Textures

Ashworth, J. R.; Mclellan, Eileen L.

doi:10.1007/978-1-4613-2347-1_5

Cited by 58 publications

(43 citation statements)

References 20 publications

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“…lOd), providing textuial evidence for partial back-reaction, but replacement is rarely complete. Ashworth (1976) and Ashworth & McLellan (1985) described and interpreted similar intergrowths as 'primary replacement' textures formed by retrograde reaction with crystallizing melt. The partial replacements clearly account for only a minor proportion of the probable water content of the melt generated in segregations, suggesting that the prograde reactions cannot be fully reversed.…”

Section: Cooling Historymentioning

confidence: 93%

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Waters

1988

Journal Metamorphic Geology

227

124

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Abstract. Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of uHzO. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aHZO is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T s 850°C is relatively small (10-20%), and probably insufficient to mobilize a partially melted rock body.Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ? Crd + Kfs + L (metapelite), Bt + Qtz + P1 = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + PI + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.

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Section: Cooling Historymentioning

confidence: 93%

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Waters

1988

Journal Metamorphic Geology

227

124

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“…Ashworth (1985) and Olsen (1985) have outlined problems * Present address: Department of Geology, James Cook University of North Queensland, Townaville, Australia. inherent in geochemical, petrological and mass-balance analysis of migmatites.…”

Section: Migmatites-previous Work and Problemsmentioning

confidence: 99%

Segregation of leucogranite microplutons during syn‐anatectic deformation: an example from the Taylor Valley, Antarctica

Allibone¹,

Norris²

1992

Journal Metamorphic Geology

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A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein-and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranitebearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10-50-m-wide 'microplutons' of xenolith-rich leucogranite.Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.

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“…Here we would like to emphasize two points: (a) anatectic migmatite was largely melt-dominated, and (b) migmatization was syntectonic. Ashworth & McLellan (1985) stated 'In the Mid-Strathspey specimen of Ashworth (1979), where structural control of segregation indicates that migmatization was syntectonic, there is some biotite fabric (girdle of (001) poles, due to rotational deformation) and a quartz shape fabric; if an anatectic origin is accepted, then the deformation must have outlasted the crystallization of the melt'. Hence it is not necessary that relic orthopyroxenes showing preferred orientation must also show signs of intracrystalline deformation.…”

Section: Proposed Structural-microstructural Evidence For Synkinematimentioning

confidence: 99%

Discussion of the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India

2002

Geol. Mag.

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Textures

Cited by 58 publications

References 20 publications

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Segregation of leucogranite microplutons during syn‐anatectic deformation: an example from the Taylor Valley, Antarctica

Discussion of the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India

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