High-grade Metamorphic Complex Research Articles

On the Rhodopean protoliths and their Middle Jurassic to Early Cretaceous evolution. A review

The Rhodope Massif represents a large high-grade metamorphic complex situated in the Balkan Peninsula, straddling the border between Bulgaria and Greece. Geological studies carried out during the last thirty years clearly established that the complex is composed of three major lithotectonic units characterized by differing protolith ages and Alpine thermotectonic evolution. The Rhodope Massif consists of Upper and Lower terranes representing continental fragments composed mainly of pre-Mesozoic magmatic rocks and covered by Early Mesozoic sedimentary sequences. These two entities are separated by an Intermediate Terrane comprising a Jurassic magmatic arc and its host rocks tectonically intercalated with Jurassic ophiolitic fragments together with the Early Mesozoic sediments. Detailed analyses of the previously published geochronological data revealed that the three main tectonic terranes were juxtaposed in the Late Jurassic–Early Cretaceous during the continental collision and northward subduction of the Lower and Intermediate terranes. These rocks experienced Middle Jurassic to Early Cretaceous high-pressure metamorphism, followed by high-temperature one accompanied by anatexis. At that time, the Upper Terrane was metamorphosed and deformed only partially. Being part of the European margin, this terrane represented an upper continental plate during the subduction, and therefore is not considered as a part of the Alpine Rhodope metamorphic complex. It is important to note that, although most of the present-day fabric and major tectonic zones in the Rhodope metamorphic complex are younger than Early Cretaceous, and therefore they postdate the geological evolution discussed in this study, they often reactivate the old boundaries between the main terranes and units.

High-grade complexes record the Late Permian-Middle Triassic arc metamorphism in the southernmost Altaids: Implications for the final closure of the Paleo-Asian Ocean

High-grade metamorphic complexes or terranes in orogens play a vital role in reconstructing orogenic dynamics and tectonic history of Earth's past owing to their multi-tectono-thermal recording capacity. The Chijin Complex, located in the southernmost Altaids and composed of intensely deformed medium- to high-grade meta-volcanic rocks including (garnet-bearing) mica-quartz schists, felsic gneisses, and amphibolitic schists and gneisses, imposes crucial constraints on the fiercely disputed timing and tectonic architecture of the Paleo-Asian Ocean's (PAO) terminal evolution. This Complex has long been interpreted as the Precambrian basement of a microcontinent in tectonic reconstructions. Our detailed field investigations, geochemical research, and detrital age spectrum analyses of the Chijin Complex reveal structural-lithological-geochemical affinities to volcanic-magmatic extrusions originating from a continental arc regime. Anti-clockwise P-T paths and high T/P geothermal gradients (~24–37 °C/km), reconstructed by thermodynamic modeling and geothermobarometry for the Complex, reveal magmatic underplating mechanism from prograde “hot” T/P metamorphism resulting from thermal intrusion and finite arc crustal thickening to in-situ retrograde isobaric cooling. Hornblende and biotite 40Ar/39Ar and zircon UPb dating demonstrates that the continental arc system was still active as during the Early to Middle Triassic (~252–237 Ma) as a result of continuing southward subduction of the PAO, thereby constraining the final amalgamation of the Altaids to the Late Triassic. This study proposes a new perspective for redefining high-grade rocks at the southernmost Altaids as arc-metamorphic complexes rather than “reputed” ancient microcontinent, as well as a potential template example for discerning tectonic nature of semblable metamorphic complexes or terranes with distinct origins in global orogens.

Quartz c-axis fabric characterization of the strike-slip ductile deformation within the Three Pagodas shear zone, western Thailand

The Three Pagodas shear zone is associated with mainland Southeast Asia deformations. The evolution of the shear zone is divided into deformation phases D1a, D1b, and D2. The first phase (D1a) involved ductile sinistral shearing associated with metamorphism. Foliations are well developed in a NW–SE with variable dips from east to west. Stretching lineations show NW–SE trending strike and flatly plunge to the south. Based on deformation patterns exhibited by outcrop and microscopic analysis, D1a shows NW–SE trending of ductile sinistral shearing. Additionally, the quantitative analysis of mylonitic quartzite exhibits an oblique angle of quartz grain shape fabric and quartz c-axis fabric asymmetry. Sinistral simple shear was confirmed by kinematic vorticity (Wk). The spatial distribution of the kinematic velocity number (Wk) of D1a shows a range between 0.93 and 0.99 at the central and western parts of the shear zone, which is higher than the 0.82–0.85 range for the eastern part of the shear zone. The strain variation within the Three Pagodas shear zone derived from the kinematic vorticity number (Wk) shows that the central and the western parts have been deformed under simple shear more than the eastern part. D1b is represented by the exhumation of high-grade metamorphic complex from sinistral transpression. The last stage (D2) involved the inversion of this strike-slip system, which is represented by dextral strike-slip faults.

