Isotopic Ages Of Zircons Research Articles

Geochemistry and Isotopic Age of Zircons from Rocks of Ultramafic Massifs in the Southern Folded Framing of the São Francisco Craton (Southeastern Brazil)

Abstract —We present the first data on the geochemistry and isotopic age of zircons from rocks of the Roseta (lherzolite and olivine orthopyroxenite) and Córrego da Areia (olivine orthopyroxenite) ultramafic massifs localized among Archean–Proterozoic metamorphic strata in the southern folded framing of the São Francisco Craton. The examined zircons differ in crystal morphology, the intensity and type of cathodoluminescence, oscillatory zoning, the distribution of trace elements, and U–Pb isotopic age. Zircon crystals from the Roseta lherzolite have an age of 800-728 Ma and show negligible cathodoluminescence. Some of them have reaction rims with an age of 663–619 Ma. The crystal cores are enriched in rare earth elements (REE) but are often depleted in U as compared with the reaction rims. The age of zircons from the Roseta olivine orthopyroxenite showing moderate cathodoluminescence is within 622.6–596.9 Ma. Zircons from the Córrego da Areia olivine orthopyroxenite show moderate to intense cathodoluminescence and often have a rhythmic oscillatory zoning. They are characterized by an uneven distribution of REE and other trace elements and have high contents of U and Th. Most of these zircons have an ancient isotopic age (2558–2100 Ma). The examined zircons are of several types: relict, xenogenic, syngenetic, and epigenetic. The Roseta and Córrego da Areia massifs are considered to be protrusive outliers of a deep-seated Precambrian mafic–ultramafic complex, which were tectonically transferred along faults into the intensely metamorphosed Archean–Proterozoic strata in the southern folded framing of the São Francisco Craton.

Composition and U–Pb (LA-ICP-MS) Isotopic Age of Zircons from Composite Dikes of Western Sangilen (Tuva-Mongolian Orogen)

Abstract —Composite dikes (mingling dikes) of Western Sangilen (southeastern Tuva) are believed to indicate a shift from tectonic contraction, related to the mountain fold system formation, to postcollisional extension that ended with the disintegration of the collisional orogen. However, the position of composite dikes in the general sequence of magmatic events was based only on their geologic location and has not been confirmed by U/Pb isotope-geochronological data. The proposed study is concerned with the geologic structure, composition, and isotopic age (485–490 Ma) of West Sangilen composite dikes. The isotopic age of the host granites from the Matut pluton is 509 Ma; thermal events of Riphean (679–934 Ma) and early Paleozoic (484 Ma) age are reflected in zircons from gneiss-granites of the Erzin metamorphic complex. The obtained data are consistent with the earlier determined sequences of West Sangilen tectonometamorphic events. Basites from the examined composite dikes are strongly different in the contents of indicator elements (Rb, Nb, Ta, Zr, and Hf) and the Nb/Y, Zr/Y, Th/Ta, Zr/Nb, and Nb/Th ratios, which is indicative of different (probably, different-depth) mantle sources. The protolith of the salic component of the composite dikes might have been rocks similar in composition to the autochthonous/parautochthonous gneiss-granites of the Erzin metamorphic complex.

The First Data on the Isotopic age of Zircons from Rocks of Roseta Ultramafic Massif, Southern Framing of Craton San Francisco (Brazil)

The First Data on the Isotopic age of Zircons from Rocks of Roseta Ultramafic Massif, Southern Framing of Craton San Francisco (Brazil)

First Data on the Isotopic Age of Zircons from Rocks of the Roseta Ultramafic Massif, Southern Margin of the São Francisco Craton (Brazil)

The results of isotope U–Pb dating of zircons from lherzolite and vein olivine orthopyroxenite composing the Roseta ultramafic massif are presented. The zircons differ in their morphological, optical, and geochemical properties and in their age. Lherzolite samples that are almost deprived of cathodoluminescence are represented by two age clusters 788 ± 15 and 638.2 ± 5.4 Ma. The zircons from the first cluster are relict. A later event recorded in the zircons from the second cluster was manifested in an intense metamict mineral transformation. The age of the zircons from vein olivine orthopyroxenite conforms to 608.9 ± 3.4 Ma. These zircons are considered to be syngenetic; therefore, their age is determined by the time of crystallization of olivine orthopyroxenites.

