OPENING LECTURE & KEYNOTE TALKS

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

OPENING LECTURES

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

OUTLINES OF WESTERN ANATOLIAN GEOLOGY

Yücel Yılmaz

İstanbul Technical University, Department of Geology

Kadir Has University

E-mail: yyilmaz@khas.edu.tr

The sequence of Western Anatolia may be divided into two parts: the lower association and the upper association. The lower association is a tectonic mosaic consisting of the different tectonic entities. From north to south these are, the Sakarya continent, the İzmir-Ankara ophiolitic suture, the Menderes Massif and the Taurides.

The Sakarya continent is a narrow continental fragment that is delimited along the northern and the southern boundaries by the ophiolitic suture zones. The İzmir-Ankara ophiolitic suture zone is the remnant of the Tethyan ocean that was totally consumed between the Sakarya continent and the Taurides. The Menderes Massif is a metamorphic complex that represents the northern margin of Tauride-Anatolide Platform.

The rocks, which will be described as the upper association or the cover makes up the succession which was formed following the final amalgamation of the tectonic entities mentioned above. These rocks are post Oligocene in age, and consist essentially of continental deposits and also partly coeval and widespread volcanic rocks.

Initially north-south trending graben basins formed during Early Miocene time. These extensional openings associated with approximately north-south trending oblique slip faults provided access for calc-alkaline, hybrid magmas to reach the surface. During the Late Miocene time two major breakaway faults began to form in the central part of western Anatolia. With respect to these low angle normal faults the lower plate was uplifted to form the Bozdağ Horst . To the end of late Miocene and beginning of the Early Pliocene the region passed through a severe phase of denudation, which produced a region wide flat lying erosional surface. The north-south extension was rejuvenated immediately after this period during which the erosional surface was disrupted and the Lower and Upper Miocene rocks were cut by approximately east-west trending normal faults .This tectonic event which is still continuing presently has led to the development of the present- east-west trending graben depressions during Plio-Quaternary time.

The data available on the timing and history of development for the east-west grabens are conflicting. There are ongoing debates on two major problems associated with the geology of the graben regions and the related structures: (1) when did the grabens begin to develop? (2) what event triggered their initiation?

In the region, three different groups of magmatic rocks may readily be distinguished: a plutonic association, an intermediate volcanic association and a basaltic association. The plutonic rocks are dated from 35 to 18m.y. The intermediate volcanic rocks formed partly contemporaneously with the plutonic rocks and are dated from 30 to 15m.y. These two groups also show close spatial and temporal association. There appears to be a brief non -volcanic interval between 15m.y and 10m.y. The magmatic activity was rejuvenated approximately 10m.y ago for a further 6m.y., continued until 4m.y. ago and then disappeared. During this late phase, the basaltic lavas, which were nearly absent previously, were formed as the dominant volcanic rocks. The only exception to this is the Kula volcanic province,which were still active during the pre historical times.

The Menderes massif is the major metamorphic culmination of Western Anatolia. It extends from the İzmir-Ankara suture in the north to the Kale-Tavas molasse basin in the south. It is roughly elliptical, with a NE-SW oriented long axis. The Massif has a complex internal structure and lithological distribution. The lowermost tectonic unit is made up of high grade gneisses and schists, commonly regarded as the Pan African representative of the Massif, The lower grade (the greenschist facies) schists, marbles and phyllites, and the lower greenschist facies recrystallized limestones and associated meta sedimentary rocks commonly cover the high grade rocks, except where the later structural rearrangement interrupt this ordering.. The cover rocks are regarded as identical units to that of the Tauride- Anatolide platform.

There are a number of radiometric and paleontological data obtained from the metamorphic rocks, and during the recent years a number of detailed studies were conducted on the Massif. Despite this, the age, mechanism of generation and development of the Massif are still widely debated.

