Mylonitic Granite Research Articles

Genesis of the Sanhetun Tellurium–Gold Deposit, Northeast China: Constraints from In Situ Elemental and Sulfur Isotopic Compositions of Pyrite

The Sanhetun tellurium–gold (Te–Au) deposit, located in the Duobaoshan polymetallic metallogenic belt (DPMB) within the eastern section of the Central Asian Orogenic Belt (CAOB), is a newly discovered small-scale gold deposit. The mineralization, with a resource of ≥4 t Au, is mainly hosted in three NNE-trending alteration zones between Early Carboniferous granitic mylonite and Lower Cretaceous volcanogenic-sedimentary formations. The genesis of formation of this deposit is poorly constrained. Here, we report the results of petrographic studies, TESCAN Integrated Mineral Analyzer (TIMA), major and trace element concentrations, and in situ S isotopes of pyrite. The results show that there are four types of pyrite: coarse-grained euhedral Py1, fine-grained quartz-Py2 vein crosscutting Py1, anhedral aggregated Py3, and anhedral aggregated Py4. The pre-ore stage Py1 contains negligible Au, Te, and other trace elements and has a relatively narrow range of δ34S values ranging from −1.20 to −0.57‰. Py2 has higher concentrations of Au and Te and distinctly high concentrations of Mo, Sb, Zn, and Mn with markedly positive δ34S values of 4.67 to 14.43‰. The main-ore stage Py3 contains high Au and Te concentrations and shows narrow δ34S values ranging from −5.69 to 0.19‰. The post-ore stage Py4 displays low Au concentrations with the δ34S values ranging from 2.66 to 3.86‰. Tellurides are widespread in Py3 and Py4, consisting mainly of native tellurium, tetradymite, tsumoite, hessite, and petzite. Especially, tetradymite commonly coexists with native gold. This study highlights the role of Te–Bi–S melt as an important gold scavenger in As-deficient ore-forming fluids.

Anisotropic effect of shear zone on groundwater potentiality in crystalline hard rock terrain

Assessment of potential groundwater recharge sites and sustainable water resource management in semi-arid crystalline rock terrain is a challenging task. Globally, analysis of remote sensing satellite imagery data for delineation of groundwater potential zones over sheared crystalline hard rock terrains has been fairly successful. But there is no existing study present at our disposal which discusses the factors controlling the inconsistent groundwater potentiality that exits along the shear zones. This study attempts to analyse the major geological factors controlling the irregular groundwater potentiality of shear zones within older crystalline rock terrain. Therefore, the study area selected for this analysis is the Purulia district of West Bengal, NE India, composed mostly of Precambrian metamorphic rocks i.e., quartzite, granite gneisses, porphyroclastic granite-gneiss, quartzo-feldspathic-granite-gneiss, mylonitic granites, quartz-biotite-granite gneiss, quartzites, carbonatites and phyllites. Satellite imagery study of IRS-P6 LISS IV standard FCC image reveals the presence of two bifurcating shear zones namely North Purulia Shear Zone (NPSZ) and South Purulia Shear Zone (SPSZ) over the study area. Careful analysis of rock structure, different lithotypes, soil thickness, electrical resistivity tomography data and water table data with an emphasis on high water table fluctuation, shows a strong spatial relation between the potentially good groundwater recharge zones and the branching/confluence sites of shear zones present in the study area. The study constructs an attempt to demonstrate the relationship between shear zone conjunctions and significant groundwater recharge sites in Precambrian crystalline fractured-rock aquifer system.

