Igneous Rock Units Research Articles

U-Pb geochronological constraints on the genesis of the Olympic Dam Cu-U-Au-Ag deposit, South Australia

The Olympic Dam Cu-U-Au-Ag deposit is hosted by the Olympic Dam Breccia Complex, a high-level, hematite-rich hydrothermal breccia system which occurs entirely within, and clearly postdates, the Roxby Downs Granite. The U-Pb zircon age of the Roxby Downs Granite is well constrained at 1588 + or - 4 Ma. Field evidence indicates that brecciation, hydrothermal alteration, and Cu-U-Au-Ag-REE mineralization in the Olympic Dam Breccia Complex were penecontemporaneous, but this is inconsistent with differing ages from previous U-Pb and Rb-Sr isotopic data on pitchblende and sericite. New SHRIMP U-Pb zircon data for three Olympic Dam Breccia Complex igneous rock units confirm that Cu-U-Au-Ag-REE mineralization was contemporaneous with brecciation and alteration and directly followed cooling of the Roxby Downs Granite by as little as several (<8) million years. Two of the dated Olympic Dam Breccia Complex rock units are interpreted as autobrecciated felsic dikes. One crosscuts laminated hematite-quartz sandstones, and the other crosscuts Cu-U-mineralized hematite- rich breccias. Zircons from these dikes yielded ages of 1592 + or - 8 and 1584 + or - 20 Ma, respectively. The third unit dated is a disrupted ash-fall tuff horizon from within a diatreme which crosscuts hematite-quartz breccias near the center of the deposit. The northern extremities of this diatreme also crosscut hematitic breccias containing a substantial Cu-U ore zone. Zircons from this tuff horizon have an age of 1597 + or - 8 Ma. The tuff and associated subaerial or subaqueous volcaniclastics were unconsolidated or only weakly lithified at the time of their incorporation into the diatreme, so the age of the tuff also reflects the age of diatreme activity. These new isotopic data indicate that the Olympic Dam Breccia Complex has an age of approximately 1590 Ma and therefore imply that brecciation closely followed emplacement and cooling of the Roxby Downs Granite. Because these breccias contain sulfide fragments and polyminerallic ore fragments, exhibit penecontemporaneous sulfide-oxide textural relationships, and show a close association between Cu + or - U + or - Ag + or - REE mineralization and Fe-rich breccias, we argue that the metals of economic interest were introduced...

Naming of igneous and metamorphic rock units in Antarctica: recommendation by the SCAR Working Group on Geology

Geologists from many countries have worked in Antarctica since the turn of the twentieth century and during the past thirty years the level of research activity on the continent has increased annually. As a result there has been a proliferation of stratigraphical names and lithostratigraphical schemes (Thomson, 1990). Furthermore, geologists from different nations are familiar with different standards and codes of nomenclature, which has resulted in a number of inconsistencies in Antarctic stratigraphical names. The SCAR Working Groups on Geology and Solid Earth Geophysics therefore recommended at their meeting in 1990 with SCAR XXII in São Paulo that, for sedimentary rocks, geologists working in Antarctica should adhere to the stratigraphical principles and recommendations proposed by Hedberg (1976). It was recognized, however, that igneous and metamorphic rocks present special problems of nomenclature. An ad hoc group of the SCAR Working Group on Geology for naming igneous and metamorphic rock units, constituted by the authors of this note, was therefore established to consider and discuss these problems and to formulate recommendations for suitable schemes that may be used internationally for Antarctica.

Early Proterozoic crustal evolution in the birimian of Ghana: constraints from geochronology and isotope geochemistry

Abstract Representative suites of Ghanaian granitoids, metavolcanics and metasediments were selected from Birimian rock units for RbSr, Pb/Pb and SmNd analyses. Isotopic data on the igneous rocks provide evidence for a major Early Proterozoic crust-forming episode which took place over a maximum time interval from ∼ 2.3 to ∼ 2.0 Ga by a differention from a depleted mantle source. There is little evidence for the involvement of significantly older (e.g. Archaean) crust in the genesis of the igneous rock units. The only exception so far found is the Winneba granitoid, which appears to contain a magmatic contribution from presumably underlying Archaean basement. SmNd model ages indicates that Birimian sediments were derived from adjacent penecontemporaneous volcanic belts. The Birimian of Ghana forms part of a major Proterozoic (Eburnean) episode of juvenile crustal accretion which has been recognised in the surrounding areas of Mauretania, Senegal, Ivory Coast, Burkina Faso, Mali and Niger, where it has been dated at 2.1–2.2 ga (abouchami et al.) The Birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents.

