Plate Tectonic Setting Research Articles

Compositional and textural study of modern beach sands in the active volcanic area of the Campania region (southern Italy)

The association among volcanism, tectonics and sedimentation may be critical for interpreting the paleogeographic and plate tectonic settings of the past. In modern and ancient sedimentary systems, volcaniclastic fragment classes have been distinguished on the basis of texture and composition. Criteria adopted for their temporal and spatial correlation with source volcanic areas, in terms of type of volcanism and particle-forming processes are still not well constrained and often a topic of controversy and misinterpretation. Here, we present data for volcaniclastic fragments occurring in modern beach sands of the Campanian coast of southern Italy, where there are two active volcanic areas, the Phlegrean Fields and Mt. Vesuvius. Despite their geographical proximity, these volcanoes are characterized by different magma compositions and eruptive mechanisms. We set out to test whether this translates into different textural and compositional characteristics of their associated volcaniclastic detritus in order to better constrain the criteria relating the type of volcanism to the nature of volcaniclastic sand fragments in the sedimentary record. Along a ~100 km coastal stretch, from the Volturno River mouth to Sorrento, 72 sand samples were collected mainly in beach berm environments. Detrital modes were defined for the medium sand fraction of each sample, where monocrystalline and lithic grains are easily identified. The analyzed samples show a mixed sedimentary and volcanic provenance. Samples from the northernmost coastal stretch show a sedimentary petrofacies at the Volturno River mouth. Bacoli marks the transition between the Apennine sedimentary and Phlegrean volcanic petrofacies. Along the Portici-Sorrento coastal stretch, volcanic lithic fragments dominate. Leucite and sanidine crystal components indicate that detritus from the Phlegrean Fields extends south from Licola to Naples Bay, whereas the Vesuvius provenance extends from Naples/Portici south to Castellammare di Stabia. Phlegrean Fields and Mt. Vesuvius volcaniclastic petrofacies have been discriminated through a different distribution of both compositions of the volcanic lithic fragments, leucite and K-feldspars content. Volcanic lithics with lathwork texture and leucite, are Mt. Vesuvius provenance source sensitive detrital grains, whereas volcanic lithics with vitric texture and K-feldspars marks the transition to the Phlegrean Fields petrofacies. The results formalize three different sandy petrofacies which were mapped from north to south as Sedimentary, Phlegrean (volcanic) and Vesuvius (volcanic) petrofacies.

Heavy Minerals for Junior Woodchucks

In the last two centuries, since the dawn of modern geology, heavy minerals have been used to investigate sediment provenance and for many other scientific or practical applications. Not always, however, with the correct approach. Difficulties are diverse, not just technical and related to the identification of tiny grains, but also procedural and conceptual. Even the definition of “heavy minerals” is elusive, and possibly impossible. Sampling is critical. In many environments (e.g., beaches), both absolute and relative heavy mineral abundances invariably increase or decrease locally to different degrees owing to hydraulic-sorting processes, so that samples close to "neutral composition" are hard to obtain. Several widely shared opinions are misleading. Choosing a narrow size-window for analysis leads to increased bias, not to increased accuracy or precision. Only point-counting provides real volume percentages, whereas grain-counting distorts results in favor of smaller minerals. This paper also briefly reviews the heavy mineral associations typically found in diverse plate-tectonic settings. A mineralogical assemblage, however, only reproduces the mineralogy of source rocks, which does not correlate univocally with the geodynamic setting in which those source rocks were formed and assembled. Moreover, it is affected by environmental bias, and by diagenetic bias on top in the case of ancient sandstones. One fruitful way to extract information on both provenance and sedimentological processes is to look for anomalies in mineralogical–textural relationships (e.g., denser minerals bigger than lower-density minerals; harder minerals better rounded than softer minerals; less durable minerals increasing with stratal age and stratigraphic depth). To minimize mistakes, it is necessary to invariably combine heavy mineral investigations with the petrographic analysis of bulk sand. Analysis of thin sections allows us to see also those source rocks that do not shed significant amounts of heavy minerals, such as limestone or granite, and helps us to assess heavy mineral concentration, the “outer” message carrying the key to decipher the “inner message” contained in the heavy mineral suite. The task becomes thorny indeed when dealing with samples with strong diagenetic overprint, which is, unfortunately, the case of most ancient sandstones. Diagenesis is the Moloch that devours all grains that are not chemically resistant, leaving a meager residue difficult or even impossible to interpret when diagenetic effects accumulate through multiple sedimentary cycles. We have conceived this friendly little handbook to help the student facing these problems, hoping that it may serve the purpose.

