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- Cuesta de la chilca (1)
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The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.
At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic-Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Rio Chanarito fault.
The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc- silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M-1 phase to the peak M-3, characterized by the reactions: Qtz + Pl + Bt +/- Ms -> Grt + Bt(2) + Pl(2) +/- Sil +/- Kfs, Qtz + Bt + Sil -> Crd + Kfs and Qtz + Grt + Sil -> Crd. The M-3 assemblage is coeval with the dominant foliation related to a third deformational phase (D-3).
The QMMC, forming the foot wall, is made up of fine-grained banded quartz - biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M-3 peak assemblage is quartz + biotite + plagioclase +/- garnet +/- sillimanite +/- muscovite +/- ilmenite +/- magnetite +/- apatite.
The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D-1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S-1 is nearly parallel to S-0. In the EPMIC, D-2 is represented by the S-2 foliation, related to the F-2 folding that overprints S-1, with dominant strike NNW - SSE and high angles dip to the E. D-3 in the EPMIC have F-3 folds with axis oblique to S-2; the S-3 foliation has striking NW - SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S-2 (D-2) is a discontinuous cleavage oblique to S-1 and transposed by S-3 (D-3), subparallel to S-1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D-2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640 degrees C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D-3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M-3 are 710 degrees C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.
The schists from the QMMC suffered the early low grade M-1 metamorphism with minimum PT conditions of ca 400 degrees C and 0.35 GPa, comparable to the fine schists (M-1) outcropping to the east. The D-2 deformation is associated with the prograde M-2 metamorphism. The penetrative D-3 stage is related to a medium grade metamorphism M-3, with peak conditions at ca 590 degrees C and 0.55 GPa.
The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.
The Big Naryn Complex (BNC) in the East Djetim-Too Range of the Kyrgyz Middle Tianshan block is a tectonized, at least 2 km thick sequence of predominantly felsic to intermediate volcanic rocks intruded by porphyric rhyolite sills. It overlies a basement of metamorphic rocks and is overlain by late Neoproterozoic Djetim-Too Formation sediments; these also occur as tectonic intercalations in the BNC. The up to ca. 1100 m thick Lower Member is composed of predominantly rhyolites-to-dacites and minor basalts, while the at least 900 m thick pyroclastic Upper Member is dominated by rhyolitic-to-dacitic ignimbrites. Porphyric rhyolite sills are concentrated at the top of the Lower Member. A Lower Member rhyolite and a sill sample have LA-ICP-MS U-Pb zircon crystallization ages of 726.1 +/- 2.2 Ma and 720.3 +/- 6.5 Ma, respectively, showing that most of the magmatism occurred within a short time span in the late Tonian-early Cryogenian. Inherited zircons in the sill sample have Neoarchean (2.63, 2.64 Ga), Paleo- (2.33-1.81 Ga), Meso- (1.55 Ga), and Neoproterozoic (ca. 815 Ma) ages, and were derived from a heterogeneous Kuilyu Complex basement. A 1751 +/- 7 Ma Ar-40/Ar-39 age for amphibole from metagabbro is the age of cooling subsequent to Paleoproterozoic metamorphism of the Kuilyu Complex. The large amount of pyroclastic rocks, and their major and trace element compositions, the presence of Neoarchean to Neoproterozoic inherited zircons and a depositional basement of metamorphic rocks point to formation of the BNC in a continental magmatic arc setting.
Mineral resource exploration and mining is an essential part of today's high-tech industry. Elements such as rare-earth elements (REEs) and copper are, therefore, in high demand. Modern exploration techniques from multiple platforms (e.g., spaceborne and airborne), to detect and map the spectral characteristics of the materials of interest, require spectral libraries as an essential reference. They include field and laboratory spectral information in combination with geochemical analyses for validation. Here, we present a collection of REE- and copper-related hyperspectral spectra with associated geochemical information. The libraries contain reflectance spectra from rare-earth element oxides, REE-bearing minerals, copper-bearing minerals and mine surface samples from the Apliki copper-gold-pyrite mine in the Republic of Cyprus. The samples were measured with the HySpex imaging spectrometers in the visible and near infrared (VNIR) and shortwave infrared (SWIR) range (400-2500 nm). The geochemical validation of each sample is provided with the reflectance spectra. The spectral libraries are openly available to assist future mineral mapping campaigns and laboratory spectroscopic analyses. The spectral libraries and corresponding geochemistry are published via GFZ Data Services with the following DOIs: https://doi.org/10.5880/GFZ.1.4.2019.004 (13 REE-bearing minerals and 16 oxide powders, Koerting et al., 2019a), https://doi.org/10.5880/GFZ.1.4.2019.003 (20 copper-bearing minerals, Koellner et al., 2019), and https://doi.org/10.5880/GFZ.1.4.2019.005 (37 copper-bearing surface material samples from the Apliki coppergold-pyrite mine in Cyprus, Koerting et al., 2019b). All spectral libraries are united and comparable by the internally consistent method of hyperspectral data acquisition in the laboratory.