Geological, petrochemical and metallogenic characteristics of the Costa Rican gold belt: Contribution to new explorations

The Costa Rican Gold Belt is located in the northwestern part of Costa Rica. The geology of this region is characterized by tectonically juxtaposed volcanic-sedimentary and igneous rock units of Tertiary age.

Field Geologist's Guide to Lithostratigraphic Nomenclature in Australia

Abstract Stratigraphic nomenclature in Australia was regulated from 1950 by four successive editions of the Australian Code of Stratigraphic Nomenclature. The International Stratigraphic Guide was adopted for stratigraphic nomenclature in Australia in 1978 and thus superseded the Australian Code. Certain provisions of the Code and certain Australian practices still pertained, however, and these have been formulated as Notes, intercalated with extracts from Chapters 2–5 of the International Guide to form the Field Geologist's Guide to Lithostratigraphic Nomenclature in Australia. Biostratigraphic and chronostratigraphic nomenclatures are excluded. The lithostratigraphic nomenclature of igneous and metamorphic rock units is also excluded, as recommendations are still being formulated. Procedures for reserving, defining, and publishing stratigraphic names are administered by the Stratigraphic Nomenclature Committee in conjunction with the Bureau of Mineral Resources through the Bureau's Central Register of S...

Field relations in the north-eastern part of the Insch mafic igneous mass, Aberdeenshire

Synopsis A total field ground magnetic survey, supplemented by a programme of bedrock exposure in shallow pits, has enabled considerable revision to be made to the position of the north-eastern margin of the Insch mass. In addition the distribution and attitude of the basic igneous rocks and their associated hornfelses and the nature of the external contact is shown to be strongly influenced by zones of shearing deformation trending between E–W and NE–SW. Magnetic modelling has shown that the external contact of the mass, and the igneous rock units within the mass dip steeply (60°–70°) southwards, and that the igneous rocks are limited to a depth of about 5 km. New petrological information indicates that narrow but discrete zones of hornfelsed derivatives of the schistose and gneissose country rocks exist, and that major trends and discontinuities observed on the magnetic map correlate with zones of shearing and mylonitization which generate the complex distribution of the basic mass and its country rocks. The external contact is rarely undisturbed. Field relations in NE Scotland indicate a regionally important episode of shearing deformation about 470 Ma in age. Later faulting on NNW-SSE or NW-SE trends has further contributed to the disruption of the basic mass and its environs.

Latest Cretaceous and early Tertiary orogenesis in south-central Arizona: Thrust faulting, regional metamorphism, and granitic plutonism

Rocks in a number of mountain ranges in southernmost central Arizona are juxtaposed by thrust faults, regionally metamorphosed, and intruded by garnet–two-mica granites. Field relations and K-Ar and U-Pb isotopic geochronology indicate that thrust faulting, metamorphism, and granitic plutonism were closely related aspects of a latest Cretaceous and early Tertiary orogenic episode. The regionally metamorphosed rocks, chiefly greenschist-facies quartzofeldspathic schists, were derived from Jurassic and Cretaceous sedimentary, volcanic, and plutonic rocks. In five ranges, these metamorphic rocks are overlain, along synmetamorphic thrust faults, by Precambrian gneiss or Late Jurassic or Late Cretaceous plutonic rocks. In three additional ranges, upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range. Seven of these eight thrust faults have single lower plates, but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides. Areal structural relations and the regional arrangement of distinctive Jurassic igneous rock units strongly suggest that one large range, the Baboquivari Mountains, and several smaller ranges are fensters in a thrust-fault system of regional extent, and that several adjacent ranges are allochthonous. The regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early Tertiary leucocratic granites characterized by various combinations of accessory biotite, muscovite, and garnet. At least two of these granites contain inherited Precambrian zircon indicative of generation by crustal anatexis. K-Ar and(or) U-Pb isotopic ages of premetamorphic granites, metamorphic rocks, thrust-zone mylonites, synmetamorphic granites, and metamorphogenic fissure veins indicate that the orogenic episode commenced in Late Cretaceous time and culminated in early Tertiary time, 58 to 60 m.y. ago. The observed and inferred close spatial and temporal relations between thrust faulting, regional metamorphism, and granitic plutonism lead to the following hypothesis for orogenesis in south-central Arizona: crustal compression caused overthrusting of crystalline rocks, resulting in crustal thickening; and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place.

Mineralogic Information from a New Airborne Thermal Infrared Multispectral Scanner

A new six-channel aircraft multispectral scanner has been developed to exploit mineral signature information at wavelengths between 8 and 12 micrometers. Preliminary results show that igneous rock units can be identified from their free silica content, and that carbonate as well as clay-bearing units are readily separable on the digitally processed images.

