Abstract

Changes in the kinds and properties of minerals in the Adirondack paragneiss are related to its progressive metamorphism. The least altered gneiss at Emeryville, New York, consists of quartz, biotite, and oligoclase, with small amounts of muscovite and microcline. This assemblage formed at about 500° C under a lithostatic load of about 5 miles. As the least altered geneiss is traced northeastward into what was a deeper, hotter environment of metamorphism the biotite and muscovite react to produce garnet; K2O, SiO2, and water are expelled. At Colton, New York, 35 miles northeast of Emeryville, temperatures of metamorphism probably reached 600° C. There, the least altered gneiss consists of quartz, calcic oligoclase, biotite, garnet, and accessory K feldspar. Migmatitic and granitized paragneiss throughout the region is enriched in K feldspar and depleted in biotite, garnet, and plagioclase.

The microcline in least altered gneiss at Emeryville occupies 1–5 per cent of the rock. It is distinctly perthitic and contains up to 2 weight per cent Na2O, 0.2 to 0.3 per cent BaO, and from 300 to 2600 parts per million Sr. With increasing grade of metamorphism gridiron twinning disappears (orthoclase appears?), and Na2O content increases to 2.4 per cent.

The K feldspar in the migmatite and granitized gneiss throughout the Emeryville–Colton region is almost wholly a perthitic microcline, with some abrupt and local variations in amounts of Na, Ba, and Sr. These variations in alkalis occur within individual bodies of granitic gneiss, and between microcline in these bodies and the microcline porphyroblasts in the enveloping paragneiss.

Plagioclase in the least altered gneiss at Emeryville averages Ab 73, forms about 40 per cent by volume of the rock and shows a separation of forumla and 2θ(131) of 1.61. Only 5 per cent of the plagioclase is visibly twinned (albite and pericline types). Most grains are sercitized. Zoning is very rare. With increasing grade of metamorphism plagioclase in the least altered gneiss becomes more abundant, coarser-grained, and profusely twinned (largely albite twinning). At Colton, plagioclase forms half the rock and is Ab 67 with forumla – 2θ(131) of 1.71.

Plagioclase in migmatite and granitized gneiss decreases in volume and Ab content as K feldspar content of these rocks increases. The average granitic gneiss at Emeryville contains about 30 per cent plagioclase of Ab 86.

A greenish-brown, Al-rich biotite constitutes 18 per cent of the least altered gneiss at Emeryville. With progressive metamorphism biotite decreases in volume, in size of unit cell, and in its content of Fe2O3, FeO, OH, Sc, and Mn. Grain size increases, as do amounts of TiO2, MgO, Ba, Cr, F, V, and probably in Ca. Color changes to deep reddish brown as a function of the increasing ratio of TiO2:FeO. Nz and density undergo small, less consistent changes. The 1M polymorphs predominate, but 2M forms are not uncommon. There is no correlation of polymorphic type with grade of metamorphism. Granitization of gneiss results in the decomposition of biotite, which forms less than 10 per cent by volume of the average granitic gneiss. Biotite of the incipiently granitized gneiss is very similar in composition to that in associated least altered gneiss.

At Emeryville garnet is confined to pegmatites and migmatitic gneiss, but from Edwards northeastward it occurs throughout the least altered gneiss as a product of regional metamorphism. The garnets associated with pegmatite near Emeryville are spessartitic almandites with 6 per cent MnO, 4 per cent MgO, and 2 per cent CaO. The first garnets to appear throughout least altered gneiss (at Edwards) contain less than 2 per cent MnO and about 6 per cent MgO. As the grade of metamorphism increases the garnets increase in abundance and in per cent MgO, Cr, Ti, V, and Zn. Complementary decreases occur in dimensions of the unit cell, in index of refraction, density, Fe2O3, FeO, MnO, Ba, Sc, Y, and Yb.

In the selective distribution of mafic elements in the paragneiss, biotite accumulated more Mg, Ba, Co, Cr, Cu, Ni, Ti, and V than did garnet. Garnet is enriched in Mn, Fe, Ca, Sc, Y, and Yb. With increasing grade of metamorphism the ratio Fe+++ Mn:Mg in garnet decreases from 8 to 5, and in biotite from 2.8 to 1.7 Fe+++:Fe++ decreases in both biotite and garnet.

SiO2:Al2O3 decreases in both total rock and in plagioclase with increasing grade of metamorphism of least altered gneiss. K2O is concentrated in biotites and K feldspar, and Na in plagioclase. With progressive metamorphism K2O:Na2O decreases in K feldspar and total rock.

The pronounced and systematic variations in concentrations and ratios of elements in minerals and total rock between Emeryville and Colton are marked by only one classical isograd. This is the reaction of quartz, biotite, magnetite, and muscovite to produce garnet. For this region, and perhaps many others, the scope and patterns of progressive metamorphism are shown in much greater detail by changes of ratios of elements and oxides in minerals. The ratio of two easily determined oxides that have opposing trends in concentration is especially definitive. TiO2:MnO in biotites changes from 16 to 520 between Emeryville and Colton. TiO2:MnO in garnets changes from .003 at Emeryville to .06 at Colton. Ratios of Fe:Mg, Mn:Mg, or Fe+++ Mn:Mg in both minerals are even more informative but also more difficult to obtain accurately.

The mineral assemblages in the paragneiss at Emeryville are complemented by diopside in adjoining dolomites, sillimanite in nearby pelitic schists, and hornblendeandesine in amphibolite interlayers. At Colton, diopside persists in the dolomitic marble, whereas in the amphibolite ortho- and clinopyroxenes have formed at the expense of about half the hornblende. These relationships indicate the transition from upper amphibolite to lower granulite facies and are accompanied by marked dehydration, decarbonation, and a basification of the total rock.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.