Mesozoic Age Research Articles

Seismic anisotropy and its relation with crust structure and stress field in the Reggio Emilia Region (Northern Italy)

SUMMARY Shear wave splitting is measured at 14 seismic stations in the Reggio Emilia region above local background seismicity and two sequences of seismic events. The good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume. It is about 60 km 2 wide and located between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the Pliocene‐Quaternary alluvium above the Mesozoic layer. The fast polarization directions (N30 ◦ E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW‐NNE) in the region and also parallel to the strike of the main structural features in the Reggio Emilia area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy. These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid-saturated microcracks aligned by the active stress field. We cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.

Late orogenic, large-scale rotations in the Tien Shan and adjacent mobile belts in Kyrgyzstan and Kazakhstan

Most of Kazakhstan belongs to the central part of the Eurasian Paleozoic mobile belts for which previously proposed tectonic scenarios have been rather disparate. Of particular interest is the origin of strongly curved Middle and Late Paleozoic volcanic belts of island-arc and Andean-arc affinities that dominate the structure of Kazakhstan. We undertook a paleomagnetic study of Carboniferous to Upper Permian volcanics and sediments from several localities in the Ili River basin between the Tien Shan and the Junggar–Alatau ranges in southeast Kazakhstan. Our main goal was to investigate the Permian kinematic evolution of these belts, particularly in terms of rotations about vertical axes, in the hope of deciphering the dynamics that played a role during the latest Paleozoic deformation in this area. This deformation, in turn, can then be related to the amalgamation of this area with Baltica, Siberia, and Tarim in the expanding Eurasian supercontinent. Thermal demagnetization revealed that most Permian rocks retained a pretilting and likely primary component, which is of reversed polarity at three localities and normal at the fourth. In contrast, most Carboniferous rocks are dominated by postfolding reversed overprints of probably “mid-Permian” age, whereas presumably primary components are isolated from a few sites at two localities. Mean inclinations of primary components generally agree with coeval reference values extrapolated from Baltica, whereas declinations from primary as well as secondary components are deflected counterclockwise (ccw) by up to ∼ 90°. Such ccw rotated directions have previously also been observed in other Tien Shan sampling areas and in the adjacent Tarim Block to the south. However, two other areas in Kazakhstan show clockwise (cw) rotations of Permian magnetization directions. One area is located in the Kendyktas block about 300 km to the west of the Ili River valley, and the other is found in the Chingiz Range, to the north of Lake Balkhash and about 400 km to the north of the Ili River valley. The timing of the ccw as well as cw rotations is clearly later than the disappearance of any marine basins from northern Tarim, the Tien Shan and eastern Kazakhstan, so that the rotations cannot be attributed to island-arc or Andean-margin plate settings — instead we attribute the rotations to large-scale, east–west (present-day coordinates), sinistral wrenching in an intracontinental setting, related to convergence between Siberia and Baltica, as recently proposed by Natal'in and Şengör [Natal'in, B.A., and Şengör, A.M.C., 2005. Late Palaeozoic to Triassic evolution of the Turan and Scythian platforms: the pre-history of the palaeo-Tethyan closure, Tectonophysics, 404, 175–202.]. Our previous work in the Chingiz and North Tien Shan areas on Ordovician and Silurian rocks suggested relative rotations of ∼ 180°, whereas the Permian declination differences are of the order of 90° between the two areas. Thus, we assume that about 50% of the total post-Ordovician rotations are of pre-Late Permian age, with the other half of Late Permian–earliest Mesozoic age. The pre-Late Permian rotations are likely related to oroclinal bending during plate boundary evolution in a supra-subduction setting, given the calc-alkaline character of nearly all of the pre-Late Permian volcanics in the strongly curved belts.

Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan Volcanic Province

The Lonar impact crater in the Deccan Traps of the Indian peninsula provides unique opportunities to study physical and chemical processes of impact cratering on basaltic targets, because terrestrial impact craters on basalts are extremely rare. Such studies are needed for determining provenance and other parameters of the excavated rocks and the cratering phenomenon that may have implications for similar crater formations in Lunar, Martian and other basaltic targets in the solar system. Considering some of these objectives, we analyzed trace elements and Nd, Sr, Pb-isotopes of impact breccia rocks and target basalts collected from the Lonar crater. Chondrite-normalized Rare Earth Element (REE) patterns in the target basalts and breccia rocks show similar light REE-enriched patterns, although in detail, the impact breccia are more fractionated in La/Sm compared to the target basalts. The target basalts also show much lower concentrations of Rb, Ba, Th, U and Pb compared to the breccia and are characterized by Rb, Ba and Pb depletions with respect to the primitive mantle-normalized Th, U, Nb, Ta and the REE contents. The breccia rocks are significantly enriched in Rb, Ba and Pb, and to a lesser extent in Th and U, compared to the target rocks. The Nd, Sr and Pb-isotopic compositions of the Lonar target basalts can be correlated with those of the Poladpur suite, one of the mid-section volcano-stratigraphic units of the Deccan traps. In contrast to the host basalts, the impact breccia rocks show more radiogenic Sr, less radiogenic Nd and higher Rb/Sr and lower Sm/Nd ratios, indicating an additional component, other than the target basalt, that must have been derived from beneath the basaltic target rocks at the impact site. The Deccan traps in western India are underlain by Archean to mid-Proterozoic cratonic rocks. The overall geochemical signatures of the impact breccia rocks, specifically, the trace element concentrations, negative ε Nd values, radiogenic Sr isotopic composition as well as the high 207Pb/ 204Pb at low 206Pb/ 204Pb indicate that a major component of the Lonar impact breccia was derived from melting of Archean basement rocks. We argue that the Archean component in the breccia cannot be from the incorporation of paleosols that are weathering products of the target basalts, or from the inter-trappean sediments that are most commonly cherts and limestones of Mesozoic age. Similarly, the possible role of eolian sediments in causing the Archean Pb-isotopic signature, identical to those of the Deccan basement, in the breccia rocks can be excluded. The basement beneath the Lonar region is believed to be similar to the Dharwar craton of peninsular India. Based on their similar Pb-isotopic compositions with the breccia rocks, we suggest the Archean Chitradurga Group of rocks of this craton to be present in the basement beneath the Deccan lavas of the Lonar region. The thickness of the basaltic target rocks at the crater-site (∼ 400 m) and the inferred crater depth (350 m–610 m), based on depth to diameter ratios in simple planetary craters (0.2–0.33), are consistent with our conclusion regarding melting and incorporation of these ancient basement rocks in the impact breccia of the Lonar crater. Using Pi-group scaling relations, the observed crater diameter, and density of the basaltic target rocks, we have estimated with reasonable approximation the diameter of the Lonar impactor to be either 70 m, 86 m or 120 m, assuming the bolide to be an iron meteorite, stony-iron meteorite, or an ordinary chondrite, respectively.

Low-Grade Metamorphism of Permian Mafic Rocks From the Gorzów Wielkopolski Block (Fore Sudetic Monocline, Nw Poland): Age and Mechanism

Low-Grade Metamorphism of Permian Mafic Rocks From the Gorzów Wielkopolski Block (Fore Sudetic Monocline, Nw Poland): Age and MechanismThe metavolcanic rocks in the Gorzów Wielkopolski area (NW Poland) are andesite-basalts and andesites derived from contaminated within-plate subalkaline basalt melts. K-Ar dating of primary K-feldspar yields an extrusion age of 285±5 Ma. This new date suggests that these rocks may be the youngest dated Permo-Carboniferous volcanic rocks within the eastern margin of the North German Basin.Petrological and micro-thermometric data indicate that the volcanic rocks were altered by a sub-greenschist facies, metamorphic process that proceeded within temperatures and pressures ranging from 195-290°C and 63-96 MPa, respectively. The major metamorphic phases, corrensite, pumpellyite, laumontite and prehnite, formed due to interaction between the original volcanic rock and low-saline Na2SO4-rich hydrothermal fluids. Numerical modelling indicates fluid flow in fractures as the most probable explanation for the low-grade hydrothermal metamorphism. Model calculations suggest that the process was of short duration, ca 140-250 years. K-Ar dating reveals three Mesozoic episodes at 121±2 Ma, 188-190 Ma and 149±4 Ma that can be ascribed to the metamorphism. These ages are similar to previously determined Mesozoic ages from altered volcanic- and clastic rocks in the North German Basin and in

