Rb-Sr Isotope Geochemistry Research Articles

Whole-rock and zircon geochemistry of the Xiaoliugou granites, North Qilian Orogen (NW China): Implications for tectonic setting, magma evolution and W–Mo mineralization

Ordovician W–Mo polymetallic mineralization-related granites are widely developed in the western section of the North Qilian Orogen, NW China. In order to shed light on the tectonic setting, magma evolution and genesis of mineralization in the orogeny, highly evolved granitic rocks (monzogranites) from the Qibao and Qiqing deposits in the Xiaoliugou ore field were studied. Typical of subduction-related A-type granites, the Xiaoliugou granitic rocks are enriched in SiO2, Rb, Th, U, Ta, Nd, Zr and Hf but relatively depleted in Ba, Sr, P and Eu. The variable initial whole rock 87Sr/86Sr ratios (0.62937–0.70543) and wide range of zircon εHf(t) values (−18 to 6) suggesting mantle–crust interaction and mixing of depleted mantle and deep crust in the source area. Whole-rock elemental and Rb–Sr isotopic geochemistry suggest slow cooling, significant fractional crystallization, weak wall-rock assimilation/contamination and intense fluid differentiation/metasomatism during the genesis of the Xiaoliugou granitic rocks. Zircon U–Pb dating yielded three age clusters: Precambrian (>540 Ma) (n = 32), Ordovician (~450 Ma) (n = 40), and Cretaceous (~135 Ma) (n = 18). Zircon internal textures indicate a range of sources including residual/detrital zircons from the magma source region and local sediments, magmatic and fluid-altered zircons, and late stage hydrothermal zircons. Zircon trace element and Lu–Hf isotopic compositions suggest fluid metasomatism during granite emplacement in the Ordovician and overprinting by a more evolved, oxidized fluid in the Cretaceous. Involvement of a late, high-temperature, highly fractionated, F-rich fluid could have enhanced W–Mo mineralization. Consequently, a two-stage genetic model is proposed for the Xiaoliugou granitic-minerogenic system involving primary mineralization in the Ordovician (~450 Ma), followed by a Cretaceous magmatic hydrothermal overprint (~135 Ma).

Insights on the evolution of the Arroio Grande Ophiolite (Dom Feliciano Belt, Brazil) from Rb-Sr and SHRIMP U-Pb isotopic geochemistry

This paper presents the first Rb-Sr isotopic geochemistry study and SHRIMP zircon U-Pb dating in the Arroio Grande Ophiolite (southeasternmost Dom Feliciano Belt in Brazil), an extension of the Uruguayan Punta del Este Terrane. The metamafic units of the ophiolite show 87Sr/86Sr ratios ranging from 0.7036 to 0.7070 (recalculated at 630 Ma), which suggest a MORB magmatic source for the amphibolite protoliths and a volcanic magmatic arc source (with possible crustal contamination) for the metagabbro protoliths. Together, these features suggest a supra-subduction zone origin for these rocks, corroborating previous studies based on bulk-rock chemistry. In the metaultramafics, the 87Sr/86Sr630 ratio of the chloritite sample (0.7152) is comparable to that of the Três Figueiras granite (0.7139), a syn-kinematic peraluminous granite related to the Ayrosa Galvão-Arroio Grande Shear Zone (which also affected the southern portion of the ophiolite, where the studied samples are located). The emplacement of the latter granite could be responsible for the metasomatism of the former serpentinites, generating talc- and chlorite-rich reaction zones. The SHRIMP U-Pb analysis of twelve chloritite zircons strengthens that assumption, as zircons with ca. 610-585 Ma (the age of the granite) were found in association with metasomatic microtextures. Zircon ages ranging from ca. 2000 to 660 Ma were also found in the chloritite. These latter ages are analogous to those found in the metasiliciclastic rocks that envelop the metaultramafic rocks of the ophiolite, so that the chloritite probably represents a blackwall reaction zone between (i) a former serpentinite, (ii) the Três Figueiras granite, and (iii) metasiliciclastic rocks. Additionally, we perform a SHRIMP U-Pb dating in a quartz-syenite melt, which are related to a tonalitic intrusion within the metasedimentary unit (the Matarazzo Marbles) of the ophiolite. The obtained concordia age of 640 Ma constrains the minimum age of ophiolitic mélange emplacement, as the tonalitic intrusion is related to continental arc magmatism (Pinheiro Machado Intrusive Suite).

