Zircon Ages Research Articles

The Burmese Jade Mines belt: origins of jadeitites, serpentinites, and ophiolitic peridotites and gabbros

Ophiolitic peridotites in Myanmar (Burma) occur along three major tectonic zones: the Kaleymyo–Nagaland suture along the Indo-Burman Ranges, the Jade Mines belt and the Tagaung–Mytkyina belt. These belts all show harzburgite–lherzolite–dunite peridotites, but the Hpakan-Taw Maw region (Jade Mines belt) also hosts jadeitites, including pure jadeite, mawsitsit (Cr-rich jadeite) kosmochlore (Cr-rich clinopyroxene) and albitite. Jadeitites with high Na and Al contents require either very unusual Al-rich, Si-poor protoliths or extensive fluid metasomatism, or both. The Hpakan jadeitites formed by Na-, Al- (and Si-) metasomatic alteration of pyroxenite–wehrlite intrusions into harzburgite–dunite from widespread fluid alteration. The fluids could have been derived from a mid-Jurassic intermediate pressure subduction event during ophiolite formation and emplacement. In the Lake Indawgyi area, normal ophiolitic peridotites, including harzburgite and dunite with pyroxenite veins, have not been jadeitized. Gabbros related to the Jade Mines ophiolite gave a U–Pb zircon age of 169.71 ± 1.3 Ma (MSWD 2.2), a similar timing to the Myitkyina ophiolite (173 Ma) to the east, suggesting that the ophiolite belts were originally continuous. The jade ‘boulders’ in the Uru conglomerate beds at Hpakan have also resulted from normal in situ serpentinization weathering processes, followed by limited fluvial mass transport processes along the Uru River. Supplementary material: U–Pb zircon data are available at https://doi.org/10.6084/m9.figshare.c.6655269

Zircon age spectra to quantify magma evolution

Abstract The past decades have seen tremendous advances in analytical capabilities regarding the sensitivity, spatial selectivity, and instrumental precision of U-Th-Pb zircon geochronology. Along with improved zircon pretreatment to mitigate the effects of Pb-loss, these advancements have resulted in the emergence of U-Th-Pb dating as the most widely used geochronometer. In parallel, it became increasingly obvious that modern analytical techniques can resolve zircon age dispersal beyond instrumental uncertainties and that this dispersion cannot be attributed to Pb-loss or inheritance. Hence, there is a pressing need to refine statistical procedures for displaying and interpreting dispersed age data from volcanic and plutonic rocks, where zircon ages were traditionally assigned to the quasi-instantaneous events of eruption and magma emplacement, respectively. The ability to resolve zircon age spectra, which often range over timescales of 103–106 years, also offers new opportunities to monitor magmatic processes, because zircon crystallization directly relates to the temperature and composition of its host melt. This relation is, at least for typical subalkaline melt compositions, well calibrated by multiple zircon saturation experiments, although absolute saturation temperatures derived from them can vary by tens of degrees. Moreover, zircon saturation thermometry is supported by the trace element and isotopic inventory of zircon, which records the thermochemical and compositional evolution of melts at high fidelity. Here, we first review the properties of true zircon age spectra that are defined by a statistically robust overdispersion relative to analytical uncertainties. Secondly, we evaluate existing models and present new models that aim to quantitatively translate the properties of zircon age spectra into parameters controlling the longevity and thermal evolution of crustal magma bodies such as magma recharge flux and duration. These developing approaches, which aspire to capture all processes that affect the formation and dispersal of zircon in dynamic crustal magma systems, have the potential to foster an improved understanding of magmatism with implications for volcanic hazard assessment, geothermal energy uses, and the origins of ore deposits.

