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.     

Precise dating of the Middle Permian: Zircon U–Pb geochronology from volcanic ash beds in the basal Gufeng Formation,Yangtze region,South China

Direct radiometric dating of the Lower/Middle Permian epochs has not been well accomplished. Shales and bedded cherts of the geologically well-documented Middle Permian Gufeng Formation are exposed in the Chaohu area, Anhui province, South China. Through detailed field examination and mapping of the Gufeng stratigraphic section, we found at least four volcanic ash beds within the basal shale strata. This new discovery indicates the existence of prominent volcanic activity during Gufeng sedimentation and provides the opportunity to precisely date the age of the Middle Permian. Zircon grains separated from two near-basal horizon yield LA‐ICP‐MS U–Pb ages of 272.0 ± 5.5 Ma (MSWD = 2.6) and 271.5 ± 3.3 Ma (MSWD = 1.7). As the first precise isotopic age (272 Ma) of the Middle Permian Gufeng Formation in South China, our data offer precise geochronological constraints for the division and correlation of Middle Permian not only in South China but also worldwide.     

Chronological link between deep-seated processes in magma chambers and eruptions: Permo-Carboniferous magmatism in the core of Pangaea (Southern Pyrenees)

In the Southern Pyrenees there are Upper Carboniferous–Lower Permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions (rhyolitic ignimbrites and andesitic flows). These basins are spatially associated with granodiorites and dacitic dykes emplaced in Variscan basement rocks. U–Pb SHRIMP dating of zircons extracted from three granodiorites, an andesitic flow, a dacitic dyke and six ignimbrites, revealed that magmatism occurred over an extended period of thirty eight million years, from ca. 304 Ma to ca. 266 Ma (Upper Carboniferous–Middle Permian). A scattering of zircon ages in each sample shows that the history of melt crystallization was complex, with more than one zircon-forming event in each magma chamber. The prolonged crystallization history was transferred to the product of the eruptions. A chronological link between the deep-seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at: ca. 309–307 Ma (Upper Carboniferous), ca. 304–296 Ma (Upper Carboniferous–Lower Permian), ca. 294–282 Ma (Lower Permian), and ca. 276 Ma (Lower Permian). The variation of zircon U/Th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the Permian. Zircons probably crystallized from melt phases related to both a felsic-intermediate metaluminous source from ca. 310–293 Ma (mostly 0.1 < Th/U < 0.6) to ca. 289–273 Ma (especially in the range 0.6 < Th/U < 1) and a mafic source (mostly 1.2 < Th/U < 1) at ca. 266–265 Ma. U–Pb zircon ages from volcanic and plutonic rocks of the Southern Pyrenees are consistent with the ages of the post-Variscan magmatism of Iberia associated with orocline generation and subduction of the Paleotethys Ocean (ca. 304–283 Ma), and in addition reveal a later magmatic event at ca. 276–266 Ma (Lower–Middle Permian). The location of the Iberian orocline in the core of Pangaea and near the western end of the subduction zone of the Paleotethys Ocean leads to the hypothesis that this later magmatic activity of the Southern Pyrenees could provide the missing link between the Variscan and Cimmerian cycles that acted sequentially in Permo-Carboniferous times.     

Isotopic and structural constraints on the late Miocene to Pliocene evolution of the Namche Barwa area,eastern Himalayan syntaxis,SE Tibet

Within the Namche Barwa area, SE Tibet, the Indus–Yarlung suture zone separates the Lhasa terrain in the north from the Himalayan unit including the Tethyan (sedimentary and volcanic rocks), Dongjiu (greenschist to lower amphibolite facies), Namche Barwa (granulite facies), Pei (amphibolite facies) and Laiguo (greenschist facies) sequences in the south. Two fault systems were distinguished in the Namche Barwa area. The former includes a top-down-to-the-north normal fault in the north and two top-to-the-south thrust zones in the south named as Upper and Lower Thrusts, respectively. The Namche Barwa and Pei sequences were exhumed southwards from beneath the Dongjiu sequence by these faults. Thus, the fault system is regarded as a southward extrusion structure. Subsequently, the exposed Dongjiu, Namche Barwa, Pei and Laiguo sequences were displaced northwards onto the Lhasa terrain by the top-to-the-north fault system, thus, marking it as northward indentation structure. Monazite TIMS U–Pb dating demonstrates that the normal fault and the Lower Thrust from the southward extrusion system were probably active at ~ 6 Ma and ~ 10 Ma, respectively. Zircon U–Pb SHRIMP and phlogopite K–Ar ages further suggest that the Upper Thrust was active between 6.2 ± 0.2 Ma and 5.5 ± 0.2 Ma. The northward indentation structures within the core portion of the eastern Himalayan syntaxis were perhaps active between 3.0 Ma and 1.5 Ma, as inferred by published zircon U–Pb SHRIMP and hornblende Ar–Ar ages. The monazite from upper portions of the Pei sequence dated by U–Pb TIMS indicates that the precursor sediments of this sequence were derived from Proterozoic source regions. Nd isotopic data further suggest that all the metamorphic rocks within eastern Himalaya (εNd = ? 13 to ? 19) correlate closely with those from the Greater Himalayan Sequences, whereas the western Himalayan syntaxis is mainly comprised of Lesser Himalayan Sequences. The two indented corners of the Himalaya are, thus, different.     

