Late Hercynian Research Articles

Hydrocarbon charge history of the Silurian bituminous sandstone reservoirs in the Tazhong uplift, Tarim Basin, China

The Tarim Basin is the largest inland basin in China. The hydrocarbon charge history of the Silurian bituminous sandstone reservoirs in the Tazhong uplift was investigated using an integrated approach, combining detailed petrographic analysis (thin section, scanning electron microscopy, and transmission electron microscopy), clay mineral x-ray diffraction characterization, and modeling of authigenic clay minerals and subsequent potassium-argon (K-Ar) dating of authigenic illite. The timing of the hydrocarbon charges of the reservoirs in the Tazhong uplift area is compared with that of the Kongquehe area within the basin. The authigenic illite ages in the Tazhong uplift area and the Kongquehe area range from 383 Ma to 204 Ma, indicating a unique age distribution pattern in the basin. The Silurian oil accumulations were primarily formed in the late Caledonian through the late Hercynian. The reservoirs within wells Qiao-1, Kongque-1, Longkou-1, and Yingnan-2, located in the western area and the eastern area of the basin, were charged around 383 to 271 Ma in the late Caledonian to the early Hercynian. The reservoirs in wells Tazhong-37, Tazhong-67, Tazhong-12, and Tazhong-32, located in the center of the basin, were charged at around 235 to 204 Ma in the late Hercynian. A correlation between the authigenic illite ages and the thickness of the Silurian bituminous sandstones suggests that the paleostructural framework is a key controlling factor. The paleo-oil pool located in and around the sedimentary centers, such as wells Qiao-1 and Kongque-1, was formed relatively early (383 Ma). The K-Ar dating results were consistent with the results of conventional hydrocarbon accumulation history analysis and highlight the differences in the accumulation timing between various oil pools in different parts of the basin.

Pyrrhotite Re–Os and SHRIMP zircon U–Pb dating of the Hongqiling Ni–Cu sulfide deposits in Northeast China

The Hongqiling (HQL) magmatic Ni–Cu sulfide deposits (Jilin Province, NE China) are located at the southern margin of the eastern Xing'an–Mongolian Orogenic Belt (XMOB) of the eastern Central Asian orogenic belt (CAOB), situated between the Siberian Craton (SC) and the North China Craton (NCC). The HQL ore-bearing mafic–ultramafic intrusions intrude into the metamorphic rocks of the lowermost Huangyingtun Formation of the Hulan Group (HLG), whose lithology is herein identified as a hornblende–zoisite gneiss. SHRIMP zircon U–Pb dating for the HLG, country rock of the HQL deposits, indicated a maximum deposition age of 272.2 ± 4.3 Ma (95% confidence level, MSWD = 2.6, n = 14). Six pyrrhotite samples separated from massive Ni–Cu sulfide ores of the Fujia (No. 7) deposit yielded a Re–Os isotopic isochron age of 208 ± 21 Ma (95% confidence level, MSWD = 2.4, n = 6), indicating that the ore-formation age was Late Triassic. Re–Os isotope analyses showed an initial 187Os/ 188Os ratio of 0.315 ± 0.050. The γ Os values ranged from + 137 to + 161 with an average of + 151, indicating that its ore-forming materials were mainly derived from mantle with possibly < 30% crustal Os contamination. Large scale magmatic Ni–Cu mineralization in eastern Jilin occurred in post-collisional tectonic setting in the Late Triassic. Our new results suggest that the ages of the Ni–Cu sulfide deposits in the CAOB within China tended to become younger from west to east, as manifested by the Late Caledonian (~ 440 Ma), through the Late Hercynian (300–265 Ma) to the Late Indosinian (225–200 Ma). Such variation could reflect the gradual closure and post-collisional orogeny between the SC and the NCC from west to east.