Paleomagnetic investigations in igneous–metamorphic rock units in eastern New England

A clustering of paleomagnetic poles for rock units of mid-Paleozoic (ca. 400 Ma) age in eastern New England indicates that a variety of lithologic types may be used to obtain virtual geomagnetic poles (VGP's) in an igneous–metamorphic terrain. These rock types include chilled margins of basaltic dikes or narrow dikes, hydrothermal alterations such as epidote veinlets, mafic rocks reset by low-grade metamorphism, and hypabyssal igneous bodies, particularly where there is evidence of hydrothermal activity.The results suggest the approach to be followed to obtain valid VGP's in similar geological belts in the Precambrian. The remanence directions in Devonian and Mississippian rock units in eastern Massachusetts and southern and northern Maine are close to other determinations of similar age within the Maritime block, demonstrating the potential usefulness of the procedures.

Paleomagnetism of igneous rocks from the Belcher Islands, Northwest Territories, Canada

Paleomagnetic results from igneous rock units on the Belcher Islands, Hudson Bay, are described. Fold tests for all units studied, as well as a contact test for the intrusive bodies, indicate that both primary (initial), and secondary (post-folding) magnetizations are present.The paleomagnetic pole position from primary directions of the oldest unit studied, the Eskimo volcanics, is situated at 40°S, 002°E (A95 = 12°) and is similar to that derived from equivalent volcanics on the mainland. The younger volcanic unit studied, the Flaherty volcanics, yielded a pole position from primary directions at 0°, 244°E (A95 = 7°). The Haig intrusions, associated with these younger volcanics, yields an almost identical pole position at 1°N, 247°E (A95 = 6°), being derived from directions which are shown to be not only pre–folding but also date from initial cooling. The Eskimo volcanics, which have been more deeply buried than the Flaherty (upper) volcanics, carry substantial components of secondary (post-folding) magnetization which yield a pole position at 19°N, 243°E (A95 = 15°), about 20° north of the pole positions derived from the youngest units.It is argued that the apparent polar wander path (APWP) constructed for the Belcher Islands is representative of the mainland Ungava Craton. Comparison with the equivalent APWP from elsewhere in North America shows that the two APWP's are at variance. Although a two-plate model could be advanced, perhaps a more conservative interpretation is to extend the existing North American APWP eastward to include the Belcher–Ungava APWP, that is, to favour a one-plate model.

The structure of the eastern part of the Insch Mafic Intrusion Aberdeenshire

Synopsis A ground magnetic survey and a programme of shallow drilling have enabled considerable revisions to be made to the position of the margin of the Insch mass at its eastern end and revealed that the distribution and attitude of the igneous rock units have been strongly influenced by shear zones trendin g ENE. New petrological information shows that, except in a restricted area, there is a lack of regular relationships between the cumulate igneous rock units and it is established that the major trends and discontinuities in the contoured magnetic map can be correlated with zones of shearing and mylonitization. Magnetic modelling indicates that the shear zones dip steeply (65–80°) to the south and that the mafic units extend no further than about 2 km in depth. Later faults on N or NW trends have also contributed to the disruption of the intrusion. These findings are discussed in relation to regional structure and it is suggested that such shearing is more important than has been recognized hitherto and may constitute a discrete and extensive episode of deformation about 470 m.y. in age.

An experimental study of the titanomagnetite solid solution series

Analysis of saturation magnetization measurements and data from the literature indicate that the cation distribution in the titanomagnetite solid solution series is temperature dependent. The ionic configuration of ferric and ferrous ions on both lattice sites of their spinel structure can be described by a modified Boltzmann relation in agreement with theoretical consideratios. Thermodynamic equilibrium isotherms for the cation distribution and the resulting variation of saturation magnetization are calculated on the basis of the experimental values. These results should be especially valuable for the interpretation of magnetic properties of rapidly cooled igneous rock units where a respective high-temperature metastable state may exist in the magnetic ore component.

Mesozoic reversal sequence

Research directed toward the development of a magnetic polarity reversal sequence for the Mesozoic has been very active during the past few years, having been stimulated by the successful delineation of a polarity reversal sequence for the Tertiary by Heirtzler et al. [1968]. This research naturally divides itself into two parts: (1) the study of the magnetic record preserved in the oceanic crust and observed as anomalies of the earth's magnetic field and (2) the magnetic record preserved in individual igneous and sedimentary rock units and observed by paleomagnetic techniques. Both aspects of this work are made more difficult by the incomplete preservation of the record which has been locally destroyed by hiatuses or by subsequent tectonic activity. Thus it has been necessary to patch together a record that has been developed independently at many different localities. The sea floor record is now fairly well known back to 153 m.y. (Late Jurassic), and studies on land‐based sections and DSDP cores have resulted in the development of polarity reversal sequences for the latest Late Cretaceous, the Late Jurassic, and the Early and Late Triassic. The following discussion summarizes and attempts to integrate the two diverse data sets.