İn this presentation major geological features and the ongoing debates on the main geological issues of western Anatolia will be summarized and some new solutions will be proposed.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

ACTIVE TECTONICS OF THE AEGEAN: Earthquake Source Parameters and Numerical Simulation of Historical Tsunamis in the Eastern Mediterranean

Tuncay Taymaz

İstanbul Technical University, the Faculty of Mines,

Department of Geophysical Engineering, Maslak TR-34469, İstanbul, Turkey

E-mail: taymaz@itu.edu.tr

Anatolia is a complex blend of multiple tectonic regimes controlled by the interaction of the Arabian, African, and Anatolia plates in one of the most rapidly deforming regions on Earth. Estimates of regional deformation and major fault movements from GPS measurements divide the area into a few major geodynamic regions including the N-S extensional region in western Turkey, a region of strike-slip extension in the northwest, the stable central interior with < 2mm/yr of internal deformation that is bound by the North Anatolian and East Anatolian fault, and a region of distributed strike-slip deformation in eastern Turkey. This diversity of neotectonic environments makes Anatolia a natural laboratory to study a variety of tectonic processes including continent-continent collision, continental escape, continental thinning, and subduction termination. In addition, the Aegean region presents an excellent opportunity to study and to constrain better quantitative models for continental deformation over a wide range of temporal and spatial scales observed in the area, explicitly on the dynamic processes of subduction and the causes and consequences of trench roll-back. Such dynamic processes are not directly measurable but must be inferred from a combination of geodynamic modeling and observational constraints, such as on slab geometry and density, the surface expression of subduction through development of topography and geologic features, and the temporal evolution of the Aegean system as a whole.

The improved focal mechanisms of earthquakes, constrained by P and SH body wave modeling as well as by first motions, show that the faulting in the western part of the Aegean region is mostly extensional in nature, on normal faults with a NW to WNW strike and with slip vectors directed NNW to NNE. There is accompanying evidence from palaeomagnetism that this western region rotates clockwise relative to stable Europe. In the central and eastern Aegean, and in NW Turkey, distributed right-lateral strike–slip is more prevalent, on faults trending NE to ENE, and with slip vectors directed NE. Palaeomagnetic data in this eastern region is more ambiguous, but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations, relative to Europe, in the south. The strike–slip faulting that enters the central Aegean from the east appears to end abruptly in the SW against the NW-trending normal faults of Greece. The kinematics of the deformation is controlled by three factors: the westward motion of Turkey relative to Europe; the continental collision between NW Greece–Albania and the Apulia–Adriatic platform in the west; and the presence of the Hellenic subduction zone to the south. As the right-lateral slip on the North Anatolian Fault enters the Aegean region it splays out, becoming distributed on several parallel faults. The continental shortening in NW Greece and Albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed E–W right-lateral shear, and thus leads to E–W shortening in the northern Aegean, which is compensated by N–S extension as the southern Aegean margin can move easily over the Hellenic subduction zone. The dynamics of the system, once initiated, is self-sustaining, being driven by the high topography in eastern Turkey and by the roll-back of the subducted slab beneath the southern Aegean. The geometry of the deformation resembles the behavior of a system of broken slats attached to margins that rotate. In spite of its extreme simplicity, a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the Aegean region, including: the slip vectors and nature of the faulting in the eastern and western parts; the senses and approximate rates of rotation; the overall extensional velocity across the Aegean; and the distribution of strain rates, as seen in the seismicity and topography or bathymetry, and using geodetic measurements.

Teleseismic inversion results showed that earthquakes along the Hellenic subduction zone can be classified into three major categories: [1] focal mechanisms of the earthquakes exhibiting E–W extension within the overriding Aegean plate; [2] earthquakes related to the African–Aegean convergence; and [3] focal mechanisms of earthquakes lying within the subducting African plate. Normal faulting mechanisms with left-lateral strike slip components were observed at the eastern part of the Hellenic subduction zone, and we suggest that they were probably concerned with the overriding Aegean plate. However, earthquakes involved in the convergence between the Aegean and the eastern Mediterranean lithospheres indicated thrust mechanisms with strike slip components, and they had shallow focal depths (h < 45 km). Deeper earthquakes mainly occurred in the subducting African plate, and they presented dominantly strike slip faulting mechanisms. Slip distributions on fault planes showed both complex and simple rupture propagations with respect to the variation of source mechanism and faulting geometry. Low stress drop values (Δσ < 30 bars) for all earthquakes implying typically interplate seismic activity in the region.