Age constraints on c. 1.9 Ga volcanism, basin evolution and mineralization at the world-class Zinkgruvan Zn-Pb-Ag(-Cu) deposit, Bergslagen, Sweden

We present improved age constraints for the world-class Zinkgruvan Zn-Pb-Ag and Cu deposit: one of the largest Zn deposits of the Fennoscandian shield, and one of the earliest large, basin-hosted Zn deposits that formed from oxidized saline brines. Secondary Ion Mass Spectrometry (SIMS) U-Pb dating on zircon is used to constrain at least two phases of c. 1.9 Ga volcanism in the Zinkgruvan area, separated by a period of fluvial sedimentation, all of which predated formation of the stratiform Zn-Pb-Ag mineralization. A 1908 ± 4 Ma age for a rhyolitic rock of the first volcanic phase is the oldest recorded U-Pb zircon age of a volcanic rock in the Bergslagen lithotectonic unit (BLU) where Zinkgruvan is located. Similarly, two identical ages of 1898 ± 5 Ma for volcanic rocks belonging to the second volcanic phase indicate that the local volcanic activity, which formed the stratigraphic footwall, ended earlier in the Zinkgruvan area than in other parts of the BLU, where intense explosive felsic volcanic and intrusive activity until c. 1891 Ma has been demonstrated. This, along with a hybrid siliciclastic-volcaniclastic (tuffitic) character of the Zinkgruvan ore host, confirms earlier interpretations that the Zinkgruvan deposit formed in an actively subsiding basin, distal to active volcanic centers in the BLU in the time range 1.90–1.89 Ga. Our results support models suggesting that basinal brine-driven hydrothermal systems in sedimentary basins distal to volcanic centers could form world-class Zn deposits as early as c. 1.90 Ga.

Constraints from the Ntaba Mhlope Syncline and the Mkomozane Homocline, Moodies Group (3.22 Ga), Eswatini, on the deformation of the southern margin of the Barberton greenstone belt

The Barberton Greenstone Belt (BGB) has been classically thought to be a first-order synclinal structure, therefore constraining vertically-dominated models of early Earth lithosphere (de-)formation. Geological mapping, strain analysis, U-Pb zircon age dating, and Raman spectroscopy of carbonaceous matter (RSCM) of the Ntaba Mhlope Syncline (NMS) and the Mkomozane Homocline (MH) close to the southeastern linear margin of the BGB in Eswatini assess this interpretation by constraining the type and timing of the dominant late-phase deformation in this region. Geologic mapping shows that the NMS is a steeply doubly plunging, highly strained syncline composed of basal Moodies conglomerates overlain by sandstones and subordinate shales and siltstones. Deformation of conglomerate clasts is markedly constrictional and of uniform orientation throughout the syncline, suggesting that regional deformation occurred in zones of ductile strain. RSCM indicates maximum peak paleotemperatures of ca. 500 °C, somewhat elevated compared to temperatures measured throughout the BGB. Shortening may have occurred contemporaneously with lower crustal extension.

Phanerozoic Tectonic and Sedimentation History of the Arctic: Constraints From Deep‐Time Low‐Temperature Thermochronology Data of Ellesmere Island and Northwest Greenland

AbstractRocks exposed along both sides of the Smith Sound in Ellesmere Island and NW Greenland record the tectono‐sedimentary evolution of the whole Phanerozoic, including two periods of mountain building—the Palaeozoic Ellesmerian Orogeny and the Palaeogene Eurekan Orogeny—and the formation of two major sedimentary basins, the Franklinian and the Sverdrup Basins. We used geo‐ and thermochronology and apatite chemistry data to unravel this evolution. Apatite fission track and (U‐Th)/He dates vary strongly from >600 to <100 Ma. We present internally consistent thermal history models, which allow to explain the data variations by a unitized exhumation and burial history. Our models suggest that the cratonic areas were buried beneath a several km‐thick succession of Franklinian Basin deposits. During the Ellesmerian Orogeny, the craton acted as sediment source, as also suggested by the composition of apatite and by U‐Pb ages of zircon contained in Devonian foreland sediments. The Ellesmerian foreland was buried by up to 4–5 km thick strata on top of the preserved sedimentary rocks. During the Triassic, the Sverdrup Basin strongly widened and extended at least ∼370 km further toward the east, as compared with previous reconstructions of the basin based on the preservation of Triassic deposits. Thermal history modeling suggests Late Cretaceous to early Cenozoic reheating, which may be caused by deposition associated with the Eurekan Orogeny and/or enhanced heat flow associated with continental breakup. Our data also show that low‐temperature thermochronology is not suitable for resolving potential strike‐slip movements along the Wegener Fault.

