Lithium, boron and chloride in volcanics and greywackes in Northland, Auckland and the Taupo Volcanic Zone

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Reyes, A.G.; Trompetter, W.J. 1998 Lithium, boron and chloride in volcanics and greywackes in Northland, Auckland and the Taupo Volcanic Zone. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 98/21 39 p.

Abstract: During magmatic differentiation processes B preferentially partitions into the glassy mesostasis of rhyolite and andesite. The behaviour of Li, on the other hand, varies with the silica content of the rock, residing in hornblende and biotite in rhyolite and staying in the glassy matrix in andesite. Lithium, B, Cl and water contents increase proportionally with the silica concentration of the volcanic rocks. Their relative proportions in andesites of the Taupo Volcanic Zone (TVZ) appear to reflect the nature of the underlying crust, the dip of the subducting slab and hence the depth and temperature of magma generation. Because of the proximity in the TVZ of the rhyolite-dominated back-arc, to the andesitic arc, trace elements in rhyolites are probably also affected by the subducting slab. This may be the reason why the relative Li, B and Cl contents of rhyolites overlap those of andesite. The B/Li ratios of rhyolites associated with the northern Rotorua and Okataina eruptive centres yield lower B/Li ratios than those from Maroa and Taupo centres in the south, where the slab subducts at a shallower angle. The MORB compositions of the basalts of both Northland-Auckland and the TVZ back-arc are contaminated by altered oceanic crust, oceanic sediments and continental crust including the Torlesse Supergroup greywackes, and remnant subduction material in the upper mantle, resulting to high B/Li ratios. Variations in the amounts and type of assimilated material produce basalts that have higher B/Li ratios in Northland-Auckland than in the TVZ. Apparently, volcanics associated with a younger subduction event as in the TVZ, contain and retain more Cl, yielding lower Li/Cl ratios for the TVZ than Northland-Auckland basalts. The B/Li ratio of greywackes from the Torlesse terrane (<1.4) is lower than that of the Waipapa terrane (>1.4) due to the presence of high-Li detrital tourmaline and granitic materials in the Torlesse terrane, on one hand, and the abundance of intermediate volcanic fragments within the Waipapa terrane, on the other. In geothermal wells in Ngawha, hydrothermal alteration yields higher B/Li ratios of >2.8 for Waipapa terrane sedimentary rocks, suggesting that thermal fluids, at least at Ngawha, add B during alteration. The Li/Cl ratios for average South and North Island greywackes are similar and may reflect similar degrees of metamorphism. In general, the relative Li, B and Cl contents in greywackes are dictated by the composition of the detrital fragments, the clay fraction, the type of clays and the metamorphic grade. During hydrothermal alteration of rhyolite in the TVZ, Cl always partitions into solution while Li and B have an affinity for the rock. However, more Li remains in the rock than B at any given temperature. The distribution coefficients of Li and B between water and rock increase with increasing temperature. The partitioning of Li between rock and solution in TVZ hydrothermal systems is mainly dictated by temperature, where as the mass distribution coefficient for B is related to the tectonic setting. It is highest for the rift-type Wairakei and lowest for the arc-type Kawerau. An increase in relative Li of the rock is associated with the formation of secondary quartz and chlorite. Boron enrichment is due to the presence of smectites, interlayered clays and leucoxene, onto which it is largely adsorbed. The decrease in the B/Cl ratio of altered rocks with temperature reflects the decrease in phyllic and argillic alteration and an increase in propylitization with temperature. The latter involves recrystallisation and a mere reshuffling of chemical constituents among hydrothermal silicates while the former is associated with rock destruction and the formation of clays. Boron is largely adsorbed onto clays hence the high B/Cl ratio of altered rocks at low temperatures. Changes in B/Li ratios with temperature in altered rhyolites are the result of the dissolution of primary followed by the formation of secondary minerals. The type and abundance of secondary minerals formed are then dictated by the composition of the altering fluid, formation permeability and temperatures. (auth)