Detailed petrological and geochemical investigations performed upon amphibolites from Eldora, Colorado have clearly demonstrated the heterogeneous nature by which rocks situated within the lower crust adapt to changes in the physico-chemical environments imposed upon them through time. The Eldora Amphibolites formed and attained both chemical and textural equilibration during the initial PreCambrian metamorphism of the Idaho Springs Formation <i>c</i>.1850my ago. Several alteration events have affected the Amphibolites since their formation. Migmatisation, with the widespread injection of leucocratic seams, occurred <i>c</i>.1750my ago. At least two phases of PreCambrian re-heating, caused by the intrusions of the Silver Plume and Pike's Peak Granites, have also taken place. The re-heating associated with the intrusion of the Silver Plume Granite <i>c</i>.1410my ago resulted in the re-setting of the K-Ar ages of the rocks. Both PreCambrian and Laramide (60-100my) mineralisation have been previously documented within other rocks from the Formation. It has been difficult to conclusively distinguish between these different aged mineralising events in many of the rocks at Eldora since both types produced similar textural and chemical changes within the amphibolites. A quartz monzonite stock which also intruded into the Amphibolites during the Laramide orogenic event has been shown to have had little noticeable effect upon the surrounding country-rocks except those within approximately 2m. of the contact. This conflicts with the classic text of Hart (1964), who postulated temperature-related modifications in an approximately 30m. wide aureole surrounding the Stock. This study has concentrated upon characterising, and establishing the relative timing of, the modifications which took place to original ferroan-pargasitic-hornblende crystals within the amphibolites. Four slightly different amphibole alteration types (termed Boundary, Microvein, Extensive single crystal and Extensive multi-crystal alterations) have been recognised and fully characterised using optical, electron microprobe and electron microscope techniques. Extensive single crystal and Extensive multi-crystal alterations are only seen in rocks situated within 2m. of the Stock. All the alteration types, irrespective of their proximity to the Stock contact are only ever present within certain parts of the rock, termed permeation zones and microfracture networks. These zones are believed to represent the routes via which fluids gained access to the rocks. The majority of the mineral alterations identified resulted from crystal-fluid interactions and are not the result of diffusive exchange associated with any re-heating (PreCambrian and/or Laramide) that may have affected the rocks. The 2m. zone surrounding the Stock is probably the product of alteration caused by the passage of metasomatic fluids emanating from within the Stock.

The Glenelg peninsular in NW Scotland contains Lewisian type rocks occurring east of the Moine thrust that have been metamorphosed and deformed along with the Moine rocks. These Lewisian rocks can be split into two facies, the Eastern Lewisian and the Western Lewisian, always separated by a thin strip of highly deformed Moine rocks. The Western Lewisian contains no recognisable meta-sediments, only a variety of meta-igneous rocks. Eclogite is rare, only occurring at one locality as pods within otherwise deformed amphibolite facies rocks. These eclogites exhibit broadly similar textures to the Eastern Lewisian eclogites, but have a slightly different chemistry, being more Si rich and Ca poor. They are formed under similar conditions of about 760°C, 15 Kb, but only one metamorphic event has been distinguished. Many of the amphibolites of the Western Lewisian have similar textures and chemistry as partly retrogressed Western Lewisian eclogite. These rocks were deformed to produce a weak fabric prior to the intrusion of further basic material that is texturally and chemically different to the eclogites. All of these rocks were then deformed and metamorphosed by the syn-Moine deformation as outlined above for the Eastern Lewisian. It is suggested that the Eastern and Western Lewisian rocks share a common history of initial high-pressure metamorphism, then the intrusion of granitic melt prior to further high pressure metamorphism. This was followed by deformation and partial retrogression before the intrusion of basic and ultrabasic dykes. The Lewisian rocks were then exhumed and the Moine rocks deposited onto them prior to further deformation and metamorphism. Radiometric dating suggests that the high pressure metamorphism occurred at approximately 1050 Ma (Grenville), but field and petrographic evidence suggests that the Lewisian rocks have undergone a similar history to the Scourian rocks of the main Lewisian outcrop and that Grenville ages for the rocks date initial syn- Moine metamorphism.

No description available.

No description available.