Significance of a late neoproterozoic – Early Cambrian southern Baltica active margin in late-stage Rodinian and early Gondwanan reconstructions

New whole-rock geochemical data indicate changing provenance of sedimentary rocks deposited on the southern margin of Baltica in the late Neoproterozoic to early Paleozoic. The oldest part of the sequence is characterized by highly weathered detritus sourced from a mafic igneous source, inferred to be the nearby Volyn Large Igneous Province (L.I.P.), in an intracontinental rift (aulacogen) setting. Neoproterozoic–Cambrian boundary sequences are weakly weathered and sourced from intermediate to felsic rocks in an active margin setting whereas the youngest parts of the sequence exhibit significant quartz accumulation consistent with deposition of recycled material in a passive margin setting. These data support published U-Pb and Lu-Hf zircon data from the same sedimentary sequence where the Neoproterozoic-Cambrian boundary strata are characterized by anomalously high Ediacaran–Cambrian zircon contribution with juvenile isotopic composition. Taken together, these data indicate a cryptic late Neoproterozoic–early Cambrian active margin along the periphery of southern Baltica. Based on a correlation of similarly aged foreland basin and accretionary complexes, and links to paleomagnetically constrained plate reconstructions it is hypothesized that this detritus was sourced from the Avalonian micro-continent and a Cambrian arc terrane that is today preserved in high-grade metamorphic complexes of the European Variscides. These findings have potentially profound implications for our understanding of the supercontinent cycle that links the Rodinia to Pangea supercontinents, in particular the role played by Baltica-derived terranes in Paleozoic collisional orogeny.

Two magma fractionation paths for continental crust growth: Insights from the adakite-like and normal-arc granites in the Ailaoshan fold belt (SW Yunnan, China)

Abstract Geochemical similarities between the continental crust and arc magmas have led to the inference that subduction zones may be the primary sites of crustal growth. Thus, it is necessary to unravel the petrogenetic mechanism(s) of granitoid generation in subduction-related settings to understand crustal growth through magmatic differentiation processes. In this study, we focused on granitoid generation in oceanic-continental subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex (Yunnan, SW China). All the studied granite samples were characterized by large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high field strength element (e.g., Nb, Ta, and Ti) depletions, similar to arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020–0.7048, εNd(t) = +0.6 to +4.2, and zircon Hf-O, εHf(t) = +10.3 to +18.1, δ18Ozircon = 5.09‰–6.65‰, isotope compositions, which overlap with those of previously reported coeval (ca. 237–235 Ma) hornblende diorite and granodiorite, the formation of which was interpreted to have originated from a mantle wedge metasomatized by a sediment-derived melt. Furthermore, the fractionation trends of some of the granitic samples and diorite-granodiorite suite overlap. They can be divided into two geochemical groups: Group 1 has intermediate to high SiO2 (66.9–73.8 wt%) and K2O (3.40–5.42 wt%) and low MgO (0.19–1.09 wt%) contents and shows depletion in heavy rare earth elements (HREEs; e.g., Yb and Y), resulting in adakite-like high Sr/Y (61–183) and La/Yb (47–90) ratios. Group 1 shows positive SiO2 versus Sr/Y and La/Yb correlations and negative SiO2 versus HREE and Y correlations, implying fractionation of a garnet-bearing assemblage. The negative correlations between SiO2 and εNd(t) and Nb/La reveal a crustal assimilation trend. Group 2 has relatively high SiO2 (72.6–76.5 wt%) and low K2O (1.93–3.82 wt%) and MgO (0.05–0.83 wt%) contents and shows depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1–10) and La/Yb (4–11) ratios. Group 2 granites show negative Gd/Yb versus SiO2 correlation, which indicates significant fractionation of an amphibole-bearing assemblage. Our results suggest that both group 1 and 2 granites were formed in a subduction setting from a common mantle-derived parental dioritic magma, but they experienced two distinct fractionation processes. While group 1 granites were likely formed by crustal assimilation and high-pressure (lower-crustal) garnet-dominated fractionation, group 2 granites were generated through low-pressure (middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We suggest that these two fractionation trends are critical to crustal growth and the development of a more fractionated (felsic) upper crust.

