1 research outputs found
Evoluci贸 del mantell subcontinental litosf猫ric del NE d鈥橧b猫ria a partir de l'orientaci贸 cristal路logr脿fifica preferent i la geoqu铆mica de xen貌lits mant猫l路lics
Zona Volc脿nica Catalana, van revelar l'exist猫ncia d'un mantell subcontinetal litosf猫ric heterogeni,
format principalment per lherzolites i harzburgites protogranulars amb espinel路la, i prioxenites en
menor proporci贸. Ambdues peridotites es van interpretar com residus de la fusi贸 parcial, que
posteriorment van experimentar diferents episodis metasom脿tics. Un metasomatime antic causat per
fluids de subducci贸 nom茅s el registren les harzburgites, mentre que un metasomatisme posterior,
relacionat amb fosos silicatats i derivats carbonat铆tics va afectar a les harzburgites i algunes
lherzolites. Les piroxenites es van relacionar amb fosos m脿fics alcalins de l'煤ltim metasomatisme. A
m茅s, aquests estudis van plantejar altres preguntes que eren necess脿ries avaluar sobre aquest mantell
subcontinental litosf猫ric europeu i que s贸n l'objectiu d'aquesta tesis: (1) relacions entre la deformaci贸
del mantell i les microestructures; (2) identificaci贸 de les associacions minerals dels sulfurs; i (3),
relaci贸 entre la deformaci贸 mant猫l路lica i els processos petrogen猫tics com la fusi贸 parcial i la interacci贸
roca-fos infiltrant. Per poder tractar aquests temes es van utilitzar t猫cniques cl脿ssiques com el
microscopi petrogr脿fic, combinades amb d'altres m茅s anal铆tiques: an脿lisis d'elements majors en
minerals per EPMA i de traces per LA-ICP-MS, an脿lisis de roca total (elements majors i tra莽a) per ICPMS,
an脿lisis d'imatges d'electrons retrodispersats per l'estudi dels sulfurs i les fases en
microestructures de reacci贸 per SEM, i la indexaci贸 de les orientacions cristal路logr脿fiques preferents
dels minerals principals per SEM-EBDS.
El nou mostreig confirma que les roques mant猫l路liques d'aquesta zona s贸n lherzolites (56%) i
harzburgites (40%) amb espinel路la i websterites oliv铆niques (4%) protogranulars. Entre les lherzolites a
m茅s, n'hi ha un 15% amb microestructura porfirocl脿stica i un 10% amb microestructura equigranular.
Les peridotites i websterites protogranulars es van equilibrar a major temperatura (1175-1025 掳C) i
pressi贸 que les lherzolites porfirocl脿stiques i equigranulars (994-803 掳C). Les associacions
mineral貌giques dels sulfurs s贸n el resultat de la desmescla de solucions s貌lides monosulf煤riques (Fe-
Ni) i interm猫dies de Cu d'alta temperatura (1000-900 掳C) al disminuir aquesta (600 掳C). Pr猫viament a
ser arrencat per la lava, aquest mantell va experiments varies etapes de fusi贸 parcial, interacci贸 rocafos,
amb diferents fraccions de fos, i deformaci贸. La indexaci贸 de les orientacions cristal路logr脿fiques
preferents va revelar l'exist猫ncia de tres f脿briques de deformaci贸 de l'olivina. Una f脿brica axial [010],
forta i coherent amb la dels piroxens en la majoria dels casos, caracter铆stica de la majoria de les
harzburgites, lherzolites i websterites protogranulars. Aquesta f脿brica es relaciona amb una
deformaci贸 tardi-Herciniana per cisalla simple o transpressi贸, i que van perdurar posteriorment per
l'annealing i el creixement cristal路l铆 afavorit per la infiltraci贸 limitada de fosos, des de la descompressi贸
herciniana fins al final del rift Cret脿cic. Les f脿briques m茅s d猫bils, ortor貌mbica i axial [100], s贸n m茅s
t铆piques de lherzolites porfirocl脿stiques i equigranulars, els piroxens de les quals tenen f脿briques
incoherents amb les de l'olivina. Aquestes f脿briques s'expliquen per una deformaci贸 transtensiva, a
esfor莽os m茅s elevats i condicions de temperatura i pressi贸 decreixent respecte la deformaci贸 anterior.
