Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co,Tibetan Plateau

Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north–northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U–Pb and [U–Th]/He dating and apatite fission track and [U–Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U–Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63–58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U–Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U–Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E–W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation.     

Partitioning of the Cretaceous Pan-Yangtze Basin in the central South China Block by exhumation of the Xuefeng Mountains during a transition from extensional to compressional tectonics?

The formation of a series of intermountain basins is likely to indicate a geodynamic transition, especially in the case of such basins within the central South China Block (CSCB). Determining whether or not these numerous intermountain basins represent a division of the Cretaceous Pan-Yangtze Basin by exhumation of Xuefeng Mountains, is key to understanding the late Mesozoic to early Cenozoic tectonics of the South China Block (SCB). Here we present apatite fission track (AFT) data and time–temperature modeling in order to reconstruct the evolution history of the Pan-Yangtze Basin. Fourteen rock samples were taken from a NE–SW-trending mountain–basin system within the CSCB, including, from west to east, the Wuling Mountains (Wuling Shan), the south and north Mayang basins, the Xuefeng Mountains (Xuefeng Shan) and the Hengyang Basin. Cretaceous lacustrine sequences are well preserved in the south and north Mayang and Hengyang basins, and sporadically crop out in the Xuefeng Mountains, whereas Paleogene piedmont proluvial–lacustrine sequences are only found in the south Mayang and Hengyang basins. AFT results indicate that the Wuling and Xuefeng mountains underwent rapid denudation post-84 Ma, whereas the south and north Mayang basins were more slowly uplifted from 67 and 84 Ma, respectively. Following a quiescent period from 32 to 19 Ma, both the mountains and basins have been rapidly denuded since 19 Ma. Both the AFT data and sedimentary facies changes suggest that the Cretaceous deposits that cover the south–north Mayang and Hengyang basins through to the Xuefeng Mountains define the Cretaceous Pan-Yangtze Basin. Integrating our results with tectonic background for the SCB, we propose that rollback subduction of the paleo-Pacific Plate produced the Pan-Yangtze Basin, which was divided into the south–north Mayang and Hengyang basins by the abrupt uplift and exhumation of the Xuefeng Mountains from 84 Ma to present, apart from a period of tectonic inactivity from 32 to 19 Ma. This late Late Cretaceous to Paleogene denudation resulted from movement on the Ziluo strike–slip fault, which formed due to intra-continental compression most likely associated with the Eurasia–Indian plate subduction and collision. Sinistral transpression along the Ailao Shan–Red River Fault at 34–17 Ma probably transformed this compression to the extrusion of the Indochina Block, and produced the quiescent window period from 32 to 19 Ma for the mountain–basin system in the CSCB. Therefore, the initiation of exhumation of the Xuefeng Mountains at 84 Ma indicates a switch in tectonic regime from Cretaceous extension to late Late Cretaceous and Cenozoic compression.     

Cenozoic thermo-tectonic evolution of the Gangdese batholith constrained by low-temperature thermochronology

Gangdese batholith in the southern Lhasa block is a key location for exploring the Tibetan Plateau uplift and exhumation history. We present the new low-temperature thermochronological data from two north–south traverses in the central Gangdese batholith to reveal their cooling histories and corresponding controls. Zircon fission track ages show prominent clusters ranging from 23.7 to 51.6 Ma, apatite fission track ages from 9.4 to 36.9 Ma, apatite (U–Th)/He ages between 9.5 and 12.3 Ma, and one zircon (U–Th)/He age around 77.8 Ma. These new data and thermal modeling, in combination with the regional geological data, suggest that the distinct parts of Gangdese batholith underwent different cooling histories resulted from various dynamic mechanisms. The Late Eocene–Early Oligocene exhumation of northern Gangdese batholith, coeval with the magmatic gap, might be triggered by crust thickening followed by the breakoff of Neotethyan slab, while this stage of exhumation in southern Gangdese batholith cannot be clearly elucidated probably because the most of plutonic rocks with the information of this cooling event were eroded away. Since then, the northern Gangdese batholith experienced a slow and stable exhumation, while the southern Gangdese batholith underwent two more stages of exhumation. The Late Oligocene–Early Miocene rapid cooling might be a response to denudation caused by the Gangdese Thrust or related to the regional uplift and exhumation in extensional background. By the early Miocene, the rapid exhumation was associated with localized river incision or intensification of Asian monsoon, or north–south normal fault.     

