Geochronological challenges posed by continuously developing tectonometamorphic systems

insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

R. A. Cliff, C. E. Bond, R. W. H. Butler, J. E. Dixon

Research output: Contribution to journalArticle

9 Citations (Scopus)
6 Downloads (Pure)

Abstract

Is netamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt. The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the Cycladic Blueschist Belt on Syros. The majority are
from samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves
an age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectono-metamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt
phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under green schist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.
Original languageEnglish
Pages (from-to)197-211
Number of pages15
JournalJournal of Metamorphic Geology
Volume35
Issue number2
Early online date15 Dec 2016
DOIs
Publication statusPublished - Feb 2017

Fingerprint

blueschist
mica
Tectonics
blueschist facies
phengite
Microstructure
Rocks
Geochronology
greenschist facies
Crystallization
Minerals
microstructure
serpentinite
Sampling
exhumation
Temperature
tectonics
closure temperature
metabasite
greenschist

Keywords

  • blueschist
  • exhumation
  • phengite
  • Rb–Sr ages
  • Syros

Cite this

@article{2fcbc73fd2de4636ba7132f7a50951c6,
title = "Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)",
abstract = "Is netamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt. The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the Cycladic Blueschist Belt on Syros. The majority arefrom samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preservesan age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectono-metamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Beltphengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under green schist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.",
keywords = "blueschist, exhumation, phengite, Rb–Sr ages, Syros",
author = "Cliff, {R. A.} and Bond, {C. E.} and Butler, {R. W. H.} and Dixon, {J. E.}",
year = "2017",
month = "2",
doi = "10.1111/jmg.12228",
language = "English",
volume = "35",
pages = "197--211",
journal = "Journal of Metamorphic Geology",
issn = "0263-4929",
publisher = "Wiley-Blackwell Publishing Ltd",
number = "2",

}

TY - JOUR

T1 - Geochronological challenges posed by continuously developing tectonometamorphic systems

T2 - insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

AU - Cliff, R. A.

AU - Bond, C. E.

AU - Butler, R. W. H.

AU - Dixon, J. E.

PY - 2017/2

Y1 - 2017/2

N2 - Is netamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt. The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the Cycladic Blueschist Belt on Syros. The majority arefrom samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preservesan age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectono-metamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Beltphengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under green schist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.

AB - Is netamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt. The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the Cycladic Blueschist Belt on Syros. The majority arefrom samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preservesan age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectono-metamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Beltphengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under green schist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.

KW - blueschist

KW - exhumation

KW - phengite

KW - Rb–Sr ages

KW - Syros

U2 - 10.1111/jmg.12228

DO - 10.1111/jmg.12228

M3 - Article

VL - 35

SP - 197

EP - 211

JO - Journal of Metamorphic Geology

JF - Journal of Metamorphic Geology

SN - 0263-4929

IS - 2

ER -