Regional variations in cleavage and fold development in North Wales

The arcuate pattern of the main Caledonian cleavage and associated fold axial plane traces in North Wales is due partly to NW-SE compression with tectonic transport to the southeast against the concealed crop of the Tan y grisiau Microgranite. Low-angle cleavage close to the microgranite is shown to be a local variant of the regional cleavage formed during the main deformation and not an earlier phase as previously supposed. Transcurrent movements along several major fault systems are also related to compression around the microgranite and the Harlech Dome block.     

The petrology and tectonic setting of Quaternary—Recent volcanic centres of Lombok and Sumbawa,Sunda arc

In the Sunda arc, only the Bali—Lombok—Sumbawa sector is apparently flanked both north and south by oceanic crust. South of Lombok Island the oceanic crust is probably of Early Cretaceous or Late Jurassic age, whereas the oldest rocks known from Lombok and Sumbawa islands are the Lower Miocene to Pliocene sediments and volcanics of the basement beneath the Quaternary—Recent volcanic centres.Three large active volcanoes form the northern parts of Lombok and Sumbawa. The volcanic rocks of Rindjani on Lombok belong to a basalt—andesite—dacite association, rich in plagioclase and hy- and Q-normative. East of Lombok, the volcanic rocks of Tambora and Sangeang Api on Sumbawa belong to a potassic ne—trachybasalt—trachy-andesite association. All three volcanoes occur only 150–190 km above the active north-dipping Benioff zone.Extinct Quaternary centres occur south of the active volcanoes on Sumbawa. Two of these centres, Soromundi and Sangenges, erupted markedly ne- and lc-normative leucitites together with andesites, dacites and trachybasalts.The volcanic composition—space—time relations in the Lombok—Sumbawa sector of the Sunda arc are not in accordance with the generalized island-arc schemata. Conventionally, potassic ne-mnormative island-arc associations are supposed to occur over the deep part of the Benioff zones, far from the trenches of mature island arcs. The SiO₂|K₂O relations of the Rindjani association are reasonably appropriate for a volcano overlying intermediate Benioff-zone depths, but both the Tambora and the Sangeang Api associations are far more potassic than would be predicted by generalized schemata, and also occur in a relatively young arc sector that apparently has developed only since Miocene time.Basalts, trachybasalts and leucitites from the Lombok—Sumbawa sector have been compared: at similar MgO contents and Mg/(Mg+Fe), the progression from hy- and Q-normative to ne- and lc-normative magmas is not marked by significant enrichment in TiO₂, Na₂O, Zr, Nb and P, but is accompanied by a substantial increase in K₂O, Rb, Sr and LREE, by increasing $\text{K}{}_2\text{O}\text{Na}{}_2\text{O}$ and by decreasing K/Rb.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from Rindjani (0.70386–0.70402) and Tambora (0.70385–0.70389) are very similar and among the lowest for the Sunda arc, but from Sangeang Api (0.70460–0.70500) are significantly higher and more variable in spite of the similar tectonic setting and petrological affinities. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of leucities tend to be higher (0.70488–0.70529).The petrogenesis of the volcanic associations of Lombok and Sumbawa cannot be readily explained. Although even the leucitites display the poverty in TiO₂ that generally characterizes volcanics from simple island-arc tectonic settings, there is very obvious uncoupling within the “incompatible elements”: enrichment in the LIL group (K, Rb, Sr but not Na) is not accompanied by similar behaviour in the group of small highly-charged ions (Ti, Zr, Nb, P). It has proved impossible to model this behaviour without invoking inhomogeneities in the source regions, both in mineralogy and in chemical composition. Similar uncoupling within the incompatible elements has also been reported from basalt groups from the Mid-Atlantic Ridge, may also occur in the Birunga province, and might not arise from processes unique to the island-arc environment.We suggest that a LIL-rich component is being progressively added to the source regions. This component could be incorporated by the crystallization of additional phases such as phlogopite or paragasite. If this component occurs deep within the mantle, it might gain passage to shallower regions either by percolating up the downgoing slab to yield the familiar arc magma zonation, or up substantial cross-arc fractures.     

The present flora and vegetation of the moraines of the Klutlan Glacier,Yukon Territory,Canada: A study in plant succession

The flora and vegetation of six ice-cored moraines of the Klutlan Glacier were analyzed in 65 plots by European plant-sociological techniques. The age of each plot was estimated from annual growth rings of shrubs or trees in the plots. Nine major vegetation types are distinguished: Crepis nana, Dryas drummondii, Hedysarum mackenzii, Hedysarum-Salix, Salix-Shepherdia canadensis, Picea-Salix, Picea-Arctostaphylos, Picea-Ledum, and Picea-Rhytidium. These contain plants aged 2–6, 9–23, 10–20, 24–30, 32–58, 58–80, 96–178, 177–240, and >163- >339 yr, respectively. Six other vegetation types are described from windthrow areas, drainage channels, volcanic tephra slopes, lake margins, fens, and drained lakes. The major vegetation types reflect a vegetational succession related to moraine age and stability, with the Crepis nana type as the pioneer vegetation developing through the other vegetation types to the Picea-Rhytidium type on the oldest moraines. Changes in species diversity and soil development, particularly humus accumulation, parallel the vegetational succession. This succession differs from patterns of revegetation of deglaciated landscapes in Alaska and British Columbia today and in Minnesota in late-Wisconsin times because of differences in climate, plant migration, and local ecology.