The Mejillonia suspect terrane (Northern Chile): Late Triassic fast burial and metamorphism of sediments in a magmatic arc environment extending into the Early Jurassic

The Mejillonia terrane, named after the Mejillones Peninsula (northern Chile), has been traditionally considered an early Paleozoic block of metamorphic and igneous rocks displaced along the northern Andean margin in the Mesozoic. However, U–Pb SHRIMP zircon dating of metasedimentary and igneous rocks shows that the sedimentary protoliths were Triassic, and that metamorphism and magmatism took place in the Late Triassic (Norian). Field evidence combined with zircon dating (detrital and metamorphic) further suggests that the sedimentary protoliths were buried, deformed (foliated and folded) and metamorphosed very rapidly, probably within few million years, at ca. 210 Ma. The metasedimentary wedge was then uplifted and intruded by a late arc-related tonalite body (Morro Mejillones) at 208 ± 2 Ma, only a short time after the peak of metamorphism. The Mejillones metamorphic and igneous basement represents an accretionary wedge or marginal basin that underwent contractional deformation and metamorphism at the end of a Late Permian to Late Triassic anorogenic episode that is well known in Chile and Argentina. Renewal of subduction along the pre-Andean continental margin in the Late Triassic and the development of new subduction-related magmatism are probably represented by the Early Jurassic Bólfin–Punta Tetas magmatic arc in the southern part of the peninsula, for which an age of 184 ± 1 Ma was determined. We suggest retaining the classification of Mejillonia as a tectonostratigraphic terrane, albeit in this new context.     

Uplift-denudation history of the Qinling orogen: Constrained from the detrital-zircon U–Pb geochronology

This paper presents a great number of detrital zircon U–Pb ages from the Middle Triassic to the Middle Jurassic sediments in the Jiyuan basin, southern North China. The results represent age spectra from 2.9 Ga to 216 Ma, with five peaks at 2.5 Ga, 1.9 Ga, 840 Ma, 440 Ma, and 270 Ma and two grains of ∼220 Ma. The ages of 2.5 Ga and 1.9 Ga are mainly derived from the Precambrian basement of the North China Block, whereas the others are typical characteristics of the Qinling orogenic belt. An important observation is that the Qinling-sourced detrital zircons become older as the strata get younger. Samples from the Middle Triassic to early Late Triassic strata are characterized by the age peak at 270 Ma, whereas the Late Late Triassic to Early Middle Jurassic samples are dominated by age peaks at 840 Ma and 440 Ma and minor grains within 800–650 Ma. Two grains of ∼220 Ma are preserved in the Late Middle Jurassic sample, which may be contributed by the Carnian deep plutons. These signatures indicate that the unroofing pattern of the Qinling orogenic belt developed by the denudation of materials from young covers to old basements and the Carnian deep plutons. Integrated with the data reported from the Hefei Basin, it is well-established that the intensity of unroofing increased from the Qinling to the Dabie orogen in the Early Jurassic, and the denudation timing of the ultra-high pressure (UHP) and high pressure (HP) rocks or Carnian plutons changed successively from the Early Jurassic in the Dabie to the Late Middle Jurassic in the Qinling orogen.     

Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin

The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~ 280 m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150 °C and ~ 300 °C removes the secondary, moderately inclined (~ 48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~ 63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~ 200 m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF + UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles.     

Constraints on the paleogeographic evolution of the North China Craton during the Late Triassic–Jurassic

This paper reports U–Pb–Hf isotopes of detrital zircons from Late Triassic–Jurassic sediments in the Ordos, Ningwu, and Jiyuan basins in the western-central North China Craton (NCC), with the aim of constraining the paleogeographic evolution of the NCC during the Late Triassic–Jurassic. The early Late Triassic samples have three groups of detrital zircons (238–363 Ma, 1.5–2.1 Ga, and 2.2–2.6 Ga), while the latest Late Triassic and Jurassic samples contain four groups of detrital zircons (154–397 Ma, 414–511 Ma, 1.6–2.0 Ga, and 2.2–2.6 Ga). The Precambrian zircons in the Late Triassic–Jurassic samples were sourced from the basement rocks and pre-Late Triassic sediments in the NCC. But the initial source for the 238–363 Ma zircons in the early Late Triassic samples is the Yinshan–Yanshan Orogenic Belt (YYOB), consistent with their negative zircon ε_Hf(t) values (−24 to −2). For the latest Late Triassic and Jurassic samples, the initial source for the 414–511 Ma zircons with ε_Hf(t) values of −18 to +9 is the Northern Qinling Orogen (NQO), and that for the 154–397 Ma zircons with ε_Hf(t) values of −25 to +12 is the YYOB and the southeastern Central Asian Orogenic Belt (CAOB). In combination with previous data of late Paleozoic–Early Triassic sediments in the western-central NCC and Permian–Jurassic sediments in the eastern NCC, this study reveals two shifts in detrital source from the late Paleozoic to Jurassic. In the Late Permian–Early Triassic, the western-central NCC received detritus from the YYOB, southeastern CAOB and NQO. However, in the early Late Triassic, detritus from the CAOB and NQO were sparse in basins located in the western-central NCC, especially in the Yan’an area of the Ordos Basin. We interpret such a shift of detrital source as result of the uplift of the eastern NCC in the Late Triassic. In the latest Late Triassic–Jurassic, the southeastern CAOB and the NQO restarted to be source regions for basins in the western-central NCC, as well as for basins in the eastern NCC. The second shift in detrital source suggests elevation of the orogens surrounding the NCC and subsidence of the eastern NCC in the Jurassic, arguing against the presence of a paleo-plateau in the eastern NCC at that time. It would be subsidence rather than elevation of the eastern NCC in the Jurassic, due to roll-back of the subducted paleo-Pacific plate and consequent upwelling of asthenospheric mantle.     