Anatomy of a world-class epizonal orogenic-gold system: A holistic thermochronological analysis of the Xincheng gold deposit,Jiaodong Peninsula,eastern China

Xincheng is a world-class orogenic-gold deposit hosted by the Early Cretaceous Guojialing granitoid in the Jiaodong Peninsula, eastern China. A zircon U–Pb age of 126 ± 1.4 Ma, together with previous data, constrain the emplacement of the Guojialing intrusion to 132–123 Ma. The granitoid underwent subsolidus ductile deformation at >500 °C following its intrusion. The small difference in age between the youngest zircon U–Pb age of unaltered granitoid (~123 Ma) and the ca. 120 Ma ⁴⁰Ar/³⁹Ar ages of sericite, associated with breccias and gold mineralization within it indicate initial rapid cooling from magmatic temperatures to those prevalent during brittle deformation and associated gold mineralization at ~220–300 °C. Evidence of a direct association between granitic magmatism and gold mineralization, such as at least localized near-magmatic depositional temperatures and metal zoning evident in undoubted intrusion-related gold deposits, is absent. The ⁴⁰Ar/³⁹Ar age of ~120 Ma coincides with the mineralization age of many other orogenic-gold deposits along the Jiaojia Fault. Sixteen zircon fission-track (ZFT) ages across the ore and alteration zones range from 112.9 ± 3.4 to 99.1 ± 2.7 Ma. The long period of cooling to the ~100 Ma ZFT closure temperatures recorded here suggests that ambient temperatures for hydrothermal alteration systems lasted to ~100 Ma, possibly because of their focus at Xincheng within the young Guojialing granitoid as it cooled more slowly below approximately 300 °C to 220 °C. However, the restricted number of auriferous ore stages, combined with the presence of cross-cutting gold-free quartz-carbonate veins, indicate that gold itself was only deposited over a restricted time interval at ~120 Ma, consistent with studies of orogenic gold deposits elsewhere. This highlights the complex interplay between magmatism, deformation and the longevity of hydrothermal systems that cause genetic controversies. Based on apatite fission-track (AFT) ages, the Xincheng gold deposit was then uplifted and exhumed to near the surface of the crust at 15 Ma, probably due to movement on the crustal-scale Tan-Lu Fault. Recognition of such exhumation histories along gold belts has conceptual exploration significance in terms of the probability of discovery of additional exposed or sub-surface gold ore bodies as discovery is as much a function of preservation as formation of the deposits.     

AFT dating constrains the Cenozoic uplift of the Qimen Tagh Mountains,Northeast Tibetan Plateau,comparison with LA-ICPMS Zircon U–Pb ages

The Tibetan Plateau (TP) is the highest plateau in the world, which has been the focus of Cenozoic geological studies. The Northeast Tibetan Plateau (NETP) is a key location to decipher the Cenozoic evolution history of the TP. Understanding the building of the Qimen Tagh Mountains located in NETP will help to constrain the development of the northern boundary of the main TP, test the existence of a Paleo-Qaidam Basin and test the eastward growth model of the TP. In this study, granite samples from the Qimen Tagh Mountains were dated by LA-ICPMS and apatite fission track (AFT). The LA-ICPMS zircon U–Pb ages give two magmatic events around ~ 405 and ~ 255 Ma from two different sites. AFT modeling shows that the initial uplift took place at ~ 40–30 Ma in these mountains, which should be controlled by the Altyn Tagh Fault. Compiling previously low-temperature thermochronometry results, it reveals that the initial Cenozoic uplift of the northern boundary of the TP (Qimen Tagh and East Kunlun mountains), soon after the India–Eurasia collision in the southern TP, has divided the Paleo-Qaidam Basin into several sub-basins. The approximate NE–E growth process occurred along the lithospheric Altyn Tagh and Kunlun faults. The current basin and range morphology of the NETP took place around ~ 8 Ma.     

