The only nepheline syenite complex presently known in the Arabian Shield is at Jabal Sawda, about 30 km S of Haql in the extreme NW of Saudi Arabia. It is a post-tectonic, composite intrusion with a crudely concentric structure. A core of leuco-nepheline syenite, a partial ring of mela-nepheline syenite, and an almost complete outer ring of alkali-feldspar syenite are the main rock units. Several mega-inclusions of porphyritic nepheline syenite, nepheline monzosyenite, malignite and ijolite are present in the leuco-nepheline syenite. The chemical composition is notable for very high values of Al2O3, Na2O, Ba, La, Nb, Sr and Zr. UPb isotope dating indicates an emplacement age of 553 ± 4 Ma, one of an increasing number of reliable Cambrian isotope dates in the northern Red Sea region.

Rubidium-strontium analyses of whole-rock samples of an Archean granite from the Owl Creek Mountains, Wyo., indicate an intrusive age of 2640 {plus minus} 125 Ma. Muscovite-bearing samples give results suggesting that these samples were altered about 2300 Ma. This event may have caused extensive strontium loss from the rocks as potassium feldspar was altered to muscovite. Alteration was highly localized in nature as evidence by unaffected rubidium-strontium mineral ages in the Owl Creek Mountains area. Furthermore, the event probably involved a small volume of fluid relative to the volume of rock because whole-rock {delta}{sup 18}O values of altered rocks are not distinct from those of unaltered rocks. In contrast to the rubidium-strontium whole-rock system, zircons from the granite have been so severely affected by the alteration event, and possibly by a late-Precambrian uplift event, that the zircon system yields little usable age information. The average initial {sup 87}Sr/{sup 86}Sr (0.7033 {plus minus} 0.0042) calculated from the isochron intercept varies significantly. Calculated initial {sup 87}Sr/{sup 86}Sr ratios for nine apparently unaltered samples yield a range of 0.7025 to 0.7047. These calculated initial ratios correlate positively with whole-rock {delta}{sup 18}O values; and, therefore, the granite was probably derived from an isotopically heterogeneousmore » source. The highest initial {sup 87}Sr/{sup 86}Sr ratio is lower than the lowest reported for the metamorphic rocks intruded by the granite as it would have existed at 2640 Ma. Thus, the metamorphic sequence, at its current level of exposure, can represent no more than a part of the protolith for the granite.« less

Rubi'dium-strontium whole-rock isochrons for eight postorogenic plutons of the northeastern Arabian Shield yield ages 630 to 575 Ma.Inclusion of one zircon U-Pb age extends the lower limit to 565 Ma.These ages plus existing data also suggest that the postorogenic granites north of the Nadj fault zone are younger (580 to 565 Ma) than those within the fault zone (630 to 595 Ma).The northern strand of the Nadj fault zone may therefore separate two fundamentally different rocks.Initial isotopic compositions of lead and strontium indicate that the granites were derived from either unevolved source materials or tha^: the source materials were not much older than the granites (less than 100 Ma).Oxygen-isotope data and chemical data, however, show that at least some of the granites were derived from an evolved pelitic source, and suggest that this component is present in the protolitn for several granites.Isotopic and chemical data for the rest of the granites are consistent with derivation from a primitive, island-arc protolith.

Research Article| March 01, 1984 Late Precambrian rifting and crustal evolution in the Northeastern Desert of Egypt Robert J. Stern; Robert J. Stern 1Programs in Geoscience, University of Texas at Dallas, Richardson, Texas 75080 Search for other works by this author on: GSW Google Scholar David Gottfried; David Gottfried 2U.S. Geological Survey, Reston, Virginia 22092 Search for other works by this author on: GSW Google Scholar Carl E. Hedge Carl E. Hedge 3U.S. Geological Survey Mission, APO New York, New York 09697 Search for other works by this author on: GSW Google Scholar Geology (1984) 12 (3): 168–172. https://doi.org/10.1130/0091-7613(1984)12<168:LPRACE>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 MailTo Tools Icon Tools Get Permissions Search Site Citation Robert J. Stern, David Gottfried, Carl E. Hedge; Late Precambrian rifting and crustal evolution in the Northeastern Desert of Egypt. Geology 1984;; 12 (3): 168–172. doi: https://doi.org/10.1130/0091-7613(1984)12<168:LPRACE>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 Basement exposures along the northwestern flanks of the Gulf of Suez record the rapid formation of continental crust during the interval 670–550 Ma. A variety of field, petrographic, petrologic, and isotopic considerations indicate that this episode of crust formation took place in an extensional tectonic setting analogous to that of the late Paleozoic Oslo Rift of Norway. Crustal evolution in this region thus contrasts with other regions of the Afro-Arabian Shield where the crust appears to have been formed by convergent margin accretionary processes and collisional tectonics. This imposes new constraints on our understanding of late Precambrian crustal evolution. 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.

