Paleomagnetic polarity sequence and radiolarian zones,brunhes to polarity epoch 20

Superposition of paleomagnetic polarity logs of seven chronologically overlapping piston cores from the central equatorial Pacific, using the established tropical radiolarian zonation as a stratigraphic reference, produced a nearly continuous correlation of magnetic and radiolarian events ranging from late Pleistocene to earliest Miocene. Twenty magnetic polarity epochs, and possibly as many as 30 polarity events, occur during this time span. Epoch 16 (reversed polarity) appears to be the longest interval (～ 14.8–17.6m.y. B.P.) among these Neogene magnetostratigraphic units. The middle/late Miocene boundary is shown to fall within latest Epoch 11 (normal) and its approximate age is between 10.5 and 11 m.y. B.P. The early/middle Miocene boundary occurs within the top of Epoch 16 at a suggested age of about 15 m.y. B.P.     

Local axial migration and spreading rate variations,East Pacific Rise, 31°S

Mapping and analysis of marine magnetic anomalies generated during the past 3.78 m.y. at the East Pacific Rise crest near 31°S reveals a history of ongoing small-scale migration of the spreading center. The axis first became curved and then broke when the curvature became too severe, forming a 10-km offset. The offset healed rapidly and the topographic axis of the rise is now continuous and essentially linear. Sea-floor spreading has occurred asymmetrically in this area with east and west flank rates of 86 and 77 mm/yr, respectively, since 2.41 m.y. ago. Total spreading rates show an overall decline from 176 to 145 mm/yr prior to the Jaramillo event, 0.9 m.y. ago. For the last 0.7 m.y. the total spreading rate has been 162 mm/yr.     

Palaeomagnetic and palaeontological dating of a section at Gioia Tauro,Italy: Identification of the Blake Event

Palaeomagnetic and palaeontological studies on samples withdrawn from a 250-m-long unorientated core from Reggio di Calabria, Italy (38°N, 16°E) are described. The lower 200 m of core penetrated off-shore clays and visual examination of the split sections of core indicated no obvious breaks in deposition. After alternating field demagnetization in 200 Oe the palaeomagnetic inclination log shows the Brunhes/Matuyama boundary at 110 m below which the Jaramillo and Olduvai Events within the Matuyama Epoch are identified. This interpretation of the magnetostratigraphy is supported by palaeontological evidence. The rate of accumulation of clay is estimated to have ranged between about 60 and 190 mm/kyr with an overall average through the Matuyama Epoch of about 90 mm/kyr. Assuming that this average rate continued through the Brunhes Epoch, the age of the top of the clay unit is estimated to be about 90,000 yr B.P. About 5 m from the unconformable top surface of the clay, a split sequence of reversed inclinations is interpreted as a record of the Blake Event, and the overall average deposition rate implies that its duration may have been as long as 50,000 yr. No other reversed event is recorded by the palaeomagnetic inclination log through the Brunhes Epoch, though there are four horizons where shallow positive inclinations are recorded.     

U(Th)Pb systematics and ages of Himalayan leucogranites, South Tibet

The age and origin of five leucogranites from the High and Tethys Himalaya, and two country-rock gneisses were investigated by UPb dating of zircon fractions and single grains, and fractions of monazite. Additionally, ThU concentrations in whole rock powders and isotopic compositions of Pb in leached K-feldspars were determined. Monazites yield ages of 16.8 ± 0.6 m.y. for the Nialam migmatite-granite, 15.1 ± 0.5 m.y. for the Lhagoi Kangri granite, 14.3 ± 0.6 m.y. for a granite from Mt. Everest, and 9.8 ± 0.7 m.y. and 9.2 ± 0.9 m.y. for two varieties of the Maja granite. These data, together with monazite ages of 21.9 ± 0.2 and 24.0 ± 0.4 m.y., determined earlier on the Makalu granite [1], substantiate a period of intracontinental granite emplacements from 24 to 9 m.y. ago, i.e. from uppermost Oligocene to late Miocene times. Such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of India with Eurasia. Furthermore, the individual ages, together with structural relationships between granites and country rocks suggest that granite formation and tectono-metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m.y. Inherited lead components, present in all granite zircons point to large proportions of Precambrian material in the magma source regions, up to 2200 m.y. old.ThU systematics between monazite and country rocks indicate that U has been leached from most of the granites after crystallisation of monazite.Zircon dating of the Kangmar granite gneiss, which occurs in a window through the Tethys Himalayan sediments, shows that this pluton, transformed to a gneiss during the Alpine orogeny, crystallised in lowermost Palaeozoic times 562 ± 4 m.y. ago.     

