Open and closed isotopic system behaviour of monazite and xenotime during crustal melting of metapelitic migmatites
Urs SchalteggerFlorian MartenotPavlína HasalováChristian A. BergemannMaria OvtcharovaJoshua H.F.L. DaviesMartin J. Whitehouse
0
Citation
0
Reference
20
Related Paper
Keywords:
Migmatite
Cite
In Sm-Nd studies aimed at constraining crustal growth history, Sm-Nd systematics are normally considered to be undisturbed at the whole rock scale during intracrustal processes such as alteration, metamorphism or even partial melting. However, if Sm-Nd systematics are disturbed during intracrustal processes, this could put wide ranging conclusions from Sm-Nd crustal studies into question. Migmatites are ideal rocks to study the chemical and isotopic fractionation which may occur during high grade metamorphic events. From this perspective, we carried out Sm-Nd, U-Pb work associated with major and trace element analyses on three outcrops of migmatites generated during the 2.0 Ga orogeny in the Central Zone of the Limpopo Belt (Botswana and South Africa). The Central Zone which is mainly composed of orthogneisses and paragneisses underwent granulite facies metamorphism followed by decompression metamorphic conditions (clockwise P T loop) at 2.0 Ga. Partial melting following the Bt breakdown reaction in high temperature, low pressure rocks did lead to different behaviour of Sm-Nd systematics for metagreywacke and metapelite. In the first case, chemical equilibrium and full Nd isotope exchange were not reached due to the effect of accessory minerals (monazite and apatite) on the compositions of the partial melt. Monazite entered the melt in preference to apatite,
Cite
Citations (2)
Cite
Citations (57)
Geochronology
Anatexis
Trace element
Leucogranite
Cite
Citations (81)
Research Article| January 01, 2009 Metamorphic rates in collisional orogeny from in situ allanite and monazite dating Emilie Janots; Emilie Janots 1Institut für Geologie, Universität Bern, Blatzerstrasse 3, CH-3012 Bern, Switzerland *Current address: Institut für Mineralogie, WWU Münster, Corrensstrasse 24, 48149 Münster, Germany Search for other works by this author on: GSW Google Scholar Martin Engi; Martin Engi 1Institut für Geologie, Universität Bern, Blatzerstrasse 3, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Daniela Rubatto; Daniela Rubatto 2Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Alfons Berger; Alfons Berger 1Institut für Geologie, Universität Bern, Blatzerstrasse 3, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Courtney Gregory; Courtney Gregory 2Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Meinert Rahn Meinert Rahn 3Hauptabteilung für die Sicherheit der Kernanlagen (HSK), CH-5232 Villingen, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (1): 11–14. https://doi.org/10.1130/G25192A.1 Article history received: 26 May 2008 rev-recd: 03 Sep 2008 accepted: 04 Sep 2008 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Emilie Janots, Martin Engi, Daniela Rubatto, Alfons Berger, Courtney Gregory, Meinert Rahn; Metamorphic rates in collisional orogeny from in situ allanite and monazite dating. Geology 2009;; 37 (1): 11–14. doi: https://doi.org/10.1130/G25192A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Icon
Orogeny
Cite
Citations (141)
Abstract The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition ( HREE , Y, Eu N /Eu* N ) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time ( P–T–t ) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c . 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐ T , medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.
Allanite
Felsic
Anatexis
Trace element
Cite
Citations (39)
Pegmatite
Sillimanite
Overprinting
Staurolite
Andalusite
Isochron dating
Cite
Citations (61)
Abstract The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.
Allanite
Staurolite
Isograd
Cite
Citations (39)
Migmatite
Titanite
Residuum
Geochronology
Overprinting
Cite
Citations (82)
Anatexis
Migmatite
Melt inclusions
Grossular
Mylonite
Cite
Citations (102)
Ages retrieved from accessory minerals in high-grade metamorphic rocks place important constraints on the timing of events and the rates of tectonometamorphic processes operating in the deep crust. In suprasolidus rocks, the dissolution and growth of zircon and monazite are strongly dependent on the P–T conditions of metamorphism and the chemistry and quantity of anatectic melt present. Along a clockwise P–T path, prograde heating above the solidus leads to episodic melt loss and changes in melt chemistry that have important implications for the dissolution and growth of zircon and monazite. In this study, phase equilibria modelling of open-system melting is coupled with experimental data on zircon and monazite solubility to evaluate the stability of these minerals at suprasolidus conditions along several schematic clockwise P–T paths. In migmatite melanosomes and residual granulites, some zircon is expected to survive heating to peak temperature and subsequent isothermal decompression, whereas monazite may be completely consumed, consistent with the observation that inherited cores are less common in monazite than in zircon. After decompression, during cooling to the solidus, new zircon and monazite growth from melt trapped along grain boundaries in melanosomes and residual granulites is expected to be limited. By contrast, leucosomes in migmatites and anatectic granites are predicted to contain mostly newly formed zircon and monazite with minimal inherited components, unless significant entrainment of these minerals from the source occurs. The preservation of cores inside newly formed zircon, as observed in many anatectic granites, demonstrates that segregation, ascent and emplacement is commonly fast enough to limit dissolution of these inherited grains.
Cite
Citations (259)