Thermotectonic history of the Chiplakot Crystalline Belt in the Lesser Himalaya,Kumaon, India: Constraints from apatite fission-track thermochronology

Fission-track ages and confined track length distribution of apatite samples separated from the Chiplakot Crystalline Belt (CCB) of the Lesser Himalayan Crystalline (LHC) zone, located to the south of the Main Central Thrust (MCT)/Munsiari Thrust (MT) in Kumaon, India, have been determined. Ages from the CCB along the Kali and Darma valleys fall in two distinct groups. In the northern part of the CCB, the ages range from 9.8 ± 0.6 to 7.6 ± 0.6 Ma with a weighted mean of 9.6 ± 0.1 Ma, while in the southern part the ages vary from 17.9 ± 0.9 to 12.9 ± 1.1 Ma with a weighted mean of 14.1 ± 0.1 Ma. The bimodal distribution of track lengths indicates that the ages are mixed ages, rather than simple cooling ages. The apatite fission-track (AFT) ages and already published structural data of the CCB suggest a complex erosional, denudation history within the upper 3–4 km of the crust of the CCB. The ages further indicate that the CCB was thrust into place earlier than the Middle Miocene i.e. at the time of development of the MCT. Since, then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation. These results have important implications for the tectonic evolution of the LHC zone to the south of the MCT/MT. The exhumation of the LHC zone in different parts of the Himalaya was not uniform. In the Kumaon Himalaya, it was not controlled, as in the Himachal Himalaya, by any major tectonic event, since it was thrust over the Lesser Himalayan Meta-sedimentary (LHMS) zone, and underwent moderate to slow erosion and exhumation.     

Preservation and exhumation history of the Harizha-Halongxiuma mining area in the East Kunlun Range,Northeastern Tibetan Plateau,China

The extent to which ore bodies are preserved in orogenic belts remains a poorly understood area of ore deposit research. Using zircon and apatite fission track analysis together with apatite (U-Th)/He dating we constrained the erosion history of the ore bodies in the Harizha–Halongxiuma mining area of the East Kunlun Range, Northern Tibetan Plateau, China. Apatite fission-track ages range from 114 ± 8 to 87 ± 6 Ma, with mean track lengths varying from 11.4 ± 1.9 to 12.9 ± 2.0 μm. Zircon fission-track ages vary from 205 ± 14 to 142 ± 7 Ma. In addition, apatite (U–Th)/He dating yielded ages of 60–56 Ma. The thermal history of Jiapigou was modelled based on the apatite fission-track data, including ages and track lengths, with constraints of zircon fission-track ages and (U-Th)/He ages. The exhumation history of the Harizha–Halongxiuma mining area was reconstructed with these age data, revealing that since the early Mesozoic the area has undergone three cooling stages: (1) rapid cooling from 175 ± 30 Ma to 100 ± 10 Ma with a cooling rate and inferred exhumation of 2.0 ± 0.8 °C/Myr and 4.3 ± 1.7 km, respectively; (2) a relatively stable stage from 100 ± 10 Ma to 40 ± 10 Ma with a cooling rate and inferred exhumation of 0.3 ± 0.1 °C/Myr and 0.5 ± 0.2 km, respectively; and (3) rapid cooling since 40 ± 10 Ma with a cooling rate and inferred exhumation of 1.2 ± 0.6 °C/Myr and 1.4 ± 0.4 km, respectively. This exhumation history is consistent with the subduction process of Pacific plate and the strike slip movements of Dunmi fault. The total exhumation after main mineralization is calculated to be 7.6 ± 3.2 km, suggesting that ore bodies in the Harizha–Halongxiuma mining area remain partially preserved.     

Cooling history and tectonic exhumation stages of the south-central Tibetan Plateau (China): Constrained by 40Ar/39Ar and apatite fission track thermochronology

