bluuebunny/crossref_metadata_2025 huggingface dataset:
Orogeny in relation to crust and upper mantle
Authors: Bott M. H. P.
Published: May 2008

Seismic observations show that young fold mountains are typically associated with thickening of the crust, and normal P-wave velocity in the top-most mantle. Gravity observations show that fold mountains are in approximate regional isostatic equilibrium. The gravity and seismic observations together show that the density in the upper mantle beneath the mountains is not strongly anomalous. In these fundamental respects fold mountains differ greatly from ocean ridges and plateau uplifts (such as western U.S.A.). Beneath ocean ridges and plateau uplifts the crust is not thickened but the upper mantle has anomalously low density and seismic velocity.

The structure of the crust and upper mantle beneath mountain ranges can be explained by crustal shortening amounting to about half the width of the range; on this hypothesis the primary cause of the mountain range is horizontal compression. Alternatively, the root may have been added to the crust by differentiation from the mantle, the mountain range being primarily formed by vertical movement The crustal-shortening hypothesis has been greatly strengthened by the near-proof that large horizontal displacements related to continental drift have affected the ocean crust, and by the increasing realization that mantle convection is a mechanically valid hypothesis. Further indirect support for crustal shortening comes from energy considerations—for instance the gravitational energy associated with dissipation of a root by vertical uplift appears to be inadequate to explain the present-day energy release by earthquakes.
The pre-Pleistocene Phanerozoic time-scale — a review
Authors: Lambert Richard St John
Published: May 2008

The data in
The Phanerozoic Time-scale
(Geological Society, 1964) are reviewed, together with data relevant to the Phanerozoic time-scale published up to mid-1968. Quaternary problems are not considered, while the Tertiary is considered to be on a comparatively well-established basis. The use of glauconite ages to establish any part of the time-scale prior to the Oligocene is not to be recommended, as early glauconites can provide anomalously high apparent ages in addition to the occurrence of low ages. No criteria for distinguishing anomalously high or low glauconite ages have yet been established. The definition of inter-system and within-system boundaries in the Mesozoic provides an acute problem, to which there is at present no solution. Reconsideration of the Cretaceous data shows reasons for preferring 95 m.y. as the age of the base of the Cenomanian, but neither of the boundaries of the Jurassic are clearly defined. New data suggest that there should be an increase in the accepted age of the base of the Triassic to 235 ± 5 m.y., but this is in part dependent upon a firm knowledge of the decay rate of
87
Rb. The current six per cent range in commonly-used values of this constant is now of over-riding importance in causing uncertainty throughout the Palaeozoic scale; Rb-Sr determinations now provide the majority of critical Palaeozoic ages. Even without this uncertainty, increases in the age of the Westphalian, the base of the Carboniferous (to 360 m.y.) and the base of the Devonian (to ?405 m.y.) seem to be indicated by new data (using the 4·7 × 10
10
y half-life for
87
Rb: if 5·0 × 10
10
y proves correct, or any other figure higher than 4·7, then there will be correspondingly greater increases in the age of these boundaries). The Devonian data are particularly tantalizing: imprecision in stratigraphical correlations is allied to spreads in radiometric ages from individual complexes to cause considerable uncertainty. There are still no useful Silurian data, nor is any part of the Ordovician other than the Caradocian dated at all. The Caradocian data can be questioned and, if a half-life of
87
Rb > 4·7 × 10
10
y. is adopted, would need reconsideration. No part of the Cambrian is satisfactorily dated, but its base (as far as it can be defined palaeontologically) appears to be now controlled by North American data as being at not more than 570 ± 10 m.y. (4·7 × 10
10
y
87
Rb half-life), confirming the most recent estimates.
Summary
Authors: Evans Percy
Published: May 2008

Until the recent publication of radiometric dates for (i) the Eifel volcanic rocks, (ii) sediments from deep-sea cores, (iii) shells from high shore-lines, and (iv) polarity changes in Pleistocene volcanic rocks, there was no adequate basis for a Pleistocene time-scale. For example, dates proposed for the beginning of the Mindel glaciation range from 200 to 1450 thousand years ago. This review attempts a synthesis of the chronological evidence for Pleistocene events, with the main emphasis on the part older than the radiocarbon limit.

Palaeomagnetic evidence from deep-sea cores and the mid-oceanic ridges is used to supplement the time-scale for geomagnetic reversals published by Cox
et al.
(1968), and attention is drawn to the uncertainty of the date shown by deep-sea cores for the beginning of the Brunhes normal epoch. Discussion of climatic evidence that can be dated palaeomagetically shows that there is no evidence of any major deterioration in climate which can be taken as the beginning of the Pleistocene. The most convenient Plio-Pleistocene boundary based on palaeontological changes has an age of slightly over two million years.

