The genesis of grey monazite
The genesis of grey monazite
Grey monazite occurs as sub-millimetre sized, ellipsoidal, authigenic concretions in Llanvirn-aged shales of the Calymene Formation, Montes de Toledo area, Central Spain. The nodules are similar to those previously described from Palaeozoic shales in Wales, Brittany and Belgium. They are Eu-rich and poor in Th by comparison with igneous monazites. They are concentrically zoned, with La, Ce- rich rims and Nd, Sm- rich cores; all the heavy REE and Y are weakly enriched in the cores. The host rocks are massive shales with thin silty horizons, deposited in a distal shelf environment near the oxic/anoxic boundary. The host shales have been metamorphosed under anchizonal conditions, with a peak temperature of 260 to 275oC. They have largely recrystallised to the assemblage quartz-chlorite-illite. A crenulation cleavage post-dates the monazite nodules. The nodules contain typically 20% inclusions with similar mineralogy to the host shale, often forming an aligned fabric parallel to the long axis of the nodule, which is interpreted as orginal bedding.
The monazite nodules grew replacively; phyllosilicates, especially illite, were replaced in preference to quartz. No authigenic illites are present within the nodules, only detrital micas with interlayer charge > 0.85. Chlorite included in nodules, as well as those in the host shale, exhibit a range of tetrahedrally coordinated aluminium contents, suggesting that nodule growth spanned a range of temperature, up to near the maximum temperature experienced by the shale.
Silicified cone-in-cone concretions occur in a zone about 280 m above the base of the Calymene Formation. They formed at about 20 m. depth and orginally had a carbonate matrix. The carbonate was replaced by quartz and chlorite under anchizonal conditions; heavy REE were mobile at this stage, and are depleted in the concretion. Monazite nodules post-date, or are syngenetic with, the chlorite + quartz alteration of the cone-in-cone. During late-stage weathering under oxidising conditions, REE were leached from the cores of cone-in-cone concretions and a rare lead mineral, corkite-hidalgoite, was formed.
University of Southampton
Windle, Stephen John
5b0078d0-97d4-4a55-81fc-a97d88156d52
1994
Windle, Stephen John
5b0078d0-97d4-4a55-81fc-a97d88156d52
Nesbitt, R.W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Windle, Stephen John
(1994)
The genesis of grey monazite.
University of Southampton, Doctoral Thesis, 214pp.
Record type:
Thesis
(Doctoral)
Abstract
Grey monazite occurs as sub-millimetre sized, ellipsoidal, authigenic concretions in Llanvirn-aged shales of the Calymene Formation, Montes de Toledo area, Central Spain. The nodules are similar to those previously described from Palaeozoic shales in Wales, Brittany and Belgium. They are Eu-rich and poor in Th by comparison with igneous monazites. They are concentrically zoned, with La, Ce- rich rims and Nd, Sm- rich cores; all the heavy REE and Y are weakly enriched in the cores. The host rocks are massive shales with thin silty horizons, deposited in a distal shelf environment near the oxic/anoxic boundary. The host shales have been metamorphosed under anchizonal conditions, with a peak temperature of 260 to 275oC. They have largely recrystallised to the assemblage quartz-chlorite-illite. A crenulation cleavage post-dates the monazite nodules. The nodules contain typically 20% inclusions with similar mineralogy to the host shale, often forming an aligned fabric parallel to the long axis of the nodule, which is interpreted as orginal bedding.
The monazite nodules grew replacively; phyllosilicates, especially illite, were replaced in preference to quartz. No authigenic illites are present within the nodules, only detrital micas with interlayer charge > 0.85. Chlorite included in nodules, as well as those in the host shale, exhibit a range of tetrahedrally coordinated aluminium contents, suggesting that nodule growth spanned a range of temperature, up to near the maximum temperature experienced by the shale.
Silicified cone-in-cone concretions occur in a zone about 280 m above the base of the Calymene Formation. They formed at about 20 m. depth and orginally had a carbonate matrix. The carbonate was replaced by quartz and chlorite under anchizonal conditions; heavy REE were mobile at this stage, and are depleted in the concretion. Monazite nodules post-date, or are syngenetic with, the chlorite + quartz alteration of the cone-in-cone. During late-stage weathering under oxidising conditions, REE were leached from the cores of cone-in-cone concretions and a rare lead mineral, corkite-hidalgoite, was formed.
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Published date: 1994
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Local EPrints ID: 462840
URI: http://eprints.soton.ac.uk/id/eprint/462840
PURE UUID: 879969d1-f5b4-44f3-a0b2-79831f674631
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Date deposited: 04 Jul 2022 20:13
Last modified: 16 Mar 2024 18:59
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Contributors
Author:
Stephen John Windle
Thesis advisor:
R.W. Nesbitt
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