On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Spear, Frank S.

doi:10.1111/j.1525-1314.1991.tb00533.x

Cited by 271 publications

(167 citation statements)

References 17 publications

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“…This oscillatory zoning is occasionally observed in highly frac tionated leucogranites (Pierrel, 1987), and a retrograde or secondary origin can be ruled out, taking into consideration that there is no textural evidence for such a process and the fact that no Mn in crease or Mg-Ca decrease toward the rim was observed. The high Mn content of this garnet im plies cry stallization at lower temperature that MICRONS prevents the internal homogeneization during growth (Spear, 1991).…”

Section: Garnetmentioning

confidence: 99%

Chemical variability of Al-Ti-Fe-Mg minerals in peraluminous granitoid rocks from Central Spain

Villaseca¹,

Barbero²

1994

ejm

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Two broad series of peraluminous granite types can be distinguished in the Central System of Spain. The first group consists of cordierite-bearing granites that are moderately peraluminous in composition (peralum inous S-type granites, PS). The second is composed of amphibolejallanite-bearing granites, weakly peraluminous in composition (peraluminous I-type, PI). These granite types evolved through complex igneous fractionation processes.Both peraluminous granite types show differences in the chemical composition of biotite and cordierite; cor dierite is common in PS granites but restricted to some highly fractionated PI granites. Garnet and ilmenite cannot be used as chemical criteria for distinguishing the two series.The Al and Na content of biotite and cordierite correlate well with the granite type (PS and PI). The higher Na content in biotite (and cordierite) of PS granites is noteworthy. When plotted in a Na vs. IV11AI diagram biotites are located in different fi elds according to the peraluminosity degree of the granite type. The estimated Fe3+j(Fe3+ +Fe 2 +) ratio of biotite is higher in PI than in PS granites, although this value is very pluton-dependent. Several compositional variations of AI-Ti-Fe-Mg (ATFM) minerals are functions of the degree of fractionation or evolution of the magma, irrespectively of its peraluminosity. Other chemical features of ATFM phases, such as XFe or XMn, can be used as fractionation indices. Thus, the more acid the magma, the higher XFe and XMn in ATFM minerals. This correlation is not linear showing a rapid increase for silica values of Si02 � 74 %. Usually, XFe garnet> XFe biotite > XFe cordierite, and XMn ilmenite == XMn garnet> XMn cordierite > XMn biotite, while XFe and XMn of whole rock are very close to biotite values. The high increase in Mn content, more marked than in Fe, in all AFM phases in the silica-rich granites (Si02 == 74 %), can be interpreted as a consequence of the incompatible behaviour of this element in highly evolved granites, that are sufficiently poor in Ti, Mg and Fe to prevent or diminish crystallization of ore-minerals, such as ilmenite.

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Section: Garnetmentioning

confidence: 99%

Chemical variability of Al-Ti-Fe-Mg minerals in peraluminous granitoid rocks from Central Spain

Villaseca¹,

Barbero²

1994

ejm

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“…Intracrystalline diffusion is relatively slow up to 700 • C (e.g. Yardley, 1977;Spear, 1991;Carlson and Gordon, 2004;Caddick et al, 2010;Ague and Carlson, 2013): Below 700 • C, garnet with a radius of a few hundred micrometres will retain compositional zoning for several tens of millions of years (Florence and Spear, 1991;Caddick et al, 2010).…”

mentioning

confidence: 99%

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

2018

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Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid-rock interaction processes. Garnet shows lowCa porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid-garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid-garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

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“…To minimize reequilibration caused by retrogressive diffusion during cooling (infinite reservoir concept of Spear [1991]), all the selected amphibolite samples have minor idiomorphic or subidiomorphic garnet distributed in a matrix composed predominantly of green hornblende, plagioclase, and ilmenite. This mineral assemblage is considered to represent peak metamorphic conditions.…”

mentioning

confidence: 99%

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt

García¹,

Arenas²,

Catalán³

et al. 1999

The Journal of Geology

103

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A B S T R A C TAnalysis of the Careó n Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age ( Ma, U-Pb) was obtained in a leucocratic gabbro. The 395 ‫ע‬ 2 whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole ( ЊC; kbar) suggested that imbrications T ≈ 650 P ≈ 11.5 developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

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On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Cited by 271 publications

References 17 publications

Chemical variability of Al-Ti-Fe-Mg minerals in peraluminous granitoid rocks from Central Spain

Chemical variability of Al-Ti-Fe-Mg minerals in peraluminous granitoid rocks from Central Spain

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt

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