Peri-Gondwanan Provenance and Geodynamic Evolution of The Guadaiza Nappe (Alpujarride Complex, Betic Cordilleras, Spain): Insights on The Paleotethyan Paleogeography

  Citation: Esteban, J.J.; Cuevas, J.; Tubía, J.M. Peri-Gondwanan Provenance and Geodynamic Evolution of The Guadaiza Nappe (Alpujarride Complex, Betic Cordilleras, Spain): Insights on The Paleotethyan Paleogeography. Minerals 2022,12, 325. https:// doi.org/10.3390/min12030325 Academic Editors: Victoria B. Ershova, Artem V. Moiseev, Andrey Khudoley and Manuel Francisco Pereira Received: 20 January 2022 Accepted: 3 March 2022 Published: 5 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article Peri-Gondwanan Provenance and Geodynamic Evolution of The Guadaiza Nappe (Alpujarride Complex, Betic Cordilleras, Spain): Insights on The Paleotethyan Paleogeography JoséJulián Esteban *, Julia Cuevas and JoséMaría Tubía Department of Geology, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Apto. 644, 48080 Bilbao, Spain; [email protected] (J.C.); [email protected] (J.M.T.) *Correspondence: [email protected]; Tel.: +34-946-01-24-53 Abstract: Based on the LA-ICP-MS U-Pb zircon ages of four metamorphic samples, we discuss the geochronology and provenance of the Guadaiza nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras, Spain). The Guadaiza nappe is composed of Triassic marbles overlying a metapelitic sequence with schists and migmatites. Zircons from a quartzite interlayered with the marbles yield a maximum depositional age of ca. 289 Ma that supports the Triassic age. The idiomorphic morphology of these Paleozoic zircon crystals and the lack of late-Variscan metamorphism (ca. 300 Ma) supports a proximal source area, and suggests that the marbles were discordantly deposited over the metapelitic sequence, along the northern margin of the Alboran microplate. The zircon patterns from the metapelitic sequence mainly yield Paleoproterozoic (ca. 1.6–2.5 Ga), Tonian–Stenian (ca. 1000 Ma), Ediacaran–Cryogenian (ca. 600 Ma) and Paleozoic (ca. 500 Ma) age clusters. These results suggest provenance from areas within the triangle bound by the West African Craton, the Metasaharan Craton and the Hun Superterrane during the Paleotethys opening (Silurian–Devonian). A Silurian–Carboniferous deposition age for the schist protoliths is constrained by the youngest detrital zircon population (ca. 443 Ma) and the Variscan age of their migmatization by an additional peak of around 299 Ma in the migmatites. Keywords: peri-Gondwana; Betic Cordilleras; Guadaiza nappe; detrital zircon; LA-ICP-MS 1. Introduction In the last decade, the technological improvement of the geochronological techniques (LA-ICP-MS, SHRIMP, SIMS, among others) and the facilities to acquire individual zircon data from sedimentary and metamorphic rocks, as the U-Pb age determination can supply the maximum depositional age or the tectonothermal events, means they have become the perfect geological tool to decipher the paleogeographic and geodynamic evolution of ancient terranes. Some of these terranes, such as the Betic Cordilleras, Kabylian, Peloritanian and Calabrian belts scattered around the Alboran Sea (western Mediterranean), form the so-called Alboran microplate [ 1 ], AlKaPeCa microplate [ 2 ], Mesomediterranean terrain [ 3 ] or Alboran Domain [ 4 ] that drifted away from northern Gondwana during the Paleotethys aperture (Silurian–Devonian). However, although it has been concluded that it conforms to a peri-Gondwanan terrane, e.g., [ 5 – 10 ], the scarcity of geochronological detrital zircon age data, the variable imprint of the Alpine tectonics that erased the vestiges of previous orogenic events, and, in other cases, the fact that the metamorphic rocks that form the basement have been dated as undifferentiated Paleozoic, makes its paleogeographic location still controversial. On this matter, some authors have suggested that during Cambrian and Ordovician times the Alboran microplate should be located in a central position of the Galatian Superterrane (formerly referred to as the Hun Superterrane, [ 5 , 11 , 12 ]), lying in between the Aquitaine and the intra-Alpine terranes [ 11 , 13 ]. Instead, other authors have Minerals 2022,12, 325. https://doi.org/10.3390/min12030325 https://www.mdpi.com/journal/minerals Minerals 2022,12, 325 2 of 16 suggested a westernmost starting localization of the Alboran microplate, in direct relationship with the Ossa-Morena (SW Iberian Massif) and Moroccan Meseta terranes [ 14 , 15 ]. Therefore, the acquisition of new geochronological data will help in providing a better understanding of the tectonic evolution of the Alpine orogenic segments