Determination of the recharge area and salinization degree of karst springs in the Lamas Basin (Turkey)

The Lamas Basin is an area covering ∼4400 km2 situated on the eastern Mediterranean coast of Turkey covered with highly karstified limestone and dolomitic limestone from the Miocene and Mesozoic age, respectively. Owing to the area’s low karstification basement, groundwater in the karst aquifer circulates deep from the surface towards the springs along the coast as well as to the submarine springs. This study aims working out the salinization level and recharge characteristics of the Lamas Basin using environmental isotopes techniques. In the study, the data collected previously to discover, in general terms, the groundwater characteristics within the area are reanalyzed to fulfil the purpose of the study. In conclusion, it is found that the down gradient karst springs discharging along the Mediterranean coast mostly contain groundwater contributions from higher altitudes with depleted δ18O and δ2H compositions. The δ18O-altitude effect was determined as approximately−0.12 ‰/100 m which may indicate sea-spray intrusion towards inland. As a result, the salinization level of coastal springs changes ranging between 1.2 % and 17.0 %. Owing to the seawater encroachment, Ca–HCO3 water type changes to Na–HCO3 or Na-Cl water by the cation exchange during the dry period. As the unique freshwater potential extends along the coastal area, the groundwater production should be exploited in a way that seawater encroachment is kept at minimum.

Phylogenetic and ontogenetic parallelisms on talo-calcaneal superposition

The process of talo-calcaneal superposition is one of the most important steps in the transformation of the reptilian foot (primitive) into the mammalian foot (modern). This change was developed in certain families of mammal-like reptiles and transitional mammals during the Mesozoic age and represents a different path in the morphological development of the foot to that of other dominant reptiles (Dinosaurs, etc.). The talo-calcaneal superposition takes place during the embryonic stage of human life and ends during the primary stages of the foetal period of the same. This study aims to demonstrate the similarities between the phylogenic and ontogenic development of the said morphological change.

Migration pathways in the Central North Slope foreland basin, Alaska USA: solute and thermal constraints on fluid flow simulations

AbstractThe North Slope foreland basin, Alaska, USA is an east–west asymmetrical trough‐shaped basin adjacent to the Brooks Range fold‐thrust mountain belt. Lower Cretaceous age rocks make up much of the sediment fill, including flysch‐like marine turbidites and shales of the Torok and Fortress Mountain formations and marine and sandstones, shales and conglomerates of the overlying Nanushuk group. Lower Cretaceous age rocks were deposited on top of a Palaeozoic and Mesozoic age passive margin sequence. We have conducted numerical simulations of fluid flow driven by topographic recharge in the Central North Slope foreland basin. These simulations are constrained by salinity estimates from well logs, location of oil and gas fields, vitrinite reflectance and heat flow measurements. Our model results indicate that there are two south to north pathways for fluid migration. The primary pathway for fluid movement is downward through the Fortress Mountain formation, then upwards along the interface between the Fortress Mountain and Torok Formation and finally northward through the permeable Nanushuk group. A smaller mass of groundwater moves along sands below the Torok formation and into offshore sediments north of Alaska. Very little meteoric water enters the underlying Palaeozoic rocks in our simulations, which could explain the presence of deep saline pore waters. Our results also show that permafrost is a primary control on the pathway and rate of fluid flow by controlling the distribution of surface recharge and discharge. For example, areas of high heat flow and low saline waters along the arctic coast may represent upward groundwater discharge because of the absence of permafrost. As surface temperatures were warmer in the Miocene, the absence of permafrost would produce a more local fluid circulation pattern and less transfer of heat energy from south to north.

Detrital zircon provenance evidence for large-scale extrusion along the Altyn Tagh fault