Thermo-tectonic history of the Issyk-Kul basement (Kyrgyz Northern Tien Shan,Central Asia)

Lake Issyk-Kul occupies a large Late Mesozoic–Cenozoic intramontane basin between the mountain ranges of the Northern Kyrgyz Tien Shan. These ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs. Several granites from the Terskey, Kungey, Trans-Ili and Zhetyzhol Ranges were dated with the zircon U/Pb method (SHRIMP, LA-ICP-MS) and yield concordant Late Ordovician–Silurian (~ 456–420 Ma) emplacement ages. These constrain the “Caledonian” accretion history of the Northern Kyrgyz Tien Shan in the amalgamated Palaeo-Kazakhstan continent. The ancestral Tien Shan orogen assembled in the Early Permian when final closure of the Turkestan Ocean ensued collision of Palaeo-Kazakhstan and Tarim. A Late Palaeozoic structural basement fabric formed and Middle–Late Permian post-collisional magmatism added to crustal growth of the Tien Shan. Permo‐Triassic cooling (~ 300–220 Ma) of the ancestral Tien Shan was unraveled using ⁴⁰Ar/³⁹Ar K-feldspar and titanite fission-track (FT) thermochronology on the Issyk-Kul granitoids. Apatite thermochronology (FT and U–Th–Sm/He) applied to the broader Issyk-Kul region elucidates the Meso-Cenozoic thermo-tectonic evolution and constrains several tectonic reactivation episodes in the Jurassic, Cretaceous and Cenozoic. Exhumation of the studied units occurred during a protracted period of intracontinental orogenesis, linked to far-field effects of Late Jurassic–Cretaceous accretion of peri-Gondwanan blocks from the Tethyan realm to Eurasian. Following a subsequent period of stability and peneplanation, incipient building of the modern Tien Shan orogen in Northern Kyrgyzstan started in the Oligocene according to our data. Intense basement cooling in distinct reactivated and fault-controlled sections of the Trans-Ili and Terskey Ranges finally pinpoint important Miocene–Pliocene (~ 22–5 Ma) exhumation of the Issyk-Kul basement. Late Cenozoic formation of the Tien Shan is associated with ongoing indentation of India into Eurasia and is a quintessential driving force for the reactivation of the entire Central Asian Orogenic Belt.     

Sedimentary response to the paleogeographic and tectonic evolution of the southern North China Craton during the late Paleozoic and Mesozoic

