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ANATECTIC MELTING IN A METAPELITIC SYSTEM: A FLUID AND MELT INCLUSION STUDY
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In this thesis, a melt and fluid inclusion study was performed to investigate both anatectic melt and fluid phase during anatexis of metapelitic rocks, by studying samples from two different geological settings, the khondalites of the Kerala Khondalite Belt, India, and the granulitic enclaves from El Hoyazo, Neogene Volcanic Province, Spain.
In khondalites (garnet-sillimanite-cordierite granulite/gneiss), melt inclusions (MI), interpreted as containing anatectic melt, were found for the first time in peritectic minerals from classic regionally metamorphosed metapelitic migmatites, partially melted at T~900°C and 6-8 kbar. A detailed study was carried out to characterize the petrographic features of MI, their chemical composition and the structures of the crystallized phases within the inclusions, by microscope observation, FESEM imaging in BSE mode, X-ray elemental mapping, EMP and Micro-Raman analyses. Peritectic garnets contain irregular clusters of hundreds of MI, from totally crystallized (nanogranites) to totally glassy, often negative-crystal in shape. Nanogranites are 5-25 μm across and contain a cryptocrystalline aggregate of Bt+Kfs+Pl+Qtz±Ap, along with trapped phases (apatite, zircon, rutile, Zn-bearing spinel and rarely ilmenite) that are likely to have favoured the entrapment of the anatectic melt during the host growth.. The grain size of crystals in nanogranite is variable, from few tens of nanometer to several microns. Partially crystallized inclusions contain a differentiated melt that occupies 60 to 20% of the inclusion area, along with Qtz, Kfs and Bt. Glassy inclusions are usually smaller (2.5–17.5 μm) than nanogranites and represent about 15% of the total amount of MI in the clusters. They contain an amorphous phase, along with the same trapped phases found in nanogranites. EMP analysis of the glass provides an ultrapotassic and rhyolitic composition, with K2O >> Na2O. The EMP totals suggest a H2O content (calculated by difference to 100%) <3 wt%. The occurrence of preserved glassy MI is an unprecedented finding, and since the mean diameter (8 μm) of glassy inclusions is smaller than those of nanogranites (13 μm), we propose that this difference in size was influential to the crystallization of melt droplets, so that most of the smaller inclusions remained glassy because of inhibited nucleation. Nanogranites were homogenized using a heating stage at controlled atmosphere, and then analyzed, to obtain a consistent set of compositional data on their bulk composition. EMP analyses on homogenized inclusions show a compositional homogeneity of the melt from all the nanogranites, with strong similarity to those of preserved glassy inclusions. Melt composition is Na-poor, and plots very far from the “minimum melt” of the haplogranitic system in the Q-Ab-Or diagram, commonly accepted as the composition of the anatectic melt produced by partial melting of metapelites. The retrieved composition, although uncommon for anatectic melts, is reported for natural rhyolites and for experimental products, and accounts for partial melting conditions with T in excess of 850°C, in agreement with the inferred PT conditions of partial melting for these rocks. Moreover, it testifies that assuming a minimum melt composition as representative of the anatectic melt is not correct in the present case study and should not be considered a general rule. The consistency of the compositional data and the careful microstructural investigation of the samples, coupled with the use of the correct techniques of MI homogenization, microchemical analyses and data correction, support for the interpretation of these data as representative of the phase trapped in MI, and the conclusion that MI in garnet from khondalites contain droplets of anatectic melt. This novel result represents therefore an important contribution to the knowledge of anatectic melt in natural rocks.
