Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?

Bell, T. H.; Forde, A.; Hayward, Nicholas

doi:10.1130/0091-7613(1992)020<0059:dscssi>2.3.co;2

Cited by 66 publications

(14 citation statements)

References 4 publications

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“…An alternative hypothesis for the formation of curved inclusion trails in garnet is that they are produced by progressive growth of garnet over successive orthogonal matrix foliations. According to this interpretation, the garnet remains stationary as the matrix deforms around it, and stages of porphyroblast growth are restricted to regimes in which deformation is not active (Bell, 1985; Bell et al. , 1992a,b; Hayward, 1992).…”

Section: Strain‐rate Calculationsmentioning

confidence: 99%

“…However, a dissenting viewpoint holds that garnet porphyroblasts remain fixed with respect to geographical coordinates during dynamothermal metamorphism, and the internal textures within a garnet porphyroblast are produced by progressive overgrowth of garnet onto successive generations of matrix foliation surfaces (e.g. Bell, 1985; Bell et al. , 1992a,b).…”

Section: Introductionmentioning

confidence: 99%

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Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

Berg

Carlson²,

Connelly

2012

Journal Metamorphic Geology

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Quantitative strain rates at outcrop scale are very difficult to obtain, but they may be estimated from crystals with curved inclusion trails by calculating rotation rates from growth rates and corresponding deflections of the internal foliation. Garnet in a quartzose pelite at Passo del Sole in the central Swiss Alps is extraordinarily valuable for calculation of strain rates during Alpine orogenesis, because the unusual zoning patterns clearly define the kinetics of its nucleation and growth. Complex concentric zoning patterns can be correlated from one crystal to another in a hand sample, based on compositional and microstructural similarities; the ubiquity of these features demonstrates that all garnet crystals nucleated at nearly the same time. Compositional bands whose radial widths are proportional to crystal size provide evidence for growth governed by the kinetics of intergranular diffusion of locally sourced nutrients. Together, these constraints increase the reliability of estimates of rates of garnet growth, and the strain‐rate calculations that depend on them. To obtain growth rates, P–T conditions during garnet crystallization were modelled in a series of pseudosections, and compositional evolution was connected to rates of garnet growth by means of an independently determined heating rate. These growth rates, combined with measured amounts of curvature of inclusion trails, indicate that the time‐averaged strain rate at Passo del Sole during Alpine metamorphism was on the order of 10−14 s−1. Strain rates calculated using rotational v. non‐rotational models are similar in magnitude. The constraints on crystallization kinetics also allow direct calculation of strain rates during individual stages of garnet growth, revealing short‐term increases to values on the order of 10−13 s−1. These higher strain rates are correlated with the growth of concentric high‐Ca or high‐Mn zones in garnet, which implies that strain softening associated with the transient passage of fluids is responsible for acceleration of deformation during these intervals.

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Section: Strain‐rate Calculationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

Berg

Carlson²,

Connelly

2012

Journal Metamorphic Geology

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“…However, there has been strong disagreement and animated debate over the geological significance of strongly curved S i trails in porphyroblasts, especially the so-called 'snow-ball' inclusion patterns. One group of workers (Bell 1985;Bell and Johnson 1989;Bell et al 1992aBell et al , 1992bHickey and Bell 1999) have strongly argued that porphyroblasts do not rotate with respect to an external frame of reference and that even 'snow-ball' type spiral inclusion trails can be reinterpreted as a product of repeated overprinting deformation and transposition of the matrix foliation around a growing and non-rotating porphyroblast. The other group supports the existing and 'traditional' view of rotation of porphyroblasts in non-coaxial flow to explain strongly folded and rotated inclusion trails in porphyroblasts (Schoneveld 1979;Vernon 1988;Busa and Grey 1992;Visser and Mancktelow 1992;Passchier et al 1992;Williams and Jiang 1999;Jiang and Williams 2004).…”

Section: Introductionmentioning

confidence: 98%

Polyphase deformation and garnet growth in pelitic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast-matrix relationship

Chattopadhyay

Ghosh

2007

J Earth Syst Sci

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Polyphase deformation and metamorphism of pelitic schists of Chorbaoli Formation of Sausar Group in and around Ramtek area, Nagpur district, Maharashtra, India has led to the development of garnet and staurolite porphyroblasts in a predominantly quartz-mica matrix. Microstructural study of oriented thin sections of these rocks shows that garnet and staurolite have different growth histories and these porphyroblasts share a complex relationship with the matrix. Garnet shows at least two phases of growth -first intertectonic between D 1 and D 2 (pre-D 2 phase) and then syn-tectonic to post-tectonic with respect to D 2 deformation. Growth of later phase of garnet on the earlier (pre-D 2 ) garnet grains has led to the discordance of quartz inclusion trails between core and rim portion of the same garnet grain. Staurolite develops only syn-D 2 and shows close association with garnet of the later phase. The peak metamorphic temperature thus coincided with D 2 deformation, which developed the dominant crenulation schistosity (S 2 ), regionally persistent in the terrain. The metamorphic grade reached up to middle amphibolite facies in the study area, which is higher than the adjoining southern parts of Sausar Fold Belt.

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“…spiral and staircase-shaped trails). Difficulty in dis- Johnson, 1989;Johnson, 1990a,b;Bell et al, 1992b;Aerden, 1995). The bulk flow is strongly partitioned tinguishing and correlating coevally developed portions of the inclusion trails makes it virtually impossible to into zones of high non-coaxial shear strain (represented by cleavage folia) that anastomose around ellipsoidal test the extent to which porphyroblasts have rotated relative to one another using the trace of S i alone.…”

mentioning

confidence: 99%

Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

Hickey

Bell

1999

Journal Metamorphic Geology

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The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals. Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre‐date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs. Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation. Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post‐date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre‐dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames. These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In con...

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Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?

Cited by 66 publications

References 4 publications

Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

Polyphase deformation and garnet growth in pelitic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast-matrix relationship

Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

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