It is generally believed that abundant water is the basic reason for the emergence of a large number of granitoid continental crust on the earth (Campbell Nader Taylor,1983; Taylor and Maclernan, 1985). K.C.Condie( 1989) proposed that plate structure should be a necessary factor for the formation of continental crust besides water. The plate structure mentioned by K.C.Condie here does not actually refer to the modern plate structure, but should be broadly understood as the mechanism of the return of the ocean crust to the mantle.
K.C.Condie put forward the above viewpoint to illustrate the development of Archean crust, based on the following research data: ① The continental crust in the early Archean (3.8~3.9Ga) was mainly composed of felsic gneiss, which was identified as the metamorphic product of tonalite, augite and granodiorite (TTG gneiss), and this gneiss contained Komatite and basalt (amphibolite). ② In geological records, the real granite appeared after 3.0Ga and was widely distributed after 2.6Ga, that is to say, granite did not exist in geological records before it was widely distributed in tonalite. (3) Archaean tonalite and granodiorite share most incompatible elements, especially the negative Ta-Nb anomaly shown by the standardized trace element composition model of the original mantle, which is very similar to the similar rocks formed in the post-Archean plate convergence zone; ④ The trend of experimental petrology research is that Archaean tonalite was formed by partial melting of amphibole or eclogite in the subsidence plate under water-rich conditions, and Archaean felsic rocks were the product of differentiated crystallization of basalt magma under relatively dry conditions; ⑤ The geothermal gradient is adiabatic before 4.0Ga, and the global heat output decreases with time; ⑥ Mantle convection may begin in the late stage of planetary accretion and absorption; ⑦ During the period of 2.7 ~ 3.0 GA, the continental crust proliferated rapidly, and cratonization became an important geological event; Before and after 2.7Ga, the continent may have gathered into several supercontinents through collision of micro-landmasses.
Considering the above eight points, around the formation and evolution history of the continental crust, with the emergence of Komatite, basalt, tonalite and granite as the main line, Kandy proposed the following development model of the Archean continental crust:
(1) during the first 50 ~1000 ma of the earth, the mantle melted strongly and gradually formed the core. Due to the establishment of adiabatic geothermal gradient and large-scale degassing of mantle, the atmosphere and ocean are produced. With the escape of radiation and volatile substances and the loss of heat, the earth's surface gradually becomes cold. Therefore, a thin and unstable Komatiite oceanic crust is formed through the ridge (Figure 9.438+0). After that, the earth continued to cool down, and basalt magma participated in the formation of ocean crust.
Fig. 9. 1 Schematic diagram of the early development of the lithospheric crust in the former Comati (according to Condie, 1989).
(2) Before about 4.0Ga, the oceanic crust and lithosphere were composed of a series of oceanic ridges and subsidence-reducing zones, and the plate was driven by Komati's subsidence to the mantle, which meant that the early crust quickly recycled with the mantle. 3.9Ga ago, a strong meteorite hit the crust, which contributed to the recycling of crustal materials. At a certain time during the period of 4.0~4.2Ga, when the ground temperature in the subduction zone dropped to such an extent that only the settled basaltic crust could be partially melted, quartz diorite magma was generated and rose, forming the earliest continental crust (Figure 9.2). At the same time, the number of submarine basalt plateaus, which are not easy to subduct, gradually increased, eventually covering a large part of the earth's surface. About three years ago, the cooling of subsidence zone caused the root of basalt plateau to transform into eclogite, and caused catastrophic subsidence of mantle. During this detachment process, wet partial melting occurred, which led to the formation of quartz diorite magma accounting for 50%~70% of the modern continental crust, and rose to form magma arc (Figure 9.3). About 2.5Ga ago, these magmatic arcs were combined into several large landmasses by collision. During the period of 2.4~2.6Ga, with the cratonization of these continents, the partial melting of the continental lower crust (mainly TTG gneiss) produced a large amount of granite magma, which invaded upward to form the upper crust dominated by tonalite. This marks the formation of the first cratons. The essence of so-called cratonization is devolatilization of the lower crust and lithosphere and melting in the crust (Campbell and Jarvis,1984; Pol-lack, 1986). Both processes redistributed large ion pro-MagmaElemental (LLI), including heat-generating elements U, th and K, which were transported from the lithosphere and lower crust to the upper crust by rising magma and fluid.
Fig. 9.2 Schematic diagram of continental crust (tonalite magmatic arc) development at the early stage of plate convergence margin (according to Condie, 1989).
Fig. 9.3 Schematic diagram of the development of eclogite roots under the early Archean basalt plateau (according to Condie, 1989).