Concordia Intercept Age Research Articles

U-Pb zircon ages for granitoid gneisses in northern Namibia and their significance for proterozoic crustal evolution of southwestern Africa

The pre-Damara (> 900 Ma) basement gneisses and granitoids of northern Namibia and southern Angola constitute the southern margin of the Congo craton and are poorly known geochronologically. We report U-Pb zircon concordia intercept ages for an augengneiss of the Epupa Complex (∼ 1800 Ma), a banded quartzofeldspathic gneiss of the Huab Complex (∼1810 Ma), a post-Huab granitic gneiss (∼1750 Ma) and for inherited zircons (∼2120 Ma) from a Damaran syenite that has a true emplacement age of 800–840 Ma. These ages suggest that the Epupa and Huab Complexes are broadly coeval, and their formation, together with the emplacement of the widespread Fransfontein granitoid suite, constitutes a significant and regionally extensive crust-forming event in northern Namibia and southern Angola (the Huabian Episode) that corresponds to the Eburnian event in western, central and eastern Africa. Older crust in excess of 2120 Ma is likely to be present in the region but has not been documented geochronologically. Similarities in age, rock association and broad geologic evolution for pre-Damara basement complexes in northern, central and southern Namibia suggest that the entire territory may have been part of the Congo craton in mid-Proterozoic times, whose southwestern portion was largely reworked during the late Precambrian Namaqua and Pan-African events.

Age and isotope geochemistry of the Archaean Pongola and Usushwana suites in Swaziland, southern Africa: a case for crustal contamination of mantle-derived magma

The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland. U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial ε Nd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age. An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial 143Nd/ 144Nd that lies off the CHUR growth curve by ε Nd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of ∼ 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS. The negative deviations of initialε Nd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial 87Sr/ 86Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of ∼ 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust. Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr∼ 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.

U‐Pb zircon ages and a revised chronology for the Tennant Creek Inlier, Northern Territory

New U‐Pb zircon ages, combined with previous Rb‐Sr data, provide a geochronological framework for the Tennant Creek Inlier of Northern Australia. Ages of 1870± 20 Ma and 1846± 8 Ma are advanced for the igneous crystallization of the Tennant Creek and Cabbage Gum Granites, respectively. A lower concordia intercept age of 248 ± 27 Ma is thought to relate to zircon dilation and Pb loss resulting from late Palaeozoic uplift, alteration and/or erosion. If previously analysed volcanics from the Bernborough Formation also lost Pb at that time, they have a primary age of 1870 ± 15 Ma. This provides a temporal estimate for deposition of the Warramunga Group, the major geological component of the Tennant Creek Inlier. At any single site, Rb‐Sr total‐rock ages are younger than U‐Pb zircon ages, possibly up to 400 Ma so for the Cabbage Gum Granite. Previously reported Rb‐Sr total‐rock ages of about 1650–1660 Ma probably define igenous crystallization of several other felsic magmas. Granite emplacement thus occurred b...

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex, Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende 40Ar/39Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262. Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the 207Pb/206Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data. Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.

A garnet-two-mica granite, Coyote Mountains, southern Arizona: Geologic setting, uranium-lead isotopic systematics of zircon, and nature of the granite source region

The 58-m.y.-old Pan Tak Granite, considered representative of the widespread early Tertiary garnet-two-mica granites of south-central Arizona, consists of biotite and muscovite-biotite granite intruded by garnet-muscovite and garnet-biotite granite and associated pegmatite. Textural evidence indicates that muscovite is a primary, magmatic mineral. The granite was intruded into Jurassic dioritic to granitic rocks and Paleozoic metasedimentary rocks during a latest Cretaceous and early Tertiary regional metamorphic episode. Part of the pluton was subsequently mylonitized during an early or middle Tertiary deformational episode of “metamorphic-core-complex” aspect. The fabrics produced by these two metamorphic episodes differ in distribution, style, orientation, and, probably, age. The zircon population of the Pan Tak Granite consists of a mixture of small euhedral crystals and larger, rounded crystals with or without overgrowths. Uranium-lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 ± 2 m.y. and a late Precambrian upper intercept. 207 Pb*/ 206 Pb* ages range from 214 m.y. for the finest fraction to 434 m.y. for the coarsest fraction. The Pan Tak discordia is interpreted as a mixing line between 58-m.y.-old magmatic zircon and an older zircon component largely or entirely of late Precambrian age. Geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but, rather, was inherited from the source material of the granite. The presence of this inherited zircon indicates that the Pan Tak magma was produced by melting of crustal rocks containing late Precambrian zircon. This source material was probably the 1.7- to 1.4-b.y.-old crystalline basement of south-central Arizona. Generation of the Pan Tak and related granites by crustal anatexis was one aspect of a latest Cretaceous to early Tertiary orogenic episode in south-central Arizona.