Between the Qiangtang Block and Yalung-Zangpo Suture Zone in the south-central Tibetan Plateau, the following geological units and suture zones have been identified from south to north: the Gangdese Granitic Belt, the Lhasa Block, the Nyainqentanghla Shear Zone, the Dangxiong–Sangxiong Tectono-granitic Belt and the Bangong–Nujiang Suture Zone. To better constrain the tectonic evolution and cooling histories of these units, ⁴⁰Ar/³⁹Ar muscovite, biotite and K-feldspar, as well as apatite fission track dating and thermochronological analysis have been carried out. The analytical results indicate that the south-central Tibetan Plateau, with the exception of the Nyainqentanghla Shear Zone, provides a record of three cooling stages at 165–150, 130–110 and ∼45–35 Ma. Fission-track data modelling also indicates that the stages of cooling were different in the different tectonic belts or blocks. Very different cooling phases occurred in the south-central Tibetan Plateau, compared with southern Tibet, as well as along the Yalung–Zangpo Suture Zone. There is no thermochronological evidence to indicate that the south-central part of Tibetan Plateau was influenced by the underthrusting of Indian Plate.The three-stage cooling history and the stages of tectonic exhumation were controlled completely by the closure of the Bangong–Nujiang Suture Zone along its eastern segment during Middle–Late Jurassic (165–150 Ma) and its western segment in the Early–Late Cretaceous (130–110 Ma), as well as by the collision between the Indian and Asian plates in the Paleogene (45–35 Ma).     

The exhumation history of collision-related mineralizing systems in Tibet: Insights from thermal studies of the Sharang and Yaguila deposits,central Lhasa

The large, newly discovered Sharang porphyry Mo deposit and nearby Yaguila skarn Pb–Zn–Ag (–Mo) deposit reside in the central Lhasa terrane, northern Gangdese metallogenic belt, Tibet. Multiple mineral chronometers (zircon U–Pb, sericite ⁴⁰Ar–³⁹Ar, and zircon and apatite (U–Th)/He) reveal that ore-forming porphyritic intrusions experienced rapid cooling (> 100 °C/Ma) during a monotonic magmatic–hydrothermal evolution. The magmatic–hydrothermal ore-forming event at Sharang lasted ~ 6.0 Myr (~ 1.8 Myr for cooling from > 900 to 350 °C and ~ 4.0 Myr for cooling from 350 to 200 °C) whereas cooling was more prolonged during ore formation at Yaguila (~ 1.8 Myr from > 900 to 500 °C and a maximum of ~ 16 Myr from > 900 to 350 °C). All porphyritic intrusions in the ore district experienced exhumation at a rate of 0.07–0.09 mm/yr (apatite He ages between ~ 37 and 30 Ma). Combined with previous studies, this work implies that uplift of the eastern section of the Lhasa terrane expanded from central Lhasa (37–30 Ma) to southern Lhasa (15–12 Ma) at an increasing exhumation rate. All available geochronologic data reveal that magmatic–hydrothermal–exhumation activities in the Sharang–Yaguila ore district occurred within four periods of magmatism with related mineralization. Significant porphyry-type Mo mineralization was associated with Late Cretaceous–Eocene felsic porphyritic intrusions in the central Lhasa terrane, resulting from Neotethyan oceanic subduction and India–Asia continental collision.     

Polyphase movement on the Lavanttal Fault Zone (Eastern Alps): reconciling the evidence from different geochronological indicators

The Lavanttal Fault Zone (LFZ) is generally considered to be related to Miocene orogen-parallel escape tectonics in the Eastern Alps. By applying thermochronological methods with retention temperatures ranging from ~450 to ~40°C we have investigated the thermochronological evolution of the LFZ and the adjacent Koralm Complex (Eastern Alps). ⁴⁰Ar/³⁹Ar dating on white mica and zircon fission track (ZFT) thermochronology were carried out on host rocks (HRs) and fault-related rocks (cataclasites and fault gouges) directly adjacent to the unfaulted protolith. These data are interpreted together with recently published apatite fission track (AFT) and apatite (U-Th)/He ages. Sample material was taken from three drill cores transecting the LFZ. Ar release spectra in cataclastic shear zones partly show strongly rejuvenated incremental ages, indicating lattice distortion during cataclastic shearing or hydrothermal alteration. Integrated plateau ages from fault rocks (~76 Ma) are in parts slightly younger than plateau ages from HRs (>80 Ma). Incremental ages from fault rock samples are in part highly reduced (~43 Ma). ZFT ages within fault gouges (~65 Ma) are slightly reduced compared to the ages from HRs, and fission tracks show reduced lengths. Combining these results with AFT and apatite (U-Th)/He ages from fault rocks of the same fault zone allows the recognition of distinct faulting events along the LFZ from Miocene to Pliocene times. Contemporaneous with this faulting, the Koralm Complex experienced accelerated cooling in Late Miocene times. Late-Cretaceous to Palaeogene movement on the LFZ cannot be clearly proven. ⁴⁰Ar/³⁹Ar muscovite and ZFT ages were probably partly thermally affected along the LFZ during Miocene times.     

