Pyroxene-anorthosite

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

Lahradoriie is the characteristic feldspar of the more basic rock types like diorite. gahbro. andesite or basalt and it is usually associated with some one of the pyroxenes or amphiholes. Lahradoriie frequently shows a heauliful play of iridescent colors due lo minute inclusions of another mineral. However, the labradorescent phenomenon has not been fully determined. The classic location for this mineral is. of course. Labrador, whence its name. Il is a constituent there of the rock anorthosite and is found in the anorthosites of the Provinces of Quebec and Ontario, and in the Adirondack region in New York Stale. 

In the few FA suite rocks that (as sampled) contain more than 1-2% mahc silicates, the mahcs are typically a mix of olivine and coarsely exsolved low-Ca pyroxene (igneous pigeonite) (Taylor et al., 1991 Papike et al., 1991, 1998). Another apparent geochemical discontinuity between the ferroan anorthositic and Mg-suites is manifested by Ni-Co systematics in olivine (Shearer et al., 2001b). Early models designed to... 

The seismic properties of the Grenville lower crust are inconsistent with pyroxene-rich or high-grade metapelites (Fig. 2). This is crucial in determining which source, mantle or crust, is responsible for generating the mid-Proterozoic meta-anorthosites, as we will discuss below. 

Longhi et al. 1999) and amphibolite (Christensen Mooney 1995 Rudnick Fountain 1995). For any of these compositions, partial melting and extraction of the anorthosite suite had to result in a general increase of restitic minerals (pyroxene and garnet). The occurrence of this process at a large scale, as the volume of mid-Proterozoic meta-anorthosites would require, would have produced a restitic bulk composition of the lower crust and a decrease of VpjVs ratios to values inconsistent with our observations. 

The Walker Anorthosite in Table 13.3 underlies the Aughenbaugh Gabbro in the Dufek Massif and was named after Walker Peak in Fig. 13.41 where a 230-m section of this formation is exposed. It is composed of a medium-grained plagioclase cumulate (An to An ). The anorthosite is white to cream colored or gray and is spotted because of the presence of coarse poikilitic postcumulus crystals of pyroxene. 

Fig. 13.51 The concentrations of copper in the Forrestal Range vary stratigraphically and reach high values in two layers at 1,080 m (Cu = 2,000 ppm) and at 1,343 m (Cu= 460 ppm) both of which have high modal pyroxene concentrations (71% at 1,080 m 46% at 1,343 m, respectively). The copper concentrations of the Stephens Anorthosite and the Lexington Granophyre are less than 100 ppm. The association between copper and pyroxene is caused by the association of sulfide minerals with layers of pyroxenite in the Forrestal Range (Data from Ford et al. 1983)...

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