In deep-sea cores, the proportions of the isotope
18
O in foraminiferal shells, the relative abundance of some temperature-sensitive foraminifera, and differences in the amount of coarser constituents in the sediments, together provide a record of cyclic changes in climate. Radiometric dating shows that these changes have a somewhat irregular period averaging about 40 000 years. Variations in the Earth’s relationship to the Sun bring about changes in insolation which in higher latitudes
2. The Pleistocene palaeomagnetic time-scale
Authors: Evans Percy
Published: May 2008

Volcanic rocks showing reversed magnetization ceased to be a geological curiosity when it was found possible to construct a palaeomagnetic stratigraphy based on the alternation of rocks with normal and rocks with reversed polarity. Dates from potassi
3. Paleomagnetic dating
Authors: Evans Percy
Published: May 2008

Application of the palaeomagnetic stratigraphy discussed in the previous section to Pleistocene problems suffers from the limitation that no easily recognizable polarity changes have been found within the past 700 t.y.–the period of greatest interest in glacial stratigraphy. For the earlier part of the Pleistocene, palaeomagnetic dating has a great potential value which is still largely unexploited. Palaeomagnetic evidenceis now being used to date events recorded in deep–sea cores the boundaries of faunal and floral zones, climatic sequences, and environmental changes revealed by lithology. So far (1969), records of about 50 cores have been published showing magnetic polarity chnages as well as palaeontological or climatic evidence, but not all are helpful for dating.

Age-determination, even in a corewhich has satisfactory magnetic stability and a number of polarity reversals, are often subject to difficulties and uncertainties. The palaeomagnetic time-scale is as yet tentative; more reference points are needed in volcanic rocks, and a few magnetic logs of deep-sea cores gives a full record of magnetic changes. The palaeomagnetic record of a core to be dated should includen at least one other magnetic parameter (e.g. intensity) in addition to the polarity changes; otherwise, brief events such as the Gilsa event or the short reversed period thought to occur within the Olduvai normal event are unlikely to be recorded. Mixing of sediment, while capableof masking a polarity change lasting say 20to 30 t.y., may still leave some indication of variations in intensity and inclination.

The recognition of brief events can be a safeguard
4. The 40 000 year cycle
Authors: Evans Percy
Published: May 2008

The palaeomagnetic time–scale discussed in sections 2 and 3 is helpful in establishing the dates of a few important changes in climate and in marine and in marine faunas which occured more than 700 t.y. ago: it is of little help in dating more recent events. The most promising evidence for determining the ages of the later climatic changes comes from depp-sea sediments.

Arrhenius (1952) and Olausson (1960 A,b) found in cores taken by the Swedish Deep-Sea Expedition of 1947–8 a record of cyclic changes in the proportion of carbonate and in the abudance of foraminifera. These changes were interpreted as indications of broad climatic chnages. Emiliani (1955A) found somewhat similar cyclic changes in the proportions of the oxygen isotope 18O contained in foraminifera preserved in deep-sea sediments. Despites uncertainties in the exact interpretation of the variations of this isotope, it is agreed that they do reflect the major changes in climate. This deduction is supported by mire recent work, especially in the Caribbean, which has shown that the 18O variations are closely parallel to variariona in the amount of coarse material in the sediments and in the proportions of temperature-sensitive foraminifera.

The various parameters which give an indication of climate are usually presnted in graphs in which climate is plotted against depth below the top of the core. There are now sufficient cores with radiometric dates to make it possible to calibrate the scale of these graphs in terms of thousands of years. Figure 9 shows a climate–time curve
5. A possible framework for a Pleistocene classification
Authors: Evans Percy
Published: May 2008

Even though the conclusion reached in section 4 is partly dependent on evidence to be discussed in sections 6 and 7, it is convenient at this stage to see how the isolation curve can best be made use of as an aid to Pleistocene classification. It mos
6. The climatic sequence in Atlantic and Caribbean deep-sea cores
Authors: Evans Percy
Published: May 2008

(i) ‘Palaeotemperature’ determinations

Oxygen-isoptope determinations on selectd foraminifera from deep-sea cores from the Atlantic region were first recorded by Emiliani in 1955, and have been continued by him in collaboration with othe
7. The deep-sea cores of the Pacific
Authors: Evans Percy
Published: May 2008

Detailed descriptions of the Swedish Deep-Sea Expedition's cores of the East Pacific were given by Arrhenius (1952), and their interpretation has been discussed further by Olausson (1961, 1965), Arrhenius (1963) and Riedel
et al
. (1963). Two c
9. Radiometric dates
Authors: Evans Percy
Published: May 2008

Although the radiometric dates bearing closely on terrestrial glaciations must provide the key points in any Pleistocene time-scale, they acquire their full usefulness only within the framework of the climatic sequence obtained from deep-sea cores
10. The climatic sequences in lacustrine boreholes
Authors: Evans Percy
Published: May 2008

In addition to the deep-sea cores already discussed, there are cores from several boring in lacustrine sediments which yield chronological evidence from climatic changes. Their interpretation depends partly on the dates of the later terrestrial glaciatio
11. Dates of high-sea-levels derived by extrapolariation from radiometric ages
Authors: Evans Percy
Published: May 2008

The potential value of sea-level fluctuations in establishing a Pleistocene chronology has been referred to in section 9(D). The direct radiometric dates of the high sea-levels of warm periods do not at present go back beyond cycle 6 (210 t.y.) and a
12. Some problems of chronology and classification
Authors: Evans Percy
Published: May 2008

Having discussed the chronological evidence obtained from a variety of sources, we may now examine several general problems which are dependent on chronology, and at the same time attempt to fill out the meagre record of Table 17 and Fig. 44. The sug
13. River terraces and shore-lines in southern England
Authors: Evans Percy
Published: May 2008