Review on the monograph Historical Minerageny volumes 1–3, 2005–2008 by N. N. Zinchuk, A. D. Savko, and L. T. Shevyrev; Voronezh State University

A major review of the deposits of mineral resources of all ages and all types on the Earth’s surface has recently been published. Data on the main types of metallic and nonmetallic deposits of the world from the Archean to the Cenozoic are systematized. The huge body of data is of considerable importance both for the comprehensive analysis of mineragenic pro� cesses and for revealing geodynamic trends in the crust and mantle, as well. The material is presented in three volumes. Volume 1 is concerned with a global review of the data on the spatiotemporal distribution of mineral resources; Vol� ume 2 is devoted to the mineral resources of ancient platforms; and Volume 3 comprises a description of deposits in mobile belts (fold regions). The descrip� tions of objects (deposits and their groups) are system� atic, complex, and comprehensive. A convenient hier� archic system is elaborated for easy orientation among the multitude of different objects. The succession of “mineragenic process” (forma� tion of metallic and nonmetallic mineral resources) is considered in two dimensions. The first coordinate— type of geodynamic settings—is expressed in the data arrangement by volumes: the first volume is devoted to global trends in the distribution of deposits; the second volume deals with deposits on platforms; and the third volume concerns deposits in orogenic belts. The sec� ond coordinate is distribution in time according to the basic orogenic epochs: Early Precambrian, Riphean (Late Precambrian), Caledonian (Early Paleozoic), Early Hercynian (Middle Devonian–Early Carbonifer� ous), Late Hercynian (Middle Carboniferous–Middle Triassic), Kimmerian (Late Triassic–Jurassic), Early Alpine (Cretaceous), and Late Alpine (Cenozoic). Voluminous factual material is generalized in the

Hydrocarbon Accumulation Conditions of Ordovician Carbonate in Tarim Basin

Abstract:Based on comprehensive analysis of reservoir‐forming conditions, the diversity of reservoir and the difference of multistage hydrocarbon charge are the key factors for the carbonate hydrocarbon accumulation of the Ordovician in the Tarim Basin. Undergone four major deposition‐tectonic cycles, the Ordovician carbonate formed a stable structural framework with huge uplifts, in which are developed reservoirs of the reef‐bank type and unconformity type, and resulted in multistage hydrocarbon charge and accumulation during the Caledonian, Late Hercynian and Late Himalayan. With low matrix porosity and permeability of the Ordovician carbonate, the secondary solution pores and caverns serve as the main reservoir space. The polyphase tectonic movements formed unconformity reservoirs widely distributed around the paleo‐uplifts; and the reef‐bank reservoir is controlled by two kinds of sedimentary facies belts, namely the steep slope and gentle slope. The unconventional carbonate pool is characterized by extensive distribution, no obvious edge water or bottom water, complicated oil/gas/water relations and severe heterogeneity controlled by reservoirs. The low porosity and low permeability reservoir together with multi‐period hydrocarbon accumulation resulted in the difference and complex of the distribution and production of oil/gas/water. The distribution of hydrocarbon is controlled by the temporal‐spatial relation between revolution of source rocks and paleo‐uplifts. The heterogenetic carbonate reservoir and late‐stage gas charge are the main factors making the oil/ gas phase complicated. The slope areas of the paleo‐uplifts formed in the Paleozoic are the main carbonate exploration directions based on comprehensive evaluation. The Ordovician of the northern slope of the Tazhong uplift, Lunnan and its periphery areas are practical exploration fields. The Yengimahalla‐Hanikatam and Markit slopes are the important replacement targets for carbonate exploration. Gucheng, Tadong, the deep layers of Cambrian dolomite in the Lunnan and Tazhong‐Bachu areas are favorable directions for research and risk exploration.

Lead Isotopic Composition and Lead Source of the Huogeqi Cu-Pb-Zn Deposit, Inner Mongolia, China

