Le systeme de classification des pegmatites granitiques actuellement en vogue est largement derive de la classification des granites selon la profondeur de leur mise en place. Nous proposons comme alternative une classification fondee sur les processus petrogenetiques responsables, et nous incluons ici les pegmatites des systemes satures et sous-satures en silice. Des venues pegmatitiques a petite echelle sont souvent formees en milieu anatectique, au cours d'une culmination metamorphique. Nous incluons une categorie recemment decrite de pegmatites anatectiques a corindon, formees dans les massifs de Iherzolite ayant subi une metasomatose mantellique prealable en presence d'une phase fluide a H 2 O + CO 2 . En revanche, la plupart des venues de pegmatite sont le produit du fractionnement d'un volume important de magma. Nous considerons les pegmatites LCT (enrichies en Li-Cs-Ta) comme membres de suites orogeniques (calco-alcalines), signalant un contexte de subduction ou de collision; par contre, nous attribuons aux pegmatites NYF (enrichies en Nb-Y-F) une affiliation aux suites anorogeniques, formees dans un contexte d'extension, et sans difference fondamentale du point de vue petrogenetique avec les essaims pegmatitiques des complexes sous-satures classiques, y inclus les complexes carbonatitiques. Les pegmatites de types LCT et NYF contiennent typiquement des composantes de croute et de manteau, mais dans le cas d'un magmatisme anorogenique, il ya eu une etape de preparation metasomatique du socle en presence d'une phase fluide a H 2 O + CO 2 avant de passer a l'etape de fusion, afin d'assurer un produit a tendance alcaline. Un signal NYF + LCT inhabituel peut se developper dans des suites shoshonitiques. Nous decrivons ensuite les pegmatites hybrides dans lesquelles un ensemble NYF change a la fin de l'intervalle de cristallisation a un assemblage de type LCT, temoignant d'une contamination locale du systeme. La possibilite existe qu'une pegmatite LCT puisse aussi etre affectee par une telle surimposition tardive, pour donner une pegmatite qualifiee de "super-LCT". Une documentation des processus petrogenetiques serait essentielle a une evaluation du potentiel economique des venues pegmatitiques.

The patterns of enrichment in felsic pegmatites ultimately depend on tectonic setting

Pegmatites and aplites: Their genetic and applied ore geology

The Late-Proterozoic Sveconorwegian pegmatite province in southern Norway and southwest Sweden hosts seven rare-element pegmatite districts with more than 5000 rare-element pegmatites. Most of these pegmatites with Niobium-Yttrium-Fluorine (NYF) signature are not related to a parental granite, but instead occur in areas of high-grade metamorphism and are the result of migmatization and local melt collection. There are three groups of pegmatites: (1) rare-element pegmatites related to H P -H T high-grade metamorphism associated with the assembly of the Sveconorwegian orogen; (2) rare-element pegmatites related to post-orogenic extension with L P -H T granulites; and (3) rare-element pegmatites related to granite magmatism during post-orogenic extension. The pegmatite formation principally comprises four periods restricted to certain tectono-metamorphic domains: (I) 1094–1060 Ma (Bamble sector); (II) 1041–1030 Ma (Idefjord terrane); (III) 992–984 Ma (Idefjord terrane, Rogaland-Hardangervidda-Telemark sector); and (IV) 922–901 Ma (Rogaland-Hardangervidda-Telemark and Bamble sectors). The observed relationships between pegmatite formation and regional high-grade metamorphism reveal that the majority of Sveconorwegian pegmatites are formed by anatexis, either by crustal stacking during different stages of continental/terrane collision (H P metamorphism) (periods I to III), or by mafic magma underplating (H T metamorphism) during orogenic extension (period IV). In several provinces that have been affected both by early H P metamorphism during continental collision and by late H T metamorphism during crustal extension, there may occur several generations of pegmatites that show mineralogical and geochemical affinity, even though they formed during several different periods. In addition, the results imply that the majority of Sveconorwegian NYF pegmatites are not necessarily formed in an anorogenic setting in relation to A-type magmatism, but in compressional or extensional orogenic settings unrelated to pluton-scale magmatism. In light of this, the genetic criteria of the pegmatite family classification scheme [NYF versus Lithium-Cesium-Tantalum (LCT)] will have to be re-evaluated.

