Archeans cratons

Labrador (Fig. 1) contains sections of the Superior and North Atlantic Archean cratons, separated by a wide tract of Paleoproterozoic rocks assigned to the... 

Direct evidence for the compositional effects of partial melt extraction is preserved in samples of upper-mantle lithosphere with a range of ages, including Archean cratonic mantle, Proterozoic subcontinental mantle, and modern oceanic mantle. Samples of upper mantle are collected as xenoliths, peridotites dredged from oceanic fracture zones, and slices of upper mantle tectonically exposed at the surface, and extensive samples exist from both oceanic and continental settings (see Chapters 2.04 and 2.05). Here, data sets are assembled for oceanic and subcontinental mantle lithosphere, and compositional trends are compared to those predicted for partial melt extraction from fertile peridotite in order to deduce the role that melt extraction has played in producing compositional variability in upper-mantle lithosphere, and to place constraints on the thermal evolution of the mantle. 

Modern oceanic mantle is defined solely by abyssal peridotites, which are samples of harzbur-gite and Iherzolite collected from fracture zones at oceanic spreading centers, and these samples are representative of shallow oceanic lithosphere that has been processed at mid-ocean ridges (see Chapter 2.04). Two types of continental mantle lithosphere are considered (i) cratonic mantle, which refers to xenoliths collected from kimberlites that sample portions of mantle beneath stable, Archean cratons and (ii) off-craton mantle, which refers to xenoliths collected from alkalic basalts that have sampled portions of the subcontinental mantle adjacent to ancient cratonic mantle (see Chapter 2.05). Also included with off-craton lithosphere are orogenic Iherzolites and ophiolites, which are slices of mantle tectonically emplaced typically at convergent margins. 

Tarney (1984) and Shaw et al. (1994) may be representative of evolved lower crust in Archean cratons lacking a high-velocity lower crust, but are unlikely to be representative of the global continental lower crust (Archean cratons constitute only —7% of the total area of the continental crust (Goodwin, 1991)). 

Of all the estimates in Table 9, that of Taylor and McLennan (1985, 1995) stands out as being the most mahc overall (Figures 15 and 16). This mahc composition stems from their model for Archean crust, which constimtes 75% of their crust and is composed of a 2 1 mixture of mafic-to felsic-igneous rocks. This relatively mafic crust composition was necessitated by their inferred low heat production in Archean crust and the inferred large proportion of the Archean-aged crust. However, such a high proportion of mafic rocks in the Archean crust is at odds with seismic data (summarized in Section 3.01.3), which show that the crust of most Archean cratons is dominated by low velocities, implying the presence of felsic (not mafic) compositions, even in the lower crust. In addition, some of the... 

Horstwood M. S. A., Nesbitt R. W., Noble S. A., and Wilson J. F. (1999) U-Pb zircon evidence fo an extesive early Archean craton in Zimbabwe a reassessment of the timing of craton formation, stabilization, and growth. Geology 27, 707-710. 

Another curious feature of A-types is their uneven distribution throughout the geological record. Rocks of this nature are encountered in Archean cratons (Section 3.11.3.2.1), though they are far subordinate to the TTG and other granitic types. Similarly, A-type magmas are volumetri-cally minor in Phanerozoic orogenic belts they comprise only 0.6% of the vast granitic batholiths of the LFB (Chappell et al., 1991), discussed in Section 3.11.4.4. 

Isotopic studies have established that the continental crust is old, in that more than half of it was formed by the end of the Archean. The distinctive igneous components of Archean cratons, coupled with evidence that the crust at this time was overall less mafic, provide clues that crustal growth mechanisms during this period... 

World annual production of natural diamonds, the cubic form of carbon, is about 110 million carats (1 carat = 200 mg). Almost all is derived from kimberlite or its weathered remnants, but Australian production is from the Argyle mine, at which the host rock is lamproite. Kimberlites are olivine- and volatUe-rich potassic ultrabasic rocks of variable geological age that typically form near-vertical carrot-shaped pipes intmded into Archean cratons. The volatile-rich component is predominantly CO2 in the carbonate minerals calcite and dolomite, and the texture is characteristically inequigranular, with large grains (macrocrysts), usually of olivine [Mg2Si04], in a fine-grained, olivine-rich matrix. 

Ballard, S., Ill Pollack, H. N. 1988. Diversion of heat by Archean cratons a model for Southern Africa. Earth and Planetary Science Letters, 85(1-3), 253-264. 

Carlson, R. W., Grove, T. L., de Wit, M. J. Gurney, J. J. 1996. Anatomy of an Archean craton a program for interdisciplinary studies of the Kaapvaal craton, southern Africa. EOS Transactions, American Geophysical Union, Tl, 273—277. 

Schmitz, M. D., Bowring, S. A. Robey, J. v. A. 1998. Constraining the thermal history of an Archean craton U-Pb thermochronology of lower crustal xenoliths from the Kaapvaal craton, southhem Africa. In 7th International Kimberlite Conference, Extended Abstracts. University of Cape Town, Cape Town, 766-768. 

Kay, I., Musacchio, G., White, D. 6 others 1999a. Imaging the Moho and Fp/Ks ratio in the Western Superior Archean craton with wide angle reflections. Geophysical Research Letters, 26, 2585-2588. 

Kay, I., Sol, S., Kendall, J.-M. 5 others 19996 Shear wave splitting observations in the Archean craton of Western Superior. Geophysical Research Letters, 26, 2669-2672. 

Padgham, W. a. Fyson, W. K. 1992. The Slave Province a distinct Archean craton. Canadian Jourrml of Earth Sciences, 29, 2072-2086. 

Fedo, C. M. Eriksson, K. A. 1996. Stratigraphic framework of the c. 3.0 Ga Buhwa greenstone belt a unique stable-shelf succession in the Zimbabwe Archean craton. Precambrian Research, 77, 161-178. 

The margin of an Archean craton structural relationships between the Zimbabwe craton and the Zambezi orogenic belt. Precambrian Research, in press. 

Kusky, T. M. Polat, A. 1999. Growth of granite-greenstone terranes at convergent margins, and stabilization of Archean cratons. Tectonophysics, 305, 43-77. 

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