Lithological Transition Research Articles

Triassic tectonics and mineral systems in the Qinling Orogen, central China

The Qinling Orogen in central China preserves the records of a complex geological and tectonic history, and also carries abundant mineral resources. Previous investigations on the Qinling Orogen have identified that the final transition from the Palaeo‐Tethys Ocean to a continental orogen occurred in the Triassic, although the timing of the oceanic closure and the tectonic affinity of the Triassic Qinling belt have been poorly constrained. In this contribution, we compile the palaeomagnetic data and geological evidence related to sedimentation, magmatism and mineral systems. The palaeomagnetic data show that the Palaeo‐Tethys Ocean in the Qinling Orogen gradually closed westward during the Triassic, terminating in the beginning of the Jurassic. Triassic strata are widely developed in the Qinling Orogen and conformably overlie the Permian strata. They are composed of carbonates and siliciclastic rocks that were strongly deformed and weakly metamorphosed during the Late Triassic to Late Jurassic. The Triassic magmatic rocks are dominated by I‐type granitoids, with subordinate S‐type and alkaline granites and minor carbonatite dykes, with a clear northward transition in lithology and geochemical features from the Mian‐Lue Suture. Different types of Triassic mineral systems have been reported from this region, including sedimentary‐hosted epizonogenic hydrothermal Hg–Sb, Pb–Zn ± Ag, Au deposits, metamorphic hydrothermal lodes of Au, Ag–Pb–Zn, Mo–Au and Mo, magmatic hydrothermal systems including porphyry Mo, breccia pipe Au, and carbonatite‐hosted Mo deposits. We infer that the Triassic Qinling Orogen witnessed synchronous oceanic plate subduction in the west and continental linkage in the east, with a transition from oceanic basin to continental orogen through gradual subduction‐related consumption of the oceanic plate. The Palaeo‐Tethys Ocean closed in a westward scissor‐like fashion in the Triassic and continental collision and extensive crustal shortening in the Jurassic, following which post‐collisional extension occurred in the Early Cretaceous. Copyright © 2014 John Wiley & Sons, Ltd.

Hydrocarbon migration and accumulation in the Upper Cretaceous Qingshankou Formation, Changling Sag, southern Songliao Basin: Insights from integrated analyses of fluid inclusion, oil source correlation and basin modelling

The Upper Cretaceous Qingshankou Formation acts as both the source and reservoir sequence in the Changling Sag, situated in the southern end of the Songliao Basin, northeast China. An integrated approach involving determination of hydrocarbon charging history, oil source correlation and hydrocarbon generation dynamic modeling was used to investigate hydrocarbon migration processes and further predict the favorable targets of hydrocarbon accumulations in the Qingshankou Formation. The hydrocarbon generation and charge history was investigated using fluid inclusion analysis, in combination with stratigraphic burial and thermal modeling. The source rocks began to generate hydrocarbons at around 82Ma and the hydrocarbon charge event occurred from approximately 78Ma to the end of Cretaceous (65.5Ma) when a large tectonic uplift took place. Correlation of stable carbon isotopes of oils and extracts of source rocks indicates that oil was generated mainly from the first member of Qingshankou Formation (K2qn1), suggesting that hydrocarbon may have migrated vertically. Three dimensional (3D) petroleum system modeling was used to evaluate the processes of secondary hydrocarbon migration in the Qingshankou Formation since the latest Cretaceous. During the Late Cretaceous, hydrocarbon, mainly originated from the Qianan depression, migrated laterally to adjacent structural highs. Subsequent tectonic inversion, defined as the late Yanshan Orogeny, significantly changed hydrocarbon migration patterns, probably causing redistribution of primary hydrocarbon reservoirs. In the Tertiary, the Heidimiao depression was buried much deeper than the Qianan depression and became the main source kitchen. Hydrocarbon migration was primarily controlled by fluid potential and generally migrated from relatively high potential areas to low potential areas. Structural highs and lithologic transitions are potential traps for current oil and gas exploration. Finally, several preferred hydrocarbon accumulation sites have been identified by this work, like Western Slope, Southern uplift, and Eastern Slope, helping reduce the risk on targeting hydrocarbon potential reservoirs in Changling Sag.

Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England

A comprehensive organic geochemical investigation of the Hawsker Bottoms outcrop section in Yorkshire, England has provided new insights about environmental conditions leading into and during the Toarcian oceanic anoxic event (T-OAE; ∼183 Ma). Rock-Eval and molecular analyses demonstrate that the section is uniformly within the early oil window. Hydrogen index (HI), organic petrography, polycyclic aromatic hydrocarbon (PAH) distributions, and tricyclic terpane ratios mark a shift to a lower relative abundance of terrigenous organic matter supplied to the sampling locality during the onset of the T-OAE and across a lithological transition. Unlike other ancient intervals of anoxia and extinction, biomarker indices of planktonic community structure do not display major changes or anomalous values. Depositional environment and redox indicators support a shift towards more reducing conditions in the sediment porewaters and the development of a seasonally stratified water column during the T-OAE. In addition to carotenoid biomarkers for green sulfur bacteria (GSB), we report the first occurrence of okenane, a marker of purple sulfur bacteria (PSB), in marine samples younger than ∼1.64 Ga. Based on modern observations, a planktonic source of okenane's precursor, okenone, would require extremely shallow photic zone euxinia (PZE) and a highly restricted depositional environment. However, due to coastal vertical mixing, the lack of planktonic okenone production in modern marine sulfidic environments, and building evidence of okenone production in mat-dwelling Chromatiaceae, we propose a sedimentary source of okenone as an alternative. Lastly, we report the first parallel compound-specific δC13 record in marine- and terrestrial-derived biomarkers across the T-OAE. The δC13 records of short-chain n-alkanes, acyclic isoprenoids, and long-chain n-alkanes all encode negative carbon isotope excursions (CIEs), and together, they support an injection of isotopically light carbon that impacted both the atmospheric and marine carbon reservoirs. To date, molecular δC13 records of the T-OAE display a negative CIE that is smaller in magnitude compared to the bulk organic δC13 excursion. Although multiple mechanisms could explain this observation, our molecular, petrographic, and Rock-Eval data suggest that variable mixing of terrigenous and marine organic matter is an important factor affecting the bulk organic δC13 records of the T-OAE.

Sediment geochemical records of environmental change in Lake Wuliangsu, Yellow River Basin, north China

We present a paleolimnological record from shallow Lake Wuliangsu in the Yellow River Basin, north China, using a short (56 cm) sediment core. Our objective was to investigate environmental changes in this semi-arid region over the past ~150 years. The sediment core was dated using 137Cs and 210Pb. We examined stratigraphic trends in core lithology, nutrients, stable isotopes (δ13C and δ15N) and trace element concentrations in the Lake Wuliangsu core to discern between natural sediments and those affected by human agency. A lithologic transition from yellow, coarse-grained sediment to grey, fined-grained sediment marked the lake’s formation about 1860. Until ~1950, sediments displayed relatively low and constant heavy metal concentrations, indicating little human influence. In the 1950s, enrichment factors (EFs) increased, reflecting greater impact of human activities. Carbon and nitrogen stable isotopes in organic matter (OM), along with heavy metal concentrations, were used to infer past shifts in trophic state and identify pollutants that came from agriculture, industry and urbanization. In the late 1950s, the first evidence for environmental change is recorded by increases in total organic carbon (TOC), total organic nitrogen (TN), TOC/TN, EFs, δ13C and a decrease in δ15N. After about year 2000, a more rapid increase in trophic status occurred, as indicated by greater total phosphorus (TP), EFs, δ15N and lower δ13C values. Changes in isotope and TOC/TN values in the lake sediments may reflect a shift in lake ecology during this period. The first increase in trophic status during the late 1950s was mainly a result of agricultural development in the catchment. In contrast, the change after ca. AD 2000 was driven largely by urban and industrial development. Agreement between paleolimnologic data from Lake Wuliangsu, and both instrumental and written records, indicates that the lake sediments provide a reliable archive for investigating the formation and environmental history of the lake.

