Pb Crystallization Ages Research Articles

Age and significance of ‘outboard’ high-grade metamorphics and intrusives of the Ross orogen, Antarctica

We present new U–Pb zircon and 40Ar–39Ar hornblende isotopic data for gneisses and undeformed intrusions from three important and widely separated localities from the northern–eastern ‘outboard’ edge of the Ross orogen in the Queen Maud and Horlick mountains, Antarctica. U–Pb zircon analyses of gneisses that have long represented possible candidates for Precambrian cratonic crust have yielded latest Late Proterozoic to early Paleozoic ages. One paragneiss from the Fallone Nunataks, near Scott Glacier yields a 546+10.6/−18.9Ma age for its youngest zircon population, indicating a maximum latest Neoproterozoic–early Paleozoic (Ross orogeny) age for deformation and fabric development. Two orthogneisses from the Bravo Hills near Shackleton Glacier yield igneous protolith crystallization ages of 535+8.5/−8.9Ma and 517+6.9/−6.5Ma, and one orthogneiss from Woodall Peak near Ramsey Glacier yields a protolith crystallization age of 503.8+6.9/−6.5Ma. Intrusion of the orthogneiss protoliths therefore occurred concomitant with other pretectonic (deformed) subduction-related magmatic rocks associated with the early Paleozoic Queen Maud batholith. The younger limit for deformation of the paragneiss and orthogneiss protoliths is constrained by U–Pb crystallization ages of post-tectonic (undeformed) intrusions of the batholith that intrude the gneisses. These intrusions yield U–Pb crystallization ages of 503.5+6.2/−6.5Ma (post-tectonic, Woodall Peak), 498.4+8.8/−7.6Ma (post-tectonic, Bravo Hills), and 484.7±8.4Ma (syntectonic to post-tectonic, Fallone Nunataks). The younger limit for deformation is further constrained in the Bravo Hills by 40Ar–39Ar hornblende analyses that yield a metamorphic cooling age of 502±7Ma for an orthogneiss. On a regional scale, magmatism commenced by ~535Ma within the Queen Maud batholith. Deformation locally extended into the Late Cambrian to Ordovician, with several areas transitioning to post tectonic magmatism and orogenic cooling by the late Middle Cambrian to Early Ordovician possibly concomitant with phases of slab roll back along the Gondwana margin.

A comparison of Nd model ages and U–Pb zircon ages of Grenville granitoids: constraints on the evolution of the Laurentian margin from 1.5 to 1.0 Ga

Terra Nova, 24, 7–15, 2012AbstractConcordance and discordance between Nd model ages and U–Pb ages is still widely misunderstood in the geological community. In the SW Grenville Province, plutonic orthogneisses often yield TDM ages significantly older than U–Pb crystallization ages, but this can be understood in a model where a series of crustal Nd isotope evolution lines departs at intervals from the depleted mantle (DM) evolution line. Seven samples with concordant U–Pb and Nd model ages have initial ε Nd on the DM line, attributed to melting of juvenile crust generated in an Elzevirian back‐arc rift zone. In contrast, plutons with discordant U–Pb and Nd model ages lie near crustal evolution lines defined by suites of basement gneisses on the flanks of the rift zone. This study shows that Nd data from large suites of geographically constrained, undated grey gneiss provide a crustal evolution framework for orogen‐scale interpretation of more scattered but precisely dated plutons.

Depositional provenance of the Himalayan metamorphic core of Garhwal region, India: Constrained by U–Pb and Hf isotopes in zircons