Geochemical characteristics of the Upper Cretaceous to Lower Eocene sedimentary rocks from the Pieniny Klippen Belt (Western Carpathians, Slovakia): implications for tectonic setting, paleoenvironment and paleoclimate

This paper presents geochemical data on sedimentary rocks from the Upper Cretaceous to Lower Eocene formations of the Pieniny Klippen Belt (Western Carpathians, Slovakia). On the basis of geochemical analyses of 15 samples, the plate tectonic setting of sediment accumulation and paleodepositional environment were reconstructed, which may help to understand a part of evolution of this orographically remarkable zone. Four main rock groups were identified within the samples studied: fine- to medium-grained sandstones, siltstones or claystones, pebbly/sandy mudstones and shales. In the Al2O3*5-SiO2-CaO*2 ternary diagram, they displayed developed sand and mud flat characteristics with light rich clay content. According to high-silica and lowsilica multi-dimensional diagrams, the investigated rocks might have originated from a continental collision tectonic setting. The Zr/Rb values (0.76–6.47) mirrored a fluctuation of hydro energy during deposition of the sediments studied. Paleoclimate indexes, such as the C-value (0.03–0.49) and Sr/Cu (3.25–202.27), revealed arid to semiarid climatic conditions during deposition of the studied sediments. In addition, the chemical data were used to constrain paleoredox conditions of the depositional environment, which proved that these sediments had been deposited mainly beneath the oxic water column with relatively low paleoproductivity, as evidenced by P/Ti with an average of 0.13 and Ba/Al with an average of 34.30.

Geology, Petrology and Geochemistry of Volcanic Rocks Around Arba Minch, Southern Ethiopia

The study area is subpart of the southern main Ethiopian Rift and geographically bounded between UTM (Universal Transverse Mercator) coordinates of 662000m-669000m latitude and 336000m-341000m longitude. The main aim of this study is to investigate the geological, petrographic and geochemical properties of volcanic rocks around Arba Minch, southern Ethiopia. Both field and laboratory analytical techniques are employed to characterize minerals and the rock types. Eight thin section samples are prepared to study petrographic properties of minerals and five best representative samples are analyzed for whole rock chemistry using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). Petrographically, samples from the porphyritic basalt show phenocrysts of olivine and plagioclase within the fine-grained groundmass. There are rounded to subrounded holes or vesicles that appear white under plane polarized light observations and dark under cross polarized light observations which further confirms vesicular nature of the basalt. Thin section sample from rhyolite outcrop shows phenocrysts of K-feldspar and quartz within tiny fine-grained groundmass of feldspar minerals. Aphanitic basalt, Porphyritic basalt, vesicular basalt, rhyolite and dolerite are the rock units found in the study area. The geochemical results of the analyzed samples indicate silica ranges of (48.5-60.1)%. Based geochemical classifications made, rock units are classified into rhyolite, basalt, hawaiite and mugherite rock types. Trace element analytical results show all samples lie on the within plate tectonic setting. The rare earth and trace element ratios and spider diagrams confirmed that most samples are derived from magmas of asthenospheric source with little crustal contamination.

Long-term drainage system evolution in the Wabe Shebele River basin (SE Ethiopia - SW Somalia)

Large river systems play an important role in Earth dynamics since they exert an influence on geological, geomorphological, and geochemical processes. On the other hand, these landforms occur in a variety of topographic and plate tectonic settings and tend to persist for 107–108 yr, resisting variations in environmental conditions. Both modern and ancient big rivers configurations suggest that the persistence of such features is much longer on passive margins with long-lasting continental tilting and long-term rainfall without interference from continental glaciation, desertification, and volcanism. The literature on the geological evolution of river systems concentrates almost entirely on large-scale river basins. Very little research has been done to understand the evolution of smaller river networks that have a much lower persistence because they are more sensitive to climatic and tectonic changes. In this work we focused on the Wabe Shebele River basin (SE Ethiopia, SW Somalia) that drains the eastern slope of the Horn of Africa. It is a medium-scale drainage system (drainage area of ~105 km2) developed on a long-lived regional slope to the SE inherited from the Early Mesozoic and influenced by tectonic structures relative to a passive continental margin and slightly affected by volcanism. By taking into consideration the present topography (swath profiles, filtered topography, slope, local relief), the river network (river longitudinal profiles, channel gradient), and the ancient landforms present in the region, we demonstrate that the Wabe Shebele River basin, despite its medium scale, is a long-lived landform persisting at least since the Oligocene. The results show that even smaller river systems can have a long-term history in favorable tectonic and topographic conditions.