The Vrondou pluton is an Oligocene high-K calc-alkaline composite granitoid body emplaced in the Lower Tectonic Unit of the Western Rhodope Massif, adjacent to its western margin, and borders the Strymon and Serres basins to the west and south, respectively. Hornblende-biotite quartz-monzonite and biotite-hornblende granite s.s. crop out in the central-western and southern areas, whereas biotite-hornblende granodiorite crops out in the southeastern areas. Clinopyroxene-hornblende monzonite and porphyritic monzonite occur in the central-eastern and northeastern areas. Minor rock types include gabbro, microgranular enclaves and amphibole-bearing lamprophyre dykes. Textural evidence, especially the abundant presence of plagioclase with disequilibrium zoning features (patchy zoning, calcic spikes) in the granitoids and their enclaves, suggests that hybridisation was responsible for their genesis. Enclaves are interpreted as magmatic, probably with a multi-stage crystallisation history, the final stage of crystallisation taking place in chemical and thermal equilibrium with their hosts, as is also indicated by the close chemical affinities between enclave and host ferromagnesian minerals. Major and trace element modelling suggest that the quartz-monzonites, granites and granodiorites are inter-connected by the fractionation of parental quartz-monzonite, and possibly that the monzonites and porphyritic monzonites are inter-connected by the fractionation of a parental monzonitic magma. Rare earth element and trace element patterns of the granitoids suggest that they have a common source and have features similar to magmas generated in active continental margins or post-collisional settings. The basic rock-types show subduction-related trace element enrichments and a garnet-absent source, and are inferred to have been generated from subcontinental mantle lithosphere, enriched during subduction. The Sr and Nd isotopic compositions of the granitoids and the gabbro rule out a single stage basic-to-acid fractionation process. Instead, it is proposed that basic magmas underwent fractional crystallisation concurrent with assimilation in a transient deep magma chamber to produce an initial quartz-monzonite batch, parental to the evolved rocks.

Subduction-zone metamorphism (SZM) is considered to be a major geochemical process on Earth of both petrological and geodynamic significance that triggers the subduction-zone magmatism and contributes to the mantle compositional heterogeneity. To understand SZM and elemental responses to SZM, detailed petrological and geochemical studies were conducted on metamorphic rocks of basaltic and sedimentary protoliths from two orogenic belts, i.e., Western Tianshan and North Qilian Mountain, in NW China. Based on the bulk-rock geochemistry of rocks from ultrahigh pressure metamorphic belt of Western Tianshan, different elemental mobility/immobility has been identified using the inter-elemental correlations. Mineral compositions have also been analyzed for the same rocks. The significant elemental hosts are phengitic muscovite, paragonite, garnet, epidote group minerals, rutile and titanite. Together with detailed petrography and considering a series of plausible metamorphic reactions, we conclude that it is the presence and stability of these minerals that largely controls the geochemical behaviors of chemical elements during SZM. In terms of both bulk-rock composition and mineral geochemistry for rocks from North Qilian Mountain, we conclude the same except the mobility of U, which may be attributed to the seafloor alteration rather than SZM. The consistent immobility of U, Th and light rare earth elements (LREEs), like high field strength elements (HFSEs), during SZM indicates that the enrichment of these elements in arc magmas is not caused by simple dehydrated aqueous fluids. Therefore, the traditionally accepted fluid flux induced-melting needs reconsideration in order to explain the arc signature in melts produced through subduction-zone magmatism. In addition, the lack of Rb/Sr-Sm/Nd (or Lu/Hf) correlation in these and other metabasites world-wide is inconsistent with the observed first-order Sr-Nd (or Hf) isotope correlation in oceanic basalts. Hence, the subducted residual ocean crust cannot be the major source materials for oceanic basalts although it can contribute to mantle compositional heterogeneity.

