Lower continental crust

In contrast to the southern volcanic zone, Parinacota volcano lies on very thick continental crust (> 70 km) in the central volcanic zone of Chile. Bourdon et al. (2000a) showed that young Parinacota lavas encompass a wide range of U-Th disequilibria. excesses were attributed to fluid addition to the mantle wedge but °Th-excesses in lavas from the same volcano are more difficult to explain. The lavas with °Th-excesses also have low ( °Th/ Th) (< 0.6) characteristic of lower continental crust characterized by low Th/U and in their preferred model. Bourdon et al. (2000a) attributed the °Th-excesses to contamination by partial melts, formed in the presence of residual garnet, of old lower crustal materials. 

Like the upper continental crust, the lower portion of the continental crust probably consists of heterogeneous distributions of various metamorphic and igneous rocks (Sims, Newsom and Gladney, 1990). Available chemical data from rock samples suggest that the lower continental crust is depleted in arsenic when compared with the upper crust (Sims, Newsom and Gladney, 1990,303-304). Specifically, Wedepohl (1995) estimated the average arsenic concentration of the lower continental crust at 1.3 mg kg-1, which is somewhat lower than his 2.0 mg kg-1 of arsenic for the upper crust (Table 3.3). For the continental crust as a whole, Wedepohl (1995, 1220) obtained an arsenic value of 1.7 mg kg-1. 

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... 

This means that the lead paradox is alive and well, and the search for the unradiogenic, hidden reservoir continues. The lower continental crust remains (in the author s opinion) a viable candidate, even though crustal xenolith data appear to be, on the whole, not sufficiently unradiogenic (see review of these data by Murphy et al. (2003)). It is not clear how representative the xenoliths are, particularly of the least radiogenic, Precambrian lower crust. Another hypothetical candidate is a garnetite reservoir proposed by Murphy et al. (2003). 

Quick J. E., Sinigoi S., and Mayer A. (1995) Emplacement of mantle peridotite in the lower continental crust, Ivrea-Verbano Zone, Northwest Italy. Geology (Boulder) 23, 739-742. 

In this chapter we review the composition of the upper, middle, and lower continental crust (Sections 3.01.2 and 3.01.3). We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes (Sections 3.01.4 and 3.01.5). Finally, we compare the Earth s crust with those of the other terrestrial planets in our solar system (Section 3.01.6) and speculate about what unique processes on Earth have given rise to this unusual crustal distribution. 

Figure 13 Comparison of different models of the trace-element composition of the lower continental crust. All values normalized to the lower-crust composition of Rudnick and Fountain (1995), which is adopted here as the best estimate of the global lower crust. Gray-shaded field represents 30% variation from this value. Trace elements are divided into the following groups (a) transition metals, (b) high-field strength elements, (c) alkali, alkaline earth, and...

Table 7 Compositional estimates of the lower continental crust. Major elements in weight percent. 

Based on seismic investigations the crust can be divided into three regions upper, middle, and lower continental crust. The upper crust is... 

Broadhurst J. R. (1986) Mineral reactions in xenoliths from the Colorado Plateau implications for lower crustal conditions and fluid composition. In The Nature of the Lower Continental Crust, Geol. Soc. Spec. Publ. (eds. J. B. Dawson, D. A. Carswell, J. Hall, and K. H. Wedepohl). Eondon, vol. 24, pp. 331-349. 

Coherson K. D., Hearn B. C., MacDonald R. A., Upton B. F., and Park J. G. (1988) Granulite xenoliths from the Bearpaw mountains, Montana constraints on the character and evolution of lower continental crust. Terra Cognita 8, 270. 

Downes H. (1993) The nature of the lower continental crust of Europe petrological and geochemical evidence from xenoliths. Phys. Earth Planet. Inter. 79(1-2), 195-218. 

Kay R. W. and Kay S. M. (1981) The nature of the lower continental crust inferences from geophysics, surface geology, and crustal xenoliths. Rev. Geophys. Space Phys. 19, 271-297. 

McCulloch M. T., Arculus R. J., Chappell B. W., and Ferguson J. (1982) Isotopic and geochemical studies of nodules in kimberlite have implications for the lower continental crust. Nature 300, 166-169. 

Rogers N. W. (1977) Granulite xenoliths from Lesotho kimberlites and the lower continental crust. Nature 270, 681-684. 

Rudnick R. L. (1992) Xenoliths—samples of the lower continental crust. In Continental Lower Crust (eds. D. M. Fountain, R. Arculus, and R. W. Kay). Elsevier, Amsterdam, pp. 269-316. 

Sachs P. M. and Hansteen T. H. (2000) Pleistocene underplating and metasomatism of the lower continental crust a xenolith study. J. Petrol. 41(3), 331-356. 

Villaseca C., Downes H., Pin C., and BarberoL. (1999) Nature and composition of the lower continental crust in central Spain and the granulite-granite linkage inferences from granulitic xenoliths. J. Petrol. 40(10), 1465-1496. 

Kay R. W. and Mahlburg Kay S. (1991) Creation and destruction of lower continental crust. Geol. Rundsch. 80, 259-278. 

It has also been recognized that much of the lower continental crust worldwide is mafic in composition (Rudnick, 1992). This leads to the... 

Figure 2 Primitive-mantle normalized minor and trace-element diagrams for (a) the upper, middle, bulk, and lower continental crust (values from Table 1), and (b) oceanic and island arc basalts and the bulk continental crust (all normalizing values are from McDonough and Sun, 1995). The oceanic basalts (N-MORB, normal mid-ocean ridge basalt and OIB, ocean island basalt) are from Sun and McDonough (1989), whereas the arc basalts are from Turner et al. (1997) (Tonga-Kermadec arc) and Pearce et al. (1995) (South Sandwich arc).

Arndt N. T. and Goldstein S. L. (1989) An open boundary between lower continental crust and mantle its role in crust formation and crustal recycling. Tectonophysics 161, 201-212. 

Turcotte D. L. (1989) Geophysical processes influencing the lower continental crust. In Properties and Processes of Earth s Lower Crust, Geophys. Monogr. Ser. 51 (eds. R. F. Mereu, S. Mueller, and D. M. Fountain). American Geophysical Union, Washington, DC, pp. 321 -329. 

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