Adakite Research Paper

868 Words 4 Pages
Introduction

Adakites are volcanic rocks that are diagnostic of high temperature, high pressure conditions, (Stevenson, 2005). Their composition ranges from felsic to intermediate, and are characterized by assemblages of plagioclase, and often amphibole, and are identified by their unique geochemical signatures. Adakite is named Adak Island, Alaska, where it was first documented,(Defant and Drummond, 1990). They are often formed through the partial melting of a young subducting slab of oceanic basalt, in the presence of garnet. While adakites can be formed under a variety of conditions, they are typically found in volcanic arcs above the shallower parts of subduction zones. Due to its similarity to Archean trondhjemites, adakite
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Low-SiO2 adakite (LSA) contains on average 56.25% SiO2, while high-SiO2 adakite (HSA) has an average of 64.80% SiO2. Adakite can range from peraluminous to metaluminous in composition, with both LSA and HSA having >15wt% Al2O3, (Martin et al. 2005). Na2O is typically >3.5wt%. Magnesium content is variable. Over 75% of adakites have MgO <4 wt%, while a high magnesium variety has been recorded with MgO >6 wt%, (Defant and Drummond, 1990). Adakite always contains an abundance of plagioclase. With the exception of high magnesium adakite, amphibole is also a common component. Clinopyroxene, othopyroxene, biotite, and opaques may also be present, but ultimately plagioclase and amphiboles compose the primary adakite mineral assemblage, (Defant and Drummond, …show more content…
Kay has suggested that high-Mg adakiteis the product of dacitic magma which has mixed with the peridoite under the hydrous conditions of the overlying hot mantle. This new equilibrium between the melted slab, and the mantle wedge would result in an andesitic composition, representative of the high-Mg adakite that has been observed. The reactions with the partially melted peridotite will lead to higher Y concentrations, and subsequently lower Sr/Y ratios in the adakite. This hypothesis is supported by the loose correlation between Mg-rich andesites and adakite, although the MgO content of these andesites is considerably higher (>10 wt %) than those found even in Mg-rich adakite, which is seldomly higher than 6 wt%. This partial melting of the peridotite mantle cannot be attributed to low-Mg adakite, due to their nature of being low in MgO, (Defant and Drummond,

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