Pre-Cryogenian crystalline rocks occur only in the eastern part of the Arabian Shield, around Jabal Muhayil in the Khida sub-terrane. These occurrences have been mapped and investigated using U-Pb zircon geochronology, bulk rock Sm-Nd and feldspar Pb isotopes. The previously unrecognized ca. 1.87 Ga Libab gneiss complex contains granitic rocks with inherited zircon up to ca. 2.5 Ga and has both Nd and Pb isotopic evidence for a Neoarchean precursor. The ca. 1.67 Su'ayra alkali feldspar granite similarly requires a Neoarchean precursor but lacks older zircon inheritance. The areally and topographically dominant Muhayil anorthosite cannot precisely be dated but has a minimum age of ca. 750 Ma based on a cross cutting gabbro and granodiorite, the latter also having ca. 1.7 Ga zircon cores. Highly retarded Pb isotopes in the anorthosite require long-term residence in a near-zero U/Pb reservoir, considered most consistent with a ca. 1.7 Ga emplacement age, though a previously suggested genetic link with the Su'ayra granite remains entirely speculative. The consistently ancient isotopic signature that occurs in the younger (850 - 750 Ma) studied samples suggest that these were influenced by Neoarchean continental crust during emplacement of the Siham magmatic arc, which developed along the southern margin of the Afif composite terrane prior to its amalgamation with the Asuir terrane to its west. Correlation with terranes in Yemen showing similar ages and isotopic character suggest a contiguous crustal block separate from the Azania microcontinent that was juxtaposed along the northward extension of the ophiolite-decorated Bayhan suture during Cryogenian closure of the Mozambique Ocean in the northern part of the East African Orogen. Whether the Neoarchean continental crust rifted off of Azania and then re-amalgamated or forms an entirely separate crustal domain remains to be tested.

Major structures are imaged on 2D reflection seismic data within the Precambrian basement of the western Rub' Al-Khali of Saudi Arabia for the first time. In this area the base-Cambrian Angudan Unconformity that defines top basement is at a depth of 1–2 km with a gentle easterly dip. Large antiformal structures are clear on 2D reflection seismic within the basement. The antiforms are separated by zones that are interpreted as low-angle faults. No basal detachment is seen and the faults pass below the base of reflection seismic at a depth of approximately 12 km. Individual folds have dimensions in the order of 20 km wavelength and 2 km amplitude and occur between 3 and 10 km depth. The structural style is interpreted as compressional by comparison with thrust systems and accretionary prisms elsewhere. Regional studies indicate that basement is comprised of a series of north-south trending oceanic Proterozoic terranes that were brought together in the Ediacaran East African Orogeny. Based on current regional tectonic maps the newly-imaged fold belt is on the east margin of the Khida-Abas terrane. Depth conversion shows the thrusts dip at approximately 10° north in the line of section. A novel combination of cross-section to structural trend obliquity estimates combined with matched filtering of potential field data indicate the fold and thrust belt trends 015°-195° with ESE vergence. This compressional belt may be some internal structure of the Khida-Abas terrane, or may be a non-outcropping accretionary prism on the margin of that terrane, or may even represent a set of amalgamated microterranes that do not outcrop along trend. Either way, these observations represent a major, previously undescribed element of this sector of the East African Orogen.

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.

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 isotopic groups: Group I (hornblende-biotite and two-mica plutons of all ages) have Pb isotopic compositions suggesting derivation from rocks of the Belt Supergroup or their metamorphosed equivalents, although Nd isotopic 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 isotopic 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. Isotopic 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.

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 elsewhere in the North Cascades.The complex was ductilely extended northwest-southeast shortly after intrusion of granite dikes at approximately-45 Ma, but before emplacement of the earliest (approximately-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.

The area covered by this work 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. The geology of the Zalm area is well understood, and this geochronologic and isotopic study confirms that the southern Afif terrain 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. The Kabid formation is the oldest in the region and comprises pelitic, arkosic and felsic high-grade paragneisses. U-Pb zircon data from a pelitic garnet-sillimanite gneiss show that this part of the continental basement in the southern Afif terrain may be as old as 1770 Ma. Isotopic nalyses indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1770 Ma, and consequently Archaean source rocks may be present within the southern Afif terrain. 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 northeast, to less radiogenic marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a facies change within a pre-collision marginal basin. Miogeosynclinal continental shelf facies of the Siham group lie unconformably 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 685-640 Ma, the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip faulting 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 volcanosedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike-slip motion at about 620 Ma ago.

