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Geology topics

J. S. Stacey

Publications and source records attributed to J. S. Stacey.

At least 19 recordsLinked to original sources
USGS2022

U-Pb zircon geochronology and geological evolution of the Halaban- Al Amar region of the eastern Arabian Shield, Kingdom of Saudi Arabia.

U-Pb zircon model ages for eleven major units from the Halaban-Al Amar region of the eastern Arabian Shield indicate three stages of evolution: (1) plate convergence, (2) plate collision, and (3) post-orogenic intracratonic activity. Convergence occurred between the western Afif and eastern Ar Rayn plates that were separated by oceanic crust. Remnants of oceanic crust now comprise the ophiolitic complexes of the Urd group. The oldest plutonic unit in the study is from one of these complexes and gave an age of 694 ± 8 Ma. Detrital zircons from the sedimentary Abt formation of the Urd group, which is intercalated with the ophiolitic rocks, were derived from source rocks with a mean age of 710 Ma. The Abt formation may be an accretionary wedge on the western margin of the Ar Rayn plate. Plate convergence was terminated by collision of the Afif and Ar Rayn plates during the Al Amar orogeny which began about 670 Ma. During collision, the Urd group rocks were deformed and in part obducted on to one or both plates. Synorogenic leucogranitoid rocks were intruded from 670 to 640 Ma. From about 640 to 630 Ma, widespread unfoliated dioritic plutons were emplaced in the Ar Rayn block, and represent the end of orogenesis related to collision. There is no definitive evidence for a significantly older basement beneath the study region.

Journal of the Geological Society↗
USGS2022

U-Pb isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield.

This area includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. U/Pb zircon data from a pelitic garnet-sillimanite gneiss show that the Kabib formation in the S of the Afif terrain may be as old as 1770 m.y. Pb and Rb/Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the NE to less radiogenic marginal arc rocks in the SW. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation. U/Pb zircon age determinations show that this 'Andean' continental margin developed before approx 720 m.y. and the emplacement of calc-alkaline plutonic rocks continued until approx 690 m.y. During the period 685-640 m.y. the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approx 640 m.y. age the Najd strike-slip faulting commenced, with a dextral phase that controlled emplacement of granite plutons as well as the development of large pull-apart grabens. Some of the latter were floored by new oceanic crust and filled with volcanosedimentary rocks of the Bani Ghayy group.-R.A.H.

Journal of the Geological Society↗
USGS2002

Servo-amplifiers for ion current measurement in mass spectrometry

A servo-voltmeter can provide a useful alternative to the d.c. amplifier or vibrating reed electrometer for the accurate measurement of mass spectrometer ion currents, and has some advantages which recommend its use in certain applications. A generalized analysis based on servomechanism theory is presented as an aid for understanding the design criteria for this type of device. Two existing systems are described and their operation and performance are examined.

Journal of Scientific Instruments↗
USGS1992

Age and nature of the basement in northeastern Washington and northern Idaho: isotopic evidence from Mesozoic and Cenozoic granitoids

K-feldspar Pb and whole rock Nd isotopic analyses from 25 Mesozoic and Cenozoic plutonic rocks and two gneisses from NE Washington and northern Idaho are used to elucidate the age and nature of the concealed cratonic basement. The plutons form two highly distinct isotopie groups: Group I (hornblende-biotite and two-mica plutons of all ages) have Pb isotopie compositions suggesting derivation from rocks of the Belt Supergroup or their metamorphosed equivalents, although Nd isotopie data can only support this model if there was a significant input of juvenile mantle-derived or ancient light REE-depleted material. Group II (hornblende-biotite of Eocene age) have highly retarded Pb isotopie compositions relative to the present day crustal average and require a source region with long-term U (and other LIL) depletion, characteristic of cratonic lower crust. A U-Pb zircon upper intercept age of ca. 2600 Ma obtained from one of the Group II samples, together with Sm-Nd data from the gneisses, indicates possible late-Archean crust at depth, which acted as a source region for Eocene extension-related plutonism. Isotopie compositions and apparent geochemical evolution do not support a direct correlation with the nearest exposed North American Craton in the Wyoming province. If it represents attenuated pre-Mesozoic craton, then it must have been accreted to the craton prior to development of the miogeocline in the Late Proterozoic. Alternatively, it may be part of the Cordilleria terrane accreted to the craton in the Early Cretaceous.

