bluuebunny/crossref_metadata_2025_split huggingface dataset:
Orogeny in relation to crust and upper mantle
Authors: Bott M. H. P. 
Published: May 2008

Summary
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

Summary

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

Summary
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
3. Paleomagnetic dating
Authors: Evans Percy 
Published: May 2008

Summary
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

Summary
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
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.
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.
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

Summary
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.