The Huogeqi orefield located on the northern side of Mt. Langshan, Inner Mongolia occurs in the Middle Proterozoic Langshan Group metamorphic rocks, and the orebodies are stratiform. In the past twenty years, many Chinese geologists have conducted researches on the Huogeqi Cu-Pb-Zn deposit, but there has been still a controversy on its origin. Some advocate that the deposit is of sedimentary-metamorphic reworking origin, some hold that it is of sea-floor SEDEX origin, and others have a preference for magmatic superimposition origin. The crux of the controversy is that there is no common understanding about the source of ore-forming materials. In this paper, the Pb isotopic compositions of regional Achaean-Early Proterozoic basement rocks, various types of sedimentary-metamorphic rocks and volcanic rocks in the mining district, Late Proterozoic and Hercynian magmatic rocks are introduced and compared with the ore-lead composition, so as to constrain the source of the ore lead. The result indicates that (1) sulfides in the ores have homogeneous Pb isotopic compositions, showing a narrow variation range. Their 206Pb/204Pb ratios are within a range of 17.027–17.317; 207Pb/204Pb ratios, 15.451–15.786 and 208Pb/204Pb ratios, 36.747–37.669; (2) the Pb isotopic compositions of the regional Achaean-Early Proterozoic basement rocks are characteristic of the old Pb isotopic composition at the early-stage evolution of the Earth, which varies over a wider range, reflecting significant differences in Pb isotopic compositions of the ores. All this indicates that the source of ore lead has no bearing on the basement rocks; (3) the sedimentary-metamorphic rocks in the mining district are characterized by highly variable and more radiogenic Pb isotopic compositions and their Pb isotopic ratios are obviously higher than those of ores, demonstrating that ore lead did not result from metamorphic reworking of these rocks; (4) Pb isotopic compositions of Late Proterozoic diorite-gabbro and Hercynian granite are higher than those of ores. Meanwhile, the Pb isotopic compositions of sulfides in the small-sized strata-penetrating mineralized veinlets formed at later stages are completely consistent with that of sulfides in stratiform-banded ores, suggesting that these veinlets are the product of autochthonous reworking of the stratiform-banded ores during the period of metamorphism and the late magmatic superimposition-mineralization can be excluded; (5) amphibolite, whose protolith is basic volcanic rocks, has the same Pb isotopic compositions as ores, implying that ore lead was derived probably from basic volcanism. So, the source of ore-forming materials for the Huogeqi deposit is like that of the volcanic massive sulfide (VMS) deposits. However, the orebodies do not occur directly within the volcanic rocks, and instead they overlie the volcanic rocks, showing some differences from those typical VMS-type deposits.

Basement faults and volcanic rock distributions in the Ordos Basin

Volcanic rocks in the Ordos Basin are of mainly two types: one in the basin and the other along the margin of the basin. Besides those along the margin, the marginal volcanic rocks also include the volcanic rocks in the Yinshanian orogenic belt north of the basin. Based on the latest collection of gravitational and aeromagnetic data, here we interpret basement faults in the Ordos Basin and its peripheral region, compare the faults derived from aeromagnetic data with those from seismic data, and identify the geological ages of the fault development. Two aeromagnetic anomaly zones exist in the NE-trending faults of the southern basin, and they are in the volcanic basement formed in pre-Paleozoic. These NE-trending faults are the channel of volcanic material upwelling in the early age (Archean-Neoproterozoic), where igneous rocks and sedimentary rocks stack successively on both sides of the continental nucleus. In the Cambrian, the basin interior is relatively stable, but in the Late Paleozoic and Mesozoic, the basin margin underwent a number of volcanic activities, accompanied by the formation of nearly north-south and east-west basement faults in the basin periphery and resulting in accumulation of great amount of volcanic materials. Volcanic tuff from the basin periphery is discovered in the central basin and volcanic materials are exposed in the margins of the basin. According to the source-reservoir-cap rock configuration, the basin peripheral igneous traps formed in the Indosinian-Early Yanshanian and Late Hercynian are favorable exploration objectives, and the volcanic rocks in the central basin are the future target of exploration.

Main progress and problems in research on Ordovician hydrocarbon accumulation in the Tarim Basin

