The first record of Eocene tuff in a Paleogene rift basin near Nantou,Western Foothills,central Taiwan

Microfossils and a U–Pb age dating on zircon grains in the tuff beds exposed in the axial part of the Tsukeng anticline along the Pinglin River in the Western Foothills near Nantou, central Taiwan, show an occurrence of the Eocene volcanics unconformably beneath the uppermost part of the Latest Oligocene Wuchihshan Formation. This is the first discovery of the Eocene tuff exposed in the Western Foothills.The proposed Miocene “Tsukeng Formation” and “Takeng Formation” of Ho et al. (1956) named for sequences exposed in the Nantou area, Western Foothills, have to be abandoned and the standard Oligocene–Miocene lithostratigraphy used commonly in the Western Foothills of northern Taiwan is properly applicable in central Taiwan. The thick pink–brown–green colored volcanics unconformably beneath the uppermost Wuchihshan Formation is named for the first time as the Pinglin Tuff which contains Late Middle Eocene calcareous nannofossils (Zone NP16) consistent with a U–Pb age dating (38.8 ± 1 Ma) on zircon grains in the tuff. The Pinglin Tuff is overlying the Middle Eocene Chungliao Formation which contains indigenous larger foraminifera Discocyclina dispansa ex. interc. sella-dispansa and calcareous nannofossils of Zones NP14–15. The Middle Eocene Pinglin Tuff and Chungliao Formation represent the Paleogene syn-rift sequence unconformably overlain by the Latest Oligocene–Miocene post-rift sequence. This is the first document with conclusive paleontological data and age dating showing an occurrence of Paleogene marine rift basin exposed in the Western Foothills. This study also confirms similar Tertiary basin architecture between the Taiwan Strait–Pearl River Mouth Basin in the NE South China Sea and the Western Foothills onland central Taiwan.     

Metallogeny,structural evolution,post-mineral cover distribution and exploration in concealed areas of the northern Chilean Andes

Mineral exploration of prospective areas concealed by extensive post-mineralization cover is growing, being very complex and expensive. The projection of rich and giant Paleocene to early Oligocene porphyry-Cu-Mo belts in northernmost Chilean Andes (17.5–19.5°S) has major exploration potential, but only a few minor deposits have been reported to date, due to the fact that the area is largely covered by post-mineral strata. We integrate the Cenozoic stratigraphic, structural and metallogenic evolution of this sector, in order to identify the most promising regions related to lesser post-mineral cover and the projection of different metallogenic belts. The Paleocene to early Eocene metallogenic belt extends along the Precordillera, with ca. 30 km wide, and includes porphyry-Cu prospects and small Cu (±Mo-Au-Ag) vein and breccia-pipe deposits. Geochronological data indicate an age of 55.5 Ma for an intrusion related to one deposit and ages from 69.5 to 54.5 Ma for hydrothermal alteration in one porphyry-Cu prospect and largest known Cu deposits. The middle Eocene to early Oligocene porphyry belt, in the Western Cordillera farther east, is associated with 46–44 Ma intrusions. It is estimated to be 40-km wide, but is largely concealed by thick post-mineral cover. The youngest Miocene to early Pliocene metallogenic belt, also in the Western Cordillera, is well-exposed and includes Au-Ag epithermal and polymetallic veins and manto-type deposits.The Oligocene-Holocene cover consists of a succession of continental sedimentary and volcanic rocks that overall increase in thickness from 0 to 5000 m, from west to east. These strata are subhorizontal in the west and folded-faulted towards the east. Miocene gentle anticlines and monocline flexures extend along strike for 30–60 km in the Precordillera and were generated by propagation of high-angle east-dipping blind reverse faults with at least 300–900 m of Oligocene bedrock offset. The thickness of cover exceeds 2000 m in the eastern Central Depression, whereas it is generally less than 1000 m in the Precordillera along the Paleocene to early Eocene porphyry-Cu belt and it can reach locally up to 5000 m in the Western Cordillera, above the middle Eocene to early Oligocene belt.In the studied Andean segment, the Miocene to early Pliocene metallogenic belt is superimposed on the Paleocene to Oligocene belts in a 40–50 km wide zone. This overlap may be explained by an accentuated migration of the magmatic front, from east to west, since ca. 25 Ma, as a consequence of subduction slab steepening after a period of magmatic lull and flat subduction from ca. 30–35 to 25 Ma. The identified areas of lesser cover thickness are prone to exploration for concealed deposits, especially along the projection of major porphyry-Cu-Mo belts.     

