Search

U-Pb isotopic analyses of zircon from the lowest structural units of the Acatlán Complex of southern Mexico indicate that Paleozoic tectonothermal events are overprinted by mid-Jurassic (175±3 to 171±1 Ma), low pressure migmatization (5–6 kb), polyphase deformation, and intrusion of felsic and mafic magmas. Ensuing rapid cooling recorded by 40Ar/39Ar muscovite, biotite and K-feldspar ages is estimated to have taken place at 21±3°C/my at exhumation rates of 0.6 mm/yr. Such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top-to-the-west kinematic data. Synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent Oaxaca terrane, where top-to-the-north, extensional shear zones occur in Paleozoic strata.

Some 8-substituted derivatives of the protoberberine alkaloids palmatine (1a) and berberine (1b) have been prepared and investigated by 1D and 2D NMR spectroscopy (H-1, C-13, N-15). Complete NMR data for the 8-hydroxy (2), 8-methoxy (3), 8-ethoxy (4), and 8-trichloromethyl (5) 7,8-dihydro derivatives of 1a and 1b, as well as X-ray data for 8-methoxydihydroberberine (3b), 8-trichloromethyldihydropalmatine (5a), and 8-trichloromethyldihydroberberine (5b), are presented. The physicochemical data for all of these compounds are reviewed and compared with previously published values.

The Eocene Buck Creek volcanic complex (central British Columbia, Canada) is a part of the Challis–Kamloops volcanic province that extends from Yukon across British Columbia to the northern United States, running approximately parallel to the margin of the North American continent. The complex is associated with the formation of a fault-bounded basin controlled by strike–slip faults. The volcanic rocks were emplaced during two short-lived (<1–2My) periods. The first 52Ma old period produced high-K calc-alkaline mafic and intermediate lavas and related subvolcanic intrusions of the Goosly Lake Formation. Their age coincides with the age of two silver deposits, which are related to the intrusions. The second period which took place 50±1Ma ago, produced high-K calc-alkaline mafic and intermediate lavas (Buck Creek Formation), and overlying intraplate tholeiitic to high-K calc-alkaline basalts (Swans Lake unit). The rocks of the three units have similar Sr and Nd isotopic ratios and mantle-normalized trace element patterns characterized by a depletion of Nb, Ta and Ti suggesting a similar mantle source, garnet-bearing subcontinental lithosphere, for the lavas of all three units. The average contents of SiO2and strongly incompatible trace elements in the lavas decrease from the Goosly Lake rocks towards the top of the complex (Swans Lake unit). The compositional changes could be, in part, related to an increase of the degree of partial melting towards the top. Volcanic rocks of similar composition and age are widespread in this part of the Eocene Challis–Kamloops belt in central British Columbia. The magmatic activities are related to subduction of the Farallon/Kula plate under the North American continent in a regime characterized by strike–slip faulting and oblique plate movement along the continental margin of western Canada. It is inferred that oblique plate convergence decoupled into a component of convergence normal to the plate margin, and a component of strike–slip faulting approximately parallel to the margin within the overriding North American plate. These events triggered a melting in the mantle wedge overlying the subduction zone and generated the parent magmas for the rocks of this part of the Challis–Kamloops belt.

Two pairs of high-field-strength elements (NbTa and ZrHf) display contrasting behaviour during the evolution of a peraluminous granitic pluton that is composed of leucomonzogranites and subordinate greisen-hosted topaz-bearing leucogranites. Zr and Hf progressively decrease with differentiation and their ratio remains the same, close to the chondritic value (∼ 36). The crystallization of zircon which contains the bulk of these two elements in the rocks, does not modify the Zr/Hf ratio. With increasing differentiation, Nb and Ta show an increase accompanied by a distinct decrease of the Nb/Ta ratio from 14 in leucomonzogranites to 2 in leucogranites and greisens. Tantalum correlates positively with Rb, Li and F and is strongly enriched in the evolved rocks, implying that an addition of internally derived mineralizing fluids and fluid fractionation played a key role in the variation of the Nb/Ta ratio in rocks with K/Rb ratios < 150. The values of the Nb/Ta ratio in the continental crust are typically lower than those of mantle and chondrites (∼ 17.5) suggesting that fluid fractionation might have been an important process during the evolution of the continental crust.

Evaluates 11 models of high-performance cross-country boots. Salomon Nordic System (SNS) Profil; New Nordic Norm (NNN) boot/binding system; SNS Profil sport system; Jalas 610; Salomon SR 511; Alfa Voss; Rossignol LTS; More.