The question of whether or not the Altyn Tagh fault is a large-scale extrusion boundary is critical for understanding the role of lateral extrusion in accommodating the Indo-Asian convergence and in building the Tibetan Plateau. Oligocene conglomerate clasts in the eastern Xorkol basin are low-grade slate, phyllite, sandstone, dacite and carbonate, and associated paleocurrent indicators evince sediment derivation from the opposing side of the Altyn Tagh fault. Matching these clasts with similar basement rocks in the North Qilian and Tuolainanshan terranes requires post-Oligocene left-lateral offset of 380 ± 60 km on the eastern segment of the Altyn Tagh fault, suggesting large-scale extrusion along the fault in the Cenozoic ( Yue, Y.J., Ritts, B.D., Graham, S.A., 2001b. Initiation and long-term slip history of the Altyn Tagh fault. International Geological Review 43, 1087–1094.). In order to further define this piercing point, the detrital zircon pattern of Oligocene sandstone from the Xorkol basin and the zircon ages of basement on the southern side of the fault were established by ion microprobe dating. Characterized by strong peaks between 850 and 950 Ma and the absence of Paleozoic and Mesozoic ages, the detrital zircon age pattern of the Oligocene sandstone matches the age distribution of zircon-bearing rocks of the Tuolainanshan terrane. This match requires 360 ± 40 km of post-Oligocene left-lateral displacement on the eastern segment of the Altyn Tagh fault, supporting as well as refining the previously reported lithology-based cross-fault match. At least one of the following three extrusion scenarios must have existed to accommodate this large offset: (1) northeastward extrusion along the Altyn Tagh–Alxa–East Mongolia fault, (2) eastward extrusion along the Altyn Tagh–North Qilian–Haiyuan fault, and (3) northeastward extrusion of northern Tibet as a Himalaya-scale thrust sheet along the North Qilian–Haiyuan fault. We prefer the first scenario inasmuch as rapidly growing evidence for Cenozoic strike-slip activity on the Alxa–East Mongolia fault and mid-Miocene exhumation of northern Tibet supports it.

40Ar-39Ar Thermochronological Constraints on the Exhumation of Ultrahigh-Pressure Metamorphic Rocks in the Sulu Terrane of Eastern China

In order to better understand the uplift history of the Sulu ultrahigh-pressure metamorphic terrane, 40Ar-39Ar thermochronological analysis was conducted on minerals extracted from both metamorphic (gneiss and retrograded eclogite) and igneous (pegmatite, granite, and mafic intrusion) rocks from this terrane. Samples of the major lithotectonic units were collected from key localities, including Rongcheng, Laoshan, Yangkou, Taohang, and Rizhao. The analyses of metamorphic rocks yielded ages for hornblende ranging from 222 to 186 Ma, biotite from 130 to 119 Ma, and K-feldspar from 112 to 108 Ma. Hornblende, biotite, and K-feldspar from igneous rocks yielded exclusively late Mesozoic ages from 137 to 97 Ma. A two-stage cooling path for the metamorphic rocks were revealed. A slow cooling (1.9 to 3.8°C/m.y.) during the period of ∼220 to ∼130 Ma was followed by fast cooling (6.8 to 16.7°C/m.y.) during ∼130 to ∼100 Ma. The time span and cooling rate of the second stage coincide with those of the analyzed post-collisional igneous rocks as well as with the formation of rift basins in the region. The study strongly indicates that extensional tectonics played an important role in the final stage of rapid exhumation of the ultrahigh-pressure rocks in the Sulu terrane.

Borehole MET-1/04 near Metlika, between the External and Internal Dinarides (SE Slovenia)

In the year 2004, a deep borehole MET/04 east of Metlika in Bela Krajina was drilled. It reached the depth of 841 meters. The aim of drilling was to catch thermal water that would be used for touristical purposes. The area where the borehole is located belongs to the transitional zone of the External to the Internal Dinarides. The first ones are characterized mainly by shallow water carbonate rocks of Mesozoic age, while the other ones are composed mainly of deep water clastic rocks also of Mesozoic age. Structurally, this zone consists of a series of the Dinaric, NW-SE oriented, thrusts along which the Internal Dinarides are thrusted over the External ones. The bore hole itself is located in the flysch sequence of Upper Cretaceous age that is deposed transgressivelly onto the Upper Jurassic limestone. The flysch, that is composed of conglomerate, breccia, calcarenite and marl, is drilled to 182,5 m of depth. Further, to the 275 of depth, there follow Lower Cretaceous limestones, that belong either to large olistolitic blocks in the base of the flysch series or to a tectonic block of the imbricated structure. To the bottom of the borehole, there follow mainly reef and perireefal limestones of Upper Jurassic age. The upper part of these is determined to be of Upper Malmian age, since the lower part could belong to Lower Malmian. In the borehole, two water bearing zones were determined. The first one is in the interval from 92,5 to 184, and the second one in the interval from 220 to 355 meters at the depth.

