Abstract The relative tectonic activity of the Bafoussam–Mamfe Region (BMR) was assessed using morphometric and geomorphic approaches. Our investigations focused on 117 drainage sub-basins delineated from a 30 m resolution Digital Elevation Model (DEM) through Geographic Information System (GIS)-based techniques. Seven geomorphic indices (Re, Rr, Bs, A F , TTSF, Vf and HI) were computed to evaluate the influence of tectonic processes on basin morphology. In order to obtain a reliable assessment of relative tectonic activity, two complementary methods, the Index of Relative Active Tectonics (IAT) and the Analytical Hierarchy Process (AHP), were applied. The IAT results indicate four classes of relative tectonic activity (very high: ∼2.56%; high: ∼25.64%; moderate: ∼52.99% and low: ∼18.81%), whereas the AHP approach yields comparable distribution (very high: ∼9.40%; high: ∼38.90%; moderate: ∼35.04% and low: ∼16.66%). Both methods consistently reveal a predominance of moderate-to-high tectonic activity (approximately 81.19% and 83.34% for IAT and AHP, respectively). These results suggest that the morphology of the BMR is strongly controlled by active tectonics, likely driven by lithospheric mantle dynamics and upper crustal deformation associated with the Cameroon Volcanic Line (CVL), as well as the reactivation of inherited Pan-African structures. These processes collectively control the present-day landscape evolution of the region.
Abstract Strata of the Cincinnati area are among the most well-preserved and exposed Upper Ordovician rocks in the world. Determining the precise temporal alignment of these extensive palaeobiological and palaeoenvironmental records with those in other basins across North America is essential to an accurate assessment of Late Ordovician events, including the formation of rare earth element-enriched phosphorites. The addition of integrated chitinozoan biostratigraphy and high-resolution chemostratigraphy to existing conodont and graptolite biostratigraphy in the type area for the regional Maysvillian Stage provides an opportunity to create a strong baseline for advancing regional and global chronostratigraphic models. The MY-14-01 core (Maysville, Kentucky) contains well-preserved and abundant chitinozoans, enabling the designation of eight biozones. Coupled high-resolution δ 13 C carb chemostratigraphy of the core and nearby sections furthers characterization of temporal signatures through the Upper Ordovician and into the basal Silurian. Portable X-ray fluorescence analysis provides the first stratigraphically comprehensive elemental characterization of the Cincinnati succession. Redefinitions of the boundaries of the Edenian, Maysvillian, and Richmondian regional stages from lithostratigraphic to biozone boundaries are proposed. This study revives and expands biozonation in the Cincinnati area and, together with detailed chemostratigraphy, provides new perspectives on the chronostratigraphic completeness of this Upper Ordovician reference area.
Analysis of fracture networks in horizontally bedded limestone and shale in the Eastern Shelf of the Permian Basin of west-central Texas was performed to investigate the lithologic and mineralogic controls on fracture networks. The results show that opening-mode fracturing is sensitive to mineralogy as a primary control and that opening-mode fracturing depends on total clay and calcite percentages. Systematically fractured beds in this study all have >80% calcite, <9% quartz and <7.5% total clay minerals, whereas beds with <80% calcite, >9% quartz and >6% clay minerals are unfractured. Depositional-texture-based lithologic classifications (grainstone, packstone, wackestone, mudstone) based on proportions of carbonate grains and carbonate mud, although extremely convenient and important for interpreting depositional environments of carbonate rocks, are not robust predictors of fracturing. The reason for this is the sensitivity of rock strength to mineralogy - in carbonate rocks, particularly the clay mineral versus calcite content. Our results show that a mineralogic threshold approach may be valuable for opening-mode fracture prediction in limestone and shale.