The Tarim Basin is the largest petroliferous basin in the northwest of China, and is composed of a Paleozoic marine craton basin and a Meso-Cenozoic continental foreland basin. It is of great significance in exploration of Ordovician. In over 50 years of exploration, oil and gas totaling over 1.6 billion tonnes oil-equivalent has been discovered in the Ordovician carbonate formation. The accumulation mechanisms and distribution rules are quite complicated because of the burial depth more than 3,500 m, multi-source, and multi-stage accumulation, adjustment, reconstruction and re-enrichment in Ordovician. In this paper, we summarized four major advances in the hydrocarbon accumulation mechanisms of Ordovician carbonate reservoirs. First, oil came from Cambrian and Ordovician source rocks separately and as a mixture, while natural gas was mainly cracked gas generated from the Cambrian-Lower Ordovician crude oil. Second, most hydrocarbon migrated along unconformities and faults, with different directions in different regions. Third, hydrocarbon migration and accumulation had four periods: Caledonian, early Hercynian, late Hercynian and Himalayan, and the latter two were the most important for oil and gas exploration. Fourth, hydrocarbon accumulation and evolution can be generally divided into four stages: Caledonian (the period of hydrocarbon accumulation), early Hercynian (the period of destruction), late Hercynian (the period of hydrocarbon reconstruction and re-accumulation), and Himalayan (the period of hydrocarbon adjustment and re-accumulation). Source rocks (S), combinations of reservoir-seal (C), paleo-uplifts (M), structure balance belt (B) matched in the same time (T) control the hydrocarbon accumulation and distribution in the Ordovician formations. Reservoir adjustment and reconstruction can be classified into two modes of physical adjustment and variation of chemical compositions and five mechanisms. These mechanisms are occurrence displacement, biodegradation, multi-source mixing, high-temperature cracking and late gas invasion. Late hydrocarbon accumulation effects controlled the distribution of current hydrocarbon. The T-BCMS model is a basic geological model to help understanding the control of reservoirs. At present, the main problems of hydrocarbon accumulation focus on two aspects, dynamic mechanisms of hydrocarbon accumulation and the quantitative models of oil-bearing in traps, which need further systemic research.

Vetupraeca n. gen. and Mucopraeca n. gen. (Cryptodonta, Bivalvia): A reappraisal of Late Devonian bivalves from the Hercynian Facies

Bivalves from the Late Devonian pelagic or Hercynian Facies of classical regions such as eastern North America and Europe have not been investigated for almost a century.A group of small, radially ribbed bivalves frequently occurs in association with ammonoids and conodonts in pelagic cephalopod limestones and shales of the latest Frasnian and early Famennian. These bivalves have traditionally been assigned to the Late Silurian genus Praecardium Barrande, 1881. Re-studying the types of the Late Devonian taxa, Cardium? vetustum Hall, 1843, Cardiola duplicata Münster, 1840, Praecardium clymeniae Beushausen, 1895, Praecardium melletes Clarke, 1904, and Praecardium multicostatum Clarke, 1904 shows that they differ significantly from Praecardium.As a result, two new genera, Vetupraeca n. gen. and Mucopraeca n. gen, are established. Furthermore, neotypes are designated for Cardiola nehdensis, Kayser, 1873 and Vetupraeca clymeniae (Beushausen, 1895), and lectotypes are chosen for Mucopraeca multicostata (Clarke, 1904) and Vetupraeca duplicata (Münster, 1840). These bivalve taxa were widely distributed in the subtropical to tropical, latest Frasnian/early Famennian outer shelf habitats of Laurussia and Gondwana.

Vetupraeca n. gen. and Mucopraeca n. gen. (Cryptodonta, Bivalvia): A reappraisal of Late Devonian bivalves from the Hercynian Facies

Bivalves from the Late Devonian pelagic or Hercynian Facies of classical regions such as eastern North America and Europe have not been investigated for almost a century. A group of small, radially ribbed bivalves frequently occurs in association with ammonoids and conodonts in pelagic cephalopod limestones and shales of the latest Frasnian and early Famennian. These bivalves have traditionally been assigned to the Late Silurian genus Praecardium Barrande, 1881. Re-studying the types of the Late Devonian taxa, Cardium? vetustum Hall, 1843, Cardiola duplicata Münster, 1840, Praecardium clymeniae Beushausen, 1895, Praecardium melletes Clarke, 1904, and Praecardium multicostatum Clarke, 1904 shows that they differ significantly from Praecardium. As a result, two new genera, Vetupraeca n. gen. and Mucopraeca n. gen, are established. Furthermore, neotypes are designated for Cardiola nehdensis, Kayser, 1873 and Vetupraeca clymeniae (Beushausen, 1895), and lectotypes are chosen for Mucopraeca multicostata (Clarke, 1904) and Vetupraeca duplicata (Münster, 1840). These bivalve taxa were widely distributed in the subtropical to tropical, latest Frasnian/early Famennian outer shelf habitats of Laurussia and Gondwana.