that surround the Alboran Sea. In this work, we present new U-Pb LA-ICP-MS detrital zircon data from four metamorphic samples of the Guadaiza nappe (western Internal Zone of the Betic Cordilleras, Spain), in order to discuss their geochronology and provenance. The presence of two different migmatization events makes the Guadaiza nappe stand out [ 16 , 17 ] as an interesting target to search for the interrelationship between old and new orogenic events. Moreover, the location of the Guadaiza nappe in the Internal Zones under the ultramafic rocks of the Betic and Rif orogens adds additional interest to the explanation of the obtained results in the oldest orogenies (Variscan, Caledonian, etc.) of the western Mediterranean. 2. Geological Setting The Guadaiza nappe, one of the nappes that forms the western part of the Alpujarride Complex (Internal Zones of the Betic Cordillera), crops out in two main areas (Figure 1): the Albornoque sector and the Guadaiza tectonic window, surrounded by the largest worldwide exposure of subcontinental lithospheric mantle, the Ronda peridotites. Other minor outcrops of Guadaiza nappe are found in the northern part of the studied region, as small tectonic windows or along the thrust contact between the Ronda peridotites and the Triassic limestones (locally marbles) of the Sierra de Las Nieves. The peridotites also rest over some other continental crustal sequences composed mainly of gneisses, metapelites and marbles belonging to the Ojen nappe and Yunquera Unit. Minerals 2022, 12, x 2 of 16 other authors have suggested a westernmost starting localization of the Alboran microplate, in direct relationship with the Ossa-Morena (SW Iberian Massif) and Moroccan Meseta terranes [14,15]. Therefore, the acquisition of new geochronological data will help in providing a better understanding of the tectonic evolution of the Alpine orogenic segments that surround the Alboran Sea. In this work, we present new U-Pb LA-ICP-MS detrital zircon data from four metamorphic samples of the Guadaiza nappe (western Internal Zone of the Betic Cordilleras, Spain), in order to discuss their geochronology and provenance. The presence of two different migmatization events makes the Guadaiza nappe stand out [16,17] as an interesting target to search for the interrelationship between old and new orogenic events. Moreover, the location of the Guadaiza nappe in the Internal Zones under the ultramafic rocks of the Betic and Rif orogens adds additional interest to the explanation of the obtained results in the oldest orogenies (Variscan, Caledonian, etc.) of the western Mediterranean. 2. Geological Setting The Guadaiza nappe, one of the nappes that forms the western part of the Alpujarride Complex (Internal Zones of the Betic Cordillera), crops out in two main areas (Figure 1): the Albornoque sector and the Guadaiza tectonic window, surrounded by the largest worldwide exposure of subcontinental lithospheric mantle, the Ronda peridotites. Other minor outcrops of Guadaiza nappe are found in the northern part of the studied region, as small tectonic windows or along the thrust contact between the Ronda peridotites and the Triassic limestones (locally marbles) of the Sierra de Las Nieves. The peridotites also rest over some other continental crustal sequences composed mainly of gneisses, metapelites and marbles belonging to the Ojen nappe and Yunquera Unit. Figure 1. Geological map of the Guadaiza nappe (modified from Esteban et al. [16]) and analysed sample locations (stars). The lithological sequence of the Guadaiza nappe (Figure 1) is characterized by an undifferentiated Paleozoic metapelitic sequence with interlayered quartzites, schists and minor amphibolites [17]. This sequence is mainly characterized by abundant folded veins of quartz (Figure 2a) and a well-developed axial plane schistosity, S2. The metamorphic Figure 1. Geological map of the Guadaiza nappe (modified from Esteban et al. [ 16 ]) and analysed sample locations (stars). The lithological sequence of the Guadaiza nappe (Figure 1) is characterized by an undifferentiated Paleozoic metapelitic sequence with interlayered quartzites, schists and minor amphibolites [ 17 ]. This sequence is mainly characterized by abundant folded veins of quartz (Figure 2a) and a well-developed axial plane schistosity, S 2 . The metamorphic Minerals 2022,12, 325 3 of 16 conditions of the metapelitic sequence range from the andalusite zone at upper levels to the sillimanite zone close to the underlaying Istan migmatites [17]. This member includes gneisses, diatexites, metatexites and medium to fine-grained leucogranites. Over the metapelitic