Introduction

Ashworth, J. R.

doi:10.1007/978-1-4613-2347-1_1

Cited by 60 publications

(26 citation statements)

References 67 publications

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“…10(b) do not follow the general trend of the granite minimum is because they formed by mechanisms other than partial melting, for example by metamorphic differentiation. Furthermore, an individual leucosome may form by partial melting but may not have a granite minimum composition, as discussed in detail by Ashworth (1985). Nevertheless, if leucosomes in general are incipient granitic plutons, the overall trend of their composition should match that of granites.…”

Section: An Leucosomesmentioning

confidence: 99%

“…Nomenclature for migmatites used in this paper is mainly after Dietrich & Mehnert (1960) modified by Ashworth (1985). Although any compositionally layered rock may be a migmatite, a more restricted use of the term migmatite refers to a rock containing parts with plutonic and metamorphic appearance (Dietrich & Mehnert, 1960).…”

Section: No Me Nclatu Rementioning

confidence: 99%

“…Nor is the spread likely to be the result of progressive melting and melt segregation (e.g. Ashworth, 1985). The first phase to be depleted during melting of these rocks would be K-feldspar, so that the partial melt composition could become more leucotonalitic as melting progressed, resulting in the spread of the leucosome composition from the granite minimum to the quartz-plagioclase cotectic.…”

Section: An Leucosomesmentioning

confidence: 99%

“…Further segregation and accumulation of such melts would form larger granitic plutons. The model of migmatites as the source for granite is far from substantiated (see Ashworth, 1985;Olsen, 1985, for further discussions). In particular, leucosomes commonly have compositions removed from the granite minimum, and intermediate stages between migmatites and granitic plutons are generally absent.…”

Section: Introductionmentioning

confidence: 99%

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Isocon analysis of migmatization in the Front Range, Colorado, USA

Olsen

Grant

1991

Journal Metamorphic Geology

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Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15-m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added'are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca)ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, K S Na + Ca, modified originally leucotonalitic leucosomes to more K-rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.

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Section: An Leucosomesmentioning

confidence: 99%

Section: No Me Nclatu Rementioning

confidence: 99%

Section: An Leucosomesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Isocon analysis of migmatization in the Front Range, Colorado, USA

Olsen

Grant

1991

Journal Metamorphic Geology

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“…Migmatites provide examples of nascent granites, but detailed studies often reveal that their leucosomes are the locations of melt depletion rather than crystallization, or at best status, crystal-melt mixes (e.g., Mehnert 1968;Ashworth 1985;Sawyer2008a, b).Insomeinstances,segregatedandhomogeneous granites within migmatitic terrains of the mid-crust can be related to specific sedimentary sources and melting reactions as exemplified by Himalayan leucogranites, (Harris et al 1995) because the melts have not migrated far from their sources. However, some recent studies (Carvalho et al 2016) have shown that leucosome can also be connected to the granite melt far from their sources.…”

Section: Introductionmentioning

confidence: 99%

Geochemical characteristics and conditions of formation of the Chah-Bazargan peraluminous granitic patches, ShahrBabak, Iran

Fazlnia¹

2017

Geologica Carpathica

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Xenoliths of garnet-biotite-kyanite schist from the Qori metamorphic complex (southern part of the Sanandaj-Sirjan zone, northeast Neyriz, Zagros orogen in Iran) in the 173.0±1.6 Ma Chah-Bazargan leuco-quartz diorite intrusion were studied. This intrusion caused these schist xenoliths to be metamorphosed to the pyroxene hornfels facies (approximately 4.5±1.0 kbar and 760±35 °C), converting them to diatexite migmatite as a result of partial melting of the xenoliths. These melts are granites in composition. Melt volumes of 20 to 30 vol. % were calculated for small patches of the peraluminous granites. It is possible that anatectic melting affected only the leucosome, such that melting was more than 20 to 30 vol. %. It is possible that a large amount of melt was not extracted due to balanced in situ crystallization, the adhesion force between melt and crystal (restite), and high viscosity of the leucosome. The Chah-Bazargan peraluminous granites are depleted in trace elements such as REEs, HFSE (Ti, Zr, Ta, Nb, Th, U, Hf, Y), Ba, Pb, and Sr. These elements are largely insensitive to source enrichment, but sensitive to the amounts of main and accessory minerals. These elements were hosted by minerals such as garnet, biotite, muscovite, K-feldspar, plagioclase, ilmenite, apatite, monazite, and zircon in the source (diatexitic migmatitic xenoliths).

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Per migma ad magma downunder

White

Chappell

1990

Geological Journal

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According to some early definitions, 'magma' had to be completely molten so that granitic rocks containing residual material from their source (restite) were not considered to be magmatic. Granites and volcanics with abundant recognizable restite were called migmatites or migmatitic rocks, and this terminology appears to be still in use by some. However, many felsic granites and volcanics commonly contain at least some restite, and for this reason we suggest that magma be defined either as melt or as rock material with enough melt for it to become mobile and intrusive or extrusive. The rocks crystallizing from these magmas are igneous. They are not migmatites. On the other hand, we suggest that migmatites be defined as rocks that did not have sufficient melt to become mobile; they retain a structural coherency even though the structures may be complex. Differences between the two rock types result from the amount of melt phase. When a critical melt fraction is reached during partial melting, there is a rapid change from migma to magma (migmatite to granite).The geochemistry and mineralogy of migmatites tell us little about reactions that produce more melt and hence about granite formation because of back reactions and other processes that occur during the cooling of migmatite. Back reactions mostly involve H,O that is not lost from the migmatitic system. Because H 2 0 is lost by processes of second boiling and pressure quenching during the crystallization of granite magmas at or near the surface, it is these rocks and their inclusions, not migmatites, that provide the most information about sources and processes of granite formation.

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Introduction

Cited by 60 publications

References 67 publications

Isocon analysis of migmatization in the Front Range, Colorado, USA

Isocon analysis of migmatization in the Front Range, Colorado, USA

Geochemical characteristics and conditions of formation of the Chah-Bazargan peraluminous granitic patches, ShahrBabak, Iran

Per migma ad magma downunder

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