NH4-bearing micas in poly-metamorphic Alpuj��rride micaschists and gneisses from the central zone of the Betic Cordillera (Spain): tectono-metamorphic and crystal-chemical constraints

The content and distribution of NH₄-bearing micas in micaschists and paragneisses of the Alpujárride complex (Betic Cordillera, Spain) are interpreted on the basis of textures, metamorphic history and crystal-chemical constraints. NH₄ is present in important amounts in early micas, recording their older diagenetic-to-metamorphic history. NH₄ was inherited by micas formed in successive metamorphic events. At similar metamorphic grade, annite is enriched in NH₄ relative to muscovite with a fractionation of 4:1. Maximum NH₄ contents were estimated in golden annite from medium-grade schists and in retrogressive annite-chlorite mixed-layers (~1.0?wt.%). A Ti-NH₄ avoidance gives rise to associated chemical changes, such as the enrichment in Mg and in ^VIAl. These data suggest that NH₄ can be a decisive factor in chemical equilibrium and element partitioning between coexisting phases, thus affecting to the commonly used thermobarometers. Estimation of the P-T conditions from texturally different muscovite-annite pairs reveals the superposition of several stages of mica growth, under different geothermal gradients.     

Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex,Betic Cordillera,Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2 km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alborán domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U–Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3 wt.%) and size (from 1.5 cm to <0.5 mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 ± 9 and 265 ± 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.     

Tectonic evolution of the Málaga Basin (Betic Cordillera). Regional implications

Abstract

During the Neogene (uppermost Aquitanian-Lower Burdigalian, Tortonian and Pliocene), three successive marine episodes took place in the present-day Malaga Basin. The first of these affected a wide area of the Belic Internal Zones and was brought to an abrupt conclusion by the westward displacement of these Zones, together with important horizontal movements associated with N70-100 direction strike-slip faults and the superposition of materials from the Campo de Gibraltar. The two other marine episodes were clearly controlled by vertical movements of NW-SE and NK-SW faults, caused by a clear E-W distension which, according to regional data, was associated with some compression in an approximately N-S direction. The area has also been affected, although to a lesser extent, by the uplift of the Betic Cordillera from the Upper Miocene to the present day.     

SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

U–Pb SHRIMP ages obtained in zircons from the Sotosalbos and Toledo anatectic complexes in Central Spain give new constraints to the evolution of the inner part of the Hercynian Iberian belt. Pre-Hercynian ages in zircons from the Sotosalbos complex (∼464 Ma) are well preserved and reveal that an age diversity of the Lower Paleozoic magmatism in the area exists, as previous data on westernmost orthogneisses yield significant older ages. Zircon ages in the pelite-derived granites from the Toledo complex also show an important Neoproterozoic age component which points to a metasedimentary protolith deposited maximally 560 Ma ago. Younger zircon populations in both complexes at ∼330 Ma in the Sotosalbos region and ∼317 Ma in the Toledo complex indicate an important diachronism between the anatectic processes in both areas but also that these processes are mainly unrelated to the generation of the later Hercynian granite batholith of Central Spain, which could be of deeper crustal derivation. In addition, as migmatization occurred late in the metamorphic cycle, after peak conditions were attained, the age of anatexis is younger than the age of the main Hercynian metamorphic event, which still is not well constrained. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Linking Palaeozoic palaeogeography of the Betic Cordillera to the Variscan Iberian Massif: new insight through the first conodonts of the Nevado-Filábride Complex

International Journal of Earth Sciences - Graphite-rich metamorphic limestones included within low-grade black schists of the lowest Nevado-Filábride tectonic unit in the Sierra de Baza...     

The metamorphic evolution of migmatites from the Ötztal Complex (Tyrol,Austria) and constraints on the timing of the pre-Variscan high-T event in the Eastern Alps

Within the Ötztal Complex (ÖC), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the ÖC, the Winnebach migmatite in the central part and the Verpeil- and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central ÖC. The P-T data range from 670–750 °C and < 2.8 kbar for the pre-Variscan event, 550–650 °C and 4–7 kbar for the Variscan event and 430–490 °C and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-Th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-Silurian metamorphic event in the ÖC.     