sequence, coarse-grained layered marbles complete the lithological sequence of the Guadaiza nappe. The Triassic age is commonly assigned to these marbles, based on stratigraphic correlations with the less metamorphic Alpujarride units of the eastern Internal Zone of the Betic Cordillera [18,19]. Minerals 2022, 12, x 3 of 16 conditions of the metapelitic sequence range from the andalusite zone at upper levels to the sillimanite zone close to the underlaying Istan migmatites [17]. This member includes gneisses, diatexites, metatexites and medium to fine-grained leucogranites. Over the metapelitic sequence, coarse-grained layered marbles complete the lithological sequence of the Guadaiza nappe. The Triassic age is commonly assigned to these marbles, based on stratigraphic correlations with the less metamorphic Alpujarride units of the eastern Internal Zone of the Betic Cordillera [18,19]. Figure 2. Field images of studied sample outcrops: (A) schists and quartzites of the metapelitic sequence (tb-2009-7), (B) Istan migmatites (tb-07-845), (C) interlayered quartzite within coarse-grained marbles (tb-18-06) and (D) Hoyo del Bote mylonitic migmatite (Gu-35-2) with elongated white quartz nodules and metapelitic restites (dark grey) that define the mylonitic foliation (SMyl). The Guadaiza nappe shows two main anatectic events recognized from tectonic arguments [17]. The oldest one is recorded by the Istan migmatites (Figure 2b) and is related to the pre-alpine regional metamorphism of the metapelitic sequence. Performed geochronological studies with the 207Pb/206Pb sequential evaporation technique [20] and U/Pb SHRIMP dating of metamorphic zircons [21] yield Variscan and late-Variscan ages of 319 ± 8 Ma and 299 ± 4 Ma, respectively. The youngest event is represented by the migmatites concentrated around a dynamothermal aureole associated with the emplacement of the overlying Ronda peridotites (Figure 1). This dynamothermal aureole, named as Hoyo del Bote migmatite [17], forms a 100–200-m-thick layer parallel to the lower contact of the peridotite sheet that cuts obliquely all the materials, including the Istan migmatites [16,22] and deflects the previous structures (S1, S2) developed in the metapelitic sequence of the Guadaiza nappe. This dynamothermal aureole is interpreted as a low-viscosity shear zone formed at temperatures high enough, ≈725 °C, to promote the partial melting of the underlaying metapelitic sequence during the hot emplacement of the Ronda peridotites at Alpine times [23]. It shows an anatexis and strain increase toward the contact to the peridotite slab. Its lower portion contains unfoliated stromatic migmatites that are progressively transformed into nebulitic migmatites rich in rounded blocks of quartz and angular restites from the metamorphic sequence and even the marbles [17]. Toward the peridotites, the coarse-grained and unfoliated migmatites are progressively transformed into protomylonites that give way to S/C mylonites and, finally, ultramylonites. The age of this second migmatization has been constrained by U-Pb SHRIMP dating on zircons at 22 Ma [21] and 22.3 ± 0.7 Ma [23]. Other additional geochronological data come from the granite Figure 2. Field images of studied sample outcrops: ( A ) schists and quartzites of the metapelitic sequence (tb-2009-7), ( B ) Istan migmatites (tb-07-845), ( C ) interlayered quartzite within coarsegrained marbles (tb-18-06) and ( D ) Hoyo del Bote mylonitic migmatite (Gu-35-2) with elongated white quartz nodules and metapelitic restites (dark grey) that define the mylonitic foliation (SMyl). The Guadaiza nappe shows two main anatectic events recognized from tectonic arguments [ 17 ]. The oldest one is recorded by the Istan migmatites (Figure 2b) and is related to the pre-alpine regional metamorphism of the metapelitic sequence. Performed geochronological studies with the 207 Pb/ 206 Pb sequential evaporation technique [ 20 ] and U/Pb SHRIMP dating of metamorphic zircons [ 21 ] yield Variscan and late-Variscan ages of 319 ±8 Ma and 299 ± 4 Ma, respectively. The youngest event is represented by the migmatites concentrated around a dynamothermal aureole associated with the emplacement of the overlying Ronda peridotites (Figure 1). This dynamothermal aureole, named as Hoyo del Bote migmatite [ 17 ], forms a 100–200-m-thick layer parallel to the lower contact of the peridotite sheet that cuts obliquely all the materials, including the Istan migmatites [ 16 , 22 ] and deflects the previous structures (S 1 , S 2 ) developed in the metapelitic sequence of the Guadaiza nappe. This dynamothermal aureole is interpreted as a lowviscosity shear zone formed at