Sm–Nd and Rb–Sr isotope geochemistry and petrology of Abu Hamamid intrusion, Eastern Desert, Egypt: An Alaskan-type complex in a backarc setting

Alaskan-type complexes are uplifted fragments of the deep levels of island arcs. These complexes usually reveal chemical diversity that cannot be assigned to a unique setting within arcs. Here we report the petrology, Rb–Sr and Sm–Nd isotopic data of the Abu Hamamid intrusion from the Eastern Desert of Egypt as an example of Neoproterozoic backarc Alaskan-type complex. The intrusion comprises clinopyroxene dunite, olivine clinopyroxenite, amphibole clinopyroxenite and gabbro. All rock units represent cumulates crystallized from a high-MgO hydrous magma with geochemical characteristics intermediate between mid-ocean ridge basalt and island arc magmas. Slab signatures of the parental magma comprise elevated H2O, 87Sr/86Sr and large ion lithophile element contents and relative depletion in Nb. High positive initial ɛNd (+6.2 to +11.0) is consistent with the Abu Hamamid magma being extracted from a highly depleted mantle source. The parental magma was generated by 10% partial melting of a mantle source modified by 10% MORB (ecolgite) derived fluid. The Abu Hamamid Alaskan-type complex and the surrounding related volcanic rocks were formed in a Neoproterozoic backarc setting. The magmas forming the lower crust of the Arabian-Nubian shield were liberated from a depleted mantle before 900Ma.

Sm–Nd, Pb–Pb, and Rb–Sr systematics of the basement in the Cigar Lake area, Saskatchewan, Canada

The Sm–Nd, Pb–Pb, and Rb–Sr isotope geochemistry of graphitic metapelitic gneisses and their altered equivalents from the Cigar Lake area (Saskatchewan, Canada) has been investigated. Some granitic gneisses were also analyzed for Pb–Pb and Rb–Sr. Sm–Nd data show that the metapelitic gneisses are composed of detritus from heterogeneous, mainly mantle-derived Archean rocks (2.5–2.6 Ga) and that the Sm–Nd system has not been significantly perturbed during subsequent alteration and metamorphic events. The Pb–Pb age for samples of the less altered graphitic metapelitic gneisses is 1.77 ± 0.03 Ga. The crustal common Pb is located on the Pb–Pb isochron, but there are different zones with high and variable U/Pb ratios (μ = 15–280). The Pb–Pb age for the granitic gneisses is 1.79 ± 0.11 Ma. The Pb isotope data show that there has been no major uranium redistribution in the basement after the Hudsonian orogeny. However, there has been a strong perturbation of the U–Pb system in the regolithic zone beneath the Athabasca cover. In some samples, uranium was added during the mineralizing event. The Rb–Sr system in the graphitic metapelitic gneisses was also affected.The 87Sr/86Sr ratio in pitchblende is 0.709. At 1.3 Ga, there is a strong contrast between the 87Sr/86Sr ratio in the Athabasca sandstones (0.706–0.710) and the 87Sr/86Sr ratio in the metapelitic gneisses from the basement (0.725–0.775). The upper zone of the regolith is characterized by a low 87Sr/86Sr ratio (0.705–0.707). The Pb–Pb and Rb–Sr data are consistent with the circulation of a fluid with a low 87Sr/86Sr ratio, derived from the sedimentary cover; this fluid passed through the most permeable zones of the basement rocks, especially the regolith. The mineralizing fluid had a 87Sr/86Sr value typical of a fluid in equilibrium with the Athabasca sandstones.

Edad y geoquimica isotopica Rb-Sr del yacimiento de cobre Mantos Blancos: relacion temporal con el magmatismo jurasico

ABSTRACT. Rb-Sr isotopic age and geochemistry of The Mantos Blancos copper deposit: temporal relation of the Jurassic magmatism. The Mantos Blancos volcanic sequence, host of the stratabound Cu (-Ag) deposit of the same name, has been studied for its Rb-Sr isotope geochemistry, to provide insights into the age and genesis of this important mineral deposit. Whole rock Rb-Sr ages of rock units characterized for their alteration pattern, allow the following generalizations: a- errorcrons from the NK unit (rocks with normal K2O contents and with little sodic alteration), yield ages of ca. 175 Ma, interpreted as the age of the host Mantos Blancos volcanic sequence; b- the ages of about 150 Ma obtained for the strongly altered ANa unit (sodic alteration), are interpreted to represent the age of hydrothermal alteration responsible for sodium metasomatism and its associated copper mineralization; c. intrusives emplaced into La Negra Formation and Mantos Blancos volcanic sequence may have been responsible for the thermal event causing the hydrothermal alteration process related to the ore mineralization. The initial 87Sr/86Sr of both NK and ANa units, and in calcite, which crystallized late in the paragenetic sequence of the ores, suggests that the hydrothermal fluids could not have been purely magmatic, nor of sea water origin, but could represent meteoric waters that have partially equilibrated with the host formations, or mixtures of different fluids.

