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| Titel |
The inverse microconglomerate test: Definition and application to the preservation of Paleoarchean to Hadean magnetizations in metasediments of the Jack Hills, Western Australia |
| VerfasserIn |
Rory Cottrell, John Tarduno, Richard Bono, Matthew Dare |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129132
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| Publikation (Nr.) |
 EGU/EGU2016-9203.pdf |
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| Zusammenfassung |
| We introduce a new paleomagnetic field test, the inverse microconglomerate test. In contrast
with traditional conglomerate tests, which target specimens that might preserve primary
magnetizations, the inverse microconglomerate test focuses on magnetic carriers having
unblocking temperatures less than peak metamorphic temperatures. These mineral carriers
are expected to carry a consistent direction of remagnetization. Hence, the inverse
microconglomerate test evaluates whether coherent magnetizations are retained on a
grain/mineral scale in a given sedimentary rock sample. By defining the remagnetization
direction, it also serves as a benchmark for comparison of magnetizations from other
grains/minerals having unblocking temperatures higher than peak metamorphic conditions
(i.e., potential primary magnetizations). We apply this new test to sediments of
the Jack Hills (JH), Yilgarn craton, Western Australia. For the JH sediments we
focus on fuchsite, a secondary Cr-mica that contains relict Cr-Fe spinels capable of
recording remanent magnetizations. We find that JH fuchsite grains retain consistent
magnetic directions at unblocking temperatures between ∼270 and 340 oC, which
defines a positive test. This direction does not reproduce a nominal 1078-1070 Ma
remagnetization reported by Weiss et al. (EPSL, 2015) that we interpret as an artifact of
inappropriate use of averaging and statistics. The thermochemical remanent magnetization
recorded by the fuchsite was most likely imparted during peak JH metamorphic
conditions at ∼2650 Ma. Our inverse microconglomerate test complements a positive
microconglomerate test and large scale positive conglomerate test conducted on JH cobbles
(Tarduno and Cottrell, EPSL, 2013), further supporting evidence that JH zircons
record Paleoarchean to Hadean primary magnetizations at high (greater than 550 oC)
unblocking temperatures (Tarduno et al., Science, 2015). More generally, the new inverse
microconglomerate test may aid in understanding the timing of peak metamorphism and
deformation in complex terrains that have undergone multiple episodes of folding. |
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