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.
A paper-tape digital recording system is described which has been used to determine the isotopic abundance ratios of strontium from geologic samples. The system costs less than $10 000 and has proved to be highly reliable during its three years of operation. More than 600 analyses have been recorded and the results processed by an IBM-360 computer. This procedure has improved the precision of analyses from this laboratory by a factor of two. Over a period of 11 months, 16 measurements on the M.I.T. strontium carbonate standard gave a mean value for ( 87 Sr/ 86 Sr) n = 0.70797 ± 0.017%. The uncertainty En the results is the standard deviation of the analyses with the 86 Sr/ 88 Sr value normalized to 0.1194. Over shorter periods of time considerably better precision is indicated.
Lead isotopes are used to distinguish between Precambrian mineralization and Mesozoic or Cenozoic mineralization in an area of Belt Supergroup rocks in northwestern Montana and northern Idaho. The Precambrian lead is characterized by a comparatively uniform isotopic composition (Pb 206 /Pb 204 = 16.15-16.73; Pb 207 /Pb 204 = 15.37-15.45; Pb 208 /Pb 204 = 35.88-36.46) and model ages of 1.5-1.2 b.y. We interpret it to be the product of approximate single-stage development, although the model ages need not be exact geochronologic ages. The possibility that some vein-type deposits formed at a later time by remobilization of this lead already in Belt rocks is also discussed. The Mesozoic or Cenozoic lead, which has undergone at least a two-stage development, is generally more radiogenic and ranges widely in isotopic composition.Deposits associated with the Osburn fault system, including those of the Coeur d'Alene mining district, and with a second belt of mineralization which extends southward from near Bonners Ferry, Idaho, and joins the Osburn fault system near Thompson Falls, Montana, contain the Precambrian type of lead. Deposits adjacent to the Kootenay Arc mobile belt in Idaho and occurrences throughout southern Lincoln and Sanders Counties, Montana, appear to be largely Mesozoic or Cenozoic. Small, isolated deposits of Precambrian and Mesozoic or Cenozoic ages have been identified in the weakly mineralized northeastern part of the study area. The ability to distinguish, by their lead isotopes, two generations of ore widely spaced in time but overlapping geographically promises to be a valuable aid in unraveling the structural history and in carrying out future prospecting within this area.
Zircons from Cretaceous and Tertiary plutonic rocks of the west-central portion of the Bitterroot lobe of the Idaho batholith contain abundant inherited cores that make precise U-Pb age determination difficult.Zircons from a tonalite emplaced along the western periphery of the batholith yield an almost concordant age of 94±1.4Ma.Monzogranite and granodiorite plutons were intruded along the northern edge of the lobe from 75 to 71 Ma.Subsequently, the volumetrically more dominant plutons in the central and western parts of the lobe were emplaced between 59 and 54 Ma; most of the intrusive activity in this area probably occurred at about 55 Ma.The most accurately determined upper-intercept age for the Cretaceous plutons is 1,779±20 Ma.This is similar to the 1,751 ±63 Ma age that was obtained for a gneissic muscovitebiotite monzogranite of probable Proterozoic age.For Tertiary rocks, upper-intercept ages are more variable, ranging from 1,812±21 Ma to 1,625±46 Ma.These results, combined with Pb isotopic data from feldspars, confirm that the magmas of the Bitterroot lobe were derived mainly from an Early Proterozoic lower continental crust.Our findings are consistent with studies in the northern and northeastern parts of the Bitterroot lobe by other workers.
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.
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.
On the basis of 13 new U/Pb ages for plutonic rocks from the southernmost portion of the Saudi Arabian Shield, the Nabitah mobile belt and orogeny are defined.The Nabitah mobile belt is a 100-to 200-km-wide, 1000-km-long, northstriking belt of intensely deformed rocks that include ophiolitic and synorogenic plutonic complexes.The age data for all but one sample are highly concordant, and it is confirmed that U/Pb systems in zircons from the region remained undisturbed until the late Tertiary.island arc formed over a west-dipping subduction zone; (2) collision between the Hijaz-Asir and Afif plates (680-640 Ma); and (3) neocratonic intraplate activity after suturing (<640 Ma).The evolution of the shield is interpreted in terms of a microplate accretion model in which the Nabitah suturing episode is only one of a number of such suturing events that occurred in the Arabian Shield.On this basis, three main stages of evolution are proposed for the shield: (1) preaccretion, during which the Hijaz-Asir island-arc assemblage was formed by plate-tectonic processes from >950 to 690 Ma;(2) plate accretion during 690-630 Ma to form the Arabian-Nubian neocraton; and (3) intraplate activity from 630 to <550 Ma, during which the Murdama molasse, Pan-African posttectonic granites, and Najd fault zone formed.The Arabian erogenic belts are discussed relative to the Pan-African Mozambique and Dahomeyan-Pharusian mobile belts of Africa.Base metals are preferentially associated with the island-arc domains, gold with the Nabitah suture zone, and tin, tungsten, lead, and silver with the Afif province of the eastern shield.
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.