Variscan granitic magmatism in the Western Carpathians with linkage to slab break-off

The Variscan basement within the Western Carpathian Alpine architecture generally consists of metaluminous/peraluminous tonalite/granodiorite massifs and high-grade metamorphic complexes of metapelites, metaultramafites, and metabasites with relics of eclogites. Unfortunately, the Variscan crystalline basement of the Western Carpathians is only fragmentally exposed. Therefore, the proposed geodynamic evolutionary model for the Variscan granites of the Western Carpathians is primarily based on granite data from the Malá Fatra Mts. with additional dating from the High Tatra Mts. The oldest magmatic age of 362 ± 4 Ma in the Malá Fatra horst was recorded in diatexites from a high-grade metamorphic complex, which is related to crustal anatexis during Variscan subduction. Subsequent collisional event and break-off of the subducted slab promoted exhumation of the diatexites within the high grade metamorphic complex and intrusion of 353 ± 3 Ma old Tournaisian tonalite. Intensive heat input after slab break-off from the rising asthenosphere generated melting of the lower crust and extensive calc-alkaline, Mg-rich granitic magmatism in a short time span from 347 ± 4 to 342 ± 3 Ma. These Visean granitic rocks caused thermal overprint on the roof metamorphic rocks, including diatexites and Tournaisian tonalites at ca. 348 ± 5.6 to 342 ± 3 Ma. The Visean granite formation was controlled by the mixing of hot magmas, which is indicated by the presence of composite oligoclase/andesine plagioclase with preserved labradorite cores, alkali feldspars with Na2O ≥ 2 wt%, zoned apatite, the presence of antiperthite, and quartz ocelli. Elevated contents of mantle-derived elements like V, Ni, Cr, Ba, high Sr/Y ratio of ~44, steep LREE and flat HREE segments of chondrite-normalised patterns document adakite-like feature of the investigated granitic rocks which resulted from melting of a mixed lower-crustal and mantle sources and crystallisation in the presence of garnet. Unusual abundance of Fe–Ti oxides in granodiorites with magmatic cooling temperatures of 735–756 °C supports high-T input from mantle. In the High Tatra Mts., diorite xenolith shows the age of 359.2 ± 3 Ma, and its host granodiorite the age of 350.1 ± 2.6 Ma. The diorite contains acicular zircons, which points to rapid exhumation. Stubby zircon of the host granodiorite shows regular, oscillatory zoning controlled by a gradual temperature decrease. The non-comagmatic relationships between diorite and host granodiorite are indicated also by a difference in zircon Th/U ratio, which is 0.2 for the host granodiorite, but 1.0 for the diorite on average. The presented data show that slab break-off could have been a mechanism that promoted Variscan granitic magmatism in the Western Carpathians.

Timing and kinematics of the Variscan orogenic cycle at the Moldanubian periphery of the central Bohemian Massif

The high-grade metamorphic complexes along the northern Moldanubian periphery of the central Bohemian Massif provide an outstanding structural record of all episodes of the collisional evolution of the Variscan Orogeny. The kinematics and timing of the orogenic processes were examined by structural and microstructural studies of middle and lower crustal rocks combined with xenotime and monazite geochronology. Four distinct tectonic events were identified. A first relict sub-horizontal fabric S1 associated with high-pressure–high-temperature metamorphism was only developed in the lower crustal rocks and was related to back-arc extension or lower crustal flow in a supra-subduction domain. This fabric was completely reworked to the subvertical foliation S2 at c. 340 Ma by major collisional thickening, leading to juxtaposition of the lower and middle crust. Thereafter, extensional collapse of the thickened orogenic system caused strong refolding to the high-temperature sub-horizontal fabric at c. 325 Ma. The region was subsequently affected by NNE–SSW-oriented horizontal shortening related to dextral shearing and the clockwise rotation of crustal blocks adjacent to the large-scale dextral shear zone (the Elbe Zone). This led to fragmentation and reorientation of the Moldanubian margin to its current position.Supplementary material: A summary of the main microstructural features related to the defined deformation phases, the results of the U–Pb laser ablation inductively coupled plasma mass spectrometry analyses of xenotime and monazite, back-scattered electron images and rare earth element compositional maps are available at https://doi.org/10.6084/m9.figshare.c.5708800.v1