Metamorphic evolution and zircon ages of a Barrovian metamorphic series in central-western Inner Mongolia, China: Insights into the tectonic evolution of the Central Asian Orogenic Belt

Metamorphic evolution and zircon ages of a Barrovian metamorphic series in central-western Inner Mongolia, China: Insights into the tectonic evolution of the Central Asian Orogenic Belt

Spatiotemporal Evolution of Central Qilian Shan (Northwest China) Constrained by Fission-Track Ages of Detrital Grains from the Huangshui River

The emergence of the Tibetan Plateau is one of the most significant geological events in East Asia. The Central Qilian Shan connects North and South Qilian Shan in the northeastern part of the Tibetan Plateau. However, the exhumation history of the Central Qilian Block from the Mesozoic to Cenozoic remains unclear. Determining the cooling ages of detrital zircon and apatite in modern river sediments is an ideal method for tracing the evolutionary processes of orogenic belts. In this study, we present the first single-grain detrital apatite (153) and zircon fission-track (108) data for the Huangshui River sediments from the Central Qilian Shan. The decomposition of the dataset revealed major Mesozoic and Cenozoic age peaks at ca. 145–93, and 11 Ma. The Central Qilian Shan entered the intracontinental orogeny stage dating back to the Cretaceous (ca. 145–93 Ma) and Late Cenozoic (ca. 11 Ma) caused by the subduction of the Neo-Tethys and Indian–Asian collision. Therefore, we propose that the geomorphic framework of the northeastern margin of the Tibetan Plateau was initially established during the Mesozoic and further consolidated in the Late Miocene.

Metamorphic evolution and tectonic significance of Neoarchean high-pressure mafic granulites in the Central Limpopo Belt, South Africa

ABSTRACT The Sand River area is in the northeastern part of the Central Zone (CZ) of the Limpopo Belt, and represents a key terrane to reveal early Precambrian tectonic processes in southern Africa. Based on petrology, U–Pb zircon ages, and major and trace element data of high-pressure (HP) mafic granulites from the Sand River area, the tectonic evolution of the Central Zone of the Limpopo Belt is re-evaluated here that allows revision of previous interpretations. The protoliths of the HP mafic granulites and amphibolites are sub-alkaline tholeiitic basalts similar to those formed at a mid-ocean ridge and intruded at 2.8 Ga. Based on the distinct mineral assemblages, three metamorphic stages (M1–M3) are recognized in the studied samples: (1) a peak HP granulite facies stage M1 (~1.17–1.32 GPa/~825–870°C), (2) a post-peak near-isothermal decompression stage M2 (~0.70–0.73 GPa/~835–880°C), and (3) a later, independent metamorphic stage M3 (~0.53–0.69 GPa/~793–801°C). M1 and M2 define together a clockwise P–T path. It reveals important information related to the Neoarchean collisional orogeny, and its later overprinted by an independent orogeny with subsequent exhumation and cooling. U–Pb zircon dating of two mafic granulites samples yielded protolith formation of the mafic granulites is prior to ~2.72 Ga, and the two metamorphic age peaks of ~2.72 (M1) and ~2.0 Ga (M3), which present the peak and post-peak cooling stages, respectively. The metamorphic evolution of HP mafic granulites in the Sand River area as reconstructed in this study suggests the Neoarchean collision of the Kaapvaal and Zimbabwe cratons and a discrete Paleoproterozoic orogenic overprint followed by post-orogenic exhumation and cooling.

