebulitic diatexites.

Banded opx-cpx granulites occur mainly as enclaves within opdalitic leucosome ( ).

1 mm

1 mm

Hbl

Pl

Cpx

Opx

Hbl

Pl+Qz

BtCpx

Pl

Hbl

Bt

CpxOpx

C-838-A

C-833-A

Fig. 3: Transmitted light, non-polarized photomicrographs from the studied rocks. Dotted pink lines outline faded domanial boundaries between leucosome and residue. (A) Relation between thin bands of residue and leucosome in the banded granulite (C-833-A), showing a leucosome depleted in mafic minerals (mostly orthopyroxene and clinopyroxene); (B) peritectic orthopyroxene and clinopyroxene in a band of leucosome relatively enriched in mafic minerals (C-838-A).

Banded Opx-Cpx granulites

Mafic layers: Hbl+Pl+Bt+Cpx+Opx+Qz

Leucocratic layers: Kfs+Pl+Qz±Cpx±Opx±Hbl

Hbl replaces Opx and Cpx in the residue (selvedge). The only

difference is that C-833-A has only few Opx and Cpx in the leucosome (<10 vol.%) when

compared to C-838-A (~25 vol.%).

Mag CpxQzPlKfsIlmBtHblOpx

A1mm

Opx Cpx

Mg (a.p.f.u)0.80

Mg (a.p.f.u)

0.82

0.84

0.86

0.88

0.90

0.92

0.95

1.00

1.05

1.10

Opx

Al (a.p.f.u)

0.02

0.04

0.06

0.08

0.10

0.12

Cpx

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

0.15 Al

Fig. 4: Compositional maps from EPMA imaging: (A) Phase map showing the texture of the banded granulite (C-838-A) with assigned phases based on quantified compositional maps. Dotted yellow rectangles indicate the position of compositional maps for clinopyroxene and orthopyroxene.

Opx and Cpx are zoned in the leucosomeEn Fs MgTs 56-48 40-46 4-6

Di Hd Jd 60-70 36-27 5-4

Hornblende (Al in the site M2=0.6 apfu; Na=0.59 apfu; Ca=1.8 apfu; and XMg=0.11)

Zoned Kfs in the leucosome: (Ab Mc ) 23-10 77-90

Mg

zirc

on/c

hondrite F

D

C-833-A C-838-A

0.2812

0.2813

0.2814

0.2815

0.2816

0.2817

0.2818

1.2 1.6 2.0 2.4 2.8

207 206Apparent Pb/ Pb (t) Ga

17

61

77

Hf/

Hf

(t)

B

La

Ce

Pr

Nd

Sm

Eu

Gd

TbDy

Ho

Er

Tm

Yb

Lu

La

Ce

Pr

Nd

Sm

Eu

Gd

TbDy

Ho

Er

Tm

Yb

Lu

010

210

310

110

410

zirc

on/c

hondrite C

207 235Pb/ U

20

62

38

Pb

/U

0.5

0.0

0.2

0.4

0 2 4 6 8 10 12

600

1000

1400

1800

2200

2600

2559 ± 66 MaƐHf = +6.52

box heights are 2 σ

2360

2440

2520

2600

WM = 2530 ± 23

MSWD = 0.77 Probability = 0.60

WM = 2506 ± 24

MSWD = 1.5 Probability = 0.14

20

72

06

Pb

/P

b

Cores Rims >1.0 Ga Rims <1.0 Ga

A

C

0.3

0.1

CoresNeoproterozoic rimsArchean rims Cores

Rims

176 177 206 238Fig. 5: U-Pb LA-ICP-MS data(Concordia and probability density diagrams), Hf/ Hf(t) ratio vs age (using Pb/ U dates), and chondrite-normalized REE diagrams for: banded granulite C-833-A (A-C) and banded granulite C-838-A (D-F). Ellipses in the Concordia diagrams are 2σ errors.

E

500 530 560 590 620 650 680 710 740 770 800

0

2

4

6

8

10

12

14

16

18

20

C8382C838BC838A

C833A

C716B

(n=39)

(n=37)

(n=41)

(n=6)

(n=63)

Age (Ma)

590 620 650 670

Protracted U-Pb zircon geochronological records prevent the determination of “ages”. However, different samples record same age peaks distribution.

176 177 Hf/ Hf allowed to constrain the crystallization age (t)

of the protolith.