The earlier sections of this review have discussed evidence which seems to make it possible to move towards a Pleistocene time-scale of world-wide application. Any attempt to apply the proposed scale to a great variety of local successions throughout
15. The Oxford area
Authors: Evans Percy
Published: May 2008

The terraces of the lower and middle parts of the Thames Valley have been dated by direct reference to base-levels of aggradation. These terraces have
16. The Severn and Avon terraces and glaciations
Authors: Evans Percy
Published: May 2008

If the argument developed in section 13 dealing with the chronology of the terraces of the Lower and Middle Thames is valid, it should be possible to make similar deductions for the Severn and Avon, and sot to progress towards a chronology for one of
17. Other regions
Authors: Evans Percy
Published: May 2008

A few other regions of special interest are discussed briefly; they are chosen because of an unsolved problem or for their archaeological interest.
19. Acknowledgements
Authors: Evans Percy
Published: May 2008

The author acknowledges with gratitude the help received from Dr W. W. Bishop, Dr W. H. Blow, Dr D. Q. Bowen, Professor E. H. Brown, Sir Edward Bullar
Introductory remarks on the transport problem
Authors: Fyfe W. S.
Published: May 2008

Synopsis

Subsurface aspects of the ore-forming process can be described in terms of standard chemical transport parameters such as T-P gradients, mineral solubilities, solvent chemistry and volume, diffusion parameters and energy flow. These necessary factors are rather easily assessed by geochemical, isotopic and structural studies. If deposition is in the surface environment, a complex array of factors will determine the success of the natural concentration process and the chances of preservation of a deposit.

Basically, most large-scale ore-forming processes involve large fluid volumes and energy sources. Many environments where these requirements are met, the modern ocean ridge and the subduction environment, are still not well understood. The need for remote-sensing techniques in the submarine environment is stressed.
Model of hydrothermal ore genesis
Authors: Elder J. W.
Published: May 2008

Synopsis

The dynamical nature of the mechanisms of operation of hydrothermal systems and their key parameters are described. A simple model suitable for a gross analysis of the role of water circulation on the solution, transportation and deposition of ores is described in detail.
Identification of ore-deposition environment from trace-element geochemistry of associated igneous host rocks
Authors: Pearce J. A. , Gale G. H.
Published: May 2008

Synopsis

The tectonic environment of formation of volcanogenic massive sulphide deposits and porphyry tin and copper deposits can be identified from the geochemical characteristics of the associated igneous rocks. The ‘stable’ trace-element geochemistry (involving Ti, Zr, Y, Nb, Cr and rare-earth elements) and geology of metabasalts related to 12 massive sulphide deposits indicate that the deposits studied fall into four distinct classes. (1) Cyprus-type, including Cyprus, Oman and Betts Cove, possibly formed during the early stages of back-arc basin development; (2) Løkken-type, including Løkken and York Harbour, possibly formed at back-arc basin spreading centres; (3) Joma-type, including Joma, Røros and Bidjovagge, possibly formed in a small ocean of Red Sea type; and (4) Gjersvik-type, including Gjersvik, Buchans, Noranda and Lynn Lake, possibly formed during an early stage of island arc evolution (Gjersvik), later during island arc evolution (the Buchans Kuroko-type deposit) or in a Precambrian setting (Noranda and Lynn Lake). Deposits related to major ocean ridge crests appear to be small and relatively uncommon, perhaps because relatively few favourable sites for ore deposition exist in such environments.

The environment of intrusion of acid-intermediate igneous rocks can be deduced by use of diagrams based on the element Nb (e.g. SiO
2
versus Nb). On this basis, tinbearing granites can be classified either as within-plate magmas (Nigeria) or as magmas from evolved volcanic arc settings (e.g. Bolivia, Cornwall and Indonesia); the latter group can be further subdivided geologically into postorogenic and back-arc extensional settings. Although the continental crust may play a significant role in the formation of tin deposits, the geochemical data presented here suggest that partial melting of tin-enriched mantle above a subduction zone may be the single most important genetic factor.
Identification of the origin of oreforming solutions by the use of stable isotopes
Authors: Sheppard S. M. F.
Published: May 2008

Synopsis

The four major different types of water — magmatic, metamorphic, sea water and/or connate, and meteoric water — have characteristic hydrogen (D/H) and oxygen (
18
O/
16
O) isotope ratios. Applied to the analysis of isotopic data on hydrothermal minerals, fluid inclusions and waters from active geothermal systems, these ratios indicate that waters of several origins are involved with ore deposition in the volcanic and epizonal intrusive environment.

Water of a single origin dominates main-stage mineralization in some deposits: magmatic — Casapalca, Peru (Ag-Pb-Zn-Cu); meteoric — Butte, Montana (Cu-Zn-Mn), epithermal deposits, e.g. Goldfield, Tonopah, Nevada (Ag-Au), Pachuca, Mexico (Ag-Au), San Juan Mountains District, Colorado (Ag-Au-Pb-Zn); sea water — Troodos, Cyprus (Fe-Cu), Kuroko, Japan (Fe-Cu-Pb-Zn). Solutions of more than one origin are important in certain deposits (magmatic and meteoric — porphyry copper and molybdenum deposits) and are present in many.

