Mylonitic Granite Research Articles

Petrogenesis of Early Paleozoic I-type granitoids in the Wuyi-Yunkai Orogen, South China: Implications for the tectono-magmatic evolution of the Cathaysia Block

• The Early Paleozoic I-type granites were generated through partial melting, magma mixing and fractional crystallization . • Mantle-derived magmas provided the heat and material for the formation of the granites. • The granites formed during an intracontinental orogeny . • The granites were related to the assembly of northern East Gondwana . The tectonic setting of the Early Paleozoic granites in the South China Block (SCB) remains enigmatic, and it is uncertain as to whether they are related to intracontinental orogenic processes or oceanic subduction. Previous studies mostly focused on Early Paleozoic S–type granitoids in the SCB, and relatively little attention has been paid to Early Paleozoic I–type granitoids. Here we present whole–rock major and trace elemental data and LA–ICP–MS zircon U–Pb age and Lu–Hf isotopic data for hornblende monzogranites, monzogranites, and mylonitic granites of I–type granites affinities that comprise the Daning and Guiling plutons in the southwest part of the Wuyi–Yunkai orogen in the SCB. These rocks contain zircon with U–Pb ages of 422–418 Ma, taken as the age of emplacement. They are rich in LREEs and moderately depleted in Eu (δEu = 0.49–0.64), with negative Ba, Sr, Nb, and Ti anomalies, and positive in Rb, Th, U, and Pb anomalies. Our studies of these granites and their abundant enclaves indicate that they formed via complicated petrogenetic processes, including the partial melting of a mixed source of meta–sedimentary rocks and meta–basalts in the lower crust, magma mingling and mixing of crust–mantle materials, and fractional crystallization. Mantle–derived magma played an important role in the formation of these Early Paleozoic granites. Although an ancient and enriched mantle source might have made a significant contribution to these Early Paleozoic granites, the input of juvenile mantle–derived magma might have been insignificant. Taking into account all the other geological evidence from the Wuyi–Yunkai region, we suggest that the tectono–magmatic events of the Early Paleozoic took place during intraplate orogenesis rather than oceanic subduction and collision, and that these events were triggered by a far–field tectonic response to the assembly of northern East Gondwana.

40Ar/39Ar dating of Paleoproterozoic shear zones in the Ellesmere–Devon crystalline terrane, Nunavut, Canadian Arctic

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the 40Ar/39Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U–Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have 40Ar/39Ar ages in the range of ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at ≈1870–1840 Ma, or ≈90 million years after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

Geochronology and genesis of the newly discovered Mengdehe orogenic-type Au deposit in the Xing’an-Mongolia orogenic Belt, NE China

The Xing’an-Mongolia Orogenic Belt (XMOB) is a tectonic window to understand the subduction-orogenesis-magmatism and corresponding Au mineralization during the Mesozoic. Although epithermal gold mineralization associated with Paleo-oceanic plate subduction has been widely reported, information on orogenic Au metallogenesis during this period is insufficient. Geochronology and genesis study of the newly discovered orognenic Au deposit named as Mengdehe located at the Heihe-Nenjiang tectonic mélange in the eastern XMOB, can provide new information about regional metallogenetic mechanisms in this region. The Mengdehe Au deposit is the first discovered mylonitic rocks-hosted and brittle-ductile shear zone-dominated Au deposit in this area. Magmatic zircon U-Pb ages from the granitic mylonite and undeformed diorite in the Mengdehe deposit constrain that the strike-slip event probably has happened between 245.8 and 208.0 Ma. Rb-Sr dating of hydrothermal pyrites from the ore-bearing altered rocks indicates that Au mineralization occurred at 209.6 ± 3.1 Ma. We consider that the Mengdehe Au deposit was formed in the Late Triassic and associated with a strike-slip event. The trace elements, and He-Ar, S and Pb isotopic compositions of the ore-forming stage pyrites suggest that the ore-forming materials were derived from crustal sources with orogenic affinity. Combined with the early Mesozoic regional tectonic evolution in NE China and deformation characteristics of wall rocks, hydrothermal minerals and diorites, we conclude that the Au mineralization of the Mengdehe deposit was in response to the tectonic transformation from compression to extension associated with the extensional collapse of a previous thickened continental lithosphere related to the final closure of the Paleo-Asian Ocean.