The eastern Jebilet successions record major palaeogeographic and tectono-sedimentary changes during the Variscan orogeny, but the chronology of key events remains incompletely constrained because of the many allochthonous carbonate units and the strong deformation. Here, three autochthonous outcrops and six allochthonous units of the eastern Jebilet nappe were sampled for microfossils, with emphasis on Vis & eacute;an foraminifers recovered from washed residues and thin sections. Forty-five out of 155 samples yielded diagnostic assemblages which allowed a more precise biostratigraphy. The base of the Kharrouba Formation is dated for the first time as at least middle Vis & eacute;an Cf5, and the middle and upper parts are Asbian Cf6 gamma 1-2. An Upper Devonian age for the pre-Vis & eacute;an deposits of eastern Jebilet is indicated by reworked Famennian conodonts collected from the base of the Kharrouba Formation. Most allochthonous units contain upper Vis & eacute;an foraminiferal associations. These new data provide improved chronological constraints on (i) the Vis & eacute;an transgression in eastern Jebilet, (ii) the emplacement of the eastern Jebilet nappe, (iii) the provenance of the nappe units and (iiii) the regional correlations between the Meseta and adjacent domains. Our results confirm a marine connection with the Palaeotethys Ocean, suggesting the proximity of the Meseta to Gondwana.
The Mazon Creek fossil site is famous for the preservation of diverse animal and plant assemblages. Among these fossils are representatives of animal groups that are rarely preserved with soft tissues. Annelida is one of these groups, and the Mazon Creek polychaete fauna comprises a disproportionately large sample of all known soft-bodied annelid fossils. Here, we describe the first new fossil annelid from Mazon Creek in over 20 years. This new species, Mazovermes magnaterminus, has a unique morphology with notably large posterior segments, giving an overall teardrop-shaped silhouette not found in any other fossil polychaete. It further preserves fine details such as the nanometre-sized chaetae, as well as muscular tissues. Fundamentally, this new species demonstrates not only a unique body organisation within this extremely morphologically and ecologically disparate phylum but also that the Mazon Creek Lagerst & auml;tte is likely to yield additional undescribed invertebrates with soft-tissue preservation upon re-investigation.
Mesozoic siliciclastic rocks of the Kutch Basin represent a unique archive of western India's evolution during Gondwana breakup. Previous studies revealed differences in provenance between individual sub-basins within this basin, but the understanding of sediment-supplying units remains hampered due to the absence of detailed geochronological studies. In this study, we present detrital zircon and rutile U-Pb data from sedimentary units of all sub-basins, as well as modern rivers draining potential source areas. The Late Triassic to Early Cretaceous succession deposited in these sub-basins is classified into the Kutch Mainland Group, Pachchham Island Group and Eastern Kutch Group. The zircon and rutile ages of all three groups reveal the absence of post-Devonian igneous as well as medium- to high-temperature tectono-thermal events in the source areas. The Pachchham Island Group exhibits a higher proportion of similar to 1615 Ma zircon and >700 Ma rutile compared to the two other groups, confirming a different evolutionary course. While similar to 1615 Ma zircon occurs in pre-Callovian samples of the Eastern Kutch Group, indicating a common source, this changes from the late Bathonian to Callovian onwards, where both the Eastern Kutch and Kutch Mainland groups show an increasing proportion of similar to 540 Ma zircons. This late Bathonian to Callovian provenance change is coeval with the uplift of a basement high, known as the Median High, causing a drainage divide which diverted sediments away from the Pachchham Island sub-basin. These findings provide new constraints on sediment provenance, routing and tectonic controls during the evolution of the Kutch Basin.
Trace fossils record in situ animal-substrate interactions and constitute key proxies for paleoecological and paleoenvironmental reconstructions, although their generally long stratigraphic ranges limit biostratigraphic applications. Recent multi-proxy studies integrating ichnological, paleontological, and geochronological data have demonstrated that such limitations can be overcome, revealing temporal reassignments driven by convergent ecological strategies, including the 'd & eacute;j & agrave; vu effect'. In the Ibiracatu region (Minas Gerais, Brazil), trace fossils attributed to bilaterian organisms were reported from successions tentatively assigned to the Ediacaran-Cambrian Bambu & iacute; Group. However, the lack of integrated stratigraphic and chronological constraints has generated uncertainty regarding their age and depositional context. Here, we combine detailed sedimentological and stratigraphic analysis, ichnological characterization, and geochronological data to establish the stratigraphic framework, depositional environments, and maximum depositional age of the trace fossil-bearing succession. Our data indicate that the Bambu & iacute; Group formed in a storm-influenced restricted marine system, whereas the ichnofossil-bearing succession developed within turbiditic deposits in a lacustrine setting. The ichnofauna comprises eleven ichnospecies, representing a mixed Scoyenia-Mermia ichnofacies, containing the Diplopodichnus association. The ichnodiversity combined with zircon provenance data indicates that the succession belongs to the Santa F & eacute; Group, deposited during the Late Paleozoic Ice Age. Therefore, its occurrence within the Bambu & iacute; Group registers a tectonically trapped record due to fault reactivation associated with glacioeustatic adjustments following ice retreat.