Paleomagnetic excursion recorded in latest Pleistocene deep-sea sediments,Gulf of Mexico

Paleomagnetic and/or micropaleontological studies have been carried out on approximately 28 sedimentary cores of latest Pleistocene age from the Gulf of Mexico. Sedimentation rates range from 9 cm to 20 cm/1000 yr. A distinct excursion in the earth's magnetic field occurs in the upper parts of 8 of 15 cores for which paleomagnetic studies were conducted and is independently correlated with planktonic foraminiferal zones. An inclination change to zero or negative inclination is often associated with a declination change. The age of the excursion was determined by extrapolation of sedimentation rates from the Z-Y paleontological boundary which is dated at 11,000 B.P. The magnetic excursion occurred between 12,500 and 17,000 yr ago within the upper part of zone Y. This falls within the age range of the Laschamp Event as originally defined.     

Unroofing history of a suture zone in the Himalaya of Pakistan by means of fission-track annealing ages

The uplift history of the Swat Valley and Hazara region of northwestern Pakistan has been established using 22 fission-track dates on apatite, zircon and sphene. A major fault, the Main Mantle Thrust (MMT) strikes east-west across the Swat Valley, separates regions of markedly differing fission-track age regimesm, and may be a suture zone separating an extinct island arc terrane on the north from the Indian plate to the south. Fission-track ages ranging from about 55 to 58 m.y. for sphene, 18 to 53 m.y. for zircon, and 9 to 17 m.y. for apatite were obtained from the region north of the MMT. To the south the fission-track age ranges are 20 to 25 m.y. for sphene, 17 to 26 m.y. for zircon, and 16 to 23 m.y. for apatite. Disparate zircon and sphene ages on each side of the MMT imply different cooling histories for each side of the fault prior to 15 m.y. Similar apatite ages on both sides of the fault imply similar cooling histories during the past 15 m.y. This may indicate that faulting ceased by 15 m.y. Mean uplift rates have been derived from the fission-track data using mainly the mineral-pair method. Uplift rates in the region north of the MMT increased from 0.07 to 0.20 mm/yr during the period 55 to 15 m.y. South of the fault, uplift rates averaged in excess of 0.70 mm/yr for the period 25 to 15 m.y. During the past 15 m.y. uplift across the MMT in the Swat Valley showsno discontinuities, ranging from 0.16 mm/yr in the south to 0.39 mm/yr in the north. A plausible interpretation for the fission-track uplift data has the MMT verging to the south with overthrusting taking place at a depth between 3.5 and 6.0 km, juxtaposing two terranes that were originally separated by a substantial, but unknown distance. In this model, regional uplift followed cessation of faulting just prior to 15 m.y.     