temperatures high enough, ≈ 725 ◦ C, to promote the partial melting of the underlaying metapelitic sequence during the hot emplacement of the Ronda peridotites at Alpine times [ 23 ]. It shows an anatexis and strain increase toward the contact to the peridotite slab. Its lower portion contains unfoliated stromatic migmatites that are progressively transformed into nebulitic migmatites rich in rounded blocks of quartz and angular restites from the metamorphic sequence and even the marbles [ 17 ]. Toward the peridotites, the coarse-grained and unfoliated migmatites are progressively transformed into protomylonites that give way to S/C mylonites and, finally, ultramylonites. The age of this second migmatization has been constrained by U-Pb SHRIMP dating on zircons at Minerals 2022,12, 325 4 of 16 22 Ma [ 21 ] and 22.3 ± 0.7 Ma [ 23 ]. Other additional geochronological data come from the granite dykes developed from partial melts arising from the dynamothermal aureole. The granite dykes that intrude into the peridotites also date the anatexis and dynamothermal aureole at around 22 Ma [23,24]. 3. Sample Location and Description Four samples from the Guadaiza nappe (Figure 1) were selected for this study. The samples were collected from the previously described members of the Guadaiza nappe: (1) a schist from the metapelitic sequence (sample tb-2009-7; Figure 2a), (2) a migmatite from the Istan migmatites (sample tb-07-845; Figure 2b), (3) a quartzite interlayered with the coarse-grained Triassic marbles (sample tb-18-06; Figure 2c) and (4) a mylonitic migmatite from the Hoyo del Bote migmatite (sample Gu-35-2; Figure 2d). The sample tb-2009-7, a fine-grained dark schist located at the upper levels of the metapelitic sequence of the Guadaiza nappe (Figure 1), comes from a strongly deformed zone with a penetrative S 2 -schistosity. S 2 is parallel to the axial plane of tight folds that deform veins of white quartz and an older S 1 -schistosity (Figure 2a), defined by quartz, biotite and graphite. Idioblastic porphyroblasts of andalusite randomly grow on S2(Figure 3a). Minerals 2022, 12, x 4 of 16 dykes developed from partial melts arising from the dynamothermal aureole. The granite dykes that intrude into the peridotites also date the anatexis and dynamothermal aureole at around 22 Ma [23,24]. 3. Sample Location and Description Four samples from the Guadaiza nappe (Figure 1) were selected for this study. The samples were collected from the previously described members of the Guadaiza nappe: (1) a schist from the metapelitic sequence (sample tb-2009-7; Figure 2a), (2) a migmatite from the Istan migmatites (sample tb-07-845; Figure 2b), (3) a quartzite interlayered with the coarse-grained Triassic marbles (sample tb-18-06; Figure 2c) and (4) a mylonitic migmatite from the Hoyo del Bote migmatite (sample Gu-35-2; Figure 2d). The sample tb-2009-7, a fine-grained dark schist located at the upper levels of the metapelitic sequence of the Guadaiza nappe (Figure 1), comes from a strongly deformed zone with a penetrative S2-schistosity. S2 is parallel to the axial plane of tight folds that deform veins of white quartz and an older S1-schistosity (Figure 2a), defined by quartz, biotite and graphite. Idioblastic porphyroblasts of andalusite randomly grow on S2 (Figure 3a). Figure 3. Textural photomicrographs of the analysed samples. (A) Detail of relations in schists between the S2 parallel to the axial plane of microfolds that deform an older S1-schistosity. The schistosity is defined by quartz biotite and graphite. Idioblastic porphyroblast of andalusite (And) growth on S2. Plane-polarized light (sample tb-2009-7). (B) Polymineralic sillimanite (Sil) flakes and parallel arrangement of quartz and plagioclase in the sample tb-07-845 from the Istan migmatites. Crossed polars. (C) Microscopic fabric of sample tb-18-06, a quartzite interlayered with marbles, showing a penetrative schistosity, S2, marked by the preferred orientation of recrystallized grains of quartz and muscovite. Crossed polars. (D) Idioblastic porphyroblast of plagioclase with inclusions of garnet and biotite. Cordierite (Crd) porphyroblasts and mosaic textures in recrystallized quartz indicate a high temperature deformation of sample Gu-35-2 (Hoyo del Bote migmatite). Crossed polars. Tb-07-845 is a sample from the Istan migmatites that corresponds to the deepest and most metamorphic zone of the Guadaiza nappe and only crops out in the eastern part of the studied zone (Figure 1). Despite its small areal extent, the Istan migmatites display Figure 3. Textural photomicrographs of the analysed samples. ( A ) Detail of relations in schists between