TY - JOUR
T1 - Exhumation dynamics of high-pressure metamorphic rocks from the Voltri Unit, Western Alps
T2 - constraints from phengite Rb–Sr geochronology
AU - Smye, Andrew J.
AU - Seman, Spencer M.
AU - Scambelluri, Marco
AU - Starr, Paul G.
AU - Federico, Laura
N1 - Funding Information:
AJS acknowledges support from the Slingerland Early Career Fellowship; AJS and PS acknowledge support from NSF grant OISE1545903. Two anonymous reviewers and editor, Othmar M?ntener, are thanked for thorough and constructive comments that served to improve the manuscript. This paper also benefited from discussions with Philip England, Horst Marschall, Timm John and Mark Caddick. Several of the figures were made with GMT (Wessel and Smith 1998).
Funding Information:
AJS acknowledges support from the Slingerland Early Career Fellowship; AJS and PS acknowledge support from NSF grant OISE1545903. Two anonymous reviewers and editor, Othmar Müntener, are thanked for thorough and constructive comments that served to improve the manuscript. This paper also benefited from discussions with Philip England, Horst Marschall, Timm John and Mark Caddick. Several of the figures were made with GMT (Wessel and Smith ).
Publisher Copyright:
© 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.
Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.
PY - 2021/2
Y1 - 2021/2
N2 - The Voltri Unit and adjacent Tertiary Piedmont Basin, Western Alps, preserve complementary bedrock and sedimentary archives of Alpine subduction and exhumation. Combined PT modeling and phengite Rb–Sr geochronology of bedrock and clast samples shows that Fe–Ti metagabbros and metasediments preserve a record of protracted high-pressure metamorphism, between ~ 50 and ~ 40 Ma. Bedrock and clast Fe–Ti metagabbros yield similar eclogite-facies peak conditions, between 23 and 25 kbar, 510–530 °C; phengite, zoned in celadonite content, with cores > 3.5 Si c.p.f.u. and rims < 3.3 Si c.p.f.u., constrains the timing of exhumation between 24 and 20 kbar to 45–49 Ma. A single impure quartzite bedrock sample records peak-P conditions of 18–19 kbar, 450–470 °C; partial phengite equilibration in this sample occurred continuously between 19 and 12 kbar, between ~ 45 and ~ 40 Ma. Exhumation-related recrystallization of high-pressure phengite to low-Si phengite in two metasedimentary samples occurred between ~ 33 and ~ 30 Ma, contemporaneous with the onset of deposition in the Tertiary Piedmont Basin and consistent with previous 40Ar–39Ar constraints on the timing of greenschist metamorphism. Combined with existing ages from the ophiolite, these data show that peak subduction-related high-pressure conditions, between ~ 18 and ~ 25 kbar, were attained at different times across the Voltri Unit, between ~ 50 and ~ 40 Ma, implying that the Voltri Unit comprises an assembly of discrete lithotectonic units that were juxtaposed prior to erosion and deposition in the Tertiary Piedmont molasse basin. The PTt data reported here support a model in which individual sheets of high-pressure material were detached from the downgoing plate, partially exhumed from peak pressures to blueschist facies conditions, while subduction continued, and were stored for > 10 Myr until subduction ceased on arrival of the European continent into the orogenic wedge. As shear tractions on the plate interface are considered to exceed available buoyancy forces for exhumation, we suggest that syn-subduction exhumation occurred along the wedge-plate interface.
AB - The Voltri Unit and adjacent Tertiary Piedmont Basin, Western Alps, preserve complementary bedrock and sedimentary archives of Alpine subduction and exhumation. Combined PT modeling and phengite Rb–Sr geochronology of bedrock and clast samples shows that Fe–Ti metagabbros and metasediments preserve a record of protracted high-pressure metamorphism, between ~ 50 and ~ 40 Ma. Bedrock and clast Fe–Ti metagabbros yield similar eclogite-facies peak conditions, between 23 and 25 kbar, 510–530 °C; phengite, zoned in celadonite content, with cores > 3.5 Si c.p.f.u. and rims < 3.3 Si c.p.f.u., constrains the timing of exhumation between 24 and 20 kbar to 45–49 Ma. A single impure quartzite bedrock sample records peak-P conditions of 18–19 kbar, 450–470 °C; partial phengite equilibration in this sample occurred continuously between 19 and 12 kbar, between ~ 45 and ~ 40 Ma. Exhumation-related recrystallization of high-pressure phengite to low-Si phengite in two metasedimentary samples occurred between ~ 33 and ~ 30 Ma, contemporaneous with the onset of deposition in the Tertiary Piedmont Basin and consistent with previous 40Ar–39Ar constraints on the timing of greenschist metamorphism. Combined with existing ages from the ophiolite, these data show that peak subduction-related high-pressure conditions, between ~ 18 and ~ 25 kbar, were attained at different times across the Voltri Unit, between ~ 50 and ~ 40 Ma, implying that the Voltri Unit comprises an assembly of discrete lithotectonic units that were juxtaposed prior to erosion and deposition in the Tertiary Piedmont molasse basin. The PTt data reported here support a model in which individual sheets of high-pressure material were detached from the downgoing plate, partially exhumed from peak pressures to blueschist facies conditions, while subduction continued, and were stored for > 10 Myr until subduction ceased on arrival of the European continent into the orogenic wedge. As shear tractions on the plate interface are considered to exceed available buoyancy forces for exhumation, we suggest that syn-subduction exhumation occurred along the wedge-plate interface.
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U2 - 10.1007/s00410-020-01767-0
DO - 10.1007/s00410-020-01767-0
M3 - Article
AN - SCOPUS:85100483909
VL - 176
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
SN - 0010-7999
IS - 2
M1 - 14
ER -