Petrogenesis of the NE Gondwanan uppermost Ediacaran-Lower Cretaceous siliciclastic sequence of Jordan: Provenance, tectonic, and climatic implications

Detrital framework modes of the NE Gondwanan uppermost Ediacaran-Lower Cretaceous siliciclastic sequence of Jordan are determined employing the routine polarized light microscope. The lower part of this sequence constitutes a segment of the vast lower Paleozoic siliciclastic sheet flanking the northern Gondwana margin that was deposited over a regional unconformity truncating the outskirts of the East African orogen in the aftermath of the Neoproterozoic amalgamation of Gondwana. The research aims to evaluate the factors governing the detrital light mineral composition of this sandstone. The provenance terranes of the Arabian craton controlled by plate tectonics appear to be the primary factor in most of the formations, which could be either directly inferred employing Dickinson‘s compositional triangles or implied utilizing the petrographic data achieved and the available tectonic and geological data. The Arabian-Nubian Shield constitutes invariably the craton interior or the transitional provenance terrane within the NE Gondwana continental block that consistently supplied sandy detritus through northward-flowing braided rivers to all the lower Paleozoic formations. On the other hand, the Lower Cretaceous Series received siliciclastic debris, through braided-meandering rivers having same northward dispersal direction, additionally from the lower Paleozoic and lower-middle Mesozoic platform strata in the Arabian Craton. The formations making about 50% of the siliciclastic sequence represent a success for Dickinson‘s plate tectonics-provenance approach in attributing the detrital framework components primarily to the plate tectonic setting of the provenance terranes. However, even under this success, the varying effects of the other secondary sedimentological and paleoclimatological factors are important and could be crucial. The inapplicability of this approach to infer the appropriate provenance terranes of the remaining formations could be ascribed either to the special influence of local intracratonic syn-rift rhyolitic extrusions, where their plate tectonic setting is not represented by the standard plate tectonics-provenance diagrams, or to the rather unusual effect of the Late Ordovician glacial event.

A global framework for the Earth: putting geological sciences in context

Abstract We propose a global framework for the Earth system to facilitate communication between the geoscience community, the public and policy makers. Geoscience research aims to understand the history and evolution of the Earth system. This combines the non-living and living parts of the Earth, especially through interactions of the lithosphere, biosphere and atmosphere as well as the other parts of the system, such as the asthenosphere, core and extraterrestrial influences. Such research considers a system that spans scales from microscopic (micrometer scale) to megascopic (many 1000 s of km scale), and from milliseconds to millions of years. To connect different parts of this immense system, we habitually use a wide range of ad hoc geological frameworks, systems and geological environment models, where different processes and features operate and combine. In consequence, one way to judge the significance of our work, and to increase its value, is to assess how the elements studied are integrated within the whole Earth system. This allows us to see what implications any study has for this greater Earth system. To do this successfully, our research needs a standard global framework to assess a study's relevance. However, such a global framework does not formally exist, and so this article looks at existing examples and proposes one that can systematically place research into a global geological context. This proposed framework has the advantage of being useful for communicating geological processes to other disciplines, and can be used for any type of Earth (or planetary) environment. This framework is a fundamental tool for geoscience communication and for outreach, especially through geological heritage (geoheritage). Geoheritage concerns the valuing and protection of geoscience and geological sites, and is a vital tool for communicating geoscience. It can be used to communicate our knowledge of global change, providing, through landscapes and outcrops, a story that renders the concepts and advances of geoscience accessible. Like our basic research, the concept of geoheritage evolves as our understanding of the Earth progresses, and these dual changes can be explained with the global framework. Geoheritage is a global activity and it needs a global framework to put sites into context. A revision of the UNESCO geological thematic studies was called for by the World Heritage Committee in 2014 (decision: 38 COM 8B.11), and this can be done with the input from the full geoscience community using this global geological framework. Thus, for research, geoscience policy and for geoheritage, a global framework is now needed. The proposed framework can place any site in its geological environment, related to its lithospheric plate tectonic setting and its history. The framework has a solid-earth bias (lithosphere), but includes all other spheres, such as the biosphere and anthropogenic activity. Extraterrestrial influences, like solar variations and metorite impacts are included. The framework is phenomenological, due to the necessity of grouping the features that we see, but these phenomena provide evidence of processes that we cannot see. The basic format is a table, a sketch of the Earth and a system diagram, three complementary and most powerful ways of depicting a system. A timeline, or stratigraphic column can be included, to show the evolution of geological history, and the table can be used as a ‘game board’ where one site migrates across from one set of conditions to another over time. The global framework allows any research site, area or subject to be set in the Earth's system, in a way that gives it context, allows comparisons and provides significance. We suggest that it can be a template for an internationally accepted version used to consolidate the necessary geoscience – geoheritage link and promote outreach.