Precambrian basement in the Canadian Cordillera: an introduction

s, 15: A4. ARMSTRONG, R. L., TAUBENECK, W. H., and HALES, P. 0.1977. Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington and Idaho. Geological Society of America Bulletin, 88: 39741 1. BALLY, A. W., GORDY, P. L., and STEWART, G. A. 1966. Structure, seismic data and orogenic evolution of southern Canadian Rockies. Bulletin of Canadian Petroleum Geology, 14: 337-381. BENNETT, V. C., and DEPAOLO, D. J. 1987. Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping. Geological Society of America Bulletin, 99: 674-685. BERRY, M. J., JACOBY, W. R., NIBLEIT, E. R., and STACEY, R. A. 1971. A review of geophysical studies in the Canadian Cordillera. Canadian Journal of Earth Sciences, 8: 788-801. BOWRING, S. A., and PODOSEK, F. P. 1989. Nd isotopic evidence from Wopmay Orogen for 2.0-2.4 Ga crust in western North America. Earth and Planetary Science Letters, 94: 217-230. BURWELL, A. D. M. 1975. Rb-Sr isotope geochemistry of lherzolites and their constituent minerals from Victoria, Australia. Earth and Planetary Science Letters, 28: 69-78. CAMPBELL, R. B. 1968. Canoe River, British Columbia. Geological Survey of Canada, Map 15-1967. CARR, S. D., and BROWN, R. L. 1990. Southern Cordilleran Lithoprobe transect lines 6 to 10: crustal structure and tectonic chronology. Southern Canadian Cordillera Transect Workshop, March 3 4 , University of Calgary, Calgary, Alta., pp. 10-18. COLLERSON, K. D., VAN SCHMUS, W. R., LEWRY, J. F., and BICKFORD, M. E. 1988. Buried Precambrian basement in south-central Saskatchewan: provisional results from Sm-Nd model ages and U-Pb zircon geochronology. In Summary of investigations 1988. Saskatchewan Geological Survey, Miscellaneous Report 88-4, pp. 142-150. COOK, F. A., SIMONY, P. S., COFLIN, K. C., GREEN, A. G., MILKEREIT, B., PRICE, R. A., PARRISH, R., PATENAUDE, C., GORDY, P. L., and BROWN, R. L. 1987. Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex. Geophysical Journal of the Royal Astronomical Society, 89: 91-

Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Research Article| October 01, 1977 Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico DANIEL S. BARKER; DANIEL S. BARKER 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar LEON E. LONG; LEON E. LONG 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar G. KARL HOOPS; G. KARL HOOPS 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar FLOYD N. HODGES FLOYD N. HODGES 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Author and Article Information DANIEL S. BARKER 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 LEON E. LONG 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 G. KARL HOOPS 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 FLOYD N. HODGES 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (10): 1437–1446. https://doi.org/10.1130/0016-7606(1977)88<1437:PARIGO>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 DANIEL S. BARKER, LEON E. LONG, G. KARL HOOPS, FLOYD N. HODGES; Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico. GSA Bulletin 1977;; 88 (10): 1437–1446. doi: https://doi.org/10.1130/0016-7606(1977)88<1437:PARIGO>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 Shallow intrusive bodies in a 65-km-long belt consist of quartz-bearing and feldspathoidal syenite, trachyte, and phonolite, all within a narrow range of composition. K-Ar ages (35 ± 2 m.y.) and modal, whole-rock chemical, Rb-Sr isotopic, and mineralogic data suggest that all the intrusive rocks were derived from one magma reservoir.Nine intrusive rock types are divisible into two groups. The older group, commonly forming sills and laccoliths, is fine- to coarse-grained and has little or no flow structure, low Rb, high Sr, and initial 87Sr/86Sr ratios of 0.703 to 0.709. The younger group, tending to form discordant sheets, is finely porphyritic and has strongly developed flow structure, high Rb, low Sr, and initial strontium ratios of 0.705 to 0.712. The younger group shows more extreme compositional variation, including the most silica-oversaturated and the most silica-undersaturated rocks. Each group contains both quartz-bearing and feldspathoidal rock types.Fractional removal of plagioclase and biotite from mugearitic liquid progressively enriched the remaining liquid in sodium while depleting it in calcium, magnesium, and titanium. Later differentiation proceeded mostly by removal of alkali feldspar. 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.

Rb-Sr isotope geochemistry of lherzolites and their constituent minerals from Victoria, Australia

Major element data and Rb, Sr and 87Sr/ 86Sr analyses for seven spinel lherzolite xenoliths and their Recent host basalt from Victoria, Australia, are presented. The exotic nature of the xenoliths is indicated by a wide spread in 87Sr/ 86Sr values (0.7035–0.7076) compared with the basalt (0.7041). Five of the lherzolites provide evidence of a thermal event in the mantle 650 m.y. ago. Equilibration temperatures calculated from the compositions of the lherzolite phases (ca. 1050°C) apparently relate to this event. Estimates of the local geothermal gradient suggest temperatures of less than 700°C in the source region before eruption of the lherzolites. Isotopic analyses of the lherzolite minerals show that orthopyroxene contains more radiogenic Sr than coexisting olivine and clinopyroxene in three of the xenoliths. The 87Sr/ 86Sr relationships between clinopyroxene and orthopyroxene suggest that internal isotopic disequilibrium has existed in the source region for up to 550 m.y.