This study is concerned with the Tertiary igneous rocks found within an area some twenty square miles in extent on the south-east side of the Skye central intrusion complex. The Tertiary rocks include basic lavas, a small intrusion of granophyre, the margin of the Cuillin Gabbro, and numerous minor intrusions. Extrusive rocks occupy an area of some seven square miles, and have an aggregate thickness of about 1,000 feet. The greater part of the sequence consists of alkaline olivine basalts, but there is also one horizon of trachybasalt and thin group of picritic flows. Pyroclastic rocks are rare, being limited to one small agglomerate vent and an isolated lens of tuff. It has been found possible to divide the sequence into several mappable units based on groups of flows possessing common petrographical features. The laves which lie within half a mile of the Cuillin Gabbro have been metamorphosed by that intrusion, while an outer zone of lower temperature alteration includes the whole of the strathaird lava outcrop. Within the thermal aureole a narrow zone of pyroxene hornfels along the contact with the Gabbro is followed outwards by a broader zone charaterized by the presence of an actinolitic amphibole. Within this pyroxene hornfels zone the lavas have been thoroughly recrystalized to high temperature mineral assemblages, but within the amphibolezone equilibrium was only locally attained, and many of the lavas still posses much of their original texture and mineralogy. The transition from unmetamorphed lava to pyroxene hornfels was not accomplished by independently under its own particular pressure-temperature conditions. On the basis of both chemical and petrographical evidence it is considered probable that a small amount of silica was added to at least some of the lavas during metamorphism. The internal structure and composition of the Cuillin Gabbro do not come within the provenence of this thesis, but some attention has been given to the relationships between the Gabbro and its country rocks, especially in the neighbourhood of camasunary. In this latter area there is an extensive development of basic minor instructions pre-dating the Gabbo, and metamorphosed in a similar manner to the nearby lavas. Most of these small intrusions are emplaced into Torridonian sandstones, which locally extend up to several hundred feet from the contact, the Torridonian shows clear evidence of having been partially melted by the high temperatures induced by the intrusion of gabbro magma, and many of the early intrusions are veined and broken into fragments by the mobilized material. The vein material is chemically more basic in composition than the sediments from which it was derived, partly due to reaction with the basic minor intrusions. Parallel to the contact of the Cuillin Gabbro, in Strathaird, there is a series of irregularly shaped outer-ops of granphyre which are thought to be parts of a single intrusion exposed in the vicinity of its roof. The granophyre is fine-grained, is unusual in containing paramorphs after tridymite, and differs chemically from other Skye granitic rocks. Through there is sometimes a well-defined, chilled margin, many of the contacts against sedimentary rocks are complex and highly xenolithin. The granophyre post-dates the Cuillin Gabbro, but is earlier than the large granitic intrusions which lie to the north and north-east of the areas. One of the most impressive features of the local geology is the great profusion of north-west dykes, all but a few of which are basic in composition. Most of the dykes are later date than the Cuillin Gabbro, but their distribution shows that they are closely related to that centre. Other minor intrusions include a few acid and basic skills which are, in general, of earlier date then the dyke swarm, while basic cone-sheets occur in the vicinity of the Cuillin Gabbro and are later than most of the north-west dykes. In Strathaird, erosion to a relatively deep level allows the structural relationships between lavas, intrusions, and pre-Tertiary sediments to be more easily examined than elsewhere in Syke. Most of the sediments are of Jurassic age, and these rest uncon-formably on Torridonian sandstones. There were several phases of warping in Mesozic times, but the main period of deformation occurred after the outpouring of the lavas and gave rise to large arcuate folds. It is believed that these folds are one result of the stresses set up by the intrusion of the Cuillin Gabbro.

During the earlier part of the Gardar Period, continental sandstones and volcanics (principally flows of olivine basalt) were laid down in the Ilḿmaussaq region; strata with a maximum thickness of over 3km. are preserved. The region was characterised by block faulting with sizeable dislocations and the stratigraphic succession on each fault block is different. The geology of each of the blocks is discussed in turn and, in a final synthesis, the successions of the blocks are correlated and a preliminary analysis made of the fault system. In addition to the basalt magma, which was erupted quietly in great quantities, a quite separate volatile-fich ultramafic magma was available in the region at intervals during the early Gardar. It was responsible for the drilling of numerous diatremes and for a phase of intense, explosive volcanic activity. There is clear evidence of the presence of a related, concealed carbonatite body, located near Qagssiarssuk, in the north of the area. The associated lampropbyric rocks in this vicinity have been subjected to calcitic carbonatisation, followed by ankeritic carbonatisation, while the country rocks have been subjected to potash feldspathisation. Necks, sills and flows of carbonatissd uncompahgrite occur. The petrography of the volcanic rocks is discussed in some detail and chemical analyses are provided. Separate studies, in which X-ray techniques were used extensively, have been made of the carbonates, the alkali feldspars and the trace elements, in rocks associated with the carbonatite. The lamprophyre-carbonatite vulcanism has many parallels in the volcanic fields of East Africa, but the presence of olivine basalt magnia in the area at the same time is an unusual feature. On occasions, the two magmas were erupted almost simultaneously, yet there is no indication of intermingling or mutual contamination.