The Influence of the Closure of the East Paleo‐Tethys Ocean on Southern South China: Evidences from Kinematics and 40Ar/39Ar Geochronology of the Rongxian Ductile Shear Zone in Southeastern Guangxi

AbstractThe Triassic was a crucial period in the tectonic evolution of the South China Block. Research on tectonic deformation during this period provides information on intracontinental orogenic mechanisms in South China. In this study, alongside thermochronological analyses, we examine the macroscopic and microscopic structural features of the Rongxian ductile shear zone, located south of the Darongshan granite in the southeastern part of Guangxi Province, on the southern margin of South China. Sinistral shear is indicated by the characteristics of rotated σ‐type feldspar porphyroclasts, stretching lineations defined by elongated quartz grains and the orientations of quartz c‐axes. LA‐ICP‐MS U‐Pb dating of zircons from two samples of granitic mylonite and one of granite yielded ages of ca. 256 Ma. Furthermore, two samples of granitic mylonite yield muscovite 40Ar/39Ar plateau ages of 249–246 Ma. These results indicate that the Rongxian ductile shear zone resulted from Early Triassic deformation of the late Permian Darongshan granite. This deformation was likely related to the closure of the eastern Paleo‐Tethys Ocean and the subsequent collision of the South China and Indochina blocks, during the early stage of the Indosinian orogeny.

Using the traditional microscope for mineral grain orientation determination: A prototype image analysis pipeline for optic-axis mapping (POAM).

This paper reports on the development of an open-source image analysis software 'pipeline' dedicated to petrographic microscopy. Using conventional rock thin sections and images from a standard polarising microscope, the pipeline can classify minerals and subgrains into objects and obtain information about optic-axis orientation. Five metamorphic rocks were chosen to test and illustrate the method. Thin sections were imaged using reflected and cross- and plane-polarised transmitted light. Images were taken at different angles of the polariser and analyser (360° with 10° steps), both with and without the full-lambda plate. The resulting image stacks were analysed with a modular pipeline for optic-axis mapping (POAM). POAM consists of external and internal software packages that register, segment, classify, and interpret the visible light spectra using object-based image analysis (OBIAS). The mapped fields-of-view and grain orientation stereonets of interest are presented in the context of whole-slide images. Two innovations are reported. First, we used hierarchical tree region merging on blended multimodal images to classify individual grains of rock-forming minerals into objects. Second, we assembled a new optical mineralogy algorithm chain that identifies the mineral slow axis orientation. The c-axis orientation results were verified with scanning electron microscopy electron backscattered diffraction (SEM-EBSD) data. For quartz (uniaxial) in a granite mylonite the test yielded excellent correspondence of c-axis azimuth and good agreement for inclination. For orthorhombic orthopyroxene in a deformed garnet harzburgite, POAM produced acceptable results for slow axis azimuth. In addition, the method identified slight anisotropy in garnet that would not be appreciated by traditional microscopy. We propose that our method is ideally suited for two commonly performed tasks in mineralogy. First, for mineral grain classification of entire thin sections scans on blended images to provide automated modal abundance estimates and grain size distribution. Second, for prospective fields of view of interest, POAM can rapidly generate slow axis crystal orientation maps from multiangle image stacks on conventionally prepared thin sections for targeting detailed SEM-EBSD studies.

Zircon U-Pb ages and geochemical characterization of granitic mylonites in the Northern Zone of the Maizuru Belt, northern Okayama area, Southwest Japan

Zircon U-Pb ages and geochemical characterization of granitic mylonites in the Northern Zone of the Maizuru Belt, northern Okayama area, Southwest Japan

Micro-scale structural and chemical characterisation of deformed rocks with simultaneous in-situ synchrotron X-ray fluorescence and backscatter diffraction mapping