Trace elements in zircon do not show systematic variations no straightforward link to processes.

176 177 Hf/ Hf in the rims are different than in the cores in (t)

C-833-A suggesting contribution of a Hf-enriched 176 177melt/fluid. In sample C-838-A, Hf/ Hf suggests (t)

that the fluid/melt that interacted with the rock had no significant contribution from breakdown of Lu-sink.

Fig. 6: Probability density diagram for the zircon U/Pb dates of the rims and single phases from the banded granulites (C-833-A and C-838-A) with other zircon rims from rocks in the same geological section.

Are the protracted records related to multiple metamorphic

events? Different stages? or domain-related reactions?

Campos Neto, M.C., Basei, M.A.S., Janasi, V., Moraes, R., 2011. Orogen migration and tectonic setting of the Andrelâdia Nappe system: an Ediacaran western Gondwana collage, south of São Francisco craton. J. S. Am. Earth Sci. 32, 393–406.

Cioffi, C.R., Campos Neto, M.C., Möler, A., Rocha, B.C., 2016. Paleoproterozoic continental crust generation events at 2.15 and 2.08 Ga in the basement of the southern Brasília Orogen, SE Brazil. Precambrian Res. 275, 176–196.

Engi, M., Lanari, P., Kohn, M., 2017. Significant Ages – An Introduction to Petrochronology. In: Kohn, M.J., Engi, M., Lanari, P. (Eds.), Petrochronology. Reviews in Mineralogy and Geochemistry 83, pp. 55–102.

McDonough, W.F., Sun, S.S., 1995. The composition of the earth. Chem. Geol. 120, 223–254.

Tedeschi, M., Pedrosa, Soares, A., Dussin, I., Lanari, P., Novo, T., Pinheiro, M.A.P., Peters, D., 2018. Protracted zircon geochronological record of UHT garnet-free granulites in the Southern Brasilia orogen (SE Brazil): Petrochronological constraints on magmatism and metamorphism. Precambr. Res. 316, 103–126.

0.1

1

10

100

1000

10000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Orthopyroxene

Clinopyroxene

Amphibole

Metamorphic zircon

Bulk rock

CR

CR

Min

era

l-B

ulk

rock/c

hondrite

0.1

1

10

100

1000

10000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Orthopyroxene

Clinopyroxene

Amphibole

Metamorphic zircon

Bulk rock

C

R

C

RMin

era

l-B

ulk

rock/c

hondrite

C-833-A Archean banded granulite

C-838-A Neoproterozoic banded granulite

Opx-Cpx-Amp observations:

C-833-A (no peritectic Opx and Cpx in the leucosome) REE signature when compared to C-838-A:- Stronger negative Eu anomalies for Amp and Cpx; - Higher variations in LREE and HREE contents;- Opx is REE depleted and has more diffuse HREE pattern;- Enrichment of HREEs in the Amp is incompatible with HREE release by Cpx breakdown.

Fig. 7: Chondrite-normalized (McDonough and Sun, 1995) REE diagrams for amphibole, clinopyroxene, orthopyroxene, zircon and the bulk rock composition.

Zrn observations:

- Variable Eu anomalies in both s a m p l e s s u g g e s t g r o w t h i n associations with different amounts and/or compositions of feldspar; - Some zircon grains from C-833-A record low HREEs comparable with values from Cpx and Amp; - LREE enrichment in some grains from C-838-A.

Metamorphic zircon grains (620-680 Ma) that crystallized in the residue, where the transition from Cpx to Amp proceeded in the absence or presence of few melt, are depleted in HREEs. This suggests that HREEs from Cpx were directly incorporated by Amp and is supported by the light Eu anomalies of these Zrn grains when compared to the HREE-rich Zrn. Eu negative anomalies in Zrn increase where Kfs crystallizes, in volumes without Cpx. In C-838-A, Amp is less enriched in HREEs because in the presence of melt, HREE likely enters the structure of Zrn. The absence of external fluid/melt as demonstrated by Hf isotopes for some Zrn reduces the mobility of REEs. For C-833-A the absence of fluids corroborates a subsolidus crystallization for part of the grains that indeed record higher temperatures (Tedeschi et al., 2018). Alternatively, the variation of trace elements in zircon could be caused by external melt/fluid components!?

At least partially, the protracted geochronological records potentially reflect domanial reactions in different stages of a prolonged metamorphic event.

Discussion