1,721,193 research outputs found
Continuous vs. discontinuous garnet growth in mylonitic micaschists from northeastern Sardinia, Italy: Evidence from LA-ICPMS trace element mapping
Full text linkGarnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the rela tionship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrich ment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history
Tectonic activity along the inner margin of the South Tibetan Detachment constrained by syntectonic leucogranite emplacement in Western Bhutan
No full textIn Western Bhutan Himalayas leucogranite dykes emplaced in
sub-vertical hybrid fractures that cut across the high-grade rocks of
the upper Greater Himalayan Sequence just below to the South
Tibetan Detachment. The granitic dykes dip to the North often
showing a mylonitic deformation with a top-down to-the-N sense of
shear. The high-angle fractures are interpreted to be related to the
evolution of the South Tibetan Detachment toward a brittle regime
of deformation. U-Pb monazite ages constrain the leucogranite
emplacement at 13.9±06 Ma implying that brittle-ductile deformation
of the South Tibetan Detachment was active at that time. NNE-SSW
to nearly E-W trending large scale antiforms and synforms mapped
in NW Bhutan affected the Greater Himalayan Sequence and South
Tibetan Detachment only after 14 Ma
Tectonic activity along the inner margin of the South Tibetan Detachment constrained by syntectonic leucogranite emplacement in Western Bhutan
No full textIn Western Bhutan Himalayas leucogranite dykes emplaced in
sub-vertical hybrid fractures that cut across the high-grade rocks of
the upper Greater Himalayan Sequence just below to the South
Tibetan Detachment. The granitic dykes dip to the North often
showing a mylonitic deformation with a top-down to-the-N sense of
shear. The high-angle fractures are interpreted to be related to the
evolution of the South Tibetan Detachment toward a brittle regime
of deformation. U-Pb monazite ages constrain the leucogranite
emplacement at 13.9±06 Ma implying that brittle-ductile deformation
of the South Tibetan Detachment was active at that time. NNE-SSW
to nearly E-W trending large scale antiforms and synforms mapped
in NW Bhutan affected the Greater Himalayan Sequence and South
Tibetan Detachment only after 14 Ma
Pressure-temperature-deformation-time (P-T-d-t) exhumation history of the Voltri Massif HP complex, Ligurian Alps, Italy
Full text linkAn integrated structural, petrological, and geochronological/ thermochronological study was undertaken to constrain the tectonic controls on the exhumation of the Voltri Massif high-pressure (HP) complex, located in the Ligurian portion of the Western Alps (Italy). Petrotextural analyses were performed to identify the pressure-temperature interval of (1) peak metamorphism (the D1-M1 stage) to eclogitic/blueschist facies conditions and (2) the main retrogressive event (the D2-M2 stage) to greenschist facies conditions. U-Pb SHRIMP dating on a zircon rim (33.8 ± 0.8 Ma) and titanite grains (29 ± 5 Ma), coupled with 40Ar-39Ar analyses on phengite (∼64 Ma to ∼34 Ma) placed temporal constraints on the exhumation path from the D1-M 1 to the D2-M2 stages at the Eocene-Oligocene boundary. Apatite fission track thermochronology, with older ages of 23.9 ± 4.9 Ma, confirms the existence of a regional, Late Oligocene to Miocene cooling/exhumation event for the Voltri Massif (the D3-M3 stage). The compilation of this pressure-temperature-deformation-time path supports a change in the exhumation history: (1) an initial stage, from the D1-M1 to the D2-M2, which was nearly isothermal with highly variable exhumation rates, and (2) a second stage, from D2-M2 to upper crustal levels, which was accomplished by cooling and moderate exhumation rates of ∼1-2 mm yr-1. This two-stage path can be reconciled with the dominant tectonic mechanisms responsible for exhumation of HP rocks in the Voltri Massif area. At the regional scale, this path is consistent with major geodynamic reorganization in the Mediterranean region at the Eocene-Oligocene time boundary, which involved a switch from synorogenic events during transpressive kinematics at the Alpine-Apennine plate boundaries, to postorogenic processes related to crustal thinning and opening of back-arc basins. Copyright 2010 by the American Geophysical Union
Age of anatexis in the crustal footwall of the Ronda peridotites, S Spain
Full text linkThis study investigates the age of anatexis of a crustal sequence constituting the footwall of the Ronda peridotite slab, in the hinterland of the Betic Cordillera (S Spain, region of Istán). These rocks represent a polymetamorphic basement involved in the Alpine orogeny and show an increase in the proportion of melt towards the peridotites. Metamorphic conditions in the migmatites vary between T. ≈. 675-750. °C at P. ≈. 0.30-0.35. GPa. The timing of metamorphism and deformation of the migmatites around the Ronda peridotites is controversial and has been previously ascribed to either the Alpine or Variscan orogenies. We present U-Pb SHRIMP dating of zircons from six samples collected across the migmatitic sequence that provide a tighter age constraint on the metamorphism. Zircon ages are related to conditions of metamorphism on the basis of the relationships between zircon microstructures and degree of melting recorded by the host rocks. Anatexis occurred during the late stages of the Variscan orogeny (≈. 280-290. Ma), as indicated by ages of euhedral, oscillatory-zoned domains or new crystals in metatexites and diatexites. Thin, U-rich zircon rims that are affected by radiation damage yield discordant scattered dates between ≈. 260 and 30. Ma, which are interpreted as reflecting a thermal and fluid overprint during the Alpine orogeny that produced recrystallization and Pb loss in Permian zircons. This study identifies a previously unknown Variscan domain within the Betic Cordillera, and indicates, in accordance with previous studies, that Variscan basements recycled during the Alpine orogeny that formed the Betic Cordillera preserve pre-Alpine mineral associations and tectonic fabrics
Zircon: the metamorphic mineral
Full text linkA mineral that forms under conditions as variable as diagenesis to deep subduction, melt crystallization to low temperature alteration, and that retains information on time, temperature, trace element and isotopic signatures is bound to be a useful petrogenetic tool. The variety of conditions under which zircon forms and reacts during metamorphism is a great asset, but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure–temperature–deformation–fluid context. Under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information.
In the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology. With the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high-grade metamorphic rocks. Since then, there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures, its capacity to protect mineral inclusions, zircon thermometry, trace element patterns and their relation to main mineral assemblages, solubility of zircon in melt and fluids, and isotopic systematics in single domains that go beyond U–Pb age determinations.
Metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks, but has become a truly indispensable mineral in reconstructing pressure–temperature–time–fluid-paths over a wide range of settings. An obvious consequence of its wide use, is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial: in this chapter, reference to published works are intended as examples and not as a compilation.
This chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture, deformation structure and mineral chemistry, including trace element and isotopic systematics. Linking this information together highlights the potential of zircon as a key mineral in petrochronology
Accessory phase control on the trace element signature of sediment melts in subduction zones
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Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism
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