U–Pb and Hf isotopes in zircon from the highly metamorphosed core of the Himalaya in the Garhwal region of India indicate that the ortho- and paragneiss units of the Greater Himalayan Sequence (oGHS and pGHS, respectively) are geochemical distinct from the older Lesser Himalayan Crystalline Sequence (LHCS). Zircon grains from Paleoproterozoic metasediment of the LHCS yield U–Pb crystallization ages from 1614±14Ma to 2679±14Ma with a single distinct peak in the age spectra at 1870Ma. εHf values from zircon in the LHCS range from 1 to −8. In contrast, zircon grains from the Neoproterozoic pGHS yield U–Pb crystallization ages from 481±10Ma to 2560±8Ma with a single distinct peak at 860Ma. εHf values from these zircons range from 13 to −21. The oGHS yields zircon grains with U–Pb crystallization ages from 416±2Ma to 2740±2Ma with a single distinct peak at 472Ma. These grains often exhibit older inherited cores ranging from ~800Ma to ~2740Ma with Ordovician rims (416Ma to 510Ma). The εHf values of zircon grains in the oGHS have two distinct modes: the ~860Ma inherited zircon cores have a similar range to the ~860Ma zircons from the pGHS (13 to −21), whereas the ~480Ma rims range from −7 to −11.The detrital zircon U–Pb age spectra from the pGHS in the Garhwal region differs significantly from the U–Pb age spectra in other parts of the range (Nepal and Bhutan). These differences indicate wide lateral variations in provenance for the various source regions of the pGHS.Comparisons of the U–Pb and Hf data from the Garhwal region with potential ~1800Ma source regions show overlap with metasediments of the Aravalli shield of India. In the pGHS the most juvenile εHf values overlap with those of the northern Arabian Nubian Shield in Israel. However, these rocks show little crustal contamination at ~800Ma, whereas the pGHS has significant crustal contamination at ~860Ma. The Ordovician oGHS has εHf values that match the average εHf values of the paragneiss implying that it was sourced from the paragneiss. Although the nature of this Ordovician igneous event is still poorly understood, it is clear that the Hf isotopes can provide further insight into the petrogenesis of the magmatic rocks associated with it.

A direct comparison of the ages of detrital monazite versus detrital zircon in Appalachian foreland basin sandstones: Searching for the record of Phanerozoic orogenic events

The provenance potential of detrital monazite was investigated by in situ measurement of 232Th– 208Pb dates of grains isolated from six Middle Carboniferous–Permian sandstones from the Appalachian foreland basin. Provenance assessment of these units was previously investigated by measuring U–Pb crystallization ages of detrital zircon (Thomas et al., 2004; Becker et al., 2005, 2006). Approximately 90% of the detrital zircon ages record Mesoproterozoic or older ages, with only 10% recording the three major pulses of tectonism (Taconian, Acadian and Alleghanian) that are the hallmark of the Appalachian Orogen. 232Th– 208Pb ages of detrital monazite, however, strongly record the complex phases of Paleozoic orogenesis. Nearly 65% of the ages record Paleozoic events, while 35% record Neoproterozoic or older ages. In several of the analyzed sandstones, detrital monazite ages record Paleozoic orogenic events that are completely missed by detrital zircon ages, demonstrating that monazite ages more accurately reflect the character of the sediment source rocks. The inferred maximum age of sediment deposition, as determined by the youngest monazite grains, is ~ 550 Ma younger for two of the analyzed sandstones compared to depositional constraints based on the youngest detrital zircon. The different physical properties and petrogenesis of zircon and monazite are interpreted to be factors for the dramatic differences in sediment provenance information provided by each mineral. The results from this study have important implications for determining sediment provenance, constraining maximum age of sediment deposition, and developing robust regional tectonic models.

SHRIMP dating and Nd isotope geology of the Archean terranes of the Uweinat-Kamil inlier, Egypt–Sudan–Libya

