TY  - JOUR
A1  - Schmidt, Alexander
A1  - Mezger, Klaus
A1  - O'Brien, Patrick J.
T1  - The time of eclogite formation in the ultrahigh pressure rocks of the Sulu terrane Constraints from Lu-Hf garnet geochronology
JF  - Lithos : an international journal of mineralogy, petrology, and geochemistry
N2  - Eclogites from the main borehole of the Chinese Continental Scientific Drilling project yield highly precise Lu-Hf garnet-clinopyroxene ages of 216.9 +/- 1.2 Ma (four samples) and 220.5 +/- 2.7 Ma (one sample). The spatial distribution of the rare earth elements in garnet is consistent with the preservation of primary growth zoning, unmodified by diffusion, which supports the interpretation that the Lu-Hf ages date the time of formation of garnet, the major rock forming mineral in the eclogites. The preservation of primary REE-zoning, despite peak metamorphic temperatures around 800-850 degrees C. indicates that the Lu-Hf chronometer is perfectly suitable to date garnet-forming reactions in high grade rocks. The range of Lu-Hf ages for eclogites in the Dabie-Sulu UHP terrane point to episodic rather than continuous growth of garnets and thus punctuated metamorphism during the collision of the North China Block and the Yangtze Block. The U-Pb ages and Hf-isotope systematics of zircon grains from one eclogite sample imply a protracted geologic history of the eclogite precursors that started around 2 Ga and culminated in the UHP metamorphism around 220 Ma.
KW  - Lu-Hf
KW  - Eclogite
KW  - Garnet
KW  - Geochronology
KW  - Ultrahigh-pressure
Y1  - 2011
U6  - https://doi.org/10.1016/j.lithos.2011.04.004
SN  - 0024-4937
VL  - 125
IS  - 1-2
SP  - 743
EP  - 756
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Willner, Arne P.
A1  - Gerdes, Axel
A1  - Massonne, Hans-Joachim
A1  - Schmidt, Alexander
A1  - Sudo, Masafumi
A1  - Thomson, Stuart N.
A1  - Vujovich, Graciela
T1  - The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure-temperature-time evolution within part of a collisional belt (Guarguaraz Complex, W-Argentina)
JF  - Contributions to mineralogy and petrology
N2  - The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2 GPa, 470 degrees C and 1.4 GPa, 530 degrees C, when the penetrative s(2)-foliation was formed. The average age of garnet crystallization of 390 +/- 2 Ma (2 sigma) was determined from three four-point Lu-Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5 GPa, 560 degrees C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An Ar-40/39 Ar plateau age of white mica at 353 +/- 1 Ma (1 sigma) indicates the time of cooling below 350-400 degrees C. These temperatures were attained at pressures of 0.2-0.3 GPa, indicative of an average exhumation rate of >= 1 mm/a for the period 390-353 Ma. Late hydrous influx at 0.1-0.3 GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284-295 Ma, the entire basement cooled below 280 degrees C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10-12 degrees C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent-continent collisional setting.
KW  - Albite-amphibolite facies
KW  - Geothermobarometry
KW  - Lu-Hf system
KW  - K-Ar system
KW  - Fission track (zircon)
KW  - Collision zone
KW  - Garnet
KW  - Potassic white mica
Y1  - 2011
U6  - https://doi.org/10.1007/s00410-010-0598-8
SN  - 0010-7999
VL  - 162
IS  - 2
SP  - 303
EP  - 327
PB  - Springer
CY  - New York
ER  -