Interpretation of hydrothermal evolution in the Qolqoleh gold deposit, southwest of Saqqez, Iran: Analysis of pyrite by LA-ICP-MS and sulfur isotopes

The Qolqoleh gold deposit is located in the northern part of the Sanandaj-Sirjan Zone (SaSZ), northwest Iran. The main host rocks include mylonitic granite and chlorite schist. Four generations of Au-bearing sulfides have been identified and are referred to as Py1 to Py4. LA-ICP-MS elemental maps and spot analyses of gold and other trace elements in Py1 to Py4 were performed to provide robust information about the ore-forming fluid and conditions of saturation. Element compositions in all generations of pyrite show heterogeneity in fluid compositions and reflect the role of a multistage hydrothermal system in their formation. The earliest hydrothermal event is related to magmatic fluids synchronous with magmatic fluids from a granitic pluton emplacement in Carboniferous time. Multistage magmatism and related heating caused reaction, replacement, and recrystallization (RRR) of original Py1, generating overgrowths of Py2 to Py4. The variation in the Au-As rim compositions shows that fluctuating temperatures occurred during overgrowths of pyrite. The ratios of Co/Ni for most Py1 to Py4 are higher than 1. This along with the high Te contents infer that the ore-forming fluids came from mainly a magmatic source with low redox conditions. The increase in Au and other elements in Py2 and Py4 shows enrichment through the coupled dissolution-reprecipitation reaction (CDRR) mechanism in response to greater non-metamorphic fluids. The bulk δ34S values range from 3.1 to 6.9‰. Two samples range from 13.8 to 14.3‰, which are compatible with a main magmatic source and to a minor metamorphic source for the ore-forming fluids. Subsequently, dextral movements caused brittle-ductile deformation and dislocation of mineralization and mylonitization. However, this study determines that the Qolqoleh deposit is a magmatic-related gold deposit.

Feldspar microboudinage paleopiezometer and its applications to estimating differential stress magnitudes in the continental middle crust (examples from west Yunnan, China)

Feldspar, which is the most abundant crustal mineral, exhibits brittle behavior while quartz and mica deform plastically by dislocation creep, under the greenschist-facies conditions (300–450 °C). Pull-apart microboudinage, which formed by repeated cycles of tensile fractures, is a common microstructure observed in feldspar porphyroclasts embedded in the ductile matrix of quartz in granitic mylonites from the Red River, Chongshan and Gaoligong shear zones (Yunnan, China). These crustal-scale shear zones, which lie in the southeastern margin of the Tibet Plateau, have played a critical role in the southeastward lateral extrusion of the Indochina composite block in response to the Indo-Asian continental collision. Here we developed a feldspar microboudinage paleopiezometer. In each feldspar porphyroclast, the differential stresses estimated from microboudins progressively increases with sequential fracturing, depicting an evolution during exhumation of the mylonite upwards the brittle-ductile transition. The results from the feldspar microboudinage piezometer and the quartz recrystallized grain-size piezometer are largely consistent within the range of uncertainties. The microboudinage piezometer, combined with results of finite strain measurements, deformation temperature estimates, and thermochronological data, can yield the valuable information about rheological properties of naturally deformed rocks. It can also be directly used or adapted to a wide range of geological problems, for example, fractured pebbles in an unlithified matrix, chromite pods in serpentinized mantle rocks, and strong boudins in a soft material at any scale.

Brittle–ductile fabrics and P – T conditions of deformation in the East Pernambuco shear zone (Borborema Province, NE Brazil)