U-Th-Pb isotope systematics related to igneous rocks and ore Pb, Mount Isa, Queensland

This study is a search for a genetic relationship between Pb sulphide ore and igneous rocks in the region of Mount Isa, Queensland. The approach involves derivation of Pb isotope initial ratios by the whole-rock isochron method, and comparison of the initial ratios (Pb206/Pb204, Pb207/Pb204 and Pb208/Pb204) with the isotopic composition of the ore Pb. Data are reported for four igneous units; Kalkadoon granodiorite, Kalkadoon adamellite, Sybella granite and Eastern Creek volcanics. The results display considerable scatter for each of the units, and reveal the effects of recent surficial loss of U. The positioning of isochrons is aided by previous Rb-Sr geochronological data wherever possible. Comparison of initial ratios and ore Pb suggests that none of the igneous rock units is co-genetic with the ore deposit. Both phases of the Sybella Granite are more radiogenic and are apparently younger than the ore Pb. The Kalkadoon Granite is possibly related to the ore through some post-emplacement process of extraction and transport of Pb (e.g. by erosion or by anatectic magma generation) to the present site of the orebodies.

Paleomagnetism of Igneous Rock Units from the Coast of Labrador

Paleomagnetic results are reported for the first time from the coast of Labrador. The rocks are from (1) 20 sites of gently folded basalts of the Mugford Series, dated 948 ± 90 m.y., near Cape Mugford, and (2) 12 gabbroic dikes in a swarm dated 2080 ± 42 m.y., near Indian Harbour. Both dates are from preliminary, published K–Ar determinations. After partial alternating-field (AF) or thermal demagnetization, nearly all samples from both rock units revealed a stable thermoremanence that is most probably of primary origin. For the Mugford basalts, a mean remanence direction relative to bedding planes was calculated, after AF treatment, from 16 stably magnetized sites ([Formula: see text] flows). The corresponding pole is 49 °N, 143 °W, with dp = 9°, dm = 11°, which is northeastward of most North American poles from rocks dated 1200-1000 m.y. The pole calculated from the 12 Indian Harbour dikes, after AF treatment, is 6 °S, 117 °W, with dp = 6°, dm = 12°.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - The shear along the Dead Sea rift

Recent surface and subsurface geological investigations in Israel and Jordan provide new data for the re-examination of Dubertret’s (1932) hypothesis of the left-hand shear along the Dead Sea rift. It is found that while none of the pre-Tertiary sedimentary or igneous rock units extend right across the rift, all of them resume a reasonable palaeographical configuration once the east side of the rift is placed 105 km south of its present position. It is therefore concluded that the 105 km post-Cretaceous, left-hand shear along the Dead Sea rift is well established. The 40 to 45 km offset of Miocene rocks and smaller offsets of younger features indicate an average shear movement rate of 0.4 to 0.6 cm a -1 during the last 7 to 10 Ma. Unfortunately, the 60 km pre-Miocene movement cannot be dated yet. Along the Arava and Gulf of Aqaba and in Lebanon the shear is divided over a wide fault zone within and outside the rift.

Paleomagnetism of the Beaver Bay Complex, Minnesota

The Beaver Bay complex is a series of intrusions of gabbroic rocks related to the Middle Keweenawan Duluth gabbro complex. The Beaver Bay ferrogabbro, one of three units into which the complex has been divided, is an iron-rich gabbro with a very prominent igneous lamination. Magnetic anisotropy caused by preferential alignment of inequant magnetite grains within the lamination appears to have deflected the direction of remanent magnetization of the ferrogabbro by about 8.5°. When samples from the ferrogabbro are eliminated, the resulting paleomagnetic pole for the Beaver Bay complex falls at 27.5°N, 170.5°W, where it forms part of a distinct cluster of paleomagnetic poles for Keweenawan igneous rock units of negative (normal) polarity. Positively magnetized Keweenawan igneous rocks, which appear to be older than the negatively magnetized units, give paleomagnetic pole positions that are scattered over a considerable area to the northeast of the cluster of negative poles. Magnetic intensities vary markedly throughout the complex, probably in response to the ordinary processes of magmatic differentiation.