Quantitative reconstruction of competing sources reveals spatial heterogeneity and complex aeolian–fluvial interactions in the Horqin Sandy Land, NE China

AbstractThe provenance and aeolian–fluvial interaction are essential for understanding the formation and evolution of sandy land. However, the dune sediment source in the Horqin Sandy Land (HQSL), the largest semi‐stable sandy land in China, has been debated. Here, we collected 900 coarse (i.e., >63 μm) and 750 fine (i.e., <63 μm) detrital zircon (DZ) grains from surface sand in different spatial locations for U–Pb age analysis. The visual comparison largely shows remarkably similar zircon age characteristics throughout the HQSL, and the quantitative results of the inverse Monte Carlo modelling show the primary detrital contribution of the Central Asian orogenic belt (~50.5%–61.3%) to the HQSL. In contrast, the southeast of the HQSL is characterized by the absolute dominance of the North China Craton (~75.8%). The DZ age spectra in the western and northern HQSL are extremely similar but significantly different from the south, indicating the spatial heterogeneity of the HQSL. The DZ ages are largely unaffected by particle size sorting, except in the southeastern HQSL (e.g., the dramatically different detrital contribution of the North China Craton to the fine [52.4%] and coarse fractions [75.8%]). We argue that the aeolian–fluvial interaction and the resulting sedimentary sorting and recycling are responsible for the DZ age characteristics of the HQSL. We confirm that the DZ ages of the HQSL are a faithful record of the tectonic‐magmatic events, for example, the North China Craton massif collision and collusion, the Paleo‐Asian Ocean subduction and closure, the Mongolian‐Okhotsk Ocean plate subduction and the paleo‐Pacific Ocean subduction and retreat. Finally, this study suggests that caution must be exercised in interpreting the paleoclimatic record and the aeolian deposit source investigations due to the spatial heterogeneity and the complex wind‐fluvial interactions in the HQSL.

Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: Context for hydrothermal and magmatic processes

Continental magmatic arcs commonly have heterogeneous and anisotropic metamorphic basement foundations hosting their plutonic roots and associated geothermal systems. Quaternary volcanic and volcaniclastic rocks of New Zealand's Taupō Volcanic Zone (TVZ) lie above and between the Torlesse and Waipapa Composite Terranes, two of New Zealand's major basement geological units. Deep geothermal and magma systems beneath TVZ interact with and/or are hosted in these Mesozoic metasedimentary crustal basement terranes rather than in Quaternary volcanic and volcaniclastic fill. New mineralogical, whole rock geochemical, O, Sr, Nd and Pb isotopic, and U-Pb zircon age data from lava-hosted xenoliths and ignimbrite-hosted lithics imply that the terrane boundary is not sub-vertical as previously thought, but instead dips gently west. Most of the non-igneous crust under the TVZ is thus composed of Torlesse Composite Terrane metasedimentary rocks, which have higher quartz, mica and SiO2 abundances, higher 87Sr/86Sr, 206Pb/204Pb and δ18O ratios, lower chlorite and MgO contents and lower 143Nd/144Nd ratios than their Waipapa Composite Terrane counterparts. This new interpretation provides important new constraints to refocus geodynamic understanding, petrological modelling, and future deep geothermal drilling.

In Situ LA-ICP-MS U-Pb Geochronology, Sr-Nd-Hf Isotope and Trace Element Analysis of Volcanic Rocks from the Gacun Volcanic-Hosted Massive Sulfide Deposit in Sichuan, China

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.

Zircon age distributions and implied tectonic settings of early Cambrian to Early Devonian sedimentation in western and central Victoria

Age distributions among the detrital zircon populations in western and central Victorian Paleozoic metasedimentary rocks suggest that most were deposited in Andean-type back-arc environments, with variable proportions of arc and continental sediment sources. Notable exceptions are the upper Cambrian Knowsley East Shale and the Silurian(?) sample of ‘Glen Creek Lithic Sandstone’, which both contain unimodal zircon populations. In the case of the Knowsley East Shale, this age is very close to the inferred depositional age, implying a possible forearc environment, with sediment derived directly from the contemporary volcanic arc. On the other hand, the implied trench environment for the ‘Glen Creek Lithic Sandstone’ is most likely inherited from its very local sediment source in the fault-bounded and mainly Cambrian Glen Creek erosional window. The only analysed rock units that may have been deposited in non-arc settings are the middle Silurian Humevale Siltstone and the Lower Devonian Norton Gully Sandstone. The zircon populations in these units carry strong continental signals, and they may have been deposited in an extensional marine basin bounded by continental blocks. The upper Silurian Grampians Group may have been deposited in a similar rift setting, but the zircon age histogram for this unit suggests that it was more proximal to an arc terrane. These assignments of depositional/tectonic environment should be regarded as indicative only, particularly since the method provides only subtle distinctions between some extensional and convergent settings. However, if the assignments are correct, there are some important implications for the geological history of this part of the Lachlan Orogen. For example, contrary to present perceptions, the Stawell Zone rocks may not have been prominently uplifted during the Ordovician, and the basin in the Bendigo Zone may have been receiving sediment input from further west in the Grampians–Stavely Zone.

