General geology of Proterozoic crust

Proterozoic belts display two groups of rocks that are distinguished on the basis of their metamorphic grade and deformation history. The first group consists of thick sequences of weakly deformed, unmetamor-phosed sedimentary and volcanic rocks that were deposited in large basins on top of Archean cratons. The second group is composed of highly deformed, high-grade metamorphic rocks that define large oro-genic belts. Both these groups contain distinctive suites of igneous rocks.

The most common lithologic assemblage in the weakly deformed parts of Early-Middle Proterozoic crust are quartzite-carbonate-shale sequences that reach thicknesses of some 10 km (Condie, 1982b). Quartz-pebble conglomerates and massive, cross-bedded sandstones also are common. Many of these sequences are intercalated with banded iron formations, cherts, and volcanic rocks. Other rock types that are either rare or absent in Archean belts appeared at this time, including extensive evaporites, phosphorous-rich sedimentary sequences, and red bed deposits (Section 3.4). These latter rocks generally are interpreted to have accumulated in stable, shallow water environments after 2.0 Ga. The appearance and the preservation of such thick sequences of sedimentary rock has been interpreted to reflect the stabilization of Precambrian continental crust during Proterozoic times (Eriksson et al., 2001, 2005) (Section 11.4.2). In the Pilbara region of northwest Australia (Fig. 11.8) the deposition of 2.78-2.45 Ga coarse clastic sedimentary rocks and volcanic sequences in a shallow platform environment in the Hamersley Basin (Trendall et al., 1991) reflects this stabilization. By

I.8 Ga, the existence of large, stable landmasses and free oxygen in the Earth's atmosphere allowed all of the well-known sedimentary environments that characterize Phanerozoic times to develop (Eriksson et al., 2005).

The highly deformed regions of Proterozoic crust are divisible into two types (Kusky & Vearncombe,1997). The first type consists of thick sedimentary sequences that were deformed into linear fold-and-thrust belts similar to those in Phanerozoic orogens (Figs 10.5, 10.19). The second type consists of high-grade gneisses of the granulite and upper amphibolite facies. Some of the largest and best known of these latter belts form the ~1.0 Ga Grenville provinces of North America, South America, Africa, Antarctica, India, and Australia (Fig.

II.19). Other belts (Fig. 11.12) evolved during the period 2.1-1.8 Ga (Zhao et al., 2002). These orogens contain large ductile thrust zones that separate distinctive ter-ranes. Some contain ophiolites (Section 2.5) that resemble Phanerozoic examples except for the lack of highly deformed mantle-derived rocks at their bases in ophio-lites older than ~1 Ga (Moores, 2002). The presence of these features reflects the importance of subduction, collision, and terrane accretion along Proterozoic continental margins (Carr et al., 2000; Karlstrom et al., 2002).

Fig. 11.12 Global distribution of 2.1-1.8 Ga orogenic belts showing selected areas of Archean and Early Proterozoic basement (afterZhao et al., 2002, with permission from Elsevier). Orogens labeled as follows: I, Trans-Hudson; 2, Penokean; 3, Taltson-Thelon; 4, Wopmay; 5, Cape Smith-New Quebec; 6, Torngat; 7, Foxe; 8, Nagssugtoqidian; 9, Makkovikian-Ketilidian; 10, Transamazonian; 11, Eburnian; 12, Limpopo; 13, Moyar; 14, Capricorn; 15, Trans-North China; 16, Central Aldan; 17, Svecofennian; 18, Kola-Karelian; 19, Transantarctic.

Fig. 11.12 Global distribution of 2.1-1.8 Ga orogenic belts showing selected areas of Archean and Early Proterozoic basement (afterZhao et al., 2002, with permission from Elsevier). Orogens labeled as follows: I, Trans-Hudson; 2, Penokean; 3, Taltson-Thelon; 4, Wopmay; 5, Cape Smith-New Quebec; 6, Torngat; 7, Foxe; 8, Nagssugtoqidian; 9, Makkovikian-Ketilidian; 10, Transamazonian; 11, Eburnian; 12, Limpopo; 13, Moyar; 14, Capricorn; 15, Trans-North China; 16, Central Aldan; 17, Svecofennian; 18, Kola-Karelian; 19, Transantarctic.

Selected exposures of Archean and early Early Proterozoic crust Selected exposures of late Early Proterozoic and Middle Proterozoic crust 2.1-1.8 Ga orogenic belt

Phanerozoic cover or ice of subcontinental scale preventing access to cratonic basement

Other crust, mostly younger than 1.8 Ga

A comparison of igneous rocks in Archean and Proterozoic belts indicates a progressive change in the bulk composition of the crust through time (Condie, 2005b). During the Early Archean, basaltic rocks were most abundant (Section 11.3.2). Later, the partial melting of these rocks in subduction zones or at the base of oceanic plateaux produced large volumes of TTG suite granitoids (Sections 11.3.2, 11.3.3). By 3.2 Ga granites first appeared in the geologic record and were produced in large quantities after 2.6 Ga.

This compositional trend from basalt to tonalite to granite generally is attributed to an increase in the importance of subduction and crustal recycling during the transition from Late Archean to Early Proterozoic times.

Large swarms of mafic dikes were emplaced into Archean cratons and their cover rocks during the Late Archean-Early Proterozoic and onwards. One of the best exposed examples of these is the 1.27 Ga MacKenzie dike swarm of the Canadian Shield, which exhibits dikes that fan out over a 100° arc and extend for more than 2300 km (Ernst et al., 2001). Some of these shield regions also contain huge sills and layered intrusions of mafic and ultramafic rock that occupy hundreds to thousands of square kilometers. These intrusions provide information on the deep plumbing systems of Precambrian magma chambers and on crust-mantle interactions. Three of the best known examples are the ~1.27 Ga Muskox intrusion in northern Canada (Le Cheminant & Heaman, 1989; Stewart & DePaolo, 1996), the —2.0 Ga Bushveld complex in South Africa (Hall, 1932; Eales & Cawthorn, 1996), and the —2.7 Ga Stillwater complex in Montana, USA (Raedeke & McCallum, 1984; McCallum, 1996). Unlike the layered igneous suites of the Archean high-grade gneiss terrains (Section 11.3.2), these intrusions are virtually undeformed.

Anorthosite massifs (Section 11.3.2) emplaced during Proterozoic times also differ from the Archean examples. Proterozoic anorthosites are associated with granites and contain less plagioclase than the Archean anorthosites (Wiebe, 1992). These rocks form part of an association known as anorthosite-mangerite-charnockite-granite (AMCG) suites. Charnockites are high temperature, nearly anhydrous rocks that can be of either igneous or high-grade metamorphic origin (Winter, 2001). The source of magma and the setting of the anorthosites are controversial. Most studies interpret them as having crystallized either from mantle-derived melts that were contaminated by continental crust (Musacchio & Mooney, 2002) or as primary melts derived from the lower continental crust (Schiellerup et al., 2000). Current evidence favors the former model. Some authors also have suggested that these rocks were emplaced in rifts or backarc environments following periods of orogenesis, others have argued that they are closely related to the orogenic process (Rivers, 1997). Their emplacement represents an important mechanism of Proterozoic continental growth and crustal recycling.

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