Zoic

PRECAMBRiAN

PRECAMBRiAN

Fig. 4.1. Synthetic geological time scale

The Hadean extends from Earth accretion until the age of the oldest known rocks: the 4.03-Ga Acasta gneisses. In other words, except for zircon crystals from Australia, direct geological memory for this 0.55-Ga long period does not exist. The Archaean-Proterozoic boundary corresponds to a period of transition in Earth dynamics, indeed present-day-like geodynamic processes started to operate since Proterozoic times (2.5Ga). These changes will be described and discussed in detail in this chapter. Finally, the Proterozoic-Phanerozoic transition is marked by the apparition and development of living beings with internal or external skeleton able to be preserved by fossil-ization.

Geographically, the early continental crust is widely distributed all over the world; on each continent it outcrops in very large domains. This appears even more obvious when the early continental crust only covered by a few kilometres of younger sediments is taken into account (Fig. 4.2).

After Earth accretion, the core rapidly differentiated from the mantle, a process that lasted less than 0.1 Ga and more probably less than 0.03Ga (Kleine et al., 2002; Yin et al., 2002; Vityazev et al., 2003). On the contrary, the crust differentiated from the mantle by a slow and progressive mechanism that is still in progress today. Different models were proposed to account for juvenile continental crust growth. Figure 4.3 shows very important differences depending on the authors, however, all clearly point to the fact that 70 to 75% (volume) of the continental crust was generated and extracted from the mantle before 2.5Ga. Consequently, the Archaean aeon appears as a period of intense magmatic activity that led to the extraction of about 3/4 of the continental crust from the mantle.

Archaen Baltic

Fig. 4.2. Distribution of Archaean provinces (adapted from Condie, 1981; and Goodwin, 1991) Exposed Archaean terrains are in dark blue, and areas underlain by Archaean rocks are in light blue. (1) Baltic Shield; (2) Ukrainian Shield; (3) Scottish Shield; (4) Greenland Shield; (5) Labrador Shield; (6) Superior Province; (7) Wyoming Province; (8) Kaminak Group; (9) Slave Province; (10) Committee Bay Block; (11) Guiana Shield; (12) Guapore Craton; (13) Sao Francisco Craton; (14) Rio de la Plata and Luis Alves Massifs; (15) Napier Complex; (16) Kaapvaal Craton; (17) Madagascar Craton; (18) Zimbabwe Craton; (19) Zambian Block; (20) Kasaï Craton; (21) Chaillu Craton; (22) Cameroon N'tem Complex; (23) Tuareg Shield; (24) Man Shield; (25) Reguibat Shield; (26) Central Africa Craton; (27) Ethiopian Block; (28) Indian Shield; (29) Yilgarn block; (30) Pilbara block; (31) Litchfield, Rul Jungle and Nanambu Complexes; (32) Sino-Korean, Tarim and Yangtze Cratons; (33) Aldan Shield; (34) Baikal, Sayan and Yienisei fold belts; (35) Anabar Shield

Fig. 4.2. Distribution of Archaean provinces (adapted from Condie, 1981; and Goodwin, 1991) Exposed Archaean terrains are in dark blue, and areas underlain by Archaean rocks are in light blue. (1) Baltic Shield; (2) Ukrainian Shield; (3) Scottish Shield; (4) Greenland Shield; (5) Labrador Shield; (6) Superior Province; (7) Wyoming Province; (8) Kaminak Group; (9) Slave Province; (10) Committee Bay Block; (11) Guiana Shield; (12) Guapore Craton; (13) Sao Francisco Craton; (14) Rio de la Plata and Luis Alves Massifs; (15) Napier Complex; (16) Kaapvaal Craton; (17) Madagascar Craton; (18) Zimbabwe Craton; (19) Zambian Block; (20) Kasaï Craton; (21) Chaillu Craton; (22) Cameroon N'tem Complex; (23) Tuareg Shield; (24) Man Shield; (25) Reguibat Shield; (26) Central Africa Craton; (27) Ethiopian Block; (28) Indian Shield; (29) Yilgarn block; (30) Pilbara block; (31) Litchfield, Rul Jungle and Nanambu Complexes; (32) Sino-Korean, Tarim and Yangtze Cratons; (33) Aldan Shield; (34) Baikal, Sayan and Yienisei fold belts; (35) Anabar Shield

Age in Ga

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