New stratigraphical results and significance of reworking based on nannofossil, foraminiferal and sedimentological records in the Lower Tertiary sequence from the northern Isparta Angle, Eastern Mediterranean

This study presents new stratigraphical data on the sediments of the Lower Tertiary sequence outcropping in the northern Isparta Angle, Southwest Anatolia. These data, which are based on field observations, foraminifera and nannofossil studies, and sedimentological findings, contribute to the paleoenvironmental and structural interpretations. Reworking of fossils that was neglected in previous work is an important factor for understanding and clarifying the Early Tertiary geological evolution of the region. This investigation particularly focuses on the significance of reworking in the stratigraphic record. The Early Tertiary unconformity-bounded unit consist of two transitional lithofacies, varying from marine clastics to carbonates. The first lithofacies mainly composed of clastics, whereas the second contains carbonate-dominated sediments. The Lower Tertiary sequence unconformably overlies an irregular surface composed of ophiolitic rocks, deep marine bedded cherts and carbonates of early Mesozoic age, Mesozoic carbonates and marine clastics and carbonates of Late Cretaceous age. The unit is unconformably overlain by Lower Miocene marine clastics. Both the unconformity and tectonic relationships between the Eocene and Lower Miocene sequences are seen in the northern study area. These exposures are proof of pre-Miocene erosion and post-Miocene deformation. In this study, two nannofossil interval zones, NP12 and NP13, were identified from fine-grained clastics and carbonates. According to benthic foraminifera and nannofossil contents, the formation is Early Eocene (Ypresian) in age. The percentages of reworked (older) fossils range from 75% in the lower levels to 15% in the higher levels of the Lower Tertiary sequence. This study demonstrates that detailed analyses of fossil reworking are needed in order to obtain more reliable results. The evidence of strong reworking, plant traces, biostratigraphical and sedimentological data indicate that a single cycle of Early Eocene sedimentation occurred in a shallow to open-marine depositional environment, becoming shallower as time progressed. The obtained data indicate a marine transgression from north to south in the northern part of the Isparta Angle during Eocene time.

Palaeomagnetism of Ordovician and Silurian rocks from the Chu-Yili and Kendyktas mountains, south Kazakhstan

Despite significant progress during the last decade, the overwhelming majority of the details of the crustal evolution of Kazakhstan remain enigmatic. The central question of whether Kazakhstan amalgamated during the Ordovician and has acted as a coherent microplate since or whether Kazakhstan is the result of continuous accretion of terranes and arcs along a major subduction system throughout the Palaeozoic is still unanswered. Defining structural units and reconstructing their interaction relative to each other and with respect to Siberia and Baltica will improve our understanding of continent formation in Central Asia. Addressing this problem, a detailed palaeomagnetic study of the Palaeozoic formations in the Chu-Yili and Kendyktas mountains, south Kazakhstan was undertaken. Detailed stepwise thermal demagnetization experiments reveal the presence of a post-folding component of magnetization with intermediate to high unblocking temperatures (component A) in almost all samples. The in situ mean directions identified in 31 sites range in declination between 150° and 260° and show intermediate and upward pointing inclinations; based on the mean inclination a late Palaeozoic to early Mesozoic age is assumed for this secondary component of magnetization. After removal of component A a second component of magnetization (component B) was identified in red coloured argillites of early Arenigian age (Kendyktas Mountains) as well as in sediments of Early Silurian and Late Silurian to Early Devonian age (Chu-Yili Mountains). Component B passes the fold test (Enkin 2003) on the 95 per cent confidence level and yields a bedding-corrected site mean direction of D = 9.2°, I =−16.9° (five sites, k = 26.9, α95 = 15.0°) for the Lower Ordovician unit and D = 346.9°, I =+23.8° (eight sites, k = 20.8, α95 = 12.4°) for the Silurian and Lower Devonian units. Based on the age of angular unconformities, the timing of regional deformation in the Agalatas area can be constrained to be Late Ordovician in age and Middle Devonian in the Chu-Yili area. Based on the mean inclination, a southerly palaeolatitude of 8.6°–8.7°/+ 7.6° can be derived for the study area during the Early to Late Ordovician and a northerly palaeolatitude of 12.4°+ 7.7°/− 6.6° for the Silurian to Early Devonian. The observed mean declinations for the post-folding component A are significantly different from the expected reference directions for the Permian based on either the apparent polar wander path for Baltica or Siberia and suggest counter-clockwise rotations of Kazakhstan with respect to both during Devonian to Permian time.