Lower Palaeozoic source rocks in Manjiaer Sag, Tarim Basin

Abstract Considering its vertical distribution characteristics, this article argues that the Lower Palaeozoic source rock in the Manjieer Sag is composed of three sets of source rocks of Middle-Lower Cambrian, Middle-Lower Ordovician, and Upper Ordovician. The source rock of the Middle-Lower Ordovician Heituwa Formation has similar sedimentary facies and developmental features to the Middle-Lower Cambrian source rock, and has abundant organic material. The Upper Ordovician source rock is poor and limited in distribution. The source rocks have different thermal evolution histories. In central and western Manjiaer sag, Middle-Lower Cambrian source rock entered oil generation peak in Late Caledonian and Early Hercynian, Middle-Lower Ordovician source rock in Late Hercynian, and Upper Ordovician source rock in Late Yanshan and Himalayan. The threefold division of the source rock is the foundation of detailed study on the derived source and the accumulation process of marine oil in Tarim platform area.

Crustal Contributions to Late Hercynian Peraluminous Magmatism in the Southern Calabria-Peloritani Orogen, Southern Italy: Petrogenetic Inferences and the Gondwana Connection

Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircon from granites of the medium-high grade Aspromonte^ Peloritani Unit, Calabria^Peloritani Orogen (CPO), southern Italy, show that one of the minor trondhjemites (313 7 3 5Ma) represents the earliest identified occurrence of Late Hercynian peraluminous igneous rocks in the CPO, predating the emplacement of the more common peraluminous leucogranodiorites by about 14 Myr. Some of the trondhjemite zircon grains contain small cores with ages of about 2 45 Ga, 625Ma and 490Ma, consistent with the presence of a sediment component in the magma. A newly dated leucogranodiorite (300 2 3 8Ma) is rich in inherited zircon. Cores with ages of about 2 36 Ga, 870Ma, 630Ma, 545Ma and 460Ma are overgrown by two generations of Hercynian igneous zircon, the first with moderate to highTh/U (up to 1 67), and the second with low Th/U (50 1).The overgrowths probably crystallized from magmas of two compositions, the first metaluminous and the second peraluminous. This could indicate either magma mixing or, more probably, crystallization in a single, evolving magma. In either case, the leucogranodiorite magma is considered to have been the product of anatexis of a metasedimentary source. Differences in the inherited zircon age spectra, and the relatively small amount of inheritance in the trondhjemite, indicate that the trondhjemite and leucogranodiorite are unlikely to be genetically related.The ages of the inherited zircons are consistent with the sedimentary component in both magmas being derived from North Africa, with a possible contribution from Pan-African granitoids similar to those exposed in southern Calabria.

Characteristics, genesis and accumulation history of natural gas in Hetianhe gasfield, Tarim Basin, China

Hetianhe gasfield in Bachu region of the Tarim Basin is mainly composed of three reservoir-caprock assembly, namely regional caprock of upper mudstone, middle mudstone and lower mudstone of the Carboniferous and reservoir of Bachu bioclastic limestone, glutenite and the Ordovician carbonate buried hill. Natural gas in Hetianhe gasfield sourced from the Cambrian source rock. It is thought that gases in Ma4 well block in the east of Hetianhe gasfield are mainly crude-oil cracked gases, while those in Ma3 and Ma8 well blocks in the west are the mixture gases of kerogen cracked gases and crude-oil cracked gases. Natural gas is rich in H2S and accumulated in multiply stages as the result of TSR. The accumulation history is divided into three stages, namely accumulation and breakage in the late Caledonian-early Hercynian, migration and dissipation in the late Hercynian and accumulation in Himalayan. The main accumulation of reformed gas reservoir is in Himalayan.

Forming mechanism of hydrocarbon reservoirs in Yingshan Formation of Yuqi block in Akekule arch, Tarim Basin

The reservoir conditions, oil and gas charge history and accumulation phases were studied for Yingshan Formation of Yuqi block, and an oil and gas accumulation model was established by using the techniques of reservoir prediction, fluorescence thin section and fluid inclusion analysis under the guidance of the theories of oil and gas accumulation. The results indicate that the main rock types in Yingshan Formation are micrite and calcarenite. The carbonate reservoirs are of cave, fracture-pore and fracture types, and their physical properties are intermediate; there are at least four oil/gas charges, i.e. late Hercynian, Yanshanian, early Himalayan and middle Himalayan (Cenozoic). The most important charge periods are late Hercynian, early Himalayan and middle Himalayan; the oil and gas accumulation model is self source—lateral expulsion of hydrocarbon—multistage accumulation, or hydrocarbon sourced from and preserved in the same old rocks—long term expulsion of hydrocarbon—multistage accumulation.

