Continental tholeiites

The problem of crustal contamination is particularly acute for low mg continental flood basalts and smaller volume continental tholeiitic basalts, both of which have low trace-element concentrations (see Sections 3.03.3.2.3 and 3.03.3.3). The issue is less critical for many smaller volume continental rocks, such as kimberlites and alkali basalts, which have much higher abundances of many trace elements. As a result of their high strontium and neodymium content, for example, the isotopic compositions of these elements in kimberlites and alkali basalts are relatively insensitive to modification during crustal contamination. Conversely, the osmium and lead concentration of basaltic magmas are so low that these isotope systems are particularly vulnerable to modification by interaction with cmstal rocks (McBride et al, 2001 Chesley et al, 2002) hence these systems provide relatively sensitive indicators of crustal assimilation. 

Few samples of continental tholeiites were used in the construction of the Ti-Zr-Y fjiagram and continental flood basalts do not plot in the within-plate setting — field D of Figure 5.1 — (Holm, 1982 Duncan, 1987). 

Figure SJ5 The Ti02-Y/Nb discrimination diagram for basalts (adapted from Floyd and Winchester, 1975), showing die fields of alkafi basalts (Aik.), continental tholeiites (Cont thol) and MORE.

A plot of FI and F2 separates MORE, volcanic-arc basalts, shoshonites and within-plate basalts (ocean-island basalts and continental tholeiites) from each other (Figure 5.19a). A plot of F2 and F3 separates island-arc tholeiites, cak-alkali basalts... 

Holm P.E., 1982, Non-recognition of continental tholeiites using the Ti-Y-Zr diagram. Contn b. Mineral. Petrol.., 79, 308-310. 

Atlegre CJ, Dupre B, Richard P, Rousseau D (1982) Subcontinental versus suboceanic mantle, II. Nd-Sr-Pb isotopic comparison of continental tholeiites with mid-ocean ridge tholeiites, and the structure of the lithosphere. Earth Planet Sci Lett 57 25-34... 

Crescent formation are tholeiites, with light lanthanide depleted distributions, and appear to be ocean floor basalts thrust against the western edge of the continent. The upper basalts are tholeiites also, but with lanthanide distributions mildly enriched in light lanthanides, similar to Hawaiian tholeiites or continental tholeiites and flood basalts. 

The tholeiites (fig. 21.25) have lanthanide distributions like those of their younger counterparts and ranging from essentially flat as is characteristic of ocean floor basalts to slightly enriched in light lanthanides as found in some continental tholeiites. The more acid volcanic rocks and the granite-related rocks... 

Lustrino M, Melluso L, Morra V, Secchi F (1996) Petrology of Plio-Quatemary volcanic rocks from central Sardinia. Per Mineral 65 275-287 Lustrino M, Melluso L, Morra V (2000) The role of lower continental crust and lithospheric mantle in the genesis of Plio-Pleistocene volcanic rocks from Sardinia (Italy). Earth Planet Sci Lett 180 259-270 Lustrino M, Melluso L, Morra V (2002) The transition from alkaline to tholeiitic magmas a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy). Lithos 63 83-113... 

Petrologic and major element studies of Cen-ozoic continental basaltic rocks have dehned two primary magma series in intraplate basaltic igneous rocks alkalic, and tholeiitic (the latter comprising, along with calc-alkalic, the subalkalic series of Iddings (1892)). Alkalic... 

Figure 6 Major element compositions of alkali basalts and continental flood basalts (a) total alkali elements versus wt.% Si02- Dashed line separating alkaline from subalkaline (tholeiitic) basalts from Irvine and Baragar (1971) and (b) wt.% Ti02 versus wt.% MgO. Data from Tables 2 and 3.

Table 2(a) Average major element, trace element and isotope data for Cenozoic continental sodic alkali basalts and associated small volume tholeiitic basalts. 

Lightfoot P. C., Hawkesworth C. J., Hergt J., Naldrett A. J., Gorbachev N. S., Fedorenko V. A., and Doherty W. (1993) Remobilization of continental lithosphere by a mantle plume major-trace-element, and Sr-, Nd- and Pb-isotopic evidence from picritic and tholeiitic lavas of the Norils k District, Siberian Trap, Russia. Contrib. Mineral. Petrol. 114, 171-188. 

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. 

The different basalt fields are subdivided according to Ti/V ratio (Figure 5.10). MORB plote between Ti/V ratios of 20 and 50, although there is considerable overlap with the fields of continental flood basalt and back-arc basin basalts. Ocean-island and alkali basalts plot between Ti/V ratios of 50 and 100. Island-arc tholeiites plot between Ti/V ratios of 10 and 20 with a small overlap onto the field of MORB, Calc-alkali lavas plot with a near-vertical trend and with Ti/V ratios between 15 and 50. 

Lecolle (1989) This diagram has not yet been widely tested but offers another means of discriminating between different types of MORB. Elemental concentrations are plotted in ppm as La/10, YV15 and Nb/8 and the three main fields are further subdivided (Figure S.ll). Volcanic-arc basalts plot in field 1 and are subdivided into calc—alkali basalts (lA) and island-arc tholeiites (IQ. Field IB is where the two plot together. Field 2 characterizes continental basalts and field 2B may define continental back-arc tholeiites, although this subdivision is based upon a single... 

Granites in oceanic arcs dominated by tholeiitic basalt Granites in oceanic arcs dominated by calc-alkalt basalt Granites in active continental margins... 

Many of the trace element and isotopic characteristics compare more closely with those of continental crustal rocks than with typical mantle-derived materials, indicating the dominance of a crustal component in the petrogenesis of these tholeiites. 

The interpretation of Sr/ Sr ratios in continental tholei-ites is ambiguous. Presently available data on the Ferrar Group do not give strong support to models involving either selective contamination by crustal-derived components or generations of the tholeiites in a heterogeneous mantle. 

Basalts, basaltic andesites, and andesites with this distribution are common in some island arcs (e.g., Jakes and Gill, 1970 Ewart et al., 1973 Taylor et al., 1969). Their presence is believed to result from melting of subducted oceanic crust. By and large, the sediment layers which lie above the ocean floor tholeiites and are derived mainly from continental material are not subducted but piled up against continental margins in some manner that prevents their modifying significantly the trace element and isotopic abundances of oceanic crustal matter in the production of this class of island arc volcanics. Nor does ocean water severely modify the lanthanide distributions in volcanics that are extruded under... 

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