Discusses the newest Nordic skis, boots and bindings as part of the `Skiing Magazine' Buyer's Guide '91. Rossignol Carbon SDC waxless ski; Fischer RCS Skating Graphite ski; Salomon SR Profil Automatic binding; Rottefella NNN bootbinding system; Poles.

Gives field-test results on several brands of cross-country skating ski boots and bindings. Interface between sole and binding; How the boots were tested; Salomon 911 Equipe Skate; Hartjes XC 2000; Tyrolia TXC 509 RF; More. INSET: What we did.;What we wish we had done (problems during testing)..

Discusses spring heli-skiing in Quebec's Chic-Choc mountains on the Gaspe Peninsula. Heli-Ski Chic-Chocs, operating out of St.-Octave-de-l'Avenir, Que.; Ouellet Bowl; Mount Logan; Mount Albert; Travel information. INSET: How high is tree line?, by J. Chase..

Gives an overview of Nordic skiing equipment for 1990. Boots and bindings for racing and backcountry skiing; Many types of skis; Poles.

Gives advice to Alpine skiers who want to begin cross-country skiing. How to choose skis; Technique, including double poling and skating; Parallel turns.

Presents the newest Nordic skis for 1989 as part of the 1989 Buyer's Guide. Brand names; Prices; Types of skis; Light touring; Stride/skate crossover; Waxless touring skis; Telemark/backcountry; Boots; Poles.

Rare Earth Element (REE) data confirm earlier suggestions from major and trace elements that the Proterozoic Burin Group in the southwestern Avalon Zone is similar to modern oceanic tholeiites, and also exhibit a systematic evolution through the sequence. The Group forms a 60-km long belt consisting of four formations of pillowed basalt, two of subaqueous volcaniclastic and minor stromatolitic sediments and pyroclastics, and a thick gabbro-quartz diorite sill, with a total thickness of about 5000 m. Basalts of the oldest formation are enriched in light (LREE) with a chondrite-normalized pattern similar to alkali basalts. REE patterns through the rest of the sequence can be matched by those of modern ocean basins, with a steady decrease in total REE and a distinct depletion in LREE at the top of the sequence. REE patterns of the gabbroic sill are similar to those of oceanic gabbros, with LREE depletion and a small positive Eu anomaly. This evolutionary pattern can be interpreted as the result of progressive ‘dynamic’ partial melting and depletion of a single mantle source region.

The Proterozoic Flin Flon Domain of the Trans-Hudson Orogen (Churchill Province of the Canadian Shield) which contains a significant amount of volcanic rocks dated at 1,886 Ma and hosts two major mining districts, is similar to Archean greenstone belts. The volcanic rocks include island arc tholeiitic (IAT) and volumetrically less important MORB-like suites. The IAT suite ranges from basalt to rhyolite and represents a differentiation sequence with a predominance of mafic types. Compared to typical Archean greenstone belts, the IAT are more depleted in incompatible trace elements and resemble modern island arc tholeiitic sequences. The arc volcanism progressively evolved to more advanced stages characterized by the presence of calc-alkaline island arc volcanics. The basalt group which resembles E-type MORB was derived from a garnet peridotite source whereas the other MORB-like basalts were generated at a shallower depth (in the spinel stability field). The association of IAT with MORB-like tholeiites suggests that the volcanism occurred in a back-arc setting and the compositional variations of the MORB-like basalts can be attributed to melting of a rising mantle diapir during back-arc spreading.

A thick (<175 m) North Mountain Basalt flow at McKay Head, Nova Scotia (Canada) shows ~25-cm-thick differentiated layers separated by ~130 cm of basalt in its upper 34m. Upper layers (~5 m below the lava top) are highly vesicular whereas lower ones are pegmatitic and contain a thin (~2 cm) rhyolite band. The layering of the flow closely resemble that of some Hawaiian lava lakes. The eesicular basalts and mafic pegmatites are inferred to be liquid-rich segregations which drained into horizontal cracks that formed within a crystalline mush. The cracks resulted from a thermal contraction associated with cooling and shrinkage of the mush. Rhyolites were formed by in situ differentiation. Gas overpressures fractured the pegmatites and gas effervescence filter pressing forced silicarich residual liquid from pegmatite interstices into the fractures creating bands. Chemical differences between the pegmatitic layers and early formed, highly differentiated upper vesicular layers may reflect a role for volatiles in the differentiation process along with crystal fractionation.