In the second part of the research the granulitic enclaves of El Hoyazo are studied. These partially melted metapelites, ripped off from the basement when anatexis was still an on-going process, contain abundant rhyolitic glass both as layers and pockets, and as glassy inclusions in almost all the minerals of the assemblage. Unlike regionally, slowly cooled rocks, in this case the anatectic conditions were frozen by the rapid ascent of the samples in a uprising magma. MI-bearing garnet is the first peritectic phase produced by partial melting at ~700°C and 5-7 kbar, and contains abundant FI with no visible evidence of post-entrapment modifications, trapped in conditions of fluid-melt immiscibility. A FI study was performed on garnets from Spl-Crd and Bt-Grt-Sil enclaves by microscope investigation, microthermometric studies, Micro-Raman analyses, mass balance calculation and TEM investigation. In Spl-Crd enclaves FI are two-phase (L+V), spherical to tubular, and often contain graphite as trapped phase. Trapped fluid is a mixture of H2O+CO2+N2±H2S±CH4, with water up to 95 mol%, while in Bt-Grt-Sil enclaves FI in garnet are one phase, and contain a CO2+N2 mixtures. In both samples FI have densities that are not consistent with the inferred trapping conditions, and suggest that despite of their primary-looking features, FI re-equilibrated during uprising. TEM investigation on Bt-Grt-Sil samples showed partially healed cracks at sub-μm scale, possible escape pathways for the leakage of fluids out of the inclusions. In Spl-Crd enclaves microchemical data acquired on MI and biotite inclusions, that occur in the same cluster along with FI, demonstrate that a water-rich leucogranitic melt was trapped along with a H2O-rich, COH phase at conditions consistent with the inferred garnet growth (c. 700°C). In garnet from Bt-Grt-Sil enclaves, the almost complete decrepitation and fluid leakage suffered by the studied FI did not allow to estimate the original composition of fluids hosted in garnet. Based on the H2O content of coexisting melt inclusions, however, the fluid is inferred to have been more CO2-rich than the fluid in the Spl-Crd enclaves. This work adds further compositional constraints to the characterization of anatexis of metapelites in the lower crust: in fact, although final results clearly show that enclaves lost part of the original components, the composition of fluid trapped in garnet from Spl-Crd enclaves is probably very close to the original, and is consistent with the composition of the coexisting melt.
Research on melt and fluid inclusions in peritectic minerals represents a new approach to the problem of partial melting in natural rocks, and the present study demonstrated that reliable petrological and geochemical information on anatexis can be collected from nano- to micron-scale objects. The dataset reported in this study widens the horizons in crustal petrology, because for the first time the crustal melt composition can be analyzed rather than assumed. Moreover, MI study in migmatites is likely to have large potentials of development, as confirmed by recent findings of anatectic melt trapped in inclusions in peritectic minerals from various migmatite terrains. As regard the fluid coexistent with anatectic melt, the exceptional occurrence of MI and FI in garnets from El Hoyazo enclaves allowed the identification and characterization of a H2O-rich fluid present during the first step of anatexis of these metapelites.Nel presente lavoro di tesi sono riportati i risultati dello studio di inclusioni fluide e di fuso silicatico effettuato sulle khondaliti della Kerala Khondalite Belt, India, e sugli inclusi granulitici di El Hoyazo, Neogene Volcanic Province, Spagna meridionale, con il fine di caratterizzare l`anatessi di rocce metapelitiche.
Nelle khondaliti (gneiss granulitici a granato-sillimanite-cordierite), inclusioni di fuso silicatico (inclusioni vetrose, o MI), interpretate come contenenti fuso anatettico, sono state rinvenute per la prima volta in fasi peritettiche di classiche migmatiti metapelitiche, caratterizzate da metamorfismo regionale con fusione parziale a T~900°C e 6-8 kbar. Le caratteristiche petrografiche delle MI, la loro composizione chimica e il loro fabric interno, dovuto alla presenza di fasi cristallizzate, sono state caratterizzate attraverso l`uso del microscopio ottico e di quello elettronico a scansione con emissione di campo (FESEM), l`acquisizione di mappe elementari ai raggi X e le analisi alla microsonda elettronica (EMP) e in spettroscopia Micro-Raman.