The earliest stage of the Variscan orogeny in SW England is marked by the formation and obduction of the Mid-Devonian Lizard ophiolite from the Rheic ocean northwestward onto the previously passive continental margin of Avalonia (Laurussia). The Lizard ophiolite comprises an almost complete thrust slice of oceanic crust (U-Pb zircon: similar to 386.8 +/- 0.3 Ma; Givetian) and upper mantle formed above a south-dipping subduction zone, with an amphibolite to greenschist facies metamorphic sole (U-Pb zircon: similar to 395 Ma; late Emsian). The Kennack Igneous Complex includes a suite of granitoids that intrude the base of the ophiolite and its metamorphic sole and records melting of diverse protoliths beneath the ophiolite during subduction and later obduction. Structurally beneath the ophiolite is a complex unit of m & eacute;lange and a series of strongly folded and thrust units of Middle Devonian - Carboniferous sedimentary rocks that form the Dodman, Veryan and Carrick thrust sheets. Thrusting propagated from SSE to NNW as distal Gramscatho Group rocks were progressively emplaced onto the passive continental margin below the Lizard ophiolite. Early crustal shortening was subsequently followed by late Carboniferous-Early Permian extensional reactivation and crustal melting that resulted in intrusion of the Cornubian granites (295-275 Ma). The tectono-stratigraphy of the Lizard ophiolite and the metamorphic sole, together with structures in thrust sheets beneath the ophiolite, are directly comparable to similar allochthonous rocks beneath the Semail ophiolite, Oman.
Field research in the Petites Pyr & eacute;n & eacute;es (France) yielded new Late Cretaceous continental microvertebrates, including some of the few known mammals from Europe from this time. They come from the well-dated late Maastrichtian Auzas Marls Formation which has yielded some of the latest European Mesozoic vertebrates. Here, we report new discoveries, including for the first time a co-occurrence of multituberculate and eutherian mammals in the Cretaceous of Western Europe, breaking previously known strong provincialism of Late Cretaceous mammals in Europe. Two new mammals are described. The kogaionid species Hainina cassagnauensis n. sp. is the first multituberculate known from the Late Cretaceous of Western Europe and the earliest record of Hainina. It makes Hainina one of the only known vertebrate genera crossing the K/Pg boundary in Europe. H. cassagnauensis n. sp. is the first described evidence for a mammal dispersal between the Eastern and Western parts of the European Archipelago at the end of the Cretaceous. Mammals from Tricout & eacute; also include an upper molar of the new eutherian cf. Azilestes yvettae n. sp. It exhibits an advanced morphology showing affinities to other specialized endemic eutherians from the Cretaceous of Europe such as Valentinella vitrollense and Azilestes ragei. These three European species have a basic zhelestid dental morphology, but their specializations suggest a new, at least subfamilial, European clade. As with other vertebrates from the European Archipelago, the kogaionid multituberculates and the zhelestid eutherians are ancient relict lineages that belong to the & laquo;old European faunal core & raquo;.