Melt- to Shear-Controlled Exhumation of Granulites in Granite–Gneiss Domes: Petrological Perspectives from Metapelite of the Neoarchean Ha-Tshanzi Structure, Central Zone, Limpopo Complex, South Africa

Abstract Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean–Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810–830 °C) exhumation from pressure of 10·0–10·2 kbar to 7·5–7·0 kbar during the Neoarchean event (2·68–2·62 Ga). The exhumation mediated by anatectic melt supports interpretation of the Ha-Tshanzi structure as a diapir-related granite–gneiss dome. During upwelling of the dome, the melt segregated into leucosome, while growth of cordierite sequestered water from the melt, assisting its crystallization at the end of the sub-isothermal decompression stage. As the rheology of the rock changed, melt-dominated deformation was transformed to solid-dominated ductile shear deformation. In contrast to the earlier sub-isothermal decompression P–T path, the gentler slope of ~75 °C kbar–1 of the decompression-cooling path marks the exhumation from pressures of ~7 kbar to pressures of 5–4·5 kbar and cooling to 600–550 °C. Dating of zircon, monazite and rutile shows that the Neoarchean evolution of the metapelite was strongly overprinted by the Paleoproterozoic event at c. 2·01 Ga. The results of the study highlight the significance of domed structures related to granitic diapirs in the exhumation of the Central Zone of the Limpopo Complex.

Metamorphic evolution of Daqingshan supracrustal rocks and garnet granite from the North China Craton: Constraints from phase equilibria modelling, geochemistry, and SHRIMP U–Pb geochronology

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.

The polyphase rifting and inversion of the Danube Basin revised

Abstract To track the evolution and formation of one of the major sub-basins of the Pannonian back-arc basin system, we re-evaluate pre-rift, syn-rift, post-rift, and inversion stages of the Danube Basin. This synthesis builds upon a new compilation of (i) geophysical measurements, (ii) current data about the substratum of the basin, (iii) recent advances in stratigraphic architecture of the basin fill, (iv) subsidence history, (v) geomorphological evolution, and (vi) new biostratigraphic and geochronological data.The history of the basin begins with the pre-rift stage represented by the pre-Neogene basement, which belongs to an Eo-Alpine mountain range uplifted at the end of the Cretaceous. The core of the mountain range was formed by pre-Alpine high-grade metamorphic complexes, with a dome-like structure. The crystalline complexes were overlain by late Paleozoic and Mesozoic cover and nappes, which were later eroded during the early Cenozoic. These pre-rift processes were triggered by (i) the late Cenozoic collision of the Alpine-Carpathian orogenic wedge with the European platform, (ii) the crustal extension resulting from the northeastward lateral extrusion of the Alpine-Carpathian-Pannonian lithospheric microplate, and (iii) the subsequent updoming of the lithospheric mantle. This stage was followed by the polyphase rifting of the basin associated with extensive volcanic activity. This syn-rift stage consists of four phases. The first syn-rift phase dated to the early Badenian (~15.2–13.8 Ma), with subsidence documented in the Blatne and Želiezovce depressions, was characterized by accommodation rates up to ~900 m·Ma-1. The second syn-rift phase dated to the late Badenian (~13.8–12.6 Ma), exhibited accelerated subsidence with accommodation rates up to ~2300 m·Ma-1, and mainly affected the Blatne depression. The third and fourth syn-rift phases took place during the Sarmatian (~12.6–11.6 Ma) and early Pannonian (~11.6–9.5 Ma) and the accommodation rates reached ~900 m·Ma-1 and ~1000 m·Ma-1, respectively. They were documented in the Risňovce, Komjatice and Gabcikovo-Győr depressions, confirming the eastward subsidence migration, up to the external zone of the middle Miocene mantle upheaval. The late Miocene post-rift stage, commonly refered to as a thermal subsidence stage (~9.5–6.0 Ma), was associated with formation of a widespread planation surface called the “Mid-mountain level” on the margins of the basin. The history of the Danube Basin terminates with the inversion stage which began after ~6.0 Ma and is still ongoing. It includes the continuous uplift of the basin margins and the subsidence of the central depocenters. The fluvial depositional systems did not behave steadily, because (i) lateral channel mobility and area of deposition increased progressively until ~4–3 Ma and caused onlaps on the base of the succession, followed by (ii) gradual confinement of the streams to the central part of the basin by uplifting basin margins, recorded by formation of river terrace staircases.