Paleomagnetic evidence for Jurassic deformation of the McCoy Mountains Formation,southeastern California and southwestern Arizona

The Mesozoic McCoy Mountains Formation is a 7.3-km-thick deformed clastic sequence exposed in six mountain ranges in southeastern California and southwestern Arizona. Interbedded with Jurassic volcanic rocks at its base, the McCoy Mountains Formation had been assigned a Cretaceous age based upon fossil angiosperm wood found in the upper third of the section. Characteristic natural remanent magnetism (NRM) from 145 oriented samples from 18 sites within the sedimentary terrane yield an in situ mean direction:I = 20.6°, D = 335.1°, α₉₅ = 7.7° (uncorrected for structural tilting). Opaque mineralogy and a failed fold test indicate that the NRM is a chemical remanence acquired post-folding. The paleomagnetic pole position calculated from the in situ mean direction falls adjacent to poles from the Summerville Formation and Canelo Hills Volcanics. We interpret these data to indicate that deformation, mild metamorphism, and resultant magnetization of the McCoy Mountains Formation occurred during Jurassic time. It is suggested that the McCoy Mountains Formation and underlying Jurassic volcanics were deposited adjacent to, and then deformed between, the North American craton and an outlying allochthonous terrane during Jurassic time.     

K-Ar chronology of the volcanic eruptions in the Canarian archipelago: Island of La Gomera

A geochronological study of the Island of La Gomera (Canaries) has been carried out by the K-Ar method. The 26 new ages obtained, together with the 17 previous determinations, show that above the main unconformity of the island, separating the « basal complex » from the later volcanic series, there is a unit of « lower old basalts » more than 10 m.y. old. Polymictic volcanic breccias were emplaced between 10 and 9 m.y. ago. The « upper old basalts » above them were formed between 9 and 6 m.y. ago, with a peak of activity around 7 m.y. After a period of erosion (6-5 m.y.), a thick series of « young basalts » associated with trachytic and phonolitic domes and flows, were rapidly emplaced between 4.5-4 m.y. ago. Finally, local basaltic activity took place 2.8 m.y. ago. The age of the basal complex is not well known, although three ages (14.6, 15.5 and 19.3 m.y.) have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex.Research funded under Project n. 1771. CAYCIT. Spain.     

The evolution of early precambrian crustal rocks at Isua,West Greenland — Geochemical and isotopic evidence

Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860 ± 240 m.y. (2 sigma error), but consideration of the initial⁸⁷Sr/⁸⁶Sr ratio constrains the possible age of formation of 3710 ± 900 m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m.y. on Isua banded ironstones.Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700–3800 m.y. old Ami?tsoq gneisses, involving fractionation of garnet during their evolution.A Pb/Pb whole-rock isochron for Ami?tsoq gneisses from Isua yields an age of 3800 ± 120 m.y. (2σ), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 ± 120 m.y. ago. A two-stage model for the U-Pb system yields an average²³⁸U/²⁰⁴Pb (μ₁) value of 9.3 ± 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U-Pb heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700–3800 m.y. ago. Attempts to apply U-Pb whole-rock dating to the Ami?tsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching.A Rb-Sr whole-rock isochron on a suite of Ami?tsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 ± 130 m.y.In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900–3700 m.y. ago.     

Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics

The Comores Islands together with the Tertiary volcanic province of northern Madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the Mozambique Channel. Potassium-argon dating of shield-building lavas confirms an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The velocity of the Somali plate over the mantle magma source is 50 mm/yr.

We use the distribution of ages along the Comores-Madagascar chain in conjunction with existing age data for the Reunion-Mascarene Plateau hotspot track to model the absolute motion of the Somali plate for the last 10 m.y. We calculate the relative motion across the East African Rift by subtracting the Somali plate absolute motion from African plate absolute motion during this period. The model predicts 320 km of total separation across the East African Rift during the past 10 m.y. which is greater than has been estimated from surface geological evidence

The geometry of older portions of the Comores and Reunion trends indicates that there was no significant relative motion between the African and Somali plates prior to about 10 m.y. ago.     

