Younger Apparent Ages Research Articles

Role of Microtexture in Isotopic Exchange and Weathering in Alkali Feldspars

Feldspars are often unretentive of radiogenic 'mAr ('mAr) and give young apparent ages using the KAr and 39Ar/'mAr methods. Similarly, feldspars frequently exchange oxygen isotopes to low temperatures and are the last mineral to close in multiphase assemblages. Because of their abundance and reactivity, feldspars in many crustal rocks approximate to the infinite reservoir of exchangeable oxygen implicit in the formulation of closure temperature, To. For both isotopic processes, Tc is often much lower than would be expected using volume diffusion coefficients determined experimentally using 'gem quality' crystals. The reasons are microtextural and are ultimately related to intracrystal phase behaviour. In nature, 4~ is lost and 180 exchanged via routes that permit rapid, non-volume diffusion or involve a non-diffusional process such as solutionredeposition. Calculations of thermal history require estimates of the effective diffusion dimension (or dimensions in 'multidomain' samples) and the effective grain shape, together with knowledge of the reaction process and the time-temperature evolution of the microtextures. Pathways for fast diffusion will be illustrated using transmission and scanning electron microscopy, and include lattice-scale dislocations, pores, and subgrain boundaries. Dislocations occur along perthite lamellae in Ab-rich ternary feldspars from syenitic rocks and granulites and in Or-rich cryptoand micro-perthites in many granites. Their development is a function of bulk composition and is a response to the need to reduce coherency strain. TEM micrographs will be presented of features in Or-rich feldspar phenocrysts from the subsolvus granite from Shap, northern England, which show that, once formed, the dislocations provide pathways into the interior of crystals which lead to dissolution and reprecipitation of feldspar far from the crystal surface. The resulting microtexture contains numerous subgrains with a network of walls and micropores between them. A similar microporous texture forms along fractures permeated by aqueous solutions; together, these features are responsible for the variable translucency of common feldspars. The textures imply abrupt decrease in effective diffusion dimension during 103, at T 350 C, leading to a reactive, easily modified, micropermeable structure. Microporous texture also develops in An-poor feldspars from hypersolvus syenites but in this compositional range dislocations do not develop as a result of coherency strain and recrystallisation is not preceeded by development of dislocations independently of fluid-crystal interactions; it may occur at T ~< 450~

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

40Ar/39Ar age constraints on the thermal history of the Archean Abitibi greenstone belt and the Pontiac Subprovince: implications for terrane collision, differential uplift, and overprinting of gold deposits