From Cadomian magmatic arc to Rheic ocean closure: The geochronological-geochemical record of nappe protoliths of the Münchberg Massif, NE Bavaria (Germany)

The Münchberg Massif in northeastern Bavaria, Germany is an allochthonous metamorphic nappe complex within the Saxothuringian Zone of the Variscan orogen. From top to bottom it consists of four major units: Hangend-Serie, Liegend-Serie, Randamphibolit-Serie and Prasinit-Phyllit-Serie, which show an inverted metamorphic gradient of eclogite- to amphibolite-facies (top) to greenschist-facies (bottom) and are separated from each other by thrust faults. New geochemical and U-Pb zircon data indicate that the four units host metasedimentary and meta-igneous rocks, which were formed at different time and in distinct geotectonic settings during the evolution of the Saxothuringian terrane between 550 and 370Ma. Mafic and felsic protoliths of the Hangend-Serie result from a bimodal magmatism in an evolved oceanic to continental magmatic arc setting at about 550Ma. These rocks represent relics of the Cadomian magmatic arc, which formed a cordillera at the northern margin of Gondwana during the Neoproterozoic. The Liegend-Serie hosts slivers of granitic orthogneisses, emplaced during magmatic events at c. 505 and 480Ma, and Early Palaeozoic paragneisses, with our samples deposited at ≤483Ma. Ortho- and paragneisses were affected by an amphibolite-facies metamorphic overprint at c. 380Ma. Granite emplacement and sediment deposition can be related to the separation of the Avalonia microterrane from the northern Gondwana margin. Amphibolite protoliths of the Randamphibolit-Serie emplaced at c. 400Ma. They show N-MORB to E-MORB signatures, pointing to their formation along an oceanic spreading centre within the Rheic ocean. Mafic igneous rocks in the Prasinit-Phyllit-Serie emplaced at nearly the same time (407–401Ma), but their calc-alkaline to tholeiitic character rather suggests formation in an intra-oceanic island arc/back arc system. This convergent margin lasted for about 30Ma until the Late Devonian, as is suggested by a maximum deposition age of 371Ma of associated phyllites, and by metamorphic Ar-Ar ages of 374–368Ma. The timing of the different magmatic and sedimentary events in the Münchberg Massif and their plate tectonic settings are similar to those estimated for other Variscan nappe complexes throughout Europe, comprising the French Massif Central and NW Spain. This similarity indicates that the Münchberg Massif forms part of an European-wide suture zone, along which rock units of different origin were assembled in a complex way during the Variscan Orogeny.

Crustal fluid flow in hot continental extension: tectonic framework of geothermal areas and mineral deposits in western Anatolia

Abstract Lithospheric thinning and crustal extension have shaped the Alpine orogen in western Anatolia since the late Oligocene, resulting in the denudation of one of Earth's largest metamorphic core complexes, the Menderes Massif. We review locations and characteristics of geothermal fields and of Miocene mineral deposits in the context of crustal structure and geodynamic processes. Thermal spring locations show a close spatial association with active fault zones; the largest geothermal areas are located in the widest graben and at fault intersections, but show little relation to volcanic activity. During the first stage of tectonic denudation in the Miocene, epithermal, porphyry-type gold and structurally controlled base-metal deposits formed synchronously with K-rich volcanic and plutonic complexes in the northern Menderes Massif. Depositional environments favoured the formation of lignite, sedimentary uranium and borate deposits. Throughout this phase of extension in a hot continental setting, secondary porosity caused by brittle faulting of metamorphic basement rocks provided the key pathways for fluids and magmas. Although the Menderes Massif has remained in a similar position relative to active plate boundaries from the Miocene to the present, three significant changes in subcontinental mantle dynamics affected the nature of hydrothermal flow. First, the partial removal of lithospheric mantle changed the primary source component of magmatic rocks and metals from metasomatized lithosphere mantle to asthenospheric mantle. Secondly, surface uplift and progressive crustal extension led to segmentation of the Miocene land surface along NNE–SSW- and east–west-orientated fault zones, which changed the overall structural control on crustal permeability. Finally, hydrothermal flow changed from locally magmatic driven, to focused flow of topographically and thermally driven fluids in the crust, with high background heat flow caused by regional upwelling of the asthenosphere. The Menderes Massif is a continental tectonic domain that has experienced rapid thinning of lithospheric mantle and crustal extension in an overall convergent plate tectonic setting. The tectonic and geodynamic framework for evolving hydrothermal activity in western Anatolia may be applicable to other ore-forming systems in hot, extending continental crust in Earth's history.