The ductile shear zone in granitoid of the Central Anatolian Crystalline Complex,Turkey: Implications for the origins of the Tuzgölü basin during the Late Cretaceous extensional deformation

The Emizözü shear zone is the west–northwest-trending ductile shear zone within the A?açören granitoid in central Turkey. Deformation that affected the granitoid along the Emizözü shear zone resulted in mylonites with mylonitic foliation and stretching lineation. The textural features of the deformed minerals suggest that mylonitization occurred under conditions of upper greenschist facies. The shear indicators, including asymmetric porphyroclasts, oblique foliation, and shear bands, suggest a down-dip (top-to-the-southwest) displacement. The orientation of stretching lineation, as well as kinematic indicators, indicates the extensional character of the Emizözü shear zone. Although it is not precisely dated, the available age constraints suggest that the zone formed at 78–71 Ma. According to field and micro-structural data, the A?açören granitoid was most likely emplaced during a regional deformation in central Turkey, and synchronously or shortly after was overprinted by the extensional Emizözü shear zone. The zone can also be correlated with the earlier stage development of the Tuzgölü basin in central Turkey.     

SHRIMP U–Pb detrital zircon dating to check subdivisions in metamorphic complexes: a case of study in the Nevado–Filábride complex (Betic Cordillera,Spain)

International Journal of Earth Sciences - U–Pb dating on inherited detrital zircons has been applied to obtain the probable maximum age of deposition of the detrital protolith of the...     

The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry

The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed2.3-2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the ArchaeanRangrim and Ryonggang Blocks of the northeastern Sino-Korean Craton. The Complex ismainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrologicaland geochemical studies show that the protoliths of the Complex are mainly assoctations ofbimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta,P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative toNd and Pb; La/Nb ratios are higher than 1(1.75 to 5.18). The trace element patterns of theamphibolites are similar to continental flood basalts formed by the Gondwana break-up, suchas those in South Karoo and Tasmania, which shows continental contamination. ε_(Nd) valuesranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yband La/Yb suggest that contamination of basaltic magma happened in the mantle, rather thanalong the conduit. Isotope ratios of ~(208)Pb/~(204)Pb, ~(207)Pb/ ~(204)Pb, ~(206)Pb/~(204)Pb, ~(143)Nd/~(144)Ndand ~(87)Sr/~(86)Sr indicate that the magma was derived from a contaminated mantle source likeDMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is thecase with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania.Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite camefrom the same parental magma, being products of strong fractional crystallization. Protoliths ofthe Kuandian Complex were formed in extensional tectonic setting during the transition fromcontinental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3or 2.4 Ga ago tectonic evolution of the Sino-Korean craton was different from that of otherwell-studied Precambrian cratons, e.g. the North American shield, European platform andAustralian continent in that strong volcanic eruption resulted in its accretion. Besides, the con-taminated magma source with a Dupal anomaly for the Complex indicates that crust-mantleconvection whose scale was similar to that of the present plate tectonics had occurred at leastbefore the formation of the Kuandian Complex (2.3-2.4 Ga B.P).     

Palaeoenvironmental implications of ferruginous deposits related to a Middle–Upper Jurassic discontinuity (Prebetic Zone,Betic Cordillera,Southern Spain)

The Middle–Upper Jurassic boundary in the westernmost Tethyan basins is marked by a discontinuity. A thin iron crust with ferruginous ooids and pisoids and an overlying ferruginous oolitic limestone lithofacies occur in a genetic relationship to this discontinuity with a reduced thickness (< 50 cm) and very local distribution in the Prebetic Zone (Betic Cordillera).The ferruginous coated grains are subdivided into two types. Type A ooids are characterised by thin, regular lamination in concentric layers enclosing a nucleus; they are dominant in the top of the iron crust (100% of the ferruginous ooids) and in the ferruginous oolitic limestone (82%). Type B ooids typically have thick, irregular lamination in a few discontinuous concentric layers enclosing a variable nucleus including bioclasts and foraminifera; they are exclusive to the ferruginous oolitic limestone (18% of the ferruginous ooids). The bulk chemical composition varies between 80% Fe₂O₃ by weight in the iron crust and 67% by weight in the coated grains. In the ferruginous ooids, the contents in SiO₂ (5.4%), Al₂O₃ (6.5%), P₂O₅ (3.6%), and CaO (4.7%) are higher than in the crust. Trace elements (V, Cr, Co, Ni, Zn, Y, Mo, and Pb) in both the crust and ooids show enriched values compared with the bulk composition of the upper continental crust. The mineral composition of the iron crust and ooids is primarily goethite, with small amounts of Al-hydroxide (bohemite) and apatite, whereas hematite is identified only in the iron crust.The Type A ooids are interpreted as having an origin related to the iron crust. Since there is no evidence to support a marine genesis for the iron crust, the possibility of a subaerial origin is presented here. The crust has characteristics (chemical and mineralogical composition) similar to those of ferruginous pisolitic plinthite (highly weathered redoximorphic soil), and goethite shows an Al-substitution range (5–10 mol%) that indicates pedogenic conditions. Soil processes under periodic hydrous conditions are suggested; groundwater soils with hydrous conditions are congruent with the formation of the Type A ferruginous ooids and pisoids. In this situation, a coastal plain with periodically flooded soils would be the likeliest scenario. Callovian shallow carbonate shelf was possibly emerged and weathered, followed by marine sedimentation during the Middle Oxfordian, associated with major flooding of the Prebetic shelf and the erosion of ferruginous pisolitic plinthite. The first marine deposit was ferruginous oolitic limestones. Fragments of iron crust and Type A ferruginous ooids were reworked and incorporated into the marine sediments. A second phase of ferruginous ooids (Type B) with clear marine features developed, benefiting from iron-rich microenvironments due to the redistribution from iron crust fragments and Type A ferruginous ooids.     