Geology of the Montigiu Nieddu metamorphic basement, NE Sardinia (Italy)

ABSTRACT A geological map at 1:10,000 scale is presented that shows the distribution of the metamorphic rock units in the area of Montigiu (Mt.) Nieddu (NE Sardinia). These units belong to the High-Grade Metamorphic Complex in the Inner Zone of the Variscan chain of Sardinia. The rocks include different types of migmatite, amphibolite, and retrogressed eclogites. The geological map and cross section show the complex tectonic and metamorphic setting of the area, whereas the metamorphic assemblages and deformation framework represent a valuable tool for the reconstruction of the P-T metamorphic evolution for the crustal sectors involved in the Variscan orogeny. The metamorphic rock units reported in the map derived from the lower and upper continental plates involved in the Variscan collision and were tectonically juxtaposed during the early Carboniferous exhumation stage.

Geochronological and geochemical study of the high-grade orthogneissic rocks from the Northern Fuyun Complex: Implications for crustal architecture and Early-Middle Paleozoic evolution of the Chinese Altai

High-grade metamorphic complexes are critical for the understanding of regional crustal configuration and accretionary history of the Altai orogenic belt. The orthogneissic rocks from the Northern Fuyun Complex, southern Chinese Altai, are investigated in this study. Zircon UPb isotopic analysis yields ages of ca. 411–398 Ma, arguing against previous views that these rocks represent old Precambrian basement. All the samples are calc-alkaline, metaluminous to weakly peraluminous (ASI = 0.80–1.06), and can be classified as I-type granitoids. They have low to medium Al2O3 and relatively high ferromagnesian compositions, and most of them exhibit weak heavy rare-earth element (HREE) fractionation and variably negative Eu anomalies. The above geochemical compositions are consistent with dehydration melting of meta-basaltic or amphibolitic crustal rocks followed by later fractional crystallization for their derivation. In addition, their positive zircon εHf(t) (average = 2.58–8.38) and mantle- to CHUR (chondritic uniform reservoir)-like whole-rock εNd(t) (−1.6 to +2.9) values indicate a relatively juvenile signature of their source rocks, which suggests that the deep crustal section of the Chinese Altai may be dominated by juvenile mantle-derived materials. The high MgO contents (2.06–3.66 wt%) of some samples and the occurrence of the mafic microgranular enclave may reflect the involvement of mantle materials in their petrogenesis, implying possible anatexis of the lower crust driven by basaltic underplating. By synthesizing magmatic, metamorphic, and structural geological evidence, we interpret that the Chinese Altai experienced a lithospheric-scale extensional event in the Devonian, which resulted in massive heat influx, emplacement of juvenile magmas, syn-orogenic crustal anatexis, and formation of mature and layered continental crust. The available data further suggest that alternating contraction and extension have probably operated on the Chinese Altai in the Early-Middle Paleozoic, which is possibly critical for the evolution of accretionary systems worldwide.

