Abstract
The U-Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany-Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U-Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U-Pb record for the older Biranup Zone preserves widespread overprinting at 1225-1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330-1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U-Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U-Pb ages of 1200-1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U-Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695-725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5-8.5 kbar and 675-725 °C. An in situ U-Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.
| Original language | English |
|---|---|
| Pages (from-to) | 283-302 |
| Number of pages | 20 |
| Journal | Precambrian Research |
| Volume | 278 |
| DOIs | |
| Publication status | Published - 1 Jun 2016 |
| Externally published | Yes |
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Grain size matters : Implications for element and isotopic mobility in titanite. / Kirkland, Christopher L.; Spaggiari, C.V.; Johnson, T. E.; Smithies, Robert H.; Danišík, Martin; Evans, N.; Wingate, M. T.D.; Clark, C.; Spencer, C.; Mikucki, E.; McDonald, B. J.
In: Precambrian Research, Vol. 278, 01.06.2016, p. 283-302.Research output: Contribution to journal › Article
TY - JOUR
T1 - Grain size matters
T2 - Implications for element and isotopic mobility in titanite
AU - Kirkland, Christopher L.
AU - Spaggiari, C.V.
AU - Johnson, T. E.
AU - Smithies, Robert H.
AU - Danišík, Martin
AU - Evans, N.
AU - Wingate, M. T.D.
AU - Clark, C.
AU - Spencer, C.
AU - Mikucki, E.
AU - McDonald, B. J.
PY - 2016/6/1
Y1 - 2016/6/1
N2 - The U-Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany-Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U-Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U-Pb record for the older Biranup Zone preserves widespread overprinting at 1225-1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330-1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U-Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U-Pb ages of 1200-1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U-Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695-725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5-8.5 kbar and 675-725 °C. An in situ U-Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.
AB - The U-Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany-Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U-Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U-Pb record for the older Biranup Zone preserves widespread overprinting at 1225-1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330-1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U-Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U-Pb ages of 1200-1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U-Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695-725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5-8.5 kbar and 675-725 °C. An in situ U-Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.
KW - Albany-Fraser orogen
KW - Closure temperature
KW - Phase equilibrium modelling
KW - Thermochronological modelling
KW - Titanite
KW - Uranium loss
UR - http://www.scopus.com/inward/record.url?scp=84962419421&partnerID=8YFLogxK
U2 - 10.1016/j.precamres.2016.03.002
DO - 10.1016/j.precamres.2016.03.002
M3 - Article
VL - 278
SP - 283
EP - 302
JO - Precambrian Research
JF - Precambrian Research
SN - 0301-9268
ER -