The structure and tectonic evolution of the Aguilón fold-nappe,Sierra Alhamilla,Betic Cordilleras,SE Spain

Detailed structural work in the Sierra Alhamilla, SE Spain, shows that the Aguilón nappe, comprising Triassic and older metasedimentary rocks, is a fold-nappe. The most prominent set of small-scale folds changes from dominantly N-vergent in the upper, right-way-up limb to S-vergent in the greatly thinned lower limb. The nappe closes to the north, and must have been emplaced in this direction. Nappe formation was accompanied by small-scale folding and extensive solution-transfer producing a pronounced differentiated crenulation cleavage. These structures overprint an earlier set of folds and cleavage, and are overprinted in turn by late N-vergent structures. The lower limb of the nappe was thinned and disrupted during continued nappe transport.Pre-Triassic schist in the core of the nappe was affected by medium-grade metamorphism of probable post-Triassic age. The contact with low-grade Permo-Triassic sediments above and below the schist coincides with a distinct change in metamorphic grade. This contact may be a post-metamorphic extensional fault that is now folded around the nappe, which suggests that nappe formation was preceded by extensional faulting. This is consistent with gravity spreading as the driving process for nappe emplacement.     

Origin and evolution of volatiles in the Central Europe late Variscan granitoids,using the example of the Strzegom-Sobótka Massif,SW Poland

Mineralogy and Petrology - The results of the new Electron Microprobe Analysis of apatite, hornblende and biotite crystals of the hornblende-biotite variety of the Strzegom-Sobótka granite...     

Stratigraphy and evolution of the Galve sub-basin (Spain) in the middle Tithonian–early Barremian: Implications for the setting and age of some dinosaur fossil sites

A review of the stratigraphy of the Galve sub-basin (western Maestrazgo Basin, eastern Spain) around the Jurassic–Cretaceous transition is presented here, based on new data acquired after extensive geological mapping and logging complemented with facies analysis, new biostratigraphic data and a revision of the published information available. The results obtained are relevant for a more detailed understanding of the tecto-sedimentary evolution of the studied basin during the transition between two stages of rift evolution (i.e., syn-rift sequences 1 and 2). In addition, new information on the age and setting of numerous dinosaur fossil- and track-sites found across the Galve sub-basin and in the northern part of the nearby Penyagolosa sub-basin is provided here. Two new lithostratigraphic units are defined and characterized, the Aguilar del Alfambra and the Galve formations. The previous stratigraphic framework considered only two lithostratigraphic units (the Villar del Arzobispo and El Castellar formations) bounded by a single regional unconformity, and this resulted in significant misinterpretations. The whitish limestones, red lutites and cross-bedded sandstones of the Aguilar del Alfambra Formation were deposited in transitional environments, ranging from coastal lutitic plains to restricted lagoons. Of particular interest are the laminated micritic-peloidal limestones with abundant fenestral porosity (supratidal ponds to intertidal flats), which preserve common dinosaur footprints. This unit is bounded by widespread unconformities and is of very variable thickness (0–450 m), controlled by extensional tectonics operating at the climax of syn-rift sequence 1 during the latest Tithonian–middle Berriasian. The overlying Galve Formation is of variable thickness (from 0 to 100 m) and is also bounded by regional unconformities described in detail here. It consists of red lutites with cross-bedded and tabular-burrowed sandstones representing channel and overflow deposits in an alluvial floodplain. The sauropod dinosaur Aragosaurus ischiaticus found in this unit has a controversial age assignment. The age of the Galve Formation is poorly constrained from late Berriasian to Hauterivian, but new biostratigraphic data presented here, combined with the correlation with the nearby Penyagolosa and Salzedella sub-basins, suggest a possible equivalence to the upper Berriasian–lower Valanginian sequence deposited during the initial stage of syn-rift sequence 2.     