In the porphyry Cu-Mo deposits the initial major ore transportation and alteration processes (K-feldspar-biotite alteration) are magmatic-hydrothermal events that occur at 750–500°C. These fluids are typically highly saline Na-K-Ca-Cl-rich brines (more than 15 wt % equivalent NaCl). The convecting meteoric-hydrothermal system that develops in the surrounding country rocks with relatively low integrated water/rock ratios (less than 0.5 atom % oxygen) subsequently collapses in on a waning magmatic-hydrothermal system at about 350–200°C. These fluids generally have moderate to low salinities (less than 15 wt % equivalent NaCl). Differences among these deposits are probably in part related to variations in the relative importance of the meteoric-hydrothermal versus the magmatic-hydrothermal events. The sulphur comes from the intrusion and possibly also from the country rocks.

Deposits in which meteoric or sea water is the dominant constituent of the hydrothermal fluids come from epizonal intrusive and sub-oceanic environments where the volcanic country rocks are fractured or well jointed and highly permeable. Integrated water/rock ratios are typically high, with minimum values of 0.5 or higher (atom % oxygen) — the magmatic water contribution is often ‘drowned out’. Salinities are low to very low (less than 10 wt % equivalent NaCl), and temperatures are usually in the range 350–150°C. The intrusion supplies the energy to drive the large-scale convective circulation system. The sulphur comes from the intrusion, the country rocks and/or the sea water.

Argillic alteration, which occurs to depths of several hundred metres, generated during supergene weathering in many of these deposits is isotopically distinguishable from hydrothermal clays.
Hydrogen and oxygen isotope evidence for sea-water-hydrothermal alteration and ore deposition, Troodos complex, Cyprus
Authors: Heaton T. H. E. , Sheppard S. M. F.
Published: May 2008

Synopsis

A model is presented for the genesis of the Cyprus massive cupriferous sulphide deposits, their associated stockwork mineralization and the metamorphism of the Troodos ophiolite complex based on D/H,
18
O/
16
O and
13
C/
12
C ratio analyses, where applicable, of 94 whole rocks and minerals, chemical, mineralogical and sulphur iso tope data.

The upper 3–5 km of the hot Troodos crust — pillow lavas, Sheeted Intrusive Complex, trondhjemites and upper gabbros — were metamorphosed to zeolite facies, temperatures of about 0 to 250° C, and greenschist-amphibolite facies, temperatures of about 250 to 450–550° C, during interaction with a deep circulating sea-water geothermal system. Minimum isotopic temperatures of 320–400°C are given for the epidote-bearing vein-halo assemblages in the upper gabbros, trondhjemites and Sheeted Intrusive Complex. If temperatures at the base of the Sheeted Intrusive Complex were higher than ∼ 500° C, either a magmatic or metamorphic water component must have been present in the hydrothermal fluids. The magmatic water could be of sea-water origin, being derived from the melting or assimilation of sea-water-hydrothermally altered roof rocks overlying the gabbroic magma chambers. Minimum water/rock ratios during pervasive alteration decrease from > 1 by weight in zeolite facies pillow lavas to ∼ 0.1 in the underlying green-schist facies rocks, but were an order of magnitude larger in the veins and fractures.

In the stockwork zones, which underlie the sea-floor sites of massive sulphide orebodies, isotopic temperatures declined from initial values greater than 300° C (with water/rock ratios > 1–5, by weight) to less than 220° C during post-sulphide later-stage vein quartz formation; these temperatures were higher than temperatures in the predominantly zeolite facies country rocks. The stockwork zones are interpreted to represent the discharge zones of the seawater-hydrothermal fluids responsible for the regional-scale mass transfer and the mineralization. At least some of the sulphide sulphur in the stockworks and orebodies is of seawater origin. Formation of massive sulphide deposits requires the coincidence of several factors, including the penetration of a brine to deep levels, where temperatures were greater than 300° C in an active spreading centre, leaching of ore metals, and discharge of the hot springs into basins where both sulphide deposition and localization could occur.
Hydrodynamic model for the origin of the ophiolitic cupriferous pyrite ore deposits of Cyprus
Authors: Spooner E. T. C.
Published: May 2008

Synopsis

The information presently available — in particular, that derived from the use of the isotopes (
87
Sr/
86
Sr, δ
18
O and δ
34
S) as geochemical tracers — indicates that a model for mineralization and metamorphism of the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus, which involves hydrothermal convection of sea water, is reasonable. During water-rock interaction in the recharge part of the cycle of convective heat and mass transfer, the pillow lavas, Sheeted Dyke Complex and uppermost gabbros were hydrothermally metamorphosed at zeolite facies to amphibolite facies conditions, and base metals (e.g. Fe, Mn, Cu, Zn, Pb, Ni, Co, Ag and Au) were leached by the fluid from the rocks. At the positions of discharge of hot fluid at the sea water-rock interface sulphide ore deposits or ferromanganoan hydroxyoxide sediments (umbers) were precipitated.