A Geophysical Study of North Park and the Surrounding Ranges, Colorado

A geophysical study in the North Park basin and surrounding mountains, Colorado illustrates the structural relationship of various sedimentary, metamorphic, and igneous rock units. Bouguer anomalies from 1330 gravity stations range from −210 mgal over Precambrian metamorphic rocks in the mountains to −260 mgal in the Walden syncline and —280 mgal in the North Park syncline. Steep gradients delineate a fault which strikes west-northwest along the north flank of the North Park syncline. Two models fitted to the gravity data show 1 to 2 km relief on this steeply dipping fault. Density contrasts between Precambrian metamorphic and igneous rocks produce anomalies of as much as 25-mgal amplitudes in the Park and Medicine Bow Ranges. A 30-km-long seismic refraction profile, parallel to the most negative Bouguer anomaly values in the North Park basin, shows velocities increasing from 2.5 to 3.4 km/sec within Tertiary rocks at depths ranging from 1.2 to 2.0 km. Mesozoic sedimentary rocks have a velocity of 4.0 to 4.5 km/sec, a very high velocity in view of the predominance of Upper Cretaceous rocks. Precambrian basement with a velocity of 6.25 km/sec underlies the profile at depths ranging from 3.5 to 4.5 km. Strong second arrivals across the profile, observed at distances of more than 14 km from the shotpoints and interpreted as SP reflections, verified the refraction model. An aeromagnetic survey shows numerous anomalies ranging from 100 to 200γ in the Park and Rabbit Ears Ranges and in the Never Summer Mountains, to 400γ in the Front Range, and to 1200γ over the Medicine Bow Range. Positive anomalies in the Park, Medicine Bow, and Front ranges overlie metamorphic rocks. Magnetic and gravity data suggest that the Never Summer Mountains are separated from the Front Range by a north-trending, steeply east-dipping reverse fault, extending beneath the Front Range along the Colorado River valley. The magnetic data indicate that this fault may connect with a possible fault that is parallel to the Laramie River valley. In the Rabbit Ears Range, a series of magnetic anomalies show that igneous rocks are present in the eastern part of the range. A northeast-trending positive magnetic anomaly, which is parallel to foliation trends reported in Precambrian rocks, extends from the Park Range across the North Park basin to the Medicine Bow Range. On the basis of this anomaly, the high seismic velocity of the Precambrian basement, and computed profiles fitted to the gravity and magnetic data, we infer that much of the basin is underlain by high-density metamorphic rock. As shown by gravity data, the deepest part of the basin is 2.7 km below sea level, resulting in a maximum relief of 6.7 km on the basement, relative to the Medicine Bow Range. A 25-mgal negative gravity anomaly and a zone of negative magnetic anomalies outline a large granitic intrusion in the Park Range, which probably extends northeast beneath the North Park basin and connects with granitic rocks in the Medicine Bow Range.

A graphic story of magmatic differentiation

The processes of magmatic differentiation can be subdivided roughly into those that operate 1) at the time of magma formation, 2) while a magma is in transit, 3) during the emplacement of a magma, and 4) when a body of magma is at rest. Many different variables, some of which are listed here, determine what processes will be dominant at any particular stage during the evolution and crystallization of a magma. A check-list of factors that may affect the crystallization history of a magma will assist petrologists in searching for models to explain the genesis of igneous rock units interpreted as consanguineous.

ISOTOPIC STUDIES IN THE ROSKRUGE MOUNTAINS, PIMA COUNTY, ARIZONA

K-Ar dating of igneous rock units in the Roskruge Mountains, Pima County, Arizona, indicates that periodic activity occurred in mid-Cretaceous time, during the Late Cretaceous–early Tertiary (Laramide) interval, and in mid-Tertiary time. The general sequence of rock types and periods of activity are very similar to those in the adjacent Tucson Mountains. The oldest date (108 m.y.) is from an andesite in the Cretaceous basement of the range. The Laramide ages (74–66 m.y.) are on a series of ash flows and an associated pluton which give initial Sr 87 /Sr 86 ratios ranging from .706 to .709. The mid-Tertiary activity includes the emplacement of a small pluton at 34 m.y., the extrusion of basaltic andesites at 23 m.y. (initial Sr 87 /Sr 86 ratio of .709), and the formation of a small well-preserved ash-flow sheet at 13 m.y. (initial Sr 87 /Sr 86 ratio of .707). The latest known igneous activity in the range is represented by basalt dikes and a flow(?) which are 10 m.y. old and have initial Sr 87 /Sr 86 ratios of .704.