In-situ measurements of structural and chemical information on deformed rocks are required to understand microphysical deformation mechanisms, the relative timing of tectono-metamorphic events, and the interplay between deformation, fluid flow, and mineral reactions. Scanning-electron microscopy is commonly used for this purpose, because it can provide compositional information (via energy- or wavelength-dispersive spectroscopy and backscattered electron imaging) and crystallographic orientation data (via electron backscatter diffraction) for large fractions of thin sections down to nanometre resolution. However, elemental and crystal-structural data are usually acquired independently, introducing the need for registration of maps with different information and increasing instrument time. In addition, measuring element concentrations to levels below 1000 ppm increases acquisition times significantly, rendering the mapping of centimetre-scale areas with micrometre-resolution or below time-costly. Here, we introduce an alternative method using coeval synchrotron X-ray fluorescence microscopy (XFM) and X-ray backscatter diffraction mapping (XBDM) for the rapid acquisition of structural and compositional information down to the ppb level on whole rock sections with a spatial resolution down to ∼1 μm per pixel. We illustrate the principles of XFM/XBDM mapping on a sample of granitic mylonite. As example for the scientific potential of the new method, it is shown that the Ti distribution in quartz and feldspar can be accurately correlated with physical structure. A discussion of the limitations and opportunities of XFM/XBDM mapping for studying deformed rocks concludes this article.

Continental‐scale shearing triggered by Oligocene subduction in Myanmar‐Indochina, SE Asia

Continental‐scale shear zones in Indochina record Cenozoic tectonic processes in SE Asia. Previous extrusion models link these shear zones to northward indentation of the Indian continent and conflict with distributed Oligocene conjugate strike‐slip pairs, formed by E‐W‐directed compression (present coordinates). This paper presents evidence of Oligocene shearing along the Mogok Metamorphic Belt in Myanmar, the western boundary of the Indochina block. The Kyanigan quarry at northern Mogok Metamorphic Belt exposes paragneiss, marble and quartzite schist with E‐W striking, which would be dragged from N‐S striking foliations of the main Mogok Metamorphic Belt by right‐lateral shearing of the brittle Sagaing Fault. After restoration, right‐lateral shearing ‘σ’ structures, cored with garnet, in paragneiss in the Kyanigan quarry are consistent with ‘σ’ and ‘δ’ structures in the Moulmein granitic mylonite to the south. U–Pb ages of metamorphic zircon of a sheared paragneiss and a leucogranite, cross‐cutting shear foliation in the Kyanigan quarry, and a biotite 39Ar‐40Ar age of mylonite at Moulmein constrain dextral shearing to 33–25.4 Ma, coeval with other 31–24.5 Ma shear zones in Indochina. The eleven NW‐striking left‐lateral shear zones and N‐S to NE‐striking right‐lateral shear zones define Oligocene conjugate fault patterns in Indochina. After restoration of ~80–45° clockwise rotation, these Oligocene conjugate shear zones, coupled with the anticlinal configuration of the Xuelongshan and Doi Inthanon‐Doi Suthep domes, reflect approximate N–S‐directed shortening. These structures to the north with coeval Myanmar‐Sumatra magmatic arc to the south demonstrate that they correspond to back‐arc contraction during northward subduction of the Indian Ocean. Oligocene continental‐scale intra‐continental shearing may be triggered by syn‐subduction compression in SE Asia.

Tectonic evolution of the Middle-Late Permian orogenic belt in the eastern part of the CAOB: Implications from the magmatism in the Changchun-Kaiyuan area

AbstractVarious magmatisms during the subduction-collision process are crucial to reveal the long-term tectonic evolution of the eastern Central Asian Orogenic Belt. In this paper, we present major and trace elements of whole-rock, zircon U-Pb dating and Hf isotope of the Shanmen pluton. Results imply that the Shanmen pluton consists of quartz diorite and mylonitic granite, with zircon U-Pb ages of 263.7–259.6 Ma. The studied quartz diorite contains high Sr/Y (51.19–90.87) and (La/Yb)N (7.82–13.62) ratios, and belongs to adakitic rocks. Coupled with the positive εHf(t) values of +5.71 to +12.8 with no obvious Eu anomaly, we propose that quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge. In contrast, the mylonitic granite has lower MgO (0.28 wt% – 0.47 wt%) contents and positive εHf(t) values of +7.79 to +10.15, indicating an affinity with I-type granite originated by partial melting of the intermediate-basic lower crust. The geochemical characteristics and lithological assemblages, along with the Permian magmatic rocks in the Changchun-Kaiyuan area displaying arc rocks affinity, propose their formation is related to the southward subduction of the Paleo-Asian Ocean (PAO). Based on this study and previous evidence, we lean towards adopting a middle-late Permian slab break-off model, wherein the PAO did not close until the late Permian.