Early Archaean rocks and geochemical evolution of the earth's crust

Remnants of Early Archaean rocks (>FX3000 m.y. old) are reported from most continents. A critical review of the radiometric data shows that few of these are well authenticated and most are very limited in extent. The oldest are predominantly plutonic gneisses of tonalitic-to-granitic composition (e.g., the basement gneisses of West Greenland, Labrador, Rhodesia and South Africa). In all cases there are inclusions of meta-volcanic and sedimentary rocks with greenstone belt affinities which probably represent crust into which the igneous parents of the gneisses were intruded.The trace element chemistry of these very old rocks is reviewed in an attempt to establish the mechanism of formation of early crust and place constraints on the chemical evolution of the earth's mantle. “Mantle-type” Sr isotope compositions show that the sialic members of both early gneisses and greenstone belts were not derived from much older crustal differentiates, either at 3800 or at 2800 m.y. ago. However, trace element ratios such as K/Rb and Sr/Ba, and rare earth element abundances, are not consistent with direct derivation of the plutonic suite from the upper mantle and also rule out a common parentage for the tonalites and granites. An origin by partial melting of metamorphosed juvenile crust with a composition range equivalent to that represented by the greenstone belts is preferred. Tonalites resulted from high-pressure melting of mafic garnet-amphibolite and at least some of the granites from low-pressure melting of more felsic (possibly even sedimentary) material.The trace element chemistry of the greenstone belt volcanics is thought to characterize the composition of early mantle melts, although the best preserved and best documented cases are about 500–1000 m.y. younger than the oldest known gneisses. The dominant type is tholeiite with low incompatible element contents and light-depleted or essentially flat rare earth patterns, features even more marked in the ultramafic komatiites which represent large degrees of melting. More evolved calc-alkaline rocks with relative incompatible and light rare earth element enrichment are also important. With the exception of the ultramafic lavas, all these types can be matched by the chemistry of present-day oceanic volcanism.It is concluded that the range of trace element variations in the earth's mantle was comparable in early Archaean times to that at the present. This is supported by mass balance calculations for the lithophile elements which have been preferentially extracted into the crust. Thus the isotope and trace element evidence of the oldest rocks argues against primary differentiation of the crust either during accretion of the earth or during its first 500 m.y. as a solid body. Crust formation has probably occurred continuously, although worldwide evidence for magmatism at around 2800 m.y. ago probably marks a particularly active period.     

Establishing the time of initial human occupation of Liang Bua, western Flores, Indonesia

Liang Bua, a limestone cave in western Flores, has an archaeological and faunal sequence known to span the last 95 ka and is the type-site for a small-bodied hominid species, Homo floresiensis. This paper describes the geomorphic history of this significant site, and presents numerical ages for key events as determined by thermoluminescence dating of sediments (using a dual-aliquot regenerative-dose protocol) and uranium-series dating of flowstones. Our age estimates indicate that Liang Bua existed as a subterranean chamber by 400 ka ago. It was subsequently exposed by the incision of the Wae Racang River, which invaded the cave around 190 ka and deposited an upward-fining sequence of water-rolled boulders and cobbles. The latter conglomerate was partially reworked around 130 ka ago and was capped by flowstones at 100 ka. The conglomerate also contains stone artefacts, which implies that the occupation of the surrounding area is at least as old as the time of emplacement of the conglomerate. After 190 ka, a complex sequence of erosion and deposition led to the accumulation of at least 11 m of sediment in the front chamber of the cave, which proved suitable for hominid occupation. Such geochronological and geomorphological information is extremely valuable for interpreting the archaeological record at Liang Bua and its wider significance.     

Drainage migration and out of sequence thrusting in Bhalukpong,Western Arunachal Himalaya,India

In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5 cm/yr in upper reaches and ∼13.5 cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1 m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1 ka) and differing incision rates of the surface of same terrace (T_2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1 ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12 mm/yr and 7 mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27 mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan.     

Age relationships between greenstone belts and “granites” in the Rhodesian Archaean craton

Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced2780 ± 30 m.y. ago.The initial⁸⁷Sr/⁸⁶Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.     

