DISCRIMINACIN TECTNICAEN ROCAS IGNEASPor Luis Lpez V.GL54C

Existe una importante diferencia entre las tres series mayores:Entonces, Qu hacemos?

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SeriesMargen de PlacaIntraplaca

CaractersticaConvergenteDivergenteOceanicaContinental

Alkalinasisisi

Tholeticasisisisi

Calco-alcalinasi

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IntroduccinUsando la qumica de basaltos (andesitas) es posible distinguir entre los diferentes (conocidos) regmenes tectnicos en que ellos se habran generado. Los diagramas fueron construidos utilizando elementos mayores y traza mediante el mtodo de anlis discriminate. Aquellos que utilizan elementos inmbiles bajo actividad hidrotermal (x ejm. Ti, Zr, Y, Nb, P) pueden ser utilizados en rocas alteradas y hasta metamosfizadas.Elementos deben ser insensitivos a procesos secundarios y fciles de medir con buena precisin.Los diagramas discriminantes NUNCA deben usarse como una prueba, solo sirven para sugerir una asociacin tectnica.

Placas Tectnicas.

LOS DIFERENTES CONTEXTOS GEOTECTONICOSIABIATVABOIBOITOIAMORBLKTOFBCABVABCRBCFBY tipos de magmatismo asociados

Subduccin bajo corteza ocenicaSubduccin bajo corteza continentalMrgenes convergentes: zonas de subduccinIABIATVABCABVAB

Intraplaca Ocenica:Hot SpotsOIBOITOIA

Rifts Mid Ocenicos (MORB)

Petrografa y Geoqumica de Elementos Mayores.Un MORB tpico es una basalto toletico de olivinos con bajo K2O (< 0.2%) y bajo TiO2 (< 2.0%).El vidrio representa las composiciones del lquido.

La secuencia de cristalizacin comn: olivino ( Mg-Cr espinela), olivino + plagioclasa ( Mg-Cr espinela), olivino + plagioclasa + clinopiroxeno

Geoqumica de Elementos Mayores de MORBs

Diagramas de variacin tipo Fenner.

N-MORB (MORB normal) Fuente manto superior empobrecidoMg# > 65: K2O < 0.10 TiO2 < 1.0E-MORB (MORB enriquecido, tambien llamados P-MORB por pluma) Fuente manto frtilMg# > 65: K2O > 0.10 TiO2 > 1.0

Elementos TrazasLas razones entre elementos incompatibles se usan para distinguir los reservorio de las fuentes. N-MORB: la razn K/Ba es alta(usualmente > 100)E-MORB: la razn K/Ba es ~ 30s

Elementos Traza: REE

Volcanismo Ocenico Intraplaca (IOB)

Islas Ocenicas y Montes Submarinos.Comnmente asociados con Hot Spots

Tipos de Magmas OIB2 series magmticas principalesSerie Toletica (tipo dominante)Basalto toletico parental de isla ocenica, OITSimilar al MORB, pero tienen diferencias qumicas y mineralgicas.Serie Alcalina Basalto alcalino parental de islas ocenicas, OIA2 sub-series alcalinas principalesSubsaturada en sliceLigeramente sobresaturada en slice (menos comn)

Elementos TrazaLos elementos traza LILE (K, Rb, Cs, Ba, Pb2+ and Sr) son incompatibles y estn enriquecidos en los magmas OIB con respecto a MORB.Para OIT, la razn K/Ba est entre 25-40Para OIA, la razn K/Ba est sobre los 20s

Entonces parece que tienen distintas fuentes.

Elementos TrazaElementos HFS (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) son incompatibles, y estn enriquecidos en OIBs > MORBsRazones entre estos elementos tambin sirven para distinguir fuentes mantlicas.La razn Zr/NbN-MORB generalmente algo altos (>30)OIBs son bajos (

Elementos Traza: REEs

Elementos Traza: REEsLa/Yb (pendiente de las REE) correlaciona con el grado de subsaturacin de slice en OIBs: Magmas altamente subsaturados: La/Yb > 30OIA: ~ 12OIT: ~ 4

Modelo para Magmatismo OcenicoDMOIBReservorios ContinentalesEM and HIMU from crustal sources (subducted OC + CC seds)

Continental Flood BasaltsLarge Igneous Provinces (LIPs)Plateaus OceanicosAlgunos riftsContinental flood basalts (CFBs)

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Table 15-1. Major Flood Basalt Provinces

NameVolumeAgeLocality

CRBMioceneNW US

KeeweenawanPrecambrianSuperior area

DeccanCret.-EoceneIndia

Paranaearly Cret.Brazil

Karrooearly JurassicS. Africa

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Setting Tectonico de CFBsHot spots continentales.Rifting continentales pueden estar asociados con hot spotsRiftsRifts fallidos (aulacogens)

Provincias Flood basalt de Gondwanaland antes de su quiebre y separacin.