Reconstructing the geological history of the Amazon River and its palaeo-drainage is of prime importance for understanding the control of tectonics and geodynamics processes on Amazonian landscape dynamics and the origin of Amazonian biodiversity. In this study, we report new stratigraphic, sedimentological and provenance constraints on the Neogene sedimentary rocks of the Madre de Dios Basin (South Peru). Our multidisciplinary dataset shows that the Neogene sedimentary rocks of the northern part of the Madre de Dios Basin (12.5-13 degrees lat S) have a different provenance than those of the southern part (13-14 degrees lat S). The sedimentary rocks of the northern part of the basin are characterized by Solimoes-like ENd (0) values (between -9.8 and -5.2) for fined grained sedimentary rocks, REE signatures suggestive of authigenic Fe-oxides formed from sulfide oxidation processes and U-Pb age distributions dominated by zircon ages between 0.9-1.3 Ga (Grenville/Sunsas), 0.5-0.7 Ga (Brasilian/Pampean), 290-252 Ma (Permian magmatism) and, to a lesser extent, zircon ages younger than 130 Ma (Andean Arc). We interpret the source of these sedimentary rocks to be the Altiplano and the Eastern Cordillera (EC). In contrast, the Neogene sedimentary rocks of the southern part were sourced from the EC and Sub-Andean Zone (SAZ) with secondary Fe-oxides derived from silicate rock weathering. Combined with previous studies, these contrasted geochemical signatures imply the existence of a palaeogeographic barrier within the Madre de Dios Basin between the Cretaceous and the Pliocene suggesting that the southern limit of the Amazonian palaeo-drainage basin was located within the Madre de Dios Basin.
The Paleocene-Eocene Thermal Maximum (PETM, similar to 56 Ma) marks a rapid, intense warming event at the Paleocene-Eocene boundary. However, the terrestrial record of the PETM is limited, and its paleoenvironmental impacts are debated. This study examines the effects of the PETM on terrestrial lake environments using mineralogical, inorganic and organic geochemical analyses of the Paleocene Kongdian Formation's organic-rich rocks from the Bohai Bay Basin. Findings show that global PETM warming drove regional aridification and evaporation, while carbonate isotopic signals (negative delta 13C, positive delta 18O) are consistent with enhanced hydrological cycling and salinity fluctuations, reflecting the interplay between global climate forcing and local hydrological responses. Nutrient influx during the PETM boosted paleoproductivity. Arid-adapted organisms like Podocarpidites and Ephedripites also became present. The PETM experienced heightened seasonal variance and climatic extremes, with elevated temperatures causing increased evaporation and salinity during dry seasons, indicating greater aridity. Seasonal precipitation likely intensified due to monsoonal rains or the tropical convergence zone's northward movement. Pollen from the Bohai Bay Basin indicates both significant episodic precipitation and increased aridity, reflecting complex climatic interactions. Increased detrital kaolinite content during the PETM suggests intensified physical weathering and erosion in kaolinite-bearing catchment areas, driven by episodic heavy rainfall. Our sedimentological observations (e.g., laminated shale cyclicity and detrital mineral assemblages) indicate sporadic heavy rainfall events, enhancing physical weathering and altering detrital sediment mineral content in the Bohai Bay Basin.
The Mineiro Belt is an example of the Tonalite-Trondhjemite-Granodiorite (TTG)-Sanukitoid transition, exemplified by the Lagoa Dourada (LDS, 2350 Ma) and the Alto Maranh & atilde;o (AMS, 2130 Ma) suites. Their opaque mineralogy, along with the amphibole chemistry, was investigated to better understand the granitic magmatism at that time. The LDS contains magnetite, ilmenite, chalcopyrite and pyrite as opaque primary phases, while the AMS includes ilmenite, pyrrhotite, chalcopyrite, pyrite, magnetite, pentlandite and sphalerite. With magnetite predominating in the LDS and ilmenite in the AMS, they should be classified as magnetite and ilmenite series granitoids, respectively. In the TTGs, the amphiboles are ferrotschermakite, whereas in the sanukitoids, they are primarily Mg-hornblende to actinolite. Despite the opaque mineralogy, the positive correlation between Fe# and AlIV in the amphiboles, along with the lower Fe# values in the ilmenite-bearing sanukitoids, suggests higher fO2 conditions compared to the magnetite-bearing TTGs. Although both opaque assemblages indicate fO2 values above the FMQ buffer, exceeding 10-18 bars for the ilmenite-bearing sanukitoids and 10-17 bars for the magnetite-bearing TTGs, their crystallization occurs close to the equilibrium reaction titanite + magnetite + quartz - Fe-Mg-Ca silicates + ilmenite. The assemblages show that the magnetite-bearing TTGs crystallized at lower temperatures (700-800 degrees C) than ilmenite-bearing sanukitoids (800 degrees C), showing a temperature dependency of fO2 once the latter register higher fO2, despite being ilmenite-series granitoids. This intriguing scenario may be due to a combination of heterogeneously metasomatized magma sources, temperature condition, fH2O and sediment entrance in the subduction, explained by the changes in crustal growth processes towards modern plate tectonics.