龙门山冲断隆升及其走向差异的裂变径迹证据

大量的低温年代学研究用来讨论龙门山晚新生代的隆升,但很少涉及其走向差异和中生代隆升。本文分别沿龙门山北、中、南段3条剖面进行了锆石和磷灰石裂变径迹测试,结合已有的热年代学数据,以期揭示整个中-新生代期间龙门山隆升历史及其时空变化。中生代以来,龙门山主要有印支期(约200 Ma)、早白垩世末(约100 Ma)、早新生代(65～30 Ma)以及晚中新世(15～9 Ma)等或快或慢的冷却事件,总体上经历了中生代至早新生代的缓慢冷却和晚新生代快速冷却2个阶段,快速剥露开始于15～9 Ma,剥蚀速率由早期的0.1 mm/a增加到0.15～0.3 mm/a左右,局部可达0.9 mm/a左右。走向上,龙门山北段相对偏小的锆石裂变径迹年龄和相对偏大的磷灰石裂变径迹年龄反映其在中生代较中、南段隆升更快,而裂变径迹年龄总体上从北段向中、南段减小,表明中、南段在新生代发生了更快的隆升。倾向上,多种热年代学数据显示新生代期间在北川断裂和彭灌断裂两侧存在明显的差异剥露,这种差异在中、南段表现比北段更为突出。龙门山晚新生代快速隆升和剥露是青藏高原区域隆升背景上叠加的冲断活动所致,而非下地壳流动驱动。     

Diachronous exhumation of HP–LT metamorphic rocks from south‐western Alps: evidence from fission‐track analysis

New fission‐track ages on zircon and apatite (ZFT and AFT) from the south‐western internal Alps document a diachronous cooling history from east to west, with cooling rates of 15–19 °C Ma⁻¹. In the Monviso unit, the ZFT ages are 19.6 Ma and the AFT ages are 8.6 Ma. In the eastern Queyras, ZFT ages range from 27.0 to 21.7 Ma and AFT ages from 14.2 to 9.4 Ma. In the western Queyras, ZFT ages are between 94.7 and 63.1 Ma and AFT ages are between 22.2 and 22.6 Ma. The Chenaillet ophiolite yields ages of 118.1 Ma on ZFT and 67.9 Ma on AFT. The combination of these new FT data with the available petrological and geochronological data emphasize an earlier exhumation in subduction context before 30 Ma, then in collision associated with westward tilting of the Piedmont zone.     

Tracing metamorphism,exhumation and topographic evolution in orogenic belts by multiple thermochronology: a case study from the Nízke Tatry Mts., Western Carpathians

A combination of four thermochronometers [zircon fission track (ZFT), zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th–[Sm])/He (AHe) dating methods] applied to a valley to ridge transect is used to resolve the issues of metamorphic, exhumation and topographic evolution of the Nízke Tatry Mts. in the Western Carpathians. The ZFT ages of 132.1 ± 8.3, 155.1 ± 12.9, 146.8 ± 8.6 and 144.9 ± 11.0 Ma show that Variscan crystalline basement of the Nízke Tatry Mts. was heated to temperatures >210°C during the Mesozoic and experienced a low-grade Alpine metamorphic overprint. ZHe and AFT ages, clustering at ~55–40 and ~45–40 Ma, respectively, revealed a rapid Eocene cooling event, documenting erosional and/or tectonic exhumation related to the collapse of the Carpathian orogenic wedge. This is the first evidence that exhumation of crystalline cores in the Western Carpathians took place in the Eocene and not in the Cretaceous as traditionally believed. Bimodal AFT length distributions, Early Miocene AHe ages and thermal modelling results suggest that the samples were heated to temperatures of ~55–90°C during Oligocene–Miocene times. This thermal event may be related either to the Oligocene/Miocene sedimentary burial, or Miocene magmatic activity and increased heat flow. This finding supports the concept of thermal instability of the Carpathian crystalline bodies during the post-Eocene period.     