Age and origin of the Dover Fault: tectonic boundary between the Gander and Avalon Zones of the northeastern Newfoundland Appalachians

The Dover Fault forms a tectonic boundary between northern portions of the Gander and Avalon Zones of the Newfoundland Appalachians. A systematic geochronological investigation across the mylonitic fault zone has been carried out to clarify the origin and history of tectonic activity along this important Appalachian structure.Zircon fractions from the mylonitic Lockers Bay Granite (Gander Zone) record individually discordant U–Pb dates, but yield a well-defined upper concordia intercept age of 460 ± 20 Ma. Hornblende (1 sample) and biotite (11 samples) from variably mylonitic Gander Zone lithologies (plutonic and metamorphic) adjacent to the fault zone record undisturbed 40Ar/39Ar age spectra with plateau ages of 395 and 365–383 Ma, respectively. Together with field and petrographic characteristics, the new geochronologic data suggest that the Lockers Bay Granite originated as an anatectic melt during high-grade regional metamorphism of the country rock terrane at approximately 460 Ma. The crystal-rich magma was subsequently emplaced into its present position thereby producing local discordance with small-scale structures in host gneisses.Following its emplacement, the Lockers Bay Granite and country rock terrane were maintained at elevated postmetamorphic temperatures for a prolonged interval until they underwent rapid strain during Acadian (Devonian) juxtaposing of the northern Gander and Avalon Zones along the Dover Fault. Sudden Acadian uplift along the fault is suggested because of the rapid cooling of the northern Gander Zone through temperatures required for argon retention in hornblende and biotite. Post-mylonite brecciation may have locally affected argon isotopic systems of phyllitic lithologies adjacent to the fault zone in the study area.

Geochronology of the Swift Current granite and host volcanic rocks of the Love Cove Group, southwestern Avalon zone, Newfoundland: evidence of a late Proterozoic volcanic–subvolcanic association

Zircon fractions from the variably deformed and metamorphosed Swift Current granite and host volcanic rocks of the Love Cove Group record individually discordant U–Pb ages with well-defined upper concordia intercept ages of 580 ± 20 and 590 ± 30 Ma, respectively. These are interpreted to be crystallization dates and indicate a late Proterozoic cogmagmatic relationship. Primary hornblende from the pluton record disturbed 40Ar/39Ar age spectra that suggest postcrystallization argon loss, probably during Acadian (Devonian) regional metamorphism. 40Ar/39Ar plateau ages of 560–566 Ma are well defined for the hornblende and are interpreted to date times of postmagmatic cooling. The similarity between zircon and hornblende dates suggests relatively rapid postmagmatic cooling. A six-point, Rb–Sr whole-rock isochron age of 548 ± 11 Ma is defined for the pluton. The slight discordancy of this date in comparison with the zircon and hornblende ages may reflect a minor disturbance of whole-rock isotopic systems during Acadian regional metamorphism.

Geochronology of archean gneisses in the Lake Helen area, Southwestern Big Horn Mountains, Wyoming

Abstract The RbSr and UPb methods were used to study gneisses in the 7 1 2 - minute Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite. A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial 87Sr/86Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source. A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The 87Sr/86Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.

Areal extent and first U-Pb age of the Pre-Damaran Abbabis complex in the central Damara belt of South West Africa (Namibia)

The Proterozoic Abbabis Complex emerges from beneath the metasediments of the Late Precambrian Damara Supergroup within mantled gneiss domes and anticlinoria over a wide area in the central granite zone of the Pan African Damara belt. U-Pb dating of composite-, size-, and non-magnetic fractions of metamict zircons from two samples of Abbabis granite-gneiss from the farm Abbabis yielded a concordia intercept age of 1925 −280 +330 Ma. It is speculated that the Abbabis basement rocks are continuous under the Damaran metasedimentary cover and may link up with the Franzfontein basement inlier of similar age. This conclusion supports the ensialic character of the northeastern branch of the Damara orogenic belt.

