38 research outputs found
The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas
International audienceIsotopic compositions in the Monts d’Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from -3.6 to + 3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = -1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs, Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from intermediate unit (-1.7‰) to internal pegmatitic unit (+ 3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallisation during the pegmatite’s consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite-petalite subtypes) observed throughout the Monts d’Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tend to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica, garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallisation at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems
Temporal relationships between Mg-K mafic magmatism and catastrophic melting of the Variscan crust in the southern part of Velay Complex (Massif Central, France)
Mg-K mafic intrusive rocks are commonly observed during the late stages of the evolution of orogenic belts. The Variscan French Massif Central has many outcrops of these rocks, locally called vaugnerites. Such magmas have a mantle-derived origin and therefore allow discussion of the role of mantle melting and crust-mantle interactions during late-orogenic processes. In the Southern Velay area of the French Massif Central, LA-ICPMS U-Pb dating on zircons and monazites from three vaugnerites and four coeval granites reveals that the two igneous suites formed simultaneously, at c. 305 Ma. This major igneous event followed after an early, protracted melting stage that lasted for 20-30 My and generated migmatites, but the melt was not extracted efficiently and therefore no granite plutons were formed. This demonstrates that widespread crustal anatexis, melt extraction and granite production were synchronous with the intrusion of vaugneritic mantle-derived melts in the crust. The rapid heating and subsequent melting of the crust led to upward flow of partially molten rocks, doming and collapse of the belt.JHS was financially supported by the Spanish grant CGL2008–02864 and the Andalusian grant RNM1595
Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: 1 the geologic record of the French Massif Central
We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust.
This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust
Petrogenesis of S-type granites : the example of the Cape Granite Suite
Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2010.ENGLISH SUMMARY: S-type granite intrusions are extremely common in the continental crust and form
from the partial melting of metasediments. Compositions of S-type granite range
from leucogranite to granodiorite and have trace element contents that globally
increase with increasing maficity (Fe + Mg). Models proposed for the formation
of S-type granite do not answer satisfactorily all petrological and compositional
requirements. In this study, S-type granite of the Cape Granite Suite (CGS), South
Africa is used to discriminate between potential sources of compositional variation.
Experimental studies show that melt produced from the partial melting of sediment
is exclusively leucocratic. On this basis, the entrainment of up to 20 wt.% of
peritectic garnet within S-type melt can be established to produce the observed
major element variations. S-type CGS locally contains garnet. This garnet is in
equilibrium with granite composition at P-T conditions (5kb and 750 C for the
core of the garnet and 3kb and 720 C for the rim) well below conditions recorded
by xenoliths from the same granite (10 kb and 850 C from a metabasite). From
this result it seems that the originally entrained garnet no longer exists in the Stype
CGS and it have been replaced by newly formed minerals (garnet, cordierite
and biotite). Considering the short time necessary to emplace granites (about
100 000 years), it appears that garnet has been compositionnally re-equilibrated through a dissolution-precipitation process. The study of trace element variations
in S-type CGS shows that most leucocratic compositions are undersaturated in
Zr and Ce compared to predictions from experimental models for dissolution of
accessory zircon and monazite in their source regions. Thus, S-type melts are
likely to be formed in disequilibrium with respect to accessory phase stability. As
a result the observed increase in trace element content with increasing maficity
indicates that accessory minerals such as zircon and monazite are co-entrained
with peritectic garnet in melt to produce the observed trace element variation in
S-type granite. Trace element disequilibrium in the CGS S-type granitoids requires
particularly short times of residence of melt within the source region. Together,
these results provide for the first time, a fully comprehensive model for major
and trace elements variations. Compositional variation in CGS S-type granite
results from source processes by a selective entrainment of peritectic and accessory
minerals. After entrainment, these minerals are likely to be re-equilibrated within