Nannostratigraphy and investigation of sedimentation conditions of the lower boundary and the upper boundary of the Aitamir Formation in the east and west Kopet Dagh,northeast of Iran

By attention to the stratigraphic value of calcareous nannoplanktons for the age determination of sedimentary beds, for the first time Late Cretaceous calcareous nannofossil taxa, their distributions and relative abundances were recorded from the lower and the upper boundary of Aitamir Formation located in northeast Iran. In the present study, biostratigraphy and paleoecological conditions were reconstructed. The Aitamir Formation comprises glauconitic sandstones and olive-green shales. In this work, samples were prepared with smear slides, and nannofossils of these boundaries are listed and figured. They were photographed under a light microscope. Based on nannoplanktons and as a result of biostratigraphic studies, the age of the lower boundary of the Aitamir Formation in the east Kopet Dagh is Early/Middle Turonian, the age of the lower boundary in the west Kopet Dagh is Late Turonian/Early Coniacian, the age of the upper boundary of the Aitamir Formation in the east Kopet Dagh is Late Santonian, and the age of the upper boundary of this Formation in the west Kopet Dagh is Early Campanian. Based on paleoecological interpretation, the Aitamir Formation was deposited in a shallow marine environment, at relatively low latitude. A deepening trend of the sedimentary basin is recognized passing from Aitamir Formation to the overlying Abderaz Formation while in the lower boundary from Sanganeh to Aitamir Formation depth decreased.     

U–Pb zircon geochronology and geochemistry from NE Vietnam: A ‘tectonically disputed’ territory between the Indochina and South China blocks

Volcanoplutonic complexes in NE Vietnam have recently been interpreted as intraplate products of the Emeishan plume. Alternatively, mafic–ultramafic rocks have been considered as dismembered Palaeotethyan ophiolites juxtaposed along a tectonic mélange zone. New U–Pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the Indochina–South China blocks. Mafic–ultramafic rocks exposed within a tectonic mélange (Song Hien Tectonic Zone) include sub-alkaline pillow basalts that define two geochemically distinct ophiolitic suites (SH-1: N-MORB-like, SH-2: transitional E-MORB-like). Both suites have geochemical signatures suggestive of crustal contamination, compatible with a volcanic passive margin/rift setting. We suggest that SH-1 basalts may correlate with the Devonian–Carboniferous Jinshajiang–Ailaoshan–Song Ma branch of the Palaeotethys and form part of the associated Dian–Qiong belt, whereas SH-2 basalts are co-magmatic with Middle–Late Permian mafic–ultramafic intrusive rocks (dolerites, gabbros, peridotites) that developed in a rift basin, most likely on the margin of the down-going South China plate during west-vergent subduction beneath Indochina. During continental orogenesis and thrust stacking, these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the Song Hien Tectonic Zone. Post-collisional relaxation led to the development of a rift basin (Song Hien rift) comprising Late Permian–Triassic volcano-sedimentary strata including < 270–265 Ma terrigenous sandstones, < 252 Ma mudstones, and c. 254–248 Ma felsic effusives. Granites and granodiorites were emplaced across NE Vietnam between c. 252 and 245 Ma in a syn- to post-collisional setting. The Late Permian–Early Triassic felsic magmatic rocks best correlate with coeval rocks in SW Guangxi and the Central and Western Ailaoshan fold belts (China) and the Truong Son fold belt (Vietnam); together they signal the final to post-collisional stages of Indochina–South China collision. We demonstrate that the analysed magmatic rocks in the Lo-Gam–Song Hien domains of NE Vietnam are not genetically linked to the Emeishan Large Igneous Province in the Yangtze block of South China, as has been previously widely proposed.     