Provenance and thermal history of the Bayan Har Group in the western-central Songpan–Ganzi–Bayan Har terrane: Implications for tectonic evolution of the northern Tibetan Plateau

Triassic turbidites dominate the Songpan–Ganzi–Bayan Har (SGBH) terrane of the northern Tibetan Plateau. U‐Pb dating on single detrital zircon grains from the Triassic Bayan Har Group turbidites yield peaks at 400–500 m.y., 900–1000 m.y., 1800–1900 m.y., and 2400–2500 m.y., These results are consistent with recently published U‐Pb zircon ages of pre‐Triassic bedrock in the East Kunlun, Altyn, Qaidam, Qilian and Alaxa areas to the north, suggesting that provenance of the Bayan Har Group may include these rocks. The similarities in the compositions of the lithic arkosic sandstones of the Bayan Har Group with the sandstones of the Lower‐Middle Triassic formations in the East Kunlun terrane to the north also suggests a common northern provenance for both. A well exposed angular unconformity between the Carboniferous–Middle Permian mélange sequences and the overlying Upper Permian or Triassic strata indicates that regional deformation occurred between the Middle and Late Permian. This deformation may have been the result of a soft collision between the Qiangtang terrane and the North China Plate and the closure of the Paleo‐Tethyan oceanic basin. The Bayan Har Group turbidites were then deposited in a re‐opened marine basin on a shelf environment. Fission‐track dating of detrital zircons from the Bayan Har Group sandstones revealed pre‐ and post‐depositional age components, suggesting that the temperatures did not reach the temperatures necessary to anneal retentive zircon fission tracks (250–300°C). A 282–292 m.y. peak age defined by low U concentration, retentive zircons likely reflects a northern granitic source. Euhedral zircons from two lithic arkoses with abundant volcanic fragments in the southern area yielded a ～237 m.y. zircon fission track (ZFT) peak age, likely recording the maximum age of deposition. A dominant post‐depositional 170–185 m.y. ZFT peak age suggests peak temperatures were reached in the Early Jurassic. Some samples appear to record a younger thermal event at ～140 m.y., a short lived event that apparently affected only the least retentive zircons.     

Rb-Sr isotope geochemistry of lherzolites and their constituent minerals from Victoria,Australia

Major element data and Rb, Sr and⁸⁷Sr/⁸⁶Sr analyses for seven spinel lherzolite xenoliths and their Recent host basalt from Victoria, Australia, are presented. The exotic nature of the xenoliths is indicated by a wide spread in⁸⁷Sr/⁸⁶Sr values (0.7035–0.7076) compared with the basalt (0.7041). Five of the lherzolites provide evidence of a thermal event in the mantle 650 m.y. ago. Equilibration temperatures calculated from the compositions of the lherzolite phases (ca. 1050°C) apparently relate to this event. Estimates of the local geothermal gradient suggest temperatures of less than 700°C in the source region before eruption of the lherzolites.Isotopic analyses of the lherzolite minerals show that orthopyroxene contains more radiogenic Sr than coexisting olivine and clinopyroxene in three of the xenoliths. The⁸⁷Sr/⁸⁶Sr relationships between clinopyroxene and orthopyroxene suggest that internal isotopic disequilibrium has existed in the source region for up to 550 m.y.     

FLUCTUATIONS OF CRILLON GLACIER SYSTEM,SOUTHEAST ALASKA

Abstract

Crillon Glacier system (Lat. 58° 37′ N, Long. 137° 23′w) consisting of two long feeding arms and a common T-shaped ending along the Fairweather Fault, with two cliffed termini, has been advancing. La Perouse's sketch map in 1786 suggests that North and South Crillon Glaciers were back far enough to have separate termini in Lituya Bay, so they must have advanced at about 30 m/yr for 150 years. More detailed maps and photos show that North Crillon terminus did advance 28 m/yr into open water positions from 1894 to 1933. Some of this 4.5 km advance may well be just the result of consolidation of two long calving ice cliffs into one deep water front.