Sandstone memory of a Late Paleozoic continental arc in southeast China (Lower Yangtze region)

Provenance analysis was carried out on Paleozoic siliciclastic rocks exposed in the South China Block (Lower Yangtze region, Chaohu and Nanjing areas) by sandstone petrography, U-Pb detrital-zircon geochronology, and zircon Hf isotopes. Sandstones of the Silurian Fentou Formation are feldspatho-quartzose with a few igneous and low-grade metamorphic rock fragments. Sandstones of the Devonian Wutong Formation are quartzose with a few quartz-rich metamorphic rock fragments. The Fentou and Wutong formations share similar detrital-zircon age spectra, with peaks mainly at 465−420 and 838−812 Ma. Zircons of Early Paleozoic age were with all likelihood derived from magmatic rocks associated with the Kwangsian orogeny, documented by a widespread unconformity across the South China Block. Neoproterozoic zircons were probably shed from Proterozoic basement of the Wuyi terrane in the Cathaysian block, interpreted to have been uplifted south of the Lower Yangtze region during the Kwangsian orogeny. The Permian Longtan Formation in Chaohu area and Nanjing area are litho-quartzose and feldspatho-quartzose, respectively, with dominant felsic and intermediate volcanic lithic fragments, indicating that detrital source areas did not change markedly after the Kwangsian orogeny. Zircon-age spectra from the Longtan Formation in the Chaohu area display a diagnostic peak at ~ 1866 Ma, indicating provenance from the Paleoproterozoic basement of the Wuyi terrane. Instead, detrital zircons in the Longtan Formation in the Nanjing area display peaks at ~ 445 and ~ 970 Ma, similar to those in the underlying Paleozoic sandstones. The Wuyi terrane in the Cathaysian block thus remained as a major source area for long, as supported by independent sedimentological and seismic evidence. The Longtan Formation in both Chaohu and Nanjing areas also contain detrital zircons with ages distributed continuously from 341 to 254 Ma and a major peak around 270 Ma, reflecting renewed intermediate and felsic magmatism in the source area. These zircons have e Hf( t ) ranging from −19.8 to +14.9, indicating melting of old crust and mixing with crustal magmas. Such a continuously magmatic source supplied sediments all across the South China Block from east to west. These and other lines of evidence including probability accumulation-curve analysis favor deposition in a convergent plate-tectonic setting. We suggest that source rocks were part of a continental arc developed along the southeast coast of China during the Late Paleozoic, shedding sediments to as far as north as the northern South China Block. According to previous studies, such continental arc would be related to the subduction of the paleo-Pacific plate. Magmatism generated at first by melting of old crust was mixed later on with felsic and intermediate magma newly generated during mantle upwelling and associated uplift. The continental arc remained as the main detrital source to the Lower Yangtze region and other parts of the South China Block until it was eventually denudated or buried by the Mesozoic times.

Geochemistry and Zircon U–Pb age of the Yao’an pseudoleucite porphyry, Yunnan Province, China

The Yao’an Pb–Ag deposit, located in the Chuxiong Basin, western Yangtze Block, is an important component of the Jinshajiang–Ailaoshan alkaline porphyry–related polymetallic intrusive belt. This complex suite of rock bodies includes a vein of pseudoleucite porphyry within deposits of syenite porphyry and trachyte. The pseudoleucite is characterized by a variable greyish, greyish-white, and greyish-green porphyritic texture. Phenocrysts are mainly pseudoleucite with small amounts of alkali feldspar and biotite. In an intense event, leucite phenocrysts altered to orthoclase, kaolinite, and quartz. Both the pseudoleucite porphyry and the syenite porphyry samples were typical alkali-rich, K-rich, al-rich rocks with high LaN/YbN ratios; enriched in light rare earth elements and large-ion lithophile elements, and depleted in high field strength elements; and with strongly negative Ta, Nb, and Ti (TNT) anomalies and slightly negative Eu anomalies—all characteristics of subduction-zone mantle-derived rock. We obtained a LA-ICP-MS zircon U–Pb age of 34.1 ± 0.3 Ma (MSWD = 2.4), which is younger than the established age of the Indian and Eurasian Plate collision. The magma derived from a Type-II enriched mantle formed in a post-collisional plate tectonic setting. The geochemical characteristics of the Yao’an pseudoleucite porphyry are powerful evidence that the porphyry’s development was closely linked to the Jinshajiang–Ailaoshan fault and to the Indian-Eurasian collision.

Mesoproterozoic base metal sulphide deposits in the Namaqua Sector of the Namaqua-Natal Metamorphic Province, South Africa: a review

