Cratons and continental evolution

 

 

Roberta L. Rudnick

 

Department of  Geology, University of  Maryland, College Park, MD 20742, USA. rudnick@geol.umd.edu

 

 

Abstract

 

Cratons are regions of stability that have persisted for billions of years.  Archean cratons are characterized by the presence of thick, chemically buoyant mantle keels, which extend to depths of 200-300 km.  Studies of cratonic mantle xenoliths reveal that these keels are composed of cold, highly refractory peridotites, which represent residua from high-degree partial melting.  It is the thickness and high viscosity of these keels that are believed to contribute to craton stability, insulating cratons from disruption by tectonic forces and the underlying convecting mantle.  A number of unanswered questions exist regarding the formation of cratons, including:  1) in which tectonic setting do these keels form?, 2)  where are the komatiitic melts that are the complement to the refractory, peridotitic residues?, 3) do all Archean regions posses mantle keels, and 4) how can these keels, once formed, be disrupted?  Contrasting cratonic evolutionary histories are provided by the Tanzanian craton, east Africa, and the North China craton.  In east Africa, the Tanzania craton developed a lithospheric keel around the time of cratonization, at 2.8 Ga.  To this day, the craton acts as an island of stability, despite Proterozoic collisional orogeny on its margins and Cenozoic rifting.  In contrast, the north China craton also developed a lithospheric keel in the Archean.  However, this keel was widely disrupted during the Mesozoic, leaving the eastern block of the North China craton a seismically and magmatically active area with high surface heat flow and producing world-class Phanerozoic ore deposits.

 

Keywords:  Archean craton, mantle keel, peridotite, xenolith, delamination

Cratons are regions of prolonged stability (³ 1 Ga) within the continents and so, by definition, cratons are Precambrian in age. Indeed, most formed in the Archean.  Some first-order observations about Archean cratons include:

 

1.             They generally occur in the center of continental masses, surrounded by younger fold belts

2.             Surface heat flow is typically low within cratons, averaging 41 ± 11 mW m^-2 (Morgan, 1984; Nyblade & Pollack, 1993)

3.             Cratons are underlain by regions of fast P- and S-velocities, extending to depths of at least 200-250 km, perhaps deeper (Jordan, 1975; James et al., 2001; Gung et al., 2003)

4.             Kimberlites from Archean cratons carry diamonds that are interpreted to be derived from the lithospheric mantle, consistent with the presence of a cool and thick keel (Boyd & Gurney, 1986)

5.             Mantle xenoliths from craton-penetrating kimberlites are highly refractory peridotites, which experienced large degrees of melt extraction (Boyd, 1989), followed by variable degrees of metasomatic overprinting (Erlank et al., 1987)

6.             Osmium isotope investigations of cratonic peridotites demonstrate that the lithospheric keel formed coincident with the overlying crust (at least within the precision of Os models ages, e.g., ± 200 Ma) (Pearson et al., 1995a; Pearson et al., 1995b; Chesley et al., 1999; Hanghoj et al., 2001; Irvine et al., 2003)

 

Thus, in general, Archean cratons are characterized by cold, thick lithosphere whose density is offset by its refractory composition, giving rise to chemical buoyancy (Jordan’s tectosphere hypothesis).  The lack of high elevation or free-air gravity anomalies over cratons suggest they are in isostatic equilibrium and have a density profile that matches those of off-craton regions (i.e., Jordan’s isopycnic hypothesis is generally correct).

Despite these well-established features of cratons, a number of fundamental questions regarding their origin and their role in continental dynamics remain unresolved. 

 

How did thick, refractory mantle keels form?