Uranium-lead isotope systematics and apparent ages of zircons and other minerals in precambrian granitic rocks, Granite Mountains, Wyoming

Zircon suites from the two main types of granite in the Granite Mountains, Wyoming, yielded concordia-intercept ages of 2,640±20 m.y. for a red, foliated granite (granite of Long Creek Mountain) and 2,595±40 m.y. for the much larger mass of the granite of Lankin Dome. These ages are statistically distinct (40±20 m.y. difference) and are consistent with observed chemical and textural differences. The lower intercepts of the zircon chords of 50±40 and 100+ 75 m.y. for the granite of Long Creek Mountain and granite of Lankin Dome, respectively, are not consistent with reasonable continuous diffusion lead-loss curves but do correspond well with the known (Laramide) time of uplift of the rocks. Epidote, zircon, and apatite from silicified and epidotized zones in the granites all record at least one postcrystallization disturbance in addition to the Laramide event and do not define a unique age of silicification and epidotization. The lower limit of ∼2,500 m.y. provided by the least disturbed epidote, however, suggests that these rocks were probably formed by deuteric processes shortly after emplacement of the granite of the Lankin Dome. The earlier of the two disturbances that affected the minerals of the silicified-epidotized rock can be bracketed between 1,350 and 2,240 m.y. ago and is probably the same event that lowered mineral K-Ar and ages in the region.

Zircon U-Pb ages of the Franzfontein granitic suite, northern South West Africa

U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed.

Geology and Age of the Wolf River Batholith, Wisconsin

The Wolf River Batholith is a large anorogenic plutonic complex in central and northeastern Wisconsin that is similar lithologically to the rapakivi massifs of Finland. Quartz monzonite, the most abundant rock type present, can be subdivided into three major lithologic units, one of which commonly has well-developed rapakivi texture. Other rock types include syenite, granite, quartz and feldspar porphyries, monzonite, and anorthosite; the anorthosite exists as totally isolated masses within the quartz monzonite and may or may not be genetically related to the main complex. Whole-rock Rb-Sr data yield an age of 1,468 ± 34 m.y., with initial Sr 87 /Sr 86 = 0.7048 ± 0.0017 (λ (Rb 87 ) = 1.39 × 10 −11 yr −1 ). U-Pb data from cogenetic zircon fractions define a concordia intercept age of 1,510 ± 15 m.y. for traditionally used values of the U 235 and U 238 half-lives or an age of 1,485 ± 15 m.y. based on recently redetermined values. Rb-Sr data also show that the syenite complex of the Wausau area is approximately, if not exactly, the same age as the Wolf River Batholith. The Wolf River Batholith is similar in age to other granites in southeastern Missouri and the southwestern United States and is apparently a representative of widespread anorogenic igneous activity that existed throughout the southern part of the North American continent 1,400 to 1,500 m.y. ago.

Zircon Ages of Precambrian Granitic Rocks, Northeastern Wisconsin

U-Pb isotopic analyses of cogenetic zircon fractions from four granitic bodies in northeastern Wisconsin yielded the following concordia intercept ages: Amberg Granite and Newingham Granodiorite, ; Hoskin Lake Granite, ; Dunbar Gneiss, Emplacement of granitic rocks in this area therefore appears to have occurred during a time interval extending over and not exceeding a few tens of millions of years, presumably as part of the Penokean orogeny. Moreover, the sequence of ages agrees with geologic evidence, with the possible exception of the problematical Dunbar Gneiss. Slight differences in slopes and intercepts of the chords defined by the zircon data indicate that no single simple mechanism can explain the observed isotopic disturbance patterns.

Zircon Age of a Precambrian Rhyolite, Northeastern Wisconsin

U-Pb isotopic analyses of three cogenetic zircon fractions from a Precambrian rhyolite of the Quinnesec Formation in northeastern Wisconsin yielded a concordia intercept age of 1905 (+30 to −10) m.y. Thus, at least part of the Quinnesec Formation is late or post-Animikie in age and is not correlative with the pre-Animikie complexes of Michigan, as has been previously believed.