The Lahanos pyritic sulphide deposit is one of the important copper deposits occurring within the submarine volcanic environment of the Eastern Pontus Ore province. It is situated 10 miles to the south of Espiye, Giresun Vilayeti. Petrographic study of the Lahanos rocks, despite their heavy alteration, shows that they belong mainly to the calc alkaline and tholeiitic basalt suites, and in addition, there are some high-alumina basalts. The intense igneous activity took place during the Mesozoic and the early and middle Tertiary periods. This phase of igneous activity culminated in the intrusion of syntectonic granites, tonolites and adamellites, and was followed by aplitic and hydrothermal phases, in the Pontid Geosyncline. The ore mineralisation, associated with dacites, is characterised by large quantities of pyrite associated with other base metal sulphides such as chalcopyrite, sphalerite, galena and other sulphides and sulphosalts. Field, underground and borehole evidence shows that the Lahanos ore body displays a zonal arrangement, which is similar to the well documented "Kuroko" type ore deposits in Japan. The results of detailed chemical and mineralogical study of the ore and country rock are given and the temperature of formation is estimated. Comparative details are given for the Murgul and Karadere deposits. The evidence supports an epigenetic hydrothermal origin for this "strata bound" deposit, but the volcanicity alone is inadequate to explain the source of the base metals, which are probably derived from the granites. The role of pyroclastic horizons of restricted permeability as a control of precipitation is discussed.

The Borrowdale Volcanic Group constitutes a major part of the Ordovician succession in the English Lake District. It comprises a suite of lavas, tuffs and ignimbrites with a maximum measured thickness of 5 km. The rocks of the Lake District are folded into a broad anticline which results in the Borrowdale Volcanics being exposed in two main outcrops, one north and the other south of a central core of older Skiddaw Slates. The northern outcrop consists almost entirely of basalts, basaltic andesites and occasional rhyolites, with very few intermediate members. In contrast, the southern outcrop is composed largely of andesites and dacites. Garnet phenocrysts, absent in the northern outcrop, are relatively abundant in the volcanic rocks of the southern outcrop. Analyses of 229 samples of lavas, ignimbrites and associated intrusives are presented together with electron microprobe analyses of selected garnet and augite phenocrysts. The southern outcrop volcanics are of calc-alkaline affinities, whereas those of the northern outcrop are transitional in character between tholeiitic and calc-alkaline. The use of La/Y ratios is shown to be particularly effective in distinguishing between members of the two suites. Detailed analytical studies on the garnet phenocrysts, especially La and Y abundances, show that crystal fractionation of garnet phenocrysts is incompatible with the geochemistry of their host rocks. It is concluded that the southern outcrop magmas evolved by some process other than crystal fractionation. A partial-melting hypothesis is proposed as an alternative, the melt being stored at depth (possibly at the crust mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. In contrast, the northern outcrop lavas are highly porphyritic and present abundant evidence of crystal fractionation. It is suggested that these rocks evolved by the fractionation of a basalt or basaltic and site parent under relatively dry conditions at shallow depth. The Borrowdale Volcanics are compared with the volcanic rocks of modern Island arcs. In particular the southward transition of magna type from tholeiitic to calc-alkaline compares with similar transitions occurring across modern island arcs. It is concluded that the Borrowdale Volcanics were erupted in an ancient Island arc at the margin of a contracting, proto-Atlantic ocean. This hypothesis is consistent with current models for the evolution of the Caledonian Appalachian orogen. The Borrowdale magmas were probably derived by the partial melting of basaltic oceanic crust carried down into the mantle on descending lithosphere plates. In the case of the southern outcrop rocks the magma- was not affected by subsequent ciystal fractionation, whereas the northern outcrop magma has undergone considerable modification by this process. Finally- the partial melting of oceanic crust is examined in the light of recent experimental studies. It is suggested that island arc tholeiitic magmas are generated at shallow depth by reactions involving amphibole breakdown. Calc-alkaline magmas are produced at greater depths by the partial melting of wet eclogite, and between these two extremes a continuum of transitional magma types could be generated.