台湾始新统—中新统沉积物源与沉积环境

台湾地区出露的始新统-中新统地层属南海北侧的范畴,其物质组成及沉积环境为揭示南海新生代早期构造沉积演化提供了关键性依据。对台湾西部麓山带中部南投粗坑地区、国姓地区以及东北海岸新港-基隆地区的始新统-中新统地层进行了岩石学、矿物学、稀土元素地球化学特征以及碎屑锆石U-Pb定年分析等研究。结果显示:台湾中部和北部从始新世到中新世经历了从陆相河流-湖泊相沉积环境到滨浅海相的环境转变,其砂岩成分成熟度随时间由老变新呈现规律性变化;沉积物源分析表明研究区沉积物在始新世-早渐新世,物源以近源中生代源区为主,碎屑锆石年龄谱系出现120 Ma和230 Ma两个主要峰值,与周边及华南沿海地区中生代火山岩时代一致;进入晚渐新世以后,锆石年龄谱系出现900 Ma及1 800 Ma等古老峰值,说明古老地块物质明显增加,这可能反映了昆莺琼古河流由南海西部到东部的物质输送对台湾地区的影响作用。     

K–Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene–Oligocene thermo-tectonic reactivation

Systematic K–Ar dating and geochemical analyses of Paleogene cauldrons in the Sanin Belt of SW Japan have been made to explore the relationship between the timing of their formation and the Paleogene subduction history of SW Japan documented in the Shimanto accretionary complex. We also examine the magma sources and tectonics beneath the backarc region of SW Japan at the eastern plate boundary of Eurasia.Fifty-eight new K–Ar ages and 19 previously reported radiometric age data show that the cauldrons formed during Middle Eocene to Early Oligocene time (43–30 Ma), following a period of magmatic hiatus from 52 to 43 Ma. The hiatus coincides with absence of an accretionary prism in the Shimanto Belt. Resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench, as a common tectono-thermal event within a subduction system.The cauldrons are composed of medium-K calc-alkaline basalts to rhyolites and their plutonic equivalents. These rocks are characterized by lower concentrations of large ion lithophile elements (LILE) including K₂O, Ba, Rb, Th, U and Li, lower (La/Yb)_n ratios, lower initial Sr isotopic ratios (0.7037–0.7052) and higher ε_Nd(T) values (?0.5 to +3.5) relative to Late Cretaceous to Early Paleogene equivalents. There are clear trends from enriched to depleted signatures with decreasing age, from the Late Cretaceous to the Paleogene. The same isotopic shift is also confirmed in lower crust-derived xenoliths, and is interpreted as mobilization of pre-existing enriched lithospheric mantle by upwelling depleted asthenosphere.Relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the SW Japan arc-trench system during the Eocene to Oligocene. Newly identified Late Eocene low silica adakites and high-Mg andesites in the Sanin Belt and Early Eocene A-type granites in the SW Korea Peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate.The backarc region was an extensional tectonic setting, and some Paleogene rift basins and Sanin Belt cauldrons occur in linear arrays. The Eocene–Oligocene Sanin-SE Korea continental arc lies on the NE extension of the East China Sea Basin, the initial stage of which probably formed by continental arc rifting. This rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab, in response to decreased convergence rate between the Pacific and Eurasian plates.     

Long-lived,stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus,Armenia and Nakhitchevan

The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, ²⁰⁶Pb/²⁰⁴Pb vs. Th/Nb, Th/Nb vs. δ¹⁸O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.     

南海北部第三系的沉积特征与生油、储油层系

南海北部指北纬16°00'至23°00'与东经108°00'至120°00'之间的海域。包括广东大陆以南、海南、台湾两岛之间的广阔大陆架和陆坡区以及北部湾。东西长约1300公里,南北宽约200至400公里。面积约40万平方公里。经过地球物理勘探普查工作及几十口钻井资料,证实该区第三纪沉积广泛分布,厚逾万米。按沉积岩厚度大于1000米所圈定的范围约在30万平方公里以上,形成了北部湾、莺歌海、琼东南、珠江口及台湾西南五个沉积盆地(图1、2)。在北部湾、琼东南、珠江口及台湾西南盆地中,分别在下第三系流沙港组、陵水组、珠江组，上第三系角尾组、韩江组发现了储油气层(表1)。     

Clockwise rotation of the Santa Marta massif and simultaneous Paleogene to Neogene deformation of the Plato-San Jorge and Cesar-Ranchería basins