I granati peritettici contengono aggregati irregolari costituiti da centinaia di MI, le quali vanno da totalmente cristallizzate (nanograniti) a vetrose, spesso con forme a cristallo negativo. I nanograniti presentano dimensioni tra 5 e 25 μm, e contengono un aggregato criptocristallino di Bt+Kfs+Pl+Qtz±Ap, oltre a fasi accessorie (apatite, zircone, rutilo, spinello e occasionalmente ilmenite) che molto probabilmente hanno favorito l`intrappolamento di porzioni di fuso durante la crescita del granato e la conseguente formazione di inclusioni. La grana delle fasi cristalline nei nanograniti è variabile, da pochi nanometri fino ad alcuni micron. Le inclusioni parzialmente cristallizzate contengono un fuso differenziato, che può occupare il 20-60% dell`area dell`inclusione, coesistente con quarzo, K-feldspato e biotite. Le inclusioni vetrose hanno solitamente dimensioni inferiori, 2.5–17.5 μm, a quelle dei nanograniti e costituiscono circa il 15 % delle inclusioni di ogni ammasso. Al loro interno è presente una fase vetrosa, solitamente coesistente con le stesse fasi accidentali riconosciute nei nanograniti. Dati microchimici sul vetro mostrano una composizione riolitica ultrapotassica, con K2O >> Na2O. Il contenuto di acqua stimato dalla differenza da 100 wt% dei totali delle analisi EMP è inferiore a 3 wt%. La presenza di inclusioni vetrose preservate in questo contesto è una scoperta senza precedenti. Il diametro medio delle inclusioni vetrose (8 μm) è inferiore a quello dei nanograniti (13 μm), e si propone che tale differenza di dimensioni abbia influito sulla cristallizzazione delle porzioni di fuso intrappolato, inibendo la nucleazione delle fasi cristalline nella maggior parte delle inclusioni più piccole.
I nanograniti sono stati rifusi attraverso l`uso di un tavolino riscaldante ad atmosfera controllata, e successivamente analizzati al fine di ottenere dati sulle loro composizioni totali. Le analisi in microsonda elettronica mostrano in tutte le inclusioni rifuse una composizione molto simile a quella delle inclusioni vetrose preservate. Avendo un chimismo povero in Na, nel diagramma Q-Ab-Or per il sistema aplogranitico questo fuso si trova molto lontano dalla composizione del “minimum melt”, comunemente accettata come rappresentativa di un fuso anatettico prodotto dalla fusione parziale di metapeliti. Tale composizione, sebbene non sia comune, è segnalata in letteratura per magmi riolitici e per prodotti sperimentali, e suggerisce che la fusione parziale di sia avvenuta a temperature superiori a 850°C, in accordo con le condizioni di anatessi proposte per queste rocce. Questi dati mostrano che assumere una composizione da “minimum melt” come rappresentativa non sia corretto nel presente caso di studio, e che non dovrebbe essere considerata quindi una regola generale. La coerenza dei dati microchimici ottenuti e l`attento studio microstrutturale dei campioni, unito all’uso delle opportune tecniche di rifusione, di microanalisi e di correzione dei dati, supportano l`interpretazione delle composizioni ottenute come rappresentative della fase intrappolata nelle inclusioni, e la conclusione che le MI nei granati delle khondaliti contengano porzioni di fuso anatettico. Questo risultato originale costituisce quindi un importante contributo alla conoscenza di fusi anatettici in rocce naturali.
