Tectonic setting basalts

The summary of the bulk chemical compositions (major elements, minor elements, rare earth elements), Sr/ Sr (Farrell et al., 1978 Farrell and Holland, 1983), microscopic observation, and chemistry of spinel of unaltered basalt clarifies the tectonic setting of Kuroko deposits. Based on the geochemical data on the selected basalt samples which suffered very weak alteration, it can be pointed out that the basalt that erupted almost contemporaneously with the Kuroko mineralization was BABB (back-arc basin basalt) with geochemical features of which are intermediate between Island arc tholeiite and N-type MORE. This clearly supports the theory that Kuroko deposits formed at back-arc basin at middle Miocene age. 

Bulk rock chemistry of hydrothermally altered midoceanic ridge basalt has been well studied and used to estimate the geochemical mass balances of oceans today (Wolery and Sleep, 1976 Humphris and Thompson, 1978 Mottl, 1983). In contrast, very few analytical data on hydrothermally altered volcanic rocks that recently erupted at back-arc basins are available. However, a large number of analytical data have been accumulated on the hydrothermally altered Miocene volcanic rocks from the Green tuff region in the Japanese Islands which are inferred to have erupted in a back-arc tectonic setting (section 1.5.3). 

Basaltic Rocks from Different Tectonic Settings... 

Morra V, Secchi F A, Melluso L, Franciosi L (1997) High-Mg subduction-related Tertiary basalts in Sardinia, Italy. Lithos 40 69-91 Morris J, Ryan J, Leeman WP (1993) Be isotope and B-Be investigations of the historic eruptions of Mt. Vesuvius. J Volcanol Geotherm Res 58 345-358 Morrison GW (1980) Characteristics and tectonic setting of the shoshonite rock association. Lithos 13 97-108... 

Sobolev and Chaussidon (1996) summarize an extensive body of data on olivine-hosted melt inclusions from basalts from a variety of tectonic settings. They concentrated on inclusions hosted in magnesium-rich olivines, with the... 

Jones G., Sano H., and Valsamijones E. (1993) Nature and Tectonic setting of accreted basalts from the Mine terrane central Japan. J. Geol Soc. London 150, 1167-1181. 

In each of the three tectonic settings discussed above - convergent margins, within-plate (oceanic and continental crust) and at rifted margins - the dominant flux from the mantle to the continental crust is basaltic. This however poses a major problem, for the continental crust is not basaltic in composition, rather, as will be shown below (Table 4.2), the average bulk composition of the continental crust is andesitic. This paradox is one of the... 

The evidence from experimental studies, geochemical and thermal modeling outlined above, converges to support the view that the most probable mechanism for TTG genesis is the partial melting of hydrated basalt. What is not agreed, however, is the tectonic setting in... 

An excellent example of the use of discriminant analysis in igneous petrology is found in the work of J.A. Pearce (1976), who employed this technique in an attempt to classify basalts on the basis of their major element chemistry (see also Section 5.2.2). The study is based upon a collection of recent basalts taken from six different tectonic environments — ocean-floor basalts, tsland-arc tholeiites, calc-alkaline basalts, shoshonites, ocean-island basalts and continental basalts. The objective of the study was to see if there is a relationship between major element chemistry and tectonic setting. 

There are a large number of discrimination diagrams applicable to basalts and basaldc andesites which use trace elements, major and minor elements and the mineral clinopyroxene. These are considered in turn. Table 5.1 classifies the different types of basalt according to tectonic setting and shows which diagrams might be useful in their identification. 

Table 5J Discrimination dia ams which may be used to determine the tectonic setting of basalts...

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