A moderate amount of vertical-axis clockwise rotation of the Santa Marta massif (30°) explains as much as 115 km of extension (stretching of 1.75) along its trailing edge (Plato-San Jorge basin) and up to 56 km of simultaneous shortening with an angular shear of 0.57 along its leading edge (Perijá range). Extensional deformation is recorded in the 260 km-wide, fan-shaped Plato-San Jorge basin by a 2–8 km thick, shallowing-upward and almost entirely fine-grained, upper Eocene and younger sedimentary sequence. The simultaneous initiation of shortening in the Cesar-Ranchería basin is documented by Mesozoic strata placed on to lower Eocene syntectonic strata (Tabaco Formation and equivalents) along the northwest-verging, shallow dipping (9–12° to the southeast) and discrete Cerrejón thrust. First-order subsidence analysis in the Plato-San Jorge basin is consistent with crustal stretching values between 1.5 and 2, also predicted by the rigid-body rotation of the Santa Marta massif. The model predicts about 100 km of right-lateral displacement along the Oca fault and 45 km of left-lateral displacement along the Santa Marta–Bucaramanga fault. Clockwise rotation of a rigid Santa Marta massif, and simultaneous Paleogene opening of the Plato-San Jorge basin and emplacement of the Cerrejón thrust sheet would have resulted in the fragmentation of the Cordillera Central–Santa Marta massif province. New U/Pb ages (241 ± 3 Ma) on granitoid rocks from industry boreholes in the Plato-San Jorge basin confirm the presence of fragments of a now segmented, Late Permian to Early Triassic age, two-mica, granitic province that once spanned the Santa Marta massif to the northernmost Cordillera Central.     

U–Pb detrital zircon geochronology and its implications: The early Late Triassic Yanchang Formation,south Ordos Basin,China

U–Pb detrital zircon geochronology has been used to identify provenance and document sediment delivery systems during the deposition of the early Late Triassic Yanchang Formation in the south Ordos Basin. Two outcrop samples of the Yanchang Formation were collected from the southern and southwestern basin margin respectively. U–Pb detrital zircon geochronology of 158 single grains (out of 258 analyzed grains) shows that there are six distinct age populations, 250–300 Ma, 320–380 Ma, 380–420 Ma, 420–500 Ma, 1.7–2.1 Ga, and 2.3–2.6 Ga. The majority of grains with the two oldest age populations are interpreted as recycled from previous sediments. Multiple sources match the Paleozoic age populations of 380–420 and 420–500 Ma, including the Qilian–Qaidam terranes and the North Qilian orogenic belt to the west, and the Qinling orogenic belt to the south. However, the fact that both samples do not have the Neoproterozoic age populations, which are ubiquitous in these above source areas, suggests that the Late Triassic Yanchang Formation in the south Ordos Basin was not derived from the Qilian–Qaidam terranes, the North Qilian orogenic belt, and the Qinling orogenic belt. Very similar age distribution between the Proterozoic to Paleozoic sedimentary rocks and the early Late Triassic Yanchang Formation in the south Ordos Basin suggests that it was most likely recycled from previous sedimentary rocks from the North China block instead of sediments directly from two basin marginal deformation belts.     