the S 2 parallel to the axial plane of microfolds that deform an older S 1 -schistosity. The schistosity is defined by quartz biotite and graphite. Idioblastic porphyroblast of andalusite (And) growth on S 2 . Plane-polarized light (sample tb-2009-7). ( B ) Polymineralic sillimanite (Sil) flakes and parallel arrangement of quartz and plagioclase in the sample tb-07-845 from the Istan migmatites. Crossed polars. ( C ) Microscopic fabric of sample tb-18-06, a quartzite interlayered with marbles, showing a penetrative schistosity, S 2 , marked by the preferred orientation of recrystallized grains of quartz and muscovite. Crossed polars. ( D ) Idioblastic porphyroblast of plagioclase with inclusions of garnet and biotite. Cordierite (Crd) porphyroblasts and mosaic textures in recrystallized quartz indicate a high temperature deformation of sample Gu-35-2 (Hoyo del Bote migmatite). Crossed polars. Tb-07-845 is a sample from the Istan migmatites that corresponds to the deepest and most metamorphic zone of the Guadaiza nappe and only crops out in the eastern part of the studied zone (Figure 1). Despite its small areal extent, the Istan migmatites display Minerals 2022,12, 325 5 of 16 great structural diversity, as indicated by the presence of highly differentiated dykes and pods of pegmatite within stromatic or phlebitic migmatite (Figure 2b). These rocks coexist with many restitic zones where sillimanite, biotite and graphite are concentrated. The analysed sample, tb-07-854, shows polymineralic sillimanite flakes that enhance a weak foliation that is defined by the parallel arrangement of slightly elongated grains of quartz and plagioclase (Figure 3b). Interlayered within the coarse-grained marbles that crown the rock sequence of the Guadaiza nappe (Figure 1), the sample tb-18-06 corresponds to a fine-grained quartzite layer about four meters thick (Figure 2c). Both marbles and quartzites show a well-defined foliation, S 2 , related to isoclinal folds. The microscopic fabric exhibits a penetrative schistosity, S 2 , marked by the preferred orientation of recrystallized grains of quartz and muscovite (Figure 3c). Sample Gu-35-2 belongs to the Hoyo del Bote migmatites that correspond to the dynamothermal aureole formed during the hot thrusting of the Ronda peridotites over the Guadaiza nappe [ 17 ]. These migmatites are mediumto coarse-grained diatexite and nebulite that are nearly isotropic and rich in cordierite and feldspar porphyroblasts. They are easily recognizable in the field by the presence of large restitic blocks of white quartz, schist and marbles from the lithological metapelitic sequence of the Guadaiza nappe. Close to the overlying Ronda peridotites, the Hoyo del Bote migmatites have developed a mylonitic foliation, S Myl , that is enhanced by the elongation of the restitic blocks ([ 22 ], Figure 2d). The characteristic metamorphic association of these migmatites consists of K-feldspar + plagioclase + biotite + cordierite. The porphyroblasts of K-feldspar usually display poikilitic texture, defined by inclusions of cordierite, quartz, sillimanite, biotite or garnet in random orientation (Figure 3d). 4. Analytical Methods Zircon crystals were separated by conventional mineral separation techniques (crushing, sieving under 125 µ m, heavy liquids and magnetic separation) in the Department of Geology of the University of the Basque Country. Zircon crystals were handpicked aleatory, mounted in epoxy resin and polished approximately to half of their thickness. The internal structure of the zircon crystals was recognized in backscattered electron (BSE) images, obtained using a scanning electron microscope (JEOL7000F-JSM; operative conditions 20 kV, 3 nA and WD 10 mm). Zircon crystals were analysed by LA-ICP-MS at the University of the Basque Country (SGIker-Geochronology and Isotope Geochemistry Facility) using a 193 nm RESOlution SE laser with a pulse energy density of ~6 J cm −2 and a frequency of 5 Hz coupled to a Thermo Fisher iCAP Qc quadrupole ICP-MS. The analytical spot size was 30 or 24 µ m, depending on zircon size, and in most of the spots the zircon crystals were completely pierced through. Data reduction was carried out using GJ-1 zircon standard for calibration [ 25 ] and Plesovice [ 26 ] and 91500 zircon [ 27 ] as secondary standards (Supplementary Material Table S1). The laboratory staff reduced the data using the Iolite 3.6 software package [ 28 , 29 ] and Vizual Age [ 30 ]. Ages in the text and the figures are quoted always with concordance between 90% and 110% and as 206 Pb/ 238 U ages. TeraWasserburg diagrams and weighted average ages were produced using Isoplot/Ex 3.0 [ 31 ], the distribution of zircon ages and the Kernel Density Estimation (KDE) plots, with bin widths of 60 and 30, respectively, were calculated using DensityPlotter 8.5 [32]. 