Maggie Hays Ni Deposit: Part 1. Stratigraphic Controls on the Style of Komatiite Emplacement in the 2.9 Ga Lake Johnston Greenstone Belt, Yilgarn Craton, Western Australia

Komatiites occur in many Archean greenstone belts and host significant Ni sulfide ore deposits. Establishing the stratigraphy and the control that stratigraphy has on the emplacement and morphology of ultramafic magmatism is crucial in understanding Archean geodynamic environments and in targeting for Ni sulfide mineralization within these environments. The ~2.9 Ga Lake Johnston greenstone belt, in the southern portion of the Youanmi Terrane of Western Australia, contains komatiite flows and related subvolcanic intrusions, mafic volcanic rocks, felsic volcanic rocks, banded iron formation, and sedimentary rocks. The stratigraphic sequence is intact, preserving original sedimentary and igneous textures and contact relationships, despite being overturned and variably deformed. This study proposes that the lithostratigraphic succession and ultramafic intrusions identified within the Lake Johnston greenstone belt record a transition from arc- to plume-dominated magmatism, accompanied by the establishment of a banded iron formation-dominated sedimentary basin. It is proposed that the rheological contrast between the felsic volcanic unit and overlying banded iron formation acted as a stratigraphic barrier, trapping ascending ultramafic magmas. The stratigraphic barrier inhibited the upward ascent of ultramafic magma causing the development of a subvolcanic magma chamber. Magma trapped beneath the banded iron formation progressively inflated and spread out along the contact, until overpressuring breached the banded iron formation and magma escaped, forming the overlying extrusive komatiites. Both the geodynamic and lithologic transitions gave rise to favorable substrate rock units and an ideal tectonic setting for formation of komatiite-hosted Ni sulfide ores.

A newly discovered earliest Triassic chert at Gaimao section, Guizhou, southwestern China

The Permian–Triassic (PTB) boundary section at Gaimao (Huaxi District, Guiyang, Guizhou Province) records a unique lithological transition between the Permian and Triassic in South China. Thus, bioclastic limestones of the Changxing Formation are overlain by radiolarian-bearing siliceous rocks of the Dalong Formation and ultimately limestone/mudstone alternations of the Shabaowan Formation. Conodont sampling of the section revealed well-preserved elements that enabled a conodont biostratigraphy to be established. The occurrences of Hindeodus julfensis and Hindeodus typicalis in Changxing Formation bioclastic limestone indicate a Late Permian age. Four conodont zones have been established in Dalong and Shabaowan Formations. In ascending order those are the Isarcicella isarcica Zone, Neogondolella (or Clarkina) planata Zone, Neogondolella krystyni Zone and Neospathodus dieneri Zone. The presence of typical Griesbachian conodonts I. isarcica and Isarcicella staeschei in upper Dalong Formation indicates that this formation's radiolarian-bearing siliceous mudrocks are of earliest Triassic age and not restricted to the Permian as previously thought. Thin section analysis reveals that the upper Dalong Formation contains spheroidal radiolarians but not a diverse assemblage of Permian holdover taxa. The new data proves that, following the end-Permian mass extinction of radiolarian, the abundance of these siliceous planktons was maintained at least locally enabling chert deposition to persist. Thus, the Gaimao section provides an exception to the ‘Early Triassic Chert Gap’, the first of its kind in Palaeo-Tethys, and indicates that the end of chert deposition does not always mark the end-Permian mass extinction.

Variable Eocene-Miocene sedimentation processes and bottom water redox conditions in the Central Arctic Ocean (IODP Expedition 302)