With the aim of constraining the influence of the surrounding plates on the Late Paleozoic–Mesozoic paleogeographic and tectonic evolution of the southern North China Craton (NCC), we undertook new U–Pb and Hf isotope data for detrital zircons obtained from ten samples of upper Paleozoic to Mesozoic sediments in the Luoyang Basin and Dengfeng area. Samples of upper Paleozoic to Mesozoic strata were obtained from the Taiyuan, Xiashihezi, Shangshihezi, Shiqianfeng, Ermaying, Shangyoufangzhuang, Upper Jurassic unnamed, and Lower Cretaceous unnamed formations (from oldest to youngest). On the basis of the youngest zircon ages, combined with the age-diagnostic fossils, and volcanic interlayer, we propose that the Taiyuan Formation (youngest zircon age of 439 Ma) formed during the Late Carboniferous and Early Permian, the Xiashihezi Formation (276 Ma) during the Early Permian, the Shangshihezi (376 Ma) and Shiqianfeng (279 Ma) formations during the Middle–Late Permian, the Ermaying Group (232 Ma) and Shangyoufangzhuang Formation (230 and 210 Ma) during the Late Triassic, the Jurassic unnamed formation (154 Ma) during the Late Jurassic, and the Cretaceous unnamed formation (158 Ma) during the Early Cretaceous. These results, together with previously published data, indicate that: (1) Upper Carboniferous–Lower Permian sandstones were sourced from the Northern Qinling Orogen (NQO); (2) Lower Permian sandstones were formed mainly from material derived from the Yinshan–Yanshan Orogenic Belt (YYOB) on the northern margin of the NCC with only minor material from the NQO; (3) Middle–Upper Permian sandstones were derived primarily from the NQO, with only a small contribution from the YYOB; (4) Upper Triassic sandstones were sourced mainly from the YYOB and contain only minor amounts of material from the NQO; (5) Upper Jurassic sandstones were derived from material sourced from the NQO; and (6) Lower Cretaceous conglomerate was formed mainly from recycled earlier detritus.The provenance shift in the Upper Carboniferous–Mesozoic sediments within the study area indicates that the YYOB was strongly uplifted twice, first in relation to subduction of the Paleo-Asian Ocean Plate beneath the northern margin of the NCC during the Early Permian, and subsequently in relation to collision between the southern Mongolian Plate and the northern margin of the NCC during the Late Triassic. The three episodes of tectonic uplift of the NQO were probably related to collision between the North and South Qinling terranes, northward subduction of the Mianlue Ocean Plate, and collision between the Yangtze Craton and the southern margin of the NCC during the Late Carboniferous–Early Permian, Middle–Late Permian, and Late Jurassic, respectively. The southern margin of the central NCC was rapidly uplifted and eroded during the Early Cretaceous.     

Late Triassic volcanic activity in South-East Asia: New stratigraphical,geochronological and paleontological evidence from the Luang Prabang Basin (Laos)

In South-East Asia, sedimentary basins displaying continental Permian and Triassic deposits have been poorly studied. Among these, the Luang Prabang Basin (North Laos) represents a potential key target to constrain the stratigraphic and structural evolutions of South-East Asia. A combined approach involving sedimentology, palaeontology, geochronology and structural analysis, was thus implemented to study the basin. It resulted in a new geological map, in defining new formations, and in proposing a complete revision of the Late Permian to Triassic stratigraphic succession as well as of the structural organization of the basin. Radiometric ages are used to discuss the synchronism of volcanic activity and sedimentation.The Luang Prabang Basin consists of an asymmetric NE-SW syncline with NE-SW thrusts, located at the contact between Late Permian and Late Triassic deposits. The potential stratigraphic gap at the Permian–Triassic boundary is therefore masked by deformation in the basin. The Late Triassic volcaniclastic continental deposits are representative of alluvial plain and fluvial environments. The basin was fed by several sources, varying from volcanic, carbonated to silicic (non-volcanic). U–Pb dating of euhedral zircon grains provided maximum sedimentation ages. The stratigraphic vertical succession of these ages, from ca. 225, ca. 220 to ca. 216 Ma, indicates that a long lasting volcanism was active during sedimentation and illustrates significant variations in sediment preservation rates in continental environments (from ∼100 m/Ma to ∼3 m/Ma). Anhedral inherited zircon grains gave older ages. A large number of them, at ca. 1870 Ma, imply the reworking of a Proterozoic basement and/or of sediments containing fragments of such a basement. In addition, the Late Triassic (Carnian to Norian) sediments yielded to a new dicynodont skull, attributed to the Kannemeyeriiform group family, from layers dated in between ∼225 and ∼221 Ma (Carnian).     

Tectonic implications of Early Cretaceous low-Mg adakitic rocks generated by partial melting of thickened lower continental crust at the southern margin of the central North China Craton