The Mesoproterozoic Namaqua Sector of the east-west trending Namaqua-Natal Metamorphic Province (NNMP) located in the Northern Cape Province is underlain by supracrustal rocks and abundant felsic intrusive rocks that have been subjected to medium to high grade metamorphism and polyphase deformation. It is the host to world class base metal sulphide resources that have been mined for more than a century. Most of these mines are no longer in production such that the Cu-Pb-Zn-Ag mines in the Aggeneys-Gamsberg Ore District (A-GOD), with a total output of 1.8 million ton (Mt) per annum, are the only current significant producers. Development of the supergiant Gamsberg zinc deposit commenced in 2015 and will add a further 4 Mt per annum (250 000 t of zinc) to the output with the mine eventually becoming one of the larger zinc producers in the world. The Namaqua Sector of the NNMP has been subdivided into several lithologically distinct, structural discontinuity bounded Subprovinces and Terranes, each with its characteristic base metal sulphide deposits. The most extensive and best mineralized of these is the Bushmanland Subprovince where the ~1300 Ma stratabound polymetallic Broken Hill-type meta-SEDEX deposits contain total resources exceeding 500 Mt of Zn-Pb-Cu-Ag ore. Further east much smaller and subeconomic stratabound deposits occur at Putsberg (Cu-Zn-Pb-Ag) and Adjoining Geelvloer (Zn-Cu-Pb-Ag-Au). Magmatic ore deposits include minor magnetite-copper skarn/iron-oxide-copper-gold (IOCG) deposits associated with syntectonic granitoids (~1200 Ma) in the east and small syntectonic gabbronorite hosted Ni-Cu-(Co) occurrences at Hondekloof (~1170 Ma) to the southwest. The mined-out Okiep Copper District in the west hosts an abundance of small, syn- to late-tectonic mafic cupriferous intrusions (~1030 Ma), which collectively contributed some 100 Mt of ore yielding ~2 Mt of blister copper. The Areachap Terrane in the far eastern part of the Namaqua sector of the NNMP has a number of stratabound volcanogenic massive sulphide (VMS) Zn-Cu (Ag-Au) deposits of variable tonnage and hosted by intermediate to mafic meta-volcanic and meta-sedimentary rocks (~1300 Ma). The largest of these, the world class Prieska deposit (47 Mt) has been mined out. The remainder of the deposits are small (1.5 to 9 Mt), of low grade and uneconomic. The Terrane has been intruded by a suite of poorly mineralized mafic to ultramafic rocks (~1180 Ma) of which the high tonnage (114 Mt) but low grade Jacomynspan Ni-Cu deposit, is the most significant. The Richtersveld and Gordonia Subprovinces are poorly mineralized whereas the Kaaien Terrane has no Mesoproterozoic mineralization of any significance. The Gordonia Subprovince hosts the small magmatic Oup Ni-Cu-(Co) deposit (~1150 Ma) and is the only massive sulphide occurrence in the Subprovince. Both domains however, have minerals deposits associated with the regionally extensive post-tectonic Namaqualand pegmatite belt (~1000 Ma) which is a resource of beryl, tantalite, wolframite, cassiterite, micas, and feldspar. The Mesoproterozoic stratabound deposits of the Namaqua Sector of the NNMP are all of similar age and terrane bounded. Their different characteristics are the result of relative position within a subduction related plate tectonic setting. The siliciclastic hosted deposits of the Aggeneys-Gamsberg Ore District are interpreted to have formed in a continental sub-basin whereas the more chemogenic-volcanogenic hosts of the Putsberg and Adjoining Geelvloer deposits have the lithological characteristics associated with a back arc basin. The VMS deposits of the Areachap Terrane are linked to a mature island arc and were located more proximal to the subduction zone. The younger magmatic, mainly Ni-Cu and Cu deposits are syn- to post-tectonic, formed over a timespan of ~150 Ma, do not appear to be terrane specific and are considered to have been generated from basaltic magma during partial melting of subcontinental lithospheric mantle. Localization of intrusive suites can be the result of crustal thinning or proximity to major zones of dislocation such as the Pofadder Shear Zone and Boven Rugzeer Shear zone. The exploration success within the Namaqua Sector of the NNMP has been significant and effectively all known deposits, apart from those of the Okiep Copper District, were discovered using basic airborne and ground geophysical methods, soil geochemistry, lithological mapping and limited remote sensing during the 1970’s and early 1980’s. Future exploration should focus on the use of advanced airborne, ground and down-the-hole geophysics supported by remote sensing, soil and lithogeochemistry, the use of heavy mineral tracers and a workable geological ore deposit model. Over the last 35 years, the area has shown a decrease in the number of operating mines and commodity production, in particular copper. Although several areas show exploration potential, the Aggeneys-Gamsberg Ore District offers the best brown fields target whereas initiation of mining of the known smaller deposits is dependent on the revival of the currently depressed commodity demand and price.

Radioactivity and geochemical characteristics of post collision granites, west Wadi Murrah area, South Eastern Desert, Egypt

Wadi Murrah area is located in the southern Eastern Desert of Egypt comprising four distinct lithological groups: Ophiolitic assemblage (dismembered ophiolitic rocks and ophiolitic mélange), arc assemblage (metavolcanics, metagabbros and older granites), post-collision granites (monzogranite, syenogranite and alkali feldspar granite), dykes and quartz veins. The post collision granites are originated from metaluminous, subalkaline magma in extensional suite. They were developed in within plate tectonic setting and have A2-type character which means that these granites were generated from apparent crustal source via fractional crystallization. The similar chondrite-normalized REEs distribution patterns of these granites reveal that they may be derived from the same source magma. Uranium and thorium were mainly controlled by primary magmatic processes. They are mainly present in accessory minerals such as allanite, zircon, sphene and apatite.

Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

The plate tectonic setting in which proto-ophiolite ‘oceanic’ lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling. Rapid deep upper-mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto-ophiolite from as deep as the mantle transition zone, thereby explaining diamonds and other 10–15 GPa UHP phases in Tibetan ophiolites.

Characterization of a Post Orogenic A-Type Granite, Gabal El Atawi, Central Eastern Desert, Egypt: Geochemical and Radioactive Perspectives

The alkali feldspar granite of Gabal El Atawi is post orogenic granite originated from subalkaline magma in extensional suite. It is developed within plate tectonic setting and has A2-type character which generated from apparent crustal source. The petrographic, geochemical and radioactive characteristics of El Atawi granite meet and fulfill the requirements of being fertile granite and it can be considered as promising uraniferous granite. Fluid inclusion studies of the altered granite elucidated two different solutions acting on the host granitic pluton. The first is NaCl-CaCl low temperature fluid with a wide range of salinity. The second is high temperature and salinity Fe-Mg-Na chloride solution. Different fractures in the granite acted as good channels for the hydrothermal fluids that leached uranium from its bearing minerals disseminated all over the host granite and redeposited it in the alteration zones.

Geology of bitumen and heavy oil: An overview

On a world-wide basis there is an estimated 5.6 trillion barrels of bitumen and heavy-oil resources which occur in over 70 different countries, with most of the heavy-oil in Venezuela and most of the bitumen in Canada. The most common plate-tectonic settings in which the heavy-oil and bitumen are found are in continental multi-cyclic marginal basins and in continental rift basins.Heavy oil and bitumen resources are largely a result of natural degradation of formerly conventional oil accumulations. The natural degradation for most is biologic in origin, with the result that the majority of heavy-oil and bitumen deposits is characteristically in younger rocks (Cretaceous and younger) and at shallow depths (usually <200m, up to 2000m maximum). At a deposit-scale it is necessary for communication to have been established between the surface and subsurface to facilitate the biologic contamination of the pre-existing light petroleum reservoirs. Communication is through loss of caprock integrity, associated with major erosion (creation of unconformities), faulting, fracturing; and, for carbonate host-rocks, karstification. Two cases are discussed to illustrate these geologic processes: 1) the influence of the sub-Cretaceous unconformity on the Cretaceous oil-sands in the Western Canada Sedimentary Basin; and, 2) the influence of faulting, fracturing and karstification on the Grosmont carbonate-bitumen deposit of northeastern Alberta.

Precambrian plate tectonic setting of Africa from multidimensional discrimination diagrams

New multi-dimensional discrimination diagrams have been used to identify plate tectonic setting of Precambrian terrains. For this work, nine sets of new discriminant-function based multi-dimensional discrimination diagrams were applied for thirteen case studies of Precambrian basic, intermediate and acid magmas from Africa to highlight the application of these diagrams and probability calculations. The applications of these diagrams indicated the following results: For northern Africa: to Wadi Ghadir ophiolite, Egypt indicated an arc setting for Neoproterozoic (746 ± 19 Ma). For South Africa: Zandspruit greenstone and Bulai pluton showed a collision and a transitional continental arc to collision setting at about Mesoarchaean and Neoarchaean (3114 ± 2.3 Ma and 2610-2577 Ma); Mesoproterozoic (1109 ± 0.6 Ma and 1100 Ma) ages for Espungabera and Umkondo sills were consistent with an island arc setting. For eastern Africa, Iramba–Sekenke greenstone belt and Suguti area, Tanzania showed an arc setting for Neoarchaean (2742 ± 27 Ma and 2755 ± 1 Ma). Chila, Bulbul-Kenticha domain, and Werri area indicated a continental arc setting at about Neoproterozoic (800-789 Ma); For western Africa, Sangmelima region and Ebolowa area, southern Cameroon indicated a collision and continental arc setting, respectively for Neoarchaean (∼2800–2900 Ma and 2687-2666 Ma); Finally, Paleoproterozoic (2232-2169 Ma) for Birimian supergroup, southern Ghana a continental arc setting; and Paleoproterozoic (2123-2108 Ma) for Katiola-Marabadiassa, Côte d'Ivoire a transitional continental arc to collision setting. Although there were some inconsistencies in the inferences, most cases showed consistent results of tectonic settings. These inconsistencies may be related to mixed ages, magma mixing, crustal contamination, degree of mantle melting, and mantle versus crustal origin.