A nearly isobaric P-T-deformation evolution path of the Cerro Negro Paleoproterozoic mylonitic igneous suites: The exhumation of the Tandilia Belt of the Rio de la Plata Craton

Two high-grade mylonites from Cerro Negro hill were studied in detail to shed light on the igneous-metamorphic evolution of the northwestern extreme of the Azul Megashear Zone (AMSZ) of the Tandilia Belt, Rio de la Plata craton. Bulk-rock compositions of a garnet-bearing hornblende gneissic mylonite (ribbon mylonite) and a xenolith of garnet-bearing granite mylonite (striped gneiss) included in the former were modeled with the GeoPs software (Xiang and Connolly, 2021) to construct P-T pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H by Gibbs energy minimization. Pseudosections were contoured for isopleths parameters and mineral isomodes of interest. Garnet, with a homogeneous composition of pyr5(gro + andr)18.5spes6.5alm70, present in the younger granite crystal mush (<8 vol% of melt for 0.6 wt% of H2O) formed at 820 °C and 7.2 kbar. These conditions were encompassed by biotite with 0.21–0.23 Ti pfu and amphibole with Mg# of 0.30–0.33. A nearly isobaric cooling is outlined during a high-grade deformational event (D1) until core with the composition of pyr5(gro + andr)10spes5alm80, in slightly zoned garnet of the granite xenolith (modeled with 0.3 wt% of H2O), yields conditions of 540–580 °C and 7.2–8.2 kbars. Rimwards, a subsequent cooling with exhumation was recorded in garnet composition of pyr3(gro + andr)17spes6alm72 to reach conditions of ∼420–440 °C and ∼6.6 kbar. The integrated P-T path holds conditions of middle crust realms (ca. 24–25 km) from the initiation of deformation (D1) coevally to, or shortly, after the younger granite crystallization with a waning apparent geothermal gradient to 20 C°Km−1. Consistent mechanisms of deformation in minerals (d) generated a subvertical E-W mylonitic foliation (D1) under granulite to amphibolite metamorphic facies conditions until greenschist metamorphic facies conditions (D2). Fragile events (D3 and D4) occurred at higher crustal levels concomitant with water influx and sporadic retrogression. Hitherto, the studied rocks experienced a somewhat different evolution path when compared to adjacent Paleoproterozoic areas around the AMSZ or from other places of the Tandilia Belt, which show gently partial exhumation P-T paths after their solidus.

Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.

Multi-stage metallogeny in the southwestern part of South China, and paleotectonic and climatic implications: A high precision geochronologic study

The South China Block (SCB) is among the large-scale W-Sn mineralized regions of the globe. The Laojunshan W-Sn-dominant ore area (LOA) in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam, and Yangtze to Cathaysia blocks, with coeval formation of giant metallic deposits. The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB. In this study, we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-scheelite mineralization. The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units. The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block (420–380 Ma) jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones. This event generated the Dulong-Song Chay granitoids, together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites (ca. 419 Ma) and pegmatites (ca. 389 Ma), which include the early-stage Sn-sulfide skarn (ca. 418 Ma) and the late-stage Sn-bearing schist (ca. 389 Ma). During the Late Permian to Late Triassic (260–220 Ma), with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east, the SCB and Indochina Block (IB) were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit. The subducted Paleo-Tethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume (270–259 Ma), which formed the Maguan diabase (ca. 260 Ma) that shows significantly older Re-Os model age of ca. 268 Ma, suggesting that the Nanwenhe mineralization is potentially derived from ca. 260 Ma source. Furthermore, the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism (ca. 230 Ma), with the main stage of scheelite-Sn-Mo mineralization (229.9, 229.8 and 219 Ma) and contemporary formation of the pegmatite (230.7 Ma). The Late Cretaceous involved two episodes of alternate extension and shortening, driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor. The evolution of the LOA consists of the NW–SE transpression ending ca. 100 Ma, the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma, the WNW–ESE transpression beginning at ca. 85 Ma and the NS extension in the later episode during the latest Cretaceous, which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization, with ages in the range of 90–89 Ma, 87–85 Ma and 83–79 Ma. We also evaluate the implications of magmatic-metamorphic-metallogenic degassing on the regional paleoclimatic history.