The status and local stratigraphical context of the Hampen Formation type section (Bathonian,Middle Jurassic)

The type section of the Hampen Formation, as currently defined, is located near Salperton (Gloucestershire); it comprises beds of Middle Jurassic (Bathonian) age. Its validity as a type section is questioned; only the uppermost 2.3 m are herein considered truly to represent the Hampen Formation. The underlying 7.4 m are re-defined as a facies variant of the Througham Formation, for which the name Daglingworth Member is proposed. A review of the local stratigraphical context clarifies the relationship of the Hampen Formation, as newly defined, and of the Daglingworth Member, to contiguous stratigraphical units.     

Paleoclimatic changes during the last 2.5 myr recorded in the Kathmandu Basin,Central Nepal Himalayas

Palynological and sedimentological studies of a series of slimes collected from a 284 m-long drill-well from the Kathmandu Basin reveal paleoclimatic records and environmental changes within the Kathmandu Valley during the last 2.5 myr. The slimes are composed of fluvio-deltaic and lacustrine sediments comprising sand beds of 66.3 m and mud beds of 218 m in length. Pollen analyses show Quercus and Cyclobalanopsis are predominant, with frequencies exceeding 70%. Pinus, Alnus and Gramineae are the next dominant taxa. Three fossil pollen zones were discriminated; each zone reflects major climatic change: Zone I, the oldest stage, indicates a cool and rather wet climate during 400 kyr from ca. 2.5 to 2.1 Ma; Zone II, the middle stage, reflects a warm and relatively dry climate without remarkable fluctuation; Zone III is characterized by seven cycles of warm-and-wet and cold-and-dry climate, which reflect the alternation of glacial and interglacial periods. The last cold maximum, 11 m deep, corresponds to the last glacial age around 20 kyr bp, judging from the ¹⁴C dating of the uppermost part of the lacustrine sediments.The Paleo-Kathmandu Lake is likely to have been initiated at around 2.1 Ma and to have been filled with black organic mud, the Kalimati Clay. The top of the Kalimati Clay is eroded and was overlain by fluvial sand after the last glacial age. The abrupt appearance of a 4 m-thick fossiliferous sand bed at the top of the middle member suggests a lowering of water level at around 1 Ma.     

Reconstructing Late Paleozoic exhumation history of the Inner Mongolia Highland along the northern edge of the North China Craton

