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PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 369

2.0 72
Phanerozoic
subduction 1 4
zones
1.5 Eclogite 3 7 High 54
Eclogite
pressure
6
Pressure (GPa) 1.0 Blueschist Amph 8 5 granulites 36 Depth (km)
Blueschist

0.5 2 Granulite 18
Greenschist

Ky
0 And Sill
0 400 800 1200
Temperature (°C)
Fig. 11.18 Pressure–temperature plot showing the published paths of the moderate temperature eclogite and high-
pressure granulite facies metamorphic rocks older than 1.5 Ga (after Collins et al., 2004, with permission from Elsevier).
1, Usagaran Belt, Tanzania; 2, Hengshan Belt, China; 3, Sanggan Belt, China; 4, Ubendian Belt, Tanzania; 5, Jianping Belt,
China; 6, Sare Sang, Badakhshan Block, Afghanistan; 7, Snowbird tectonic zone, Canada; 8, Lapland Granulite Belt. Thick
arrows refer to Usagaran/Ubende eclogites. Field for Phanerozoic subduction zone metamorphism and metamorphic
facies indicated by thick gray curves. Aluminosilicate polymorph fields plotted for reference. And, andalusite; Ky, kyanite;

Sill, sillimanite; Amph, amphibolite.

tonic processes similar to those operating during the Together these observations strongly suggest that
Phanerozoic (de Wit, 2004; Parman et al., 2004). Some plate tectonic mechanisms became increasingly impor-
Archean greenstone belts have been interpreted as tant after the Late Archean. Most authors link this
ophiolites, although these interpretations typically are development to the increased stability of continental
controversial. The oldest unequivocal examples are crust during this Eon (Section 11.4.2). Nevertheless, it
Early Proterozoic in age and support interpretations is important to realize that many uncertainties about
that seafloor spreading and associated ocean crust for- Proterozoic tectonics remain. The age of formation

mation was an established mechanism of plate tecton- of the rock units and the timing of regional meta-
ics by this time. One of the best preserved and least morphism, deformation, and cooling are poorly
equivocal of the Early Proterozoic examples is the Pur- known in many regions. The sources of magmatism
tuniq Complex (Scott et al., 1992; Stern et al., 1995) of and the amounts and mechanisms of crustal recycling
the Trans-Hudson Orogen between the Hearne and also commonly are unclear. In addition, there contin-
Superior cratons of northern Canada (Plate 11.1a ues to be a need for high resolution paleomagnetic
between pp. 244 and 245). Other examples occur in the and geochronologic data to enable accurate recon-
Arabian-Nubian Shield (Kröner, 1985) and the Yavapai- structions of the continents and oceans during Pro-
Mazatzal orogens of the southwestern USA (Condie, terozoic as well as Archean times (Section 11.5.3).
1986). The presence of these features suggests that com- These data are crucial for determining the tectonic
plete Wilson cycles (Section 7.9) of ocean opening and evolution of Proterozoic lithosphere and for detailed
closure and ophiolite obduction were occurring by at comparisons between Proterozoic and Phanerozoic
least 2.0 Ga. orogens.