The Litchfield Pluton in South-Central Maine: Carboniferous Alkalic Magmatism in northern New England, USA

The Litchfield pluton is a poorly exposed 7 km2 composite alkalic intrusive complex that cuts previously deformed and metamorphosed Silurian turbidites in south-central Maine. The pluton includes a variety of alkaline syenites, including the type locality of “litchfieldite”, a coarse-grained cancrinite, sodalite, and lepidomelane bearing nepheline syenite first recognized over 150 years ago and common in many petrologic collections. A new U-Pb zircon age of 321 ± 2 Ma from the nepheline syenite is interpreted to represent the crystallization age of the plutonic complex. A new biotite 40Ar/39Ar age of 239 ± 1 Ma from the syenite is similar to previously published mica ages from the surrounding country rocks and dates the time of regional cooling in the area below ~ 300°C. Whole rock geochemical analyses from rocks of the Litchfield pluton are compatible with strongly alkaline A-type granitoid rocks that formed in a within plate or continental rift tectonic setting. The age and geochemical characteristics of the alkalic igneous rocks near Litchfield are consistent with a model that invokes the generation of a small volume of alkalic magma beneath south-central Maine during a period of Carboniferous transcurrent tectonism in the northern Appalachian orogen.

Geochemistry of sedimentary rocks from Permian–Triassic boundary sections of Tethys Himalaya: implications for paleo-weathering, provenance, and tectonic setting

The geochemical characteristics of two sections—the Permian–Triassic boundary (PTB) Guryul Ravine section, Kashmir Valley, Jammu and Kashmir, India; and the Attargoo section, Spiti Valley, Himachal Pradesh, India—have been studied in the context of provenance, paleo-weathering, and plate tectonic setting. These sections represent the siliciclastic sedimentary sequence from the Tethys Himalaya. The PTB siliciclastic sedimentary sequence in these regions primarily consists of sandstones and shales with variable thickness. Present studied sandstones and shales of both sections had chemical index of alteration values between 65 and 74; such values reveal low-to-moderate degree of chemical weathering. The chemical index of weathering in studied samples ranged from 71 to 94, suggesting a minor K-metasomatism effect on these samples. Plagioclase index of alteration in studied sections ranged from 68 to 92, indicating a moderate degree of weathering of plagioclase feldspars. The provenance discriminant function diagram suggests that the detritus involved in the formation of present studied siliciclastic sedimentary rocks fall in quartzose sedimentary and felsic igneous provenances. These sediments were deposited in a passive continental margin plate tectonic setting according to their location on a Si2O versus K2O/Na2O tectonic setting diagram.

Grenvillian-aged reworking of late Paleoproterozoic crust of the southern North Australian Craton, central Australia: Implications for the assembly of Mesoproterozoic Australia

LA–ICP–MS U–Pb monazite and zircon geochronology from metapelites and migmatitic orthogneiss along the central southern margin of the North Australia Craton (NAC), central Australia, reveal the presence of a regional-scale Grenvillian-aged (ca. 1130Ma) deformation system. Grenvillian-aged deformation extends over a strike length of over 110km from the southern Aileron Province in the vicinity of Alice Springs to the Teapot Granite Complex within the Warumpi Province. There is also evidence for Grenvillian-aged migmatisation and resetting of monazite further west in the Mount Liebig area, which may extend the Grenvillian-aged footprint to a strike-length of at least 250km. Deformation associated with Grenvillian-aged reworking produced map-scale east–west trending folds and strongly foliated, steeply dipping shear zones that define the structural architecture of the interface region between the Aileron and Warumpi Provinces. Shallow, westerly-plunging folds associated with partial melting have evolved into shear zones and mylonites that record south-side up movement. Phase equilibria modelling of age-constrained garnet–biotite±sillimanite±cordierite-bearing metapelites from the southern Aileron and Warumpi Provinces suggest Grenvillian-aged metamorphism reached temperatures in the range of 775–820°C and pressures of 5–5.5kbar, corresponding to thermal gradients of ∼130–165°Ckbar−1. The extensive Grenvillian-aged system reworks and overprints late Paleoproterozoic (ca. 1650–1630Ma) high-grade metamorphic rocks. The Grenvillian-aged system occurs above a south dipping lithospheric-scale interface that has been geophysically imaged to depths of at least 200km. This feature was interpreted to represent a fossil subduction zone of late Paleoproterozoic age. However, the presence of regional-scale Grenvillian-aged deformation in the crust above this feature suggests that it may instead be Grenvillian-aged. If that is the case, deformation along the southern margin of the NAC may record suturing of the NAC with the Grenvillian-aged Musgrave Province to the south. This would effectively place the long-lived, Late Mesoproterozoic ultrahot orogen that is the Musgrave Province in southern central Australia in an upper plate tectonic setting, linked to the convergence of the NAC with the South Australian Craton.