Es relacionen amb la extensi贸 tardi Alpina que va formar les conques ne貌genes. Pr猫viament a la
deformaci贸 herciniana, la hist貌ria evolutiva de les peridotites va poder ser diferent, i en part
condicionada per processos dicot貌mics. El metasomatisme i possible fusi贸 parcial en una zona de
subducci贸 de les harzburgites, no va ser registrat per les lherzolites. Aquestes deriven d'una fusi贸
parcial menys intensa, per貌 la moda m茅s baixa de l'olivina i m茅s alta de l'ortopirox猫 de les que
indiquen les traject貌ries de fusi贸, suggereix la interacci贸 amb un volum important d'un fos ric en SiO2,
茅s a dir, un metasomatisme camuflat. La infiltraci贸 post herciniana d'un volum limitat de fosos
alcalins, especialment durant el Cret脿cic superior, va afavorir la conservaci贸 de les f脿briques tipus
axial [010], tamb茅 va metasomatitzar de forma cr铆ptica les harzburgites i algunes lherzolites. Tots
aquests processos s贸n anteriors a la deformaci贸 transtensional tardi Alpina que va afectar tant a les
lherzolites metasomatitzades com les no metasomatitzades. Nom茅s serien post-cinem脿tiques algunes
de les microestructures espor脿diques de reacci贸, degut a una interacci贸 limitada roca-fos just abans
de la fragmentaci贸 i transport del mantell per la lava.Previous studies on mantle xenoliths enclosed in Neogene-Quaternary basaltic rocks from the Catalan
Volcanic Zone revealed a heterogeneous subcontinental lithospheric mantle formed mainly of
protogranular spinel lherzolites and harzburgites with minor pyroxenites. Both peridotites were
thought to represent residues after partial melting of a Primitive Mantle followed by subsequent
metasomatic episodes. An earlier metasomatism caused by subduction-related fluids was only
registered by harzburgites. A later metasomatism caused by alkaline silicate melts and carbonatite
derivatives was registered by harzburgites and some lherzolites. Pyroxenites were related to the mafic
alkaline melts of the later metasomatism. However, other topics remained to be explored in this part
of the European lithospheric mantle, among which the following are the aims of this study: (1)
relationships between deformation and microstructrures, (2) identification of sulphide mineral
associations; and, (3) relationships between mantle deformation and petrogenetic processes (partial
melting, melt-rock interaction). To approach these issues, we enlarged the xenolith sampling and used
classical techniques as the petrographic microscope, combined with EPMA (major element analyses of
minerals), ICP-MS (whole-rock major and trace element analyses), LA-ICP-MS (mineral trace element
analyses), SEM (backscattered images of sulphides and of mineral overgrowth microstructures) and
SEM-EBSD (indexing of the crystal-preferred orientations of the main minerals).
The new sampling confirms that protogranular anhydrous spinel lherzolites (56%) and harzburgites
(40%) with minor olivine websterites (4%) are the main mantle lithologies. Porphyroclastic and
equigranular lherzolites are 15 and 10%, respectively, of the total lherzolites. Protogranular peridotites
and websterites were equilibrated at higher temperature (1175-1025 掳C) and pressure than
porphyroclastic and equigranular lherzolites (994-803 掳C). Sulphide mineral associations resulted from
the breakdown of high temperature (1000-900 掳C) Ni-Fe monosulphide solid solutions and Cu
intermediate solid solution at temperature < 600 掳C. Previous to the lava transport, the lithospheric
mantle of this area experimented multi-stage processes of melt depletion, melt-rock interaction, at
different melt/rock ratio, and deformation. Indexation of crystal-preferred orientations revealed three
olivine deformation fabrics. Most protogranular harzburgites, lherzolites and one websterite show
strong olivine [010]-fiber fabric and coherent pyroxene fabrics. These fabrics are related to late
Hercynian simple shear or transpression, and to subsequent annealing and grain growth, favoured by
the limited percolation of melts, from the late Hercynian decompression to the Cretaceous rifting.
Weaker olivine orthorhombic and rare [100]-fiber fabrics are more characteristic of porphyroclastic
and equigranular lherzolites, the pyroxenes of which show mostly incoherent fabrics. These fabrics are
explained by transtensional deformation at higher stress and decreasing T and P with respect to the
[010]-fiber type, during late Alpine extensional movements that led to the formation of Neogene
basins. The evolution history of both peridotites, before the common Hercynian deformation, could
have been different and conditioned by a dichotomy in processes. The earlier subduction-related
metasomatism, and possible melt depletion, of harzburgites is not registered by lherzolites. Lherzolites
are also the result of less severe melt depletion, but the most fertile of them, with olivine and
orthopyroxene mode under and over melt depletion trends, respectively, suggest SiO2-rich melt (fluid)-
rock interaction at high melt fraction (i.e., stealth metasomatism). Post-Hercynian percolation of a
limited volume of alkaline silicate melts and derivatives, during the Cretaceous, contributed to
preserve the olivine [010]-fiber fabric and to metasomatize cryptically harzburgites and some
lherzolites. These petrogenetic processes pre-dated the late Alpine transtensional deformation that
affected both metasomatized and non metasomatized lherzolites. Only sporadic mineral overgrowth
microstructures, due to limited melt-rock interaction shortly before the transport of the xenoliths by
the lavas, seem to be post-kinematic