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Megacrystic zircon with planar fractures in miaskite-type nepheline pegmatites formed at high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland)
-
Urs Schaltegger
,
Alexey Ulianov
Veröffentlicht/Copyright:
10. Januar 2015
Abstract
Trace element, Hf, and O isotopic composition and U-Pb geochronological data are reported for zircon megacrysts found in miaskitic (zircon, biotite, plagioclase-bearing) nepheline syenite pegmatites from the Finero complex in the Northeastern part of the Ivrea-Verbano Zone, southern Alps. Zircon from these pegmatites was reported to reach up to 9 cm in length and is characterized by ~100 μm spaced planar fractures in different directions. Small volumes of these highly evolved alkaline melts intruded into the lower crust and were emplaced within amphibole peridotites and gabbros between 212.5 and 190 Ma. A zircon crystal of 1.5 cm size records a systematic core-to-rim younging of 4.5 Ma found by high-precision CA-ID-TIMS 206Pb/238U dating of fragments, and of 8.7 Ma detected by laser ablation ICP-MS spot dating. Volume diffusion at high temperatures was found to be insufficient to explain the observed within-grain scatter in dates, despite the fact that the planar fractures would act as fast diffusion pathways and thus reduce effective diffusion radii to 50 μm. The U-Pb system of zircon is therefore interpreted to reflect an episodic protracted growth history.
These high-pressure miaskites probably formed by episodic, low-degree decompression melting of a metasomatically enriched mantle source and subsequent crystallization in the lower crust at volatile saturation with explosive volatile release, evidenced by their brecciated texture in the field and by the occurrence of planar fractures in zircon. They point to the existence of a long-lived period of heat advection in the deep crust by highly differentiated melts from enriched, lithospheric mantle.
Keywords: Miaskitic pegmatite; zircon megacrysts; U-Pb; planar fractures; southern Alps; diffusion modeling; volatile explosions
Received: 2013-10-15
Accepted: 2014-6-7
Published Online: 2015-1-10
Published in Print: 2015-1-1
© 2015 by Walter de Gruyter Berlin/Boston
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Schlagwörter für diesen Artikel
Miaskitic pegmatite;
zircon megacrysts;
U-Pb;
planar fractures;
southern Alps;
diffusion modeling;
volatile explosions
Artikel in diesem Heft
- The American Mineralogist at 100 years, and a mineralogy renaissance
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- Mineral evolution and Earth history
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- The rapid expansion of environmental mineralogy in unconventional ways: Beyond the accepted definition of a mineral, the latest technology, and using nature as our guide
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- The structure of water-saturated carbonate melts
- In-situ measurements of fluorine and chlorine speciation and partitioning between melts and aqueous fluids in the Na2O-Al2O3-SiO2-H2O system
- High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites
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- Tourmaline as a petrogenetic monitor of the origin and evolution of the Berry-Havey pegmatite (Maine, U.S.A.)
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- Optical absorption anisotropy of high-density, wide-gap, high-hardness SiO2 polymorphs seifertite, stishovite, and coesite
- The system Na2CO3-FeCO3 at 6 GPa and its relation to the system Na2CO3-FeCO3-MgCO3
- Chlorine-hydroxyl diffusion in pargasitic amphibole
- The use of cation-cation and anion-anion bonds to augment the bond-valence model
- AIM analysis and the form of the bond-valence equation
- Atomistic simulation on mixing thermodynamics of calcite-smithsonite solid solutions
- Combined geochemistry and geochronology constrains coupled subduction of oceanic and continental crust in the Huwan shear zone, central China
- Near-infrared investigation of folding sepiolite
- First-principles study of sulfur isotope fractionation in pyrite-type disulfides
- Tetrataenite in terrestrial rock
- Thermo-compression of pyrope-grossular garnet solid solutions: Non-linear compositional dependence
- Phase relationships in the system K2CO3-CaCO3 at 6 GPa and 900–1450 °C
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