Reassessing the PT conditions of Neoproterozoic collisional metamorphism and partial melting in southernmost Brazil

Partially equilibrated textures and partial melting products are key features in the study of high-grade metamorphic complexes, but their complexity limits the advancement of knowledge about the tectono-thermal history of ancient orogens. This difficulty is apparent in the wide interval of conditions (720–1000 °C/3–10 kbar) determined by previous studies in a high-grade complex formed during the amalgamation of West Gondwana in southernmost Brazil. In order to understand the orogen, we reanalyse the metamorphic evolution of the Várzea do Capivarita Complex through field data, petrography, whole-rock and mineral chemistry, and thermodynamic modelling. Peak metamorphic conditions (M1) are constrained with diverse mineral assemblages (Bt-Crd-Sil-Grt-Ilm/Bt-Sil-Grt-Ilm/Bt-Grt-Ilm) and overlap at 800 °C/4.5 kbar, while the partially equilibrated domain (Crd-Hc-Sil) indicates post-peak conditions (M2) of 680 °C/2.8 kbar. Models predict leucosome volume up to 20%, in agreement with field and petrographic estimates, attesting the migmatitic character. Leucogranitic veins correlating petrographically and compositionally with leucosome and melt estimates reinforce this character and the thermodynamic modelling, even with low correlations, which are explained by inherited minerals. Thus, M1 and M2 conditions indicate exhumation from depth of 16 to 10 km and high geothermal gradient, which are interpreted as due to anomalous mantle heat flow and rapid exhumation facilitated by large volumes of melt.

Cenozoic multi-metamorphism, shear deformation and geological significance of Ailaoshan high-grade metamorphic complex, western Yunnan, China

Cenozoic multi-metamorphism, shear deformation and geological significance of Ailaoshan high-grade metamorphic complex, western Yunnan, China

Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand

Abstract The NW–trending Three Pagodas shear zone exposes a high–grade metamorphic complex named Thabsila gneiss in the Kanchanaburi region, western Thailand. The quartz mylonites within this strike–slip zone were selected for strain analysis. 2–dimensional strain analysis indicates that the averaged strain ratio (Rs) for the lower greenschist facies increment of XZ– plane is Rs = 1.60–1.97 by using the Fry’s method. Kinematic vorticity analysis of the quartz mylonites in the shear zone showed that the mean kinematic vorticity number of this increment is Wk = 0.75–0.99 with an average at 0.90 ±0.07. The results implied that the quartz mylonites within the Three Pagodas shear zone have a dominant simple shear component of about 72% with a small pure shear component. A sinistral shear sense is indicated by kinematic indicators from macro– to micro–scale. We conclude that the Three Pagodas shear zone deformed in the process of sinstral shear–dominated transpression, which is similar to the Mae Ping shear zone in the north.

Neoproterozoic complexes of the shelf cover of the Dzabkhan terrane basement in the Central Asian Orogenic Belt

The formation stages of high-grade metamorphic complexes and the related granitoids of the Dzabkhan terrane basement are considered. The age data (U–Pb method, TIMS) of zircons from the trondhjemite block of the eastern part of the Dzabkhan terrane, which is directly overlain by the dolomite sequence of the Tsagaan Oloom Formation, are given. Trondhjemites yield the U–Pb zircon age of 862 ± 3 Ma. In their structural position, they are assigned to typical postmetamorphic formations that determine the formation and cratonization of rocks of the host block. The geochronological study of trondhjemites gives grounds to distinguish fragments of the continental crust in the Dzabkhan terrane basement, the formation of which occurred at different periods of time: ∼860 and ∼790 Ma. Geological–geochronological and Sm‒Nd isotope–geochemical studies indicate that the Dzabkhan terrane basement is not a single block of the Early Precambrian continental crust, but a composite terrane, comprising Neoproterozoic ensialic and island-arc structural and compositional complexes. Correlation of Sr isotopic characteristics with the 87Sr/86Sr variation curve in the Neoproterozoic and Cambrian seawater shows that carbonate deposits accumulated at the eastern margin of the Dzabkhan terrane near the end of the Neoproterozoic, 700–550 Ma, and in the central part of the terrane in the Early Cambrian, 540–530 Ma.

Rodinian granulites from southern Qiangtang terrane: Implications for tectonic evolution of the Tibetan Plateau