This paper reports new whole-rock geochemical, Sr–Nd–Pb isotopic, and zircon U–Pb and Hf isotopic data for Early Cretaceous intrusive rocks in the Sanmenxia–Houma area of central China, and uses these data to constrain the petrogenesis of low-Mg adakitic rocks (LMAR) and the spatial extent of the influence of the deeply subducted Yangtze slab during the Triassic evolution of this region. New zircon laser-ablation inductivity coupled plasma mass spectrometry (LA-ICP-MS) U–Pb data indicate that the early- and late-stage southern Quli, Qiligou, and Gaomiao porphyritic quartz diorites, the Canfang granodiorite, and the northern Wangmao porphyritic quartz monzodiorite were emplaced during the Early Cretaceous (~ 130 Ma) and the late Early Cretaceous (~ 116 Ma). These rocks are characterized by high Na₂O/K₂O, Sr/Y, and (La/Yb)_n ratios as well as high Sr concentrations, low Mg^# [molar 100 × Mg/(Mg + Fe^2 +_tot)] values, and low heavy rare earth element and Y concentrations, all of which indicate an LMAR affinity. The samples have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7054–0.7095), and low ε_Nd(t) (− 11.90 to − 22.20) and ε_Hf(t) (− 16.7 to − 32.7) values, indicative of a lower continental crust origin. The presence of Neoproterozoic (754–542 Ma) and inherited Late Triassic (220 Ma) metamorphic zircons within the late Early Cretaceous LMAR and the relatively high ²⁰⁶Pb/²⁰⁴Pb ratios of these rocks suggest that they formed from primary magmas derived from partial melting of Yangtze Craton (YC) basement material that had undergone ultrahigh-pressure metamorphism. In contrast, the presence of Paleoproterozoic and Archean inherited zircons within early Early Cretaceous LMAR in this area and the relatively low ²⁰⁶Pb/²⁰⁴Pb ratios of these rocks are indicative of derivation from primary magmas generated by partial melting of the thickened lower continental crust of the North China Craton (NCC). These rocks may have formed in an extensional environment associated with the upwelling of asthenospheric mantle material. The presence of YC basement material within the NCC in the Sanmenxia–Houma area suggests that the deeply subducted Yangtze slab influenced an area of ~ 100 km in lateral extent within the southern margin of the central NCC during the Triassic.     

Geology,geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc,SW China

The Hongshan Cu-polymetallic deposit is located in the southern Yidun arc in southwestern China, where both subduction-related (Late Triassic) and post-collisional (Late Cretaceous) porphyry–skarn–epithermal mineralization systems have been previously recognized. In this study, two distinct magmatic events, represented by diorite porphyry and quartz monzonite porphyry, have been revealed in the Hongshan deposit, with zircon SHRIMP U–Pb ages of 214 ± 2 Ma and 73.4 ± 0.7 Ma, respectively. The 73 Ma age is comparable to the Re–Os ages of 77 to 80 Ma of ore minerals from the Hongshan deposit, indicating that the mineralization is related to the Late Cretaceous quartz monzonite porphyries rather than Late Triassic diorite porphyries. The Late Triassic diorite porphyries belong to the high-K calc-alkaline series and show arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U and depletion in HFSEs, indicating that they were formed during the westward subduction of the Garzê–Litang Ocean. In contrast, the Late Cretaceous quartz monzonite porphyries show shoshonitic I-type geochemical characteristics, with high SiO₂, K₂O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr–Nd–Pb isotopic compositions (average (⁸⁷Sr/⁸⁶Sr)_i = 0.7085; ε_Nd(t) = − 6.0; ²⁰⁶Pb/²⁰⁴Pb = 19.064, ²⁰⁷Pb/²⁰⁴Pb = 15.738, ²⁰⁸Pb/²⁰⁴Pb = 39.733) suggest that the quartz monzonite porphyries originated from the partial melting of the ancient lower crust in response to underplating of mafic magma from subduction metasomatized mantle lithosphere, possibly triggered by regional extension in the post-collisional tectonic stage. The S isotopic compositions (δ³⁴S_V-CDT = 3.81‰ to 5.80‰) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.014 to 18.809, ²⁰⁷Pb/²⁰⁴Pb = 15.550 to 15.785, and ²⁰⁸Pb/²⁰⁴Pb = 38.057 to 39.468) of ore sulfides indicate that the sulfur and metals were derived from mixed mantle and crustal sources. It is proposed that although the Late Triassic magmatic event is not directly related to mineralization, it contributed to the Late Cretaceous mineralization system through the storage of large amounts of sulfur and metals as well as water in the cumulate zone in the mantle lithosphere through subduction metasomatism. Re-melting of the mantle lithosphere including the hydrous cumulate zone and ancient lower crust during the post-collisional stage produced fertile magmas, which ascended to shallow depths to form quartz monzonite porphyries. Hydrothermal fluids released from the intrusions resulted in porphyry-type Mo–Cu ores in and near the intrusions, skarn-type Cu–Mo ores in the country rocks above the intrusions, and hydrothermal Pb–Zn ores in the periphery.     