In 2004, Integrated Ocean Drilling Program Expedition 302 (Arctic Coring Expedition, ACEX) to the Lomonosov Ridge drilled the first Central Arctic Ocean sediment record reaching the uppermost Cretaceous (~ 430 m composite depth). While the Neogene part of the record is characterized by grayish–yellowish siliciclastic material, the Paleogene part is dominated by biosiliceous black shale-type sediments. The lithological transition between Paleogene and Neogene deposits was initially interpreted as a single sedimentological unconformity (hiatus) of ~ 26 Ma duration, separating Eocene from Miocene strata. More recently, however, continuous sedimentation on Lomonosov Ridge throughout the Cenozoic was proclaimed, questioning the existence of a hiatus. In this context, we studied the elemental and mineralogical sediment composition around the Paleogene–Neogene transition at high resolution to reconstruct variations in the depositional regime (e.g. wave/current activity, detrital provenance, and bottom water redox conditions). Already below the hiatus, mineralogical and geochemical proxies imply drastic changes in sediment provenance and/or weathering intensity in the hinterland, and point to the existence of another, earlier gap in the sediment record. The sediments directly overlying the hiatus (the Zebra interval) are characterized by pronounced and abrupt compositional changes that suggest repeated erosion and re-deposition of material. Regarding redox conditions, euxinic bottom waters prevailed at the Eocene Lomonosov Ridge, and became even more severe directly before the hiatus. With detrital sedimentation rates decreasing, authigenic trace metals were highly enriched in the sediment. This continuous authigenic trace metal enrichment under persistent euxinia implies that the Arctic trace metal pool was renewed continuously by water mass exchange with the world ocean, so the Eocene Arctic Ocean was not fully restricted. Above the hiatus, extreme positive Ce anomalies are clear signs of a periodically well-oxygenated water column, but redox conditions were highly variable during deposition of the Zebra interval. Significant Mn enrichments only occur above the Zebra interval, documenting the Miocene establishment of stable oxic conditions in the Arctic Ocean. In summary, extreme and abrupt changes in geochemistry and mineralogy across the studied sediment section do not suggest continuous sedimentation at the Lomonosov Ridge around the Eocene–Miocene transition, but imply repeated periods of very low sedimentation rates and/or erosion.

Subsurface compressional structures and facies transitions imaged by seismic reflection data, eastern margin of Richardson Trough, Peel Plateau, Yukon

Abstract Knowledge of the deformation front of the Richardson Anticlinorium is an important component of assessments for regional hydrocarbon potential; but sparse outcrop in the Peel Plateau, Yukon has made geological interpretations difficult and the results of reconnaissance mapping uncertain. To date, a single fault has been inferred to define the eastern limit of deformation. In this paper, we present stratigraphic and structural interpretations of multi-channel seismic reflection data collected in an early phase of hydrocarbon exploration. Early Tertiary deformation is expressed as a series of thrust faults, folds and back thrusts, not a single through-going fault. Deformation has been concentrated on several lithologic transitions: a pinch out of syn-rift sandstones and the transition from carbonate platform to basinal shales. The Middle Ordovician to Middle Devonian eastern margin of the Richardson Trough is imaged as clinoforms and a carbonate buildup; clinoforms can be mapped about 75 km along strike. Integration with aeromagnetic data allows us to construct a map of subsurface structures along the eastern deformation front associated with the Richardson Anticlinorium. Faulted anticlines and facies pinch outs are potential traps although any play in this region remains hypothetical.

Spatial organization of process domains in headwater drainage basins of a glaciated foothills region with complex longitudinal profiles

Lithologic transitions and glaciations create complex longitudinal profiles that control contemporary erosion and deposition processes. In areas with these characteristics, traditional morphometric approaches for predicting process domains, such as area‐slope plots, can be augmented by considering other predictors measured from high resolution lidar‐derived digital elevation models (DEMs). Ordinal logistic regression was used to model the distribution of hillslope, swale, colluvial channel, and fluvial channel domains, as identified during field surveys. The study area was a glaciated region of the Rocky Mountain foothills with a complex lithostructural setting. Relationships between domains and a suite of geographic information system–derived descriptors were explored. Predictors included profile anomalies measured at the reach and basin scale using a normalized stream length–gradient (SL/k) index. Drainage area was the dominant factor controlling domains. A model with area as the only predictor was 82% accurate. Reach slope relations were not consistent. A model that also included lithology and basin‐scale SL/k index variation was 87% accurate. Domain transitions had larger area thresholds in basins with resistant conglomerate versus sandstone or shale formations and where SL/k index was more variable along a profile. In a restricted model of hillslope, swale, and colluvial channel domains, profile curvature measured over 100 m was also related to domain occurrence. A model for regional‐scale mapping applications with six additional predictors was 95% accurate. The results showed that ordinal logistic regression can be used to predict and map process domains in regions with complex physiography using descriptors measured from high ‐resolution DEMs.