Early crust of the Podolia Domain of the Ukrainian Shield: Isotopic age of terrigenous zircons from quartzites of the Bug Group

Detrital zircons of the ancient metasedimentary rocks bear important information about the early crust of the Earth. In this work, the early crust in the west of the Ukrainian Shield was studied using U-Pb dating of terrigenous zircons from metasedimentary rocks (quartzites) of the lower parts of the Bug Group (Kosharo-Aleksandrovka Formation) of the Podolia Domain and through Sm-Nd isotopic investigation of these rocks. The Nd model age of rocks is 3.4–3.1 Ga. Detrital zircons were studied in two samples of quartzites. The cathodoluminescent images of most zircons support their clastic origin. More than 150 zircon grains were studied on an ion microprobe. The isotope age of zircons is 3775–2000 Ma. These results were compared with previous results of study of terrigenous zircons from garnet schists of the Zaval’e quarry located closely to the studied area. It is concluded that both Paleoarchean and Meso- to Neoarchean rocks were destroyed during formation of terrigenous rocks of the Bug Group. The different amount of ancient zircons in quartzites and garnet schists indicates the different remoteness of the most ancient rocks from sedimentary basins.

Polygenesis of mafic-ultramafic complexes: Isotope-geochronological and geochemical evidence from zircons of the Berezovka massif rocks (Sakhalin Island)

Results of comprehensive isotope-geochronological (U-Pb dating; SHRIMP II) and geochemical (LA-ICP-MS) studies of zircons from different rocks of the Berezovka polygenetic mafic-ultramafic massif of the East Sakhalin ophiolite association are presented. The massif includes three proximal but genetically autonomous structure-lithologic complexes of different ages: protrusion of ultramafic rocks of restite nature, gabbroid intrusion breaking through it, and contact reaction zone located along their boundaries. The isotopic age of zircons in the massif as a whole and in its individual rocks varies over a broad range of values. The zircons belong to several populations according to their age (Ma) and other features: relict and xenogenous (~3100-990, 70–410, and ~395-210) and syngenetic (~200-100, ~90-65, and ~30-20). They differ in grain size and morphology, optical and cathodoluminescence images, and trace-element patterns. By morphology, the grains are divided into short-prismatic crystals with well-developed faces and edges, long-prismatic crystals with well-developed faces and edges, prismatic crystals with slightly resorbed faces and edges, prismatic crystals with strongly resorbed faces and edges, and intensely resorbed grains totally or partly lacking faceting. The ages of zircons depend inversely on the contents of La, Ce, and Yb, total contents of REE, (Ce/Ce*)n, and (Eu/Eu*)n. Some grains are characterized by abnormal REE and trace-element patterns due to their epigenetic redistribution. The wide scatter of the intermediate ages of relict and xenogenous zircon grains, their resorption and disturbed optical and geochemical features are probably due to the nonuniform rejuvenation of their isotope systems and variations in other parameters, caused by the effect of younger mafic melt and its fluids, whose crystallization gave rise to a gabbroid intrusion dated at 170–150Ma. The obtained data on the isotopic age and other properties of zircons from the rocks of the Berezovka massif agree with the geological model of its polygenesis.