Hydrocarbon Accumulation and Distribution Characteristics of the Silurian in the Tazhong Uplift of Tarim Basin

Hydrocarbon accumulation of the Silurian in the Tazhong uplift of Tarim basin is characterized by “two sources and three stages”. “Two sources” means that the hydrocarbons are derived from two source rocks of the Cambrian and Middle-Upper Ordovician. “Three stages” means that asphalt and movable oil undergoes three hydrocarbon accumulation stages, i.e., Late Caledonian, Late Hercynian, and Yanshanian-Himalayan. The formation of asphalt resulted from the destruction of the hydrocarbons accumulated and migrated in the early stages. The present movable oil, mostly derived from Middle-Upper Ordovician source rock, resulted from the hydrocarbons accumulated in the late stage. There are three types of reservoirs, i.e., anticline structural, stratigraphic lithological, and lava shield reservoirs in the Tazhong uplift. Hydrocarbon accumulation of the Silurian in the Tazhong uplift is controlled by the three factors. (1) The background of uplift structure. Around the ancient uplift, the compounding of many types makes up the composite hydrocarbon accumulation areas. (2) Effective cover. The show of oil gas including asphalt, heavy crude oil, and normal oil is quite active in the Silurian. Asphalt and heavy crude oil are distributed under the red mudstone member and movable oil is distributed under the gray mudstone member. (3) High quality reservoir bed. Sandstone is distributed widely in the Tazhong area. Reservoir pore space can be divided into three types: a) secondary origin-primary origin pore space; b) primary origin-secondary origin pore space, and c) micropore space. Porosity is 3.3–17.4%, and permeability is (0.1–667.97) × 10 −3 μm 2.

Pressure–temperature evolution of the late Hercynian Calabria continental crust: compatibility with post‐collisional extensional tectonics

AbstractThe late Hercynian tectonic evolution of the Calabria crust is characterized by peak metamorphic conditions up to 800 °C and 1000 MPa, and coeval mid‐crustal granitoid emplacement at 304–300 Ma. To check if a post‐collisional extensional framework, similar to that of other Hercynian massifs, can explain petrologic data, we model the pressure–temperature evolution of the crust during extension following granitoid emplacement. Model parameters are constrained by petrologic, geochemical and structural data. Computed P–T paths are characterized by nearly isothermal decompression followed by isobaric cooling, which show a good fit to petrologic P–T paths for duration of extension between 5 and 10 Ma. The model results, therefore, support an interpretation of the magmatic and metamorphic evolution of the Calabria crust in terms of the late Hercynian extension. In this framework, slab break‐off is a reasonable explanation for the common evolution of the southern European Hercynian massifs.

Comparison of Permian palynological assemblages from the Junggar and Tarim Basins and their phytoprovincial significance

Comparison of Permian palynofloras between the Junggar and Tarim Basins has resulted in stratigraphic correlation and new knowledge about the phytoprovincial change of Tarim. The Junggar regime was part of the Subangara area since the early Late Carboniferous through the Permian, whereas the Tarim block belonged to the Euramerican Province prior to the Early Permian (Asselian–Sakmarian). A dramatic change from the Euramerican Province to the Subangara area occurred not later than the onset of the Middle Permian. Palynological data supports the assumptions that the collision between the Junggar block and the Kazakhstan–Siberia blocks was during the late Hercynian (not later than in the Early Carboniferous) and that between Tarim with Junggar was during the Early Permian (possibly prior to the Artinskian). Both regions were basically under the control of a semiarid climate during the Permian but Tarim was somewhat drier than Junggar, with both possibly straddling subtropical to warm temperate latitudes (ca. 25°–35° N.).

