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The fate of zircon during UHT-UHP metamorphism: isotopic (U/Pb, O-18, Hf) and trace element constraints



Rok publikování 2016
Druh Článek v odborném periodiku
Časopis / Zdroj Journal of Metamorphic Geology
Fakulta / Pracoviště MU

Přírodovědecká fakulta

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Klíčová slova Hf isotopes; trace elements; ultra-high-pressure; U-Pb dating; zircon
Popis Garnet-clinopyroxene ultra-high-pressure (UHP) rocks from the northern Bohemian Massif contain zircon with micro-diamond inclusions. Trace element concentrations, oxygen and hafnium isotopic composition and U-Pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution. Diamond-bearing zircon mantle domains with relicts of oscillatory zoning have uniform Th/U ratios (similar to(0.1-0.2), high-Ti contents (110-190ppm, corresponding to temperatures of at least 1100 degrees C), and some (two of 17 mantle analyses) preserve steep heavy rare earth element (HREE) patterns with Yb-N/Gd-N=10-11, with a weak negative Eu anomaly. These signatures are consistent with crystallization from a melt under UHP/ultra-high-temperature (UHT) conditions. Some of the bright-cathodoluminscence (CL) rims preserve Th/U and Ti values characteristic of the zircon mantles, but others show elevated Th/U ratios of similar to 0.3-0.4 and lower Ti contents (20-40ppm; only 13ppm in a rare low-CL outer rim). As they feature flat HREE patterns and negative Eu anomalies and commonly make embayments and truncate the mantle zoning, we suggest that they have formed through recrystallization in the solid state during exhumation of the rock, when both garnet and plagioclase were stable. The three zircon domains, that is, cores, mantles and rims, yield U-Pb concordia ages of 340.9 +/- 1.5, 340.3 +/- 1.5 and 341.2 +/- 3.4Ma respectively. When linked to the previously reconstructed P-T path of the rock, the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of similar to 140km (UHP) to similar to 80km (HP) to a minimum rate of 1.5cmyr(-1). The zircon cores are heterogeneous in terms of Th/U ratio (below 0.1 but also above 0.2) and REE characteristics, and their epsilon Hf values scatter between -15.7 and +4.8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale. The non-equilibrated epsilon Hf values as well as a large range of the Hf-depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon. Consequently, the uniform and young U-238/Pb-206 ages may represent (near-)complete resetting of the U-Pb geochronometer during the UHP-UHT event at c. 340Ma through dissolution-reprecipitation process. In contrast to Hf, the oxygen isotope composition of zircon is homogeneous, ranging between 7.8 parts per thousand and 9.6 parts per thousand VSMOW, reflecting a source containing upper crustal material and homogenization at UHP-UHT conditions. Our study documents that continental crust was subducted to mantle depths at c. 340Ma during the Variscan orogeny and was subsequently very rapidly exhumed, implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique.
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