New U-Pb isotopic data on the age of metamorphic and igneous rocks of the western margin of the Selenga-Stanovoi orogenic belt

New data on the isotopic age of zircons from metamorphic and igneous rocks are given for the Berezitovoe and Kirovskoe deposits located in the eastern margin of the Selenga-Stanovoi orogenic belt. The zircons were studied with the LA-ICP-MS method in the State Key Laboratory of Continental Dynamics, Northwestern University, Sian, Shansi province, People’s Republic of China. The results allowed us to reveal three age epochs in formation of the eastern part of the Selenga-Stanovoi orogenic belt (1.87 Ga; 380–330 Ma; 138–125 Ma), which have significant importance for understanding of the geological structure of the south-eastern framework of the North Asian craton.

Composition and age of metaorthopyroxenite xenoliths and host enderbite gneisses in the Pobuzhie granulite complex, Ukrainian Shield

The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite, diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to 6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence, the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the Novopavlovsk complex in the Azov Region.

Characteristic features and U-Pb isotopic ages of zircons in a Middle proterozoic granite pluton from Baoban Area, Hainan Province, China

The crystal form, chemical composition and U-Pb isotopic composition of various zircon fractions in a Middle Proterozoic granite pluton from the Baoban area suggest that the zircons are typically magmatogenic in nature, and different from those of sedimentary and epigenetic origins. The various zircon fractions yielded an age of about 1440.87Ma, which may represent the ge of zircon crystallization. The so called Baoban group migman’te is, as a matter of feet, a Middle Proterozoic granite pluton.

UPb zircon and monazite ages of the La Angostura granite and the orogenic history of the northwest Argentine basement

In the Cordillera Oriental of northwest Argentina, plutonic complexes of varying surface extent are aligned from NNE to SSW: Cañaní, Tipayoc, Fundición, Santa Rosa de Tastil, La Poma, Palermo, Cachi, La Paya, Molinos. They intrude the pelitic-psammitic series of the latest Precambrian-Early Cambrian Puncoviscana Formation in many places. Intrusion of the La Angostura granite (15 km south of Molinos) is believed to be post-kinematic to a second deformation (D 2) in the Sierra de Cachi. U-Pb isotopic ages of zircons and monazites have been determined on the La Angostura granite, formerly considered as Precambrian. The lower concordia intercept age of 453 + 25/−27 Ma is interpreted as the time of zircon crystallization during emplacement of the granitic complex. Within the limits of error, this Late Ordovician intercept age is confirmed by a monazite crystallization age of 462 ± 1 Ma. Thus, a Precambrian age for the granite at La Angostrua and for the minimum age of sedimentation of the Puncoviscana Formation in this area of the Cordillera Oriental must be excluded.

U-Pb isotope systematics and age of uranium mineralization, Midnite Mine, Washington

Uranium ores at the Midnite mine, near Spokane, Washington, occur in phyllites and calcsilicates of the Proterozoic Togo Formation, near the margins of an anomalously uraniferous, porphyritic quartz monzonite of Late Cretaceous age. The present geometry of the ore zones is tabular, with the thickest zones above depressions in the pluton-country rock contact. Analyses of high-grade ores from the mine define a 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron indicating an age of mineralization of 51.0 + or - 0.5 m.y. This age coincides with a time of regional volcanic activity (Sanpoil Volcanics), shallow intrusive activity, erosion, and faulting. U-Th-Pb isotopic ages of zircons from the porphyritic quartz monzonite in the mine indicate an age of about 75 m.y., hence the present orebodies were formed about 24 m.y. after its intrusion. The 51-m.y. time of mineralization probably represents a period of mobilization and redeposition of uranium by supergene ground waters, perhaps aided by mild heating and ground preparation and preserved by a capping of newly accumulated, impermeable volcanic rocks. It seems most likely that the initial concentration of uranium occurred about 75 m.y. ago, probably from relatively mild hydrothermal fluids in the contact-metamorphic aureole of the U-rich porphyritic quartz monzonite.Pitchblende, coffinitc, pyrite, marcasite, and hisingerite are the most common minerals in the uranium-bearing veinlets, with minor sphalerite and chalcopyrite. Coffinitc with associated marcasite is paragenetically later than pitchblende, though textural and isotopic evidence suggests no large difference in the times of pitchblende and colfinite formation.The U-Pb isotope systematics of total ores and of pitchblende-coffinite and pyrite-marcasite separates show that whereas open system behavior for U and Pb is essentially negligible for large (200-500 g) ore samples, Pb migration has occurred on a scale of 1 to 10 mm (out of pitchblende and coffinite and into pyrite and marcasite). Also, long-term continuous leakage of radioactive daughters of 238 U (probably 222 Rn) has occurred on scales of from approximately 100 mu m approximately 10 cm. The isotopic composition of unsupported radiogenic Pb in pyrite-marcasite seems to depend on the mineralogical microenvironment of the grains, so that the radiogenic Pb in pyrite-marcasite intimately intermixed with pitchblende-coffinite tends to be deficient in 206 Pb, and the radiogenic Pb in pyrite-marcasite in gangue tends to have excess 206 Pb. These systematics probably reflect differences between the average distances of Pb and 222 Rn diffusion since the formation of the ores.

