Models of garnet differential geochronology

Kohn, Matthew J.

doi:10.1016/j.gca.2008.10.004

Cited by 124 publications

(105 citation statements)

References 56 publications

(108 reference statements)

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“…Because of the strong partitioning of Lu into garnet, Lu tends to concentrate in garnet cores relative to rims and relative to the bulk rock (e.g., Kohn, 2009). This effect is observed in the three samples subjected to Lu-Hf geochronology, albeit to variable extents.…”

Section: Linking Lu-hf Ages To Geological Processesmentioning

confidence: 99%

Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen

2014

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Granulitized eclogite-facies rocks exposed in the Ama Drime Massif, south Tibet, were dated by Lu-Hf garnet geochronology. Garnet from the three samples analyzed yielded Lu-Hf ages of 37.5 ± 0.8 Ma, 36.0 ± 1.9 Ma, and 33.9 ± 0.8 Ma. Eclogitic garnet growth is estimated at ca. 38 Ma, the oldest age for burial of the lower Indian crust beneath Tibet reported from the central-eastern Himalaya. Granulite-facies overprinting followed at ca. 15-13 Ma, as indicated by U-Pb zircon ages. Unlike ultrahigh-pressure eclogites of the northwest Himalaya, the Ama Drime eclogites are not characteristic of rapid burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having already reached at least ~60 km thickness by the late Eocene.

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Section: Linking Lu-hf Ages To Geological Processesmentioning

confidence: 99%

Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen

2014

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“…3) that predict garnet growth over a relatively small P-T interval just before reaching peak pressure conditions, that is, only above 500°C. If garnet grew over a period of several tens of Myr, the data points would show a higher scatter, that is., a higher MSWD (e.g., Kohn 2009). …”

Section: Constraints On Protracted Metamorphism and Garnet Growthmentioning

confidence: 99%

Timing of high-pressure metamorphic events in the Bulgarian Rhodopes from Lu–Hf garnet geochronology

Kirchenbaur

Pleuger

Jahn-Awe

et al. 2011

Contrib Mineral Petrol

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Within the Mediterranean realm, the Rhodopes represent a nappe stack of oceanic and continental fragments assembled along the Eurasian continental margin during the Alpine orogeny. The timing of the high-pressure (HP) metamorphism has long been ambiguous, lacking detailed geochronological and geochemical control on subduction-exhumation and nappe stacking processes. Here we apply the Lu-Hf and Sm-Nd chronometers to a suite of representative eclogite samples covering two different key units of the Rhodopean nappe stack: (1) the Kimi Complex (Upper Allochthon) and (2) the Middle Allochthon. In addition to geochronology, we also determined whole rock Hf and Nd isotope compositions as well as major and trace element concentrations in order to constrain the nature of the eclogite protoliths. Two HP metamorphic events were revealed by Lu-Hf geochronology: (1) a Lower Cretaceous event in the Upper Allochthon (126.0 ± 1.7 Ma) and (2) an Eocene event in the Middle Allochthon (44.6 ± 0.7 Ma; 43.5 ± 0.4 Ma; 42.8 ± 0.5 Ma), at conditions of ca. 700°C/20-25 kbar. Our new data provide direct evidence for multiple subduction events in the Rhodopes. Exhumation and subsequent thrusting of the Middle Allochthon on the Lower Allochthon can be narrowed down to the time span between 42 and 34 Ma. In a broader tectonic context, the Eocene ages for the HP metamorphism support the view that the Rhodopes represent a large-scale tectonic window, exposing the deepest nappe units of the Hellenides.

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“…Lu-Hf garnet geochronology has become increasingly valuable as a routine tool to date metamorphic events with the improvement of the technology over the past few decades (Duchêne et al 1997;Lapen et al 2003;Skora et al 2006;Kohn 2009;Mulcahy et al 2009;Herwartz et al 2011;Zirakparvar et al 2011;Smit et al 2013;Baxter and Scherer 2013). Garnet is well suited for combined chronological and thermobarometric research because of its ability to preserve original chemical and isotopic compositions in relatively low-and medium-temperature metamorphic rocks (e.g., Spear and Selverstone 1983;Ague and Carlson 2013;Massonne 2013;Baxter and Caddick 2013).…”

Section: Introductionmentioning

confidence: 99%

Combined geochemistry and geochronology constrains coupled subduction of oceanic and continental crust in the Huwan shear zone, central China

Hao

Vervoort

2014

American Mineralogist

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Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. In general, many kilometers of oceanic lithosphere are ultimately consumed prior to the subsequent continental slab subduction and collision. The exposure of the eclogites derived from oceanic subduction and continental subduction at the surface of Earth today record provide different P-T-t records of the subduction process. The Huwan shear zone in the Hong'an orogenic belt, marking a former ocean-continent transition zone, has been the focus of many studies on subduction-related high-pressure metamorphism. In this study, Lu-Hf garnet, U-Pb zircon, and ArAr mica ages are combined with geochemical data to understand the origin of two coexisting eclogite bodies exposed along the Xuehe River in the Huwan Shear zone. In total, the results indicate that the two eclogites have different protoliths but experienced a similar metamorphic history. This observation requires new tectonic model for the coupled subduction of oceanic and continental crust in subduction zones. Combined geochemistry and zircon U-Pb geochronology suggest distinct oceanic and continental affinities for the eclogite protoliths. The Lu-Hf dates of 261.5 ± 2.4 Ma of the continental-type eclogite and 262.7 ± 1.7 Ma of the oceanic-type eclogite reflect garnet growth and are interpreted to closely approximate the age of eclogite-facies metamorphism. Therefore, both the geochemically oceanic-and continental-type eclogites underwent the same episode of Permian eclogite-facies metamorphism. The Permian Lu-Hf ages of ca. 262 Ma and the obtained Triassic Ar-Ar ages (~240 Ma) of the oceanic-type and continental-type eclogites imply coupled subduction and exhumation of oceanic and continental crustal materials in the Hong'an orogenic belt during the Permian and the Triassic. Though limited, the geochemical and geochronological results of this study, together with the discrepant Carboniferous dates for the nearby eclogites of previous studies, apparently suggest that the Huwan shear zone was not always a single coherent unit but instead comprises different tectonic slices that were metamorphosed at different times before final assembly. Some slices of the oceanic and continental crust underwent two subduction cycles during the Carboniferous and the Permian, whereas some eclogites registered only a single subduction-exhumation loop during the convergence between the South China Block and the North China Block in the Huwan shear zone. The consistent ages of the oceanic-and continental-type eclogites disfavor the traditional mélange model that high-pressure rocks are dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels, forming a tectonic mélange.

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Models of garnet differential geochronology

Abstract: This is an author-produced, peer-reviewed version of this article.

Cited by 124 publications

References 56 publications

Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen

Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen

Timing of high-pressure metamorphic events in the Bulgarian Rhodopes from Lu–Hf garnet geochronology

Combined geochemistry and geochronology constrains coupled subduction of oceanic and continental crust in the Huwan shear zone, central China

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