Furthermore, in this presentation potential source regions and Tsunami generation in the Mediterranean will be summarized and new numerical simulations will be presented since historical documents provide valuable information about earthquakes and tsunamis. Tsunami is a very large sea wave triggered by underwater earthquake, volcanic activities or landslides. These waves have unusually long-wavelength in excess of 100 kms, generated in the open ocean/sea and transformed into a series of catastrophic oscillations on the sea surface close to coastal zones. There is a long record of tsunami occurrences and damaging tsunamis have been observed repeatedly in the oceans and seas. The sources of tsunamis are still active and tsunamis are expected to occur in the future. Tsunamis could be even more catastrophic than past events, due to steadily growing occupation of the coasts for the economic development of the coastal countries in the last fifty years. Protection from natural disasters and mitigation of their effects on environment and societies are becoming more important issues all over the world. Moreover, there have been many destructive earthquakes in the Mediterranean region throughout the recorded history and many of them are rather well documented and studied. The understanding the geometry and evolution of potential seismogenic regions and the source rupture process along the main tectonic zones have crucial implications on the tsunami generation. The overall results of numerical simulations verified that damaging historical tsunamis along the Hellenic subduction zone are able to threaten particularly the coastal plains of Crete and Rhodes islands, SW Turkey, Cyprus, Levantine, and Nile Delta–Egypt regions. Thus, we cautiously recommend that special care should be considered in the evaluation of the tsunami risk assessment of the eastern Mediterranean region for future studies.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

KEYNOTE TALKS

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

HEAVY MINERALS AND PLACER DEPOSITS – AN OVERVIEW

Harald G. Dill

Federal Institute for Geosciences and Natural Resources,

Hannover, Germany

E-mail: dill@bgr.de
Homepage: www.hgeodill.de

Heavy minerals (HM) are common constituents of conglomerates, sand and sandstones, including pyroclastic deposits equivalent in grain size. But HM are less widespread in argillaceous and calcareous sediments where the energy regime is rather low and thus conditions to accumulate HM are less favorable. These mineral grains, with specific gravities higher than that of most rock-forming minerals (2.35 to 2.84 gr/cm3) are concentrated by water and wind but not accumulated by pure glacial processes for lack density-relate mineral separation. HM analysis of interest to academicians and geologists from various fields of applied research, can help in unraveling the extrabasinal (e.g. source area weathering) and intrabasinal processes (e.g. hydraulic processes ) affecting sediment transport. The strong points of HM analysis lie in the fields of provenance, paleoenvironmental and diagenetic studies. This peculiar type of sedimentary petrography can assist during the assessment of secondary porosity upon deep burial which is essential for hydrocarbon exploration, help in constraining the P-T regime of host rocks and better understand the unroofing story of mineral resources such as pegmatites which often impose great difficulties to get a full picture due to their extraordinary grain size and nature of minerals. A variegated spectrum of analytical methods is used for the identification of HM. The routine petrographic and ore microscopy are supplemented by SEM- EDX and EMPA. The CAMSIZER technique has proved to be a successful tool to study morphometric and granulometric issues and discriminate so the way of transport of HM and accumulation of placer minerals. Single grain dating by means of LA-ICP-MS techniques or fission track dating provide more accurate age data of HM and give an insight into the erosion history, uplift of source rocks and basin subsidence. Micro-Raman spectrometry of mineralogical characteristics of HM concentrates from placer deposits improves efficiency of separation of zircon and rutile concentrates. While in most sediments, HM are present only as accessory minerals, in placer deposits ilmenite, rutile, “leucoxene”, zircon, monazite, xenotime, garnet, chromite and magnetite become valuable heavy minerals (VHM) and may achieve economic grades in eluvial, residual, colluvial, alluvial-fluvial, littoral-coastal and aeolian placer deposits. Gold, platinum group minerals (PGM), cassiterite, scheelite, Nb-Ta oxides and gemstones (diamonds, sapphire, ruby, topaz, chrysoberyl, tourmaline, spinel s.s.s.) are recovered from residual to marine placer deposits. Joint operations of geomorphology and sedimentology may enhance the search of continental placer deposits. Landform types established throughout terrain analysis are cast in the role of “ore guides” for placer deposits or play a key role in finding the primary deposit or source rocks which HM have been derived from in the different morphoclimatic zones These parameters can also contribute to determine the human impact (ceramic goods, Pb-Fe-Au slags and mining residues) on placer deposition in man-made landscapes. It has implications for exploration by providing pathfinder elements/ minerals and is of relevance for the cultural history of the study area in terms of archaeometallurgy and ancient manufacturing. Irrespective of post-depositional alteration placer deposits may be used in explaining and predicting the juxtaposition of sediment-hosted mineralization by applying sequence stratigraphic concepts. HM were concentrated from the Precambrian, with its gold paleoplacer deposits through the most Recent. Large placer deposits - commodities are given in brackets - are located in the coastal areas of India (Ti, Zr, REE), Australia (Ti, Zr), New Zealand (Fe) Mozambique (Ti, Zr), Canada (Ti, Zr), South Africa (Ti, Zr), Kenia (Ti, Zr), Namibia (REE, diamonds), Brazil (REE), Sierra Leone (Ti), Malaysia (REE, Sn, Nb-Ta), Indonesia (Sn, Nb-Ta), Thailand (Sn) and Madagascar (Ti, REE) or large lakes such as in Malawi (Ti). Au-PGM- and gemstone placers are exploited in ancient cratons as well as modern fold belts.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