Emplacement history of the Tatoosh volcanic-plutonic complex, Washington: Ages of zircons

The Tatoosh volcanic-plutonic complex, exposed in and near Mount Rainier National Park, Washington, is one of a series of Tertiary igneous bodies that are exposed along the axis of the Pliocene to Holocene High Cascades volcanic chain of the Pacific Northwest. It is possible that these Tertiary bodies are the eroded roots of an ancestral calc-alkalic arc. The Tatoosh complex was emplaced in an Eocene and Oligocene volcanic terrane that evidently was oceanic in character. Isotopic ages of zircons from plutonic, hypabyssal, and volcanic phases of the complex show that Tatoosh igneous activity, probably fed by repeated injections of magma from a deep source, spanned some 12 m.y. The earliest activity was emplacement of a dike and sill swarm about 26 m.y. ago. Subsequently, about 25 m.y. ago and again about 22 m.y. ago, magma broke through to the surface to form thick welded tuffs. Magmatism culminated with the rise of the core of the pluton (in part invading its own earlier volcanic and hypabyssal cover) in two stages, 17.5 and 14.1 m.y. ago.

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and Reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and Reply

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons

Early Paleozoic ophiolite complexes crop out in three main northeast-trending belts in Newfoundland. Zircons separated from trondhjemite that occurs as a late-stage differentiate within ophiolite yielded nearly concordant ages; this indicated formation of the Bay of Islands belt about 508 m.y. ago (Late Cambrian time) and the Betts Cove belt about 463 m.y. ago (Middle Ordovician time). The difference in age supports the idea that each ophiolite belt is the product of a separate spreading episode in independent, narrow, newly rifted ocean basins, marginal basins, or rifts.

Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and Reply

Research Article| August 01, 1975 Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and Reply: REPLY James M. Mattinson James M. Mattinson 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar Author and Article Information James M. Mattinson 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1975) 3 (8): 479. https://doi.org/10.1130/0091-7613(1975)3<479b:EPOCON>2.0.CO;2 Article history First Online: 02 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 James M. Mattinson; Early Paleozoic ophiolite complexes of Newfoundland: Isotopic ages of zircons: Comment and Reply: REPLY. Geology 1975;; 3 (8): 479. doi: https://doi.org/10.1130/0091-7613(1975)3<479b:EPOCON>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 No Abstract Available. 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.

U-Pb isotopic ages of zircons from Precambrian rocks of South Greenland

Results of age determinations on zircons from samples of gneiss, granite, syenite and diorite from the Ketilidian mobile belt of South Greenland are presented. The lead isotopes were determined by a direct ionisation method. As three of the four zircon concentrates gave more or less discordant results, their interpretation is difficult. However, the data agree with the view that late Ketilidian granite emplacement took place roughly 1800 m.y. ago and not around 1600 m.y. age as the previously available K/Ar dates on pre-Gardar rocks seemed to indicate.