scholarly journals Apatite recrystallisation during prograde metamorphism, Cooma, southeast Australia: implications for using an apatite – graphite association as a biotracer in ancient metasedimentary rocks

2007 ◽  
Vol 54 (8) ◽  
pp. 1023-1032 ◽  
Author(s):  
A. P. Nutman
2021 ◽  
Author(s):  
Christopher Barnes ◽  
Jarosław Majka ◽  
David Schneider ◽  
Mattia Gilio ◽  
Matteo Alvaro ◽  
...  

<p>            The Seve Nappe Complex (SNC) of the Scandinavian Caledonides represents portions of the Baltican margin that were subducted to mantle depths. Eclogite-bearing sub-units of the SNC provide a record of this important step in orogen development. One such sub-unit is the Vaimok Lens of the SNC in southern Norrbotten. The Vaimok Lens constitutes eclogites hosted within metasedimentary rocks that reached ultra-high pressure (UHP) conditions in the Cambrian/Early Ordovician period. The metasedimentary rocks are typically composed of quartz, white mica, garnet, plagioclase, biotite, clinozoisite, apatite and titanite, and show a pervasive ‘S2’ foliation that developed during exhumation. Garnet is recognized as a relic of prograde metamorphism during subduction, whereas the other minerals represent retrogressive metamorphism during exhumation. To resolve the timing of prograde metamorphism, Lu-Hf geochronology was conducted on metasediment-hosted garnet that preserves prograde, bell-shaped Mn-zoning with a chemical formula of Alm<sub>69-59</sub>Grs<sub>32-24</sub>Sps<sub>13-2</sub>Prp<sub>5-2</sub>. The results indicate garnet growth at 495.3 ± 2.6 Ma. Quartz-in-garnet (QuiG) elastic geobarometry was also conducted on garnet from the same sample, providing pressures of 0.9-1.3 GPa, calculated at 500-700°C. Six samples were obtained for in-situ <sup>40</sup>Ar/<sup>39</sup>Ar geochronology, targeting white mica defining the S2 foliation. Samples can be classified as: 1) low-strain (n: 3), with large (>400 µm width), undeformed micas that are chemically homogeneous (X<sub>Cel</sub>: 0.24-0.35), which yielded a weighted average <sup>40</sup>Ar/<sup>39</sup>Ar population of 470.5 ± 5.9 Ma; 2) high-strain (n: 3), with small (<300 µm width) mica fish with heterogeneous chemistry (X<sub>Cel</sub>: 0.03-0.27), which provided weighted average <sup>40</sup>Ar/<sup>39</sup>Ar populations of 447.6 ± 2.6 Ma and 431.1 ± 4.1 Ma. An additional sample from the basal thrust of the lens that contains large (>300 µm width), homogeneous (X<sub>Cel</sub>: 0.24-0.34) mica was also dated, yielding a population of 414.1 ± 5.8 Ma. Altogether, the data indicates that the Vaimok Lens was subducting by c. 495 Ma. The lens underwent post-decompression cooling at c. 470 Ma, possibly decompressing to 0.9-1.3 GPa by this time. This would equate to an exhumation rate of 3-9 mm/yr. Imbrication of the SNC in southern Norrbotten is taken to be c. 447 Ma. Scandian deformation was active by c. 431 Ma and led to overthrusting of the SNC onto subjacent nappes by latest c. 414 Ma. Both the timescale for subduction and the rates of exhumation for the Vaimok Lens reflect subduction-exhumation dynamics of large UHP terranes. Furthermore, the timing of imbrication and Scandian deformation in southern Norrbotten is similar to estimates along strike of the SNC. These results indicate that the SNC acted as a large UHP terrane that underwent a ~25 Myr cycle of subduction and exhumation during the late Cambrian/Early Ordovician, before being deformed and partially dismembered in subsequent accretionary and collisional events.</p><p> </p><p>Research funded by National Science Centre (Poland) project no. 2014/14/E/ST10/00321 to J. Majka.</p>


Lithos ◽  
2019 ◽  
Vol 336-337 ◽  
pp. 40-53
Author(s):  
Jaime D. Barnes ◽  
Sarah C. Penniston-Dorland ◽  
Gray E. Bebout ◽  
William Hoover ◽  
Grace M. Beaudoin ◽  
...  

Author(s):  
Adam A. Garde ◽  
Brian Chadwick ◽  
John Grocott ◽  
Cees Swager

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Garde, A. A., Chadwick, B., Grocott, J., & Swager, C. (1997). Metasedimentary rocks, intrusions and deformation history in the south-east part of the c. 1800 Ma Ketilidian orogen, South Greenland: Project SUPRASYD 1996. Geology of Greenland Survey Bulletin, 176, 60-65. https://doi.org/10.34194/ggub.v176.5063 _______________ The south-east part of the c. 1800 Ma Ketilidian orogen in South Greenland (Allaart, 1976) is dominated by strongly deformed and variably migmatised metasedimentary rocks known as the ‘Psammite and Pelite Zones’ (Chadwick & Garde, 1996); the sediments were mainly derived from the evolving Julianehåb batholith which dominates the central part of the orogen. The main purpose of the present contribution is to outline the deformational history of the Psammite Zone in the region between Lindenow Fjord and Kangerluluk (Fig. 2), investigated in 1994 and 1996 as part of the SUPRASYD project (Garde & Schønwandt, 1995 and references therein; Chadwick et al., in press). The Lindenow Fjord region has high alpine relief and extensive ice and glacier cover, and the fjords are regularly blocked by sea ice. Early studies of this part of the orogen were by boat reconnaissance (Andrews et al., 1971, 1973); extensive helicopter support in the summers of 1992 and 1994 made access to the inner fjord regions and nunataks possible for the first time.A preliminary geological map covering part of the area between Lindenow Fjord and Kangerluluk was published by Swager et al. (1995). Hamilton et al. (1996) have addressed the timing of sedimentation and deformation in the Psammite Zone by means of precise zircon U-Pb geochronology. However, major problems regarding the correlation of individual deformational events and their relationship with the evolution of the Julianehåb batholith were not resolved until the field work in 1996. The SUPRASYD field party in 1996 (Fig. 1) was based at the telestation of Prins Christian Sund some 50 km south of the working area (Fig. 2). In addition to base camp personnel, helicopter crew and the four authors, the party consisted of five geologists and M.Sc. students studying mafic igneous rocks and their mineralisation in selected areas (Stendal et al., 1997), and a geologist investigating rust zones and areas with known gold anomalies.


2019 ◽  
Author(s):  
William A. Matthews ◽  
◽  
Marie-Pier Boivin ◽  
Kirsten Sauer ◽  
Daniel S. Coutts

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