The Preveli nappe of Crete (Uppermost Unit) is derived from Permo-Triassic sediments and volcanics. Structural data, deformation microfabrics and petrological constraints suggest that subduction of the Preveli rocks was related to ESE-directed D2 shearing under epidote-blueschist-facies conditions (T = 360 +/- 40 degrees C and P > 1.0 GPa). New U-Pb ages of rutile from blueschist (132 +/- 12 and 135 +/- 10 Ma, 2 sigma) suggest that subduction and related HP-LT metamorphism occurred during the Early Cretaceous (Eohellenic phase). Ar-39-Ar-40 dating of ferri-winchite and Rb-Sr dating of phengite yielded 125 +/- 10 Ma (1 sigma) and 131 +/- 7 Ma (2 sigma), respectively, which also reflect the subduction stage. Further Rb-Sr dating of phengite and albite, coupled with trace element data (B, Li), revealed four growth stages, which are younger. They range from 120 to 90 Ma and are attributed to fluid-assisted shearing and reactivation of the main foliation at still deep structural levels (>1 GPa). Alpine emplacement of the Preveli nappe on top of the Pindos Unit was accommodated by brittle top-to-the west thrusting and west-vergent D3 folding. The age of this event has been constrained at 31 +/- 9 Ma (2 sigma), by U-Pb dating of calcite. Despite the uncertainty, this age confirms that the Preveli nappe was emplaced after the deposition of the Paleogene Pindos flysch. The new data suggest that the Preveli nappe is derived from the Rhodope-Strandja or from the Sakarya Zone of Turkey. When travelling towards its recent position on Crete, the Preveli nappe should have passed the Cyclades, where similar rocks are exposed.
We integrate provenance, sedimentological and tectonic analyses to reconstruct sediment sources, transport pathways and hydroclimatic evolution of the Rotliegend Group in the Norwegian North Sea. The study also evaluates implications for basin development and reservoir quality in this key reservoir interval. Detrital zircon U-Pb geochronology from nine wells integrated with facies analysis, dipmeter- and image-derived palaeocurrent data and a palinspastic restoration reveals that sediment sources were closer to the basin than today and that inherited structural highs compartmentalized drainage and aeolian fields. Facies trends record a transition from arid dune fields to semi-arid mixed dune, interdune, flash-flood and playa-lake systems, reflecting increasing hydrological connectivity and base-level rise during the early Permian. Detrital zircon spectra define three main provenance domains with Caledonian-derived aeolian sand transported from the west-northwest, Sveconorwegian-sourced alluvial-fan systems along the eastern basin margin and hybrid basement-fed systems along northern and southern highs. Mixed zircon-age populations indicate substantial recycling of Devonian Old Red Sandstone basin fills and convergence of multiple sediment transport pathways within the basin interior. Converging transport pathways enabled interaction between long-distance and short-range detrital zircon input, whereas sustained pathway partitioning maintained distinct first-order provenance domains at basin scale. Provenance influences reservoir quality through its control on sediment composition, texture and depositional facies, with quartz-rich Caledonian-derived aeolian deposits retaining anomalously high porosities and feldspathic Sveconorwegian-derived alluvial-fan deposits displaying consistently lower porosity. Integrating provenance, facies, palaeocurrents and restored basin geometry therefore provides a predictive framework linking sediment transport, tectonic inheritance, and reservoir distribution in ancient continental basins.