Evolution of faulting and paleo-stress field within the Ottawa graben, Canada

Abstract We relate families of faults in the Ottawa graben to periods in the tectonic evolution of eastern North America. The Ottawa graben is a seismically active structure that coincides with a 55 km wide topographic depression extending from near Montreal through Ottawa. It is part of the St. Lawrence rift system that also includes the seismically active Saguenay graben. The Cambrian to Ordovician sedimentary rocks that overlie Grenvillian basement rocks in the Ottawa area are cut by several generations of brittle faults. Relative ages of faults and associated structures are determined, or inferred, from field studies of key outcrops. Three periods of faulting are defined. The orientations of the paleo-stress fields associated with each period are modelled using fault and fault surface lineations orientation data. The oldest generation of faults (D 1 ) formed in response to a stress field in which the greatest principal stress ( σ 1 ) was horizontal and oriented NW. D 2 and D 3 occurred when σ 1 was oriented WNW and SW, respectively. Hence the paleo-stress field underwent a counterclockwise rotation from D 1 to D 3 . The D 2 and D 3 periods involved the reactivation of faults and the development of new faults. D 1 structures are kinematically congruent with the compressional direction associated with the closing of the Iapetus Ocean during Paleozoic time. Emplacement of Cretaceous carbonatite dykes coincided with D 2 , which occurred during the Mesozoic when the stress regime was associated with the opening of the Atlantic Ocean. The relative timing of these structures supports both Paleozoic and Mesozoic ages of mineralization events within the Ottawa graben. D 3 structures are consistent with the post-Cretaceous stress field in eastern North America. Recent seismicity within the Ottawa graben may involve reactivations of faults belonging to the families identified in this paper.

Palaeogene igneous rocks reveal new insights into the geodynamic evolution and petroleum potential of the Rockall Trough, NE Atlantic Margin

AbstractData acquired from petroleum exploration well 164/7‐1 drilled in the UK sector of the Rockall Trough have yielded fresh insights into the igneous and thermal history of this frontier region. The well targeted a large four‐way dip closed structure of presumed Mesozoic age named ‘The Dome Prospect’. The structure is now known to have a magmatic, rather than a purely structural origin, which was the preferred pre‐well interpretation. The well encountered 1.2 km of Palaeocene age basaltic lavas, overlying Late Cretaceous mudstones which were intruded by over 70 dolerite sills ranging from <1.5‐ to 152‐m thick. 40Ar/39Ar dating of the dolerite intrusions indicates an Early Palaeocene age (63–64±0.5 Ma), which are among the oldest 40Ar/39Ar dates recognised in the North Atlantic Igneous Province. Radiometric dating of the overlying basaltic lavas proved unsuccessful, because of excessive alteration. Biostratigraphic dating of underlying and overlying sedimentary strata was utilised to constrain the age of the lavas to Late Paleocene to Early Eocene age (∼55 Ma). Despite being related to two distinct events separated by ∼8 Ma, the intrusives and extrusives are compositionally similar. The basaltic rocks from well 164/7‐1 possess Sr–Nd isotopic, major and trace‐element geochemical compositions similar to other volcanic and intrusive rocks of the British Tertiary Igneous Province and represent partial melts of both lithospheric and asthenospheric mantle associated with the proto‐Icelandic mantle plume head.Joint consideration of thermal maturity, potential fields and 3D seismic data indicate a deeper igneous body in addition to the sills encountered in well 164/7‐1. Jack‐up and arching mechanisms associated with both scales of intrusive body are believed to have developed the dome structure. The preferred interpretation is of a mafic laccolith, 17 km in diameter, ∼7 km thick, intruded at 64.5 Ma, situated ∼2.5 km below the bottom of the well. 3D thermal modelling suggests that all of Tranche 52 was thermally affected by the intrusion of the magmatic body. The thermal aureole, between 27 and 51 km in diameter, is not thought to play an important role in the hydrocarbon prospectivity of the surrounding Tranches in the NE Rockall Basin.Results show that hydrocarbon exploration prospects that are circular in map view should be interpreted with caution on volcanic continental margins. In sedimentary basins, where salt domes and shale diapirs are absent and igneous rocks prevalent, periclinal structures such as ‘The Dome Prospect’ should undergo a thorough multi‐disciplinary risk assessment.