the magma, through a dissolution-reprecipitation process. In addition, it appears
that the construction of large S-type granitic bodies occurs through successive
addition of magma batches of different composition that originates directly from
the source region.AFRIKAANSE OPSOMMING: S-tipe granietinstrusies is baie algemeen in die kontinentale kors en vorm deur
die gedeeltelike smelting van metasedimente. Samestellings van S-tipe graniete
strek vanaf leukograniet tot granodioriet en het spoorelementsamestellings wat
global toeneem met ’n toenemende mafiese component (Fe + Mg). Modelle wat
voorgestel is vir die formasie van S-tipe graniete beantwoord nie bevredigend al
die petrologiese en komposisionele benodigdhede nie. In hierdie studie word S-tipe
graniete van die Kaapse Graniet Suite (CGS), Suid Afrika, gebruik om te diskrimineer
tussen potensiele bronne van komposisionele variasie. Eksperimentele studies
wys dat smelt, geproduseer van die gedeeltelike smelting van sedimente, uitsluitlik
leukokraties is. Op hierdie basis kan bewys word, dat die optel-en-meevoering van
tot 20 wt% van peritektiese granaat in S-tipe smelt, die waargeneemde hoofelement
variasies kan produseer. S-tipe CGS bevat lokale granaat. Hierdie granaat is in
ekwilibrium met die graniet samestelling by P-T kondisies (5kb en 750circC vir die
kern van die granaat en 3kb en 720circC vir die rand) ver onder kondisies waargeneem
by xenoliete van dieselfde granite (10kb en 850circC van ’n metabasiet). Van
hierdie resultaat kan afgelei word dat die oorspronklike opgetel-en-meegevoerde
graniet bestaan nie meer in die S-tipe CGS en dat dit vervang is deur nuutgevormde
minerale (granaat, kordieriet en biotiet). As in ag geneem word die kort tyd wat nodig is om graniete in te plaas (omtrent 100 000 jaar), wil dit voorkom dat
granaat se samestelling geherekwilibreer word deur ’n oplossings-presipitasie proses.
Die studie van spoorelement variasies in S-tipe CGS wys dat meeste leukokratiese
samestellings is onderversadig in Zr en Ce in vergelyking met voorspellings deur
eksperimentele modelle vir die oplossing van bykomstige zircon en monasiet in hulle
brongebiede. Dus is S-tipe smelte meer geneig om gevorm te word in disekwilibrium
in verhouding tot bykomstige mineraalstabilileit. Met die gevolg is dat die
waargenome toename in spoorelementinhoud met toename in mafiese component
wys dat bykomstige minerale, soos zirkoon en monasiet, word saam opgetel-enmeegevoer
met peritektiese granaat in smelt om die waargenome spoorelement
variasie in S-tipe graniete te verklaar. Spoorelement disekwilibrium in die CGS
S-tipe granitoide benodig veral kort tye van residensie van die smelt binne die
brongebied. Saam gee hierdie resultate vir die eerste keer ’n algehele antwoord vir
hoof- en spoorelement variasies. Variasie in samestelling in CGS S-tipe graniete is
die resultaat van bronprosesse deur ’n selektiewe optel-en-meevoer van peritektiese
en bykomstige minerale. Na die optel-en-meevoer van hierdie minerale word hulle
geherekwilibreer binne die magma deur ’n oplossings-presipitasie proses. Addisioneel
wil dit voorkom of die konstruksie van groot S-tipe granietliggame plaasvind
deur opeenvolgende toevoegings van magma lotte van verskillende samestellings wat
direk uit die brongebied kom
Mica-liquid trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)
International audienceWe constrain the genetic relation between granite and pegmatite parental melts from the Variscan Saint Sylvestre leucogranite complex and associated pegmatite bodies (Massif Central, France) through compositions of micas. Using mica trace element concentrations and available partition coefficients for Li, Rb, Ba, Cs and F, we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas. Biotite in leucogranites and pegmatites ranges mostly from Fe-biotite to siderophyllite. More evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite. White micas have homogeneous compositions, from muscovite to Li-phengite. In granites, biotite and muscovite trace element distributions are clustered. In pegmatites, mica trace element contents globally follow differentiation from the least to the most evolved body. The reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in Li, Rb, Cs and depletion in Ba that suggest a similar origin. However, micas are shown to have selectively equilibrated with the last melt (or fluid) in contact and so trace element concentrations of early magmatic liquids are rarely preserved. Inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization. Leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent-daughter genetic relation. The data and the modelling suggest that they represent non-cogenetic melts generated by discrete episodes of partial melting. Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt/magma. We suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization
Mica crystallisation as a marker of pegmatite composition
