7/23/2019 DeJong Etal 243-220 Ma Ar/Ar Ages Korean Collision Belt, GSK Abstract 2013

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7/23/2019 DeJong Etal 243-220 Ma Ar/Ar Ages Korean Collision Belt, GSK Abstract 2013

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101 2 3 ~ [ ~ ~ A i h ~ J

243-220 Ma

40

Ar/

39

Ar plateau ages of laser probed mineral

single

grains from

Korean

collision belts:

phases

of

fast cooling

and exhumation due to slab detachment?

I I

Koenraad de Jong Grlles Ruffet Seokyoung an

School of Earth and Environmental Sciences

Seoul

National University Keuntie@snu.ac.kr

Geosciences

Rennes University

of Rennes

Rennes

France

The Gyeonggi massif is

the

only Korean Paleoproterozoic high-grade gneiss terra ne sig

nificantly affected by Permo-Triassic metamorphism. We sampled the massif's uppermost

part for isotopic dating,

as

well

as the

overlying

Imjingang

belt,

to the

North, and

the

Hongseong belt

to the

West. Both belts comprise younger metamorphic rocks containing rare

lenses

of

mafic rocks, some

with

relics

of

high-pressure metamorphism. We focused on

40

Ar/

39

Ar laser-probe step-heating

of

fabric-forming minerals produced

by

retrograde re

crystallisation during the main phase deformation concomitant with exhumation. We obtained

middle

to

late Triassic 1o plateau ages ( 50-80

39

Ar release) for single grains of muscovite,

biotite and amphibole.

Retrograde hornblende from a corona-textured garnet-clinopyroxene-rich mafic granulite

Misan

formation, Yeoncheon group, lowermost Imjingang belt) yielded a saddle-shaped age

spectrum, 70 of which

has

a single step age of 260 1

Ma

and a

base

formed by four concordant

steps with 242.82.4

Ma

as

weighted mean age ( 15

39

Ar). Muscovites from strongly retro

gressed and ductily deformed rocks in

the top of

the Gyeonggi massif yielded plateau ages

of: 242.81.0

Ma

and 240.31.0 Ma (chlorite-mica schists) and 219.70.9

Ma

(mylonitic

quartzite). The hornblende age indicates

that

retrograde hydration of the high-pressure gran

ulite occurred around 240-245

Ma.

Our age data also imply that at least part

of

the ductile

deformation fabrics in the Gyeonggi massif's top formed around this time too.

In

the Hongseong

belt we obtained slightly saddle-shaped age spectra on two hornblendes from foliated gar

net-bearing corona-textured amphibolite (Neoproterozoic Deogjeongri orthogneiss). Both

spectra's flat saddle minima have identical plateau ages

of

230.11.0 and 229.81.0

Ma,

con

cordant with the 228.11.0

Ma

plateau age of biotite in the slightly older amphibolite. These

ages are only marginally younger than published 230-255

Ma

CHIME and

SHRIMP

U-Pb ages

of accessory minerals in

the

uppermost Gyeonggi massif and

the

Imjingang and Hongseong

belts. Concordance of hornblende and mica ages in these areas and near agreement with U-Pb

ages points

to

rapid cooling and exhumation, during at least part

of

the tectonic history. The

Hongseong belt's

40

rr r

ages show

that

exhumation occurred

~

10 million years later than

that of the Imjingang belt's high-pressure rocks. But the 220 Ma-old muscovite in mylonitic

quartzite implies extended or renewed recrystallisation in low-angle normal shear zones in

the northern Gyeonggi massif's uppermost part. Exhumation thus occurred

in

steps

not

coeval

throughout the Korean collision belt. The late Triassic metamorphic mineral ages overlap with

233-226 Ma-old syenite--monzonite--granite--gabbro magmatism with high-K calc-alkaline and

shoshonitic affinity, suggesting fast exhumation may have occurred by late Triassic slab detach

ment,

following plate collision in Paleozoic-Mesozoic boundary times.