Geochronology and geochemistry of Late Cretaceous igneous intrusions and Mo–Cu–(W) mineralization in the southern Yidun Arc,SW China: Implications for metallogenesis and geodynamic setting

The Sanjiang Tethyan Metallogenic Domain (STMD) is an important part of the Tethyan giant metallogenic belt. The Yidun Arc is a part of the STMD in the eastern Tibetan Plateau. Recently, four newly discovered Mo–Cu–(W) ore deposits related to granitic intrusions were found distributed along the north-south strike in the southern Yidun Arc, which are identified as the Xiuwacu, Relin, Hongshan, and Tongchanggou deposits herein. These four deposits formed along high-angle north-northwest or north-west strike-slip faults, with vein-type and porphyry-type Mo–Cu mineralization developed in the intrusions. Molybdenite Re–Os and zircon U–Pb dating together with zircon Hf isotopes and whole-rock geochemistry of the intrusions were studied to discern the relationship between mineralization and magmatism, metallogenesis, and tectonic settings. Molybdenite from skarn-type mineralization at the Hongshan deposit has a Re–Os isochron age of 81.2 ± 2.6 Ma (MSWD = 1.3, n = 5) consistent with previously published zircon U–Pb ages and Re–Os ages of porphyry-type Mo mineralization. These results indicate that the Hongshan is a Late Cretaceous porphyry-skarn Cu–Mo deposit. Zircon U–Pb ages of the granitic intrusions in the Xiuwacu, Relin, and Tongchanggou deposits varying from ~ 87.4 Ma to ~ 82.7 Ma. Combined with published molybdenite Re–Os age spectrum (~ 85 Ma to ~ 81.2 Ma), it is proposed that the Mo–Cu–(W) mineralization in the Shangri-La region is spatially, temporally, and probably genetically related to the Late Cretaceous granitic intrusions. The Relin, Hongshan, and Tongchanggou intrusions have high SiO₂ (65.2–70.0 wt.%), Sr (363–905 ppm), Sr/Y (22–72), and La/Yb (37–69) ratios, and low Y (11.6–17.0 ppm) and Yb (0.97–1.59 ppm), which displayed adakitic affinities. Their low MgO (0.66–1.44 wt.%), Mg# (25–46), variable negative zircon ε_Hf(t) values (− 7.9 to − 2.3), and Proterozoic two-stages Hf model ages (T_DM2 = 1.13–1.62 Ga) suggest that they were probably dominantly derived from partial melting of thickened lower continental crust. According to the tectonic evolution of the Bangong Meso-Tethys Ocean during the Late Mesozoic, the Late Cretaceous igneous event and mineralization in the Yidun Arc likely formed under a late- or post-collision extensional environment, probably related to the collision between the Lhasa and Qiangtang terranes during the Late Cretaceous.     

REE and isotope (Sr,S, and Pb) geochemistry to constrain the genesis and timing of the F–(Ba–Pb–Zn) ores of the Zaghouan District (NE Tunisia)

The F–(Ba–Pb–Zn) ore deposits of the Zaghouan District, located in NE Tunisia, occur as open space fillings or stratabound orebodies, hosted in Jurassic, Cretaceous and Tertiary layers. The chondrite-normalized rare earth element (REE) patterns may be split into three groups: (i) “Normal marine” patterns characterizing the wallrock carbonates; (ii) light REE (LREE) enriched (slide-shaped) patterns with respect to heavy REE (HREE), with small negative Ce and Eu anomalies, characteristic of the early ore stages; (iii) Bell-shaped REE patterns displaying LREE depletion, as well as weak negative Ce and Eu anomalies, characterizing residual fluids of subsequent stages. The ⁸⁷Sr/⁸⁶Sr ratios (0.707654–0.708127 ± 8), show that the Sr of the epigenetic carbonates (dolomite, calcite) and ore minerals (fluorite, celestite) are more radiogenic than those of the country (Triassic, Jurassic, Cretaceous, lower Miocene) sedimentary rocks. The uniformity of this ratio, throughout the District, provides evidence for the isotopic homogeneity and, consequently, the identity of the source of the mineralizing fluids. This signature strongly suggests that the radiogenic Sr is carried by Upper Paleozoic basinal fluids.The δ³⁴S values of barite, associated to mineralizations, are close to those of the Triassic sea water (17‰). The δ³⁴S values of sulfide minerals range from − 13.6‰ to + 11.4‰, suggesting two sulfur-reduced end members (BSR/TSR) with a dominant BSR process.Taking account of the homogeneity in the Pb-isotope composition of galenas (18.833–18.954 ± 0.001, 15.679–15.700 ± 0.001 and 38.690–38.880 ± 0.004, for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios respectively), a single upper crustal source for base-metals is accepted. The Late Paleozoic basement seems to be the more plausible source for F–Pb–Zn concentrated in the deposits. The genesis of the Zaghouan District ore deposits is considered as the result of the Zaghouan Fault reactivation during the Late Miocene period.     

