MAGM-5: Metamorfismo En La Corteza
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MAGM-5: Metamorfismo en la corteza Coastal Accretionary Prism of Central Chile (~34°-41°S): Review and preliminary investigation Gonzalo Galaz1, Alvaro Torres1, Matias Nunez2, Paula Rojas1, Mauricio Contreras1. (1) Departamento de Geología, Facultad de Ingeniería, Universidad de Atacama, Copiapó, Chile (2) Departamento de Geología, Facultad de Ingeniería, Universidad Santo Tomás, Santiago, Chile Introduction The Paleozoic metamorphic basement in the Coastal Cordillera of central Chile can be subdivided into two almost continuous and roughly N–S belts: the Western Belt (HP–LT) comprising low temperature–high pressure rocks and the Eastern Belt (LP–HT) comprising high temperature–low pressure rocks. The HP–LT Western Belt is composed of:(i) a Late Cambrian– Devonian passive margin and/or Carboniferous trench-fill turbidites, consisting mainly of low grade psammites and pelites (Hervé, 1988); (ii) low to high grade metabasite lenses derived from the subducting oceanic plate (Willner, 2005; Hyppolito et al., 2004b); (iii) minor serpentinitic lenses formed in a suprasubduction zone setting (González-Jiménez et al., 2014); (iv) local oceanic chert layers (Hervé, 1988); (v) massive sulphide layers (Willner, 2005); and (vi) continental ferruginous high–grade garnet–mica schist (Hyppolito et al., 2014a). On the other hand, the Eastern Beltis lithologically very homogeneous and predominantly made up of (i) latest Devonian–Permian, pelitic–psammitic trench to forearc turbidites (Hervé, 1988); and (ii) rare quartz-rich calcsilicate rocks (Willner, 2005). Several authors have interpreted these metamorphic belts as part of a Carboniferous–Permian accretionary prism (Hervé, 1988; Willner, 2005; Hyppolito et al., 2014a; and references therein). The Eastern Belt represents the shallower levels of an accretionary prism and is considered the less deformed retro–wedge, whereas the Western Belt was derived from the lowermost part of the basal accretionary wedge. Structural characteristics Structures in both belts display a sequence of three sets of structures. They are given different subscripts: numerical in the Eastern Belt (e.g. S1, S2, S3, etc.) and alphabetic in the Western Belt (e.g. Sa, Sb, Sc, etc.). The Eastern Belt shows three phases of penetrative deformation (e.g. Richter et al., 2007; Palape, 2014; Hyppolito et al., 2014a). The earliest structures (D1) are preserved as rare intrafolial, isoclinal, generally upright F1 folds, oriented ~ENE and NNW, which have subhorizontal, NNW–SSE trending, F1 fold axes, a subvertical axial plane, S1 foliation oriented ~ NW– SE and NNW–SSE. The second set of structures (D2) is defined by open to tight, recumbent, SW–vergent, F2 folds, with subhorizontal to E– and ~NE–dipping. This folding produced an associated penetrative axial plane, S2 foliation oriented ~NW–SE/27°N, which in turn generated an L1/2 intersection lineation that plunges 22° to the east. A local third set of structures (D3) was observed in highly deformed zones as recumbent, NW–SE F3 folds associated with a locally developed crenulation S3 foliation. The microstructural analysis of the Western Belt shows three phases of penetrative deformation (e.g. Willner, 2005; Richter et al., 2007; Palape, 2014; Hyppolito et al., 2014a). The first set of structures (Da) occurs as recumbent and isoclinal Fa folds, with axes plunging gently to the ~ENE and ~NW, which have an axial planar Sa foliation, oriented~NE–SW. The second set of structures (Db) is defined by ubiquitous, isoclinal, upright to recumbent, SW–vergent, Fb folds, which have a very penetrative axial planar, ~NW– SE/30°N, Sb foliation. The recumbent and upright Fb folds, have axial planes oriented ~ENE with fold axes trending ~NNW and plunging gently ~NW. The third set of structures (Dc) is defined by NW–SE trending axial planes and W–vergent, open–gentle Fc folds. The Fc folds are accompanied by a penetrative, axial plane, Sc transposition foliation oriented ~NW–SE/50°NE. A ubiquitous, ~WNW, stretching lineation (Lb/c) is parallel to both the Fb and Fc fold axes. Thermal event in the Eastern Belt There are multiple evidences suggesting a thermal event, associated with the intrusion of Paleozoic Batholith, affecting exclusively the rocks that make up the Eastern Belt. This thermal event is evidenced by the presence in samitic-pelitic metasediments of staurolite, andalusite and in some cases cordierite, biotite, sillimanite, garnet and muscovite (Fig. 1). Several authors propose that this thermal event occurred between two dynamic deformational events that affected both metamorphic belts, i.e., after the first deformational event (post-S1 and Sa) and prior to the second deformational event (pre-S2 and Sb)(Aguirre et al., 1972; Hervé et al., 1984; Hyppolito et al., 2015). However, there are differences related to the timing of the formation of certain porphyroblasts associated with the thermal event: (i) andalusite formed syn- and post-S1 (Glodny et al., 2008), (ii) staurolite formed syn- and post-S1 (Hyppolito et al., 