Paleoproterozoic (1.90–1.86 Ga) arc volcanism in the Flin Flon Belt,Trans-Hudson Orogen,Canada

Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO₂), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial _Nd=–0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial _Nd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation.     

Confronting scale in watershed development in India

The issue of scale is examined in the context of a watershed development policy (WSD) in India. WSD policy goals, by improving the natural resource base, aim to improve the livelihoods of rural communities through increased sustainable production. It has generally been practiced at a micro-level of less than 500?ha, as this was seen to be a scale that would encourage participative management. There has been some concern that this land area may be too small and may lead to less than optimal hydrological, economic and equity outcomes. As a result there has been a move to create guidelines for meso-scale WSD of above 5,000?ha in an endeavour to improve outcomes. A multidisciplinary team was assembled to evaluate the proposed meso-scale approach. In developing an adequate methodology for the evaluation it soon became clear that scale in itself was not the only determinant of success. The effect of geographical scale (or level) on WSD is determined by the variation in other drivers that will influence WSD success such as hydrological conditions, land use and available institutional structures. How this should be interpreted at different levels in the light of interactions between biophysical and socio-economic scales is discussed.     

A study of surface features relating to brittle and semibrittle fracture

Many distinct types of fracture surface markings are found in a wide variety of materials. Those found in rocks can be classified as hackle and rib markings and further sub-divided according to geometrical structure. The geometry, occurrence, micro- and macrostructure of hackle markings are examined in detail. It is found that hackle markings consist of a series of microfractures whose orientations vary according to zonal relationships and that their geometry is controlled by the properties of the interfaces (e.g., bedding).