Bordered by the Longmu Co-Shuang Hu-Lancangjiang suture zone in the north and east, as well as the Bangong Nujiang suture zone in the south and west, the southern Qiangtang terrane is one of the least-studied blocks in the Tibetan region. The early tectonic evolution of the southern Qiangtang terrane has remained a controversy for a long time. Here we report granulites from the Tongka high-grade metamorphic complexes in the southern Qiangtang terrane. The mafic granulites occur as small lenses in the felsic granulites and leucogranites with peak mineral assemblage including garnet (core) + diopside + low An plagioclase + quartz. The retrograde assemblage is represented by garnet (rim) + pargasite + high An plagioclase + quartz. The fine-grained aggregates of feldspar + garnet + aluminosilicates + rutile around reddish-brown biotite relics in the felsic granulites are interpreted as products of partial melting of biotites. Thermobarometric estimates for the peak assemblages of the mafic rocks are P = 17.5 kbar and T = 811 °C, whereas the retrograde assemblages of the mafic rocks experienced decompression to P = 10.4 kbar, at T = 674 °C. Zircon texture and SIMS U-Pb data reveal that the felsic granulites were derived from Achaean to Proterozoic sedimentary rocks that underwent high-temperature metamorphism and transformation to paragneisses or granitoids at 1100 to 1000 Ma. During 950–900 Ma, the crystalline rocks were further buried deeply to be transformed into the felsic granulites. We therefore interpret the Tongka high-grade metamorphic complexes to be a portion of the Rodinian continental collision belt. Evidently some domains of the Rodinian continental crust were thickened during the Grenville events, similar to the more recent processes in the Himalaya and Tibetan plateau. The Tongka high-grade metamorphic complexes which form the basement of the southern Qiangtang terrane was subsequently disrupted and involved in the formation of the great Tibetan plateau during the Cenozoic times. Our work thus highlights the Precambrian history of the southern Qiangtang terrane.

PALEOARCHEAN MAFIC ROCKS OF THE SOUTHWESTERN SIBERIAN CRATON: PRELIMINARY GEOCHRONOLOGY AND GEOCHEMICAL CHARACTERIZATION

The Siberian craton consists of Archean blocks, which were welded up into the same large unit by ca 1.9 Ga [Gladkochub et al., 2006; Rojas-Agramonte et al., 2011]. The history of the constituent Archean blocks is mosaic because of limited number of outcrops, insufficient sampling coverage because of their location in remote regions and deep forest and difficulties with analytical studies of ancient rocks, which commonly underwent metamorphic modifications and secondary alterations. In this short note, we report data on discovery of unusual for Archean mafic rocks of ultimate fresh appearance. These rocks were discovered within southwestern Siberian craton in a region near a boundary between Kitoy granulites of the Sharyzhalgai highgrade metamorphic complex and Onot green-schist belt (Fig. 1). Here we present preliminary data on geochronology of these rocks and provide their geochemical characterization.

EARLY NEOPROTEROZOIC CRUST FORMATON IN THE DZABKHAN MICROCONTINENT, CENTRAL ASIAN OROGENIC BELT

The Dzabkhan microcontinent was defined by [Mossakovsky et al., 1994] as a cratonic terrane with an early Precambrian basement that combines highgrade metamorphic complexes of the Songino, Dzabkhan, Otgon, Baidarik, Ider and Jargalant Blocks. However, early Precambrian ages have so far only been recognized in the Baidarik and Ider blocks [Kozakov et al., 2007, 2011; Kroner et al., 2015].

Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints

In the early Cambrian Bithia Formation in the Variscan foreland of Sardinia, a Middle Ordovician granitic intrusion (478–457Ma) is hosted by marly metasedimentary rocks that were affected by high-temperature (HT) metamorphism. A detailed structural–petrographical transect was conducted through the granitic intrusion and its host rocks. Field data and relationships between HT/low-pressure (LP) mineral assemblages in the metasedimentary rocks (Grt+Wo+Ves in carbonate lenses and And in pelite) demonstrate that the study area was affected by a polyphase HT overprint (I: T=520–620°C at XCO2=0.1, P: 0.2–0.4GPa; and II: T=600–670°C at XCO2=0.1, P=0.2–0.4GPa) that pre-dates the Variscan tectonic, metamorphic, and igneous phases.In the Canigò or Canigou Massif (Eastern Pyrenees), the Somail Massif (Montagne Noire), and the Ruitor Massif (Internal Massifs, NW Alps), Middle Ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by Variscan amphibolite-to-granulite-facies metamorphism and subsequent Alpine low-grade metamorphism. Comparisons of associated igneous and metasedimentary rocks in the Sardinia foreland with the High-Grade Metamorphic Complex in the Variscan Axial Zone and the Canigou Massif indicate a convergent Middle Ordovician evolution that was overprinted by HT Variscan metamorphism.