Geochronological and geochemical studies of mafic and intermediate dykes from the Khao Khwang Fold–Thrust Belt: Implications for petrogenesis and tectonic evolution

Zircon U–Pb, mica 40Ar/39Ar ages and geochemistry of the Permo-Triassic mafic to intermediate dyke swarms at the south-western margin of the Indochina Terrane, central Thailand, are reported here and used to decipher the timing of the Sukhothai-Indochina & Sibumasu-Indochina collisions during the Permo-Triassic stages of the Indosinian Orogeny. The mafic dyke swarms in the folded layers of the Khao Khwang Fold–Thrust Belt (KKFTB) were emplaced between the Late Permian and the Late Triassic. The volcanic rocks range from slightly tholeiitic to mostly calc-alkalic, but can be subdivided into three different volcanic groups on the basis of trace and incompatible element abundances such as Ni, Cr, P, Co, and Th. However, all the groups present similar chemical footprints and are enriched in large ion lithophile elements (LILEs) (Rb, Ba, Sr, Pb) and light rare earth elements (LREEs), and depleted in HFSE such as Nb, and Ti highlighting the volcanic arc nature of the system. Isotopically, the three groups are characterized by subtle differences in εNd(t) values (from + 3.2 to + 5.2) and initial 87Sr/86Sr ratios (from 0.7056 to 0.7067). The KKFTB mafic dykes share a few geochemical characteristics of the mafic dykes from the Chiang Khong volcanic suite in the Sukhothai terrane, and from the Loei volcanic belt in northern Indochina. These geochemical features suggest that the KKFTB mafic dykes, and the volcanic rocks in central-northern Thailand, were likely emplaced in a similar orogenic setting. The rocks of Group III are interpreted to have intruded from the Early Triassic (255 ± 6 Ma) to the Late Triassic (207 ± 2 Ma), and were probably sourced from a more crustally contaminated magma.     

Chronology and geochemistry of Mesozoic granitoids in the Bengbu area,central China: Constraints on the tectonic evolution of the eastern North China Craton

We performed zircon U–Pb dating and analyses of major and trace elements, and Sr–Nd–Pb isotopes for granitoids in the Bengbu area, central China, with the aim of constraining the magma sources and tectonic evolution of the eastern North China Craton (NCC). The analyzed zircons show typical fine-scale oscillatory zoning, indicating a magmatic origin. Zircon U–Pb dating reveals granitoids of two ages: Late Jurassic and Early Cretaceous (²⁰⁶Pb/²³⁸U ages of 160 Ma and 130–110 Ma, respectively). The Late Jurassic rocks (Jingshan intrusion) consist of biotite-syenogranite, whereas the Early Cretaceous rocks (Huaiguang, Xilushan, Nushan, and Caoshan intrusions) are granodiorite, syenogranite, and monzogranite. The Late Jurassic biotite-syenogranites and Early Cretaceous granitoids have the following common geochemical characteristics: SiO₂ = 70.35–74.56 wt.%, K₂O/Na₂O = 0.66–1.27 (mainly < 1.0), and A/CNK = 0.96–1.06, similar to I-type granite. The examined rocks are characterized by enrichment in light rare earth elements, large ion lithophile elements, and U; depletion in heavy rare earth elements, Nb, and Ta; and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7110) and low ε_Nd (t) values (? 14.40 to ? 22.77), indicating a crustal origin.The occurrence of Neoproterozoic magmatic zircons (850 Ma) and inherited early Mesozoic (208–228 Ma) metamorphic zircons within the Late Jurassic biotite-syenogranites, together with the occurrence of Neoproterozoic magmatic zircons (657 and 759 Ma) and inherited early Mesozoic (206–231 Ma) metamorphic zircons within the Early Cretaceous Nushan and Xilushan granitoids, suggests that the primary magmas were derived from partial melting of the Yangtze Craton (YC) basement. In contrast, the occurrence of Paleoproterozoic and Paleoarchean inherited zircons within the Huaiguang granitoids indicates that their primary magmas mainly originated from partial melting of the NCC basement. The occurrence of YC basement within the lower continental crust of the eastern NCC indicates that the YC was subducted to the northwest beneath the NCC, along the Tan-Lu fault zone, during the early Mesozoic.     