Research Article| May 01, 1984 Geochronologic and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield John S. Stacey; John S. Stacey 1U.S. Geological Survey, M.S. 37, 345 Middlefield Rd., Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Carl E. Hedge Carl E. Hedge 2U.S. Geological Survey Mission, Jeddah, Saudi Arabia Search for other works by this author on: GSW Google Scholar Author and Article Information John S. Stacey 1U.S. Geological Survey, M.S. 37, 345 Middlefield Rd., Menlo Park, California 94025 Carl E. Hedge 2U.S. Geological Survey Mission, Jeddah, Saudi Arabia Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1984) 12 (5): 310–313. https://doi.org/10.1130/0091-7613(1984)12<310:GAIEFE>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation John S. Stacey, Carl E. Hedge; Geochronologic and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield. Geology 1984;; 12 (5): 310–313. doi: https://doi.org/10.1130/0091-7613(1984)12<310:GAIEFE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report zircon U-Pb, feldspar common Pb, whole-rock Sm-Nd, and Rb-Sr data from sample Z-103, a fine-grained granodiorite from the Jabal Khida region of the Saudi Arabian Shield (lat 21°19′N; long 44°50′W). The measurements yield conclusive evidence for continental crust of early Proterozoic age (∼1,630 Ma) at that locality. Furthermore, lead-isotope data indicate an even earlier, perhaps Archean, crustal history for the source of the lower Proterozoic rocks. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-grade metamorphic rocks and migmatites in the Farmington Canyon Complex were derived from igneous and sedimentary rocks possibly, as old as 3000 m.y. They were probably metamorphosed about 2600 m.y. ago, and they were severely metamorphosed, migmatized, and intruded by quartz monzonite 1790 m.y. ago. A very high initial Sr87 /Sr86 ratio of 0.769 indicates that the quartz monzonite magma was derived by melting of the more leucocratic parts of the layered gneisses. On and just west of Antelope Island, plutons of granite were emplaced 2020 m.y. ago.

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.

Research Article| January 01, 1983 Lead-isotopic compositions of diverse igneous rocks and ore deposits from southwestern New Mexico and their implications for early Proterozoic crustal evolution in the western United States JOHN S. STACEY; JOHN S. STACEY 1U.S. Geological Survey, 345 Middlefield Road, Mail Stop 37, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar DAVID C. HEDLUND DAVID C. HEDLUND 2U.S. Geological Survey, Denver Federal Center, Mail Stop 905, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN S. STACEY 1U.S. Geological Survey, 345 Middlefield Road, Mail Stop 37, Menlo Park, California 94025 DAVID C. HEDLUND 2U.S. Geological Survey, Denver Federal Center, Mail Stop 905, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1983) 94 (1): 43–57. https://doi.org/10.1130/0016-7606(1983)94<43:LCODIR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation JOHN S. STACEY, DAVID C. HEDLUND; Lead-isotopic compositions of diverse igneous rocks and ore deposits from southwestern New Mexico and their implications for early Proterozoic crustal evolution in the western United States. GSA Bulletin 1983;; 94 (1): 43–57. doi: https://doi.org/10.1130/0016-7606(1983)94<43:LCODIR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb zircon measurements from some of the basement rocks in southwestern New Mexico have ages that range from 1,650 to 1,450 m.y.The largest ore deposits occur near Silver City in the southwest part of the region studied. They are associated with Laramide alkali to calc-alkaline plutons, and their lead-isotopic compositions are the least radiogenic in the region. This lead exhibits lower-crustal-upper-mantle characteristics, and for the major producing porphyry copper deposits, the 206Pb/204Pb ratios are less than 18.0.Lead from more silicic mid-Tertiary volcanic rocks and associated ore deposits has somewhat higher 208Pb/204Pb ratios that reflect a greater crustal involvement in their origin. In a regional trend to the northeast, ore lead becomes more radiogenic, and at Hansonburg, 200 km from Silver City, lead in the Mississippi-type deposits is clearly derived from upper-crustal sources.On the 207Pb/204Pb-206Pb/204Pb plot, data from all of the rocks and ores that we have analyzed form an array that lies below the average orogene curve of Doe and Zartman (1979). A common source is implied for the lead, the isotopic composition of which was similar to that found in 1,750-m.y.-old stratiform deposits as far apart as Pecos, New Mexico, and Jerome, Arizona. Such a composition indicates that over a large region of the southwestern United States, continental crust developed between 1,750 and 1,450 m.y. ago, possibly in an island-arc environment.Basement rocks that are ∼ 1,750 m.y. old extend northward through Colorado to Utah. Galena data obtained in previous studies show that the fraction of older sialic lead in those rocks increases toward the Archean craton in Wyoming. The crust apparently developed southward from Wyoming in stages at 2,400 m.y. ago or before, 2,100 m.y. ago, and 1,750 m.y. ago, with incorporation of older sialic material in each stage as far south as Milford, Utah. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

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 Precambrian 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 approximately 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 caldera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to 1,500-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 1,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to 1,500-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 1,800-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.

The digital system described herein can record data from several mass spectrometers on a single magnetic tape. Programs have been written for isotopic abundance calculations of lead, thorium, uranium, strontium, and argon analyses. The programming techniques successfully recognize and reject spurious data. Both chart and digitally recorded data are presented for seven analyses of the U.S. National Bureau of Standards common lead standard SRM 981. Standard deviations of the ratios of the digital data range from 0.018% for the 207Pb/206Pb to 0.038% for the 208Pb/204Pb. The equivalent hand-processed values are 0.044 and 0.066%, respectively. The measured ratios are offset from their absolute values almost entirely by a fractionation component of about 0.16% per mass unit. Other bias introduced by the system is shown to be very small and probably not greater than 0.01% for the digital data and 0.05% for chart-read data. Calibration of the vibrating reed electrometer amplifier with current flowing through the feedback resistor has considerably improved the analytical precision and reduced the system bias.

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 to204Pb. 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.

A gas-source mass spectrometer has been constructed for the precise measurement of lead isotope ratios. Sixteen analyses on 4 different preparations of the same galena made over a period of 2 months gave 95% confidence limits (per analysis) of 206Pb/204Pb= 0.080%, 207Pb/206Pb= 0.042%and208Pb/206Pb= 0.046%.