The Precambrian terranes of north east Africa are still poorly known. It is generally believed that the oldest rocks in the region are the charnockitic gneisses of the Uweinat massif, located at the triple junction between Libya–Egypt–Sudan, because they have yielded Rb–Sr ages around 2.6 Ga and Nd T DM model ages around 3.0–3.2 Ga. Here we confirm that these rocks are indeed Archean with SHRIMP U–Pb zircon ages as old as 3.0 Ga, but we also report older rocks to the east, in the neighboring Gebel Kamil region. This area contains a metaigneous complex formed of tonalite–trondhjemite–granite (TTG) and gabbro-diorite (GbD) gneisses with a whole-rock Sm–Nd isochron age of 3.16 ± 0.16 Ga, average Nd T CR of 3.17 ± 0.04 Ga, and average ɛNd (3.2 Ga) of 3.4 ± 0.3. The oldest TTG gneisses, which are also the oldest rocks found to date in north east Africa, contain large magmatic zircons with SHRIMP U–Pb crystallization ages peaking at 3.22 ± 0.04 Ga, closely matching the Sm–Nd and the Nd T CR model ages. These ages are interpreted to represent arc-magmas produced between 3.1 Ga and 3.3 Ga. Other TTGs have younger Archean zircon ages that form a continuum between 3.1 Ga and 2.55 Ga, with three peaks at around 2.97 Ga, 2.85 Ga, and 2.6–2.7 Ga; these rocks were apparently generated from the older TTGs during repeated events of crustal recycling. The crust was stable from 2.55 Ga to 2.0 Ga, when an intense thermal event generated rims of variable thickness over the Archean zircons. Neither the reworking from 3.1 Ga to 2.55 Ga, nor the metamorphism at 2.0 Ga involved the addition of juvenile material to the crust of this area, which behaved as an almost closed system from 3.1 Ga to 0.75 Ga, when the intrusion of Pan-African I-type granitoids with a juvenile component began.

Thermal gradient and timing of high‐T–low‐P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia

AbstractThe Wongwibinda Metamorphic Complex (WMC) is a high‐temperature, low‐pressure (HTLP) belt in the southern New England Orogen. It is characterized by a high metamorphic field gradient exposed in variably metamorphosed siliceous turbidites. The Abroi Granodiorite and the Rockvale and Tobermory adamellites, S‐type granitoids of the Hillgrove Plutonic Suite, intrude the metaturbidites. Six samples of metaturbidite were studied from an ∼3 km long field traverse. Integrated petrography, mineral chemistry, and mineral equilibria modelling indicate a peak metamorphic temperature of 350–450 °C in the lowest grade rocks and ∼660 °C in the highest‐grade rocks. Maximum pressure does not exceed 3.5 kbar anywhere, implying a maximum depth of 12 km and indicating an average vertical gradient of at least 55 °C km−1, though our calculations suggest this is not linear. Metamorphic isograds show no apparent relationship with distance to the exposed margins of the Hillgrove Suite granitoids. Electron microprobe U–Th–Pb monazite data indicate a date of 296.8 ± 1.5 Ma for the thermal peak of the HTLP metamorphism. Laser ablation inductively coupled plasma mass spectrometry indicates a zircon U–Pb crystallization age of 290.5 ± 1.6 Ma for the Abroi Granodiorite, confirming that the pluton post‐dates the peak HTLP metamorphism. Consequently, magmatic advective heat transfer from depth via emplacement of a large volume of granitoid is unlikely to be the key local driver of the high‐grade metamorphism. It is concluded that published evidence of an extensional geodynamic setting around the Carboniferous–Permian boundary supports conductive heat transfer as the key driver of HTLP metamorphism for the WMC. It is not possible to exclude magmatic advective heat transfer via emplacement of mantle derived basaltic magmas in the deeper crust.

Mesoproterozoic juvenile mafic–ultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province): SHRIMP U–Pb geochronology and Nd–Sr constraints of the Figueira Branca Suite

Abstract The Figueira Branca Suite (FBS) comprises a layered mafic–ultramafic complex which together with mafic–felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80–1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic–sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587–1522 Ma) and Santa Helena (1485–1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U–Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5 ± 8.0 Ma (MSWD = 1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9 ± 6.9 Ma (MSWD = 0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9 ± 2.8 Ma (MSWD = 1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207 Pb/ 206 Pb crystallization age of 1541 ± 23 Ma (MSWD = 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd–Sr isotopic analyses of the FBS mafic–ultramafic rocks and coeval gabbro showed comparable ɛ Nd(1.42Ga) values (+3.0 to +4.7) and variable ɛ Sr(1.42Ga) ones (−39.1 to −8.1). These data plot in the depleted field quadrant of the Nd–Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40 Ar– 39 Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222 ± 5 Ma while a nearby troctolite yielded plateau biotite ages of 1275 ± 4 Ma and 1268 ± 4 Ma. These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40 Ar– 39 Ar plateau age of 1781 ± 15 Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539–1510 Ma and 1452–1322 Ma 40 Ar– 39 Ar age-patterns, respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50 m.y.), as estimated by the zircon–titanite–hornblende–biotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40 Ar– 39 Ar biotite age (1322 Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province.

Understanding the roles of crustal growth and preservation in the detrital zircon record

Crustal evolution studies using detrital minerals must consider the potential for bias introduced at the time of formation of the continental crust, through its preservation in subsequent supercontinental cycles and during erosion and reworking of sedimentary material. To investigate the extent of these biases, and our ability to extract global information from local studies, this study presents in situ U–Pb, O and Hf isotope data in detrital zircons from the Scottish Highlands for comparison with greater Gondwana and Laurentia. Zircon crystallisation ages range from 3.8–1.1Ga, and they group into three episodes (at ∼2.8, 1.8 and 1.2Ga), peaking within 100Ma of the ages of known supercontinents (Superia, Nuna and Rodinia). They are therefore consistent with preservation due to continental collision and supercontinent stabilisation. The Hf model ages fall between 4.2–1.4Ga, and they also group into three significant juvenile extraction events at ∼3.3, 2.2 and 1.7Ga, of which only the ∼3.3Ga peak has been observed elsewhere in Laurentia or Gondwana. There is a link between the distribution of U–Pb crystallisation ages and model Hf ages indicating typical residence times of ∼600Ma between the formation of new crust and its reworking in later magmatic events. Individual Hf model ages appear to form continua within each crystallisation event, suggesting that the generation of new continental crust is a continuous process, even though the record is then biased by the development of supercontinents.

Polyphase thrusting and dyke emplacement in the central Southern Alps (Northern Italy)

The Triassic succession of the central Southern Alps (Italy) is stacked into several units bounded by south-verging low-angle thrust faults, which are related to two successive steps of crustal shortening. The thrust surfaces are cut by high-angle extensional and strike-slip faults, which controlled the emplacement of hypabissal magmatic intrusions that post-date thrusts motions. Intrusion ages based on SHRIMP U–Pb zircon dating span between 42 ± 1 and 39 ± 1 Ma, suggesting close time relationships with the earliest Adamello intrusion stages and, more in general, with the widespread calc-alkaline magmatism described in the Southern Alps. Fission-track ages of magmatic apatites are indistinguishable from U–Pb crystallization ages of zircons, suggesting that the intrusion occurred in country rocks already exhumed above the partial annealing zone of apatite (depth < 2–4 km). These data indicate that the central Southern Alps were already structured and largely exhumed in the Middle Eocene. Although we describe minor faults affecting magmatic bodies and local reactivations of older structures, no major internal deformations have occurred in the area after the Bartonian. Neogene deformations were instead concentrated farther south, along the frontal part of the belt.

HAFNIUM ISOTOPIC COMPOSITIONS OF ZIRCON FROM ADIRONDACK AMCG SUITES: IMPLICATIONS FOR THE PETROGENESIS OF ANORTHOSITES, GABBROS, AND GRANITIC MEMBERS OF THE SUITES

Zircon crystals separated from selected samples of Adirondack (New York) anorthosite, gabbro, and granitic members of the AMCG suite, previously dated by SHRIMP U–Pb methods, have been analyzed for Hf isotopic composition. The resulting calculated Hf T DM values for both granitic rocks and anorthosites are ca. 1350–1400 Ma, consistent with previously determined Nd T DM , significantly greater that the U–Pb crystallization ages of 1155–1160 Ma. They indicate involvement of older lithosphere. The range of initial e Hf for anorthosites falls within the range observed for non-anorthosite lithologies ( ca. +3 to +8) and suggests some consanguinity in their petrogenesis. On the basis of bulk compositions, it is likely that granitic members of the suite were formed by partial melting of pre-existing crust or mantle lithosphere (or both). Anorthosite may have formed from fractionation of basalt derived from previously enriched mantle lithosphere. The fact that the Hf isotopic characteristics ( e.g. , T DM ) of zircon from the charnockites, mangerites, and granites entirely overlap those of zircon from the anorthosites and gabbros suggests that the sources ( e.g. , enriched mantle and lower crust) were characterized by similar Hf isotopic compositions and a limited range of Lu/Hf at ~1100 Ma. Such a relationship might be expected if ~1350 Ma arc development produced relatively homogeneous crust and sub-arc mantle in terms of Hf isotopic composition and Lu/Hf values. This is feasible because changes of e Hf or e Nd in such a crust–mantle system in only ~150–200 Ma (difference between T DM and U–Pb ages) would be less than 1 e Hf unit.

The anatomy of a Gondwanan terrane: The Neoproterozoic–Ordovician basement of the pre-Alpine Sebeş–Lotru composite terrane (South Carpathians, Romania)

The Sebeş–Lotru terrane in the South Carpathians mountain range comprises a lower, Neoproterozoic metamorphic unit (Lotru) and an upper, Ordovician metamorphic unit (Cumpăna) that were juxtaposed during the Variscan orogeny. Two orthogneisses from the Lotru metamorphic unit yield U/Pb LA-ICP-MS zircon crystallization ages of 549.3 ± 3.8 Ma and 587.5 ± 3 Ma, respectively. Two orthogneisses from the Cumpăna metamorphic unit yield zircon crystallization ages of 458.9 ± 3.5 Ma and 466.0 ± 4.2 Ma, respectively. High U zircons from two other orthogneisses from the Cumpăna metamorphic unit have ages ranging from 400 Ma to 320 Ma, which are interpreted to reflect protracted zircon recrystallization during the regionally significant Variscan collisional event. Detrital zircons from a metasedimentary gneiss in the Cumpăna metamorphic unit have ages ranging from ~ 0.5 Ga to 2.8 Ga. The 0.5 Ga age constrains the maximum sediment deposition age to be late Cambrian. The source most compatible with the range of Precambrian detrital ages in the Sebeş–Lotru terrane is northeastern Gondwana. The Sebeş–Lotru terrane was part of a continental subduction/collision system as a lower plate after about 400 Ma and reached peak metamorphic conditions between 350 and 320 Ma (e.g. Medaris et al., 2003). A cross-cutting granite vein has a zircon U–Pb crystallization age of 321.5 ± 3.1 Ma, which constrains minimum age of ductile deformation during the Variscan collision in this region. The trace of the Rheic suture within the South Carpathians is located between the Ordovician upper part of the Sebeş–Lotru terrane and the Drăgşan pre-Alpine terrane of the Danubian domain.

In situ zircon U–Pb and Lu–Hf isotope systematic on magmatic rocks: Insights on the crustal evolution of the Neoproterozoic Goiás Magmatic Arc, Brasília belt, Central Brazil

The Mara Rosa arc constitutes the northern portion of the Goiás Magmatic Arc, an important tectonic component of the Brasília Belt that represents the result of the convergence of the Amazonian and San Francisco cratons between 900 and 600 Ma. New in situ zircon U–Pb and Lu–Hf isotope data by LA-MC-ICP-MS on representative magmatic rocks permitted to characterize three Neoproterozoic magmatic events in the Mara Rosa arc. Zircons from a metavolcanic rock yielded a U–Pb crystallization age of 916 ± 5 Ma with positive ε Hf values (+ 8 to + 12) suggesting that the host magma derived from a depleted mantle. Two orthogneisses showed heterogeneous zircon populations: the youngest zircons from the two samples gave U–Pb crystallization ages of 792 ± 8 Ma and 811 ± 7 Ma respectively; the abundant inherited zircons (with Meso- to Paleoproterozoic U–Pb ages) showed, for both samples, negative ε Hf and old T DM values, suggesting an important contribution of Paleoproterozoic to Archean crust in the petrogenesis of the host magmas. Zircons from post-orogenic qz-diorite yielded a crystallization age of 638 ± 4 Ma, with a variation in ε Hf values (+ 1 to + 8) suggesting that the host magma involved both a juvenile and a recycled older crustal components.

Zircon Hf Isotopic Evidence for Mixing of Crustal and Silicic Mantle-derived Magmas in a Zoned Granite Pluton, Eastern Australia

Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite, New England batholith, eastern Australia, are consistent with magma mixing between two silicic melts, each derived from isotopically distinct sources. Although zircons from three zones within the Walcha Road pluton give a U^Pb crystallization age of 249 � 3 Ma, zircon populations from each zone have a range in Hf. Zircons from the mafic hornblende^biotite monzogranite pluton margin and intermediate zones have eHf �þ 5t oþ11, whereas those from the more felsic centre of the pluton have eHf �þ 7t o þ16, representing a total variation of 11 eHf units. The Lu^Hf depleted mantle model ages range from � 650 to 250 Ma, with the younger zircons present only in the felsic pluton centre.The variation in eHf indicates the involvement of silicic melts from at least two sources, one a crustal component with a Neoproterozoic model age and the other a primitive mantle-derived component with model ages similar to the U^Pb crystallization age of the pluton. The zircons reflect the isotopic compositions of the different proportions of crustal-derived silicic melt, relative to mantle-derived silicic melt, between melt generation and final pluton construction. The Walcha Road pluton is considered to have formed by incremental assembly of progressively more felsic melt batches resulting from mixing, replenishment and crystal^melt separation, with final pluton construction involving mechanical concentration as zones of crystal mush. The zoned pluton and, more broadly, the Moonbi supersuite provide examples of magma mixing by which the more silicic units have more juvenile isotopic compositions as a result of increasing proportions of residual melt from basalt fractionation, relative to crustal partial melt.

New insights into the southern margin of the Archean–Proterozoic boundary in the north-central United States based on U–Pb, Sm–Nd, and Ar–Ar geochronology

New geophysical analysis of the Precambrian basement in Minnesota–Iowa–Wisconsin indicates that an Archean–Proterozoic boundary (Spirit Lake trend) previously recognized in NW Iowa can be continued eastward into central Wisconsin and farther east as the Spirit Lake tectonic zone (SLtz). To test the age of Paleoproterozoic crust south of this structure, several subsurface samples of Precambrian basement from the north-central United States have been analyzed or re-examined using modern techniques of U–Pb, Sm–Nd, and 40Ar/ 39Ar geochronology. The results fill in a major data gap for the region and show that all U–Pb crystallization ages for samples south of the SLtz are geon 17 (1700–1800 Ma). Bedrock core samples from eastern Nebraska are ca. 1760–1800 Ma, and two samples from SE South Dakota, immediately south of the SLtz, yield ages of 1762 ± 28 Ma (Vermillion) and 1733 ± 2 Ma (Elk Point). Xenoliths from impact breccia in the Manson structure in north-central Iowa yield a similar age of ca. 1705 ± 30 Ma and metagabbro from SE Minnesota yields an age of 1760 ± 9 Ma. Farther to the northeast, zircons from Paleoproterozoic gneiss in the basement of Manitoulin Island, only a few km south of the Superior craton in Ontario, also yield a geon 17 age (1714 ± 10 Ma). Sm–Nd model ages ( T DM) for samples immediately south of the SLtz fall in the range 1.9–2.2 Ga, indicating limited involvement of Archean crust. In contrast, Sm–Nd T DM ages for samples north of the SLtz typically range from 2.5 to 3.0 Ga, for both Paleoproterozoic plutons and Archean gneisses. Ion microprobe analyses of zircons from the Elk Point and Manson samples also show the presence of geon 16 overgrowths, indicating a strong regional thermal overprint during geon 16 accretion. This is supported by mid-geon 16 hornblende 40Ar/ 39Ar ages for samples from SE South Dakota and SE Minnesota. Although no U–Pb ages are available for juvenile basement beneath the ca. 1760 Ma granite–rhyolite suite of southern Wisconsin, south of the SLtz, Sm–Nd model ages are typically ca. 1.9–2.0 Ga, consistent with basement to the rhyolites being geon 17 in age. Collectively, the data require that most, if not all, of the Paleoproterozoic crust immediately south of the SLtz formed during geon 17 and probably represents eastward continuation, from Colorado, through Nebraska, of the Yavapai crustal province in the SW United States. Penokean (geon 18) crustal rocks are limited mainly to northern and central Wisconsin, east-central Minnesota, and northern Michigan. These results also show that medium grade (>500 °C) tectonothermal effects of the subsequent geon 16 (≈Mazatzal) orogeny to the south continue into the north-central United States. Both terranes probably also continue eastward into Ontario, Canada and farther east into protolith of the Grenville Province.

U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada

U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analysis of detrital zircon in 11 sandstone samples from the late Paleo-proterozoic Dubawnt Supergroup of the Baker Lake and Thelon Basins are presented here. Alluvial fan and braided stream facies of the ca. 1.84–1.78 Ga Baker Sequence yield age spectra with significant groupings at 2.70 and 2.60 Ga, reflecting provenance chiefly from local sources in the northwestern Hearne domain of the western Churchill Province. Age spectra are consistent with sed-imentological and stratigraphic information, which indicate provenance from the proximal uplifted shoulders of an intracontinental rift that developed in response to inboard effects of the Hudsonian orogeny (collision of Superior craton into western Churchill Province). The unconformably overlying 1.76–1.74 Ga Whart Sequence exhibits a more varied provenance and has an abundance of 2.0–1.9 Ga detrital zircon. These ages match those of synorogenic plutons in the Taltson-Thelon orogen (collision of Slave craton into the western Churchill Province). Provenance from the Taltson-Thelon orogen is supported by easterly paleocurrents and Rb-Sr cooling ages from mafic dikes that indicate reactivated collision of the Slave craton and related uplift and denudation coeval with deposition of the Whart Sequence. The depositional age of the Whart Sequence is constrained by intercalated rhyolite flows with conventional 207 Pb/ 206 Pb crystallization ages of 1757.6 ± 3.4 Ma and 1753.0 ± 1.7 Ma (2σ). The thermal effects of this collision also are recorded in isotopic data from basement rocks throughout the western Churchill Province and in syndepositional to postdepositional faults that parallel the main boundary faults of the Slave indentor. Late reactivation and associated metamorphism, faulting, and basin development could be the result of the inboard effects of subduction and related collision along the southern margin of Laurentia. The detrital zircon geochronology of the Barrens Sequence supports recycling of local supracrustal sequences and weathering of underlying granitic gneisses. Zircons of Hudsonian affinity (ca. 1.9–1.8 Ga) are nearly absent, which argues against a distant eastern source region for the Barrens Sequence. The Dubawnt Supergroup detrital zircon age data set shows concentrations ca. 2.75–2.6 Ga, 2.1–1.9 Ga, and 1.85–1.75 Ga, which are concordant with the known ages of major juvenile crust–forming events leading up to final amalgamation of Late Archean and Paleoproterozoic supercontinents. Detritus of this age was shed cratonward and captured by interior basins, such as the Baker Lake and Thelon Basins, during major collisional events that reworked the margins of the western Churchill Province (e.g., Hudsonian and Thelon-Taltson orogenies).

Isotopic connections between basement rocks exposed in the St. Francois Mountains and the Arbuckle Mountains, southern mid-continent, North America

Two distinct crustal provinces have been identified in the southern mid-continent based on U–Pb crystallization ages. Both contain large volumes of undeformed granite and rhyolite, with minor amounts of metamorphic rock and mafic intrusions. The Eastern Granite-Rhyolite province is characterized by felsic rocks with crystallization ages of 1,470 ± 30 Ma and exposures are restricted to the St. Francois Mountains in southeastern Missouri. Similarly, the Southern Granite-Rhyolite (SGR) province is characterized by felsic units with ages of 1,370 ± 30 Ma with primary exposures in the eastern Arbuckle Mountains of southern Oklahoma. Within the SGR province three magmatic pulses can be identified starting at 1,400, 1,370, and 1,340 Ma. Although the crystallization ages are different, the Sm–Nd isotopic signatures are similar for the units exposed in these areas as well as the buried basement in between. Depleted mantle model ages for rocks within the Arbuckle Mountains range from 1,530 to 1,430 Ma with ɛNd(t) values of +3.2 to +4.1 while units of the St. Francois Mountains range from 1,550 to 1,430 Ma and +4.5 to +4.7. Comparison of Sm–Nd isotopic data also indicate similarities between the 147Sm/144Nd and 143Nd/144Nd ratios for the rock units in these areas suggesting a common source.

Age and geochemical constraints for partial melting of granulites in Estonia

The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.

Two neoproterozoic crustal accretion events in the Brasília belt, central Brazil

New U–Pb and Sm–Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiás magmatic arc in western Goiás constrain the geological evolution of this juvenile crust in the western Brasília belt. Orthogneiss rock samples have U–Pb crystallization ages of 804±6, 669±3, 662±12, 634±8, 630±5, and 637±20 Ma and show ε Nd( T) values between +2.8 and −15.1. Rock units with negative ε Nd( T) are more frequent in the eastern part of the studied area to the south of Anicuns, which indicates the presence of older continental crust in that part of the arc. Metagranitoids have ages of 821±10, 810±10, 792±5, 790±12, 782±14, 748±4, and 614±5 Ma and ε Nd( T) values between +5.1 and −3.7. The data presented here, combined with those in the literature, suggest that igneous activity in the Goiás magmatic arc took place in two episodes: between ca. 0.89 and 0.8 Ga, probably in intraoceanic settings, and between ca. 0.66 and 0.60 Ga, likely in an active continental margin at the end of the Brasiliano orogeny.

Age, Origin and Cooling History of the Coronel Joao Sa Pluton, Bahia, Brazil

RECEIVED APRIL 2, 2002; ACCEPTED AUGUST 8, 2004ADVANCE ACCESS PUBLICATION OCTOBER 22, 2004In north-east Brazil, Archean and Paleoproterozoic cratonic blocksare enclosed within a network of Brasiliano-age (0 7–0 55Ga)metasedimentary foldbelts. The unfoliated Coronel Joa˜o Sa´ grano-diorite pluton, which contains magmatic epidote and stronglyresorbed clinopyroxene, intrudes the Sergipano Foldbelt. Zirconsyield a concordant U–Pb crystallization age of 625 2Ma;titanite ages are approximately 621Ma. Discordant zircons suggestinheritance from at least two magma sources of ages 2 2Ga. Model calculations based on diffusion parameters andRb–Sr isotope data from separated minerals indicate that the plutoncooled ata rate of 36 C/Myr. Whole-rock element compositionsandinitialSr–Ndisotopic compositionsthat areheterogeneous onalllength scales suggest magma mixing. Trace-element concentrationsand Nd isotope data argue against a contribution from a contem-poraneous mantle-derived magma. Values of magmatic e

Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: evidence for tilting during uplift

The Quottoon plutonic complex (part of the great tonalite sill) was emplaced into eastern parts of the Coast shear zone along the west flank of the Coast Mountains. U–Pb crystallization ages range from 72.3 to 55.5 Ma. A regional compilation of K–Ar hornblende dates from the tonalite sill indicates a west to east decrease from ~60 Ma to ~54 Ma across the intrusive suite. Paleomagnetic samples were collected at 160 sites ([Formula: see text] 8 samples per site) along six transects across the Quottoon complex between the Skeena River and Willard Inlet. Directions of characteristic remanent magnetization (ChRM) were successfully determined by principal component analysis of thermal demagnetization data for about half of the samples collected. Site-mean ChRM directions were determined for 56 sites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Island transects, using criteria of [Formula: see text] 4 sample ChRM directions per site and 95% confidence limit (α95) [Formula: see text] 25°. For Filmore Island and Willard Inlet, few sites provided adequate determinations of site-mean ChRM directions, but many sample ChRM directions were well determined and consistent. The mean direction from the Skeena River – Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rotated declinations and inclinations that are shallower than the expected direction by up to 23°. Consistent with expectations for east-side-up tilting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites from the eastern part of the transect. The magnitude of east-side-up tilt varies along strike: the Skeena River to Quottoon Inlet segment experienced little or no tilt; Steamer Passage south of Portland Inlet records 15° tilt; north of Portland Inlet, tilts are 40° for Wales Island and 35° for Filmore Island; farther north, tilt decreases to 12° for Willard Inlet. Tilting was driven by Eocene extension of the Coast Mountains, with tilted crustal panels bounded by northwest-striking, east-side-down normal faults and northeast-striking transfer faults or shear zones. The ACCRETE seismic transect imaged east-dipping reflectors down to ~15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must underlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40° tilt of crustal blocks at Wales and Filmore islands.