Fabrics of the East Pernambuco shear zone (EPSZ) were studied via microstructural analysis, mineral chemistry and isochemical phase diagram modelling to constrain the pressure and temperature conditions of deformation during shearing. Granitic mylonites show fractured feldspar porphyroclasts embedded in a fine-grained, recrystallized quartzo-feldspathic matrix. These mylonites grade laterally into banded ultramylonites characterized by stretched feldspar clasts alternated with recrystallized quartz bands. Fractures in these ultramylonites are filled by phyllosilicates. The mineral chemistry of the feldspars points to systematic changes between porphyroclasts, grains within fractures and fine-grained mixtures. Quartz crystallographic fabrics in the mylonites suggest activation of prism slip, while the ultramylonites show the activation of both rhomb and basal slip systems. Thermodynamic modelling suggests that the mylonites were formed at 4.75 ± 0.25 kbar and 526 ± 9°C, while the ultramylonites yield conditions of 5.9 ± 1 kbar and 437 ± 17°C. These observations suggest that the EPSZ records a heterogeneous path of strain accommodation, marked by decreasing temperature from its western sector to its eastern termination. The differences in metamorphic conditions are consistent with a transitional, brittle–ductile strain regime. Such characteristics indicate that the EPSZ is a Neoproterozoic shear belt nucleated and heterogeneously exhumed at the brittle–ductile transition, possibly in an intracontinental setting. Supplementary Material: EPMA analysis of feldspars in Caruaru and Gravatá domains and T-X(O 2 ) pseudosections are available at https://doi.org/10.6084/m9.figshare.c.5125957

Kinematics, strain patterns, rheology, and geochronology of Woka ductile shear zone: Product of uplift of Gangdese batholith and Great Counter Thrust activity

The Woka ductile shear zone is located at the southern margin of the middle section of the Gangdese magmatic belt and southeast of Lhasa. This article is a systematic study of the Woka ductile shear zone and provides kinematics, strain patterns, rheology, temperature environment, and geochronological data of the shear zone, which have great significance for understanding of the uplift and extension process of the southern section of the Gangdese. All shear indicators, such as asymmetrical folds, rotated porphyroclasts, and S–C fabrics, developed in the shear zone indicate the shear sense of top‐to‐the‐northwest with the shear nature of detachment. Mineral deformation thermometer and EBSD quartz fabric analysis indicate that the shear zone has experienced early high‐temperature (550–650°C) deformation and post‐superimposed low temperature (380–420°C) deformation. The differential stress (σ) of the ductile shear zone was 28.08–46.73 MPa. Strain rates (έ) ranged from 10−10 to 10−14 s−1 under a high‐temperature environment (600°C) and 10−12 to 10−16 s−1 under a low‐temperature environment (420°C). Three‐dimensional strain ellipsoid measurements of the Woka ductile shear zone indicate that all deformed samples appear as prolate ellipsoids (LS‐type tectonites), which represent an elongate strain. The average kinematic vorticity of the Woka ductile shear zone is 0.93, implying that the shear zone was dominated by simple shear. The thickness of the ductile shear zone was reduced by 12–28%. Combined with the U–Pb zircon age of the protolith of granitic protomylonite, the 40Ar/39Ar sericite age of granitic mylonite (22.38 ± 0.31 Ma) can represent the shear age. The Woka shear zone studied in this article may have been formed by the Great Counter Thrust (GCT) activity and uplift of the Gangdese batholith during the transforming period of the Lhasa Terrane from extrusion to a lateral extension.

Provenance of neoproterozoic ophiolitic mélange sediments in the brasilia belt, central Brazil