South Crillon front in the other arm of the “T”, where no junction of fronts is involved, also advanced into Crillon Lake 10 m/yr between 1929 and 1961. This implies that the basic cause was some increased snowfall on the high mountains. At the same time glaciers from lower peaks just to the east of Glacier Bay lost nearly all accumulation by a rising snowline and receded 5 to 11 km.

Since 1926 the Lituya Bay terminus has been more or less stable where it lies in water; however, the land portion has advanced 25 m/yr since 1948. Between about 1920 and 1961 a large gravel delta appeared and grew to 145 × 10⁶ m³, so this slow ice advance on one side of the front may be due to ice shearing over the new land of the fan where ablation is slower than watercliff calving.

Prehistoric advances of Crillon Glacier system are recorded in stratigraphic deposits dated by C-14. Trees buried in the prominent extensive outer moraine system around Crillon Lake and Lituya Bay were annihilated in the maximal push of the Little Ice Age 400 to 1000 years ago. The first advance came 1500 to 1800 years ago when logs became buried in glacial outwash, now under Little Ice Age till. Because it is regional, this was a climatic event involving a lowering snowline due to cooler summers.

Still further back, in hypsithermal time, these Crillon Glacier termini were probably as retracted as today, because many outcrops of buried forest 3000 to 9000 years old occur in alluvial deposits in Lituya Bay. Underneath all these is outwash gravel and till recording the last push of Wisconsin ice, ending at least 9000 years ago. Still earlier Wisconsin advances of North Crillon Glacier carried the Lituya Bay ice tongue a little further to sea, yet all through the pleistocene, unglaciated refugia endured between the ice streams.     

Geochronology of Gran Canaria,Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km³ was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km³) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km³ of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km³. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.     

Fission track ages of Hymalayan muscovites (Kathmandu Valley,Nepal)

Summary Himalayan muscovites from pegmatites of Kathmandu Valley, Nepal yield an age of 2–23 m. y. by fission track (f.t.) method. The uranium concentration is estimated to be 10^–10 atom/atom.     

Paleomagnetism and accretionary tectonics of northern Sikhote Alin

The results of the paleomagnetic investigation of the sediments pertaining to the Silasinskaya Formation of the Kiselevka–Manoma terrane within the Sikhote Alin orogenic belt are presented. The ancient prefolding magnetization component is revealed: Decs = 271.7°, Incs = 52.2°, Ks = 13.5, and a ^95s = 5.1° (positive fold and reversal tests); and the coordinates of the corresponding paleomagnetic pole for ~103 ± 10 Ma are calculated: Plat = 26.3°, Plong = 70.5°, dp = 4.8°, and dm = 7.0°. As a result of this study, the geodynamical settings and paleolatitudes of the formation of three objects in the northern part of Sikhote Alin orogen are established: (a) the Kiselevskaya Formation of the Kiselevka–Manoma terrane was formed 133 Ma ago at 19° N under the seamount condition on the Izanagi Plate; (b) the Silasinskaya Formation of the Kiselevka–Manoma terrane was formed 103 Ma ago at 35° N under the oceanic island arc conditions; and (c) the Utitskaya Formation of the Zhuravlevsk–Amur terrane was formed 95 Ma ago at 54° N in the active continental margin conditions. It is found that the transform continental margin of Eurasia developed in the time interval from 105 to 65 Ma ago in the regime of a left-lateral submeridional shear from 30° to 60° N. The complete attachment of the studied rocks of the Kiselevka–Manoma terrane to the Eurasia’s margin (to the Zhuravlevsk–Amur terrane) occurred at the boundary of 60–70 Ma. Simultaneously, the sense of the displacement in the submeridional shears changed from left-lateral to right-lateral with the formation of pullapart type basins (Lake Udyl’).