5. LA-ICP-MS Results 5.1. Sample tb-2009-7 (Schist) Two hundred and one zircon crystals were analysed, of which only one hundred and seventy-four, with Concordia values ranging between 90% and 110%, were considered (Supplementary Material Table S1). Mostly, analysed zircon crystals are xenomorphic and display rounded morphologies (Figure 4). BSE images shows zircon crystals with single and composite structure (cores overgrown by rims). The cores display rounded, eroded and resorbed morphologies with mostly structureless or patchy zonings, while the rims display oscillatory and homogenous zoning. Single zircon crystals have oscillatory Minerals 2022,12, 325 6 of 16 and mainly homogenous zonings. Measured Th/U ratios are between 1.71 and 0.01. Mostly, zircons display Th/U ratios > 0.1 (n − 163) and only 11 zircon analyses have Th/U ratios < 0.1 (n −11) (Figure 5). Minerals 2022, 12, x 6 of 16 and resorbed morphologies with mostly structureless or patchy zonings, while the rims display oscillatory and homogenous zoning. Single zircon crystals have oscillatory and mainly homogenous zonings. Measured Th/U ratios are between 1.71 and 0.01. Mostly, zircons display Th/U ratios > 0.1 (n − 163) and only 11 zircon analyses have Th/U ratios < 0.1 (n − 11) (Figure 5). Figure 4. Backscattered electron images and optical photomicrographs of analysed zircons from tb2009-7 sample (schist). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). Figure 5. U vs Th compositional diagram of analysed zircons. Although age distribution patterns of the analysed zircons are scattered from 256 to 2704 Ma (Figure 6a), about 79% of them yield Precambrian ages that mostly group around Neoproterozoic (50%; ca. 551–998 Ma) and Paleoproterozoic (20%; ca. 1.6–2.5 Ga) peaks, with minor Mesoproterozoic (6%; ca. 1.0–1.1 Ga) and Archean (3%; ca. 2.5–2.7 Ga) populations. The remaining 21% analyses correspond to Paleozoic ages. Three main populations can be peaked from KDE plots at ~ 500 Ma, ~ 600 Ma and ~ 1 Ga (Figure 6a). The calculated weighted average 206Pb/238U ages of the populations result in mean values of 494  3 Ma (n − 8), 599  3 Ma (n − 13) and 1010  5 Ma (n − 6). A weighted mean age of 443  4 Ma arising from the youngest four zircon crystals can be considered as the Figure 4. Backscattered electron images and optical photomicrographs of analysed zircons from tb-2009-7 sample (schist). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). Minerals 2022, 12, x 6 of 16 and resorbed morphologies with mostly structureless or patchy zonings, while the rims display oscillatory and homogenous zoning. Single zircon crystals have oscillatory and mainly homogenous zonings. Measured Th/U ratios are between 1.71 and 0.01. Mostly, zircons display Th/U ratios > 0.1 (n − 163) and only 11 zircon analyses have Th/U ratios < 0.1 (n − 11) (Figure 5). Figure 4. Backscattered electron images and optical photomicrographs of analysed zircons from tb2009-7 sample (schist). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). Figure 5. U vs Th compositional diagram of analysed zircons. Although age distribution patterns of the analysed zircons are scattered from 256 to 2704 Ma (Figure 6a), about 79% of them yield Precambrian ages that mostly group around Neoproterozoic (50%; ca. 551–998 Ma) and Paleoproterozoic (20%; ca. 1.6–2.5 Ga) peaks, with minor Mesoproterozoic (6%; ca. 1.0–1.1 Ga) and Archean (3%; ca. 2.5–2.7 Ga) populations. The remaining 21% analyses correspond to Paleozoic ages. Three main populations can be peaked from KDE plots at ~ 500 Ma, ~ 600 Ma and ~ 1 Ga (Figure 6a). The calculated weighted average 206Pb/238U ages of the populations result in mean values of 494  3 Ma (n − 8), 599  3 Ma (n − 13) and 1010  5 Ma (n − 6). A weighted mean age of 443  4 Ma arising from the youngest four zircon crystals can be considered as the Figure 5. U vs. Th compositional diagram of analysed zircons. Although age distribution patterns of the analysed zircons are scattered from 256 to 2704 Ma (Figure 6a), about 79% of them yield Precambrian ages that mostly group around Neoproterozoic (50%; ca. 551–998 Ma) and Paleoproterozoic (20%; ca. 1.6–2.5 Ga) peaks, with minor Mesoproterozoic (6%; ca. 1.0–1.1 Ga) and Archean (3%; ca. 2.5–2.7 Ga ) populations. The remaining 21% analyses correspond to Paleozoic ages. Three main populations can be peaked from KDE plots at ~500 Ma, ~600 Ma and ~1 Ga (Figure 6a). The calculated weighted average 206 Pb/ 238 U ages of the populations