Mélanges are a chaotic mixture of different rock assemblages that can be associated with orogenic belts and suture zones. The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, which contains a poorly understood ophiolitic mélange. In order to better understand the sedimentation that was associated with this mélange, isotopic U–Pb and Lu–Hf analyses on single zircon grains, and Sm–Nd on whole rock samples was conducted on mélange metasedimentary rock (garnet-mica-quartz schists and quartzites) and basement (mylonitic granites) assemblages. The quartzite samples have a maximum age of deposition of about 930 Ma, while their negative ƐNd (T) values and TDM, −7.78 to −11.57 and 1.70 to 2.07 Ga, respectively, are similar to other metasedimentary rock units interpreted to have formed in a passive margin environment. ƐHf and Hf TDM model values from the quartzite samples range from −12.72 to 8.99 and 2.56 to 1.36 Ga, respectively. The mylonitic granite has an age of ~2.1 Ga, ƐHf values ranging from −0.97 to 2.18, and TDM model values ranging from 2.38 to 2.53 Ga, indicating that it was derived from juvenile Paleoproterozoic basement. Results of the garnet-mica schists reveal that they represent two different rock units, defined by two different periods of sedimentation. The first group of garnet-mica schist has a maximum age of deposition of 672 Ma and ƐNd (T) values and TDM -7.90 to −7.62 and 1.82 to 1.88 Ga, respectively. ƐHf and TDM model values range from −23.50 to 11.69 and 3.23 to 1.45 Ga, respectively, and indicate the presence of similar zircon populations as the passive margin sequence and basement zircon population; there is also a contribution of a younger zircon population derived from older reworked crustal material. The second group of garnet-mica schists is marked by a maximum age of deposition of 616 Ma and ƐNd (T) values and TDM ranging from 0.61 to 0.22 and 1.16 to 1.25 Ga, respectively. ƐHf and TDM model values range from −8.24 to 16.04 and 2.97 to 0.79 Ga, respectively. These results also indicate the presence of passive margin and basement zircon populations, but the younger zircon populations are derived from juvenile sources. The depositional periods of the garnet-mica schists occurred between roughly 800 to 650 Ma and 650 to 600 Ma, and indicate that the rocks from the Brasília Belt register the presence of different depositional environments and basins, which opened, closed and were later dismembered. The quartzites, mylonitic granites and ophiolites most likely represent blocks that were introduced into the mélange by sedimentation or tectonic processes.

The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India

Abstract Amphibolite facies supracrustal rocks interleaved with granite mylonites constitute a shallowly dipping carapace overlying granulite facies anatectic basement gneisses in the Giridih-Dumka-Deoghar-Chakai area that spans ~11,000 km2 in the Chottanagpur Gneiss Complex (CGC). Steep N-trending tectonic fabrics in the gneisses include recumbent folds adjacent to the overlying carapace. The basement and carapace are dissected by steep-dipping sinistral shear zones with shallow/moderately plunging stretching lineations. The shear zones trend NNE in the north (north-down kinematics) and ESE in the south (south-down kinematics). Chemical ages in metamorphic monazites in the lithodemic units are overwhelmingly Grenvillian in age (1.0–0.9 Ga), with rafts of older domains in the basement gneisses (1.7–1.45 Ga), granitoids (1.4–1.3 Ga), and the supracrustal rock (1.2–1.1 Ga). P-T pseudosection analysis indicates the supracrustal rocks within the carapace experienced postthrusting midcrustal heating (640–690°C); the Grenvillian-age P-T path is distinct from the existing Early Mesoproterozoic P-T path reconstructed for the basement gneisses. Quartz opening angle thermometry indicates that high temperature (~600°C) persisted during deformation in the southern shear zone. Kinematic vorticity values in carapace-hosted granitoid mylonites and in steep-dipping shear zones suggest transpressional deformation involved a considerable pure shear component. Crystallographic vorticity axis analysis also indicates heterogeneous deformation, with some samples recording a triclinic strain. The basement-carapace composite was extruded along an inclined channel bound by the steep left-lateral transpressional shear zones. Differential viscous extrusion during crustal shortening coupled with the collapse of the thickened crust caused midcrustal flow along flat-lying detachments in the carapace.

Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics

This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type-1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type-2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530°C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.

Tourmaline microboudinage: An indicator of its host rheology

Columnar tourmaline in Cenozoic granitic mylonites from the Chongshan metamorphic zone (CMZ), Yunnan, China has deformed, at 450–300 °C, by tensile fractures normal to the foliation and lineation. Sequential fracturing and separation of the fragments during progressive cooling and exhumation upward the brittle-ductile transition produced a series of microboudins with decreasing length/width ratios. Here we present a method to apply the microboudinage paleopiezometry, combined with strain analyses, microstructural observations and thermochronological data to provide constraints on strain rates (ε˙), differential stresses (σ) and their correlations with temperature for the polycrystalline quartz matrix in which tourmaline grains have been fragmented into microboudins. The results yield a flow law of ε˙ = Aσnexp(-Q/RT) with n = 4, Q = 184 kJ/mol and A = 7.180 × 10−7 MPa-ns−1 for naturally deformed quartzite. The differential stresses calibrated by this flow law are very close to the geometrical mean of the lower and upper bounds given by Luan and Paterson (1992) and Gleason and Tullis (1995), respectively. Utilization of this flow law to the present-day crust of the CMZ provides a reliable interpretation for the distribution of focal depths and electrical resistivity structure. The microboudinage piezometer also permits to estimate the mechanical strengths of rocks deformed in the elastic-brittle regime.

