Continental alkali basalts

Figure 5 Average Ce/Pb and Nb/U from selected kimberlites, continental alkali basalts, and continental flood basalts. Present-day continental crust values from Condie (1993), and MORE and OIB values from Hofmann (1997)...

Figure 7 Initial 8n(J versus Sr/ Sr for continental alkali basalts, (a) Sodic alkali basalts, (b) High-Ti potassic alkali basalts. Present-day MORE, EMI, EM2, HIMU mantle sources from Hofmann (1997). (c) Low-Ti potassic alkali basalts. EA = European astbenospberic reservoir (data from Table 2).

Geochemical studies of aUcah basalts have been aided over the past 10-15 yr by an explosion in the hterature of ICP-MS-based trace-element and radiogenic-isotope data for continental alkali basalts. Representative chemical analyses of the two main aUcah basalt groups, sodic and potassic (KjO/NajO <1 and 1, respectively), are listed in Tables 2(a) and (b), respectively, derived from the average composition of lavas at each locality having <51 wt.% SiOa and >5 wt.% MgO. No attempt was made to account for crystal... 

Fitton J. G. and Dunlop H. M. (1985) The Cameroon Line, West Africa, and is bearing on the origin of oceanic and continental alkali basalt. Earth Planet. Sci. Lett. 72, 23-38. 

Eorbes RB (1963) Ultrabasic inclusions from the basalts of the Hut Point area, Ross Island. Bull Volcanol 26 13-21 Eorbes RB, Turner DL, Carden JR (1974) Age of trachyte from Ross Island, Antarctica. Geol 2 297-298 Euta K, LeMasurier WE (1983) Nd and Sr isotopic studies on Cenozoic mafic lavas from West Antarctica Another source for continental alkali basalts. Contrib. Mineral. Petrol. 83 38-44... 

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.

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. 

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

Continental Extrusive Igneous Rocks High-Ti alkali basalts... 

Mantle xenoliths are fragments of the Earth s mantle, usually a few centimeters across, brought rapidly from great depths during volcanism to arrive at the surface in their unmodified state. They represent a very important source of information about the mantle. They are most commonly found in kimberlites and alkali basalts. The most abundant xenoliths are from kimberlites, which sample the deep mantle beneath ancient continental crust. Alkali basalts sample shallower mantle from beneath both continents and oceans. As will be seen below, the mineralogical... 

Oceanic-arc — dominated by tholeiltic basalts Oceanic-arc — dominated by calc-alkali basalts 5-ActivB continental margin , , ... 

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... 

Figure SJl The La/lO-Y/lS-Nb/8 discrimination diagram for basalts (after Cabanis and LecoUe, 1989). The plotting coordinates are shown around the margin of the diagram the broken lines are drawn normal to the sides of the triangular diagram. Field 1 contains volcanic-arc basalts, held 2 continental basalts and held 3 oceanic basalts. The subdivisions of the fields arc as follows I A, calc-alkali basalts 1C, volcanic-arc rholeiites IB is an area of overlap between lA and 1C 2A, continental basalts 2B, back-arc basin basalts (although this is less well defined) 3A, alkali basalts from intercontinental rift 3B, 3C, E-type MORB (3B enriched, 3C weakly enriched), 3D, N-type MORB,...

The Nb/Y-Zr/(P205 X 10 000) discrimination diagram for basalts (adapted from Floyd and Winchester, 1975), showing the helds of continental and oceanic alkali basalt and tholeiidc basalts. The boundary line and plotting coordinates are taken 6rom Winchester and Floyd (1976—Figure 11). 

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... 

Fig. 21.10. Comparison diagram for highly differentiated, continental, alkali-rich rocks. No. 1 is a Norwegian carbonatite (Loubet et al., 1972), 2 an African kimberlite (Fesq et al., 1975), 3 and 6 alkali-rich American basalts (Kay and Gast, 1973), 4 an alkali carbonate, and 5, 6, and 8 alkali-rich lavas from Africa (Gerasimovskiy et al., 1972). These examples were chosen for their extremely differentiated lanthanide distributions and are richer in lanthanides than more average materials of their rock classes.

Kimberlites and carbonatites rise to the crustal surface from great depths, often with unusually explosive violence. It has been suggested that alkali basalts come from depths intermediate to those of kimberlites and continental or island tholeiites. Thus, there appears to be a trend of increasing light lanthanide enrichment of lavas with increasing depth of source. This apparent trend cannot yet be interpreted in terms of mantle composition in any general way. The relative importance of hypothesized gas extraction of lanthanides and crystal-liquid equilibria for producing severely fractionated lanthanide distributions is unknown. 

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