New U-Pb CA-ID TIMS zircon ages implicate the Franklin LIP as the proximal trigger for the Sturtian Snowball Earth event

The Franklin igneous event (ca. 718 Ma) emplaced basalts and mafic intrusions across Arctic Laurentia just prior to the onset of the ca. 717–661 Ma Sturtian Snowball Earth event. Given the close association in time between these two events, it has been widely argued that the Franklin event contributed to the initiation of global glaciation. However, a lack of accurate high-precision geochronology has hampered establishing the timeframe of Franklin magmatism and discerning its precise temporal and possible causative relationship to glaciation. Here we present new high-precision U-Pb CA-ID TIMS zircon ages for two dykes and two sills from Baffin Island, Canada, and Avannaata, northwestern Greenland, related to the Franklin event. Combined with field relationships showing that the dykes crosscut the sills, these four dates indicate magmatic activity over 1.10±0.74 Myr at ca. 718 Ma, immediately prior to Sturtian glaciation, which we estimate to have initiated at 717.19 +0.26/−0.33 Ma. Our results are consistent with recent high precision U-Pb CA-ID TIMS age estimates for the Franklin magmatic activity in the central and western parts of the province. Combining our new dates with the other high precision CA-ID-TIMS ages yields a duration for the Franklin event of 2.14±0.75 Myr which is shorter than the previous estimates but somewhat longer than that for typical Phanerozoic large igneous events.

The extraordinary Namche Barwa sediment factory in the Eastern Himalayan Syntaxis

AbstractThe Namche Barwa Syntaxis (NBS) is one of the most productive detrital factories on Earth. Previous studies have shown that the NBS supplies large amounts of sediment to the Brahmaputra River, although the sources and controlling factors of sediment production have not been ascertained in detail. This study presents petrographic and heavy‐mineral data for 43 sand samples collected in the Yarlung and Parlung river catchments covering the entire NBS and surrounding areas. Combined with U–Pb ages of detrital zircons, our data indicate that 89 ± 11% of Yarlung River sediments downstream of the NBS are produced in the Yarlung and Parlung gorges. The annual sediment flux of the Yarlung River increases by a factor of 20 within ca. 250 km from upstream of the NBS (ca. 10 Mt) to downstream (ca. 200 Mt/a). The Yarlung and Parlung gorges, representing only ca. 1% of the Yarlung‐Brahmaputra catchment area, contribute 74 ± 9% of the total Brahmaputra sediment flux. Average interannual erosion rates in the Yarlung and Parlung gorges corresponding to these fluxes are calculated to be 9.2 ± 1.2 mm/a and 6.5 ± 2.1 mm/a respectively. Focused erosion of the Namche Barwa Complex and Yarlung Suture Zone in the gorge, where high‐grade metamorphic rocks are exposed, is a consequence of high channel steepness (ksn values up to 800–1800), high stream power and extreme events including frequent earthquakes and landslides. The coupling between surface erosion and tectonic uplift in the Yarlung Gorge is in full agreement with the tectonic aneurism model of NBS evolution.