Discussion of ‘The Highland Boundary Fault and the Highland Border Complex’ by B. J. Bluck, Scottish Journal of Geology , 46 , 113–124

P.W. Geoff Tanner writes: In this paper, Bluck has failed to provide a balanced and accurate assessment of the progress that has been made over many years to find a robust, modern geological interpretation of the Highland Border. This is no trivial matter, for the rocks exposed in this narrow strip of ground retain many of the clues to understanding the causes, and subsequent development, of the Caledonian c . 470 Ma Grampian Event, and the role played by the Highland Boundary Fault in continental-scale plate-tectonic reconstructions (Strachan & Dewey 2003; Tanner 2008). The Highland Boundary Fault is considered by Bluck to define the contact between the Southern Highland Group (Dalradian) and the ‘Highland Border Complex’. However, at outcrop this boundary is variously seen as a fault (North Esk), a lithological transition (Keltie Water), or the sole of an ophiolite (Innellan), and there is no evidence of a major, through-going fault between Arran and Stonehaven, as conventionally depicted (Tanner 2008). Conflicting interpretations of Highland Border geology have generated two diametrically opposed models, essentially summarized below, variations notwithstanding, as A & B. In model A (Curry et al . 1984; Bluck 1985), the rocks between uncontested Southern Highland Group (Dalradian) to the NW and the unconformable Old Red Sandstone to the south, were assigned to the ‘Highland Border Complex’ (HBC). The latter was once ‘considerably separated’ from the Dalradian block, and acquired a northerly dip and younging direction before the two were amalgamated, following the Grampian Event, by movement on the Highland Boundary Fault. In model B (Tanner & Sutherland 2007), the HBC was divided into two parts: the Trossachs Group (now the youngest part of the Dalradian Supergroup), and an overlying, allochthonous unit, the Highland Border Ophiolite (HBO). The main differences between the two models are: all of the …

Magnetostratigraphy of the Northern Tian Shan foreland, Taxi He section, China

TheTian Shan range formed in the late Cenozoic in response to the northward propagation of deformation related to the India^Eurasia continental collision. Precise timing of theTian Shan uplift is required to understand possible mechanisms of continental lithosphere deformation and interactions between climate, tectonism and erosion.Here, we provide magnetostratigraphic age control on the northern ChineseTian Shan foreland successions. A thorough rock magnetic analysis identifies haematite- and magnetite-bearing alluvial fan deposits in the upper portion of the sampled strata as more reliable palaeomagnetic recorders than magnetite-bearing £uvial and lacustrine deposits that are often maghaemitized in the lower part of the record. As a result, a robust correlation to the geomagnetic polarity time scale is obtained from6 to 2Ma while a tentative correlation is proposed from6 to 16Ma. Sediment accumulation rates increase from155 to 260mMyr-1 at 3.9 ± 0.3Ma.This change coincides with a gradual lithologic transition from £uvial (sandstonedominated) to alluvial fan (conglomerate-dominated) deposits that likely records an approaching erosional source related to tectonically increased subsidence rather than diierential compaction. Clear evidence for growth strata starting at an estimated age of ~2Ma provides a minimum age for folding.These results are compared with previous magneotstratigraphic studies from the same and other sections of the northernTian Shan foreland basin fill, thus enabling a critical assessment of the reliability of magnetostratigraphic dating and the significance of sediment accumulation rate variations with respect to facies variations and growth strata. Our results in theTaxiHe section provide a sequence of events that is consistentwith enhanced tectonic forcing starting at ~4Ma, although a climatic contribution must be considered given the close relationship of these ages with the Pliocene climate deterioration.