LITHOSPHERIC MAPPING, METALLOGENESIS AND THE EVOLUTION OF CONTINENTS

W.L. Griffin

CCFS/GEMOC, Dept of Earth Sciences, Macquarie Univ., NSW 2109, Australia

E-mail: bill.griffin@mq.edu.au

21st-century mineral exploration, and fundamental research into Earth’s evolution, both need predictive models, based on global 4-D tectonic analysis using multiple datasets (geophysical, geodynamic, geochemical) and an understanding of how the crust and the mantle interact. A lithosphere-scale approach can give us a better understanding of the genesis and evolution of both mantle and crust. It also will produce better models for the genesis and location of ore deposits, and a fundamental understanding of the processes that produce the diversity of many deposit systems.

An ongoing program of Global Lithospheric Architecture Mapping is integrating geophysical data (including global seismic tomography, gravity, magnetics, EM….) with mantle petrology and geochronology (from xenoliths and exposed sections) and crustal evolution (U/Pb, Hf and oxygen isotopes of zircon) to map the boundaries and histories of specific lithospheric blocks. With ca 75% of Earth’s continental area mapped, we can draw several major conclusions. (1) At least 70% of the continental crust was generated in Archean time, and has been variably reworked since; (2) Peaks in the age distribution of igneous rocks do not correspond to peaks in the generation of new crust; (3) The buoyant subcontinental lithospheric mantle (SCLM) did not exist prior to ca 3.5 Ga ago; most was generated between 3.0-3.5 Ga; (4) Once formed, the Archean SCLM is very difficult to destroy, due to its strength and buoyancy; (5) Much of the SCLM beneath younger areas is Archean in origin, but has been progressively reworked to lower its Mg#, and its overall buoyancy, with effects on its tectonic stability.

This lithosphere mapping approach has immediate implications for mineral exploration, particularly for large magma-related ore systems. Magmas originating in Earth’s convecting mantle must pass through the stagnant SCLM (ultramafic rocks, from ~40-250 km depth) on their way to the surface. The role of this uppermost mantle layer in ore genesis is not fully understood, but it is critical to building effective exploration models. In one view, most lithospheric mantle is passive – simply the buoyant raft on which the continental crust rides. However, it also acts as a storage facility for ore-forming elements introduced by magmas and fluids from below, and these elements can be picked up by later ascending magmas. The lithospheric mantle’s 3D architecture (variations in thickness and composition, major faults) also may control the emplacement and evolution of such magmas; mapping of these features is therefore critical to our understanding of lithospheric evolution, and to mineral exploration.

Keywords: lithosphere mapping; crust-mantle interaction; ore systems; continental crust; lithospheric mantle

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 DEPARTURES FROM THE STANDARD MODEL OF PORPHYRY ORE DEPOSITS

Eric S. Cheney

Department of Earth and Space Science, University of Washington

Seattle, United States

E-mail: vaalbara@uw.edu

The standard model (SM) for porphyry ore deposits (Jerome, 1966; Lowell and Guilbert, 1970) does not fit all. An appreciation of departures from SM is important in successful exploration.