Another sea area separated from the Panthalassic Ocean in the Norian,the Late Triassic: The lowest Sr isotopic composition of the Ishimaki limestone in central Japan

Mt. Ishimaki is the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan. The Ishimaki limestone is thought to be seamount-type limestone. The P₁ elements of the conodonts Norigondolella navicula and Ancyrogondolella quadrata found in the limestone indicate it is of Norian age. The Sr isotopic compositions of 45 Ishimaki limestone samples ranged from 0.7055 to 0.7077. Eighteen of these samples had lower Sr isotopic compositions than the lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. The Sr isotopic compositions in the limestone block are generally lower at the base of the block and higher at the top. The present Sr isotopic compositions of the Ishimaki limestone are unlikely to have been reduced by post-depositional alteration because most of the limestone samples had a low amount of Mn (<300 ppm) or high Sr/Mn ratios (>2) and the conodont elements had low (1–2) CAI (conodont alteration index) values. Additionally, there was little acid-insoluble residue. Thus, the low Sr isotopic compositions are thought to represent the strontium of the past ambient seawater. The low Sr isotopic compositions are in complete disagreement with the generally recognised range of seawater Sr isotopic compositions in the Norian stage of the Late Triassic (0.7075–0.7078) because the depositional environment of the Ishimaki limestone was closed or semi-closed from the Panthalassic Ocean. Therefore, the Sr isotopic composition of the limestone differs from that of the Panthalassic seawater. The low Sr isotopic compositions were probably affected by Sr inflows from mafic oceanic crust by hydrothermal fluid circulation or from hinterlands surrounded by mafic rocks by river water circulation.     

Late Quaternary climatic changes,and associated human responses,during the last ~ 45 000 yr in the Eastern and adjoining Western Cape,South Africa

Palaeoenvironmental evidence indicative of former climatic conditions in the Eastern and adjoining Western Cape during the last ~ 45 000 yr is presented and summarised. Interstadial conditions began before 43 000 BP but were succeeded by stadial conditions at ~ 24 000 BP. These climatic phases are designated the Birnam Interstadial and the Bottelnek Stadial after the type sites at which they were identified in the Eastern Cape. The Bottelnek Stadial apparently equates with the Last Glacial Maximum. Late Glacial warming was apparent by 18/17 000 BP. Sea level rose markedly by ~ 14 000 BP. Climatic oscillations marked the end of the Late Glacial. The Early Holocene was drier than the Late Holocene and, at least in the Drakensberg, there was marked aridity in the mid-Holocene. Human responses to these climatic events are briefly described.     

Recurrent rare earth element mineralization in the northwestern Okcheon Metamorphic Belt,Korea: SHRIMP U–Th–Pb geochronology,Nd isotope geochemistry,and tectonic implications

Rare earth element (REE) mineralization is hosted within Neoproterozoic alkaline metaigneous rocks in the northwestern part of the Okcheon Metamorphic Belt (OMB), a polymetamorphosed fold-and-thrust belt transecting the Paleoproterozoic Gyeonggi and Yeongnam Massifs in the southern Korean Peninsula. The principal carrier phase of REEs is allanite. Allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz, K-feldspar, biotite, britholite, apatite, fergusonite, andradite, magnetite, zircon, titanite and fluorite. Electron microprobe analysis of allanite clearly distinguishes sample-to-sample variations in total REEs, Ca, Al, Fe and Y but the textural varieties from each rock sample do not show marked differences in those elements. Sensitive high-resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization. Allanite grains from REE ores yielded Late Ordovician (444.6 ± 8.0 Ma), Permian to Triassic (ca. 300–220 Ma) and Early Jurassic (199–183 Ma) ²⁰⁸Pb/²³²Th ages. These multiple age components often coexist in single grains showing slight differences in backscattered electron brightness. The Ordovician components have distinctly higher Th/U than the younger domains in the same rock sample. The cores and rims of zircon from a syenite hosting REE ore bodies yielded Neoproterozoic (858.2 ± 6.3 Ma) and Early Jurassic (ca. 190 Ma) ²⁰⁶Pb/²³⁸U ages, respectively. The Early Jurassic ages (194–187 Ma) also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and REE mineralization. The Neoproterozoic zircon inheritance (weighted mean = 853.9 ± 3.8 Ma) in these granites is in sharp contrast to the dominant Paleoproterozoic inherited zircon from the widespread earliest Middle Jurassic granites enclosing the mineralized zone. The geotectonic significance of the Late Ordovician event recorded in the allanite, as well as in detrital zircon from the OMB, is still unclear but its temporal coincidence with intraplate volcanism and arc-related igneous activity, respectively, reported from the southwestern edge of the adjacent Taebaeksan Basin and the southwestern Gyeonggi Massif is noteworthy. The following Permian–Triassic and Early Jurassic mineralization events are probably linked to the continental suturing between the North and South China blocks and subsequent post-orogenic magmatism, and arc magmatism resulting from the paleo-Pacific plate subduction, respectively. Sub-grain Sm–Nd isotopic analyses of allanite by laser ablation multiple collector ICPMS yielded initial ε_Nd values plotting along the Nd isotopic evolution path of the Neoproterozoic metaigneous rocks, indicating that REEs originating from the host rock have been recycled during multistage mineralization events. The profound differences in inherited zircon ages and Nd isotopic compositions between the Early and Middle Jurassic granites may reflect the presence of a major thrust-bounded crustal structure beneath the OMB.     

