Producción CyT
XXII Congreso Geológico Argentino - Internal structure of zircon from Mid to Late Devonian volcanic rocks and granites in NW Patagonia
Congreso
Fecha:
2024Editorial y Lugar de Edición:
Asociación Geologica ArgentinaResumen *
INTERNAL STRUCTURE OF ZIRCON FROM MID TO LATE DEVONIAN VOLCANIC ROCKS AND GRANITES IN NW PATAGONIAC. Mark Fanning(1), Carlos W. Rapela(2*), Robert J. Pankhurst(3),, Francisco Hervé(4), Mauricio Calderón(5)(1)Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia(2*)Centro de Investigaciones Geológicas (CIG), CONICET, Universidad Nacional de la Plata, Diagonal 113 Nº 275, 1900 La Plata, Argentina (3)Visiting Research Associate, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK(4)Carrera de Geología, Facultad de Ingeniería, Universidad Andrés Bello, Sazie 2119, Santiago, Chile(5) Facultad de Ingeniería, Universidad del Desarrollo, Av. Plaza 680, Las Condes, Santiago, Chile)(*) E-mail: crapela@cig.museo.unlp.edu.arZircon from volcanic and subvolcanic rocks often has internal structures indicating rapid crystallisation at shallow depths. “Axial worm-like cavities” consistent with trapped vapour phases (e.g., Fanning et al., 1988) occur in Famatinian rhyolites associated with Ordovician K-bentonites in the Precordillera (Fanning et al., 2004). Often overlooked, these structures are found in zircon in high level intrusions as well as in volcanic rocks and ash fall tuffs. They are an important indication, perhaps the only evidence, of volcanic/subvolcanic events registered in detrital zircons. In the proto-Pacific margin of a Devonian accretionary orogen in NW Patagonia, meta-sandstones interbedded with back-arc pillow lavas contain zircon with high Th/U ratios and such internal axial cavities (Hervé et al., 2016; Rapela et al., 2021, 2024). The detrital zircon age patterns have prominent peaks at 388 ± 2 Ma and 392 ± 3 Ma (Rapela et al., 2021). Here we identify coeval cordilleran arc granites in the foreland as a plausible source of these zircons.In the foreland Early to Mid Devonian granites are usually affected by a D2 deformation, whereas a very late Devonian porphyritic biotite granite is undeformed with large K-feldspar phenocrysts. We have examined the internal zircon structures of two representative samples; cathodoluminiscence (CL) images and transmitted light photographs are shown in Fig. 1. FO19096 (Route 234, beside the Río Collón Cura). Most zircons from this weakly foliated biotite granite are euhedral elongate (up to 6.5:1) crystals, some showing pyramidal terminations but most are broken fragments. Central cavities are seen in some grains. Some have length-parallel zoning, with oscillatory zoned tips (e.g., Fig. 1a, grain 19) but others have homogeneous and/or anastomosing central areas (grain 18) that also suggest rapid crystallization. A 206Pb/238U age of 389 ± 3 Ma (Middle Devonian) was interpreted as the time of igneous zircon crystallisation (Rapela et al., 2024). FO19093 (Cordón de la Piedra Santa). Zircons from this porphyritic biotite granite are mainly euhedral elongate crystals, some with pyramidal terminations; others are broken fragments of such grains (Fig. 1c,d). In several grains worm-like central cavities indicate rapid crystallisation at a high crustal level (Fig. 1d). The CL further supports this interpretation, with length-parallel zoning, in places anastomosing, as is common in volcanic to subvolcanic zircon. Grains with pyramidal terminations show oscillatory zoned tips (Fig. 1c, grains 3, 5). A 206Pb/238U age of 357 ± 2 Ma was considered to constrain the main period of igneous zircon crystallisation close to the Devonian–Carboniferous boundary (Rapela et al., 2024). The inferred Early to Middle Devonian geodynamic setting is that of a cordilleran continental arc with deformed granites having a mixed mantle-crust isotopic signature (references above and others therein). Granites of this age interval are conspicuous, and the zircons of a typical sample (FO19096) show internal structures indicating intrusion at a high (subvolcanic) level). An obvious inference is that there would have been significant volumes of coeval volcanic sequences. This is consistent with the occurrence of Devonian detrital zircon with internal axial cavities in the meta-sandstones of the Pacific Domain. The younger undeformed crust-derived granite FO19093 also shows zircon structures indicating high level intrusion. Since there is no present outcrop record of volcanic rocks, they were most probably eroded during uplift of the accretional orogen. This study illustrates the value of volcanic/subvolcanic zircon structures in identifying the source of detrital zircons in sedimentary sequences. Figure 1. Zircon cathodoluminesence and transmitted light images for FO19096 (a, b) and FO19093 (c, d) . Fanning, C. M., Flint, R. B., Parker, A. J., Ludwig, K. R. & Blissett, A. H., 1988. Refined Proterozoic evolution of the Gawler craton, South Australia, through U-Pb zircon geochronology. Precambrian Research 40: 363-386.Fanning, C.M., Pankhurst, R.J., Rapela, C.W., Baldo, E.G., Casquet, C. & Galindo, C., 2004. K-bentonites in the Argentine Precordillera contemporaneous with volcanism in the Famatinian arc. Journal of the Geological Society 161: 747–756.Hervé, F., Calderon, M., Fanning, C.M., Pankhurst, R.J., Fuentes, F., Rapela, C.W. & Marambio, C., 2016. Devonian magmatism in the accretionary complex of southern Chile. Journal of the Geological Society 173: 587–602.Rapela, C.W., Hervé, F., Pankhurst, R.J., Calderón, M., Fanning, C.M., Quezada, P., Poblete, F., Palape, C. & Reyes, T., 2021. The Devonian accretionary orogen of the North Patagonian cordillera. Gondwana Research 96: 1–21.Rapela, C.W., García, M., Hervé, F., Pankhurst, R.J., Calderón, M., Fanning, C.M. & Verdecchia, S.O., 2024. Late Paleozoic magmatism and foreland deformation associated with opening and closing of marginal basins in the North Patagonian Andes. Journal of the Geological Society 181: https://doi.org/10.1144/jgs2023-171.Research was mainly supported by (Chile) ANID Exploration project 13220141 to FH and MC, CONICET grant PUE 229016010083 to CWR and additional research funds from CMF.S04 Magmatismo Sudamericano Información suministrada por el agente en SIGEVAPalabras Clave
Devonian magmatism Degasssification structuresZircon Patagonia