Abstract
AbstractThis study presents monazite and rutile U–Pb and hornblende and biotite40Ar/39Ar geochronological data for high-grade rocks of the eastern Grenville-aged Rayner orogen at Mount Brown in order to analyse the extent and degree of Pan-African-aged reworking. Monazite from paragneiss yields U–Pb ages of 910 Ma for larger granular grains and 670–630 Ma for smaller globular beads around garnet porphyroblasts or hosted by symplectites. Rutile from leucogneiss yields U–Pb ages of 520–515 Ma. Hornblende and biotite from different rock types yield40Ar/39Ar plateau ages of 744 and 520–505 Ma, respectively. Combining these results with published zircon U–Pb age data suggests that granulite facies metamorphism occurred at 910 Ma, with a local low-temperature fluid flow event at 670–630 Ma and thermal reworking at 520–505 Ma. The older age of 744 Ma may reflect cooling or partial resetting of the hornblende40Ar/39Ar system, indicating that Pan-African-aged reworking did not exceed temperatures much higher than the hornblende Ar closure temperature. These data also suggest that the complete isotopic resetting of some minerals may occur without the growth of new mineral phases, providing an example of the style of reworking that is likely to occur in polymetamorphic terranes.
Highlights
IntroductionThe Rayner orogen, consisting of the Rayner Complex and the Eastern Ghats Belt between the Indian craton (including the Napier Complex in East Antarctica) and the Ruker craton of East Antarctica, is a large Grenville-aged (i.e. late Mesoproterozoic to early Neoproterozoic; c. 1000–900 Ma) orogen that extends for > 2000 km and has a maximum width of > 500 km (Fig. 1)
The Rayner orogen, consisting of the Rayner Complex and the Eastern Ghats Belt between the Indian craton and the Ruker craton of East Antarctica, is a large Grenville-aged orogen that extends for > 2000 km and has a maximum width of > 500 km (Fig. 1)
This study presents the results of combined monazite/rutile U–Pb and hornblende/biotite 40Ar/39Ar dating of metamorphic rocks collected from Mount Brown during the 2014–15 austral field season in order to determine the extent and degree of Pan-African-aged reworking in the eastern part of the Rayner orogen
Summary
The Rayner orogen, consisting of the Rayner Complex and the Eastern Ghats Belt between the Indian craton (including the Napier Complex in East Antarctica) and the Ruker craton of East Antarctica, is a large Grenville-aged (i.e. late Mesoproterozoic to early Neoproterozoic; c. 1000–900 Ma) orogen that extends for > 2000 km and has a maximum width of > 500 km (Fig. 1). No new zircon growth or Pb loss associated with this event has been identified within high-grade rocks from Mount Brown (Mikhalsky et al 2015, Liu et al 2016). This led Mikhalsky et al (2015) to suggest that the Pan-African-aged tectonothermal event became weaker towards inland areas. This study presents the results of combined monazite/rutile U–Pb and hornblende/biotite 40Ar/39Ar dating of metamorphic rocks collected from Mount Brown during the 2014–15 austral field season in order to determine the extent and degree of Pan-African-aged reworking in the eastern part of the Rayner orogen. The new data indicate that the Mount Brown area did undergo Pan-Africanaged reworking, of relatively low grade, leading to no or partial resetting of the hornblende 40Ar/39Ar system and the complete resetting of the rutile U–Pb and biotite 40Ar/39Ar isotopic systems, without resulting in the growth of new metamorphic minerals
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