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.

Supported by various field geologic and petrologic data, the contents of Pb, U, Th, Rb, and Sr and the isotopic compositions of Pb and Sr for upper Cenozoic volcanic rocks of the Yellowstone Plateau volcanic field are consistent with the hypothesis of derivation of the basaltic and rhyolitic magmas by partial melting of distinct source regions in the upper mantle and lower crust, respectively. All the basalt samples analyzed but one have systematically lower values of 2ø?Pb/2ø4Pb and 87Sr/86Sr than the rhyolites. The values of 2ø6pb/:ø4Pb are smaller, and 8?Sr/6Sr are mostly larger than known values in oceanic basalts. In all but one case, the values of:ø?Pb/:ø½Pb are higher than expected from an extrapolation of known values in oceanic basalts to less radiogenic values of:ø6pb/2ø½Pb. Because there are no xenoliths, phenocrysts are only moderate to sparse in abundance, REE patterns are low and flat at the radiogenic end of lead isotopic compositions, several values of Rb/Sr are low, and 80% of the basalt samples form a well-developed secondary isochron separate from the rhyolites, we favor an interpretation for basalt genesis wherein isotopic signatures of most mafic magmas were attained in a continental 'keel' of mantlelike character about 2.6 b.y. old or somewhat older attached to the crust, and these signatures were unaltered by magma passage through the crust. At the very least, the current data continue to cast serious doubt as to the inevitability of crustal contamination for basaltic magma intruding the continental environment and postulate that much can be learned about the mantle under continents through the study of continental basalts. One basalt unit with an unusually low value of:ø?Pb/:ø½Pb and an 8?Sr/6Sr less than 0.704 may represent subcontinental 'keel'-derived magma that rose finaltered to the surface. Our data also are not consistent with formation of this rhyolite-basalt association primarily by such processes as crystal fractionation, separation of immiscible silicate liquids from a common parental magma, or fractional melting of a homogeneous source. Rather as a conceptual model, we envision large mafic intrusions to have been injected into the lower crust resulting in rhyolite generation through partial anatexis of the adjacent wall rocks which probably had a :ø6pb/2ø½Pb < 17 and ?Sr/SaSr > 0.709; a model that has much in common with that proposed by Holmes (193!). All the other hypotheses listed have the necessary added complication that either the basalt or the rhyolite or both become contaminated after the two magma types separated, have problems accounting for the lack of igneous rocks of intermediate compositions or production of such large volumes of rhyolitic material (~ 5000 km3), and fail to explain why rhyolitic magma is not a more common occurrence in the ocean basin. We appeal to bouyancy of rhyolites to generate a barrier for basalt magma migration and account for the great preponderance of rhyolite relative to basalt at the surface. Furthermore, the complex isotopic picture in the rhyolites indicates that many of these magmas interacted with the upper crustal geologic units that they traversed. The interactions involved diverse processes, probably including reacton with hydrothermal fluids or hydrothermally altered rocks at high levels as well as by contamination with Phanerozoic sedimentary and Precambrian crystalline rocks at deeper levels. At the very least, we feel our study adds a cautionary note to the currently increasingly popular hypothesis that differentiation of basalt or gabbro magmas to rhyolite or granite (as distinct from tonalite or dacite) is a common occurrence and is therefore an important continential building process. Models for formation of rhyolite and granite predominantly by reworking of crust (anatexis) must still be considered. The primitive Archean mantle of the region was characterized by higher Rb/Sr, U/Pb, and Th/U values than are typical of modern suboceanic mantle. The mantle residuum within the continental subcrustal lithosperic 'keeF that resulted from the Archean crustal differentiation event probably was depleted in Rb/Sr and U/Pb, a,d the crust was correspondingly enriched in these ratios. The crust probably was further differentiated by an Archean high-grade metamorphism, during or after the primary event, into a granulitic lower crust depleted in U/Pb and Rb/Sr and a lower-grade upper crust enriched in these ratios.