The Inner Mongolia Highland (IMH), along the northern edge of the North China Craton, was considered to be a long-standing topographic highland, whose exhumation history remains elusive. The aim of this study is to reveal Late Paleozoic exhumation processes of the IMH based on an integrated analysis of stratigraphy, petrography of clastic rocks, and U–Pb ages and Hf isotopes of detrital zircons from Permian–Triassic succession in the middle Yanshan belt. The results of the study show that the Benxi Formation, which was originally regarded as a Late Carboniferous unit, proves to be Early Permian in age because it contains detrital zircons as young as ∼298 Ma. The Lower Shihezi Formation is demonstrated to be a unit whose age spans the boundary of the Middle and Upper Permian, constrained by a U–Pb age of 260 ± 2 Ma from a dacite layer. Clastic compositions of conglomerate and sandstone change markedly, characterised by the predominance of sedimentary components in the Benxi–Shanxi Formations, by large amounts of volcanic clastics in the Lower and Upper Shihezi Formations, and by the presence of both metamorphic and igneous clastics in the Sunjiagou–Ermaying Formations. Sedimentary clastics include chert, carbonate, sandstone and quartzite, which may have been derived from Proterozoic to Lower Paleozoic sedimentary covers. Volcanic clasts were directly related to volcanic eruptions, while granite and gneiss grains were sourced from exhumed Late Paleozoic intrusive rocks and basement rocks. Detrital zircon U–Pb ages can be divided into five populations: 2.6–2.4 Ga, 1.9–1.7 Ga, 400–360 Ma, 325–290 Ma and 270–250 Ma. Precambrian detrital zircons are typically subrounded to rounded in shape, implying a recycling origin. Late Paleozoic zircons show oscillatory zones and their Th/U ratios >0.4, suggesting a magmatic origin. Most Phanerozoic zircons have negative ε_Hf(T) values of −3.2 to −25.5, which are compatible with those of Late Paleozoic plutons in the IMH. The results indicate that the IMH may have been covered with Proterozoic to Lower Paleozoic sedimentary strata, which then underwent subsequent erosion and served as provenances for adjacent Late Paleozoic basins. Vertical changes in both clastic compositions and detrital zircon ages in Permian–Triassic strata imply an unroofing process of the IMH. Three phases of the IMH uplift are distinguished. The first-phase uplift commenced 325–312 Ma and resulted from magmatic intrusion related to southward subduction of the Paleo-Asian Ocean. The second-phase uplift took place in the Middle Permian and may be attributed to crustal contraction related to the collision of the North China Craton and the Southern Mongolia terrane. The third-phase uplift happened at the end of the Permian, and may have been induced by upwelling of calc-alkali magma under an extensional setting.     

Provenance of sediments from Mesozoic basins in western Shandong: Implications for the evolution of the eastern North China Block

This paper reports LA–ICP–MS U–Pb dates and in situ Hf isotope analyses of detrital zircons from the Mesozoic basins in western Shandong, China, with the aim to constrain the depositional ages and provenances of the Mesozoic strata as well as the Mesozoic tectonic evolution of the eastern North China Block (NCB). The Mesozoic strata in western Shandong, from bottom to top, include the Fenghuangshan, Fangzi, Santai and Wennan formations. Most of the analyzed zircon grains exhibit oscillatory growth zoning and have relatively high Th/U ratios (generally 0.2–3.4), suggesting a magmatic origin. Zircons from the Fenghuangshan Formation in the Zhoucun Basin yield six main age populations (2489, 1854, 331, 305, 282, and 247 Ma). Zircons from the Fangzi Formation in the Zhoucun and Mengyin basins yield eight main age populations (2494, 1844, 927, 465, 323, 273, 223, and 159 Ma) and ten main age populations (2498, 1847, 932, 808, 540, 431, 315, 282, 227, and 175 Ma), respectively, whereas zircons from the Santai Formation in the Zhoucun and Mengyin basins yield nine main age populations (2519, 1845, 433, 325, 271, 237, 192, 161, and 146 Ma) and six main age populations (2464, 1845, 853, 277, 191, and 150 Ma), respectively. Five main age populations (2558, 1330, 609, 181, and 136 Ma) are detected for zircons from the Wennan Formation in the Pingyi Basin. Based on the youngest age, together with the contact relationships among formations, we propose that the Fenghuangshan Formation formed in the Early–Middle Triassic, the Fangzi Formation in the Middle–Late Jurassic, the Santai Formation after the Late Jurassic, and the Wennan Formation after the Early Cretaceous. These results, together with previously published data, indicate that: (1) the sediments of the Fenghuangshan Formation were sourced from the Precambrian basement and from late Paleozoic to early Mesozoic igneous rocks in the northern part of the NCB; (2) the sediments of the Fangzi and Santai formations were sourced from the Precambrian basement, late Paleozoic to early Mesozoic igneous rocks in the northern part of the NCB, and the Sulu terrane, as well as from Middle–Late Jurassic igneous rocks in the southeastern part of the NCB; and (3) the Wennan Formation was sourced from the Tongshi intrusive complex, the Sulu terrane, and minor Precambrian basement and Early Cretaceous igneous rocks. The evolution of detrital provenance indicates that in the Early–Middle Triassic, the northern part of the NCB was higher than its interior; during the Late Triassic to Early Jurassic, the eastern NCB was uplifted, resulting in a period of non-deposition; and an important transition from a compressional to an extensional tectonic regime occurred during the Middle–Late Jurassic. The presence of Neoproterozoic and Triassic detrital zircons in the Fangzi Formation sourced from the Sulu terrane suggests that large-scale sinistral strike-slip movement along the Tan-Lu Fault Zone did not occur after the Middle Jurassic (ca. 175 Ma).     