Microstructural and Geochronological Analyses of Mesozoic Ductile Shear Zones in the Western Gyeonggi Massif, Korea: Implications for an Orogenic Cycle in the East Asian Continental Margin

In response to orogenic cycles, the ductile shear zone records a complex crustal deformation history. In this study, we conducted a microstructural analysis of two NW–SE trending ductile shear zones (Deokjeok Shear Zone (DSZ) and Soya Shear Zone (SSZ)) in the Late Triassic post-collisional granites along the western Gyeonggi Massif in the Korean Peninsula. The DSZ, overlain by the Late Triassic to the Early Jurassic post-collisional basin fill (Deokjeok Formation), has asymmetric microstructures indicative of a top-down-to-the-northeast shear. Depending on the structural position, the SSZ, which structurally overlies the Deokjeok Formation, exhibits two contrasting styles of deformation. The lower portion of the SSZ preserves evidence of top-up-to-the-southwest shearing after top-down-to-the-northeast shearing; on the other hand, the upper portion only indicates a top-up movement. Given the primary deformation mechanisms of both quartz and feldspar, the deformation temperatures of DSZ and SSZ were estimated at ~300–350 °C and ~350–400 °C, respectively, indicative of the mid-crustal condition. New zircon U-Pb isotopic ages from mylonitic granite in the SSZ and volcanic rocks in the Deokjeok Formation, combined with previously published geochronological data, indicate that the post-collisional granites and volcano-sedimentary sequence were nearly contemporaneous (ca. 223–217 Ma) and juxtaposed because of the Late Triassic orogenic collapse and subsequent new orogenic event. In this study, we highlight the role of the extensional DSZ as a detachment propagated into the middle crust during the Late Triassic orogenic collapse. Our results report a deformational response to a transition from the collisional Songrim Orogeny to the subduction-related Daebo Orogeny in the western Gyeonggi Massif. This, in turn, provides essential insight into cyclic mountain building/collapse in the East Asian continental margin during the Mesozoic time.

40Ar/39Ar geochronology constraints on formation of the Tuwaishan orogenic gold deposit, Hainan Island, China

The Tuwaishan deposit is an orogenic gold deposit that is structurally controlled by the NE-trending Gezhen shear zone in the western part of Hainan Island, south China. It is part of a belt of numerous similar deposits hosted in Mesoproterozoic granitoids and lesser Paleoproterozoic to Mesoproterozoic schists. The Tuwaishan gold orebodies comprise auriferous quartz veins and disseminated gold in sulfidized granitic mylonite. The veins consist of early quartz-pyrite-arsenopyrite-pyrrhotite with minor gold, middle stage quartz-gold-base metal sulfides, and barren late quartz-carbonate. New 40Ar/39Ar dating of muscovite from wallrock schists yielded a minimum age of 356.8 ± 0.9 Ma, suggesting the youngest possible age estimate for amphibolite facies regional metamorphism of the Baoban Group country rocks. The 40Ar/39Ar dating of sericite from auriferous quartz veins yielded a weighted mean age of 243.4 ± 2.3 Ma, representing the ore-forming age of the Tuwaishan gold deposit. The formation of the shear-hosted gold deposits along the Gezhen gold belt was likely related to the Early to Middle Triassic convergence between the Indochina and South China Blocks. Based on the widely accepted metamorphic devolatilization model for orogenic gold deposit formation, most Au tends to be released into the ore-forming fluid in the source region when country rocks were firstly metamorphosed from greenschist to amphibolite facies. Therefore, it is highly unlikely that the immediate country rocks of the Baoban Group, which were at amphibolite facies by early Carboniferous, were the source for fluids and metals along the Gezhen shear zone.

Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

Late Devonian to early Carboniferous magmatism in the western Songliao–Xilinhot block, Northeast China: Implications for eastward subduction of the Nenjiang oceanic lithosphere

The timing and direction of narrowing and closure of the Nenjiang Ocean (NJO) by plate subduction remain poorly constrained. This paper presents geochemical, isotopic, and geochronological results for Late Devonian to early Carboniferous intrusive rocks in the western Songliao–Xilinhot block to constrain the evolution of subduction in the NJO region. The Late Devonian syenogranites and monzogranites formed at 366–360 Ma and are classified as A‐type granites. The wide range of εHf(t) values (−17.62 to +8.13) and TDM2 ages (789–2,208 Ma) of the syenogranites indicate that the primary magma was derived from partial melting of multiple sources, including both young accreted continental crust and old lower crust rocks. The early Carboniferous granitic mylonites formed at 342 Ma and are I‐type granites. Their high SiO2 and K2O contents, εHf(t) values of +6.61 to +9.86, and young TDM2 ages of 675 to 857 Ma suggest a preexisting juvenile crustal material source region. The early Carboniferous hornblende gabbros formed at 334 Ma and are part of an appinite suite. The gabbros have low silica and high MgO contents, E‐MORB‐like trace element features, εHf(t) values of +1.34 to +8.97, and young TDM1 ages of 595 to 902 Ma, which together suggest a primitive magmatic source including juvenile MORB and/or enriched lithospheric mantle. Magmatism in the western Songliao–Xilinhot block formed a late Palaeozoic magmatic arc stretching along the eastern Hegenshan–Heihe suture. A new model of the evolution of subduction in the NJO region and associated coeval magmatic events is proposed that includes westward subduction of NJO oceanic crust during the Late Devonian at 366–360 Ma, bidirectional subduction during the early Carboniferous at ~342 Ma, and break‐off/delamination of the eastward subducting slab at ~334 Ma.

The effect of high temperature and pressure on rock friction coefficient: a review

The coefficient of friction is a parameter of the mechanical state which has an important role in the shear failure of rock in the fields of earth sciences. In order to analyze the influence of temperature and pore pressure on the coefficients of friction between solid and gouge, data from studies carried out in both China and international contexts are applied, analyzed and discussed. The survey results show that the coefficient of friction mainly depends on the pore pressure, pressure and temperature. Under the supercritical water state, the framework of silicate minerals might be substantially disintegrated, which suggests a possible change in the frictional resistance from stick slip to stable sliding. The temperature range of 300–450 °C could constitute as the critical threshold for shear failure to collected rocks (i.e., gabbro gouge, pyroxene gouge, gabbro solid, biotite gouge, structured gouge, granitic mylonite, mylonite, plagioclase gouge, granite gouge, pre-fractured granite, westerly granite).

Plastic deformation and rheology of the sinistral strike‐slip ductile shear zone in the Bayan Nuru area, Alxa Zuoqi, NW China: Product of subduction of Paleo‐Pacific Plate

The Bayan Nuru ductile shear zone is located in the Yabulai–Bayan Nuru tectonic belt of the middle part of the southern Central Asian Orogenic Belt (CAOB) and east of the Langshan tectonic belt. This article is the first report on the Bayan Nuru ductile shear zone, and provides dynamic, kinematic, and geochronological data of the shear zone. Accurately determining the shear orientation and estimating the shearing age of the Bayan Nuruductile shear zone, has important reference value and significance for understanding the tectonic evolution in Alxa during “Yanshan movement” period. All shearing signatures, including asymmetrical folds, rotated porphyroclasts, “domino” structures, “mica fishes,” S–C fabrics, and asymmetric stress shadows, developed in the shear zone, indicate sinistral shear. The temperature environment at the time of formation of the shear zone was determined by the mineral deformation thermometer and EBSD quartz C‐axis fabric analysis. The temperature was 350–550°C. The differential stress (σ) was estimated to be 44.31–97.94 MPa and the strain rate(έ) to be 10−14 s−1(350°C) and 10−10s−1(550 °C). The average kinematic vorticity of the Bayan Nuru NNE‐trending ductile shear zone is 0.93, indicating that the shear zone was dominated by simple shear. The thickness of the ductile shear zone was reduced by 7%–22%. Combined with the age of granitic mylonite (289.53 ± 0.74 Ma) and the NNE‐trending spread feature, the Bayan Nuru ductile shear zone in the western part of the Langshan fault zone may have been formed in the context of continuous compressional intraplate deformation, caused by the subduction of the Paleo‐Pacific Plate at 170–150 Ma.

