Igneous rocks, basaltic silicic

Geologically and genetically, clay minerals are difficult to define simply and adequately, but broadly they are layer lattice silicates of secondary origin. In the same classification are the micas, talc, chlorites, and serpentines which are not strictly clay minerals. In this context, secondary origin means that mineral formation has arisen from the weathering of primary or igneous rock, e.g. granites and basalts. 

The ultimate source of most cations as weU as the silicate dissolved in rivers and the ocean is igneous rock. Granites are light-colored acidic rocks basalts are dark-colored with high concentrations of metal ions. These rocks originate from deep within the Earth, where at one time they were in a molten state. They are made of minerals like feldspar, mica, and quartz. Feldspars are the pink, green, and white minerals visible in granite ... 

A silicate melt that has the composition of basalt, a common igneous rock. 

Three major types of rocks are found in Earth s crust igneous rocks, formed by solidification of molten rock (e.g., basalt) sedimentary rocks (e.g., sandstone, which is cemented sand), formed by deposition of dissolved or suspended substances from oceans and rivers and metamorphic rocks (e.g., marble), formed by the action of heat and pressure on existing rocks. Figure 18.2 gives the average composition of Earth s crust. The most abundant substances in rocks are silicates, which are composed of silicon, oxygen, and positive metal ions (Section 18.5). The more than 2000 kinds of known minerals fall into a few major classes (Table 18.1). 

Basalt Dark, fine-grained igneous rock composed mostly of plagioclase (a calcium-sodium silicate mineral) and pyroxene (a magnesium iron silicate mineral). 

The crust extends from the surface of the earth to the Mohorovicic s (Moho) discontinuity, which is the first plane of unconformity, or boundary, between the mantle and crust. The crust is divided into the oceanic cmst and the continental crust. The oceanic crust is mainly composed of basic and ultrabasic rocks (e.g., basalt, gabbro, and peridotite) overlain by marine sediments. Continental crust is more siliceous (av. Si02 66 %) than the oceanic crust (av. Si02 50 %) and the average chemical composition of the oceanic crust is roughly equivalent to basalt granite = 1 1. It is also composed of various types of rocks (igneous rocks, metamorphic rocks, sedimentary rocks). 

Table 20. Selected lead isotope data on basalts and silicic igneous rocks of Cenozoic age in the Rocky Mountain region. United States ...

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). 

We believe that the Butcher Ridge Igneous Complex is unique. It may represent mixing in a hypabyssal setting of two magmas, one silicic, derived by melting of crustal rocks, and one basaltic (precursor of Ferrar Dolerite) derived from the mantle. 

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. 

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