Erosional modification and gully formation at Meteor Crater, Arizona: Insights into crater degradation processes on Mars

Hydrogeological modification of Meteor Crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation, snow melting, and possible groundwater discharge. The crater wall has an exceptionally well-developed centripetal drainage pattern consisting of individual alcoves, channels, and fans. Some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs; broad gullies occur along the crater corner radial faults. Deeply incised alcoves are well developed on the soft Coconino Sandstone exposed on the middle crater wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and faults, which are favorable locales of erosion due to preferential rock breakup from faulting, and groundwater flow/discharge; these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow, causing erosion and crater degradation. Channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor. Caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall. In addition, lake sediments on the crater floor provide significant evidence of a past pluvial climate, when the water table was higher, and groundwater may have seeped from springs on the crater wall. Although these hydrological processes continue at Meteor Crater today, conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift. This climate shift and the hydrological modifications observed at Meteor Crater provide insights for landscape sculpturing on Mars during various parts of its history.

Sedimentary record from Lake Hovsgol, NW Mongolia: Results from the HDP-04 and HDP-06 drill cores

The Hovsgol Drilling Project retrieved Pleistocene sediment section with the basal age of ca. 1 Ma from the Hovsgol rift basin, NW Mongolia. Detailed lithologic data on drill cores is presented and compared with analogous sediment facies in the radiocarbon-dated records of the last glacial–interglacial transition. Drill cores from two sites, presently in 239 m and 235 m water depth, represent somewhat different depositional settings. The shorter HDP-06 drill core (26 m) at the base of the gentler SE slope of the rift basin contains lithologic evidence for several recent lake lowstands on the order of −200 m. The longer HDP-04 drill core (81 m) some 8 km away at the base of the steep NW underwater slope is composed of finer sediments and contains at least 10 characteristic transitions from calcareous to carbonate-free (diatomaceous) layers. These lithologic transitions are interpreted here as signals of repeated Pleistocene lake transgressions in the Hovsgol basin. Transgressions appear to have been associated with lower sedimentation rates and with the deposition of thin turbidite beds at the drill site. Comparison of drill core lithology with the available seismic data shows reasonable agreement in terms of the number of lowstand events and the general trends of changing lake level. HDP-04 drill core retrieved shallow-water facies containing sand and carbonate oolites deposited at the time of the most dramatic mid-Pleistocene regression of the lake. At ca. 24 m core depth, this interval corresponds to a major basinwide angular unconformity apparent in the seismic pattern. Lake Hovsgol, a smaller sister rift lake of the grand Lake Baikal, has a confined local catchment, which makes it very sensitive to regional variations in the effective moisture. Consisting primarily of calcareous mud, the sedimentary record of Lake Hovsgol provides a unique regional sedimentary archive. Future multi-proxy studies of the Hovsgol sedimentary records will allow constraint of the mid-late Pleistocene history of the hydrologic budget in the Baikal region of continental interior Asia.

Evolução sedimentar da Formação Rio do Rasto (Permo-Triássico da Bacia do Paraná) na porção centro sul do estado de Santa Catarina, Brasil

Between the Late Carboniferous and the Early Triassic, the south portion of the Gondwana Supercontinent witnessed the development of intracratonic basins related with the coeval Sanrafaelic Orogeny. The continuous subsidence and consequent generation accommodation space resulted in the formation of a large confined water body and the accumulation of a transgressive-regressive sequence between the Late Permian and the Early Triassic. The progradational nature of the upper portion of this sequence, represented by de Rio do Rasto e Piramboia formations, culminated with the complete filling of the water body. In the south-eastern portion of Santa Catarina State (Southern Brazil), the Rio do Rasto Formation overlies the Teresina formation and is overlain by the Piramboia formation, both contacts being characterized by lithological transition. The lower portion of the Rio do Rasto Formation is characterized by architectural elements deposited in offshore environments subject to storm action. A marked change of the color of the pelitic facies, from gray to red and purple occurs at the top of the unit. At this stratigraphic level, there is also a predominance of deltaic and eolian architectural elements. The intercalation of nearshore and offshore architectural elements can be explained by the strong auto cyclic character expected in deltaic depositional systems and by the variation of relative rates of base-level rise. The occurrence of eolian architectural elements interbedded with subaquatically deposited sediments on the second third of the unit can be considered an evidence of continentalization to the top, materialized as the transition to the Piramboia desert system.

Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision

In order to decipher element mobility in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks during subduction and exhumation of continental crust, major-trace elements and Sr–Nd isotopes were systematically investigated for two continuous core segments of about 3 m length from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The segments are composed of lithological transitions between UHP eclogite and granitic gneiss. The eclogite exhibits a large variation in major and some trace elements such as LILE (e.g., Rb, Ba and K) and LREE, but a relatively limited range in HFSE and HREE. This suggests high mobility of LILE and LREE but immobility of HFSE and HREE during continental collision-zone metamorphism. Some eclogites have andesitic compositions with high SiO 2, alkalis, LREE, and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidenced by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence of melt-induced element mobility in the UHP metamorphic rocks. In particular, large variations in the abundance of such elements as SiO 2, LREE and LILE occur at the contact between eclogite and granitic gneiss, indicating their mobility between different slab components. Petrographic observations also show the presence of felsic veins on small scales in the UHP metamorphic rocks, demonstrating the occurrence of hydrous melt in local open-systems during the continental collision. As a whole, nevertheless, the protolith nature dictates the geochemical differences in both eclogite and granitic gneiss between the two core segments because mass transport during the subduction-zone metamorphism is principally dictated by the lithological differences at contact. The eclogite and granitic gneiss from the first core segment have high ε Nd( t) values, whereas those from the second core segment show relatively low ε Nd( t) values in concordance with majority of UHP metaigneous rocks outcropped along the Dabie–Sulu orogenic belt. Thus contrasting origins of bimodal igneous rocks were involved in the continental collision, demonstrating that the subducted continental crust is the magmatic product of active rifting margin during supercontinental breakup in the middle Neoproterozoic.

Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps

Mode and tempo of an expanded Paleocene Eocene Thermal Maximum section in the Venetian Pre-Alps

Effects of Geologic Conditions on Rock Quarry Operations, Istanbul, Turkey

In Istanbul, about 50 rock quarries have been opened for the production of specification quality concrete aggregates. Formations exploited include Catalca granite, Kucukkoy sandstone, and Cebecikoy, Hereke, and Kirklareli limestones. In this study, engineering geology parameters affecting the quality of aggregate production and the operational characteristics of rock quarries in these rock types are investigated. Faults, schists that are intruded cut by granites, and microgranites that formed by segregation at the edges of granite intrusions have a negative effect on aggregate quality and operations at quarries in Catalca granite. Dikes, faults, folding, weathering, and lithologic transitions in Kucukkoy sandstone and Cebecikoy limestone have a significant effect on the quality and operational characteristics of those quarries. In Hereke limestone, differential weathering, dolomitization, karstification, and tectonic contacts create similar problems. Lateral and vertical lithological changes, overburden units, karstic openings, and clay-filled fractures cause geologic problems for operations in the Kirklareli formation. Overall, these lithologic and operational problems account for approximate productive reserve losses in these quarries of 15–30 percent for Catalca granites, 12–70 percent for Kucukkoy sandstone, 5–20 percent for Cebecikoy limestone, 5–30 percent for Hereke limestone, and 5–50 percent for Kirklareli limestone. More carefully planned quarrying of these rock types can substantially reduce these losses for more productive operations and the production of construction aggregates of consistently higher quality.

The Neoproterozoic Mwashya–Kansuki sedimentary rock succession in the central African Copperbelt, its Cu–Co mineralisation, and regional correlations