A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes

A Cenozoic tectonic reconstruction is presented for the Southwest Pacific region located east of Australia. The reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for Pacific–Australia and Lord Howe Rise–Pacific relative plate motion. The reconstruction is based on a conceptual tectonic model in which the large-scale structures of the region are manifestations of slab rollback and backarc extension processes. The current paradigm proclaims that the southwestern Pacific plate boundary was a west-dipping subduction boundary only since the Middle Eocene. The new reconstruction provides kinematic evidence that this configuration was already established in the Late Cretaceous and Early Paleogene. From ∼ 82 to ∼ 52 Ma, subduction was primarily accomplished by east and northeast-directed rollback of the Pacific slab, accommodating opening of the New Caledonia, South Loyalty, Coral Sea and Pocklington backarc basins and partly accommodating spreading in the Tasman Sea. The total amount of east-directed rollback of the Pacific slab that took place from ∼ 82 Ma to ∼ 52 Ma is estimated to be at least 1200 km. A large percentage of this rollback accommodated opening of the South Loyalty Basin, a north–south trending backarc basin. It is estimated from kinematic and geological constraints that the east–west width of the basin was at least ∼ 750 km. The South Loyalty and Pocklington backarc basins were subducted in the Eocene to earliest Miocene along the newly formed New Caledonia and Pocklington subduction zones. This culminated in southwestward and southward obduction of ophiolites in New Caledonia, Northland and New Guinea in the latest Eocene to earliest Miocene. It is suggested that the formation of these new subduction zones was triggered by a change in Pacific–Australia relative motion at ∼ 50 Ma. Two additional phases of eastward rollback of the Pacific slab followed, one during opening of the South Fiji Basin and Norfolk Basin in the Oligocene to Early Miocene (up to ∼ 650 km of rollback), and one during opening of the Lau Basin in the latest Miocene to Present (up to ∼ 400 km of rollback). Two new subduction zones formed in the Miocene, the south-dipping Trobriand subduction zone along which the Solomon Sea backarc Basin subducted and the north-dipping New Britain–San Cristobal–New Hebrides subduction zone, along which the Solomon Sea backarc Basin subducted in the west and the North Loyalty–South Fiji backarc Basin and remnants of the South Loyalty–Santa Cruz backarc Basin subducted in the east. Clockwise rollback of the New Hebrides section resulted in formation of the North Fiji Basin. The reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere, a change in plate kinematics or slab–mantle interaction.     

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.     

Three-dimensional seismic attenuation structure beneath the Taiwan region and its tectonic implication

Three-dimensional (3-D) attenuation structures were determined for Taiwan region by inversion of earthquake intensity data set. The seismic intensity which is assumed to be a measure of the maximum acceleration of the S-wave at the seismic station is used to estimate the attenuation structure. The intensity data set consists of about 5500 intensity readings for 1410 earthquakes reported by the Central Weather Bureau of Taiwan from 1987 to 2010. The 3-D attenuation maps consisting of five layers were constructed to a depth of 74 km.The obtained model reveals the following features: first, a high absorption zone exists in the westernmost Okinawa trough. These high-attenuation anomalies seem to extend from upper crust to mantle beneath the Yilan Plain and Kueishantao Island. We interpret the high absorption zone as the presence of the relatively hot lower crust and uppermost mantle in response to the local opening of the Okinawa trough. Second, strong lateral variation of seismic attenuation is observed from the source region of the Chi–Chi earthquake in the central Taiwan. Most aftershocks were occurred beneath the Western Foothills which is characterized by a transition from high attenuation to low attenuation between the Chelungpu fault and Chuchih fault. However, the region east to the Chuchih fault is relatively quiescent. This study suggests that the high Vp/Vs and lateral variation of attenuation features existing beneath the Western Foothills could be associated with porous materials containing fluid which might affect the generation of the Chi–Chi earthquake.     

Detrital zircon analysis from the Neoproterozoic–Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo,Argentina: Evidence of a rift and passive margin system?

Metamorphic basement and its Neoproterozoic to Cambrian cover exposed in the Sierra de Pie de Palo, a basement block of the Sierras Pampeanas in Argentina, lie within the Cuyania terrane. Detrital zircon analysis of the cover sequence which includes, in ascending order, the El Quemado, La Paz, El Desecho, and Angacos Formations of the Caucete Group indicate a Laurentian origin for the Cuyania terrane. The lower section represented by the El Quemado and La Paz Formations is interpreted as having an igneous source related to a rift setting similar to that envisioned for the southern and eastern margins of Laurentia at approximately 550 Ma. The younger strata of the El Desecho Formation are correlative with the Cerro Totora Formation of the Precordillera, and both are products of rift sedimentation. Finally, the Angacos Formation and the correlative La Laja Formation of the Precordillera were deposited on the passive margin developed on the Cuyania terrane. The maximum depositional ages for the Caucete Group include ca. 550 Ma for the El Quemado Formation and ca. 531 Ma for the El Desecho Formation. Four different sediment sources areas were interpreted in the provenance analysis. The main source is crystalline basement dominated by early Mesoproterozoic igneous rocks related to the Granite-Rhyolite province of central and eastern Laurentia. Possible source areas for 1600 Ma metamorphic detrital zircons of the Caucete Group include the Yavapai-Mazatzal province (ca. 1800–1600 Ma) of south-central to southwestern Laurentia. Younger Mesoproterozoic zircon is likely derived from Grenville-age medium- to high-grade metamorphic rocks and subordinate igneous rocks that form the basement of Cuyania as well as the southern Grenville province of Laurentia itself. Finally, Neoproterozoic igneous zircon in the Caucete Group records different magmatic pulses along the southern Laurentian margin during opening of Iapetus and break-up of Rodinia. Northwestern Cuyania terrane includes a small basement component derived from the Granite-Rhyolite province of Laurentia, which was the source for detrital zircons found in the middle Cambrian passive margin sediments of Cuyania.     