New-type metasedimentary rock-hosted stratiform Cu deposits occur in the Jianglang Dome. They are hosted by the Late Neoproterozoic Liwu Group and were unusually induced by an epigenetic magmatic-hydrothermal system. However, the source characteristics of fluids and metals remain poorly understood. Here, we employed Re-Os dating and He-Ar-S-Pb isotopes to determine their mineralization age and origin. Chalcopyrite Re-Os isotopic dating defines an isochron age of 162.7 +/- 3.2 Ma and a crust-like initial Os-187/Os-188 ratio of 1.421 +/- 0.021 (n = 4). Sulphide He-Ar isotope (n = 20) yields low R/Ra ratios of 0.052-0.144, high Ar-40/Ar-36 ratios of 479-2463 and low Ar-40*/He-4 ratios of 0.013-0.804, with calculated He-mantle values of 0.69-2.20 wt.%. In situ chalcopyrite sulphur isotope exhibits positive delta S-34(V-CDT) values of 3.87-9.50 parts per thousand (n = 72). Together with similar lead isotope data of chalcopyrite separates (n = 40) and ca. 164 Ma granites (n = 4), as well as low residual gravity anomalies in this region, our integrated data indicate an epigenetic magmatic-hydrothermal mineralization at ca. 163 Ma. The ore-forming fluids were dominantly crust-derived, with minor air-saturated water and negligible mantle input (0.69-2.20 wt.%). This is most likely attributed to the low proportion (total <5 vol%) of sandwiched metabasic rocks in the metalliferous Liwu Group. All the investigated orebodies show source homogeneity, in contrast to classic sediment-hosted stratiform Cu deposits with isotopic heterogeneity. Further, our findings imply significant mineral exploration potential in the Jianglang Dome and analogous domes in the eastern Songpan-Ganze Orogen.
Intra-terrane shear zones (ITSZs), though widespread across Proterozoic mobile belts, remain underexplored in their structural evolution and tectonic significance. This study explores the anatomy of the North Purulia Shear Zone (NPSZ), a similar to E-W trending, steeply dipping ITSZ within the Chotanagpur Granite Gneiss Complex (CGGC), eastern India. Integrated structural, kinematic, anisotropy of magnetic susceptibility (AMS) and microstructural analyses reveal that the NPSZ nucleated within megacrystic granite gneiss during late-Grenvillian tectonism (similar to 0.9-1.1 Ga), contemporaneous with Rodinia assembly. Field and AMS data demonstrate a sub-simple shear regime with a sinistral strike-slip component and magnetic fabrics transition from mixed oblate-prolate in host gneiss to only oblate in mylonitic zones. Recrystallized quartz grain aspect ratios increase significantly toward the core, coupled with elevated strain rates and decreasing flow stress. Deformation temperatures inferred from microstructural observation coupled with quartz dynamic recrystallization mechanisms suggest dominance of dislocation creep, which implies that the shearing took place at deep crustal level. The absence of precursor fractures or dykes and alignment of feldspar megacrysts suggest that the nucleation of the NPSZ was facilitated by anisotropy produced by the magmatic/sub-magmatic gneissic fabric, where interstitial smaller-sized quartz-rich domains in between K-feldspar megacrysts acted as viscous pathways. These findings not only provide first-order insights into ITSZ nucleation mechanisms but also reaffirm the CGGC as a site of late-Grenvillian crustal thickening related to the suturing of North and South Indian cratons during Rodinia amalgamation. The NPSZ thus emerges as a key intra-terrane tectonic structure recording Rodinia-linked crustal reworking within the Indian shield.