Multi-Stage Mineralization of the Early Yanshanian Granites from the Central Nanling Region, South China: Implications from REE Geochemistry

Please refer to the attachment(s) for more details.     

P–T evolution and tectonic significance of lawsonite-bearing schists from the eastern segment of the southwestern Tianshan,China

Robust quantification of pressure (P)–temperature (T) paths for subduction-related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down-going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur in voluminous host metasedimentary schists, information about the P–T evolution of these rocks in the eastern segment is limited, precluding a full understanding of the development of the belt as a whole. In this study at Kekesu in the eastern segment of the SW Tianshan, we use microstructural evidence and phase equilibrium modelling to quantify the peak and retrograde P–T conditions from two lawsonite-bearing micaschists and an enclosed garnet–epidote blueschist; for two of the samples we also constrain the late prograde P–T path. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, polymineralic aggregates included in garnet are interpreted as pseudomorphs after lawsonite. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, titanite and ilmenite, the maximum P–T conditions for the peak stage are 18.0–19.0 kbar at 480–485°C. During initial exhumation, the retrograde P–T path passed through metamorphic conditions of 15.0–17.0 kbar at 460–500°C. For garnet–glaucophane micaschist TK33, which is mainly composed of garnet, glaucophane, phengite/paragonite, albite, chlorite, quartz, relict lawsonite and minor epidote, with accessary titanite, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are >24.0 kbar at 400–500°C. During exhumation, the P–T path passed through metamorphic conditions of 17.5–18.5 kbar at 485–495°C and 14.0–17.5 kbar at 460–500°C. For garnet–epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary titanite, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of ~20.0 kbar at ~445°C to P_max conditions of ~21.5 kbar at 450–460°C and T_max conditions of 19.5–21.0 kbar at 490–520°C. During exhumation, the rock passed through metamorphic conditions of 17.5–19.0 kbar at 475–500°C, before recording P–T conditions of <17.5 kbar at <500°C. These results demonstrate that maximum recorded pressures for individual samples vary by as much as 6 kbar in the eastern segment of the SW Tianshan, which may suggest exhumation from different depths in the subduction channel. Furthermore, the three samples record similar P–T paths from ~17.0 to 15.0 kbar, which suggests they were juxtaposed at a similar depth along the subduction interface. We compare our new results with published information from eclogites in the same area before considering the wider implications of these data for the orogenic development of the belt as a whole.     

40Ar–39Ar dating of volcanic rocks of the Shyok suture zone in north–west trans-Himalaya: Implications for the post-collision evolution of the Shyok suture zone

⁴⁰Ar–³⁹Ar geochronological studies carried out on the Khardung volcanics of Ladakh, India and our earlier Ar–Ar results from the volcanics of the Shyok suture along with the available geological and geochemical data provide good constraints for post-collision evolution of the Shyok suture zone. Whole-rock samples from the Shyok volcanics yielded disturbed age-spectra and we have demonstrated earlier that the youngest tectonic event in the Shyok suture zone responsible for the thermal disturbance of these samples is Karakoram fault activation at ～14 Ma. Contrastingly whole-rock samples from the Khardung volcanics, which are in tectonic contact with these Shyok volcanics, and are exposed in the form of thick rhyolitic and ignimbritic flows, yielded undisturbed age-spectra and good plateau-ages. The whole-rock plateau-ages of two rhyolite samples are 52.8 ± 0.9 and 56.4 ± 0.4 Ma. We interpret these ages to be the time and duration of emplacement of these volcanics over thickened margin of the continental crust, which appears to be coeval with the initiation of the collision between the Indian and Asian plate. The lesser extent of post-emplacement isotopic re-equilibration in these samples unlike the Shyok volcanics indicate that these samples were present in different tectonic settings, away from the Karakoram fault, at the time of deformation in the Shyok suture zone. We propose that the two volcanic belts of contrasting nature were brought together in juxtaposition by the Karakoram strike slip faulting at ～14 Ma.     

Metamorphic P–T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex,NE China

Mineralogy and Petrology - In the Gotsu area of the c. 200&nbsp;Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic...