Showing papers on "Geochronology published in 1996"

Ultrahigh-pressure metamorphic rocks in the Dabieshan-Sulu region of China

Abstract: 1. A Review on Researches of UHP Rocks in the Dabieshan-Sulu Region Cong Bolin, Wang Qingchen. 2. Regional Geology Wang Qingchen, et al. 3. Structural Geology Wang Qingchen, et al. 4. Petrography of UHPM Rocks and Their Country Rock Gneiss Zhang Ruyuan, et al. 5. Major and Trace Element Geochemistry of Eclogites and Related Rocks Zhai Mingguo, Cong Bolin. 6. Isotopic Geochronology Li Shuguang. 7. Mineralogy of UHP Rocks Zhang Ruyuan, et al. 8. Metamorphic Evolution of UHPM Rocks Cong Bolin, et al. 9. Tectonic Evolution of UHP Rocks Wang Qingchen, et al. Appendix. Reference. Epilogue. Index.

The early Neoproterozoic sedimentary Succession B of northwestern Laurentia: Correlations and paleogeographic significance

Abstract: The Mesoproterozoic–Neoproterozoic stratigraphic record of ancestral North America (Laurentia) comprises three, unconformity-bounded sedimentary successions that are termed, from oldest to youngest, A, B, and C. Recent and ongoing detailed stratigraphic studies of Succession B, along with improved geochronology, allow extension and refinement of existing correlation schemes for northwestern Canada and Alaska. Succession B strata include the Shaler Supergroup of the Amundsen Basin, Mackenzie Mountains supergroup of the Mackenzie Mountains fold belt, Pinguicula group of the Wernecke Mountains inlier, Fifteenmile group of the Ogilvie Mountains inliers, and the lower Tindir Group of Tatonduk inlier. The Katakturuk Dolomite, in the northeast Brooks Range of Alaska, is included with Succession B on the basis of platformal character, geochronology, and inferred paleogeographic affinity. The framework for regional lithostratigraphic correlation of Succession B is built on recognition of four distinctive lithostratigraphic assemblages: two thick stromatolitic platformal carbonate assemblages separated by two largely subaerial siliciclastic assemblages. The correlation is supported by geochronology of detrital zircons from the upper quartzarenite assemblage, which indicates a maximum age of ca. 1000 Ma for the lower part of Succession B. These rocks are interpreted to be remnants of a northwesterly trending (present coordinates) early Neoproterozoic basin-margin promontory (Amundsen-Ogilvie-Mackenzie platform) that developed within an intracratonic basin on the northwest margin of Laurentia. The Neoproterozoic stratigraphic record of northwestern North America bears striking similarity to contemporaneous stratigraphy on other continents, particularly in the Amadeus Basin and Adelaide fold belt of central and southern Australia. Reconstructions of the Neoproterozoic supercontinent juxtapose the eastern margin of ancestral Australia against the western margin of Laurentia during the time these strata were being deposited. The Amundsen-Ogilvie-Mackenzie platform consequently may represent a segment (of the margin) of a large intracratonic basin that rifted apart with the breakup of the supercontinent during the latest Proterozoic. This hypothesis provides a template for future sequence stratigraphic, chemostratigraphic, biostratigraphic, paleomagnetic, and geochronologic comparisons and has implications for predictive economic geology in both areas. Capitalization of group and supergroup indicates formalization according to the International Stratigraphic Code. Noncapitalization indicates that the names have not been formalized.

U-Pb Monazite Geochronology of Granitic Rocks from Maine: Implications for Late Paleozoic Tectonics in the Northern Appalachians

Abstract: Crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern Maine constrain the timing of final ductile motion of a major shear zone to approximately 280 Ma. U-Pb monazite ages of granite at Brunswick ($$278 \pm 2$$ Ma) and evolved granitic pegmatites in Topsham (275-269 Ma) provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation. These granitic rocks were evidently generated during the final high temperature event experienced by this portion of the Appalachian orogen. During this time, the host rocks record initial rapid cooling (c. 16°C/m.y.) followed by slower cooling (c. 4°C/m.y.) into the Mesozoic. The similarity of ages of granite in Brunswick and pegmatites in Topsham permit the rocks to be related to the same magmatic system. The spatially associated Sebago batholith, the largest exposed pluton in New England, has a concordant U-Pb monazite age of $$293 \pm 2$$ Ma. Thus the Sebago batholith cannot be ...

Detrital zircon geochronology of the Alexander terrane, southeastern Alaska

Abstract: U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0–3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian–Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian–Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.

Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism

Abstract: We report U—Pb and 207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. Higher P—T conditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages between 526 ±34 and 557 ±2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.

The upper Paleocene-lower Eocene stratigraphic record and the Paleocene-Eocene boundary carbon isotope excursion: implications for geochronology

Abstract: Abstract The late Paleocene carbon isotope excursion which was first identified at ODP Site 690, is currently regarded as the best means for an exact correlation at a specific point in space and time between deep sea and continental stratigraphies. Yet its position relative to biostratigraphic datums in several outcrop and deep sea sections is apparently inconsistent. Based on the integration of magnetostratigraphic, biostratigraphic (calcareous nannofossils and planktonic foraminifera) and carbon isotopic data from the North (DSDP Sites 549 and 550) and South (ODP Site 690) Atlantic Ocean, we show that in order to understand the temporal relationships and sequence of late Paleocene to early Eocene biotic and isotopic events, it is necessary to construct a composite reference section. We infer from this that in addition to the well documented ‘classic’ carbon isotope excursion which occurs in mid-Biozone NP9, a younger isotopic excursion, yet undocumented, occurs in lower Biozone NP10. The 55 Ma age estimate in Chron C24r (0.66) used as a calibration in the construction of the latest geomagnetic polarity time scale in DSDP Hole 550 lies at an unconformity (separating stratigraphic levels low in Zone NP9 from low in Zone NP10) with the result that the position of the NP9/NP10 zonal/chronal boundary is higher/younger in Chron 24r. Because the sections recovered from DSDP Sites 550 and 690 probably do not overlap over the upper NP9-lower NP10 zonal interval, and because the former section is unconformable at the NP9/NP10 zonal boundary, there is currently no means of establishing satisfactorily a numerical chronology for Chron C24r, which means that the age estimates for the upper Paleocene to lowermost Eocene biostratigraphic and isotopic events await documentation of bio-, magneto-, and isotopic stratigraphy in continuous stratigraphic section(s).

High-precision U–Pb monazite geochronology of the c. 806 Ma Grampian Shear Zone and the implications for the evolution of the Central Highlands of Scotland

Abstract: Two pegmatites and mylonitic rocks from the Grampian Shear Zone yield U–Pb monazite ages of 806 ± 3 Ma, 808 –9 +11 Ma, and 804 –12 +13 , respectively. These confirm that shearing and pegmatite crystallization was essentially synchronous. Lower intercepts of c. 440 Ma indicate disturbance during Ordovician reworking. These new data question the concept of a single latest Proterozoic–early Palaeozoic orogeny in the Grampian terrane and the significance of the Great Glen Fault.

Middle Miocene extension in the Gulf Extensional Province, Baja California: Evidence from the southern Sierra Juarez

Abstract: New geologic mapping, structural studies, and geochronology of Miocene volcanic and sedimentary rocks in the southern Sierra Juarez, Baja California, shed light on the extensional history of the Gulf Extensional Province prior to sea-floor spreading in the Gulf of California. The southern Sierra Juarez is underlain by lower–middle Miocene rocks including fluvial strata, intermediate composition volcanic deposits, basalt lava flows and cinder cones, and dacite pyroclastic deposits and lavas that nonconformably overlie the Cretaceous Peninsular Ranges batholith. The 40 Ar/ 39 Ar geochronology indicates that basaltic rocks are 16.90 ± 0.05 Ma and dacite pyroclastic deposits are between 16.69 ± 0.11 Ma and 15.98 ± 0.13 Ma. These strata were subsequently cut by two generations of faults. First generation faults comprise a dominant set of north-south–striking, west-dipping normal faults, a secondary set of north-south–striking, east-dipping normal faults, and a lesser set of variably oriented strike-slip faults. All three fault sets are temporally and spatially related and were produced by east-west extension. The dominant west-dipping faults, which are antithetic to and oblique to the east-dipping Main Gulf Escarpment, may have been a precursor or an early phase accommodation zone along the escarpment. West-dipping normal faults are cut by a 10.96 ± 0.05 Ma dacite hypabyssal intrusion, thus bracketing the age of east-west extension between 15.98 ± 0.13 Ma and 10.96 ± 0.05 Ma. Hence, this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated Miocene extension within Baja California. Second generation faults, which are comprised of east-west–striking strike-slip faults that cut first generation faults and associated northwest-striking, northeast-dipping normal faults, may be related to early development of the Transpeninsular Strike-slip Province. Global plate reconstructions suggest that transtensional motion between the North American and Pacific plates along the western margin of Baja California began during middle Miocene time, coeval with east-west extension in the southern Sierra Juarez. This observation supports a hypothesis that middle Miocene transtensional plate motion was partitioned into two components: a strike-slip component parallel to active faults along the western offshore margin of Baja California, and an extensional component normal to the margin, but located in what is now the Gulf Extensional Province. Hence, the onset of extension within the circum-gulf region was in response to plate boundary processes.

Pleistocene coastal terraces of Kaikoura Peninsula and the Marlborough coast, South Island, New Zealand

Abstract: Pleistocene marine terraces along the Marlborough coast, South Island, New Zealand, have been re‐examined with detailed stratigraphic observations, accurate height data, and amino acid and thermo‐luminescence (TL) geochronology. Marine terraces range in age from c. 220 ka (oxygen isotope stage 7) to c. 60 ka (oxygen isotope stage 3), in the area from Cape Campbell to Conway River. At Kaikoura Peninsula, five marine terraces are preserved. The marine fauna, loess stratigraphy, and amino acid dating of Tawera spissa, from the Kaikoura I (highest) terrace and from the highest terrace at Haumuri Bluffs (Tarapuhi Terrace), indicate a correlation to oxygen isotope substage 5c, with an age of 100 ka. North of the Clarence River, marine terraces (including the Parikawa Formation) are correlated to oxygen isotope substage 5e of the last interglacial. TL dating of loess supports this interpretation. The Winterholme Formation terrace at Kekerengu is reinterpreted as a last glaciation fluvioglacial terrace g...

Stratigraphy and U – Pb zircon geochronology of Kidd Creek: implications for the formation of giant volcanogenic massive sulphide deposits and the tectonic history of the Abitibi greenstone belt

Abstract: Within the context of a structural–stratigraphic analysis of the giant Kidd Creek Cu–Zn–Ag deposit, precise U–Pb zircon ages have been obtained for five critical rock units associated with the depo...

Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events

Abstract: The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.

SHRIMP U-Pb geochronological constraints on Archean volcanic-hosted massive sulfide and lode gold mineralization at Mount Gibson, Yilgarn Craton, Western Australia

Abstract: The base metal-rich Mount Gibson gold deposits are hosted within the ductile Mount Gibson shear zone in the Retaliation greenstone belt of the Murchison province of the Archean Yilgarn craton. The deposits are unique in the context of the Yilgarn, as they exhibit two main styles of hydrothermal alteration and sulfide mineralization, consistent with overprinting of synvolcanic, volcanic-hosted massive sulfide-style mineralization by synorogenic, shear-hosted lode gold mineralization. On the basis of model ages based on sulfide Pb isotope ratios, these two events are separated by approximately 300 m.y.Subvolcanic intrusive rocks from within the host sequence to mineralization at the Orion Two and Hornet deposits have SHRIMP II zircon U-Pb ages of crystallization of 2929 + or - 3 and 2934 + or - 6 Ma, respectively. These ages are within error and provide a minimum age for the host sequence, and consequently, the volcanic-hosted massive sulfide mineralizing event, at Mount Gibson. On the basis of geochemical evidence, the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude, and consequently ca. 2930 Ma is probably the approximate age of the host rocks to mineralization. The regional granitoid, which intrudes the base of the greenstone belt, has a crystallization age of 2935 + or - 3 Ma.A post-tectonic granite porphyry, which crops out approximately 25 km north-northeast of the Mount Gibson mine site, has a SHRIMP II zircon U-Pb crystallization age of 2623 + or - 7 Ma. This granitoid has identical field relationships, similar whole-rock geochemistry, and similar Pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the Deep South pit and is consequently interpreted to be cogenetic and contemporaneous. The 2623 + or - 7 Ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at Mount Gibson.A morphologically and chemically distinctive population of zircons extracted from the sample of 2934 Ma altered felsic schist from the Hornet deposit gives a SHRIMP II zircon U-Pb age of 2627 + or - 13 Ma. These zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at Mount Gibson. The interpreted mineralization age is within error of the inferred age of the postmineralization, post-tectonic granitoid. The age is also within error of the majority of absolute ages for gold mineralization from across the Yilgarn craton, which generally fall in the range 2630 + or - 10 Ma.SHRIMP zircon U-Pb geochronology thus provides strong support for the two-stage model proposed for mineralization at Mount Gibson. Synvolcanic volcanic-hosted massive sulfide-style mineralization occurred at ca. 2930 Ma and was overprinted by lode gold mineralization some 300 m.y. later, at ca. 2630 Ma.

Geochemistry and timing of post-metamorphic dyke emplacement in the Mersin Ophiolite (southern Turkey): New age constraints from40Ar/39Ar geochronology

Abstract: The Mersin ophiolite, which is a relic of the late Cretaceous Neotethyan ocean domain in the eastern Mediterranean, is situated on the southern flank of the central Tauride belt. The ophiolite body is cross-cut at all structural levels by numerous mafic dyke intrusions. The dykes do not intrude the underlying melange of platform carbonates. Therefore, dyke emplacement post-dates the formation of the opholite and metamorphic sole but pre-dates the final obduction onto the Tauride platform. The post-metamorphic dyke swarms suggest the geochemical characteristics of Island Arc Tholeiites (IAT). 40Ar/39Ar geochronology of the post-metamorphic microgabbroic-diabasic dykes cutting both mantle tectonites and metamorphic sole revealed ages ranging from 89.6 ± 0.7–63.8 ± 0.9 Myr old, respectively, indicating widespread magmatic activity during the Late Cretaceous-early Palaeocene in the Neotethyan ocean. These data suggest that island arc development in the Neotethyan ocean in southern Turkey was as early as Late Cretaceous.

Thermotectonic evolution of the Eastern Segment of southwestern Sweden: tectonic constraints from U-Pb geochronology

Abstract: Abstract The Eastern Segment forms a complex crustal terrane that occupies the southeastern part of the Southwest Scandinavian Domain adjacent to the less deformed 1.65–1.85 Ga Transscandinavian Igneous Belt (TIB). New U-Pb data from the Eastern Segment indicate that orthogneisses and granitoid rocks have protolith ages equivalent to the later magmatic phases of the TIB and that the earliest thermotectonic episodes in the Eastern Segment occurred between 1.70 and 1.61 Ga during the Gothian Orogeny. The comparable ages for Eastern Segment gneisses and TIB rocks represent permissive evidence for the hypothesis that the orthogneiss protoliths intruded into the western margin of Fennoscandia and are not exotic with respect to the pre-Gothian craton. The eastern limit of Gothian deformation is thus interpreted to be an intracratonic deformation front that coincides with the later Sveconorwegian deformation front along the Protogine Zone south of Lake Vänern. The post-Gothian anorogenic period was interrupted, in at least the eastern part of the Eastern Segment, by a thermal and magmatic event at 1.47 Ga that generated granitic dykes and (re)crystallized titanite. Sveconorwegian U-Pb ages and field observations are compatible with a thrusting and exhumation of western supracrustal terranes over the Eastern Segment during the Sveconorwegian Orogeny, followed by isostatic unroofing and cooling of the lower deck at 950–930 Ma. Available, albeit limited, Sveconorwegian titanite ages in the Eastern Segment young from east to west, ranging from 950 to 930 Ma, and are substantially younger than those dated in the western terranes of SSD.

The Parry Sound domain: a far-travelled allochthon? New evidence from U–Pb zicon geochronology

Abstract: We report U–Pb zircon ages for metaplutonic and metasedimentary rocks from three lithotectonic assemblages within the Parry Sound allochthon of the Central Gneiss Belt, southwestern Grenville Orogen: the basal Parry Sound, interior Parry Sound, and Twelve Mile Bay assemblages. Magmatic crystallization ages for granitic to tonalitic gneisses from the basal Parry Sound assemblage fall in the range 1400–1330 Ma. Younger intrusions include the Parry Island anorthosite dated at 1163 ± 3 Ma and a crosscutting mafic dyke bracketed between 1151 and 1163 Ma. Dated at a tonalitic gneiss from the overlying interior Parry Sound assemblage is slightly younger than the older group of rocks from the basal Parry Sound assemblage. 207Pb/206Pb ages for zircons from a quartzite of the basal Parry Sound assemblage range from 1385 Ma to the Neoarchaean. An absolute maximum age for this quartzite is 1436 ± 17 Ma. In contrast, detrital zircons from a quartzite of the Twelve Mile Bay assemblage constrain the age of deposition at...

K—Ar dating of the Anakie Metamorphic Group: Evidence for an extension of the Delamerian Orogeny into central Queensland

Abstract: The Anakie Metamorphic Group of central Queensland is a multiply deformed sequence of psammitic to pelitic metasedimentary and mafic igneous rocks. Although the depositional age is still uncertain, this paper presents K‐Ar dates from four samples which indicate that the sequence was deformed at about 500 Ma in the Middle Cambrian, and thus forms part of the Delamerian‐Ross Orogen of southern Australia and eastern Antarctica. Lithologically and structurally similar rocks in the Lolworth‐Ravenswood Province, farther north in Queensland, may also be part of this belt. The significance of an older age of about 540 Ma from a fifth sample is uncertain. A sixth sample gave a younger age of about 470 Ma and may be partly reset.

Geochronology of Mesozoic tholeiitic magmatism in Antarctica: implications for the development of the failed Weddell Sea rift system

Abstract: Abstract In Antarctica, Mesozoic tholeiitic magmatic rocks have been divided previously into the Ferrar and Dronning Maud Land magmatic provinces. In both provinces, K-Ar geochronology has established a spectrum of Jurassic ages, which supported relatively long-lived coeval magmatic episodes within each province. However, recent Ar-Ar geochronology has indicated that the Ferrar magmatism occurred over a short interval (176.6 ± 1.8 Ma). To further constrain the age relationships within the two provinces, new Ar-Ar geochronology has yielded the following ages from three different regions: NE Heimefrontfjella (Dronning Maud Land), basaltic lava 172.4 ± 2.1 Ma, dolerite still 182.1 ± 1.9 Ma; 10 dolerites from the Theron Mountains (Coats Land), 172.6 ± 1.7 to 181.9 ± 2.5 Ma, although two dolerites with low K plagioclase gave ages of c. 190 Ma; Dufek Intrusion (Transantarctic Mountains) 182.5 ± 2.4 Ma. The Dufek intrusion is an extremely large layered gabbroic complex, which is an integral part of the Ferrar Magmatic Province, but is slightly older than the basalts in this province; suggesting an early intrusive episode followed by somewhat later volcanic episodes. A similar age distribution has also been demonstrated in NE Heimefrontfjella. The new Ar-Ar ages confirm a short-lived but widespread Mid-Jurassic magmatic episode within Antarctica. This magmatic episode is c. 25 Ma older than the earliest evidence for the separation of Africa and Antarctica, suggesting that the plume responsible for the magmatism in Dronning Maud Land and the Karoo province in southern Africa was not the driving force for continental separation. However, the magmatism in Dronning Maud Land and Coats Land in Antarctica and the postulated position of the plume correlate with the position of the failed Weddell Sea Rift System. The plume may have provided the driving force for this rift system, but, it did not possess sufficient energy to drive continental separation.

A Middle Silurian-Early Devonian Magmatic Arc in the Qinling Mountains of Central China: A Discussion

Abstract: Lerch et al. (1995) documented an early-to-middle Paleozoic magmatic arc in the Qinling orogenic belt of central China by detailed field mapping, geochemistry, and geochronology in the Heihe area of North Qinling. Their U-Pb analyses of zircons from I-type granites developed in the Heihe area suggested short-lived arc activity from Middle Silurian to Early Devonian time. This result is very interesting. The termination of calc-alkaline magmatism in the Devonian is plausible and consistent with our studies on geochemistry and geochronology of a mafic pluton developed in Lerch et al's. (1995) Zone 2: we reported a Sm-Nd mineral isochron (pyr + 2 Plag + WR) age of 402.6 + 17.4 Ma for the Lajimiao noritegabbro that intruded into the metabasaltic lava developed in the south of the Shangxian area (Li et al. 1989), then demonstrated its island arc origin using trace elements and Sr, Nd Isotopes (Li et al. 1994a). Along the south marigin of Zone 2, metamorphic andesite interbedded with metabasaltand intruded by mafic and granitic plutons-was found recently in the Heihe area. Their major and trace element compositions show island arc andesite characteristics (W. Sun et al. 1995). The 40Ar/ 39Ar age of 426.0 + 1.3 Ma (Li et al. unpub. age) for an amphibole from a metabasalt sample shows that these rocks underwent low-amphibolite facies metamorphism in the Middle Silurian. The Caledonian metamorphic age was regarded as the time of collision between this volcanic arc and the North China Craton (Li et al. 1992). The tectonic system of the North Qinling area between 426 and 400 Ma might be similar to that of the Andes in South America, i.e., an island arc that had already accreted to the continent while the subduction of oceanic crust continued.

Mid‐Cretaceous oroclinal bending of New Zealand terranes

Abstract: Recently published results and new data suggest that the Jurassic‐Cretaceous magmatic rocks of the Median Tectonic Zone of New Zealand, and the Cretaceous Separation Point Batholith that locally intrudes it, were emplaced subparallel to the Mesozoic Gondwana margin. Together, they provide a valuable piercing point on the Alpine Fault for 118 Ma. They also lie almost exactly parallel to mean extension lineations in exhumed metamorphic core complexes and extension directions indicated by fault‐bounded Cretaceous sedimentary basins and dike swarms in the overlying cover. Continental extension and subsequent breakup in the Tasman Sea and the eastern Bounty Trough was towards the northeast, almost perpendicular to the overall trend of the Gondwana margin but parallel to the margin‐related rocks in the central sector. Together, these relationships suggest almost 90° of rotation and major dextral shear. New geochronology now constrains the rotation to the period between the intrusion of the Separation P...

Zircon U/Pb and Pb/Pb geochronology of the Rastenberg granodiorite, South Bohemian Massif, Austria

Abstract: A combined zircon typology, zircon Pb-Pb evaporation, and conventional U-Pb study of the late- to post-tectonic Rastenberg granodiorite yields the following results: Typological investigations show two distinguishable zircon populations. Type l: subtype S24 ofPupin, colourless to slightly pink, clear to turbid, often with cores, few to abundant inclusions, long prismatic; type 2: subtype S4 ofPupin, colourless to reddish or slightly pink, clear to slightly turbid, no visible cores, abundant inclusions, tabular habit, short prismatic. At least 4 different zircon-forming events can be distinguished: Inherited cores with ages around 623±22Ma and single ages > 1206Ma from type 1 zircons imply the reworking of rocks derived from Cadomian and Proterozoic to Archean crust. Ages around 353±9Ma from type 1 zircons are interpreted as timing a first magma formation or the onset of a long-lasting magma-generating event during the Variscan plutonism in the South Bohemian pluton. The actual intrusion of the granodioritic magma into the middle crust took place around 338±2Ma (type 2 and rims of type 1 zircons). Only type 1 zircons are found as inclusions in large K-feldspar phenocrysts providing evidence that these phenocrysts have grown before the 338Ma event and may be as old as 353 Ma.[⇃]

Geochronology of the mid-German crystalline rise west of the River Rhine

Abstract: The mid-German crystalline rise has its westernmost exposures at the western margin of the Rhine graben in the southern Pfalz and the northern Alsace. The outcrops are made up of granitoid rocks and minor volcano-sedimentary sequences. Radiometric ages obtained by U/Pb, Pb/Pb, Sm/Nd and Rb/Sr analyses of the igneous rocks from this area range from ∼433 to ∼325 Ma thus covering a time span from the Silurian to the end of the lower Carboniferous. Because the investigated rocks are — according to their chemical composition — largely related to subduction zone environments, the following three geodynamical scenarios are postulated, always taking subduction of oceanic crust beneath the mid-German crystalline rise into account: (a) subduction of the Rheic ocean during the Silurian from the north; (b) subduction of the Rhenohercynian ocean during the late Devonian (∼369 Ma) from the north; (c) subduction of the Saxothuringian ocean during the lower Carboniferous (∼334 Ma) from the south.

Zircon U-Pb geochronology of plutonic rocks from the Antarctic Peninsula: Confirmation of the presence of unexposed Paleozoic crust

Abstract: The Antarctic Peninsula represents the southernmost segment of a magmatic arc once present along the western margin of Gondwanaland. Zircon U-Pb geochronology of a suite of calc-alkaline granitoids from the west coast of Graham Land, northern Antarctic Peninsula, resulted in ages for the rocks ranging from 117.0 ± 0.8 to 73.6 ± 0.4 Ma. Discordant zircon populations from peraluminous granites reveal inheritance characteristics resulting from entrainment of older crustal materials. Upper intercept ages of discordant zircon populations in four samples indicate that the age of assimilated material is approximately late Paleozoic. This inherited component is present in four samples from Bone Bay, Charlotte Bay, and Stonington Island and thus may extend for ∼600 km along the west coast of Graham Land. A possible source for the inherited components is the late Paleozoic(?) sediments of the Trinity Peninsula Group. However, a late Proterozoic upper intercept age for the Charlotte Bay granodiorite in western central Graham Land indicates that the age of the rocks assimilated by this suite of plutons is not uniformly late Paleozoic and might include unexposed basement rocks. Finally, an upper intercept age of 431 ± 12 Ma from a granite clast in metaconglomerate from Horseshoe Island provides further evidence for the presence of mid-Paleozoic basement in Graham Land. These results confirm the already postulated minimum early Paleozoic age for Antarctic Peninsula basement.

U–Pb and Rb–Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland

Abstract: New geochronological data which clarify the timing of syn-orogenic magmatism and regional metamorphism in the Connemara Dalradian are presented. U–Pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 Ma whereas the earliest, basic magmatism has been dated previously at 490 Ma; a fine-grained, fabric-cutting granite contains discordant zircons which also appear to be 465 Ma old. Arc magmatism in Connemara therefore spanned a period of at least 25 Ma. Recent U–Pb data on titanite from central Connemara which gave a peak metamorphic age of 478 Ma are supplemented by U–Pb data on titanite and monazite from metamorphic veins in the east of Connemara which indicate that low- P, high- T regional metamorphism continued there to 465 Ma, i.e. at least 10 Ma later than in the central region dated previously. New Rb–Sr data on muscovites from coarse-grained segregations in different structural settings range from 475 to 435 Ma: in part this range probably also reflects differences in age from west to east, with three ages close to 455 Ma from the eastern area, which is also the site of the lowest pressure metamorphism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1–2 Ma. The new dates emphasize the complexity in the spatial and temporal distribution of high-level regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the southern part of Connemara. The later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism, and were localized in the eastern part of the region.

Geochronological constraints on the age of komatiites and nickel mineralisation in the Lake Johnston greenstone belt, Yilgarn Craton, Western Australia

Abstract: SHRIMP U‐Pb zircon geochronology has been carried out on two volcanic units from the Maggie Hays nickel prospect. A sample from the footwall to the komatiites that host the nickel mineralisation gives an age of 2921 ± 4 Ma, whereas a sample from the hangingwall gives an age of 2903 ± 5 Ma. These results constrain the age of the primary nickel mineralisation and the host komatiites.