By calculation of a Rayleigh number for the conditions of hydrothermal activity it is shown that the fluid dynamic requirements for vigorous free convection were satisfied. It is deduced that the ore deposits developed within the active zone of a spreading oceanic ridge, and took ∼ 10
5
yr to form. Available geochemical evidence is discussed to demonstrate that the hydrothermal convection model is chemically, as well as fluid-dynamically, reasonable.
Origin and emplacement of ophiolites
Authors: Gass I. G.
Published: May 2008

Synopsis

By definition, an ophiolite suite is a particular sequence of basic and ultrabasic igneous rocks. This sequence seemingly coincides with that deduced for the oceanic lithosphere from geophysical and geochemical evidence. This comparison is reviewed and the ways in which fragments of ancient sea-floor may have been emplaced upon continental margins are discussed.
Mineralization at destructive plate boundaries: a brief review
Authors: Garson M. S. , Mitchell A. H. G.
Published: May 2008

Synopsis

Three main types of destructive plate boundary are recognized — island arc type, continental margin type and collision type — each displaying distinctive mineralization.

The first, characterized by an ensimatic island arc magmatic belt developed within oceanic crust, has associated deposits, which include diorite-type porphyry copper and gold, Kuroko-type zinc-copper-lead sulphides, mercury, native sulphur-pyrite and Besshi-type cupriferous sulphides.

The second, characterized by a continental margin type magmatic belt developed within continental crust, includes associated porphyry copper and molybdenum and, locally, tin-tungsten deposits. Where complete, mineral zonation away from the plate margin into continental crust comprises copper-molybdenum, silver-lead-zinc, tin-tungsten and antimony. A modification of this type of plate margin results from the formation of a marginal basin by rifting and outward migration of incipient continental margin mountain belts bordered on the ocean side by a subduction zone. Granite plutons with associated tin, tungsten and fluorite are emplaced in this setting.

Centres of porphyry copper mineralization may be controlled locally by transform faulting at both types of plate margin.

Tectonic wedges of oceanic crust and upper mantle are scraped up with overlying sediments and emplaced above subduction zones that border magmatic arcs. These wedges may contain mineral deposits previously formed in oceanfloor spreading-related settings, including Cyprus-type massive sulphides, podiform chromite, platinum, magnesite and asbestos.

The collision type of destructive plate boundary is formed during and following the final stage of subduction of ocean floor between two continents, between two island arcs or between a continent and island arc. Slices of tectonically emplaced oceanic crust and upper mantle, forming ophiolites, are obducted or thrust on to the continental crust or inactive island arc on the subducting plate to form elongate belts at the junction of the two plates. Collision belts contain mineral deposits, interpreted as having formed at oceanic ridges and magmatic arcs, and may also include tin-tungsten, iron-titanium ores associated with anorthosites in the lower continental crust, native silver-nickel-cobalt arsenides, gemstones and strata-bound uranium-vanadium-copper mineralization in molasse derived from the associated mountain belts.
Porphyry copper deposits
Authors: Hunt J. P.
Published: May 2008

Porphyry copper deposits are large crustal sulphur anomalies containing copper and other metals. Much of the sulphur is oxidized and, along with a major portion of the copper, deposited from fluids evolved from certain calc-alkaline melts of intermediate composition. These melts are emplaced with characteristic textures as dykes and stocks in and above the cupolas of small granitic batholiths. The depth of emplacement is shallow, and the intrusions are comagmatic with overlying andesite—rhyolite volcanics. Intrusion and mineralization are relatively rapid events and occur towards the end of previous episodes of vulcanism and intrusion. Porphyry copper deposits are clustered in both space and time on regional, continental and global scales. More may have formed in the past 65 million years than in earlier geologic periods, especially in certain apparently favourable continental regions. Sources of sulphur, copper and of the associated magmas are unknown. Much additional mapping of continental and oceanic margins is needed to explain the abnormal, or unusual, coincidence of several igneous and tectonic processes probably responsible for the formation of porphyry copper deposits.
The Planes-San Antonio pyritic deposit of Rio Tinto, Spain: its nature, environment and genesis
Authors: Williams D. , Stanton R. L. , Rambaud F.
Published: May 2008

The San Antonio stratiform cupriferous deposit of Rio Tinto, discovered in the early 1960s, is now being exploited as an eastward extension of the bedded pyritite sheet in the old Planes mine, which is there underlain by a stockwork feeder pipe.

The Planes pipe consists of sulphur-rich—copper-poor sulphide ore formed by the almost complete replacement of a stockwork conduit which traverses Lower Carboniferous felsic pyroclastic rocks. Overlying the pipe, and extending far beyond its margin, is the genetically related Planes-San Antonio cupriferous pyritite, a ‘sedimentary-volcanogenic’ deposit precipitated as a chemical sediment derived from sea-floor hot springs associated with the Lower Carboniferous volcanic island arc of southwest Iberia. Most of the pyritite bodies of Rio Tinto were deposited directly above or close to their underlying feeder stockworks. The Planes-San Antonio pyritite displays well-preserved stratification, slump structures and interdigitated tuff bands, and was mainly precipitated comparatively distant from the nourishing Planes stockwork.

Across-layer base-metal zoning in the Planes-San Antonio sheet is usually characterized by an increase in copper, and to a lesser extent in zinc and lead, towards the base of the deposit. Much of this enrichment may have been due to late hypogene processes involving the ascent of juvenile-meteoric solutions which leached base metals from the underlying volcanic pile and deposited them as chalcopyrite, sphalerite and galena by partial replacement of the lower part of the pre-existing pyrite-rich stratiform sulphide sheet.