Base and precious metal mineralization in Middle Jurassic rocks of the Lesser Caucasus: A review of geology and metallogeny and new data from the Kapan,Alaverdi and Mehmana districts

The polymetallic Cu–Au–Ag–Zn ± Pb, Cu–Au and Cu deposits in the Kapan, Alaverdi and Mehmana mining districts of Armenia and the Nagorno–Karabakh region form part of the Tethyan belt. They are hosted by Middle Jurassic rocks of the Lesser Caucasus paleo-island arc, which can be divided into the Kapan Zone and the Somkheto–Karabakh Island Arc. Mineralization in Middle Jurassic rocks of this paleo-island arc domain formed during the first of three recognized Mesozoic to Cenozoic metallogenic epochs. The Middle Jurassic to Early Cretaceous metallogenic epoch comprises porphyry Cu, skarn and epithermal deposits related to Late Jurassic and Early Cretaceous intrusions. The second and third metallogenic epochs of the Lesser Caucasus are represented by Late Cretaceous volcanogenic massive sulfide (VMS) deposits with transitional features towards epithermal mineralization and by Eocene to Miocene world-class porphyry Mo–Cu and epithermal precious metal deposits, respectively.The ore deposits in the Kapan, Alaverdi and Mehmana mining districts are poorly understood and previous researchers named them as copper–pyrite, Cu–Au or polymetallic deposits. Different genetic origins were proposed for their formation, including VMS and porphyry-related scenarios. The ore deposits in the Kapan, Alaverdi and Mehmana mining districts are characterized by diverse mineralization styles, which include polymetallic veins, massive stratiform replacement ore bodies at lithological contacts, and stockwork style mineralization. Sericitic, argillic and advanced argillic alteration assemblages are widespread in the deposits which have intermediate to high-sulfidation state mineral parageneses that consist of tennantite–tetrahedrite plus chalcopyrite and enargite–luzonite–colusite, respectively. The ore deposits are spatially associated with differentiated calc-alkaline intrusions and pebble dykes are widespread. Published δ³⁴S values for sulfides and sulfates are in agreement with a magmatic source for the bulk sulfur whereas published δ³⁴S values of sulfate minerals partly overlap with the isotopic composition of contemporaneous seawater. Published mineralization ages demonstrate discrete ore forming pulses from Middle Jurassic to the Late Jurassic–Early Cretaceous boundary, indicating time gaps of 5 to 20 m.y. in between the partly subaqueous deposition of the host rocks and the epigenetic mineralization.Most of the described characteristics indicate an intrusion-related origin for the ore deposits in Middle Jurassic rocks of the Lesser Caucasus, whereas a hybrid VMS–epithermal–porphyry scenario might apply for deposits with both VMS- and intrusion-related features.The volcanic Middle Jurassic host rocks for mineralization and Middle to Late Jurassic intrusive rocks from the Somkheto–Karabakh Island Arc and the Kapan Zone show typical subduction-related calc-alkaline signature. They are enriched in LILE such as K, Rb and Ba and show negative anomalies in HFSE such as Nb and Ta. The ubiquitous presence of amphibole in Middle Jurassic volcanic rocks reflects magmas with high water contents. Flat REE patterns ([La/Yb]_N = 0.89–1.23) indicate a depleted mantle source, and concave-upward (listric-shaped) MREE–HREE patterns ([Dy/Yb]_N = 0.75–1.21) suggest melting from a shallow mantle reservoir. Similar trace element patterns of Middle Jurassic rocks from the Somkheto–Karabakh Island Arc and the Kapan Zone indicate that these two tectonic units form part of one discontinuous segmented arc. Similar petrogenetic and ore-forming processes operated along its axis and Middle Jurassic volcanic and volcanosedimentary rocks constitute the preferential host for polymetallic Cu–Au–Ag–Zn ± Pb, Cu–Au and Cu mineralization, both in the Somkheto–Karabakh Island Arc and the Kapan Zone.     