result in mean values of 494 ± 3 Ma (n − 8), 599 ± 3 Ma (n − 13) and 1010 ± 5 Ma (n − 6). A weighted mean age of 443 ± 4 Ma arising from the youngest four zircon crystals can be considered as the Minerals 2022,12, 325 7 of 16 youngest zircon population. Th/U analyses with values < 0.1 are mainly concentrated around the zircon age population of ~600 Ma (Figure 7). Minerals 2022, 12, x 7 of 16 youngest zircon population. Th/U analyses with values <0.1 are mainly concentrated around the zircon age population of ~ 600 Ma (Figure 7). Figure 6. Tera-Wasserburg, Kernel Density Estimation (KDE) plots and histograms of age distributions of analysed zircons of Guadaiza’s samples. (A) tb-2009-1, (B) tb-07-845 and (C) Gu-35-2. Figure 6. Tera-Wasserburg, Kernel Density Estimation (KDE) plots and histograms of age distributions of analysed zircons of Guadaiza’s samples. (A) tb-2009-1, (B) tb-07-845 and (C) Gu-35-2. Minerals 2022,12, 325 8 of 16 Minerals 2022, 12, x 8 of 16 Figure 7. Th/U vs 206Pb/238U Age (Ma) of analysed zircons of Guadaiza nappe´s samples. 5.2. Sample tb-07-845 (Istan Migmatite) Two hundred and forty-seven zircons were analysed by LA-ICP-MS and only one hundred and ninety-four yielded concordant ages between 156 Ma and 3009 Ma (Supplementary Material Table S1). Analysed zircon crystals are idiomorphic and display prismatic or bipyramidal morphologies (Figure 8). Most of the zircons display a composite core–rim structure. The composite zircons have an eroded or resorbed detrital inclusionsfree core overgrown by a lighter and a concentric poikilitic rim structure (Figure 8). Single zircon crystals display mainly continuous oscillatory or homogenous zonings. Although the measured Th/U ratios vary widely between 1.312 and 0.01 (Figure 5), most analyses (n − 142) yield Th/U ratios >0.1 and 52 zircon analyses have Th/U ratios <0.1. Age distribution patterns (Figure 6b) show that 48% of the analysed zircons are Paleozoic and that 3% have a Mesozoic origin. There are 49% that are Precambrian in age and mostly yield Neoproterozoic (30%; ca. 548–998 Ma) and Paleoproterozoic (12%; ca. 1.7–2.5 Ga) ages, with the rest being Mesoproterozoic (5%; ca. 1.0–1.6 Ga) and Archean (2%; 2.6–3.0 Ga). Four main populations are peaked at ~ 300 Ma, ~ 500 Ma, ~ 600 Ma and ~1 Ga (Figure 6b). The calculated weighted average 206Pb/238U ages of the populations raises values of 299  2 Ma (n − 10), 500  4 Ma (n − 9), 595  4 Ma (n − 13) and 1003  9 Ma (n − 6). Th/U analyses with values <0.1 are concentrated within ~ 300 and ~ 600 Ma aged zircon populations (Figure 7). Figure 7. Th/U vs. 206Pb/238U Age (Ma) of analysed zircons of Guadaiza nappe’s samples. 5.2. Sample tb-07-845 (Istan Migmatite) Two hundred and forty-seven zircons were analysed by LA-ICP-MS and only one hundred and ninety-four yielded concordant ages between 156 Ma and 3009 Ma (Supplementary Material Table S1). Analysed zircon crystals are idiomorphic and display prismatic or bipyramidal morphologies (Figure 8). Most of the zircons display a composite core–rim structure. The composite zircons have an eroded or resorbed detrital inclusionsfree core overgrown by a lighter and a concentric poikilitic rim structure (Figure 8). Single zircon crystals display mainly continuous oscillatory or homogenous zonings. Although the measured Th/U ratios vary widely between 1.312 and 0.01 (Figure 5), most analyses (n −142) yield Th/U ratios >0.1 and 52 zircon analyses have Th/U ratios < 0.1. Minerals 2022, 12, x 9 of 16 Figure 8. BSE images and optical photomicrographs of analysed zircons from tb-07-845 sample (migmatite). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). 5.3. Sample Gu-35-2 (Hoyo Del Bote Migmatite) One hundred and seventy-five zircons were analysed by LA-ICP-MS. As in the Istan migmatite sample, the analysed zircon crystals are idiomorphic and display mainly bipyramidal morphologies (Figure 9). Single and composite core–rim structure zircons are observed. The crystals with composite structure have eroded and resorbed cores and sometimes are overgrown by a light and thin rim. Due to their thinness these rims could not be dated by LA-ICP-MS. Measured Th/U ratios are in between 2.4 and 0.01. Most zircons display Th/U ratios >0.1 (n − 128) and only 13 zircon analyses have Th/U ratios < 0.1 (Figure 5). Age distribution patterns (Figure 6c) from 192 to 3019 Ma show that 79% of the analysed zircons are Precambrian in age and mostly yield Neoproterozoic (55%; ca. 543–1000 Ma) and Paleoproterozoic (18%; ca. 1.6–2.4 Ga) ages, with the rest being Mesoproterozoic (4%; ca. 1.0–1.6 Ga) and Archean (2%; ca. 2.5–3.0 Ga). The remaining analyses give Paleozoic (19%; ca. 278–524 Ma) or Mesozoic (2%; ca. 192–206 Ma) ages. Two main populations can be peaked at 585  3 Ma (n − 9) and 978  7 Ma (n − 5). Th/U analyses with values <0.1 mainly come from the zircon population aged at 585  3 Ma (Figure 7). Figure 8. Cont. Minerals 2022,12, 325 9 of 16 Minerals 2022, 12, x 9 of 16 Figure 8. BSE images and optical photomicrographs of analysed zircons from tb-07-845 sample (migmatite). