Calvet, F. Fifth-order cyclicity and organic matter contents relationship Lower Eocene, Pyrenees. ISSN: Marfil, R. ISSN: X. Bover-Arnal, T.
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Pablo D. Giacosa 2. Roca , Argentina. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage S3.
The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic between upper Cambrian and lower Silurian.
The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones.
The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. These rocks have been interpreted as part of a Palaeozoic basement, but both the relationships between them and their correspondence with an orogenic cycle are still unclear.
Recently, Heredia et al. These authors also proposed a two-stage development for the Gondwanan orogeny. The older stage was related to subduction and was Middle Devonian-Early Carboniferous in age. Later, a collisional stage of late Carboniferous-early Permian age closes the Gondwanan orogenic cycle.
This new data help placing the North Patagonian basement in the context of the Palaeozoic orogenic cycles recently proposed by Heredia et al. Geographic location of the studied area modified from Ramos ; B. Geological context of the studied area with the location of the geological maps of figures 2A and 2B. Modified from Leanza et al. The high-grade metamorphic rocks comprise mainly migmatites and gneisses with minor schists Fig. Modified from Cucchi and Leanza See location in figure 1B.
The age of these high-grade metamorphic rocks has been matter of discussion in recent years. A Neoproterozoic age was proposed by Dalla Salda et al. However, more recent studies constrained the maximum deposition age of the protolith in this area in ca. Additionally, Serra-Varela et al. This discrepancy between this age data will be discussed later in this contribution. The high-grade metamorphic rocks crop out as roof pendants or xenoliths within the Devonian plutonic units and the contact between them is locally sharp, indicating a pre-Devonian age for these metamorphic rocks Serra-Varela et al.
Both high-grade metamorphic and igneous rocks are covered by Paleogene volcanic and volcaniclastic rocks from the Huitrera Formation Escosteguy and Franchi, ; Ravazolli and Sesana, Recently, Serra-Varela et al. Thus, the Devonian igneous rocks are deformed by a single orogenic cycle in late Palaeozoic times while the high-grade metamorphic rocks are affected by at least two different orogenic cycles in early and late Palaeozoic times.
The most common metamorphic rocks are gneisses and schists in close association with metatexites and diatexites in the sense of Sawyer, These types of rocks show transitional contacts between them. The Devonian igneous rocks comprise large volumes of granodiorites and tonalites. Minor gabbroic rocks are found in the area and have a transitional contact with granodiorites, suggesting that they are genetically and temporarily related.
The metasedimentary rocks are predominantly gneisses with minor schists Fig. We distinguish one main metamorphic assemblage in schists Fig. Alternate gneisses and schists; B. Stromatic structure in a metatexite. Note the stromatic layering is parallel to the principal foliation S 2 of the metasedimentary rocks; D.
Schollen diatexites. Rafts of metasedimentary rocks with incipient migmatization; E. Schlieren diatexites with f low banding structure with aligned biotite schlierens; F. Photomicrography of the leucosome of a schollen diatexite with quartz, microcline, plagioclase, biotite and muscovite; G. Photomicrography of a schlieren in thin section defined by dotted line; H.
Symplectites between muscovite and quartz. All mineral abreviations according to Siivola and Schmid Commonly the metasedimentary sequences include alternate layers of schists and gneisses, most likely ref lecting the original sedimentary bedding.
Chlorite, epidote, sericite and prehnite are the common low-temperature association replacing the main assemblages. Among the metatexites, only stromatites are recognized Fig. The stromatic layering is generally parallel to the main foliation of the metasedimentary rocks. Leucosomes are between 0. These rocks are composed of three parts: 1 a dark grey part with similar characteristics to the metasedimentary rocks.
This would represent the mesosome; 2 A white part containing quartz, plagioclase and K-feldspar. This part represents the leucosome of the metatexite and has a well-developed medium grained granular texture; 3 A black part, which is formed mainly by biotite.
These biotite crystals are larger and more abundant than those observed in the mesosome. Also, this sector is enriched in accessory minerals such as apatite and zircon. It is located next to the leucosomes and represents melanosomes. In this case garnets constitute poikiloblasts with numerous inclusions of quartz.
Melanosomes in this sample are formed by biotite and cordierite where biotites reach 1. Near the contact with leucosomes, garnets are fragmented and most of them show a plagioclase rim. The first type has well defined leucosomes with a dominant igneous texture and coarse grains.
Leucosomes contain quartz, microcline and plagioclase in high proportion and biotite, muscovite in low proportion Fig. These leucosomes have the highest percentage of K-feldspar.
Mesosomes are found as rafts of metasedimentary rocks with incipient migmatization. Mafic selvedges are frequently developed between mesosomes and leucosomes. Frequently in the contact between mesosome and leucosome, mafic selvedges are developed.
They are only a few micrometer thin and are composed of biotites and quartz with large amount of accessory minerals such as zircons and Fe-Ti oxides. Crystals of biotite tend to follow the main orientation of the mesosome although discrete folds can be recognized in some sectors. They are interpreted as the result of deformation during leucosome growth Sawyer, Schlierens are composed by biotite and ilmenite Fig. They present flow banding where the biotite schlierens are aligned.
These rocks present a well-developed granoblastic texture. Both in schlieren and schollen leucosomes is common to find myrmekites as an intergrowth of quartz and K-feldspar. It is also common to find poikilitic crystals of feldspar and plagioclases containing rounded inclusions of quartz and biotites. These microstructures have been identified as representing different stages on melt crystallization Sawyer, Finally, symplectites between micas muscovite and biotite and quartz are recognized Fig.
This rock is coarse-grained, with quartz, plagioclase, K-feldspar and biotite as primary minerals and apatites and zircons as accessory minerals. K-feldspars and plagioclases are pseudomorphically replaced by sericite. Biotites are replaced by chlorite, epidote and prehnite.
Intracrystalline deformation is common in quartz with microstructures such as undulatory extinction, chessboard subgrains and static recrystallization Fig. Secondary paragenesis and microstructures in the anatectic granite; B. Microphotograph of the schist from the country rock of the anatectic granite, showing the S 2 regional foliation; C. Low-grade metamorphic paragenesis in Devonian granitoids; D. Photomicrograph of a biotite-rich schist with S 2 foliation and relict S 1 foliation defined mainly by graphite; E.
Field photograph of S 2 affected by D 3 crenulation; F. Photomicrography of folded S 2 and incipient S 3 crenulation cleavage defined by opaque minerals along pressure-solution bands; G.
Discrete centimetric brittle-ductile shear bands affecting Devonian igneous rocks; H. Top block with NE tectonic transport direction. This granite presents a transitional contact with schists. Next to the plutonic body, a sector rich in biotite is found. These biotites are larger than the ones in the granite and are associated to epidote. As accessory phases, zircons and monazite are found. Allanite with epidote rims is found as inclusions in biotite Fig.
Manual de petrologia metamorfica y su contexto geodinamico
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