Nella seconda parte del progetto di ricerca sono stati studiati gli inclusi granulitici di El Hoyazo. Queste metapeliti, parzialmente fuse, sono state rimosse dalla bassa crosta mentre l`anatessi stava avendo luogo, e contengono abbondante vetro riolitico in livelli e sacche, oltre che in inclusioni vetrose in quasi tutti i minerali della paragenesi. Diversamente dalle rocce sottoposte a metamorfismo regionale e raffreddate lentamente, in questo caso i caratteri microstrutturali dovuti alla fusione parziale sono stati “congelati” dalla rapida risalita dei campioni in un magma in eruzione. Il granato che contiene inclusioni vetrose è stato la prima fase solida prodotta dalla fusione parziale a ~700°C e 5-7 kbar, e, oltre alle MI, contiene numerose inclusioni fluide (FI) intrappolate in condizioni di immiscibilità fuso-fluido e che non presentano evidenze di modificazioni successive alla loro formazione. Lo studio delle FI nei granati degli inclusi a Spl-Crd e a Bt-Grt-Sil è stato condotto attraverso la loro caratterizzazione petrografica, studi microtermometrici, analisi in spettroscopia Micro-Raman, calcoli di bilanci di massa e studi al microscopio elettronico a trasmissione (TEM). Negli inclusi a Spl-Crd le FI sono bifasiche (L+V), da sferiche a tubulari, e spesso contengono grafite come fase intrappolata. Il fluido contenuto nelle inclusioni è una miscela di H2O+CO2+N2±H2S±CH4, con abbondante H2O, fino a 95 mol%. Negli inclusi a Bt-Grt-Sil, le inclusioni fluide nel granato sono monofasiche e contengono una miscela gassosa a CO2+N2. In entrambi i campioni le inclusioni fluide presentano densità in disaccordo con le proposte condizioni di intrappolamento, e suggeriscono che, nonostante il loro aspetto primario preservato, esse si siano riequilibrate durante la risalita. Lo studio al TEM dei granati negli inclusi a Bt-Grt-Sil ha mostrato, alla scala sub micrometrica, la presenza di fratture parzialmente rinsaldate, interpretabili come possibili vie di fuoriuscita di componenti dalle inclusioni fluide. Negli inclusi a Spl-Crd, i dati raccolti sulle biotiti e le inclusioni vetrose, entrambi coesistenti con le inclusioni fluide, dimostrano che un fuso leucogranitico e una fase COH, entrambi ricchi in acqua, sono stati intrappolati a condizioni di temperatura in accordo con quelle proposte per la formazione del granato (c. 700°C). Negli inclusi a Bt-Grt-Sil, invece, la quasi totale decrepitazione delle inclusioni fluide e la conseguente perdita di componenti non hanno permesso la caratterizzazione della composizione originale del fluido intrappolato nel granato. Tuttavia, sulla base dei contenuti in acqua delle inclusioni vetrose coesistenti, si ipotizza che il fluido fosse in origine più ricco in CO2 di quello presente negli inclusi a Spl-Crd. Il presente studio fornisce ulteriori dati sulla caratterizzazione della fusione parziale di metapeliti nella bassa crosta. Infatti, sebbene i risultati mostrino che gli inclusi hanno perso parte dei componenti originali, la composizione del fluido negli inclusi a Spl-Crd molto probabilmente è simile a quella primaria, in accordo con la composizione del fuso coesistente
Lo studio di inclusioni fluide e di fuso silicatico in minerali peritettici rappresenta un nuovo tipo di approccio al problema della fusione parziale in rocce naturali, e questa tesi dimostra che è possibile ottenere risultati validi e rappresentativi, sia dal punto di vista petrologico che geochimico, attraverso lo studio di campioni come i nanograniti, con dimensioni micrometriche o inferiori. Il set di dati ottenuto in questo studio amplia gli orizzonti dello studio petrologico della crosta, in quanto per la prima volta la composizione dei fusi crostali può essere analizzata invece che ipotizzata. Lo studio delle MI nelle migmatiti rappresenta inoltre un campo di studio con grandi potenziali di sviluppo, come è stato recentemente confermato dall`individuazione di ulteriori esempi di nanograniti intrappolati in minerali peritettici di differenti basamenti migmatitici. Per quanto riguarda il fluido coesistente col fuso anatettico, l`eccezionale presenza di MI e FI nei granati di El Hoyazo ha permesso l`identificazione e la caratterizzazione del fluido ricco in acqua che era presente in queste metapeliti all`inizio dell`anatess
Mesoarchean melt and fluid inclusions in garnet from the Kangerlussuaq basement, Southeast Greenland
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No full textGarnet brought to the surface by late Miocene granitoids at La Galite Archipelago (Central Mediterranean,
Tunisia) contains abundant primary melt and fluid inclusions. Microstructural observations
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melting at ~800 °C and 0.5 GPa, under fluid-present conditions. The trapped melt is leucogranitic
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room temperature, and contain a CO2-dominated fluid, with minor H2O, N2 and CH4. Siderite and
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and are interpreted as products of a post-entrapment carbonation/hydration reaction between the
fluid and the host during cooling. The fluid present during anatexis is therefore inferred to have been
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No full textProduction of metaluminous melt during fluid-present anatexis of the Maghrebian basement (La Galite Islands, Central Mediterranean Sea)
No full text- …
