Ocean-ridge granites

Fig. 3. Tectonic discrimination diagram of Rb versus Y + Nb from Pearce et al. (1984) and modified by Christiansen and Keith (1996) for syn-collision granites (syn-COLG), volcanic arc granites (VAG), within plate granites (WPG), and ocean ridge granites (ORG). The diagram suggests the granites and pegmatite were contaminated by a crustal component.

Ocean-Island Basalt Ocean-Island Tholeiite -Ocean-Ridge Granite... 

Figure 4. Radiogenic Ca enrichments measured on whole rocks as reported hy Marshall and DePaolo (1989), shown here as 8c, values with 1-sigma uncertainties and plotted versus (a) K/Ca and (h) Enj value. The inferred value for seawater is also plotted in (a). Mid-ocean ridge basalts have no measurable enrichment of Ca/ Ca relative to the initial value for the Earth (151.016), as expected for magma derived from the Earth s mantle. Many granitic rocks, especially those with high K contents and low e j values, have significantly elevated Ec, values.

The production of broadly granitic magmas from juvenile, mantle-derived materials is most easily demonstrated in oceanic settings remote from continental influences, for example, (i) the development of the volumetricaUy minor plagio-granites of mid-ocean ridge systems, now... 

Today, the more external part of the crust or lithosphere constitutes the superficial covering of the earth. Two kinds of crust are easily distinguished by composition, thickness and consistency continental crust and oceanic crust. Continental crust has a thickness that, in mountain chains, may reach 40 kilometers. It is composed mainly of metamorphic rock and igneous blocks enriched with potassium, uranium, thorium and silicon. This forms the diffuse granitic bedrock of 45 % of the land surface of the earth. The oceanic crust has a more modest thickness, in the order of 5-6 kilometers, and is made up of basaltic blocks composed of silicates enriched with aluminium, iron and manganese. It is continuously renewed along mid-ocean ridges (cf Table 2.2). 

The CT-2 flows and dikes plot in area 4a and extend toward the part of Fig. 14.22 that contains mantle-derived basalts on oceanic islands and mid-ocean ridges. Accordingly, the CT-2 flows and dikes formed from mantle-derived basalt magma that assimilated granitic rocks of Paleozoic age at intermediate depth in the continental crust. 

Water transformed wet sediment and basalt into granite, which rose into great mountain ridges and pulled up other rocks attached to it as well. They keep floating up and up, collecting more granite and forcing denser plates back down into the ocean. 

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