Provenances of the Mesozoic sediments in the Ordos Basin and implications for collision between the North China Craton (NCC) and the South China Craton (SCC)

To constrain the provenance of the Ordos Basin and the evolution history of the Qinling Orogen Belt from the Triassic to the Jurassic, 10 samples from the Dongsheng area and 28 samples from the Yan’an area were analyzed for U–Pb ages and Lu–Hf and Sm–Nd isotopic compositions. The results indicate that Middle Jurassic sediments in the Dongsheng area were derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane. Mesozoic sediments in the Yan’an area consist of two parts. One part is derived from the North China Craton (NCC), which has U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga, and Hf model ages of ∼2.8 Ga. The other part is derived from the Qilian–Qinling Orogenic Belt, which has U–Pb age groups of 600–1500 Ma and 100–500 Ma, and Nd and Hf isotopic model ages of less than 2.2 Ga. Combining the U–Pb ages with the Hf and Nd isotopic model ages, Mesozoic detrital zircons with U–Pb age groups of ∼1.8 Ga and ∼2.5 Ga in the Yan’an area are found to also be derived from the Khondalite Belt, Langshan Mountain and the Yinshan Terrane, not from the Trans-China Orogen Belt. From the late–Late Triassic sediments of the Yan’an area, the low average values of the Hf (2.03 Ga) and Nd (2.03 Ga) model ages and the characteristic age population of 600–1500 Ma reveal that the main collision or continental subduction between the NCC and the South China Craton (SCC) occurred in the late–Late Triassic. After the main collision or continental subduction, the proportion of sediments from the Qinling–Qilian Orogenic Belt began to decrease (recorded in the early Jurassic samples), which may be in response to the gradual slowing of the uplift speed of the Qinling Orogenic Belt. In the early-middle Jurassic, the sediments have a main U–Pb age population of 100–500 Ma, low detrital zircon Hf model ages (average value is 1.17 Ga) and low whole rock Nd model ages (average value is 1.13 Ga), which suggests that the Qilian–Qinling Orogenic Belt may have a fast uplift history in the early-middle Jurassic.     

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.     

Zircon U-Pb geochronology and geochemistry of the intrusions associated with the Jiawula Ag-Pb-Zn deposit in the Great Xing’an Range,NE China and their implications for mineralization

Located in the eastern section of the Central Asian Orogenic Belt, the Jiawula Ag-Pb-Zn deposit is classified as a volcanic to subvolcanic related vein-type ore deposit. New U-Pb zircon geochronology, whole-rock geochemistry, mineral chemistry, and Sr-Nd isotope data are presented for the intrusions in the Jiawula deposit in order to evaluate the timing, petrogenetic type of the granitoid rocks, origin and evolution of magmatism, geodynamics, and to establish its relationship with lead-zinc mineralization. Zircon SHRIMP U-Pb analyses yield weighted mean ages of 150.1 ± 1.8 Ma for quartz porphyry, 148.8 ± 2.2 Ma for syenite porphyry, and 145.3 ± 1.9 Ma for monzonite porphyry, indicating a Late Jurassic (Yanshanian) magmatic event. An earlier magmatic event (Indosinian) occurred during the Late Permian to Early Triassic from ca. 254 Ma to ca. 247 Ma and is represented by granodiorite (254 ± 2 Ma), dacite porphyry (252.9 ± 4.8 Ma), and diorite porphyry (278 ± 4.1 Ma). Both the Indosinian and Yanshanian igneous rocks are classified as I-type granitoids. The late Jurassic intrusions are highly fractionated and characterized by negative anomalies of Eu, Sr, P, and Ti. The hypabyssal intrusions have initial ⁸⁷Sr/⁸⁶Sr values between 0.70458 and 0.70522, and ε_Nd(t) values of −3.4 to −0.2, indicating relatively older crust in Jiawula among more juvenile crust in this area. Magma generation in Jiawula is linked to juvenile lower crustal and slightly enriched mantle sources. The ∼250 Ma magmatic episode in Jiawula might be related to the subduction of the Mongol-Okhotsk oceanic plate towards the south beneath the Erguna massif. The ∼150 Ma magmatic event occurred after the closure of the Mongol-Okhotsk Ocean followed by the change in subduction direction of the Paleo-Pacific plate. Varying temperature, stronger fractionation and higher oxygen fugacity related to the magmatic-hydrothermal transition caused Pb-Zn mineralization.     

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.