Monazite in the eclogite and blueschist of the Svalbard Caledonides : its origin and forming-reactions

• High-pressure low-temperature rocks from Svalbard are an excellent target for studying metamorphic reactions in Phanerozoic subduction zones. This study reveals the presence of monazite in an eclogite and a blueschist from the Vestgotabreen Complex, southwestern Svalbard. In order to investigate the monazite-forming reaction, we obtained pressure-temperature estimates coupled with U-Pb and Lu-Hf dating. Combined geothermobarometry allows to constrain three evolutionary stages of garnet growth in the eclogite: nucleation (1.6 & PLUSMN; 0.3 GPa at 460 & PLUSMN; 60 & DEG;C), peak-pressure (2.3 & PLUSMN; 0.3 GPa at 507 & PLUSMN; 60 & DEG;C), and peak-temperature (2.1 & PLUSMN; 0.3 GPa at 553 & PLUSMN; 60 & DEG;C). A zircon age of 482 & PLUSMN; 10 Ma is interpreted to belong to the prograde part of the pressure-temperature path. Monazite forms inclusions within garnet rims, or it is surrounded by allanite and apatite, altogether forming pseudomorphs of a tabular shape in the matrix. Textures, geothermobarometry and geochronology support the conclusion the monazite formed under high-pressure conditions at 471 & PLUSMN; 6 Ma. We propose that the monazite crystallization in the eclogite happened due to a decomposition of accessory phases during the decompression after peak-pressure of the metamorphic cycle. Monazite in the blueschist occurs as inclusions in garnet cores and gives an indicative age of 486 & PLUSMN; 6 Ma, which is interpreted to reflect the prograde growth of the garnet. Lu-Hf garnet dating resolves an age of peak-pressure metamorphism in the blueschist at 471.1 & PLUSMN; 4 Ma under conditions of 2.0 & PLUSMN; 0.03 GPa and 500 & PLUSMN; 30 & DEG;C. The Vestgotabreen Complex provides evidence for an early Ordovician modern-style subduction system in the proximity of the Baltica margin. Hence, this study also supports the tectonic models that favour a mixed Baltican and Laurentian provenance of south-western Svalbard.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

Nyckelord

Eclogite

Blueschist

Monazite

High-pressure low-temperature metamorphism

Vestgotabreen Complex

Svalbard

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