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). 5.3. Sample Gu-35-2 (Hoyo Del Bote Migmatite) One hundred and seventy-five zircons were analysed by LA-ICP-MS. As in the Istan migmatite sample, the analysed zircon crystals are idiomorphic and display mainly bipyramidal morphologies (Figure 9). Single and composite core–rim structure zircons are observed. The crystals with composite structure have eroded and resorbed cores and sometimes are overgrown by a light and thin rim. Due to their thinness these rims could not be dated by LA-ICP-MS. Measured Th/U ratios are in between 2.4 and 0.01. Most zircons display Th/U ratios >0.1 (n − 128) and only 13 zircon analyses have Th/U ratios < 0.1 (Figure 5). Age distribution patterns (Figure 6c) from 192 to 3019 Ma show that 79% of the analysed zircons are Precambrian in age and mostly yield Neoproterozoic (55%; ca. 543–1000 Ma) and Paleoproterozoic (18%; ca. 1.6–2.4 Ga) ages, with the rest being Mesoproterozoic (4%; ca. 1.0–1.6 Ga) and Archean (2%; ca. 2.5–3.0 Ga). The remaining analyses give Paleozoic (19%; ca. 278–524 Ma) or Mesozoic (2%; ca. 192–206 Ma) ages. Two main populations can be peaked at 585  3 Ma (n − 9) and 978  7 Ma (n − 5). Th/U analyses with values <0.1 mainly come from the zircon population aged at 585  3 Ma (Figure 7). Figure 8. BSE images and optical photomicrographs of analysed zircons from tb-07-845 sample (migmatite). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). Age distribution patterns (Figure 6b) show that 48% of the analysed zircons are Paleozoic and that 3% have a Mesozoic origin. There are 49% that are Precambrian in age and mostly yield Neoproterozoic (30%; ca. 548–998 Ma) and Paleoproterozoic (12%; ca. 1.7–2.5 Ga) ages, with the rest being Mesoproterozoic (5%; ca. 1.0–1.6 Ga) and Archean (2%; 2.6–3.0 Ga). Four main populations are peaked at ~300 Ma, ~500 Ma, ~600 Ma and ~1 Ga (Figure 6b). The calculated weighted average 206 Pb/ 238 U ages of the populations raises values of 299 ± 2 Ma (n − 10), 500 ± 4 Ma (n − 9), 595 ± 4 Ma (n − 13) and 1003 ±9 Ma (n − 6). Th/U analyses with values < 0.1 are concentrated within ~ 300 and ~ 600 Ma aged zircon populations (Figure 7). 5.3. Sample Gu-35-2 (Hoyo Del Bote Migmatite) One hundred and seventy-five zircons were analysed by LA-ICP-MS. As in the Istan migmatite sample, the analysed zircon crystals are idiomorphic and display mainly bipyramidal morphologies (Figure 9). Single and composite core–rim structure zircons are observed. The crystals with composite structure have eroded and resorbed cores and sometimes are overgrown by a light and thin rim. Due to their thinness these rims could not be dated by LA-ICP-MS. Measured Th/U ratios are in between 2.4 and 0.01. Most zircons display Th/U ratios > 0.1 (n − 128) and only 13 zircon analyses have Th/U ratios < 0.1 (Figure 5). Minerals 2022, 12, x 10 of 16 Figure 9. BSE images and optical photomicrographs of analysed zircons from Gu-35-2 sample (mylonitic migmatite). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). 5.4. Sample tb-18-06 (Quartzite Interlayered with Marbles) One hundred and seventy-two zircon crystals were analysed and only one hundred and thirty-four were considered (Supplementary Material Table S1). While zircon crystals mainly show idiomorphic shapes in the migmatites (Hoyo del Bote and Istan) and xenomorphic in the schists, the zircon crystals in the quartzite of the Guadaiza nappe display both xenomorphic and idiomorphic zircon populations in similar proportions (Figure 10). The xenomorphic zircon population display a composite structure but most are a single structure. The latter display homogenous or oscillatory zonings, whereas those with a composite structure display a rounded or resorbed core with homogeneous or oscillatory zoning overgrown by a thin and lighter rim. Measured Th/U ratios are in between 0.02– 3.9 and 0.01–0.56 for the xenomorphic and idiomorphic zircon populations, respectively (Figure 5). Figure 10. BSE images and optical photomicrographs of analysed zircons from tb-18-06 sample (quartzite). Ellipses mark analytical spot locations and the text shows spot names (italics) and 208Pb/236U age in Ma (bold). Figure 9. 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