The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina

Combined geological, geochronological, geochemical and geophysical studies have led to identification of a large (∼300 km long, ∼5 km wide) N–S trending belt of metagabbros in the province of La Pampa, south-central Argentina. This belt, though only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, stands out in the geophysical data (aeromagnetics and gravity). Modeling of the aeromagnetic data permits estimation of the geometry of the belt of metagabbros and surrounding rocks.The main rock type exposed is metagabbros with relict magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected these rocks, i.e., during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros.The whole-rock Sm–Nd data suggest a juvenile source from a depleted mantle, with model ages ranging from 552 to 574 Ma, and positive Epsilon values of 6.51–6.82. A crystallization age of 480 Ma is based on geological considerations, i.e. geochronological data of the host rocks as well as comparisons with the Las Aguilas mafic–ultramafic belt of Sierra de San Luis (central Argentina).The geochemical studies indicate an enriched MORB and back-arc signature.The La Pampa metagabbros are interpreted to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc (very poorly exposed in the western part of the study area). The extensional event was ´aborted´ by the collision of the Cuyania terrane with Pampia-Gondwana in the Middle Ordovician, causing deformation and metamorphism throughout the arc–back-arc region.The similarities between the La Pampa metagabbros and the mafic–ultramafic Las Aguilas belt of the Sierra de San Luis are very conspicuous, for example, the age (Lower Paleozoic), geochemical signature and timing of metamorphism (dated at ca. 465 Ma in the study area), which allow definition of a single, mafic back-arc belt in central Argentina, from San Luis to La Pampa.     

Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid and episodic Neogene deformation in the Andes of central Perú

In the Ayacucho basin of central Perú the regional Quechua II contractional deformation is bracketed by ⁴⁰Ar/³⁹Ar isotopic age determinations to a maximum duration of about 300,000 years, and probably less than 150,000 years, centered on 8.7 Ma. The strongly deformed Huanta Formation beneath the Quechua II angular unconformity was deposited during a period of extension that began before 9.05 ± 0.05 Ma. Deposition of a thick succession of alluvial fan deposits interbedded with flows of basaltic andesite in the Tingrayoc Member continued up to about 8.76 ± 0.05 Ma with the later part of the sedimentary record reflected by lacustrine deposits of the Mayocc Member. The upper limit on contractional deformation is constrained by an age of 8.64 ± 0.05 Ma on a unit of tuff near the base of the Puchcas volcanics, which in places was deposited upon near-vertical beds of the Huanta Formation. The Ayacucho Formation was deposited, locally unconformably, upon the Puchcas volcanics beginning slightly before 7.65 ± 0.10 Ma.Extended periods of neutral to tensional stress interrupted by rapid well-developed pulses of contractional deformation demonstrate the episodic behavior of Andean orogeny in Perú. The very short duration for the Quechua II event implies that driving forces for episodic deformation may be related to coupling along the orogen boundaries and strain accumulation and release mechanisms in the continental crust instead of much longer-term variations in the configuration of converging plates.     

Detrital zircon ages and geochronological constraints on the Neoproterozoic Puga diamictites and associated BIFs in the southern Paraguay Belt,Brazil

