Continental crust intermediate

Cullers, R.L., and J.L. Graf, 1984b, Rare Earth Elements in Igneous Rocks of the Continental Crust Intermediate and Silicic Rocks - Ore Pe-trogenesis, in Rare Earth Element Geochemistry, ed. P. Henderson (Elsevier, Amsterdam) pp. 275-316. 

The Earth s mantle is overlain with crust. The crust is divided into two main types the silicate-rich (felsic to intermediate) continental and the thinner mafic oceanic crusts. Oceanic crusts are located in the deeper parts of the ocean floor and have an average thickness of about 5 km. Continents and some shallow seas are typically underlain with continental crust. Continental crusts have an average thickness of about 40 km, but are up to 65 km thick in high mountainous areas (Press and Siever, 2001, 439 Figure 3.1). 

The weathering of surface rocks has had a critical role in the chemical evolution of the continental crust for most of the Earth s history. In the presence of air and water, mafic minerals tend to rapidly weather into iron (oxy)(hydr)oxides, clays, and other silicate minerals, and at least partially water-soluble salts of alkalis (sodium and potassium) and alkaline earths (calcium and magnesium). In contrast, quartz in felsic and intermediate igneous rocks is very stable in the presence of surface air and water, which explains why the mineral readily accumulates in sands and other sediments. 

Continental crust Thtfelsic to intermediate outer rock layers of the Earth that constitute large portions of the continents. The maximum thickness of the continental crust is about 65 km. Compared with continental crust, oceanic crust is much thinner and more mafic. 

The familiar continental crust of the Earth on which most of us live is of unique importance because it formed the platform above sea level on which the later stages of evolution occurred leading to the appearance of Homo sapiens. The conditions for the production of massive granitic cmsts are probably unique to the Earth and require three or more stages of derivation from a primitive mantle composition. The Earth has transformed less than 0.4% of its volume to continental cmst of intermediate composition and less than 0.2% of its volume into granitic continental crust (i.e., upper continental cmst) in over 4000 million years, so that the process is inefficient. The highland feldspathic crust of the Moon, about 12% of lunar volume, formed in contrast within a few million years. 

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. 

Granite itself is too siliceous to be representative in composition of the average continental crust. A mixture of half granite and half basalt is sometimes used to approximate the composition. There are rocks of intermediate acidity (e.g., diorites, andesites) whose compositions approach the average of continental crust, but they are not overwhelmingly abundant. The crust is a complicated mixture of many rock types, mostly metamorphosed substantially from their original igneous or sedimentary textures. 

Fig. 21.25. Comparison diagram for Precambrian rocks of northeastern Minnesota interpreted by Arth and Hanson (1975) as possibly representing the formation of continental crust from mantle-derived material. Nos. 4 and 5 are early stage tholeiitic basalts, nos. 2, 3, and 6 are intrusive and extrusive granite-related rocks of main, intermediate stages, and no. 1 is a late-stage, intrusive, granite-related rock. Intermediate and late-stage rocks may have derived from material of the approximate composition of tholeiite. but at depths where garnet was a stable residual mineral, accounting for the severe depletion of the granite-related rocks in heavy lanthanides.

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