If the refractory peridotite xenoliths that are carried in cratonic kimberlites are representative of the lithospheric keel, and if these peridotites formed as residues of a single melting event, they represent residues of up to 40% melt extraction (Lee and Rudnick, 1999; Walter, 1999).  Thus the equilibrium melt would have komatiitic composition.  If the peridotitic keel is at least 180 km thick, the amount of complementary komatiite melt would have a thickness of ~70 km.  Clearly, such large volumes of komatiite are not present in Archean cratons.  The scarcity of complementary melt requires separation of the melts and residues followed by efficient recycling of the melts, leaving the buoyant residue beneath the craton.  The whereabouts of the complementary melts and how they were removed from their residue is a major outstanding issue related to craton formation.  Some have suggested that the melts formed at high pressure where they were denser than the residue and thus never rose to the surface (Nisbet & Walker, 1982; Boyd, 1989).  Another possibility includes extrusion of the komatiitic melts in an ocean basin followed by tectonic accretion of the residues during convergent margin recycling. 

 

Are all Archean regions underlain by cratonic keels? 

Another relevant question is whether all regions of Archean crust developed with a thick, underlying lithospheric keel.  Continental crust that formed without such a stabilizing keel may have been prone to recycling and hence lost from the geologic record (e.g., Bowring & Housh, 1995).  Recently, Os isotope investigations of the Mojavia Block in the SW US showed that this block is underlain by Archean-aged mantle lithosphere, but that this lithosphere is more similar to post-Archean mantle lithosphere than the refractory mantle found beneath cratons (Lee et al., 2001).  The relatively higher density of this type of mantle lithosphere limits the thickness it can attain and hence may make it more easily deformed during continental orogeny, as witnessed by the multiply deformed and overprinted Mojavia block.  It remains unclear, however, how much Archean continental crust may have formed in the absence of a thick tectospheric keel and whether other examples of non-tectospheric Archean lithosphere persist today.

 

Once formed, do cratons last forever?  Contrasting views from Tanzania and Eastern China

The thick, chemically buoyant mantle keel beneath Archean cratons is considered to be mechanically strong, in part because it is likely to be anhydrous (Pollack, 1986), and thus contributes significantly to the relative stability of cratons.  The presence of this strong lithospheric keel has given rise to the notion that cratons may be impossible to disrupt.  Indeed, even cratons caught in the middle of a continental rift, such as the Tanzanian craton in East Africa, appear to retain their coherency, forcing extensional deformation to their margins (Nyblade & Brazier, 2002). 

However, the eastern block of the North China craton is an exception.  Here, a thick lithospheric keel developed in the Archean (Gao et al., 2002) and persisted at least through the Paleozoic, as Ordovician kimberlites carry cratonic peridotites and diamonds (Menzies et al., 1993; Griffin et al., 1998).  Beginning in the Mesozoic, the eastern block of the craton became destabilized, as witnessed by the onset of voluminous Mesozoic magmatism.  Some of thse Mesozoic magmas are high Mg andesites and dacites (and their intrusive equivalents) and have unusual chemical compositions (high Ni and Cr, strongly fractionated REE with (La/Yb)_n up to 30), similar to those attributed to slab melts (Gao et al., 2004).  However, unlike slab melts, the North China magmas are isotopically evolved and carry Archean and Proterozoic zircon xenocrysts.  These magmas are interpreted to represent melts of mafic lower crust from the north China craton that foundered into the mantle after conversion to eclogite during Mesozoic collisions (Gao et al., 2004).  The evolved Nd and Sr isotopes and ancient zircons are thus inherited from their source region, while the high Mg#, Ni and Cr resulted from interaction of the magmas with mantle peridotite during their ascent.

Today, the eastern block of the North China craton is characterized by high heat flow, active seismicity and is underlain by thin and relatively hot mantle lithosphere that is indistinguishable from modern convecting mantle (Menzies et al., 1993; Griffin et al., 1998; Gao et al., 2002).  That is, this block is in no way “cratonic”.  The reasons for the destabilization of the eastern block of the North China craton remain enigmatic. Hypotheses are many and varied and include: delamination of the
 

cratonic lithosphere associated with the Triassic collision of the Yangtze craton with the North China craton its southern margin (the same event responsible for creation of the UHP metamorphic rocks), and collision during the Solonker orogen on the northern margin of the craton; disruption of the cratonic lithosphere due to initiation of subduction of the Pacific plate; and continental-scale rifting, initiated in the Mesozoic.  This interesting area is in need of continued, multidisciplinary study in order to unravel its development.

 

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