The Paraguay belt comprises a thick sedimentary succession deposited on the southwestern border of the Amazonian Craton and the Rio Apa Block. The base of the succession in the southern Paraguay belt is marked by a level of glacially derived deposits from the Puga Formation associated with banded iron formations, which has been assumed to be end-Cryogenian in age (635 Ma) by previous authors is spite of the lack of geochronological data. Here we present the first U–Pb SHRIMP ages on detrital zircon grains separated from the matrix of six samples of these diamictites two different localities (Puga Hill and Bodoquena area). U–Pb ages determined from two samples (ca. 130 grains) of Puga Hill show a large variation between 970 Ma and 2100 Ma. Rocks with these ages can be found in the Amazonian Craton suggesting that it is the most probable source of the sediments. Detrital zircons (ca. 230 grains) from the Bodoquena area (about 200 km south of Puga Hill) range from 706 to 1990 Ma. The 1760 Ma source is significantly more important in these samples, comprising more than 70% of analyzed grains, and indicates provenance from the adjacent Rio Apa Block. The youngest zircon was dated at 706 ± 9 Ma, thus constraining the maximum depositional age for the Puga Formation. Possible sources for this younger population could be either the juvenile Mara Rosa magmatic arc in the Brasilia belt, or the rocks from the Laurentian external fold belts located to the west of the sampled area in Neoproterozoic paleogeographic reconstructions. The maximum depositional age of the diamictites (and associated BIFs), together with cap carbonate carbon and strontium isotope data (δ¹³C = ? 5.0 and ⁸⁷Sr/⁸⁶Sr = 0.7077) in Puga Hill, indicate that they were deposited after 700 Ma, suggesting that they may represent the end-Cryogenian event.     

Timing and metal sources for carbonate-hosted Zn-Pb mineralization in the Franklinian Basin (North Greenland): Constraints from Rb-Sr and Pb isotopes

Carbonate-hosted Zn-Pb (± barite, fluorite) occurrences in the Franklinian Basin of North Greenland were studied using the Rb-Sr method, applied to sphalerite, and combined Sr and Pb isotope analysis of ore and gangue minerals, to place constraints on their age and genesis. The occurrences are located in the easternmost part of the basin in Peary Land and Kronprins Christian Land, and are hosted by Upper Ordovician to lowermost Silurian dolostones of the Turesø Formation.Sphalerite samples from the Zn-Pb occurrence at Børglum, in Peary Land, reflect undisturbed Rb-Sr systems and return an isochron age of 388 ± 4 Ma (MSWD = 1.6, ⁸⁷Sr/⁸⁶Sr_i = 0.70930 ± 1). Sphalerites and their fluid inclusion fractions from an adjacent Zn-Pb occurrence, at Tvilum, exhibit various degrees of isotopic disturbance caused by secondary fluid overprint. They yield no age, however, reconstructive modelling of their Rb-Sr and Pb-Pb signatures indicates original isotope systematics consistent with the Børglum data. Likewise, due to a lack of suitable samples, no Rb-Sr age could be determined for a third occurrence in southeastern Peary Land at Løgum, however, the time-integrated Pb isotopic evolution of fluorite and calcite from this mineralization over c. 390 Ma is compatible with modelled initial signatures for Tvilum and Børglum ores. Consequently, a contemporaneous formation at c. 390 Ma can be assumed for all studied locations in Peary Land. For the Zn-Pb occurrence in Kronprins Christian Land farther east, a Rb-Sr sphalerite age of c. 360–365 Ma has been obtained through a combined isochron and paleomixing line approach.The absolute ages obtained at the studied occurrences, corresponding to Middle to Upper Devonian ages, are in no conflict with the age of the host dolostones of the Turesø Formation, which placed a maximum age limit for the mineralization. Actually, the fact that mineralization was emplaced at least 50 Ma after deposition of the host rocks, which would have been completely lithified by then, warrants the classification of the studied occurrences as epigenetic Mississippi Valley-Type, rather than diagenetic Irish-type. Furthermore, the obtained ages are contemporaneous to the Middle or Upper Devonian to the Lower Carboniferous Ellesmerian Orogeny, indicating that the mineralization likely formed from basinal brines expelled by tectonism and/or hydraulic head caused by Ellesmerian orogenic uplift, as previously suggested for the Polaris carbonate-hosted Zn-Pb deposit (Cornwallis Island, Canada).Pb isotope systematics of base metal mineralization in the Franklinian Basin point to principal metal sources located in the crystalline basement and in basement-derived clastic sediments. These two reservoirs define radiogenic and unradiogenic end components from which lead was mobilized and mixed in different proportions during discrete periods of hydrothermal activity. Distinct thorogenic Pb isotope signatures indicate that specific local sources (lower crustal basement, carbonate rocks and possibly organic-rich shales) were also involved in mineralization.     

Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakorum Fault (N Ladakh)

This paper investigates the age, P–T conditions and kinematics of Karakorum Fault (KF) zone rocks in the NW part of the Himalaya–Karakorum belt. Granulite to greenschist facies assemblages were developed within the KF zone during strike-slip shearing. The granulites were formed at high temperature (800 °C, 5.5 kbar), were subsequently retromorphosed into the amphibolite facies (700–750 °C, 4–5 kbar) and the greenschist facies (350–400 °C, 3–4 kbar). The Tangtse granite emplaced syn-kinematically at the contact between a LT and the HT granulite facies. Intrusion occurred during the juxtaposition of the two units under amphibolite conditions. Microstructures observed within the Tangtse granite exhibit a syn-magmatic dextral S–C fabric. Compiled U–Pb and Ar–Ar data show that in the central KF segment, granulite facies metamorphism occurred at a minimum age of 32 Ma, subsequent amphibolite facies metamorphism at 20–18 Ma. Further shearing under amphibolite facies (650–500 °C) was recorded at 13.6 ± 0.9 Ma, and greenschist-facies mica growth at 11 Ma. These data give further constrains to the age of initiation and depth of the Karakorum Fault. The granulite-facies conditions suggest that the KF, accommodating the lateral extrusion of Tibet, could be at least a crustal or even a Lithosphere-scale shear zone comparable to other peri-Himalayan faults.     

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.     

The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent,with a Neoproterozoic metamorphic overprint

We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons.     

Eastward termination of the Solonker–Xar Moron River Suture determined by detrital zircon U–Pb isotopic dating and Permian floristics

The Permian Solonker–Xar Moron River Suture in South Mongolia and Inner Mongolia of China represents a major tectonic boundary in Asia. The position of its eastward continuation in northeastern China has been debated for many years. In order to resolve this debate, we measured detrital zircons of the Cisuralian (Early Permian) plant fossil-bearing Hesheng Formation in the Yanbian area, Jilin Province. The detrital zircons have ages of ca. 2541–2535 Ma, 1897–1832 Ma, 458–452 Ma, and 390–280 Ma. We therefore conclude that the depositional age of the Hesheng Formation is younger than ca. 280 Ma; this is consistent with paleontologic data that indicates an Artinskian–Early Kungurian age. The presence of Neoarchean and Paleoproterozoic zircons suggests that the Hesheng Formation may have a North-China affinity; the absence of Neoproterozoic and Pan-African zircons preclude detrital sources from the Jiamusi–Mongolia Block during the Cisuralian. This, combined with the Permian floristic and stratigraphic data, provides a clue that the Solonker–Xar Moron River Suture likely extends to the Wangqing–Hunchun region, in eastern Jilin Province.     

Stratigraphy of the Koobi Fora Formation (Pliocene and Pleistocene) in the Ileret region of northern Kenya

Exposures of Pliocene and Pleistocene deposits in the Ileret region include all eight members of the Koobi Fora Formation and represent at least 533 m of section from the base of the Lonyumun Member to the top of the Chari Mb. This thickness is more than twice that reported by earlier workers (∼260 m) and compares well with the composite section of the Koobi Fora Formation (525 m). Thus the Ileret region provides a very complete and representative outcrop of the Koobi Fora Formation. As Koobi Fora Formation member boundaries are placed at the base of specified tuffs, not all member boundaries can be recognized in the Ileret region. For this reason, the stratigraphy is described in terms of the following members (thicknesses in parentheses): Lonyumun (110 m), undifferentiated Moiti and Lokochot (104 m), Tulu Bor (54 m), Burgi (55 m), KBS (74 m), Okote (45 m), and Chari (93 m). Moreover, the boundary between the Okote Member and the KBS Member in the Ileret region is placed at a prominent caliche which must closely approximate the age of the Okote Tuff in its type section.     

Ether lipids from the Lower and Middle Triassic at Qingyan,Guizhou Province,Southern China

Ether lipids such as non-isoprenoid mono and dialkyl glycerol ethers (MAGEs and DAGEs) and archaeol have been found in carbonate rocks and mud rocks deposited during the Early and Middle Triassic (250–240 Ma). The oldest previous discovery of ether lipids is from Cretaceous black shale deposited during the Albian (112 Ma). Paleoenvironmental reconstruction using ether lipids has therefore not been conducted on pre-Cretaceous sediments. Archaeol derives from archaea (e.g. methanogenic, methanotrophic, halophilic and thermophilic archaea). The non-isoprenoid DAGEs and MAGEs likely derive from sulfate-reducing bacteria (SRB), which are restricted to anaerobic environments. The presence of ether lipids thus indicates that anoxic conditions expanded in the depositional and/or water column environment in the Qingyan area during the Early–Middle Triassic. These ether lipids may be useful for the reconstruction of paleoredox conditions and paleoecosystems in the area. Furthermore, their occurrence suggests that paleoenvironmental reconstruction using ether lipids may be possible throughout the Mesozoic and Cenozoic.     