40Ar/39Ar mineral age spectra of granitic and metamorphic rocks, in conjunction with existing conventional zircon geochronology, indicate that at least two major late Archean thermal events affected tectonic blocks of the Abitibi Southern Volcanic Zone (SVZ) and the juxtaposed Pontiac Subprovince. The earlier thermal activity (2690–2670 Ma) was accompanied by the intrusion of voluminous syntectonic plutons and caused low-pressure, greenschist-facies metamorphism in the SVZ and intermediate-pressure metamorphism in the Pontiac Subprovince. The second thermal event (2660–2630 Ma) was coeval with the emplacement of syncollisional, S-type garnet–muscovite granites in the Pontiac Subprovince and the higher grade Lacorne block of the Abitibi SVZ, and reset the K–Ar systems in preexisting rocks.Magmatic amphibole from the syntectonic Round Lake batholith (~2695 Ma U–Pb zircon age) of the Abitibi SVZ has a slightly disturbed Ar release spectrum with an upper plateau age of 2669 ± 6 Ma, signifying that the low-grade Round Lake block cooled through 500 °C at a slow rate. Amphiboles in syntectonic batholiths from the higher grade Lacorne block and the Pontiac Subprovince have substantially disturbed Ar release spectra, with high-temperature steps giving apparent ages of 2681 ± 4 to 2679 ± 4 Ma; these overlap zircon ages of 2690–2670 Ma, indicating relatively rapid cooling through the amphibole blocking temperature.Metamorphic rocks (amphibolites) from the Lacorne block and the Pontiac Subprovince contain amphiboles with substantially disturbed 40Ar/39Ar release spectra and higher temperature step ages of 2677 ± 6 to 2670 ± 5 Ma, representing the minimum formation age. Fine-grained muscovite and biotite (180–250 μm) from mica schists also have disturbed Ar release patterns, but much younger apparent ages at high-temperature release steps (2581–2523 Ma for muscovite, 2562–2455 Ma for biotite) than the amphiboles.Coarse-grained muscovites from pegmatites associated with syncollision, S-type garnet–muscovite granites (2644 ± 13 Ma) in the Lacorne block and Pontiac Subprpvince show undisturbed or slightly disturbed Ar release spectra and magmatic δ18Oquartz–muscovite = 1.8–3.5‰, with total integrated ages of 2615 ± 10 to 2594 ± 7 Ma (Lacorne) and 2572 ± 6 Ma (Pontiac), respectively, indicating different uplift rates for the two terranes. Amphiboles (~2680 Ma) from metamorphic rocks in the Lacorne block and Pontiac Subprovince and from the Round Lake batholith are disturbed, whereas coarse-grained muscovites from the pegmatites (2644 ± 13 Ma) are relatively undisturbed. This indicates that the disturbance of the amphiboles may have been caused by a thermal event that preceded or was coeval with the emplacement of the garnet–muscovite granite suite, rather than being a grain-size effect.These results are consistent with a model whereby early subduction of oceanic lithosphere beneath the Abitibi SVZ (2740–2680 Ma), and separately under the Pontiac Subprovince, was responsible for syntectonic batholiths and the first thermal event. Collision with the Abitibi SVZ and local underthrusting of the Pontiac Subprovince at about 2670–2630 Ma caused the second major thermal event and partial melting of the underthrust Pontiac-type metasediments to form the garnet–muscovite granites. Later differential uplift exposed the entire Pontiac Subprovince and the Lacorne block as a tectonic window of underthrust Pontiac in the Abitibi SVZ. Resetting of several isotopic systems, including apparent younger ages of gold mineralization, is probably related to this late collisional, tectonothermal overprinting event. Fluid and (or) thermal events at ≥275 °C influenced the Kirkland Lake – Cadillac fault down to 2513 ± 10 Ma, as indicated by a plateau age of postkinematic biotite in the fault. The fault was intermittently reactivated over a period of 440 Ma, from ~2690 Ma to ≤2250 Ma.

The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40 Ar/ 39 Ar mineral dates of the Penmynydd schists

The 'Penmynydd' schists of SE Anglesey include several high P /low T lithologies including: (1) metasedimentary quartz–phengite schist; and (2) mafic blueschist with barroisite/crossite assemblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengite and three amphibole concentrates have been analysed by incremental-release 40 Ar/ 39 Ar dating methods and display discordant age spectra with anomalously young apparent ages recorded in low T increments. Ages increase systematically throughout intermediate T portions of the analyses to define high T plateaux of c . 550–560 Ma (barroisite/crossite-rich concentrates) and c. 580–590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discordant spectrum with intermediate T increments defining ages similar to those recorded by barroisite/crossite and high T increments giving ages similar to those of actinolite. The amphibole and phengite ages are interpreted as dating post-metamorphic cooling following a regional low- P Ml greenschist metamorphism ( c . 580–590 Ma) and a high- P M2 metamorphism ( c . 550–560 Ma). 40 Ar/ 39 Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of c. 550–560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.

Preliminary Report on Age of Uraniun-Ore Deposition and Host-Rock Formation at Lilljuthatten, Sweden: ABSTRACT

Analyses of 6 mineralized and 4 unmineralized samples from drill holes in the uranium ore deposit at Lilljuthatten, Sweden, yield an apparent Pb-Pb age of 425 ± 6 m.y. (errors at 95% confidence level). This age corresponds with the youngest phase of the Caledonian orogeny in central Sweden, indicating that the ore deposit probably formed during this event. The age of the host granite has not been determined quantitatively because the Caledonian event has disturbed (and in some places completely reset) all the isotopic systems investigated thus far. Four new and 7 published Rb-Sr analyses approximate an isochron of 1,540 m.y., but two samples from near the ore zone have been reset to much younger apparent ages. Isotope systematics in the Th-Pb and U-Pb systems are highly complex, but U-Pb data for a few samples suggest a host-rock age of approximately 1,500 m.y. If this age and thorium immobility are assumed, there is a suggestion that the host granite was extremely rich in uranium (30 to 50 ppm) at the time of intrusion and that the ore deposit may have formed by concentration of uranium from the host granite. End_of_Article - Last_Page 570------------

Dating of adularia and the relationship of hydrothermal events in the Lizard complex, Cornwall

SummaryA new total degassing argon-40/argon-39 age determination is given for crystals of fresh adularia from hydrothermal veins within the amphibolites near Porthkerris Point, the Lizard, Cornwall. The apparent age of 211±2 Ma obtained is regarded as a close minimum age for crystallization of the adularia. There appears to have been only one phase of crystallization of primary hydrothermal adularia, about 210–220 Ma, in the granulites and amphibolites in the vicinity of Porthkerris Point and in the gabbro of Dean quarry. Another hydrothermal event occurred c. 160–170 Ma. Younger apparent ages have been attributed to hydrothermal activity at one or more subsequent periods.

Age Determinations from Andean Peru: A Reconnaissance Survey

Research Article| July 01, 1974 Age Determinations from Andean Peru: A Reconnaissance Survey J. W. STEWART; J. W. STEWART 1“Promisa” Mineraçāo e Prospecçoes Minerais S.A., P.O. Box 56002, Rio de Janeiro, Brazil Search for other works by this author on: GSW Google Scholar J. F. EVERNDEN; J. F. EVERNDEN 2U.S. Arms Control and Disarmament Agency, Washington, D.C. Search for other works by this author on: GSW Google Scholar N. J. SNELLING N. J. SNELLING 3Institute of Geological Sciences, 64–78 Gray's Inn Road, London WC1X 8NG, England Search for other works by this author on: GSW Google Scholar GSA Bulletin (1974) 85 (7): 1107–1116. https://doi.org/10.1130/0016-7606(1974)85<1107:ADFAPA>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 J. W. STEWART, J. F. EVERNDEN, N. J. SNELLING; Age Determinations from Andean Peru: A Reconnaissance Survey. GSA Bulletin 1974;; 85 (7): 1107–1116. doi: https://doi.org/10.1130/0016-7606(1974)85<1107:ADFAPA>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 More than 60 new K-Ar age determinations for mica, hornblende, and potassium feldspar together with a few Rb-Sr determinations on mica and potassium feldspar are presented.Micas from two gneiss samples within the Arequipa batholith intrusive belt have yielded ages of ∼660 m.y. Five samples of granitic rocks, widely scattered in the Coastal and Eastern Cordilleras, have given ages ranging from 447 to 251 m.y. In the Eastern Cordillera, stratigraphic control is sufficient in one case to confirm intrusion in Late Ordovician or Early Silurian times and, in another case, Late Permian time.Triassic and Early Jurassic apparent ages are known from the Coastal, Western, and Eastern Cordilleras; the parent rocks are both igneous and metamorphic. The metamorphic rocks are probably Paleozoic or older, and the apparent ages must be the result of Mesozoic or later disturbances. It is tentatively suggested that intrusion of granitic rocks occurred in Late Triassic or Early Jurassic times.The Coastal batholith yields a spectrum of apparent ages from ∼100 to 10 m.y. Fewer ages are available from the Arequipa batholith, and they show a more limited range – from 80 to 50 m.y. The Cordillera Blanca batholith gives very young apparent ages of from 12 to 3 m.y.Mineral ages from across the Coastal batholith get progressively younger from southwest to northeast. However, because virtually all the Andean ages have been-determined by the K-Ar method, they do not necessarily indicate a northeastward migration of intrusive foci. Very limited Rb-Sr data indicate initial 87Sr/86Sr ratios in the range 0.704 to 0.706 for the granitic rocks of the Coastal batholith. 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.

Age of the northern part of the Nelson batholith

Nelson plutonic rocks in Slocan Mining Camp were emplaced at least 160 m.y. ago (base of the Upper Jurassic). K–Ar apparent ages for intrusions of lamprophyre and Valhalla plutonic rocks are indistinguishable from those for Nelson plutonic rocks. Younger apparent ages were obtained for altered samples and indicate at least one period of alteration less than 120 m.y. ago.