Alkali metasomatism as a process for trondhjemite genesis: evidence from Aspromonte Unit, north-eastern Peloritani, Sicily

Rocks of trondhjemitic composition are widespread in the North-Eastern Peloritani Belt within the Aspromonte Unit, a Hercynian medium- to high-grade metamorphic complex intruded by late-Hercynian peraluminous granites and later affected by MP/LT Alpine metamorphism. Among these trondhjemitic bodies, the Pizzo Bottino trondhjemites form one of the largest, outcropping over about 6 km2 and up to 400 m thick. These rocks display concordant to discordant relationships with associated metamorphic rocks and are often cut by late-Hercynian leucogranitic dykes. The field, petrographic and geochemical features of these trondhjemites are consistent with an igneous origin. Petrographic and geochemical evidences suggest that the trondhjemitic character of the Pizzo Bottino rocks is due to an alkali metasomatism process involving cationic exchange of Na and Ca for K and consequent replacement of K-feldspar by oligoclase in the original granitoids. The major and trace element contents of the Pizzo Bottino trondhjemites are in fact comparable to those of the peraluminous late-Hercynian granitoids from the southern Calabrian-Peloritani Arc (CPA), when the elements directly involved in the alkali metasomatism process (Na, Ca, K, Sr, Ba, Rb) are not considered. The behaviour of REE elements, plus Th and U, also seems to be partially controlled by metasomatic processes, because their abundances vary with the K/Na ratio. Metasomatism seems to be the only viable mechanism involved in the genesis of the Pizzo Bottino trondhjemites. Other trondhjemite generation processes such as fractionation from basaltic parents and partial melting of metabasaltic or metasedimentary sources are ruled out on geological, petrographic and isotopic (Sr, Nd) grounds. Lastly, regional considerations place the metasomatic event during the late Hercynian, after the emplacement of the original granitoids and preceding the intrusion of the leucogranitic dykes, which are not affected by metasomatism.

Hydrocarbon charging histories of the Ordovician reservoir in the Tahe oil field, Tarim Basin, China.

The Ordovician reservoir of the Tahe oil field went through many tectonic reconstructions, and was characterized by multiple hydrocarbon chargings. The aim of this study was to unravel the complex charging histories. Systematic analysis of fluid inclusions was employed to complete the investigation. Fluorescence observation of oil inclusions under UV light, and microthermometry of both oil and aqueous inclusions in 105 core samples taken from the Ordovician reservoir indicated that the Ordovician reservoir underwent four oil chargings and a gas charging. The hydrocarbon chargings occurred at the late Hercynian, the Indo-Sinian and Yanshan, the early Himalaya, the middle Himalaya, and the late Himalaya, respectively. The critical hydrocarbon charging time was at the late Hercynian.

Hydrocarbon charging histories of the Ordovician reservoir in the Tahe oil field, Tarim Basin, China

The Ordovician reservoir of the Tahe oil field went through many tectonic reconstructions, and was characterized by multiple hydrocarbon chargings. The aim of this study was to unravel the complex charging histories. Systematic analysis of fluid inclusions was employed to complete the investigation. Fluorescence observation of oil inclusions under UV light, and microthermometry of both oil and aqueous inclusions in 105 core samples taken from the Ordovician reservoir indicated that the Ordovician reservoir underwent four oil chargings and a gas charging. The hydrocarbon chargings occurred at the late Hercynian, the Indo-Sinian and Yanshan, the early Himalaya, the middle Himalaya, and the late Himalaya,respectively. The critical hydrocarbon charging time was at the late Hercynian.

Alpine and late-hercynian geochronological constraints in the Argentera Massif (Western Alps)

Located in the southern part of the Western Alps, the Argentera massif belongs to the paleo-European basement of the external domain. It experienced a polyphased deformation history, Hercynian and Alpine. The Alpine history is characterized by the development of a network of ductile shear zones. An 39Ar−40Ar study of single grains of Hercynian muscovites and Alpine phengites allowed to constrain various events. A muscovite cooling age at ca 296-299 Ma is proposed for the Argentera granite, clearly later than the estimated cooling age at ca 310-315 Ma for the low-pressure anatexis. Neocrystallized phengites collected within an Alpine shear zone (Frema Morte) crosscutting the late Hercynian Argentera granite yielded an age at 22.2 ± 0.3 Ma (1σ). This is one of the first unequivocal absolute age constraint of a late Alpine metamorphism in the external crystalline massifs of the Western Alps. Our P-T conditions estimates indicate a regional temperature at ca 350°C for pressure at 0.35-0.4 GPa for this Alpine metamorphism. These estimates imply a minimum burial of 14 km for the Argentera massif, which could result from the overloading imposed by the internal nappes emplacement, probably between 28 Ma and 22.2 Ma.