A notable feature of the pyritite is the abundance of colloform and framboidal pyrite. The mean isotopic composition of the sulphide sulphur of the Planes stockwork (
δ
34
S ≈ + 10‰) is distinctly heavier than that of the stratiform Planes-San Antonio pyrite (
δ
34
S ≈ + 2.7‰).

The cupriferous pyritic mass of Planes-San Antonio exhibits some of the features characteristic of pyritic deposits formed during the initial stages of arc development (‘Cyprus’ or ‘ophiolite’ type ores). It was actually formed, however, at a late stage of arc evolution, during a period of relative quiescence within the waning episodes of submarine explosive felsic volcanism, and it displays environmental traits analogous to those of the Japanese ‘Kuroko’ ores.
Kuroko deposits: their geology, geochemistry and origin
Authors: Sato Takeo
Published: May 2008

Synopsis

The Kuroko deposits of Japan are predominantly massive, stratiform sulphide ores with associated underlying stockwork ores of Miocene age in the Green Tuff volcanic sequences. They formed during a very limited period around 13 m.y. ago in association with the late-stage felsic rocks of the mafic to felsic volcanic cycle that began in the late Oligocene or early Miocene. They occur in narrow zones parallel to island arcs. The Benioff zone is at about 150-km depth below the Kuroko zones, which were areas of high heat flow during the Miocene.

The major constituents of the deposits are pyrite, chalcopyrite, sphalerite, galena, barite and quartz. They are zoned vertically. Gypsum and/or anhydrite commonly occur as separate bodies beside the sulphide-barite bodies. It is thought that the ores were precipitated on the Miocene sea-floor from saline solutions in a weakly acidic, reducing environment at about 200–250°C as a result of falling temperatures. The zoning of minerals may have resulted from changes in density of the ore solution-sea water mixtures as precipitation proceeded.

Studies of light stable isotopes suggest that the mineralizing solutions were predominantly coeval sea water. Lead isotope data, together with base-metal contents of the host rocks, however, indicate that leaching by circulating sea water cannot have been the dominant mechanism. So, magmatic water probably played an important role in the mineralization. The following model is therefore proposed. About 20–25 m.y. ago, as the Pacific plate was being subducted, a large amount of basic to intermediate magma was generated and differentiated in the lower crustal region. Magmatic fluids produced at the last stage of differentiation, about 13 m.y. ago, rose to the sea-floor and, on mixing with sea water, produced the Kuroko deposits. Subsidence of the sedimentary basins before the felsic volcanism could have been the result of volume decrease during solidification of underlying magma.
Volcanogenic mineralization at Avoca, Co. Wicklow, Ireland, and its regional implications
Authors: Platt J. W.
Published: May 2008

A description is given to the geological setting of the stratiform pyritic orebodies at Avoca in Ireland and a volcanogenic origin is proposed on the basis of comparison with stratiform orebodies located in similar volcanic host rocks in other parts of the Caledonian orogen and elsewhere. The Ordovician volcanic sequence in southeast Ireland is interpreted as a fossil island arc complex, which was formed above a contemporary margin of southeasterly subduction, which locally defined the boundary of the evolving Caledonian orogenic belt in southeast Ireland.

The Avoca ore deposits are located within a sequence of calc-alkaline lavas, pyroclastics and sediments at a level marked by the predominance of felsic eruptives. The stratigraphy of the host rocks is described in some detail and related to the general geology of the Avoca belt. Massive pyritic ores and disseminated sulphide ores of ‘stringer type’ can both be recognized at Avoca. The massive ore shows a compositional gradation suggestive of a primary stratigraphy; a Cu-rich pyritic zone is overlain by a zone of Pb-Zn-Cu pyritic ore.

The interpretation of the genesis of the orebody is complicated by its deformed state, which is typical of other volcanogenic orebodies within the various Caledonian provinces. A genetic model can be proposed, however, by using the descriptions of the undeformed ‘Kuroko’ deposits in Japan as a standard for comparison.

The individual orebodies of Avoca type contain 5 000 000–6 000 000 tons of ore, and these are associated with extensive zones of low-grade disseminated ore in acid pyroclastic host rocks. Sequences of manganiferous and dolomitized sediments are stratigraphically closely related to the ores.

The principal geological features of the Avoca orebodies having been defined, it is deduced that there is a strong possibility that similar orebodies lie hidden in the volcanic rocks of the southeast Irish Caledonides and that an aggressive exploration programme could lead to new discoveries.
Rifting in east Africa and large-scale tectonic processes
Authors: Oxburgh E. Ronald
Published: May 2008

The East African Rift System has many of the geophysical characteristics of a mid-ocean ridge and is laterally continuous with the Red Sea and Gulf of Aden spreading centres. Although all three were initiated simultaneously on continental crust in pre-Miocene times, that in East Africa did not evolve oceanic crust.

Seismic, thermal and geochemical evidence suggest that the lithosphere under much of Africa is about 300 km thick, whereas under the rift it is today 40–50 km. Thin lithosphere can result either from mechanical necking or from the upward migration of the isotherm defining the lithosphere/asthenosphere boundary. Both processes are important in the evolution of a continental rift into a spreading ocean, but the limited extension in East Africa indicates that here the latter must dominate. It is, however, difficult to transfer sufficient heat into the lithosphere from below in the time available; conductive processes are too slow, and transfer by magma, although fast enough, requires more mass addition to the lithosphere than is compatible with the limited surface extension. Hot volatiles might transfer heat to the lithosphere from below and depress melting temperatures without adding significantly to its mass.