Early Jurassic tectonomagmatic stages in the western North Patagonian Region, Argentina: insights from the Comallo volcanic sedimentary complex

ABSTRACT The Comallo volcanic sedimentary Complex in the western North Patagonian Region begins with the development and collapse of Plinian-type eruptive columns of rhyolitic and dacitic composition associated to vent at approximately 192 Ma. Synchronous deposits are originating in alluvial and lagoon environments. The ƐHf(t) values obtained in zircon grains indicate possible derivation by melting Paleoproterozoic and Mesoproterozoic crust. Upwards in the sequence appears basaltic and andesitic lava flows, as well as pyroclastic deposits produced during the eruptions. This volcanism is related to sustained pyroclastic fountains with a U-Pb zircon age of 185 Ma. The volcanic rocks were possibly derived from Paleoproterozoic, Mesoproterozoic, and Neoproterozoic crust affected by juvenile magmas in volcanic-arc environments. Chemical and isotopic variations are mainly due to changes in the local and regional tectonic conditions, the thickness of the continental crust, as well as the dip and age of the subducted oceanic crust.

Development of a mid-Neoproterozoic arched rift in eastern South China as a response to the Rodinia breakup

The initial time (before or after ∼ 820 Ma) and geodynamic mechanism (bottom-up rising mantle plume or top-down deep subduction) of Rodinia breakup remain unresolved. This issue has impacted our current understanding of the tectonic evolution of South China. Some scholars believe that separation of the South China Craton (SCC) from the Rodinia supercontinent and contemporary SCC rifting began at ca. 860 Ma and was induced by a subduction-related extension event. In addition, other researchers consider that the SCC rifting occurred after ca. 820 Ma. We show that renewed Rodinia breakup within or along the margin of South China occurred ca. 820–740 Ma, which is supported by early studies and our newly obtained U–Pb zircon ages of 755.4 ± 1.0 and 801.6 ± 5.2 Ma from basalts and mafic intrusive bodies in an arched (arc-shaped) belt. Major elemental variations and calculated datasets for the rift-related igneous rocks suggest a mantle plume origin for the dated rocks, which indicates that the SCC rifting was driven by a rising mantle plume and formed several rifts. This arched belt, including < 820 Ma sedimentary and igneous rocks within it, may well have been the eastern part of the original Nanhua Rift, i.e., the prototype of the Nanhua Rift should be a “T” shape. Owing to a barren magmatic area at the “T” junction, the Nanhua Rift must have split into the Nanhua Rift and arched rift. We suggest that the first stage of rifting event during 860–825 Ma may occurred in the interior Yangtze or Cathaysia Block and was associated with a subduction-related extension event.

Granulite facies metasedimentary rocks: Insight into the formation of the East Kunlun continental arc

The metamorphism and dynamic processes of the Early Paleozoic continental margin arc in the East Kunlun orogen are still unclear. This study investigated the metasedimentary rocks with granulite facies in the Qingshuiquan area in the eastern section of the East Kunlun orogen. The kyanite is not found in the pelitic granulite. The phase equilibrium modeling indicates a middle-pressure type with metamorphic conditions of 0.7 to 0.8 GPa and 800 to 840°Cfor pelitic granulite. UPb dating of zircons from metasedimentary rocks yielded metamorphic ages of 490-480 Ma and detrital zircons ages of 1700-1500 Ma. REE patterns of the metasedimentary rocks are enriched LREEs and distinct negative Eu anomalies, the εNd values vary from −1 to −16. These results imply that the Precambrian supracrustal rocks were buried into the middle-lower crust level and granulite facies metamorphism occurred in Late Cambrian-Early Ordovician.Synchronous (520-480 Ma) magmatic rocks in Qingshuiquan and adjacent areas indicate that subduction of the Proto-Tethyan oceanic crust led to the formation of arc magmatic rocks. Meanwhile, it induced some sedimentary rocks to underthrust or return flows into the middle-lower crust of the upper plate and underwent granulite facies metamorphism.

Tectonic Evolution of the JLJB, North China Craton, Revisited: Constraints from Metamorphism, Geochemistry and Geochronology of the Ji’an Group and Related Granites

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB.

