Multistage Exhumation History of Ultra-cool Oceanic (U)HP eclogites: New evidence from - PowerPoint Presentation

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Multistage Exhumation History of Ultra-cool Oceanic (U)HP eclogites: New evidence from the Nagaland Ophiolite Complex (NOC), NE India

EGU2020-9583,

Vienna, Austria

Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721 302, India

S.K.

Bhowmik

& M.

Rajkakati

Nagaland Ophiolite Complex

Where?

SE-ern margin of the YTSZ; Tri-junction of Indian-Burmese-Asian Plates

Nagaland Ophiolite Complex: Why Important

?

Presence of an array of HP

Metic rocksContrasting T-t history

Cold Mature

vs

Subduction

Infancy

Subduction architectureTiming of metamorphism

Thewati

Objectives

To establish UHP signature in the Nagaland oceanic eclogites on the basis of conventional thermobarometry and phase equilibria

modelling.To constrain multistage exhumation history of the eclogites, including rare documentation of recurring burial and exhumation history of these rocks.

To reconstruct subduction channel dynamics within the Neo-Tethys.Study Materials: Eclogites from two locations in the NOC, both from the structurally lowermost unit: Thewati and Mokie areas.

Nagaland Eclogites: Mode of Occurrences

Eclogites occur as meter-scale

lensoidal bodies within a lawsonite-blueschist facies metamorphosed oceanic sequence of basalt-chert-limestone.

Mokie eclogiteThewati eclogite

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M1 cycle: Textural & Compositional Support s

Prograde (M1PR at 13.6 kbar, 495 oC): EBS facies prograde burial

Growth zoning of Grt

1

Grt

1

(

C):

M

1PR Grt1 (IR):

M1P

Ep

1+Omp+Jd1+Spn1

+Gln±Ab→Gln-out→Spn-out/Ru-in→Jd-out→Grt

1

+Omp+Ep

1

+Ru+

+

Ph+Qtz

Omp

(C→R): Progressive

Jd

-enrichment

Grt

1

inclusion assemblage sequence

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M1 cycle: Textural and Phase Equilibrial

Modelling Supports Peak (M1P at 30.3 kbar, 700

o

C

): UHP Epidote eclogite facies; Assemblage: Grt

1

+

Omp

+ Ep

1+Rt+Ph+CoeRelics of M1P Omp(P) in arecrystallised Omp+Ep+Sph matrix

M1PR

→M1P P-T path

M1PR

M

1P

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M1 cycle: Textural and Compositional Support

Retrograde (M1R through 19.7kbar, 570

o

C

to 19.1kbar, 470

o

C

): Decomposition of

Grt+Rt+Omp+Coe

/Qtz to Ep2+Quad-rich Omp ±Jd+Sph+Chl+ Si-rich Ph assemblages

Matrix Omp+Ep

2+Sph assemblage, locally with Grt

1 relics. Matrix Omp Jd

-poor,

Quad-rich compared to peak

Omp

, now protected as

inclusions

in inner rim of Grt

1

Vein network of Omp+Ep

2

+Ph±Jd in Grt

1

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M2 cycle : Textural and Compositional Supports

Prograde & Peak (M2PR & M2P at 16.2 kbar, 590 o

C

):

Formation

of

idioblastic Grt

2

as an overgrowth on partially decomposed Grt1

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M2 cycle: Textural, Mineral Equilibria and Phase Equilibria Modelling Supports

Retrograde (M2R at 4.1 kbar, 270 oC

):

Developments

of

Pmp

with or without

Agt

,

Ab, Chl, Ttn and Ph as veins in Grt and matrix Omp

Mokie Eclogites: 3 Cycles of Metamorphism (M1-M

2-M3)

M3 cycle: Textural, Activity Corrected Mineral

Equilibria and Phase Equilibria Modelling Supports

Prograde (M

3PR

at 7

kbar

, 300

o

C

) →Peak (M3P at 7 kbar, 330 oC) : Pmp→Lws and Lws →Ep Reaction Pathways Partial rim of Lws around

Pmp

Relics of Lws

in Ep

Metamorphic P-T Paths of Nagaland Eclogites

Thewati Eclogite (T-EC) (after R19)

Mokie Eclogite (M-EC) (This study)

Other abbreviations used: PD: Pmp-Difacies metabasalt; M-LBS: MokieLawsonite blueschist facies metabasalt

Key Conclusions

Mokie and Thewati eclogites in the NOC, NE India represent rare pieces

of Latest Triassic to Jurassic aged oceanic crust that were exhumed from the deepest section within fossil subduction channels worldwide.

Thewati eclogite reveals a two stage exhumation: an initial one along a steep dP/dT gradient followed by exhumation along a much gentler

gradient.

Mokie eclogites record a more complicated, looping metamorphic P-T

path, r

eflecting recurring burial exhumation history of the easternmost

Neo-Tethys. The M1 cycle along a CW P-T path is related to a cold intra-oceanic subduction system within the Neo-Tethys. The M

2 cycle (also along a CW path) reflects an eclogite facies

prograde burial and heating (∆T>100

oC) and a subsequent exhumation

and cooling (∆T>300

o

C

) history. This can be linked with collision

tectonics involving a possible exotic continental fragment.

The M

3

cycle (again along a CW P-T path) reflects a lawsonite blue-

schist facies prograde burial event, indicating a shift to cold oceanic

system again. The event marks the tectonic juxtaposition of the

eclogites with the host blueschists.

Thanks