Rifts have been attributed to convective mantle plumes or to membrane stresses in the lithosphere. If the lithosphere is thick, mantle plumes are unlikely to generate rifts in the time available.
Structural development of the East African Rift system
Authors: Shackleton Robert M.
Published: May 2008

As the purpose of this volume is to outline the background to hominid evolution in East Africa, this paper emphasises the influence of structural development on palaeo-environments rather than the mechanism or causes of rifting.
Structural and volcanic evolution of the Gregory Rift Valley
Authors: King Basil C.
Published: May 2008

The East African Rift System extends southwards from the Red Sea for a distance of about 3000 km. Within East Africa it is represented by the Western Rift and the Eastern Rift, of which the Gregory Rift Valley is that part lying within Kenya and northern Tanzania.

The Gregory Rift Valley was initiated in early Miocene times as a downwarp along the continental watershed on a land surface having considerable relief (up to 2000 m). Subsequent faulting produced a graben about 80 km wide and some 450 km in length, to the north and south of which the faults splay outwards over much broader zones.

Volcanic activity was not confined to the Rift, but developed both to the west in the Miocene and to the east from Plio-Pleistocene to Recent times. Within the Rift itself volcanism commenced in the north and extended to the south in the Pliocene, while both tectonic and volcanic activity tended later to restriction within a central zone, which from Lake Hannington northwards formed a narrow axial trough.

Episodes of volcanism and sedimentation largely or wholly infilled the downwarping trough, but only in the Pleistocene, by uplift of the shoulders and downfaulting of the floor, was the morphology of the present Rift Valley achieved. The altitude of the sub-volcanic surface averages about 1500 m along the shoulders of the Rift; its greatest depth below the floor may be southwards from near Lake Baringo, corresponding to an absolute displacement of some 4 km. About 8 km of crustal extension over about 8o km of rift width may have occurred, but there is no evidence for crustal separation.
Character of Quaternary volcanism in the Gregory Rift Valley
Authors: Williams Laurence A. J.
Published: May 2008

The latest stages in the evolution of the rift in Kenya were marked by intense volcanic activity along the graben and massive eruptions across the plateau to the east. Within the rift, trachytic volcanoes south of the equator have calderas at least 10 km across. They erupted more abundant ash-fall and ash-flow deposits than the volcanoes in a northern group which are characterised by smaller calderas. Flood trachytes occupy much of the rift floor in central and southern Kenya, but fissure basalts become increasingly more prominent to the north where they are conspicuous in late eruptions across the caldera volcanoes. Variations in the nature of almost exclusively basaltic activity east of the rift produced impressive linear multicentre ranges in some areas, thin yet extensive flows in others, and one field dominated by maar formation. In northern Tanzania, strongly alkaline volcanism is superimposed on both the rift floor and plateau regimes mentioned above. Late caldera collapse on some trachytic shields was accompanied by build-up of large nephelinite cones and a complex volcano having a broad basaltic lava pile surmounted by phonolites and trachytes. Volcanic and tectonic processes combined to exert a profound influence on patterns of drainage and sedimentation throughout the region, thereby both creating and destroying environments favourable for occupation by early man.
Geophysical investigations and the Rift Valley geology of Kenya
Authors: Khan M. Aftab , Swain Christopher J.
Published: May 2008

During the last decade there has been considerable activity in geophysical investigations in the East African Rift under the auspices of the Upper Mantle Project. Studies in the fields of explosion and earthquake seismology, gravity, magnetism, heat flow and electrical conductivity have considerably increased our understanding of the deep structure and processes beneath the Rift. The gravity data also throw light on the near-surface geology. A gravity map of Kenya at 1:2000000 has just been compiled at the University of Leicester using data from over 6500 stations, most of which have been recently established in the Rift area. The regional field can be represented by a seventh order trend surface with a ‘saddle’ centred at Marsabit in the north. From the centre of this, the regional field rises north-westwards towards Lake Turkana and falls south-westwards towards a pronounced bowl-shaped minimum centred in the Narok-Naivasha region near the culmination of the Kenya dome. The trend surface correlates inversely with the topography and may be attributed to deep structure. The residual gravity field of the Rift Valley region contains a number of features which are clearly related to the visible geology.

