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Petrology and SHRIMP U-Pb zircon geochronology of Cordilleran granitoids of the Bariloche area, Argentina [查看] A.Castro、I.Moreno-Ventas、C.Fernández、G.Vujovich、G.Gallastegui、N.Heredia、R.D.Martino、R.Becchio、L.G.Corretgé、J.Díaz-Alvarado、P.Such、M.García-Arias、D.-Y.Liu

A petrological and geochronological study of Cordilleran granitoid intrusions in the Bariloche area (Argentina) point to a complex time-compositional evolution of magmatic processes in relation with oblique subduction of the Phoenix plate below the South America active margin during Jurassic times.The observed geochemical variations in both major and trace elements, together with the textural and mineralogical relations, point to a roughly defined, overall process of magmatic “filtering” linking all the intrusive batholithic rocks of the Bariloche area. These data suggest that the composition of the parental magma that underwent fractionation may be an intermediate magma with SiO2=58-60wt%,MgO=2.5 wt%, FeO=6.5 wt%, CaO=6.1. These are coincident with the typical compositions of evolved andesites. Magnetite, amphibole and plagioclase are the main phases involved in the fractionation process. According to Hbl thermobarometry, fractionation may have taken place, at least in part, at shallow pressures (P ¼ 0.5e1.5 kbar), possibly at the level of emplacement. The coupled observations of the two pressure dependent ratios, namely Sr/Y and La/Yb are pointing to a low-pressure, lowtemperature final fractionation dominated by only Pl. The geochronologic study by UePb SHRIMP zircon determinations of 14 samples from granites, tonalites and diorites yield a broad range of about 20 Ma,between 150 and 170 Ma at the Medium Jurassic. The batholith was accomplished by a protracted magmatic activity that lasted for about 20 Ma. This time is much longer than the time elapsed from intrusion to complete crystallization of shallow magma chambers. It is concluded that amalgamation of discrete magma pulses is the dominant process that built-up the batholith. The observed structures suggest that the fractures conditioning the emplacement of the magma batches were arranged en échelon and show a right-stepping. The resulting geometry is compatible with the activity of a largescale,sinistral, NeS trending, strike-slip fracture zone permitting the emplacement of each magma pulse.This major, strike-slip fault system should be deeply entrenched in the crust to allow intruding magmas generated and fractionated at depth. Because batholith generation is a direct consequence of subduction,structural relations and ages can be used to constraint the plate motion relations during Jurassic in this region of the South America active margin.

Cambrian ensialic rift-related magmatism in the Ossa-Morena Zone(Évora–Aracena metamorphic belt, SW Iberian Massif) Sm-Nd isotopes and SHRIMP zircon U-Th-Pb geochronology [查看] M.Chichorro、M.F.Pereira、M.Díaz-Azpiroz、I.S.Williams、C.Fernández、C.Pin、J.B.Silva

The Late Ediacaran (c. 560–550 Ma) Série Negra sediments of the Évora–Aracena metamorphic belt, Ossa-Morena Zone, SW Iberian Massif, preserve a record of the erosion of an Avalonian–Cadomian magmatic arc and subsequent related turbiditic sedimentation. Detrital zircon from the Série Negra is characterized by predominantly Ediacaran and Cryogenian ages, with few Paleoproterozoic and Archean cores, and a marked lack of Grenvillian ages. These features, when combined with the metasediments' enrichment in LREE (La/Yb=14), negative Eu-anomalies, low 147Sm/144Nd values (0.121) and negative εNd550=−5.5, indicate that the protolith Série Negra sediments were derived from a continental magmatic arc. A period of Late Cadomian (ca. 560–540 Ma) tectonism was followed by an extended episode of widespread bimodal magmatism related to Cambrian (ca. 540–500 Ma) rifting. This tectonic inversion is expressed in the geological record by a regional Early Cambrian unconformity.SHRIMP zircon U–Th–Pb ages from four felsic orthogneisses from the Évora Massif record Cambrian (527±10 Ma, 522±5 Ma, 517±6 Ma and 505±5 Ma) crystallization ages for their igneous protoliths. This confirms the existence of widespread Lower Paleozoic igneous activity in the Ossa-Morena Zone: (i) a Lower Cambrian (ca. 535–515 Ma) igneous–felsic dominated–sedimentary complex (with calc-alkaline signature and associated carbonate and siliciclastic deposition), and (ii) a Middle Cambrian–?Ordovician (ca. 515–490 Ma) igneous–bimodal–sedimentary complex (with calc-alkaline and tholeiitic signatures and associated dominant siliciclastic deposition, but also carbonate sediments).The Cambrian felsic magmatism was characterized by negative Eu-anomalies, (La/Lu)N=0.8–11, 147Sm/144Nd=0.1289–0.1447 and εNd500 ranging from −1.5 to −0.8. A tendency towards peraluminous compositions suggests late fractionation, low degrees of partial melting, or the mixing of crustal and mantle-derived material in the magma source region. Some felsic rocks possibly represent the last residual melts of hightemperature,zircon-undersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids.The transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic N–E-MORB products (ThN/TaNb1.0). The as yet undated (Cambrian–?Ordovician) E-MORB amphibolites have 147Sm/144Nd=0.1478–0.1797 and εNd500 values ranging from +6.4 to +7.3, while the N-MORB amphibolites have 147Sm/144Nd=0.1818–0.1979 and εNd500 values of +5.8 and +7.0,reaching a maximum of +9.1. In contrast, other amphibolites have a negative Ta-anomaly (1.35bThN/TaNb2.41) reminiscent of lavas from “orogenic” settings or alternatively, typical of crustally-contaminated within-plate magmas. These “VAB-like” amphibolites have 147Sm/144Nd values ranging from 0.1639 to 0.1946 and εNd500 values of +3.5 to +5.2, suggesting derivation by crustal assimilation processes. The subalkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling.

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