Journal of Geology↗
USGS1991

Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, north Cascade Range, Washington and British Columbia

The Skagit Gneiss Complex forms a more-or-less continuous terrane within the northern, more deeply eroded part of the North Cascade Range. The complex comprises abundant plutons intruded at mid-crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic-arc origin. A plethora of syntectonic pegmatite, small plutons, and granitic dikes gives the complex a migmatitic aspect. U-Pb zircon ages from gneissic plutons within and near the Skagit Gneiss Complex indicate magmatic crystallization between 75 and 60 Ma. Deformation, recrystallization, and migmatization in part postdate intrusion of the 75-60 Ma plutons. This latest Cretaceous and earliest Tertiary plutonism and migmatization may reflect thermal relaxation following early Late Cretaceous orogeny documented else-where in the North Cascades. The complex was ductilely extended northwest-southeast shortly after intrusion of granite dikes at ∼45 Ma, but before emplacement of the earliest (∼34 Ma) plutons of the Cascade arc. Outcrops of Late Cretaceous and earliest Tertiary plutons, migmatites of the Skagit Gneiss Complex, and rocks with young ductile deformation are roughly coextensive, all apparently marking a region of greater middle Eocene unroofing. Unroofing was apparently contemporaneous with east-west extension in the Okanogan region to the east and north-south and northwest-southeast strike-slip faulting within the North Cascades.

Washington↗
USGS1988

Precambrian ophiolites of Arabia: Geologic settings, U Pb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $ ̆ 1250 Ma "> 1250Ma . The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900 km-long Nabitah mobile belt was dated by the zircon U Pb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Precambrian Research↗
USGS1987

Arabian Shield ophiolites and Late Proterozoic microplate accretion

Fragments of Late Proterozoic ocean crust and mantle (ophiolites) occur within six major fault zones that mark sutures between crustal blocks (microplates) that were accreted between about 630 and 715 Ma to form the Arabian Shield. We report new U-Pb zircon ages for ophiolitic gabbro, diorite, and plagiogranite that range from 840 to 700 Ma and establish these complexes among the oldest proven ophiolites. By dating the ophiolitic rocks we are able to monitor the magmatic ages of sea-floor spreading events during accretion of the Arabian Shield. Comparison of the ophiolitic dates with the ages of the adjacent crustal blocks provides a more complete basis for plate-tectonics reconstruction of the shield than has previously been possible. Our new zircon ages confirm earlier Sm-Nd mineral isochrons for ophiolites of the northwestern Arabian Shield, show that the ophiolites are among the oldest oceanic rocks in each terrane, support near- or within-arc tectonic settings for the ophiolites, suggest that the Bir Umq suture extends north along the Nabitah mobile belt into the northern shield, and suggest that older (>1250 Ma) continental crustal material is locally present in the dominantly “oceanic” western shield. We suggest that Indonesia and Alaska provide Phanerozoic analogues for stages in the accretion history.

Geology↗
USGS1986

U-Pb isotopic evidence for accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

The region covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrane, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrane. The geology of the area is well understood, and this geochronologic and isotopic study confirms that the Afif terrane was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. U-Pb zircon age data from a pelitic garnet-sillimanite gneiss of the Kabid formation shows that this part of the continental basement in the Afif terrane may be as old as 1830 Ma. Isotope data indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1830 Ma, and require that Archean rocks exist at depth beneath the Afif terrane. Lead isotope data reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic, marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a fades change within a pre-collision marginal basin. Miogeosynclinal continental shelf fades of the Siham group lie unconcormably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this "Andean" continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 690-640 Ma, the continental Afif microplate collided with the Asir terrane as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip orogen commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcano-sedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike slip motion at about 620 Ma ago.