Research Article| January 01, 1980 Age of the basement rocks of southwest Montana H. L. JAMES; H. L. JAMES 1U.S. Geological Survey, Port Townsend, Washington 98368 Search for other works by this author on: GSW Google Scholar C. E. HEDGE C. E. HEDGE 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (1): 11–15. https://doi.org/10.1130/0016-7606(1980)91<11:AOTBRO>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 MailTo Tools Icon Tools Get Permissions Search Site Citation H. L. JAMES, C. E. HEDGE; Age of the basement rocks of southwest Montana. GSA Bulletin 1980;; 91 (1): 11–15. doi: https://doi.org/10.1130/0016-7606(1980)91<11:AOTBRO>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 Rb-Sr analyses of a suite of quartzo-feldspathic gneisses that are interlayered with beds of marble, quartzite, and amphibolite in the Ruby and Tobacco Root Ranges and the Gallatin River canyon of southwest Montana show that the age of metamorphism of these strata occurred about 2,750 m.y. ago. The 13 samples analyzed are from rock units that have in the past been assigned stratigraphically to the Pony Group, Cherry Creek Group, and Dillon Granite Gneiss. Except for two samples of anomalous composition, the data define a linear array on an isochron diagram that has a best-fit value of 2,762 ± 113 m.y. Inclusion of other published data for the Tobacco Root Range yields a best-fit value of 2,730 ± 85 m.y. This age corresponds closely to that of the principal metamorphic-plutonic epoch of the Bear-tooth Mountains, to which the term "Bear-tooth orogeny" has been applied. It also demonstrates that the major Precambrian metasedimentary sequences of the region are of Archean age. 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.

87Sr/86Sr ratios of basalts from islands in the Indian Ocean (0.7040) are higher than those of basalts dredged from the Mid-Indian Ocean Ridge (0.7034). The sources of the island basalts have apparently not been in equilibrium with the source of the ridge basalts for roughly 109 years. Both ridge and island basalts in the Indian Ocean are higher in87Sr/86Sr than are rocks from similar settings in the eastern Pacific.

Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Late Cretaceous batholith, quartz monzonite ore, Precambrian isotope age, partial-melting source material model; Isotope ratios of Pb in feldspar range widely from pluton to pluton: 16.9-18.1 for 206/204, 15.4-15.7 for 207/204, and 37.7-38.5 for 208/204.Whole-rock initial Sr-87/Sr-86 is 0.706-0.709. Variation in Pb isotope ratios within Butte Quartz Monzonite, plus differences in ratios between megacryst and groundmass feldspars in Donald pluton suggest incomplete isotopic mixing; similarity of Butte ore Pb to feldspar Pb of Butte Quartz Monzonite, therefore, has genetic significance. Isotope data suggest that batholith rocks result from partial melting of lower crust or upper mantle; assimilation of upper crustal material is necessary if source material resembled oceanic tholeiites.

Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths (St. Kevin Granite and granite of Cross Creek), and various minor intrusive rocks. A whole-rock Rb-Sr isochron age on the St. Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St. Kevin and numerous other rocks are either about the same as the St. Kevin whole-rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than St. Kevin Granite yield mica ages within analytical error of the St. Kevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than St. Kevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite, in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the St. Kevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor.

The Rb/Rbabundance ratio for a reference sample of rubidium sulfate has been determined as 2.5995 _ 0.0015 (95 per cent confidence interval) on a mass spectrometer with a 12-inch radius of curvature and a triple-filament mode of ionization. The abundance ratio was measured for 27 natural silicate samples which represent a wide geographic distribu- tion. Isotopic ages for these samples range from 20 m.y. to 2600 m.y., and a variety of minerals and geologic origins are represented. No variations in the atomic abundance ratio of Rb/Rb 87 larger than that accounted for by experimental error were observed. The absolute ratio remains to be determined. Introduction. Within recent years the Rb-Sr method of age determination has become one of the most widely used tools of geochronology. The method is based on the decay scheme Rb 87 - Sr  + (/), and in common practice it is assumed that the ratio Rb/Rbis constant in present- day rubidium. Nier's (1950) value of 2.591 for this ratio is currently being used in all labora- tories. Brewer (1938) found no variations greater than 1 per cent in a limited survey that included five natural silicates. Jamieson and $chreiner (1957, p. 263), however, reported Rbabun- dances for three African lepidolites with a differ- ence of 6 per cent. This variation led them to conclude that the Rbabundance must be de- termined for each mineral used for age deter- mination. This paper presents measurements for 27 natural silicate samples, no variations as large as the experimental error of 0.2 per cent having been observed in the relative abundance ratios of Rb/Rb . Experimental procedure. The silicate min- erals biotite, potash feldspar, muscovite, and lepidolit e, which have been most commonly used for Rb-Sr age determinations, were available to us from previous geochronologic investigations. Coarse-grained material from pegmatites re- quired little preliminary treatment; however, many of the mineral samples were concentrated from a variety of igneous and metamorphic rocks (Table 1). The samples were decomposed in platinum