Abundances of U have been determined in minerals of the spinel peridotite xenoliths in basaltic rocks from Hoggar (Southern Algeria) by the fission-track method. Olivine and spinel have the lowest U abundances (∼ 0.10–0.13 ppm) followed by orthopyroxene (∼ 0.22–0.24 ppm). The clinopyroxene has a large variation of U (0.23–0.74 ppm). The contents of U in amphibole are within the range of those of clinopyroxene (∼ 0.30–0.37 ppm). Although most of the U in the xenoliths is homogeneously distributed in the major rock-forming minerals, the whole-rock U abundances are at least twice as high as estimates of the upper mantle. The high contents of U even in rocks without amphibole is probably due to metasomatism by a fractionated fluid phase which took place before or during the recrystallization. The host basaltic rocks are not genetically related to the spinel peridotite.

The ophiolitic complex of Northern Pindos (Greece) contains ocean-floor basalts and low-Ti mafic rocks. The former rocks are similar to recent mid-ocean ridge basalts with a light REE depletion and a La/Yb ratio < 2. The low-Ti rocks resemble boninites in their high Mg and very low Ti and Zr contents and in their REE patterns which have convex-downwards shape with a slight light REE enrichment. However, their Zr/Ti, Ti/V and Zr/Y ratios are lower than in boninites. Both rock-types could be generated by dynamic partial melting of a rising upper mantle diapir. Slight enrichment in light REE, Sr, Rb and Ba in low-Ti rocks could be the result of either metasomatic or alteration processes. Although a subduction zone origin of the sequence is possible, the geochemical data do not necessarily imply such a setting.

Abstract: In the Paleozoic basalts of Nova Scotia (Canada) metamorphosed to a greenschist facies grade, U and Th are closely associated with immobile elements (e.g. Zr and Nb). The coherence of these elements with K, typical of igneous rocks is, however, absent, U and Th are apparently not affected by greenschist facies metamorphism as are alkali and alkali earth elements and their variation thus reflects primary magmatic processes. It seems that during the early stages of metamorphism, U and Th were retained together with several other incompatible elements (Zr, Nb, La and Ce) in stable secondary phases or alternatively these elements may be held in primary accessory minerals such as zircon and apatite.

Abstract: The Society Islands of French Polynesia (south-central Pacific Ocean) are formed predominatly by basaltic lavas of alkaline affinities. These intraplate rocks are either Ne- or Hy-normative. Ne-normative basaltic lavas are lower in Si and heavy rare earth elements and higher in Ti, V and Sr than Hy-normative. The differences are attributed to the contrasting behaviour of amphibole during melting of the upper mantle, which, in turn, may be related to the variation in the depth of melting. Compared to pyrolite, the upper mantle source of the lavas was enriched in incompatible elements.

Abstract: The Neoproterozoic Ougda magmatic complex occurs within platformal carbonate rocks in the western part of the Pan-African fold belt of the Tuareg shield (NW Africa). It is composed of - 800 Ma old, relatively high P-T (i.e., Grt + Cpx-bearing: P > 5 kbar; T≈900'Q, tholeiitic mafic/ultramafic cumulates and related rocks intruded by intermediate to mafic calcalkali plutons (e.g., Cpx+Hbl-bearing gabbro) and dikes. Apparent contrasts in structural level of crystallization indicate that the calc-alkali rocks are significantly younger than the tholeiites, which temporally correlate with a period of regional extension in this part of Africa. Intrusion of the calc-alkali rocks may have occurred during the formation of an arc after the tholeiitic rocks had been (diapirically?) emplaced within the shelf carbonates, and prior to (> 630 Ma) the Pan-African orogeny. Data reported herein indicate that the Ougda complex records the inception and demise of a Neoproterozoic ocean basin. Similar crustal sections have been described from collisional (e.g., Aleutian islands) and extensional (e.g., Ivre-Verbano zone) settings, indicating that processes operating in both environments can generate nearly indistinguishable igneous suites; the prevalence of shallow-level calc-alkali rocks in both settings may mask the presence of more mafic, tholeiitic rocks at depth.

Abstract: The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO3 content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO2 by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.

AbstractThe spinel peridotite inclusions in basalts from Sardinia are upper-mantle residues affected by metasomatism which led to an enrichment particularly of U and light REE. The metasomatism took place prior to the recrystallization which produced the primary mineral assemblage of the inclusions. The compositional variations imply that the xenoliths are residual after at least two melting events.

AbstractUltramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REEdue to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.