An oceanic core complex preserved in the Squanga Lake ophiolite, northern Atlin terrane, Yukon

The Squanga Lake ophiolite of southern Yukon exhibits a non-Penrose pseudostratigraphy with mantle tectonites separated from upper crustal rocks by an extensional detachment zone (Squanga Lake detachment zone, SLDZ), compatible with formation in an oceanic core complex. Spinel, orthopyroxene and clinopyroxene mineral compositions indicate dynamic re-crystallisation during exhumation of ophiolitic mantle rocks. The rocks and structures of the SLDZ also preserve evidence of exhumation: 1) clasts of primitive olivine-cumulates in the SLDZ mélange represent excised lower oceanic crust; 2) structures in the SLDZ indicate ductile to brittle conditions; 3) temperatures along the SLDZ span from upper- to lower-amphibolite facies conditions. We obtained 248.77 ± 0.22, 248.53 ± 0.41, and 248.84 ± 2.09 Ma U-Pb zircon ages from upper crustal gabbro, a gabbro fragment within the SLDZ mélange and a dyke within the SLDZ implying syn-kinematic magmatism. Calc-silicate alteration associated with cross-cutting diabase dykes yielded a UPb titanite age of 247 ± 10.5 Ma. All igneous rocks show similar geochemical signatures with ubiquitous subduction input. Our data indicate that Squanga Lake ophiolite preserves an oceanic core complex, formed in a back-arc basin environment.

Active thrust tectonics along the western slope of the Central Andes southernmost Pampean flat-slab segment (∼33°S, Chile): The Cariño Botado fault system

Along the western slope of the Central Andes in the southernmost part of the Pampean flat-slab subduction zone segment (∼33°S), crustal Quaternary west-vergent reverse faults are found, but little is known about them. To better understand and characterize these faults, grouped as the Cariño Botado thrust fault system (CBF), we undertook a multidisciplinary study focused on remote sensing and field geomorphic and geologic mapping, sedimentary relationships, and fault characterization. Optically Stimulated Luminescence (OSL) and UPb dating samples were obtained from faulted deposits to constrain the timing of fault motion. We combined the mapping, fault relationship descriptions, and age control to understand this system better and quantify tectonic rates here for the first time. The main CBF is best expressed with Late-Cenozoic hanging wall fault rocks over thrusting Quaternary coarse-grained alluvial fan deposits, and the fault zone is found over a large area and with an altered fault zone (∼0.5–2 km wide). The principal reverse fault segment of the CBF is well exposed in creeks, with an average dip of 40°E and an associated vertical throw (up to the east) of the hanging wall from ∼3.2 m up to ∼30 m. Five OSL samples, taken from tilted and untilted alluvial fan deposits adjacent to the CBF's main fault, yielded ages between 2.5 ± 0.2 ka and 9.0 ± 0.7 ka, supporting Holocene activity and ∼4.9 m of fault slip in the last ∼8.7 ka. Ninety-nine detrital zircons from fluvial terrace deposits in the study area were dated using UPb ICPMS, giving mainly Oligo-Miocene ages. The youngest ages of detrital zircons are Quaternary (n = 3; ∼0.9–4.4 Ma), which indicates that CBF activity could have started as early as ∼0.9 Ma producing an accumulated hanging wall uplift of ∼130 m. The alignment of the fault profiles, creek and river knickpoints, and alluvial and fluvial deposits allows the most recent activity of the main CBF to be traced between 5 km and up to 15 km along the fault trace. Based on fault length, slip characteristics, and earthquake scaling relationships, earthquakes up to Mw 7.5 could be associated with the fault. Because of the clear relationship between the river incision, distribution of fluvial terraces, variation in sinuosity and stream power, and the location of the faults, we suggest that the tectonic uplift controls the hanging wall fluvial incision from ∼0.9 Ma to the present. The Quaternary (at least 0.9 Ma to present) active CBF shares geometric, structural, and rupture timing similarities with the nearby better-characterized San Ramón Fault system. The latter is located ∼50 km to the south along the eastern border of Santiago, the capital city of Chile. Both fault systems share a comparable morpho-structural setting with a consistent average N-S trend. Significant seismic hazard is associated with both the CBF and the San Ramon System, which may be linked structurally and temporally regarding previous surface ruptures and major earthquakes, and therefore remain important research targets.