Time-strain evolution of shear zones from petrographically constrained Rb–Sr muscovite analysis

Some isotopic ratios in minerals like monazite, apatite, and mica can record the thermal response to mylonitization, but such systems may fail to track the time of low- to medium temperature fabric realignment (recrystallization). New analytical methods that allow spatially resolved measurement isotopic ratios in distinct microstructures offer the potential to characterize the time-strain evolution of mid-crustal shear zones over a temperature range conducive to capturing the growth timing of those structures. Here we present high spatial resolution electron backscatter diffraction (EBSD), mineral chemistry, and in situ Rb–Sr dates from two texturally distinct muscovite populations in a mid- to upper-greenschist facies granitic mylonite from Top Up Rise, Western Australia. Coarse-grained muscovite fish retain a primary magmatic composition, displaying microstructural evidence of mechanical (kink and folds) and crystal-plastic deformation. The muscovite fish yield a Rb–Sr isochron date of 1614±75 Ma that either records the crystallization of the granitic protolith or cooling from high-grade metamorphic overprinting as constrained by in situ garnet Lu–Hf dates in related metapelites (1696±43 Ma and 1670±36 Ma). Conversely, fine-grained muscovite texturally associated with shear bands is chemically distinct, display microstructural evidence of fluid-assisted dynamic recrystallization and yield a distinctly younger isochron date of 609±14 Ma. The Rb–Sr dates from the coarse-grained muscovite fish cannot be unequivocally linked to mylonitization as they are indistinguishable from regional metamorphic processes. Muscovite grains behaved as rigid porphyroclasts during later medium-temperature mylonitization, leading to fish geometries, whilst maintaining intact Rb–Sr systematics. However, recrystallized/neoblastic fine-grained muscovite records the timing of medium-temperature mylonitization, establishing a direct time–strain relationship. These results highlight the potential for significant diachronicity in mineralogical components of mylonitic fabrics, demonstrating the need to establish direct time-strain relationships in order to accurately reconstruct deformation histories.

Kinematic vorticity analysis of the Deh Zaman mylonitic granite based on deformed feldspar grains

Kinematic vorticity analysis of the Deh Zaman mylonitic granite based on deformed feldspar grains

Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes

The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.

Fluid-enhanced grain-size reduction of K-feldspar from a natural middle crustal shear zone in northern Beijing, China

Grain-size reduction may lead to a transition from grain-size insensitive to grain-size sensitive deformation of rocks in the middle crustal level, thereby affecting the lithosphere's rheology significantly. However, the mechanisms of grain-size reduction of sheared rocks have long been debated. In this study, granitic mylonites from the Shuiyu shear zone of the Yunmengshan metamorphic core complex were sampled to investigate the mechanisms of grain-size reduction of K-feldspar during shearing in the middle crust. The mylonites are characterized by a typical microstructure of coarse-grained K-feldspar porphyroclasts surrounded by fine-grained matrix of oligoclase, K-feldspar, and quartz. Deformation temperatures are constrained from 400 °C to 550 °C, estimated using the feldspar and quartz microstructures, crystal-preferred orientation patterns of recrystallized quartz, and a two-feldspar geothermometer. Here, we show that the derivation of fine grains in the matrix from the K-feldspar porphyroclasts was accomplished via fluid-assisted subgrain rotation recrystallization. Walls of either well-organized or tangled dislocations constituted the boundaries of micron-scale subgrains along the margins of porphyroclasts. Hydrolytic weakening enhanced the dislocation motions to form subgrains. Furthermore, the subgrain rotation recrystallization promoted the replacement reaction of K-feldspar by sodium-rich fluid via increasing the areas of fluid/grain interfaces. The newly formed oligoclase and quartz were subsequently deformed by diffusion creep.