Is the HP–UHP Hong’an–Dabie–Sulu orogen a piercing point for offset on the Tan–Lu fault?

The Tan–Lu fault is a major strike-slip fault in eastern China that appears to offset the high-grade rocks of the Hong’an–Dabie–Sulu orogen left-laterally ∼540 km. We evaluate models for the collision between the South and North China blocks, published radiometric dates recording HP–UHP metamorphism and exhumation in the Hong’an–Dabie and Sulu terranes, and the timing of sinistral motion on the Tan–Lu fault to evaluate whether UHP rocks provide a piercing point for offset on the Tan–Lu fault. UHP metamorphism in Hong’an–Dabie was concurrent with Sulu based on U–Pb dating of coesite-bearing domains of zircon at 244 ± 5–226 ± 2 Ma for Hong’an–Dabie and 243 ± 4–225 ± 2 Ma for Sulu. Retrograde metamorphism began c. 220 Ma for both Hong’an–Dabie and Sulu, but retrograde zircon growth ended c. 214 Ma in Hong’an–Dabie and continued until c. 202 Ma in Sulu based on U–Pb dating of zircon domains external to coesite-bearing domains. Structures in Sulu are rotated 25° counter-clockwise from, but are broadly similar to, Hong’an–Dabie suggesting the two areas have a common Triassic orogenic history that pre-dates motion on the Tan–Lu fault, and that is consistent with paleomagnetic studies. We constructed a pre-Cretaceous restoration of the Hong’an–Dabie–Sulu belt that moves the Sulu terrane south, aligning the suture and the eclogite-facies isograd, and rotates Sulu c. 25° clockwise to re-align structures with Hong’an–Dabie. Our restoration is supported by published data and shows that the Hong’an–Dabie–Sulu orogen is a piercing point for post-collisional offset on the Tan–Lu fault and that these regions shared a common subduction–exhumation history. The Tan–Lu fault did not play a significant role in the Hong’an–Dabie–Sulu collision and likely developed later, in the Early Cretaceous.     

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica ⁴⁰Ar/³⁹Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~ 700 °C to ~ 60 °C in less than 60 Myr (136–80 Ma) at variable rates ranging from ~ 50 °C/Myr to ~ 13 °C/Myr, (2) the Hengshan detachment fault accommodated ~ 8–12 km of total slip at variable slip rates from 0.14 to 1 mm/yr during tectonic exhumation, (3) the footwall has been tilted ~ 26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80 Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80 Ma.     

青藏高原中-南部新生代构造演化的热年代学制约

青藏高原新生代以来的隆升过程及特征长期以来广存争议.岩体中不同单矿物所记录的中低温热年代学信息适用于揭示较新年代地质体的隆升过程,可以为之提供有效制约.在青藏高原部分岩浆岩与变质岩露头区原位采集15块样品,利用锆石与磷灰石裂变径迹等热年代学结果为青藏高原中生代末期以来的隆升过程提供约束.其中,所获10块样品的锆石裂变径迹数据年龄范围为182~33 Ma,分别记录了渐新世之前青藏高原内不同块体间相互碰撞及高原内不同地区的构造热事件.特别是沿雅鲁藏布江缝合带分布的3个样品,锆石裂变径迹年龄结果一致显示始新世末期-渐新世早期该带存在一期显著的构造热事件.该构造热事件暗示在约36~33 Ma沿雅江缝合带发生过强烈的陆-陆硬碰撞.所获14块样品的磷灰石裂变径迹年龄范围为70.4~5.0 Ma,综合热史反演结果显示青藏高原南部中新世中晚期以来存在整体性隆升,特别是从上新世开始隆升速率显著加快.磷灰石裂变径迹年龄在空间分布上具有向高原东南部变年轻的趋势,表明青藏高原东南部在上新世以来的构造隆升较其他地区要强烈,暗示印度-亚洲板块碰撞驱动机制对该时期的高原隆升具有控制作用.此外,青藏高原中部在白垩纪末期-始新世可能即已隆升至相当高度,此后至今保持了相当低的剥蚀速率.      