Geochemical and isotopic approach to maturity/source/mixing estimations for natural gas and associated condensates in the Thrace Basin,NW Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH₄, thermogenic C₂₊ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH₄ is bacteriogenic (Calculated δ¹³C_C1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH₄. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ¹³C_C1–C = −35.80‰; Hamitabat Field) and Group-3 (δ¹³C_1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C₂₊ gases. The Group-2 C₂₊ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ¹³C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C₂₊ gases, produced from one field, were generated from isotopically light marine kerogen (δ¹³C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C₂₊ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ¹³C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R_eq = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.     

Evolution of K-rich magmas derived from a net veined lithospheric mantle in an ongoing extensional setting: Geochronology and geochemistry of Eocene and Miocene volcanic rocks from Eastern Pontides (Turkey)

The Eocene and Miocene volcanic rocks between the cities of Trabzon and Giresun in the Eastern Pontides (NE Turkey) erupted as mildly and moderately alkaline magmas ranging from silica-saturated to silica-undersaturated types. ⁴⁰Ar-³⁹Ar dating and petrochemical data reveal that the studied volcanic rocks are discriminated in two: Lutetian (Middle Eocene) mildly alkaline, (basaltic rocks: 45.31 ± 0.18 to 43.86 ± 0.19 Ma; trachytic rocks: 44.87 ± 0.22 to 41.32 ± 0.12 Ma), and Messinian (Late Miocene) moderately alkaline volcanic rocks (tephrytic rocks: 6.05 ± 0.06 and 5.65 ± 0.06 Ma). The trace and the rare earth element systematic, characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios in the Eocene basaltic and trachytic rocks, high LREE/HREE ratios in the Miocene tephrytic rocks, and different degrees of depletion in Nb, Ta, Ti coupled with high Th/Yb ratios, show that the parental magmas of the volcanic rocks were derived from mantle sources previously enriched by slab-derived fluids and subducted sediments. The Sr, Nd and Pb isotopic composition of the Eocene and Miocene volcanic rocks support the presence of subduction-modified subcontinental lithospheric mantle. During the magma ascent in the crust, parental magmas of both the Eocene and Miocene volcanic rocks were mostly affected by fractional crystallisation rather than assimilation coupled with fractional crystallisation and mixing. The silica-undersaturated character of the Miocene tephrytic rocks could be attributed to assimilation of carbonate rocks within shallow-level magma chambers. The parental magmas of the Eocene volcanic rocks resulted from a relatively high melting degree of a net veined mantle and surrounding peridotites in the spinel stability field due to an increase in temperature, resulting from asthenospheric upwelling related to the extension of lithosphere subsequent to delamination. The parental magmas for the Miocene volcanic rocks resulted from a relatively low melting degree of a net veined mantle domain previously modified by metasomatic melts derived from a garnet peridotite source after decompression due to extensional tectonics, combined with strike-slip movement at a regional scale related to ongoing delamination.     

塔里木盆地新生代海相沉积问题

基于前人文献,对塔里木盆地新生代海相沉积问题进行梳理,进而探讨该盆地新生代海侵的次数和范围以及海退的时限、原因。研究表明,新生代,塔里木盆地至少经历古近纪的阿尔塔什晚期至齐姆根早期(古新世早期至古新世晚期)、卡拉塔尔期—乌拉根期(始新世中期)、巴什布拉克中期(始新世晚期至早渐新世)等三期海侵;塔里木盆地中新世仍有海相地层这一认识获得广泛认可仍需更多的地质证据来支持。塔里木盆地海侵范围在卡拉塔尔—乌拉根组沉积时期达到最大,向东可达玛扎塔格地区,在盆地北缘和南缘分别可以到达库尔勒以东地区和洛浦县阿其克以东地区。由于受到全球海平面变化和构造运动的共同影响,副特提斯海新生代从塔里木盆地退却的沉积记录包括齐姆根组顶部、乌拉根组顶部、巴什布拉克组第四段和第五段,时间上分别对应于古新世晚期、始新世中晚期和早渐新世。     