Rocks of the Neoproterozoic Mwashya Subgroup (former Upper Mwashya) form the uppermost sedimentary unit of the Roan Group. Based on new field and drill hole observations, the Mwashya is subdivided into three formations: (1) Kamoya, characterized by dolomitic silty shales/siltstones/sandstones and containing a regional marker (the “Conglomerate de Mwashya” bed or complex); (2) Kafubu, formed by finely bedded black carbonaceous shales; and (3) Kanzadi, marked by feldspathic sandstones. Rocks of the Mwashya Subgroup are overlain by the Sturtian age Grand Conglomérat diamictite (equivalent to the Varianto/Brazil and Chuos/Namibia diamictites), and conformably overlie rocks of the Kansuki Formation (former Lower Mwashya), a carbonate unit containing volcaniclastic beds. New geochemical data confirm the continental rift context of this magmatism, which is contemporaneous with rift-related volcanism of the Askevold Formation (Nosib Group, Namibia). A gradational lithological transition between rocks of the Kansuki and the underlying Kanwangungu Formations, and similar petrological composition of these two formations, support the hypothesis that the Kansuki is the uppermost unit of the carbonate-dominated Dipeta/Kanwangungu sequence, and does not form part of the Mwashya Subgroup. Base metal deposits, mostly hosted in rocks of the Kansuki Formation, include weakly disseminated early-stage low-grade Cu–Co mineralisation, which was reworked and enriched, or initially deposited, by metamorphic fluids associated with the Lufilian orogenic event.

Lithostratigraphic position and petrographic characteristics of R.A.T. (“Roches Argilo-Talqueuses”) Subgroup, Neoproterozoic Katangan Belt (Congo)

The Neoproterozoic Katangan R.A.T. (“Roches Argilo-Talqueuses”) Subgroup is a sedimentary sequence composed of red massive to irregularly bedded terrigenous-dolomitic rocks occurring at the base of the Katangan succession in Congo. Red R.A.T. is rarely exposed in a continuous section because it was affected by a major layer-parallel décollement during the Lufilian thrusting. However, in a number of thrust sheets, Red R.A.T. is in conformable sedimentary contact with Grey R.A.T which forms the base of the Mines Subgroup. Apart from the colour difference reflecting distinct depositional redox conditions, lithological, petrographical and geochemical features of Red and Grey R.A.T. are similar. A continuous sedimentary transition between these two lithological units is shown by the occurrence of variegated to yellowish R.A.T. The D. Strat. “Dolomies Stratifiées” formation of the Mines Subgroup conformably overlies the Grey R.A.T. In addition, a transitional gradation between Grey R.A.T. and D. Strat. occurs in most Cu–Co mines in Katanga and is marked by interbedding of Grey R.A.T.-type and D. Strat.-type layers or by a progressive petrographic and lithologic transition from R.A.T. to D. Strat. Thus, there is an unquestionable sedimentary transition between Grey R.A.T. and D. Strat. and between Grey R.A.T. and Red R.A.T. The R.A.T. Subgroup stratigraphically underlies the Mines Subgroup and therefore R.A.T. cannot be comprised of syn-orogenic sediments deposited upon the Kundelungu (formerly “Upper Kundelungu”) Group as suggested by Wendorff (2000). As a consequence, the Grey R.A.T. Cu–Co mineralisation definitely is part of the Mines Subgroup Lower Orebody, and does not represent a distinct generation of stratiform Cu–Co sulphide mineralisation younger than the Roan orebodies.

The buried caldera of Misti volcano, Peru, revealed by combining a self-potential survey with elliptic Fourier function analysis of topography

This survey proposes a new approach to identify buried caldera boundaries of a volcanic cone, combining (1) a systematic elliptic Fourier functions (EFF) analysis on the contour lines based on the external shape of the edifice with (2) self-potential (SP) measurements on volcano flanks. The methodology of this approach is to investigate the relationships between (1) vertical morphological changes inferred from EFF analysis and (2) lateral lithological transition inside the edifice inferred from SP/elevation gradients. The application of these methods on Misti volcano in southern Peru displays a very good correlation. The three main boundaries evidenced by hierarchical cluster analysis on the contour lines coincide with the two main boundaries characterised by SP signal and with a secondary SP signature related with a summit caldera. In order to explain these results showing a very good correlation between morphologic and lithologic changes as function of elevation, caldera boundaries have been suggested. The latter would be located at an average elevation of (1) 4350–4400 m, (2) 4950–5000 m, and (3) 5500–5550 m. For the lowest boundary in elevation, the coincidence with the lateral extension of the hydrothermal system inferred from SP measurements suggests that caldera walls act as a barrier for lateral extension of hydrothermal systems. In the summit area, the highest boundary has been related with the summit caldera, inferred by a secondary SP minimum and geological evidence.