Paleozoic and Mesozoic granitic rocks in the Hotont area, central Mongolia.

The granitic rocks of the Hotont area were classified into the Late Paleozoic Delgerhaan batholithic complex and Mesozoic Egiindavaa stocks, the latter of which is further divided into normal granitoids and Sn-rich granitoids. The Delgerhaan granitoids are weakly magnetic, their oxygen fugacity being at QMF buffer, while the Egiindavaa granitoids are more reduced belonging generally to ilmenite series. The Sn-rich granites are completely magnetite free and most reduced. The Delgerhaan granitoids are shoshonitic, whereas the Egiindavaa granitoids are shoshonite to high-K series. Both the granitoids are similar in most of chemical components in the Harker’s diagrams, but the Delgerhaan granitoids are richer in P2O5 and poorer in Fe2O3, TiO2 and MgO than the Egiindavaa granitoids. Both the granitoids seem to be generated within continental crust in islandarc setting and not fractionated. The Sn-rich granite may be a later intrusion and generated in anorogenic tectonic setting, and well fractionated. Weak Au-mineralizations are related to the Late Paleozoic Delgerhaan granitic complex. The Late Paleozoic granitoids are too well exposed to have primary base-metal deposits, and the Mesozoic granitoids are not oxidized enough to have base metal mineralizations. Sn and W mineralizations and some Ti-Ta-Nb occurrences are possibly associated with reduced granitoids of the Mesozoic age, particularly with the Sn-rich granites, which may have formed in anorogenic setting.

Petrography and Paleoenvironmental Conditions During Formation of the Lignite Deposit at Prosilio, Northern Greece

This study focuses on the petrography of the lignite deposit at Prosilio, which is in the northern part of the larger basin of Prosilio-Polyracho-Trigoniko. The western part of this basin, which is in the center of our study, belongs to the larger graben structure that contains many other well-known lignite basins in Greece (e.g., Florina-Ptolemaida-Kozani-Servia-Elassona). The sediments in the basin are mainly limnic of Neogene age, surrounded by strata of Mesozoic or older age. The results in this study originated from core samples taken extensively within the lignite beds and were augmented with surface samplings from a recent mine face at Prosilio. The study consists of a macroscopic description of the lignite beds, followed by a microscopic description based on 16 polished samples. The results were interpreted on the basis of the groundwater index (GWI), vegetation index (VI), gelification index (GI) and tissue preservation idex (TPI). The above indices provided a clue as to the conditions of peat formation in the swamp. Specifically, the ratio of VI/GWI helped determine the hydrogeology and the availability of nutrients in the swamp. The ratio of TPI/GI indicated the paleoenvironmental conditions in relation to the water table. The above ratios suggested that the initial (parent) vegetation was that of open water. The conditions then changed in an alternating manner towards telmatic and limnotelmatic, with a few minor and local exceptions, although the dominance of the latter appeared to have prevailed.

Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97–158 Ma basaltic underplating and granulite-facies metamorphism

U-Pb zircon dating by LA-ICP-MS and SHRIMP for one olivine pyroxenite yields complex age populations including Mesozoic ages of 97–158 Ma and 228 ± 8.7 Ma, Early Paleozoic ages of 418–427 Ma, Paleoproterozoic age of 1844±13 Ma, Neoarchean age of 2541 ± 54 Ma and middle Archean age of 3123 ± 4.4 Ma. The 97–158 Ma and 228 ± 8.7 Ma zircons show typical igneous oscillatory zonation in CL images, suggesting two episodes of magmatic events. Overlapping of the 97–158 Ma ages with that of granulite xenoliths indicates that the Mesozoic granulite-facies metamorphism was induced by heating from the basaltic underplating at the base of the lower crust. Both processes lasted at least from about 158 to 97 Ma. Ages of 418 –427 Ma could be records of the subduction of Mongolia oceanic crust under the North China craton. Ages of 1.84 Ga, 2.54 Ga and 3.12 Ga correspond to the three important crust-mantle evolutionary events in the North China craton, and imply preservation of Precambrian lower crust in the present-day lower crust.