A Late-Eocene palynological record from the Hoh Xil Basin,northern Tibetan Plateau,and its implications for stratigraphic age,paleoclimate and paleoelevation

The Hoh Xil Basin, lying in the central Tibetan Plateau, is key to understanding the Cenozoic tectonics, paleoelevation and paleoclimate changes that have occurred in the Tibetan Plateau since the collision of the Indian and Asian tectonic plates. However, the stratigraphic age and paleoelevation indicated by the sediments of the Hoh Xil Basin remain hotly debated. Here we report on one palynological record from the TTH-C section, extracted from the Yaxicuo Group (the stratigraphic unit between the Fenghuoshan and Wudaoliang groups), and analyze its implications for stratigraphic age, paleoclimate and paleoelevation in the Hoh Xil Basin. The record shows that palynological taxa are mainly dominated by xerophytic Ephedripites, Nitrariadites (Nitrariapollis) and Chenopodipollis, with few ferns and conifers. Rich morphologies correspond well with those in the Xia Ganchaigou Formation (Fm) of the Qaidam Basin to the north. Palynological percentages are well correlated with the middle member of the Xia Ganchaigou Fm in the Qaidam Basin as well as the lower member of the Mahalagou Fm in the Xining Basin to the northeast. The ages of the middle member of the Xia Ganchaigou and lower member of the Mahalagou Fms from these two basins are both identical to the Bartonian Stage (~ 40–37 Ma) of the Late Eocene, according to their respective high-resolution magnetostratigraphic dating. This means that the age of the Yaxicuo Group at least covers the Bartonian Stage. Besides the Qaidam and Xining basins, the palynological assemblages of the TTH-C section are also similar to those of three other sites (the Jiuquan, Tu-ha and Hetao basins), indicating similarly arid climates dominated by a northwestern Chinese subtropical high, and a relatively low paleoelevation in the Hoh Xil Basin (mostly < 2000 m a.s.l.) in the Late Eocene.     

A 60-million-year Cenozoic history of western Amazonian ecosystems in Contamana,eastern Peru

We provide a synopsis of ~ 60 million years of life history in Neotropical lowlands, based on a comprehensive survey of the Cenozoic deposits along the Quebrada Cachiyacu near Contamana in Peruvian Amazonia. The 34 fossil-bearing localities identified have yielded a diversity of fossil remains, including vertebrates, mollusks, arthropods, plant fossils, and microorganisms, ranging from the early Paleocene to the late Miocene–?Pliocene (> 20 successive levels). This Cenozoic series includes the base of the Huchpayacu Formation (Fm.; early Paleocene; lacustrine/fluvial environments; charophyte-dominated assemblage), the Pozo Fm. (middle + ?late Eocene; marine then freshwater environments; most diversified biomes), and complete sections for the Chambira Fm. (late Oligocene–late early Miocene; freshwater environments; vertebrate-dominated faunas), the Pebas Fm. (late early to early late Miocene; freshwater environments with an increasing marine influence; excellent fossil record), and Ipururo Fm. (late Miocene–?Pliocene; fully fluvial environments; virtually no fossils preserved). At least 485 fossil species are recognized in the Contamana area (~ 250 ‘plants’, ~ 212 animals, and 23 foraminifera). Based on taxonomic lists from each stratigraphic interval, high-level taxonomic diversity remained fairly constant throughout the middle Eocene–Miocene interval (8-12 classes), ordinal diversity fluctuated to a greater degree, and family/species diversity generally declined, with a drastic drop in the early Miocene. The Paleocene–?Pliocene fossil assemblages from Contamana attest at least to four biogeographic histories inherited from (i) Mesozoic Gondwanan times, (ii) the Panamerican realm prior to (iii) the time of South America’s Cenozoic “splendid isolation”, and (iv) Neotropical ecosystems in the Americas. No direct evidence of any North American terrestrial immigrant has yet been recognized in the Miocene record at Contamana.