1.3–0.9 Ga Oaxaquia (Mexico): Remnant of an arc/backarc on the northern margin of Amazonia

Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goiás arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goiás arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.     

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) – Tso Morari transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55 Ma and metamorphosed by ca. 48–40 Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10 km) by 40–30 Ma in the Tso Morari dome (northern section of the transect) and by 30–20 Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.Metamorphism started in the High Himalayan nappe prior to the Late Oligocene.High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26 Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19 Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (<10 km) between 20 and 6 Ma, while its southern front reached this depth at 10–5 Ma.     

Two types of ultrapotassic plutonic rocks in the Bohemian Massif — Coeval intrusions at different crustal levels

We present U–Pb zircon age determinations of two Variscan ultrapotassic plutonic rocks from the Moldanubian Zone (Bohemian Massif). Equant, multifaceted zircons without inherited cores from a two-pyroxene–biotite quartz monzonite of the Jihlava Pluton yielded a precise age of 335.12 ± 0.57 Ma, interpreted as dating magma crystallization. The majority of both tabular and prismatic grains from the amphibole–biotite melagranite (“durbachite”) from the T?ebí? Pluton plot along a discordia intersecting the concordia at 334.8 ± 3.2 Ma; prismatic zircon grains commonly contain inherited cores and yield an upper intercept age of 2.2 Ga, indicating early Proterozoic inheritance. We therefore suggest that both types of the ultrapotassic plutonic rocks from the Bohemian Massif crystallized at ca 335 Ma, and the previously published ages higher than ca 340 Ma for “durbachites” were biased by a small amount of unresolved inheritance. The ultrapotassic magma emplacement in the middle crust was related to rapid exhumation of a deep crustal segment, considered as isothermal decompression between high-pressure (～ 340 Ma) and medium-pressure (～ 333 Ma) stages recorded in granulites. Mineral assemblages as well as external and internal zircon morphology suggest that the Jihlava intrusion was deep and dry, whereas the T?ebí? intrusion was shallow and wet. Low ε_Hf values of zircons (? 4.4 to ? 7.5) in both rock types suggest a similar source with a predominant crustal component. However, inherited grains in the T?ebí? melagranite indicate its contamination with crustal material during emplacement, and thus possibly a slower rate of exhumation and/or of magma ascent through the crust.     

Tectonic evolution of the North Qinling Orogen from subduction to collision and exhumation: Evidence from zircons in metamorphic rocks of the Qinling Group

The North Qinling Block (NQB) is an important segment of the Qinling Orogen in Central China. Here we report the results from SIMS geochronology and oxygen isotopes, as well as LA-MC-ICPMS Hf isotopic analyses on zircon grains from a suite of metamorphic rocks (felsic gneisses, garnet plagioclase amphibolites, and retrograde eclogite dikes) in the Qinling Group of the NQB. The age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483–501 Ma, followed by 454–470 Ma retrograde metamorphism. These results are generally coeval with the periods of 500–480 Ma for peak metamorphism and 460–420 Ma for retrograde metamorphism previously obtained from the HP/UHP metamorphic rocks of the NQB. During the prograde and retrograde metamorphism, widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons. The fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low Th/U ratios, positive ε_Hf(t) (> 5) and extremely δ¹⁸O (10.01–13.91‰) values in metamorphic zircons. In contrast, the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high U and Th contents and Th/U ratios and enriched Hf (ε_Hf(t) = − 5.42 to − 0.18) and oxygen isotope composition (δ¹⁸O around 8‰). Geochronological and geochemical features of the magmatic cores of the clear core-rim textured zircons demonstrate that the protoliths of the gneisses were intermediate-acid volcanic rocks erupted before Neoproterozoic (800 Ma), which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes, with the oldest magmatic zircon formed at 789 Ma. The protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism. Combined with the previous studies, the Qinling Group experienced overall subduction in the Early Paleozoic. The NQB as represented by the Qinling Group was most likely a discrete micro-block in the Neoproterozoic, and underwent deep subduction in the Cambrian (483–501 Ma) and exhumation in Ordovician (454–470 Ma). We propose that the NQB preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading, and micro-continent formation to deep subduction and exhumation.