Crush-origin pseudotachylyte in granitic mylonites of continental exhumed Ailaoshan-Red River shear zone southeastern Asia

Tectonic pseudotachylytes have provided valuable insights into the evolution of seismic activity during crustal deformation. However, the origin and formation processes of pseudotachylytes are still debated. Thin pseudotachylyte veins were first recognized within the granitic mylonites of the continental exhumed Ailaoshan-Red River strike-slip shear zone, southeastern Asia, which record the deformation characteristics from ductile (mylonite) to brittle deformation (ultracataclasite). Mylonitization resulted in the recrystallization of quartz aggregates/ribbons, fine-grained plagioclase, and the concentration of biotite into biotite-rich foliation planes. The pseudotachylyte veins have various angles oblique to the host rock foliation, which record superimposition of the latter brittle deformation onto former ductile fabrics. Observations of the microstructure and quartz clast size distribution (obey power-law distribution) suggest that crushing was the dominant process in the formation of pseudotachylyte in the samples. Plagioclase within the pseudotachylyte vein presents a complex texture of high-strength nanocrystalline to partly amorphous material. Detailed microstructure observation revealed that amorphous materials form by a solid-state amorphization process and is different from friction melts. The changes in composition between plagioclase clasts and amorphous materials (enriched in Ca, but depleted in Na) provide evidence of mechanochemical effect that accompanies fault motion and comminution process. When a sufficient amount of amorphous material is formed, it has great significance in fault weakening through the coseismic slip of the strain partition.

Branches of the Karakoram fault in Eastern Pamir

ABSTRACT The tectonic evolution of the Karakoram fault in the Pamir Plateau is controversial because the timing and kinematics of several branches of the fault system are unclear. Here we conduct field observations on the East Pamir fault, the North Muskol shear zone, and the Xindi-Waqia fault in the eastern Pamir, and obtain zircon U-Pb and biotite 40Ar/39Ar ages from these ductile shear zones. All three faults/shear zones are hosted by granitic mylonites. The East Pamir fault is a northwest-striking, normal-dextral shear zone that cuts the Rushan-Pshart suture. Zircon U-Pb dating yields a mean age of 201.5 ± 0.5 Ma for the mylonites in this shear zone. Biotite 40Ar/39Ar analysis yields two weighted mean plateau ages, 10.65 ± 0.13 Ma, and 11.04 ± 0.15 Ma. An undeformed granitic vein that cuts the ductile shear zone gives a mean zircon U-Pb age of 9.6 ± 0.3 Ma. We interpret that the granitic protolith crystallized at 201.5 Ma, and the East Pamir fault underwent rapid cooling and exhumation at ca. 11 Ma and ceased by 9.6 Ma. The North Muskol shear zone is an east-striking extensional shear zone along the northern boundary of the Shatput-Muztaghata dome. Two mylonitic samples show mean 206Pb/238U ages of 17.9 ± 0.2 Ma and 17.6 ± 0.3 Ma, which are interpreted as the timing of crystallization of the granitic protolith. The Xindi-Waqia fault is a northwest-striking ductile shear zone and brittle fault that bounds the Muztaghata dome. We obtain two zircon U-Pb ages (231.8 ± 1.1 Ma, 235.9 ± 0.9 Ma) and two biotite 40Ar/39Ar plateau ages (between 6.60 ± 0.10 Ma and 6.87 ± 0.10 Ma) from the mylonites in this fault, and interpret that the granitic rocks were emplaced in the Late Triassic and exhumed in the late Miocene. The newly obtained U-Pb ages, combined with the previously published data, indicate that Rushan-Pshart suture in Pamir marks the closure of an ocean basin within the Palaeo-Tethys Ocean, and cannot be correlated with the Bangong-Nujiang suture in Tibet. We establish a systematic correlation in which the two Palaeo-Tethyan sutures (Tanymas-Jinshajiang and Rushan-Pshart) in Pamir represent the western extensions of the Jinshajiang and Longmu Co-Shuanghu sutures in the Tibetan Plateau, respectively, and the Central Pamir is the western extension of the Tianshuihai-North Qiangtang terrane. This indicates that the dextral slip along the Karakoram fault system in east-central Pamir is 140 ± 10 km, and the mean slip rate since the onset of the Karakoram fault at ca. 18 Ma is 7.8 ± 0.5 mm/yr. Furthermore, our analyses suggest that the North Muskol shear zone was active at ca. 18 Ma before the arrival of the Karakoram fault system then. The East Pamir fault must have been active at ca. 11 Ma, but ceased by ca. 9.6 Ma, and the slip along the Karakoram fault system migrated northeastward and primarily accommodated by the Tashkurgan fault soon after 9.6 Ma. The Xindi-Waqia fault is the southern extension of the ductile shear zone of the Kongur Shan extensional system, as they are highly similar in geometry, kinematics, and timing. Overall, as the Karakoram fault system propagated northward in Pamir during the Cenozoic, crustal deformation migrated northeastward among the Karakoram fault branches from Ladakh to the Kashgar-Yecheng transfer system, and a complex evolution path can be portrayed.