Relacin entre las provincias de Etendeka y el plateau de Paran

Columbia River Basalt Group

Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir.

Magmatismo de Arco de IslasIslas volcnicas a lo largo de zonas de subduccin.Se diferencia de otras provincias magmticas por:Composicin ms diversa y silicea.Basalto generalmente subordinado.Ms explosivo. Strato-volcanes.

Ocano-ocano Arco de Islas (IA)Ocano-continente Arco Continental oMargin Continental Activo (ACM)

Estructura de un Arco de Islas

Rocas Volcanicas de Arcos de IslasAmplio espectro.Alta proporcin de andesita basltica andesita.La mayora de la andesitas ocurren en zonas de subduccin

Elementos Mayores y Series MagmticasToletico (MORB, OIT)Alcalino (OIA)Calc-Alkaline (~ restringuido a zonas de subduccin)

Elementos Traza: REEsPendiente en las series presentadas es similar, but height varies with FX due to removal of Ol, Plag, and PyxPendiente (+) bajo-K DMFuente manto heterogeneo HREE planas, no hay granete profundo.

MORB-normalized Spider diagramsOIB intraplaca tiene este cototo.

Magmatismo de Arco Continental

Magmatismo de Arco Continental

Modelo para Arcos Continentales

UTILIZACION DEDIAGRAMAS

MORBs are chemically distinct from basalts of other petrogenetic associations

Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulationFrom textures and experiments on natural samples at low pressure the common crystallization sequence is:All analyses are of glasses, so that only liquid compositions are represented

Note the very low content of K2O and that all analyses are quartz-hypersthene normative (although olivine is common in the mode)Decrease in MgO and relative increase in FeO early differentiation trend of tholeiites

Patterns are compatible with crystal fractionation of the observed phenocryst phases

Removal of olivine can raise the FeO/MgO ratio, and the separation of a calcic plagioclase can cause Al2O3 and CaO to decrease

SiO2 is a ~ poor fractionation index (as wed suspected)

Na2O K2O TiO2 and P2O5 are all conserved and the concentration of each triples over FX rangeThis implies that the parental magma undergoes 67% fractionation in a magma chamber somewhere beneath the ridge to reduce the original mass by 1/3

There must be incompatible-rich and incompatible-poor source regions for MORB magmas in the mantle beneath the ridges (related to the lower and upper mantle reservoirs?)N-MORB (normal MORB) taps the depleted (incompatible-poor) upper mantle sourceMg# > 65: K2O < 0.10 TiO2 < 1.0E-MORB (enriched MORB, also called P-MORB for plume) taps the deeper (incompatible-richer) mantleMg# > 65: K2O > 0.10 TiO2 > 1.0Major elements are not the best way to make these distinctions, which must be substantiated by trace element and isotopic differences

Also see two typesN-MORB has depleted trendE-MORB has non-depleted trend

More enigmatic processes and less voluminous than activity at plate marginsNo obvious mechanisms that we can tie to the plate tectonic paradigmAs with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB

41 well-established hot spots Estimates range from 16 to 122Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Runion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)Note that ocean island tholeiites (represented by the Kilauea and Mauna Loa samples) overlap with MORB and are not unlike E-MORBThe alkaline basalts have steeper slopes and greater LREE enrichment, although some fall within the upper MORB field

Note also that the heavy REEs are also fractionated in the OIB samples (as compared to the flat HREE patterns in N- and E-MORB). This indicates that garnet was a residual phase

These melts must have segregated from the mantle at depths > 60 kmIn Chapter 10, I argued that MORB tholeiites probably originated in the depleted upper mantle, and alkali basalts in the enriched lower mantle reservoir. Now it appears that E-MORB and ocean island tholeiites also have a source in the deeper reservoir (high % PM can still tholeiite from an enriched source)

Note mantle flow directions (induced drag), isolated wedge, and upwelling to back-arc basin spreading system Benioff-Wadati seismic zone (x x x x)Volcanic Fronth is relatively constant depth is importantAll three series occur in SZ setting, yet something about SZ is different that CALC-ALKALINE

Calc-alkaline magma series is used as yet another synonym to orogenic suite by some workers

Since other magma series can occur at subduction zones, I recommend that we use the term calc-alkaline strictly to denote a chemical magma series, not a tectonic association