A reinterpretation of the crustal N‐isotope record: evidence for a 15N‐enriched Archean atmosphere?

AbstractA new compilation of N‐isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ15N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ15N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ15N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ15N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ15N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ15N does not increase with decreasing N content; accordingly, high δ15N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ15N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ15N is +30‰ to + 42‰.

MACROCRYSTALS OF Pt Fe ALLOY FROM THE KONDYOR PGE PLACER DEPOSIT, KHABAROVSKIY KRAY, RUSSIA: TRACE-ELEMENT CONTENT, MINERAL INCLUSIONS AND REACTION ASSEMBLAGES

Euhedral macrocrystals of Pt–Fe alloy from the Kondyor PGE placer, Khabarovskiy Kray, eastern Siberia, Russia, have a relatively constant composition of Pt2.4–2.6Fe, tin and antimony contents up to 0.3 wt.%, and an unusually low content of all PGE except Pt. The millimetric crystals contain inclusions of fluorapatite, titanite, phlogopite, magnetite, ilmenite and iron–copper sulfides. The macrocrystals have a complex gold-rich rim, with four groups of gold alloy: tetra-auricupride (the most abundant gold-bearing phase), Au–Ag (98–54 wt.% Au), Au–Ag–Cu–Pd and Au–Pd–Cu alloys. The inner part of the reaction rim hosts a variety of PGE minerals, such as stannides, antimonides and tellurobismuthides of Pd and Pt. Stannides occur as copper-bearing (taimyrite–tatyanaite series) and copper-free compounds (atokite–rustenburgite series). The main antimony mineral is Sn-bearing mertieite-II. Tellurobismuthides are represented by Te-rich sobolevskite and an intermediate member of the moncheite–insizwaite solid-solution series. The reaction rim also hosts several unknown phases, such as Pd7Bi3, Pd3Bi, Bi2O3•3H2O, and a phosphocarbonate of thorium. The coarse crystals occur in eluvial and alluvial placers within the Kondyor massif, a zoned clinopyroxenite–dunite intrusion of Paleozoic or Mesozoic age. The inclusion assemblage suggests that the macrocrystals of Pt– Fe alloy are associated with late apatite – magnetite – phlogopite clinopyroxenite bodies, and possibly formed in a late–magmatic pegmatitic environment. The Pt–Fe alloy crystals have undergone hydrothermal overprint by NaCl-rich solutions bearing gold – silver – palladium – copper, which produced a wide variety of intermetallic compounds as well as Pd-bearing intermetallic phases. The aqueous solutions also had a minor but characteristic bismuth – tin – antimony – tellurium component.

The sulfur isotopic evolution of Phanerozoic seawater based on the analysis of structurally substituted sulfate in carbonates

One hundred fifty-six biogenic calcites and 96 whole rock carbonate samples of Paleozoic and Mesozoic age have been analysed for their sulfur isotopic composition. Recorded in structurally substituted sulfate (SSS) in the calcite lattice, the sulfur isotopic composition is thought to reflect the sulfur isotope value of ambient seawater sulfate. Possible alterations of the signal have been assessed, confirming the validity of this approach and the reliability of the resulting temporal record.The new sulfur isotope data substantially enhances our knowledge about secular variations of the global sulfur cycle during the Phanerozoic. Specifically for the Paleozoic, 177 new measurements of SSS in stratigraphically well-constrained samples provide a coherent and detailed dataset. This is extended through the entire Mesozoic via 75 measurements of δ34SSSS, largely for biogenic calcites. Long-term changes and short-term oscillations in the sulfur isotopic composition of seawater sulfate have been observed, which are thought to reflect changes in the fractional burial of sulfide sulfur.Our new sulfur isotope data measured for structurally substituted sulfate in carbonates, and additional data from Cenozoic foraminifera and marine barites, provide a new temporal record for the sulfur isotopic composition of Phanerozoic seawater sulfate. This new record is superior in its continuity and time resolution to currently available datasets and should replace them in future considerations of Earth System Evolution.