New Zircon U‐Pb Ages of Mylonitic Granite in the Baluntai Area of the Central Tianshan Block and their Geological Implications

New Zircon U‐Pb Ages of Mylonitic Granite in the Baluntai Area of the Central Tianshan Block and their Geological Implications

Grain-size reduction of feldspar and flow of deformed granites within the Gaoligong shear zone, southwestern Yunnan, China

Mineral deformation and rock flow mechanism in the lithosphere are related to the rheological behavior and weakening mechanism of the continent. Natural deformation behaviors of feldspars are not well understood due to the complexity of their mineral compositions, crystal structures, as well as changing deformation conditions. The refined microstructure, fabric and composition of major minerals in the deformed granitic rocks within the Gaoligong shear zone (GLGSZ), southwestern Yunnan, China, were studied. With increasing mylonitization, two fabric types of end-members have been distinguished (type-I banded granitic mylonite and type-II banded ultramylonite). The two types of deformed granitic rocks have the same mineral assemblage, but different mineral modes. The type-I banded granitic mylonite has a greater proportion of K-feldspar (mostly present as porphyroclasts)>plagioclase>quartz±biotite, however, the type-II banded ultramylonite has a greater proportion of fine-grained plagioclase>K-feldspar>quartz±biotite. The crystallographic preferred orientation (CPO) patterns of quartz combined with two-feldspar geothermometer, confirm that the quartz grains in the type-I and type-II granitic rock have undergone high-temperature dislocation creep deformation. The K-feldspar grains in the matrix of type-II banded ultramylonite show a dominant (100) [010] slip system with dislocation creep recrystallization, while the fine-grained plagioclase grains present a weak CPO pattern with superplastic flow. The K-feldspar porphyroclasts show grain-size reduction associated with mineral composition and fabric transformation. The myrmekite formation with the fine-grained neocrystallization of plagioclase and quartz significally replaced the K-feldspar porphyroclasts. Finally, the fine-grained neocrystallization plagioclases were formed further into the high strain localized ultramylonites with superplastic flow.

U–Pb zircon geochronology of the Hida gneiss and granites in the Kamioka area, Hida Belt

AbstractA new U–Pb dating and oxygen isotope analysis of zircons collected from a granitic mylonite and an undeformed granite in the Kamioka area, in the Hida Belt of southwest Japan, was conducted using a sensitive high‐resolution ion microprobe (SHRIMP) to restrict the timing of the mylonitization in the Funatsu Shear Zone, which is situated on the eastern and southeastern margins of the Hida Belt. Here, undeformed granite intrudes into the granitic mylonite deformed by mylonitization in the Funatsu Shear Zone. The granitic mylonite and the undeformed granite yielded U–Pb zircon ages of 242.6 ±1.9 Ma and 199.1 ±1.9 Ma, respectively. The granitic mylonite and the undeformed granite also yielded zircon oxygen isotope ratios (δ18OVSMOW) of 7.74 ±0.37 ‰ and 5.74 ±0.17 ‰, which suggests that these rocks are derived from different magmas. Therefore, the timing of the mylonitization in the Funatsu Shear Zone is constrained to be at least 242.6–199.1 Ma, which is consistent with other data from the Tateyama area. The U–Pb zircon ages of the banded gneiss in the Kamioka area also reveals that the protolith is a sedimentary rock deposited at approximately 256 Ma, and regional metamorphism occurred at 245.0 ±6.6 Ma, which indicates that the mylonitization in the Funatsu Shear Zone occurred after the metamorphism in the Hida Belt. These geochronological and geochemical data give new insight into the relationship between the Hida Belt and the eastern margin of the Asian continent: the geochronological and geochemical data in this study support the possibility that the Funatsu Shear Zone is comparable with the Cheongsan Shear Zone located at the center of the Ogcheon Belt on the Korean Peninsula.