The temporal and spatial distributions of hydrothermal alteration assemblages are important because they host hypogene ore, are more voluminous than the ore, determine geophysical and remote sensing responses, and influence the amount and location of supergene effects. The concentric hydrothermal alteration patterns of SM are characteristic of small plutons intruded into mostly feldspathic rocks. For example, at Bingham, USA, the silicate alteration assemblages inside the pluton do not occur in adjacent quartzites and carbonate rocks. The SM dealt primarily with quartz monzonitic plutons, in which the dominant mineral in K-alteration is K-feldspar; in tonalitic rocks the dominant mineral in K-alteration is biotite (phlogopite).

In batholithic settings, hydrothermal alteration patterns can be inside-out compared to SM. For example, at Butte, USA, fresh quartz monzonite grades inward into increasingly intense biotitic alteration, inside and above which is superimposed quartz-sericite-pyrite alteration.

Uneroded Cu deposits show that originally the ore zone was not a hollow cylinder, but an inverted cup, as in porphyry Mo deposits. Examples are Bingham, Resolution, and Butte in USA, El Teniente in Chile, etc.

In SM, porphyry systems were considered sulfidic. However, the common occurrence of anhydrite (in K-alteration) and of alunite (in advanced argillic alteration) and sulfur isotopic studies indicate H2S/SO42- < 0.5 to 1. Accordingly, porphyry systems can generate structurally higher high-sulfidation epithermal deposits.

Not all porphyry systems are still upright. Due to post-ore faulting, Bingham, USA is tilted 15⁰; Yerrington, USA is horizontal. Other deposits are displaced by faulting (Butte and San Manuel/Kalamazoo, USA).

The intensity of supergene enrichment depends mostly on the intensity and location of hypogene quartz-sericite-pyrite alteration. Some supergene enrichment blankets formed prior to the present erosion surface and climate. Some blankets are no longer horizontal.

Porphyry Cu-Mo-Au deposits and their geologic settings are relevant to other commodities. Porphyry deposits of Au, W, Sn, and F exist. Porphyry Ag and U might exist. The Buffalo fluorite deposit of South Africa in 2.05 Ga Bushveld Granite has characteristics of a porphyry Mo, including porphyritic rocks, a shell of quartz veins, an overlying shell of fluorite veins, and unidirectional solidification textures (USTs).

Exploration for porphyry deposits is increasing driven by Au. Porphyry Cu-Au and Au deposits have g/t Au/% Mo x 10 > 30. Porphyry Au deposits are here defined as g/t Au/%Cu > 1.2.

Porphyry deposits of all types are rarely preserved in pre-Paleozoic rocks and, thus, commonly are poorly recognized by indigenous geologists. Regionally metamorphosed porphyry deposits also are rare and poorly recognized.

Keywords: porphyry ores; alteration zoning; ore shell; porphyry Au; post-ore deformation

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

IMPACT OF SUBAERIAL VOLCANISM ON THE CARBON CYCLE

Martin Palmer

National Oceanography Centre, Southampton

University of Southampton and

Natural Environment Research Council,

United Kingdom

E-mail: m.palmer@noc.soton.ac.uk

The impact of subaerial volcanism on the carbon cycle has generally been studied in the context of the impact of carbon dioxide released by degassing, which leads to warming of the climate. Over the past few years, however, there has been growing evidence that subaerial volcanic activity can also act as a sink for carbon dioxide and thus lead to climatic cooling. Several studies have shown that enhanced carbon dioxide uptake takes place due to increased silicate weathering of fresh volcanic material, and that the scale of this process may be more than sufficient to offset the carbon dioxide added to the atmosphere by degassing. Most subaerial volcanic activity occurs in close proximity to the oceans, so a high proportion of freshly erupted material is rapidly transported into the marine environment. Hence, there is ample opportunity for reaction of highly reactive fresh volcanic material with seawater. Indeed, both laboratory and field studies have shown that addition of volcanic ash to the oceans may act as a fertilizer that stimulates biological production and increases drawdown of carbon dioxide from the atmosphere.

Over the past five years we have been carrying out a study of the impact of volcanic activity on Montserrat, West Indies on the carbon cycle. This work has involved studies on the island, laboratory and field studies of the impact on the local marine biology, research cruises to study the diagenesis of fresh volcanic material in marine sediments and, most recently, IODP leg340 to study the long term record of the impact of volcanic activity in the region. In addition to the processes outlined above, we have shown that dissolved oxygen in marine sediment pore waters is depleted to zero within a few mm of the sediment-water interface in areas containing high concentrations of fresh volcanic material. Oxidation of organic carbon in marine sediments is an important means by which carbon dioxide sequestered by biological activity is returned to the ocean-atmosphere and the efficiency of organic carbon oxidation is highly dependent on its exposure time to dissolved oxygen. Hence, most organic carbon buried in sediments lying beneath well-oxygenated bottom water is oxidised. The intensity of oxygen uptake by tephra is such, however, that a thin layer of tephra on the sea floor permanently isolates organic carbon in the underlying sediments from dissolved oxygen. In addition, results from IODP340 (March-April 2012) indicate that there is enhanced organic carbon burial in volcanic-rich sediments that may be linked to organic carbon complexes with reactive iron. While the area of ocean floor covered by tephra is small in the modern ocean, it is estimated that some super eruptions covered ~10% of the Earth’s surface in tephra. Deposition of such a large area of tephra is not, however, sufficient to impact organic carbon preservation on its own. For example, it is now apparent that the Toba eruption had only a small impact on climate. It may be significant, however, that much of the Toba tephra was deposited in the deep, well-oxygenated waters of the Indian Ocean. Hence, enhanced preservation of the low concentrations of organic carbon in these sediments would have minimal impact on atmospheric CO­2 levels. In contrast, tephra from the Late Ordovician super eruptions was largely deposited in shallow equatorial seas, at a time when the oceans had lower dissolved O2 concentrations than today. Under these conditions the potential for enhanced organic carbon preservation, and lowering of atmospheric CO2 was much greater. It may be significant, therefore, that the Late Ordovician is marked by several positive carbon isotope excursions, marking enhanced organic carbon burial, and culminated in the intense Hirnantian glaciation.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

THE CENTRAL-WESTERN MEDITERRANEAN: ANOMALOUS IGNEOUS ACTIVITY IN AN ANOMALOUS COLLISIONAL TECTONIC SETTING

Michele Lustrino

Dipartimento di Scienze della Terra
Università degli Studi di Roma La Sapienza,

Italy

E-mail: michele.lustrino@uniromal.it

The central-western Mediterranean area is a key region for understanding the complex interaction between igneous activity and tectonics. In this talk the specifi c geochemical character of several "subduction-related" and "intraplate-like" Cenozoic igneous provinces are described with a view to identifying the processes responsible for the modifi cations of their sources. Different petrogenetic models are reviewed in the light of competing geological and geodynamic scenarios proposed in the literature. During the Cenozoic widespread anorogenic magmatism, genetically (but not geologically) unrelated to recent supra-subduction zone modification of its mantle source, also developed within the central-western Mediterranean and surrounding regions.

Subduction-related plutonic rocks occur almost exclusively in the Eocene– Oligocene Periadriatic Province of the Alps while relatively minor plutonic bodies (mostly Miocene in age) crop out in N Morocco, S Spain and N Algeria. Igneous activity is otherwise confi ned to lava fl ows and dykes accompanied by relatively greater volumes of pyroclastic (often ignimbritic) products. Overall, the igneous activity spanned a wide temporal range, from middle Eocene (such as the Periadriatic Province) to the present (as in the Neapolitan of southern Italy). The magmatic products are mostly SiO2-oversaturated, showing calcalkaline to high-K calcalcaline affi nity, except in some areas (as in peninsular Italy) where potassic to ultrapotassic compositions prevail. The ultrapotassic magmas (which include leucitites to leucite-phonolites) are dominantly SiO2-undersaturated, although rare SiO2-saturated (i.e., leucite-free lamproites) appear over much of this region, examples being in the Betics (southeast Spain), the northwest Alps, northeast Corsica (France), Tuscany (northwest Italy), southeast Tyrrhenian Sea (Cornacya Seamount) and possibly in the Tell region (northeast Algeria).

Excepted for the Alpine case, subduction-related igneous activity is strictly linked to the formation of the Mediterranean Sea. This Sea, at least in its central and western sectors, is made up of several young (less than 30 Ma) V-shaped back-arc basins plus several dispersed continental fragments, originally in crustal continuity with the European plate (Sardinia, Corsica, Balearic Islands, Kabylies, Calabria, Peloritani Mountains). The bulk of igneous activity in the central-western Mediterranean is believed to have tapped mantle ‘ wedge ’ regions, metasomatized by pressure-related dehydration of the subducting slabs. The presence of subduction-related igneous rocks with a wide range of chemical composition has been related to the interplay of several factors among which the pre-metasomatic composition of the mantle wedges (i.e., fertile vs. refractory mineralogy), the composition of the subducting plate (i.e., the type and amount of sediment cover and the alteration state of the crust), the variable thermo-baric conditions of magma formation, coupled with variable molar concentrations of CO2 and H2O in the fl uid phase released by the subducting plates are the most important.

Compared to classic collisional settings (e.g., Himalayas), the central-western Mediterranean area shows a range of unusual geological and magmatological features. These include: a) the rapid formation of extensional basins in an overall compressional setting related to Africa-Europe convergence; b) centrifugal wave of both compressive and extensional tectonics starting from a ‘ pivotal ’ region around the Gulf of Lyon; c) the development of concomitant Cenozoic subduction zones with different subduction and tectonic transport directions; d) subduction ‘ inversion ’ events (e.g., currently along the Maghrebian coast and in northern Sicily, previously at the southern paleo-European margin); e) a repeated temporal pattern whereby subductionrelated magmatic activity gives way to magmas of intraplate geochemical type; f) the late-stage appearance of magmas with collision-related ‘ exotic ’ (potassic to ultrapotassic) compositions, generally absent from simple subduction settings; g) the relative scarcity of typical calcalkaline magmas along the Italian peninsula; h) the absence of igneous activity where it might well be expected (e.g., above the hanging-wall of the Late Cretaceous– Eocene Adria– Europe subduction system in the Alps); i) voluminous production of subductionrelated magmas coeval with extensional tectonic régimes (e.g., during Oligo-Miocene Sardinian Trough formation).

The anorogenic magmatism generally postdate the subduction-related igneous activity in several regions. A common sub-lithospheric mantle source component is identified for most of the anorogenic igneous rocks of the region. This has geochemical affinities to the source of HIMU oceanic island basalts and to the European Asthenospheric Reservoir (EAR), the Low Velocity Component (LVC) and the Common Mantle REservoir (CMR) of previous workers. It must be stressed that similar geochemical characteristics cannot be considered proofs for derivation from the same physical mantle source.

Global and local seismic tomography studies of the mantle beneath the central-western Mediterranean but also for easternmost circum-Mediterranean regions have revealed a range of P- and Swave velocity anomalies, some of which have been related to the presence of mantle plumes. Detailed local tomography experiments in the Massif Central of France and the Eifel region of central Germany suggest that, locally, there are diapiric upwellings rooted within the upper mantle which induce adiabatic decompression melting and magma generation. These velocity anomalies should be attributable to the presence of fluid or partial melt or to different lithologies (generically defined as pyroxenites/eclogites or olivine-poor lithologies) or to difference in grain-size of the peridotitic matrix, and significant thermal anomalies are not necessary to explain the petrogenesis of the anorogenic magmas. The geochemical and isotopic characteristics of the sub-lithospheric mantle beneath the circum-Mediterranean area reflect the introduction of recycled crustal components (derived from both oceanic and continental lithosphere) or frozen basaltic melts coming from the deep upper mantle. This sub-lithospheric mantle is subsequently partially melted in a variety of geodynamic settings related to lithospheric extension, continental collision and orogenic collapse, and contemporaneous subduction, slab roll-back and slab-window formation.