The Blaini Formation, an important marker horizon in Indian stratigraphy, preserves the imprints of the Neoproterozoic glacial event. Although the sedimentology and genesis of the Blaini Formation have been extensively investigated, its syn-sedimentary and tectonic deformation styles remain insufficiently understood. This study presents the first report of syn-sedimentary deformation structures, such as load-and-flame structure, clastic vein, syn-sedimentary fault, and sand dyke, in the Neoproterozoic Blaini Formation. The development of these syn-sedimentary structures is attributed to the basinal instability. New mapping reveals the previously unreported juxtaposition of two couplets, each comprising diamictite and dolostone, within the Blaini Formation. Several lines of evidence from the mesoscopic-scale structures and the outcrop pattern reveal that the juxtaposition of the two diamictite-cap dolostone couplets is due to the tectonic imbrication of the Blaini Formation during the Himalayan Orogeny. The diamictite and dolostone beds, occurring in the two couplets, belong to the same stratigraphic levels, respectively. Our observations reveal that the Blaini Formation was deformed by two coaxial fold groups, brittle faults, and brittle-ductile and ductile shear zones during the Cenozoic Himalayan deformation. Any estimate on the thickness of the Blaini Formation is susceptible to significant overestimation without accounting for isoclinal folding and thrust-induced duplication. The Main Boundary Thrust is interpreted as an imbricate structure comprising repeatedly folded and thrust-bounded horses.
The metamorphic architecture of the Variscan basement in the Tatra Mountains provides evidence for nappe tectonics during Variscan continental collision in the Late Palaeozoic. We reconstruct the metamorphic history of the lower unit using phase equilibrium modelling, zirconium-in-rutile geothermometry and in situ LA-ICP-MS Th-U-Pb monazite geochronology of metapelites. We also present new U-Pb zircon ages from upper unit granitoids. In the lower unit, the peak assemblage in staurolite-kyanite schists is garnet + muscovite + biotite + staurolite + kyanite + plagioclase + rutile + quartz. Structurally higher, a kyanite-fibrolite zone is marked by loss of staurolite and abundant kyanite, commonly replaced by fibrolitic sillimanite. P-T conditions increase from 600-640 degrees C and 6-8 kbar in the staurolite-kyanite zone to 640-655 degrees C and 6.5-8.5 kbar in the kyanite-fibrolite zone. Monazite ages show downward younging from 342-332 Ma in the kyanite-fibrolite zone to 338-315 Ma in the underlying staurolite-kyanite zone, revealing a temporally and structurally inverted metamorphic sequence. These ages are younger than zircon ages in upper unit granitoids (353-346 Ma), indicating prograde metamorphism in the lower unit overlapped with late granitoid intrusion above. The inverted metamorphic sequence and spatially decoupled thermal histories of the upper and lower units suggest that nappe stacking played a dual role: accommodating crustal shortening and driving crustal re-equilibration through partial melting and melt migration. These processes are critical for the long-term rheological evolution of orogenic belts and for understanding the coupling between deformation, metamorphism and plutonism in thickened continental crust.
This study examines the potential influence of deformation on the systematics of Rb-Sr geochronology in mica phases under different conditions. Biotite and muscovite porphyroclasts in deformed specimens were characterized using electron backscattered diffraction, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry to quantify spatial variations in crystal lattice orientations, element concentrations and in situ Rb-Sr geochronology. S29, a specimen subjected to deformation at greenschist facies conditions, is characterized by a spread in in situ Rb-Sr two-point isochron spot dates, which exhibit a strong inverse correlation with lattice deformation. As such, these Rb-Sr dates are interpreted to record partial re-equilibration controlled by deformation. Rb-Sr data from white mica in a specimen (NP17-58), which was deformed at lower amphibolite facies conditions, define a single population isochron. No correlation between lattice distortion and Rb-Sr spot dates is noted. Finally, two biotite porphyroclasts and matrix grains in a specimen (AC4), deformed at upper amphibolite facies conditions, define unique, single population Rb-Sr isochrons. The Rb-Sr systematics of the older porphyroclast are interpreted to be mainly temperature-controlled. In contrast, the Rb-Sr systematics for the younger porphyroclast and matrix grains are interpreted to reflect fluid-mediated resetting. The results of this study demonstrate that the multi-faceted influences on Rb-Sr systematics make isolating the effect of deformation difficult. Due to the complexity of the Rb-Sr systematics in deformed specimens, careful consideration of the mica phase analysed, as well as the temperatures, fluids and deformation experienced throughout the rock's history, needs to be accounted for.