The axial region is associated with an undulating high along which lie a number of prominent caldera volcanoes—Suswa, Menengai, Silali, Emuruangogolak and Lomi. Further north the most spectacular high occurs at the syenite plug, El Moiti, which is probably the remnant of a trachyte volcano. The flanking negative anomalies can usually be correlated with low density lavas and sediments—notably at Lake Turkana, near Lokichar, the Kerio Valley and further south at Londiani and Ol Kalou, but lows are notably absent over the Baringo and Suguta basins. Quantitative interpretation requires an estimate of the true regional, as opposed to the synthetic regional of trend surface analysis, and is often extremely difficult to make, due to lack of geologic data. Nevertheless, several east-west profiles have been modelled, yielding, amongst other parameters, estimates of the thickness of sediments and volcanics.
Taphonomical background to fossil man-problems in palaeoecology
Authors: Hill Andrew
Published: May 2008

Taphonomy deals with the various processes affecting animals from their deaths to their possible fossilisation. The particular importance of work of this kind to the elucidation of features of fossil hominid behaviour and ecology is stressed. The subject of taphonomy is re-defined in terms of the differences existing between fossil assemblages and living communities of animals. A brief review is given of some of the factors to be considered in the interpretation of assemblages of terrestrial fossil vertebrates, particularly those that have some connection with hominids and hominid activity.
A statistical approach to temporal biostratigraphy
Authors: Shuey R. T. , Brown Frank H. , Eck G. G. , Howell F. Clark
Published: May 2008

Various statistics (similarity coefficients) can be constructed for faunal assemblages which vary more or less regularly as these assemblages change. Methods are described for placing assemblages on an evolutionary scale, the units of which are equal amounts of evolutionary transformation in the species makeup of assemblages. The greatest single factor involved in the scaling appears to be time; the effects of zoogeography are also discussed. Characteristics of different similarity coefficients differ, and some of these characteristics are discussed.

Applying our method to faunal assemblages from East African hominid sites, reasonable correlations can be established between Olduvai Gorge, Tanzania, the Shungura Formation, Ethiopia, and the Koobi Fora Formation, Kenya.
Allometry and Hominid studies
Authors: Wood Bernard A.
Published: May 2008

When the shape of a bone changes during growth the relationship between its shape and its size can be expressed mathematically and the phenomenon is known as allometric or relative growth. The same concept applies to samples of adult bones of different mean overall size. It is likely that some of the morphological distinctions between early hominids are due to size related allometric features. However, even if such differences can be credited with an allometric basis their significance for hominid taxonomy is uncertain.

This paper reviews the development of allometric theory and deals with technical advances which have improved our understanding of the theory and application of allometry.

Three relevant applications of allometry are considered. These are its use in attempts to compensate for allometrically based intra-group differences which distort attempts to differentiate between taxonomic groups; its role in taxonomy and its use as a tool to increase our understanding of the relationships between function and morphology in hominids.
The first geologists—the archaeology of the original rock breakers
Authors: Isaac Glynn Ll.
Published: May 2008

What good is a broken stone—then or now? How and when did this venerable practice begin? Did the Stone Age start with a big bang or a series of little taps? Aspects of East Africa’s long record of stone tools are considered from a geological and an evolutionary perspective.
Fossil hominids from the Laetolil Beds, Tanzania
Authors: Leakey Mary D. , Hay R. L. , Curtis G. H. , Drake R. E. , Jackes M. K. , White T. D.
Published: May 2008

Remains of 13 early hominids have been found in the Laetolil Beds in northern Tanzania, 30 miles south of Olduvai Gorge. Potassium-argon dating of the fossiliferous deposits gives an upper limit averaging 3–59 Myr and a lower limit of 3–77 Myr. An extensive mammalian fauna is associated. The fossils occur in the upper 30 m of ash-fall and aeolian tuffs whose total measured thickness is 130 m.
Geochronological problems and radioisotopic dating in the Gregory Rift Valley
Authors: Fitch Frank J. , Hooker Paul J. , Miller John A.
Published: May 2008

Geochronology is the science of geological time and has a wider connotation than radioisotopic dating. The two major tools of geochronology: stratigraphical palaeontology and radioisotopic dating are equally fallible and are best used in conjunction. Basic large-scale geological mapping and the erection of local rock-stratigraphies are necessary pre-requisites for the detailed geochronological analysis of an area. In the Gregory Rift Valley these requirements are beginning to be met at such places as Olduvai, Baringo, East Lake Turkana and Omo. Conventional K-Ar age determination of volcanic rocks and minerals has been used widely in the dating of East African Cenozoic successions. It is important that the theoretical and practical limitations of this otherwise very successful technique are fully appreciated. K-Ar and
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Ar
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Ar isochron dating are developments of the K-Ar technique that can be used to resolve some of the difficulties inherent in the conventional method. Isochron dating of rocks from Olduvai, the plateau phonolites of Kenya, East Lake Turkana and the Omo illustrates this point. Arguments are presented for rejecting discrepant apparent ages of less than 2.0 m.y. from the KBS Tuff, East Lake Turkana. It is suggested that fission track dating of volcanic zircons will resolve this controversy and provide independent time-scales for many East African and other volcanic successions in future.
Caledonian correlations: Britain and Scandinavia
Authors: Nicholson Robin
Published: May 2008

While there is widespread evidence of climactic late-Silurian thrusting in the southern Scandinavian Caledonides the scale of comparable early Ordovician (Grampian) movements is not yet established. Only in the lower, eastern, marginal nappes is there evidence of continuous sedimentation from late Precambrian up into lower Palaeozoic. In the metamorphic allochthon no fossils older than Tremodocian have been discovered. Barrovian metamorphism which seems to vary from nappe to nappe is widely developed: no genuine LT/HP metamorphism is recorded. The North Sea basin, largely founded on Old Red Sandstone (ORS) deposits of the terminal Caledonian stage, covers the junction between the two distinct Caledonian provinces of Britain and Scandinavia.