Geochronology and geochemistry of the Ediacaran orthogneisses from the north Shahrekord (Sadegh‐Abad), Sanandaj‐Sirjan Zone: Insights into magmatic evolution of the Iranian basement

The metamorphosed Sadegh‐Abad granite from the north Shahrekord Metamorphic Complex (NSMC) of Sanandaj‐Sirjan Zone in west Iran shows evidence of ductile deformation. The augen to banded mylonitic granite (orthogneiss) involves sharp contacts with other rock units, including high‐grade metamorphic rocks, such as eclogite, amphibolite, and garnet amphibolite as well as schists. U–Pb zircon dating reveals that the crystallization of the granitic magma occurred at 568 ± 14 Ma (Late Neoproterozoic, Ediacaran). The Sadegh‐Abad mylonitic granite comprises high SiO2 (67–78 wt.%) and Na2O (3.23–7.79 wt.%) contents and low CaO (0.73–1.61 wt.%) and variable K2O concentrations (0.08–4.6 wt.%). The chondrite‐normalized REE diagram shows enrichment in LREE relative to HREE (Ce/Yb)N = 1.90 to 6.12), a negative Eu anomaly (Eu/Eu* = 0.10–0.66), and nearly flat HREE patterns (Tb/Yb)N = 0.94–1.44). These are interpreted to have originated from a continental crustal source. This is further supported by the high concentration of the alkali elements, U, Th, Rb, and the high 87Sr/86Sr(i) isotopic ratios (0.7086 to 0.7163). The lower HREEs abundances, negative Eu anomalies, and the low contents of mafic minerals may infer that partial melting of continental crust has taken place in low‐pressure conditions within the plagioclase stability field, possibly of pre‐existing calc‐alkaline bodies in the lower continental crust. The charge of heating probably occurred via the injection of mafic magmas into the tonalitic and granodioritic continental crust.

Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China

Abstract The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%–65%) transpression and top-to-NW shear sense as a result of NE–SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism &amp;lt;a&amp;gt; slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal &amp;lt;a&amp;gt; slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.

Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan

AbstractWe present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modelling give estimated peak P–T conditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks.