Sensitive High-resolution Ion Microprobe Ages Research Articles

Examination of the accuracy of SHRIMP U–Pb geochronology based on samples dated by both SHRIMP and CA-TIMS

Abstract. Estimations of the reproducibility of U–Pb ages from SHRIMP (Sensitive High-Resolution Ion MicroProbe) instruments are based on data from studies that are nearly 2 decades old. Since that time, refinement of analytical procedures and operational improvements have reduced the historically identified uncertainties of SHRIMP U–Pb analysis. This paper investigates 36 SHRIMP thermal ionisation mass spectrometry (TIMS) double-dated “real-world” geologic samples from a variety of igneous rock types to better understand both geological and analytical sources of disagreement between the two dating methods. Geoscience Australia's (GA) use of high-precision chemical abrasion thermal ionisation mass spectrometry (CA-TIMS) for chronostratigraphy in Australian sedimentary basins has produced a substantial selection of precisely dated zircons, which we can use to cross-correlate the SHRIMP and CA-TIMS ages throughout the Phanerozoic. A total of 33 of the 36 ages were reported with external SHRIMP uncertainties less than 1 % (95 % confidence). Six of eight cases where the CA-TIMS age was outside the SHRIMP uncertainty envelope were in samples where the 95 % confidence interval of the reported SHRIMP age was below 0.66 % uncertainty, suggesting that SHRIMP analyses of untreated zircon with smaller uncertainties are probably overoptimistic. The mean age offset between SHRIMP and TIMS ages is 0.095 %, but the distribution appears bimodal. Geological explanations for age discrepancies between SHRIMP and CA-TIMS are suggested by considering intrusive and extrusive age results separately. All but one sample where the SHRIMP age is more than 0.25 % older are volcanic. This offset could be explained by the better single-grain age resolution of TIMS, allowing identification and exclusion of antecrysts from the eruptive population, while SHRIMP does not have a sufficient single-grain precision to deconvolve these populations – leading to an apparent older SHRIMP age. In contrast, SHRIMP ages from plutonic rocks – particularly plutonic rocks from the early Paleozoic – are typically younger than the CA-TIMS ages from the same samples, most likely reflecting Pb loss from non-chemically abraded SHRIMP zircons, while chemical abrasion of zircons prior to TIMS analysis destroyed or corrected these areas of Pb loss.

Geochronological systematics of the Huayna Potosí, Zongo and Taquesi plutons, Cordillera Real of Bolivia, by the K/Ar, Rb/Sr and U/Pb methods

The Huayna Potosi, Zongo and Taquesi are Triassic plutons located at the core of the Real Cordillera of Bolivia. In this paper, several Rb-Sr and K-Ar ages obtained in the past at the Sao Paulo Geochronology Laboratory, yet unpublished, will be presented, along with newer U-Pb Sensitive High-Resolution Ion Microprobe (SHRIMP) determinations made in the same laboratory, allowing us to redefine the geologic history of this part of the Central Andes. Rb/Sr analyses of some low grade metapelitic country rocks of the early Paleozoic (Amutara and Cancaniri Formations) yielded a Rb-Sr isochron age of 344 ± 38 Ma, indicating the action of an early Gondwanide regional event. A five-point Rb-Sr isochron from a granite outcrop of the Huayna Potosi pluton yielded an age of 224 ± 28 Ma. In addition, an important Ar loss in micas was detected in the Zongo granitoids and their country rocks, recording a thermal event that opened this isotopic system in the Oligocene. Newer U-Pb SHRIMP zircon ages of ca. 221 Ma were obtained in two other granitic outcrops of the Huayna Potosi granite. They confirmed its Triassic crystallization age, and a similar U-Pb SHRIMP age of 221.9 ± 1.5 Ma was obtained for one sample of the Taquesi pluton. For the Zongo pluton, many of the zircon grains obtained from one sample of its Kuticucho facies yielded extremely high uranium content, which produced reverse discordant apparent ages. However, due to the fair alignment of the analytical points in the Concordia diagram, possibly corresponding to a linear correlation, we made a regression calculation and the interception of the Concordia curve resulted in a rather imprecise age of 220 ± 20 Ma. Our conclusion was that the final magmatic crystallization and the intrusion of plutons in the central part of the Cordillera Real of Bolivia have occurred close to 221.5 ± 2.0 Ma, in late Triassic times. Finally, the U-Pb SHRIMP ages obtained in inherited zircon xenocrysts from the four available granitic rocks yielded very different ages, and many of them are related to previous magmatic episodes of the Andean Tectonic System. A few other age measurements indicated sources related to much older Proterozoic magmatic events associated with rocks from the Andean basement.

Nature of Late Mesoproterozoic to Early Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance

The western margin of the Gyeonggi massif, southern Korean Peninsula, has preserved N–S trending Neoproterozoic and sporadic Late Mesoproterozoic metaigneous rocks. Here we present the results from systematic field mapping, sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating, and whole-rock geochemical analyses of the Mesoproterozoic and Early Neoproterozoic metaplutonic rocks in the Hongseong area, together with previously published data from the western Gyeonggi massif. The SHRIMP ages of these rocks are categorized into three groups: (1) Late Mesoproterozoic (ca. 1.25–1.15Ga), (2) Early Neoproterozoic (ca. 900–770Ma), and (3) late Early Neoproterozoic (ca. 762–730Ma). The geochronological and geochemical features of the Late Mesoproterozoic rocks suggest that they were possibly formed in association with convergent plate motion. The Early and late Early Neoproterozoic rocks are interpreted to arc-related orogenic and rift-related post-orogenic environments, respectively. These age results and the tectonic signatures provide insight into the convergence process along the margins of the Rodinia supercontinent.

U-Pb zircon dating using Nd-YAG (213 nm) Laser ablation-ICP-MS, and evaluating the consistency with SHRIMP dating

Laser ablation–inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a powerful tool for rapid and precise U-Pb zircon age dating. Here, we report U-Pb ages for detrital zircons determined using 213 nm Nd-YAG LA-ICP-MS analysis, and compare these results with those determined using a sensitive high-resolution ion microprobe (SHRIMP) to evaluate approaches for detrital zircon age dating during provenance analysis. Consistency of the system was assessed by measuring four well-characterized standard zircon samples (Plešovice, SL13, AS3, QGNG), with weighted mean measured 206Pb/238U and 207Pb/206Pb ages of the samples only slightly offset from the reference ages and with acceptable MSWD values, barring the 207Pb/206Pb age of the Plešovice zircon, which had a low 207Pb signal intensity. Detrital zircon grains were re-measured by LA-ICP-MS to assess consistency between the LA and SHRIMP systems, with 207Pb/206Pb ages for zircons older than 1000 Ma in excellent agreement with SHRIMP ages; typical offsets were between −3.9% and 0.3%. In addition, zircons younger than 1000 Ma had more accurate 206Pb/238U than 207Pb/206Pb ages. Both LA-ICP-MS and SHRIMP techniques produce results that are in excellent agreement, barring a few analyses that were affected by zircon heterogeneity or zoning.

Constraints on the Timing of Co-Cu Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks

The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206 Pb/ 238 U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.

Neoproterozoic plutonic rocks from the western Gyeonggi massif, South Korea: Implications for the amalgamation and break-up of the Rodinia supercontinent

We report sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages, and whole-rock chemical and isotopic (Sr–Nd) compositions from tonalite–trondhjemite–granodiorite (TTG) and alkaline plutons in and around the “Hongseong-Imjingang suture” along the western margin of the Gyeonggi massif, South Korea. The Neoproterozoic TTG and the alkali plutons from the Hongseong area of the Gyeonggi massif yield zircon U–Pb ages of ca. 841–822 Ma and ca. 751–746 Ma, respectively. Based on the geochemical and Nd–Sr isotope data on these plutons, together with the previously reported SHRIMP age of ca. 900 Ma from a tonalite, TTG plutons of ca. 900–820 Ma are interpreted as the products of arc-related magmatism. Their geochemical signatures include enrichment in LREE, negative Nb, Ta and Ti anomalies and relatively low ɛNd(T) values between −0.5 and +5.0. The TTG plutons are coeval with those recently identified in and around the Yangtze block in China. In contrast, the alkali plutons from the Hongseong area, together with the previously reported ones around the Imjingang belt, represent anorogenic-type magmas that can be attributed to the disruption of Rodinia. They display significant negative Eu anomalies, and are strongly depleted in Sr, P and Ti. This transition from arc setting at ca. 900–820 Ma to continental rift setting at ca. 750–740 Ma suggests a regional tectonic link for the amalgamation and disruption of Rodinia in Northeast Asia.

A polyphase metamorphic evolution for the Xitieshan paragneiss of the north Qaidam UHP metamorphic belt, western China: In-situ EMP monazite- and U–Pb zircon SHRIMP dating

In-situ electron microprobe (EMP) U–Th–Pb monazite-, sensitive high-resolution ion microprobe (SHRIMP) zircon analyses, metamorphic phase equilibrium (Domino/Theriak)- and geothermobarometric calculations are performed on kyanite/sillimanite-bearing garnet biotite gneisses forming part of the dominant rock association in the Xitieshan ultra-high pressure metamorphic belt, north Qaidam, western China. Results are consistent with the following complex polyphase tectono-metamorphic evolution.The kyanite/sillimanite bearing garnet biotite gneisses contain monazite ages of 938±23Ma and zircon SHRIMP ages of 945±7Ma, referring to a Neoproterozoic metamorphism, i.e. similar to the age of the Jinning orogeny in the Yangtze block of southern China. This correlation suggests that the paragneiss has affinities with the Yangtze block (South China block). The Neoproterozoic monazites were found inside coarse grained porphyroblastic garnets containing amphibolite facies mineral inclusion assemblages.The kyanite/sillimanite-bearing garnet biotite gneisses also contain early Paleozoic monazite ages of 422–425Ma and 455–460Ma, detected in amphibolite facies mineral assemblages associated with matrix minerals. Using phase equilibrium- and geothermobarometric calculations, PT conditions of 560–610°C/5.8–7.0kbar and 610–675°C/4.6–6.5kbar were calculated respectively for both amphibolite facies assemblages. The early Paleozoic ages of 422–425Ma and 455–460Ma were detected in 8 monazite grains from the investigated paragneiss samples. Based on the Y and Eu contents variation of the early Paleozoic monazite domains (measured by EMP), the 422–425Ma monazite ages are interpreted to have formed during an amphibolite facies tectono-metamorphic overprint that post-dates (U)HP metamorphism and can thus be related to exhumation of previously deeply subducted rocks. Alternatively, the 455–460Ma monazite ages are interpreted to represent the age of the prograde subduction zone metamorphism.We conclude therefore that the mineral assemblage of the kyanite/sillimanite-bearing garnet–biotite gneiss (and associated retro-eclogite) was formed during an early Paleozoic subduction/collision event, which involved late Proterozoic (938–945Ma) crystalline basement inliers of minimal mid amphibolite facies grade. Early Paleozoic deep subduction towards (U)HP depths occurred around 455–460Ma followed by retrograde amphibolite facies metamorphism at 422–425Ma during exhumation.

Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico

Stratigraphic relationships, detrital zircon provenance, U-Pb and 40 Ar/ 39 Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early–Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian–Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 ± 3.0 Ma), Early to Middle Jurassic 40 Ar/ 39 Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196–161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian–Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.

The Eocene bimodal Piranshahr massif of the Sanandaj–Sirjan Zone, NW Iran: a marker of the end of the collision in the Zagros orogen

Abstract: The bimodal Piranshahr massif is composed of coeval but geochemically unrelated mafic (40.7 ± 0.2 Ma zircon U–Pb sensitive high-resolution ion microprobe (SHRIMP) age) and A-type felsic rocks (41 ± 0.5 Ma Rb–Sr and 41.3 ± 0.8 Ma zircon U–Pb SHRIMP age). The mafic rocks consist of two geochemical types of gabbros that derived from different magmas. The more abundant gabbros are moderately alkaline, have ratios of large ion lithophile elements to REE and high field strength elements to REE similar to those of intraplate mantle magmas, 87 Sr/ 86 Sr 41 Ma ≈ 0.7036 and ε(Nd) 41 Ma ≈ +7.2. The less abundant gabbros have calc-alkaline affinities, 87 Sr/ 86 Sr 41 Ma ≈ 0.7043 and ε(Nd) 41 Ma ≈ +4.78. Felsic rocks are metaluminous A2-type annite–fayalite–hedenbergite, hypersolvus, leucocratic, coarse-grained agpaitic syenites, pulaskites and granites, with 87 Sr/ 86 Sr 41 Ma ≈ 0.7048 and ε(Nd) 41 Ma ≈ +3.6–4.5. Syenites, pulaskites and granites are genetically related. Pulaskites probably represent alkali-enriched water-rich residual melts from which an F-rich vapour phase was later separated. Granites were probably generated during open-system processes, in which F-rich hydrous alkaline fluids released from the syenites acted upon pre-existing felsic rocks. The c . 41 Ma age of the post-collisional Piranshahr massif indicates that the related collision probably occurred at 50–60 Ma (i.e. Late Palaeocene or Early Eocene), thus resolving a much debated question.

The age of the Nantuo Formation and Nantuo glaciation in South China

AbstractA U–Pb sensitive high‐resolution ion microprobe (SHRIMP) age of 654.5 ± 3.8 Ma from an ash bed immediately below the Nantuo Formation in South China provides the lowest age constraint for the Nantuo glaciation, which has been correlated with the global ‘Marinoan’ glaciation. A U–Pb SHRIMP age of 636.3 ± 4.9 Ma from a fallout tuff within the basal layer of the Nantuo Formation, along with the existing age of 635.2 ± 0.6 Ma from its overlying Doushantuo cap carbonate, suggests that the traditionally defined Nantuo Formation may have been deposited in a short time period at the end of the Nantuo glaciation. In combination with available ages globally, the data support a relatively short duration and rapid termination of ‘Marinoan’ glaciations.

Late Cretaceous truncation of the western Idaho shear zone in the central North American Cordillera

The Mesozoic boundary separating accreted oceanic rocks and continental North America in west-central Idaho forms a prominent right-angle bend, the origin and kinematic significance of which have had a great influence on models for the evolution of the North American Cordilleran orogen. Based on structural and geochronologic relations, we conclude that the bend in the steep crustal boundary is formed by the intersection of two shear zones with different ages. Structural fabrics of the north-striking western Idaho shear zone are overprinted by penetrative structures of the west-northwest–striking Orofino shear zone. U-Pb sensitive high-resolution ion microprobe ages on metaplutonic rocks and late synkinematic leucocratic dikes constrain the age of deformation in the Orofino shear zone to ca. 90–70 Ma, distinctly younger than ca. 120–90 Ma magmatism and deformation within the western Idaho shear zone. The Orofino shear zone is interpreted as a southwest-vergent contractional relay structure that linked basement-cored foreland thrusts of the Middle and Southern Rocky Mountains with the deep crustal duplex of the Shuswap complex in the Canadian Rockies. Truncation ca. 90 Ma terminated Late Cretaceous margin-parallel translation of terranes along the western Idaho shear zone and required displacement to step outboard to poorly understood structures to the west.

Provenance of Cambrian conglomerates from New Zealand: implications for the tectonomagmatic evolution of the SE Gondwana margin

The oldest rocks in New Zealand are the Mid- to Late Cambrian intra-oceanic island arc rocks of the Takaka terrane (Devil River arc). The provenance of Cambrian conglomerates stratigraphically above the exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental crust. The Dead Goat Conglomerate contains two distinct groups of igneous clasts: (1) intermediate to felsic volcanic clasts with moderately enriched light rare earth element (LREE) and high field strength element (HFSE) contents and positive ϵNd 500 (+2.1) that were derived from a medium-K calc-alkaline source, probably the main sequence of the Devil River arc; (2) dioritic to metagranitic plutonic clasts strongly enriched in LREE and HFSE and with ϵNd 500 of +3.5 to +5.9 that were derived from a high-K arc source, probably the uppermost units of the Devil River arc. This is consistent with a new U–Pb sensitive high-resolution ion microprobe age of 496 ± 6 Ma. The Lockett Conglomerate also contains two distinct groups of igneous clasts: (1) ultramafic to intermediate igneous clasts identified as boninitic to transitional low-K calc-alkaline arc-related rocks based on depleted REE and HFSE abundances; (2) ‘I’-type metagranitoid clasts derived from a distinct Andean type continental margin, as indicated by ϵNd 500 as low as −7.1. Both conglomerates contain sandstone clasts derived from a common old, multi-cycle continental source with ϵNd 500 of −14.2 to −15.7, and no suitable source has been found in present-day New Zealand. The new provenance data from these conglomerates constrain the time of accretion of the Devil River arc to the palaeo-Pacific Gondwana margin and provide new information on the structural evolution of the accretionary event.

Discussion on Mesozoic extensional structures of the Fangshan tectonic dome and their subsequent reworking during collisional accretion of the North China Block Journal , Vol. 163 , 2006, 127–142

Yu Wang writes: On the basis of descriptions of the stratigraphic sequences and structural features in the field, and interpretations of published data, Yan et al . (2006 a ) reach the surprising conclusion that early Mesozoic (early–middle Triassic) ESE-oriented extension resulted in formation of the Fangshan metamorphic core complex. If correct, the authors have made an important contribution to understanding of the tectonic evolution of North China. However, the way in which they cite previously published work to support their model, and discrepancies between their findings in the field and those of other workers raise a number of problems that will be discussed below. ### Citing of published work. Yan et al . (2006 a ) cited many references (e.g. Lei et al . 1994; Niu et al . 1994; Yu & Zhang 1996; Chen 1999; Fu 1999; Meng et al . 2003) in support of their conclusions about early Triassic extension and formation of metamorphic core complexes, as well as Late Cretaceous normal faulting. In many cases, however, the cited references do not lend support to their interpretations. For example, the Du Shan granite and surrounding Archaean and Proterozoic metamorphic rocks (Yu & Zhang 1996) contain no evidence for extension or development of metamorphic core complex features, but instead record NE-trending, strike-slip ductile shearing, not top-to-the-SE extension. The Malanyu dome is an old metamorphic sequence (Chen 1999), and no one has previously interpreted it as an extensional dome associated with SE-directed extensional shear. Zhang et al . (1991) did not examine any aspect of the geology in the area of the so-called Yuerya metamorphic core complex. Furthermore, no dome has been identified here by any previous geologists; instead, it is a c . 175–174 Ma (U–Pb sensitive high-resolution ion microprobe ages, Luo et al . 2001) granitic sheet-like …

Does the Great Valley Group contain Jurassic strata? Reevaluation of the age and early evolution of a classic forearc basin

The presence of Cretaceous detrital zircon in Upper Jurassic strata of the Great Valley Group may require revision of the lower Great Valley Group chronostratigraphy, with significant implications for the Late Jurassic-Cretaceous evolution of the continental mar- gin. Samples (n 5 7) collected from 100 km along strike in the purported Tithonian strata of the Great Valley Group contain 20 Cretaceous detrital zircon grains, based on sensitive high-resolution ion microprobe age determinations. These results suggest that Great Val- ley Group deposition was largely Cretaceous, creating a discrepancy between biostratig- raphy based on Buchia zones and chronostratigraphy based on radiometric age dates. These results extend the duration of the Great Valley Group basal unconformity, provid- ing temporal separation between Great Valley forearc deposition and creation of the Coast Range Ophiolite. If Great Valley forearc deposition began in Cretaceous time, then sedi- ment bypassed the developing forearc in the Late Jurassic, or the Franciscan subduction system did not fully develop until Cretaceous time. In addition to these constraints on the timing of deposition, pre-Mesozoic detrital zircon age signatures indicate that the Great Valley Group was linked to North America from its inception.

Geology, Geochemistry and U-Pb SHRIMP Age of the Tacloban Ophiolite Complex, Leyte Island (Central Philippines): Implications for the Existence and Extent of the Proto-Philippine Sea Plate

Abstract. The oceanic basement of the Central Philippines is exposed in ophiolitic massifs the age and origin of which remain debated. The Tacloban Ophiolite Complex (TOC) outcrops as a NW-SE trending massif in the northeastern portion of Leyte Island, Central Philippines. It is unconformably overlain by sedimentary sequences dated to Late Miocene-Pliocene and Pleistocene volcaniclastic deposits on its eastern and western flanks, respectively. Field, petrographic and trace element data suggest a subduction-related origin for this ophiolite. Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb dating of zircons from a gabbro yielded Early Cretaceous magmatic age for the TOC, which is very much older than a previously reported whole rock K-Ar derived Eocene age. The Early Cretaceous age of the TOC limits its possible progenitor to the proto-Philippine Sea Plate. Correlation with other Cretaceous ophiolites in Central Philippines reveals the possible extent of the proto-Philippine Sea Plate remnants now exposed onland.

Sm–Nd and U–Pb dating of high‐pressure granulites from the Złote and Rychleby Mts (Bohemian Massif, Poland and Czech Republic)

AbstractIn the Orlica‐Śnieżnik complex at the NE margin of the Bohemian Massif, high‐pressure granulites occur as isolated lenses within partially migmatized orthogneisses. Sm–Nd (different grain‐size fractions of garnet, clinopyroxene and/or whole rock) and U–Pb [isotope dilution‐thermal ionization mass spectrometry (ID‐TIMS) single grain and sensitive high‐resolution ion microprobe (SHRIMP)] ages for granulites, collected in the surroundings of Červený Důl (Czech Republic) and at Stary Gierałtów (Poland), constrain the temporal evolution of these rocks during the Variscan orogeny. Most of the new ages cluster at c. 350–340 Ma and are consistent with results previously reported for similar occurrences throughout the Bohemian Massif. This interval is generally interpreted to constrain the time of high‐pressure metamorphism. A more complex evolution is recorded for a mafic granulite from Stary Gierałtów and concerns the unknown duration of metamorphism (single, short‐lived metamorphic cycle or different episodes that are significantly separated in time?). The central grain parts of zircon from this sample yielded a large spread in apparent 206Pb/238U SHRIMP ages (c. 462–322 Ma) with a distinct cluster at c. 365 Ma. This spread is interpreted to be indicative for variable Pb‐loss that affected magmatic protolith zircon during high‐grade metamorphism. The initiating mechanism and the time of Pb‐loss has yet to be resolved. A connection to high‐pressure metamorphism at c. 350–340 Ma is a reasonable explanation, but this relationship is far from straightforward. An alternative interpretation suggests that resetting is related to a high‐temperature event (not necessarily in the granulite facies and/or at high pressures) around 370–360 Ma, that has previously gone unnoticed. This study indicates that caution is warranted in interpreting U–Pb zircon data of HT rocks, because isotopic rejuvenation may lead to erroneous conclusions.

Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia

Research Article| October 01, 2005 Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia A.M. Fioretti; A.M. Fioretti 1Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, Corso Garibaldi 37, I-35137 Padova, Italy, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar L.P. Black; L.P. Black 2Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar J. Foden; J. Foden 3Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia Search for other works by this author on: GSW Google Scholar D. Visonà D. Visonà 4Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information A.M. Fioretti 1Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, Corso Garibaldi 37, I-35137 Padova, Italy, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia L.P. Black 2Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia, and Visiting Fellows at Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia J. Foden 3Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia D. Visonà 4Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy Publisher: Geological Society of America Received: 02 Mar 2005 Revision Received: 03 May 2005 Accepted: 03 May 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (10): 769–772. https://doi.org/10.1130/G21671.1 Article history Received: 02 Mar 2005 Revision Received: 03 May 2005 Accepted: 03 May 2005 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.M. Fioretti, L.P. Black, J. Foden, D. Visonà; Grenville-age magmatism at the South Tasman Rise (Australia): A new piercing point for the reconstruction of Rodinia. Geology 2005;; 33 (10): 769–772. doi: https://doi.org/10.1130/G21671.1 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 A U-Pb zircon sensitive high-resolution ion microprobe age of 1119.4 ± 8.5 Ma, obtained from a quartz syenite dredged from the South Tasman Rise (Australia), provides the first direct evidence of the presence of Grenville-age magmatic rocks along the central part of the hypothesized Australia–East Antarctica conjugate margin of Laurentia. The distinctive mineralogy and geochemistry of the rock and its Sm-Nd and Pb isotopic signatures 1) indicate that it represents a juvenile Grenville-age addition to the crust, 2) support a correlation with the Grenville magmatic province of the western United States, and 3) set a unique pivotal point for a precise reconstruction of Rodinia. The resulting scenario implies the presence of a new magmatic province crossing the East Antarctic craton, the extension of Proterozoic belts of southwest Laurentia to East Antarctica, and appears consistent with the Australia–western United States (AUSWUS) model. This tectonic setting envisages a near-local source for the ubiquitous Grenville-age detrital zircon population in marginal Neoproterozoic to early Paleozoic Gondwanan sequences and suggests a possible direct source for the widespread Grenville-age inherited zircon component observed in most Paleozoic granites in northern Victoria Land (Antarctica) and east Tasmania (Australia). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations

Two distinctive volcanic ash beds were found in the terminal Proterozoic Doushantuo Formation in south China. The lower ash bed, ;2.5 m above the cap carbonate at the base of the Doushantuo, yields a U-Pb zircon age of 621 6 7 Ma, providing the closest upper limit for a correlative of the Marinoan glaciation. The upper ash bed, near the Doushantuo-Dengying boundary, yields a U-Pb zircon age of 555.2 6 6.1 Ma, providing for the first time a direct age determination for a prominent negative d 13 C excursion (#25‰) above the Marinoan glacial level. This excursion, if interpreted to be of glacial origin, is much younger than the Gaskiers Formation (ca. 580 Ma) in Newfoundland, and perhaps the fifth or sixth such level in the Neoproterozoic. That interpretation, however, is not supported by the proliferation of organisms within strata encompassing the negative d 13 C excursion in south China and globally, by the lack of a ca. 555 Ma glacial record, and by the absence of stratigraphic evidence for sea-level change. The data call for alternative paleoceanographic models for the origin of Neoproterozoic d 13 C excursions not clearly related to glaciation.

Paleoproterozoic Magmatic Provenance of Detrital Zircons, Porongos Complex Quartzites, Southern Brazilian Shield

Provenance investigation using detrital zircon U-Pb SHRIMP (sensitive high-resolution ion microprobe) age dating for six quartzite samples (167 spot determinations on 166 grains) indicates that metasedimentary rocks of the Porongos Complex from the southern Brazilian shield were derived almost entirely from Paleoproterozoic sources. Intensive study of igneous and metamorphic rocks in this portion of southwestern Gondwana employing SHRIMP geochronology over the past five years provides important evidence for provenance investigation of zircon grains. Ages of magmatic sources of the zircon detritus in the deformed quartz sandstones show an eight-peak distribution (known equivalent rocks in parenthesis): 2470 Ma (Neto Rodrigues Gneiss), 2350 Ma (Santa Maria Chico Complex Granodiorite), 2200 Ma (Encantadas Complex Tonalite), 2140 Ma (Paso Severino Dacite, Miséria Mylonite), 2100 Ma (Sierra Azul Granite), 2080 Ma (Chacofy Tonalite, Villa Monica Monzogranite), and 2040 and 2020 Ma (Itapema Monzogranite). Ages between 2260 and 2000 Ma correspond to the Trans-Amazonian Cycle, and were already known in the basement of the region, but the 2470 Ma and 2350 Ma ages are a new contribution regarding the tectonic evolution of the southern Brazilian shield. Tectonic scenarios for the Porongos basin fill are restricted to two different environments: (1) cratonic cover, if the fill is near 1500 Ma; or (2) passive margin if the age of fill is near 1000 Ma (this explanation requires additional studies). We have thus elucidated a major problem in the provenance of detrital zircon in the southern Brazilian shield, because we now know that the protolith sediments of the Porongos Complex were deposited under stable tectonic conditions and derived from Paleoproterozoic sources. This occurred after cratonization of the crust and formation of supercontinent Columbia following the end of the Trans-Amazonian Cycle. Nearly all zircon analyses have Th/U ratios higher than 0.2. Thus, only magmatic crystals survived the sedimentary processes. We show that detailed study of detrital zircon is a powerful tool for the understanding of the provenance of sandstones.

U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho

Three stratigraphically well defined rocks from the glaciogenic Scout Mountain Member, Neoproterozoic Pocatello Formation, southern Idaho, yielded sensitive, high-resolution ion-microprobe (SHRIMP) U-Pb zircon ages that constrain the age of the upper diamictite and its cap carbonate to between ca. 710 and 667 Ma. (1) Zircons from an epiclastic plagioclase-phyric tuff breccia immediately below glaciogenic Scout Mountain Member diamictite on Oxford Mountain, just north of the Utah border, yield a SHRIMP U-Pb concordia age of 709 ′ 5 Ma. (2) A porphyritic rhyolite clast from the upper Scout Mountain Member diamictite at Portneuf Narrows, south of Pocatello, yields a concordia age of 717 ′ 4 Ma. (3) The simple igneous zircon population from a reworked fallout tuff bed in the uppermost Scout Mountain Member, 20 m above the upper diamictite and its cap carbonate and immediately below a second cap-like carbonate, has a concordia age of 667 ′ 5 Ma. These data support previous interpretations that the Scout Mountain Member glaciation scoured nearby volcanic highlands composed of the bimodal Bannock Volcanic Member and suggest that the volcanism was 717 ′ 4 Ma. This age is close to, but distinctly older than, ca. 685 Ma U-Pb SHRIMP ages from the lithostratigraphically correlative Edwardsburg Formation in central Idaho. These data imply that the major rifting phase in this part of western Laurentia spanned 717-685 Ma rather than 800-750 Ma, as previously suggested. Further, because the Scout Mountain succession has been correlated with the Sturtian glacial phase on the basis of lithostratigraphy plus C and Sr isotope values in the carbonates, these data suggest that the Sturtian glacial epoch may have lasted until 670 Ma.