International audienceMicas are ubiquitous minerals within granitic and pegmatitic magmas. The presenceand the composition of micas are believed to depend strongly on the magma theycrystallised from. The Upper Carboniferous Variscan St Sylvestre LeucograniteComplex (SSLC), located in the western Massif Central (Limousin, France) iscomposed of five main granitic facies: a biotite-only facies (Brame), a two-mica facies(St Sylvestre); a coarse grained two-mica facies (St Goussaud); a fine grained two-micafacies (Châteauponsac) emplaced as sills within the Brame facies; finally amuscovite-only facies (Sagne). Pegmatitic bodies within the SSLC are enriched inrare-elements (e.g. Li, Be, Nb, Ta) and most of them are intrusive within theSt-Sylvestre facies. Pegmatites in the SSLC can be divided in six groups depending ontheir chemistry, mineralogy and internal structuration. I.) No rare-element-rich phase(not sampled); II.) Be bearing phase only; III.) Be and Nb rich; IV.)Be, Nb and Ta richphases; V.) Be, Nb, Ta-rich phases and Li-enrichment; VI.) Li-rich phase (lepidolite,petalite). From I to VI the volume ratio between aplitic and pegmatitic texture increases.Sampling of micas within the pegmatites have been realised accordingly to thisclassification.We analysed major and trace elements using both electron microprobe and LaserAblation ICP-MS, within primary micas (biotite, muscovite and lepidolite) from bothgranite and pegmatite within the St Sylvestre leucogranitic complex. Within the granite,biotite and muscovite do not vary significantly in composition in regard of granitefacies. Interestingly, biotite within pegmatite records a considerable change incomposition from biotite in granite towards lithium-rich biotite in regard of pegmatitefacies, changing from annite-phlogopite composition to zinnwaldite (in type-Vpegmatite). On the contrary, muscovite is very homogenous and its composition hasonly a slightly higher Li-mica content than granitic muscovite. Experimental studies ofmica crystallisation from a pegmatitic magma suggest that compositional changesobserved in biotite can be directly related to the differentiation of pegmatitic magmas.Behind the apparent compositional homogeneity provided by major elements withinmuscovite, trace element contents reveal compositional changes. These changes arevery similar to the variation of trace elements contents observed in biotite in agreementwith the pegmatite type, yet with significantly lower concentration for most elementsconsidered here. It appears that both biotite and muscovite record compositionalvariability within pegmatitic magmas. Both biotite and muscovite record a subsequentdecrease of Ba, Sr and Sc during differentiation while contents of Li, Be, Nb, Rb, Ta, Csand Zn increase significantly with differentiation degree within both biotite andmuscovite.Differences of trace element concentration between biotite and muscovite can either bethe consequence of partition coefficient between biotite and muscovite or a diachronouscrystallisation of muscovite and biotite. Additionally the reduced variability inmuscovite structural formulae emphasizes a lack of sensitivity of muscovite in regard ofmagmatic differentiation
Effect of anorthite on granite phase relations: Experimental data and models
International audienceNew experimental data on the effect of anorthite (An) on liquidus phase equilibria in the system Qz–Ab–Or are presented. The data were obtained for 5 wt% An added to variable Qz/Ab/Or compositions at 300 MPa and under H2O-saturated conditions. Crystal–liquid equilibria were determined for 13 synthetic glass compositions made from gels in experiments performed between 660 and 750 °C in cold-seal pressure vessels. Forward and reversal experiments were systematically conducted on each composition to demonstrate equilibrium. A total of 51 charges was examined. Three crystalline phases, quartz, alkali feldspar and plagioclase appear on the H2O-saturated liquidus surface. The determined minimum liquidus 5 wt% An “piercing” point (39% Qz, 33% Ab, 28% Or) is shifted away from the Ab apex toward the Qz–Or sideline when compared with the An-free 300 MPa H2O-saturated minimum. This shift is of the same type as that observed at 100 MPa in the same system and at 200 MPa in a rhyolitic system. The new experimental results are used to test both empirical and thermodynamic models for silicic magmas. Empirical models reproduce reasonably well the new experimental data, although more sophisticated calculations schemes appear to be required to improve their accuracy. The new experimental results in the haplogranodiorite system are not well reproduced with the model of Holland and Powell (2001), mainly because plagioclase stability appears greatly enhanced in the model. Rhyolite-MELTS satisfactorily reproduces the Qz-, Pl- and Af-liquid phase equilibria, but model H2O solubilities are significantly lower and crystallization temperatures higher than in experiments
The granite-pegmatite connection : insight from mica trace element chemistry