Open-File Report↗
USGS1983

Distribution of oceanic and continental leads in the Arabian-Nubian Shield

New common lead data for feldspar, whole-rock, and galena samples from the Arabian-Nubian Shield, together with data from previous work, can be divided into two main groups. Group I leads have oceanic (mantle) characteristics, whereas group II leads have incorporated a continental-crustal component of at least early Proterozoic age. The group I leads are found in rocks from the Red Sea Hills of Egypt and the western and southern parts of the Arabian Shield. Group II leads are found in rocks from the northeastern and eastern parts of the Arabian Shield, as well as from the southeastern Shield near Najran. They are also found in rocks to the south in Yemen, to the east in Oman, and to the west at Aswan, Egypt. This distribution of data suggests that the Arabian-Nubian Shield has an oceanic core flanked by rocks that have developed, at least in part, from older continental material. Two mechanisms are suggested by which this older lead component could have been incorporated into the late Proterozoic rocks, and each may have operated in different parts of the Shield. The older lead component either was derived directly from an underlying early Proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent. New U-Pb zircon data indicate the presence of an early Proterozoic basement southeast of Jabal Dahul in the eastern Arabian Shield. These data, together with 2,000-Ma-old zircons from the Al Amar fault zone, verify the implication of the common lead data that at least a part of the eastern Arabian Shield has an older continental basement. Because continental margins are particularly favorable locations for development of ore deposits, these findings may have important economic implications, particularly for tin, tungsten, and molybdenum exploration. ?? 1983 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1981

A five-collector system for the simultaneous measurement of argon isotope ratios in a static mass spectrometer

A system is described that utilizes five separate Faraday-cup collector assemblies, aligned along the focal plane of a mass spectrometer, to collect simultaneous argon ion beams at masses 36–40. Each collector has its own electrometer amplifier and analog-to-digital measuring channel, the outputs of which are processed by a minicomputer that also controls the mass spectrometer. The mass spectrometer utilizes a 90° sector magnetic analyzer with a radius of 23 cm, in which some degree of z -direction focussing is provided for all the ion beams by the fringe field of the magnet. Simultaneous measurement of the ion beams helps to eliminate mass-spectrometer memory as a significant source of measurement error during an analysis. Isotope ratios stabilize between 7 and 9 s after sample admission into the spectrometer, and thereafter changes in the measured ratios are linear, typically to within ±0.02%. Thus the multi-collector arrangement permits very short extrapolation times for computation of initial ratios, and also provides the advantages of simultaneous measurement of the ion currents in that errors due to variations in ion beam intensity are minimized. A complete analysis takes less than 10 min, so that sample throughput can be greatly enhanced. In this instrument, the factor limiting analytical precision now lies in short-term apparent variations in the interchannel calibration factors.

International Journal of Mass Spectrometry and Ion↗
USGS1980

A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation. The low207Pb/204Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low207Pb/204Pb and208Pb/204Pb characteristics. In the eastern part of the shield, east of longitude 44??20???E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear207Pb/204Pb-206Pb/204Pb relationship together with distinctly higher208Pb/204Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100??300 m.y. (2??). The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions. ?? 1980 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1979

Genesis of ore deposits in the San Juan Volcanic Field, Southwestern Colorado-lead isotope evidence

In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Pre-cambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization. Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb ∼ 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids. When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caklera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to l,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to l,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to l,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to l,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex. © 1979 Society of Economic Geologists, Inc.

Colorado↗
USGS1979

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820-760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.

Contributions to Mineralogy and Petrology↗
USGS1979

An evaluation of the zircon method of isotopic dating in the southern Arabian Craton, Kingdom of Saudi Arabia

A zircon study has been made on 11 samples of igneous rocks from the Saudi Arabian craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance, which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leuco-adamellite bodies in the southern part of the An Nimas batholith yield ages in the time range 820-760 m.y. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas batholith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could have been more episodic than previously assumed.

Open-File Report↗
USGS1969

Some triple-filament lead isotope ratio measurements and an absolute growth curve for single-stage leads

Triple-filament analyses of three standard lead samples are used to calibrate a mass spectrometer in an absolute sense. The bias we measure is 0.0155 percent per mass unit, and the precision (for 95% confidence limits) is ±0.13% or less for all ratios relative to 204 Pb. Although its precision is not quite so good as that of the lead-tetramethyl method in the analysis of large samples, the triple-filament method is less complex and is an attractive alternative for smaller sample sizes down to 500 μg. Triple-filament data are presented for six possibly single-stage lead ores and one feldspar. These new data for ores are combined with corrected tetramethyl data for stratiform lead deposits to compute absolute parameters for a universal single-stage lead isotope growth curve. Absolute isotopic ratios for primeval lead have been determined by Oversby and because all the previous data for both meteorites and lead ores were similarly fractionated, the absolute value of 238 U 204 Pb= 9.09 ± 0.06 for stratiform leads is little different from the value 8.99 ± 0.05 originally computed by Ostic, Russell and Stanton. Absolute values for lead isotope ratios for all interlaboratory standard samples presently available from the literature are tabulated.

Earth and Planetary Science Letters↗