/pdf/the-sveconorwegian-pegmatite-province-thousands-of-29dq7odxnf.pdf

The Sveconorwegian Pegmatite Province – Thousands of Pegmatites Without Parental Granites

The Cap de Creus pegmatite field exhibits a well-defined regional zonation of four types of granitic pegmatite: barren K-feldspar-rich pegmatites (type I), beryl–columbite-subtype pegmatites (type II), beryl–columbite–phosphate-subtype pegmatites (type III) and albite-type pegmatites (type IV). There is an increase in the albite content, relative to K-feldspar, with increasing pegmatite evolution. Type-I microcline-rich pegmatites have low contents of albite, whereas in type IV, albite is the only feldspar. The K/Rb value in K-feldspar decreases progressively from type-I to type-III pegmatites. In addition, there is a progressive enrichment in P, Rb, Cs, Y, Sr, Ga, Ce, W and Sn from type-I to type-IV pegmatites. Concentrations of Pb and Zr decrease in K-feldspar from type-I to type-III pegmatites. K-feldspar from the Cap de Creus pegmatites exhibits higher contents in Ba, Rb, Cs, Nb, Y, Sn, V and Cr than albite. Albite is richer in Sr, Zr, Cu, As, Ce and Ga. Pb, Zn, W, Co and Ni have similar affinity for K-feldspar and albite. Muscovite exhibits similar geochemical evolution to K-feldspar. These progressive trends suggest a common origin for all of the Cap de Creus pegmatites by fractionation of the same parental magma. The high P content of feldspars, up to 1.67 wt.% P 2 O 5 in K-feldspar and 1.75 wt.% in albite, indicates buildup in this element from the earliest stages of pegmatite crystallization. Trace-element variation in feldspars and muscovite reflects enrichments in the melt; a higher content correlates with characteristic minerals of these elements. In particular, the phosphorus content of albite is proposed as a useful mineralogical guide to niobium–tantalum-rich oxide mineralization.

Geochemistry of feldspars and muscovite in granitic pegmatite from the cap de creus field, catalonia, spain

Compositional variation in columbite-group minerals was studied
from the beryl-columbite pegmatite at Scheibengraben, Marsikov,
Northern Moravia, Czech Republic. The pegmatite consists of
five textural-paragenetic units, from the least to the most
evolved: volumetrically dominant coarse-grained unit,
subordinate graphic and blocky units and a minor cleavelandite
unit. Saccharoidal albite unit is rather randomly distributed
within the dike. It replaces and/or crosscuts all other units
except the cleavelandite unit. Columbite-group minerals are the
dominant Nb,Ta-oxide phases in all units. They are associated
with other Nb,Ta-oxide minerals: minerals of the pyrochlore
subgroup and fersmite in the coarse-grained unit, and minerals
of the microlite subgroup, ferrotapiolite and rynersonite in
the cleavelandite unit. The extreme Nb-Ta [Ta/(Ta+Nb) = 0.06 to
0.97 (microlite 0.99)] and appreciable Fe-Mn [Mn/(Mn+Fe) =
(ferrotapiolite 0.22) 0.35 to 0.90] fractionations in
columbite-group minerals differ from those observed in beryl
pegmatites examined to date, but they are comparable with those
in some highly fractionated, complex, Li-rich pegmatites. High
activity of F (facilitated by low contents of B, P and Li in
the pegmatite melt) very likely maximized such an extensive
Nb-Ta fractionation, over and above differential solubilities
of columbite and tantalite in pegmatite melt. The apparent
reversal of Nb-Ta and Fe-Mn fractionation found in columbite
from the saccharoidal albite unit seems to be an artefact from
early units (particularly the coarse-grained one), which were
extensively replaced by saccharoidal albite.

Extreme variation and apparent reversal of Nb-Ta fractionation in columbite-group minerals from the Scheibengraben beryl-columbite granitic pegmatite, Maršíkov, Czech Republic

Four types of pegmatites comprise the zoned pegmatite field in the eastern sector of the Albera Massif. Type I is represented by barren pegmatites with graphic textures; type II comprises transitional varieties with Li-Fe-Mn phosphates, Be (chrysoberyl) and scarce Nb-Ta and U oxide minerals; type III consists of pegmatites with significant zones of replacement containing Li-Fe-Mn phosphates, beryl and more abundant Nb-Ta oxide minerals; and type IV, muscovite-quartz-albite pegmatites are highly mineralized with Be, Nb-Ta and HREE. REE mineralization is strongly related to abundance of graphite in the late pegmatite units and in the host-rock. The individual pegmatite types are distributed within four subparallel zones concentric around anatectic muscovite-biotite leucogranites, with type I within the granites or close to the contact, and type IV pegmatites in the outermost areas. The zoning from type I to type IV could relate to fractionation processes which generated the pegmatites and is characterized by an enrichment of Mn, Ta, Na, Li, P, Be and REE. According to the pegmatite distribution and their fractionation trends, we propose an origin by differentiation of a granitic melt.

The Albera zoned pegmatite field, Eastern Pyrenees, France

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The Albera zoned pegmatite field, Eastern Pyrenees, France