International audienceMicas are ubiquitous minerals within granitic and pegmatitic rocks. Thepresence and the composition of micas are believed to depend stronglyon the magma they crystallised from. We studied major and traceelements, using both electron microprobe and Laser Ablation ICP-MS,within primary micas (biotite, muscovite and lepidolite) from the differentgranitic phases of the St Sylvestre leucogranite complex (WesternFrench Massif Central), and associated pegmatitic plugs intrusive in thegranite. The granite can be biotite-only, biotite-muscovite or muscoviteonly.Compositions of granitic micas do not vary significantly exceptfor the muscovite-only facies that has a slightly higher F-rich content.Pegmatitic biotite is quite homogenous within single bodies but recordsa considerable compositional change within the entire pegmatitic fieldranging from F-poor (similar to granite) towards F-rich. Pegmatitic muscoviteappears rather homogeneous in major element composition withonly a slightly higher Li-mica content than most granitic muscovite.Trace elements contents in muscovite are lower than in biotite but compositionaltrends are very similar to trends observed in biotite contents.While the variation observed within micas from the pegmatite can berelated to a magmatic differentiation, facies variation within the granitemay be related to changes in magma parameters such as f(O2) and thusare probably not related to differentiation processes. From here it appearsthat granite and pegmatite, even though spatially associated, mayfollow different evolutions and may not be genetically linked despitecommon characters. Differences of trace element concentration betweenbiotite and muscovite in pegmatite can either be the consequence of partitioncoefficient between biotite and muscovite or a diachronous crystallisationof muscovite and biotite. Additionally the reduced variabilityin muscovite structural formulae emphasizes a lack of sensitivity ofmuscovite with respect to magmatic differentiation
Multi-batch, incremental assembly of a dynamic magma chamber: the case of the Peninsula pluton granite (Cape Granite Suite, South Africa)
International audienceThe S-type Peninsula Pluton (South Africa) exhibits substantial compositional variability and hosts a large variety of mafic and felsic magmatic enclaves with contrasting textures and compositions. Moreover, the pluton is characterized by mechanical concentrations of K-feldspar megacrysts, cordierite and biotite, generating a complex array of magmatic structures including schlieren, pipes, and spectacular sheeted structures. Chemical evidence indicates that the pluton is constructed incrementally by rapid emplacement of numerous magma pulses. Field, and textural data suggest that magmatic structures form by local flow at the emplacement level of highly viscous crystal-rich magmas (i.e. crystallinity up to 50 vol.%) through magma mushes assembled from older batches. At the time of arrival of relatively late magma batches, some areas within the pluton had achieved crystal fractions that allowed the material to act as a solid, whilst maintaining enough melt to prevent formation of sharp intrusional contacts. Magmatic structures represent "snapshots" of processes that operate in multiphase crystal-rich mushes and their genesis is due to mechanical and thermal instabilities in the crystal-rich magma chamber that are triggered by the emplacement of pulses of new magma derived from the melting of a compositionally variable metasedimentary source
Successive geotherms, Granitic production and evolution of the lower crust in a post collisional context
International audiencePost-collisional context is commonly associated with production of large amount of peraluminous granitic magmas produced from the melting of crustal material. The variability of the produced granite commonly varies from leucogranitic to granodioritic and are mostly peraluminous. The South-Eastern French Massif Central (EFMC) region record several evidence for crustal melting revealed by migmatitic and granitic bodies providing ~30 Ma history of peraluminous granite production. Previous thermobarometric studies provides records for two successivves melting event: 1) a biotite stable event at ~314Ma (720°C and 5 kb) 2) a bitotite breakdown melting event constrained at ~301Ma (850°C 4kb) This suggests geotherms evolution from 45 °C/km to 70°C/km in 13Ma. A thermodynamic modelling approach considering a 20km thick pile of crustal material undergoing successive geotherm evolving from 25°C/km to 70°C/km with starting conditions between 3 and 10 kb. Along this evolution our approach allows successive melt extraction and the monitoring of melt compositional variability, residuum evolution and mineral phases modal and compositional variabities according to depth, geotherm and composition of the source. Over the 5 crustal sources used as starting composition for the model, 305 individual partial melting reactions are triggered. Melt and peritectic phases produced provide a variability that suggest the importance of source composition in matter of granite production for some element and ratios (K/Na, XMg). Compared to regional granites (EMCF). Most of the granites produced in the post collisional context of the EFMC can be reproduced by either melt only (leucogranite) or melt in addition to peritectic material produced along with melt (granite to granodiorite). In the same way, the relative-time constrain provided by the approach shows that it is possible to produce simultaneously heterogeneous granitic magmas in respect to source protolith and depth. Identically, the residual crust undergo an very variable evolution depending on protolith leading to an heterogeneous granulitic lower crust