Biogenic components of the Baltic Sea sediments

The contents of biogenic components in 1511 samples of the Baltic Sea sediments (depth range 0–5 cm) are studied, and maps of their distribution are compiled. The sediments contain < 13.03% C_org, < 1.33% N, < 9.0% SiO_2am, < 5.0% CaCO₃, and < 1.45% P. The maximum and elevated contents of components are found in the mud of the sea deeps. The more fraction < 0.01 mm the sediments contain, the higher are the contents of components. Four facies types of carbonaceous mud, precursors of shales, have been recognized: (1) shallow-water (lagoon) lime sapropel, (2) carbonaceous mud of the shallow-water Gulf of Finland, (3) carbonaceous mud of the middle-depth Baltic Sea, and (4) laminated carbonaceous metal-bearing mud. The latter type of mud is strongly enriched in manganese and ore-forming trace elements, which points to its formation in the stagnant environment. In composition the Baltic Sea mud is similar to petroliferous mudstones of the Bazhenov Formation in West Siberia and to ancient black shales.     

A review of the geological characteristics and mineralization history of iron deposits in the Altay orogenic belt of the Xinjiang,Northwest China

In this review, we describe the geological characteristics and metallogenic–tectonic origin of Fe deposits in the Altay orogenic belt within the Xinjiang region of northwestern China. The Fe deposits are found mainly within three regions (ordered from northwest to southeast): the Ashele, Kelan, and Maizi basins. The principal host rocks for the Fe deposits of the Altay orogenic belt are the Early Devonian Kangbutiebao Formation, the Middle to Late Devonian Altay Formation, with minor occurrences of Lower Carboniferous and Early Paleozoic metamorphosed volcano-sedimentary rocks. The principal mineral-forming element groups of the deposits are Fe, Fe–Cu, Fe–Mn, Fe–P, Fe–Pb–Zn, Fe–Au, and Fe–V–Ti. The Fe deposits are associated with distinct formations, such as volcanic rocks, skarn deposits, pegmatites, granite-related hydrothermal vein mineralization, and mafic pluton-related V–Ti-magnetite deposits. The Fe deposits are most commonly associated with volcanic rocks in the upper Kangbutiebao Formation, in the volcano-sedimentary Kelan Basin, and in skarn deposits at several localities, including the lower Kangbutiebao Formation in the volcano-sedimentary Maizi Basin, and the Altay Formation at Jiaerbasidao–Kekebulake region. Homogenization temperatures of fluid inclusions in the prograde, retrograde and sulfide stages of the skarn type deposit are mainly medium- to high-temperature (cluster between 200 and 500 °C), medium-temperature (cluster between 200 and 340 °C) and low- to medium temperature (cluster between 160 and 300 °C), respectively. Ore fluids in the sedimentation period in the volcano-sedimentary type deposit are characterized by low- to medium temperature (with a peak around 190 °C), low to moderate salinity (3.23 to 22.71 wt.% NaCl equiv). Ore fluids in the pegmatite type deposit are characterized by low- to medium temperature (with a peak at 240 °C), low salinity (with a peak around 9 wt.% NaCl equiv). An analysis of the isotopic data for Fe deposits from the Altay orogenic belt indicates that the sulfur was derived from several sources, including volcanic rocks and granite, as well as bacterial reduction of sulfate from seawater. The present results indicate that different deposit types were derived from various sources. The REE geochemistry of rocks and ores from the Fe deposits in the Altay orogenic belt suggests that the ore-forming materials were derived from mafic volcanic rocks. Based on isotopic age data, the timing of the mineralization can be divided into four broad intervals: Early Devonian (410–384 Ma), Middle Devonian (377 Ma), Early Permian (287–274 Ma), and Early Triassic (c. 244 Ma). The ore-forming processes of the Fe deposits are closely related to volcanic activity and the emplacement of intermediate and felsic intrusions. We conclude that Fe deposits within the Altay orogenic belt developed in a range of tectonic settings, including continental arc, post-collisional extensional settings, and intracontinental settings.