The relatively homogeneous composition of the anorogenic igneous rocks in terms of incompatible trace-element content and Sr–Nd–Pb isotopic composition is unexpected, considering the variable lithospheric structure of this large area and the different tectono-thermal histories of the various districts. In order to reconcile the geochemical characteristics with a statistical sampling model, it would be necessary to propose volumes of the enriched regions much lower than the sampling volumes for each volcano (that is, less than 10 cubic metres), or alternatively, efficient magma blending from larger areas. The data are consistent with a relatively well-stirred and mixed sub-lithospheric upper mantle, in the solid state, which is also hard to understand. This contrasts with the situation under oceans where magma blending from diverse sources and sampling theory can explain the compositional statistics.

To summarize, these salient central-western Mediterranean features, characterizing a late-stage of the classic ‘ Wilson Cycle’ offer a ‘ template ’ for interpreting magmatic compositions in analogous settings elsewhere.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

MANTLE PROCESSES TOLD BY OPHIOLITIC CHROMITITES

Shoji Arai

Department of Earth Sciences, Kanazawa University, Kanazawa

Japan

E-mail: ultrasa@staff.kanazawa-u.ac.jp

Chromitites, composed mainly of chromian spinel (chromite) (and olivine), in peridotite are “durable” after their formation; they are resistant against weathering, alteration, subsolidus recrystallization and reaction with invading melt. In contrast, peridotites are easily changeable with a change of condition; e.g., garnet peridotite changes to spinel peridotite with a decrease of pressure, and harzburgite is frequently converted to dunite through reaction with invading melt. Chromitites are, therefore, a good marker of the processes that formed them or that they experienced.

Podiform chromitites are frequently found within the uppermost mantle peridotite, i.e., the mantle part of ophiolites or mantle-derived peridotite massifs. They can be formed by a process of harzburgite/melt reaction coupled with melt mixing. This process most effectively occurs within a moderately high Cr/Al system (e.g., within cpx-bearing harzburgite) at low pressures. In a low Cr/Al system (e.g., within lherzolite), the degree of spinel-oversaturation is not so high in the mixed melt because of the low curvature of the olivine-spinel cotectic boundary. In an extremely high Cr/Al system (e.g., within highly depleted harzburgite), low contents of Al and Cr in orthopyroxene, which are in part the source of chromitite, make the size of resultant chromitite small. Podiform chromitites can be formed at any settings if magma is actively supplied in moderately depleted harzburgite (= clinopyroxene-bearing harzburgite). The low-P origin of ordinary podiform chromitites is consistent with the presence of pargasite in primary silicate inclusions in their spinel.

Finding of diamond and other ultrahigh-pressure (UHP) minerals in some podiform chromitites (Tibet and the Polar Urals) has casted a serious question about the framework of podiform chromitite genesis on us. Coesite lamellae in spinel from the Luobusa (Tibet) UHP chromitite (Yamamoto et al., 2009) clearly show that the spinel has experienced a UHP condition, possibly precluding a xenocrystal origin of the UHP minerals. Some features of UHP chromitites can be explained by compression/heating of ordinary low-P igneous chromitites; that is, the UHP chromitite is possibly a deep recycled material. It may emerge at the upper mantle beneath spreading centers, which are also the locus of low-P igneous chromitite genesis, by mantle convection. Both the recycled UHP chromitites and low-P igneous chromitites can thus coexist within the upper mantle beneath the spreading center. Diamond was possibly formed by reduction of CO2, which was trapped during a downward travel of chromitite in shallow part of the mantle. PGE-bearing alloys, which are only the PGM observed in the Luobusa chromitite, can be formed by decomposition of PGE-sulfides (and arsenides), which are common in the low-P chromitite. Nodular texture is found both in the low-P chromitite and in the Luobusa UHP chromitite. Spinel nodules show aggregation of individual spinel grains in the low-P one, but they are agglutinated and show brittle fractures filled with olivine in the UHP one. This clearly indicates a difference in subsolidus history; the UHP one possibly has longer duration after igneous formation than the low-P one. Stability relations, especially low-P limits, can constrain P-T trajectory of the UHP chromitites, which in turn may give constraints on the style of mantle convection, two-layer or whole-mantle.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 



__________________________________________________________________________________________________

 

footer footer footer footer footer footer footer footer footer footer footer footer footer

__________________________________________________________________________________________________

 

Top banner image is after Taymaz et.al. (2007) The Geological Society of London, ISBN: 978-1-86239-239-7)