2015), y; (iii) andalusite (And2) formed syn- and post-S2 (Hervé et al., 1984). On the other hand, there are some inconsistent theories about the metamorphic-structural evolution of the Eastern Belt, specifically in the related to the timing of the overprinted thermal event: (i) contemporary to the D1 deformation event (syn-S1) that affected both metamorphic belts (Martin et al., 1999), and; (ii) syntectonic and after to the D2 deformation event (syn- and post-S2)(Willner et al., 2005). The major differences in the theories about the genesis and metamorphic-deformational evolution of the Accretionary Prism of Central Chile generated the need to formalize this petrological study in its Eastern Belt. Our preliminary results imply a thermal event that overprints the first deformational event that affected both metamorphic belts, and in turn, was affected by the second deformational event that suffered both belts (post-S1/Sa and pre-S2/Sb). Geodynamics constraints on Accretionary Prism of Central Chile Our petrological- structural data, together with those of previous works, suggests that the Upper Paleozoic tectonic evolution of the Accretionary Prism of Central Chile can be summarized as follows: (i) Early Carboniferous deposition of passive–margin and/or trench–fill psammitic/pelitic sediments on the SW–margin of Gondwana, which make up the Eastern Belt and the slices intercalated with mafic oceanic rocks in the Western Belt (Hervé et al., 2013); (ii) Mississippian–Pennsylvanian common initial frontal accretion of 917 MAGM-5: Metamorfismo en la corteza the wedge associated with very low–grade metamorphism (M1 and Ma: 280–250°C and ~5.5 kbar) and first event deformation (D1, Da), which affected both belts (Willner, 2005; Hyppolito et al., 2014a); (iii) Middle Mississippian deeper burial through the subduction channel of specific HP slices of the Western Belt,which reached the high-grade peak conditions (Mb): (1) a garnet– mica schist and an amphibolite at Punta Sirena locality (~34°40S) at ~13 kbar/580–540°C and ~10 kbar/540–500°C, respectively (Hyppolito et al., 2014a), and; (2) a deepest slice composed of a garnet-amphibolite at Los Pabilos locality (~40°57S) at 16.5–11 kbar/760–600°C (Willner et al., 2004), indicating that these rocks reached eclogite facies; (iv) Lower Pennsylvanian (~320 Ma; Ar/ Ar age) initiation of exhumation by return flow of the deeper HP slices (Willner, 2005; Hyppolito et al., 2014a). The HP slices at Punta Sirena were tectonically juxtaposed during ~316 Ma (Ar/Ar age; Hyppolito et al., 2014a); (v) Upper Pennsylvanian–Lowest Permian emplacement of calcalkaline intrusives that generated a short thermal metamorphic overprint M2 (peak: 720–400°C and 3.5–2.5 kbar), which affected exclusively to the Eastern Belt between 301–296 Ma (Ar/Ar ages; Willner et al., 2005; Hyppolito et al., 2015) or 306–286 Ma (Rb/Sr; Glodny et al., 2008). This magmatic event was contemporaneous at least partly to; (vi) Upper Pennsylvanian–Lowest Permian (305–292 Ma; Ar/Ar ages) subsequent emergence of the HP unit from the subduction channel is then incorporated within the growing accretionary wedge (Willner et al., 2005; Hyppolito et al., 2014a). In this tectonic setting, basal accretion take places associated with a second deformational event (Db), evidencing by pervasive transposition Sb foliation, that led to tectonic juxtaposition between HP and low-grade slices of the Western Belt as is indicated by a common greenschist-facies retrograde overprinting at 4–3 kbar and 400–300°C (Willner, 2005). This is consistent with estimated retrograde conditions in the Eastern Belt (~2.8 kbar and 388–344°C) associated with a second deformational event (D2) associated with an S2 penetrative foliation (Willner, 2005; Hyppolito et al., 2015); (vii) Upper Triassic termination of basal accretion activity suggested by a shallow granite intruded into the Western Belt at ~224 Ma (Pb/Pb age; Willner et al., 2005); Acknowledgments Thanks for the valuable comments provided by Drs. J. D. Keppie (UNAM) and M. Ayaz Alam (Universidad de Atacama). References Aguirre, L., Hervé, F., Godoy, E., 1972. Distribution of metamorphic facies in Chile–an outline. Kristalinikum 9, 7-19. Glodny, J., Echtler, H., Collao, S., Ardiles, M., Burón, P., Figueroa, O., 2008. Differential Late Paleozoic active margin evolution in South-Central Chile (37°S-40°S) - the Lanalhue Fault Zone. Journal of South American Earth Sciences 26, 397-411. González-Jiménez, J.M., Barra, F., Walker, R.J., Reich, M., Gervilla, F., 2014. Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies, Central Chile, International Geology Review 56 (12), 1466-1483. Hervé, F., 1988. Late Paleozoic subduction and accretion in Southern Chile. Episodes 11, 183-188. Hervé, F., Kawashita, K., Munizaga, F., Bassei, M., 1984. Rb–Sr isotopic ages from late Paleozoic metamorphic rocks of central Chile. Journal of the Geological Society of London141, 877-884. Hervé, F., Calderón, M., Faning, C.M., Pankhurst, R.J., Godoy, E., 2013. Provenance variations in the Late Paleozoic accretionary complex of central Chile as indicated by detrital zircons.