Decoupled U-Pb date and chemical zonation of monazite in migmatites: The case for disturbance of isotopic systematics by coupled dissolution-reprecipitation

Result code in IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F00025798%3A_____%2F20%3A00000010" target="_blank" >RIV/00025798:_____/20:00000010 - isvavai.cz</a>

Result on the web

<a href="https://www.sciencedirect.com/science/article/pii/S0016703719306787" target="_blank" >https://www.sciencedirect.com/science/article/pii/S0016703719306787</a>

DOI - Digital Object Identifier

<a href="http://dx.doi.org/10.1016/j.gca.2019.10.024" target="_blank" >10.1016/j.gca.2019.10.024</a>

Result language

angličtina

Original language name

Decoupled U-Pb date and chemical zonation of monazite in migmatites: The case for disturbance of isotopic systematics by coupled dissolution-reprecipitation

Original language description

Monazite is an abundant accessory mineral in metasedimentary rocks and their anatectic products. Trace element analysis combined with U-Pb dating of monazite is widely used to reconstruct P-T-t histories of high-grade metamorphic terranes. This approach relies on interpreting U-Pb dates, which requires understanding the processes that cause isotopic disturbances. We present in-situ laser ablation split-stream (LASS) results of U-Pb dating and trace element spot analyses of single monazite grains from anatectic granite samples from a HT-LP migmatitic turbidite terrane exposed in the Sierra de Quilmes, NW Argentina. This terrane was part of the back-arc and remained close to anatectic temperatures for most of the &#8764;510–440 Ma Famatinian Orogeny, in the Western Gondwana margin. Sampled outcrops display evidence of remelting of the anatectic granites, marked by leucosomes and confirmed by zircon chronology that define multiple thermal peaks and anatexis events during &#8764;60 Myr duration of the arc. U-Pb monazite chronology of these granite samples reveals a continuum of dates between &#8764;500 and 450 Ma recording most of the duration of the Famatinian event and matching the range of zircon growth ages, but without their multiple, well-defined age populations. Despite each sample yielding a reasonable Famatinian monazite mean date, the results reveal that the isotopic system has been perturbed. Data for each rock sample yield similar date spread for both the cores and the well-defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zonation. We interpret these random, within-grain date variations, as a result of coupled dissolution-reprecipitation reactions between monazite grains and ambient fluids, most likely silicate melts. These reactions occurred during a single orogeny marked by long-lasting, high-temperature metamorphic history. During coupled dissolution-reprecipitation reactions, ingrown radiogenic Pb was remobilized and/or redistributed within the grain giving rise to unsupported Pb. This process was likely significant here because dissolution of apatite into silicate melts during migmatization stabilized monazite, preventing their dissolution but not preventing their reaction with the melt. Redistribution of radiogenic Pb caused by the coupled dissolution-reprecipitation reactions gave rise to meaningless individual dates spreading along the 50 Myr duration of the thermal event associated with the orogeny. In this case, rather than reacting with fluids from a distinct event, monazite dates were perturbed by events occurring during the same orogeny.

Czech name

—

Czech description

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Type

J_imp - Article in a specialist periodical, which is included in the Web of Science database

CEP classification

—

OECD FORD branch

10505 - Geology

Project

<a href="/en/project/GA13-16315S" target="_blank" >GA13-16315S: Prograde metamorphism, crustal thickening and lower crustal flow: new concept of building of crustal root in Variscan orogen</a><br>

Continuities

P - Projekt vyzkumu a vyvoje financovany z verejnych zdroju (s odkazem do CEP)

Publication year

2020

Confidentiality

S - Úplné a pravdivé údaje o projektu nepodléhají ochraně podle zvláštních právních předpisů

Name of the periodical

Geochimica et Cosmochimica Acta

ISSN

0016-7037

e-ISSN

—

Volume of the periodical

269

Issue of the periodical within the volume

January

Country of publishing house

GB - UNITED KINGDOM

Number of pages

14

Pages from-to

398-412

UT code for WoS article

000502332400021

EID of the result in the Scopus database

2-s2.0-85075211994