A. E. J. Engel

PROGRESSIVE METAMORPHISM AND GRANITIZATION OF THE MAJOR PARAGNEISS, NORTHWEST ADIRONDACK MOUNTAINS, NEW YORK PART II: MINERALOGY

The progressive metamorphism and partial granitization of a belt of quartz-mica feldspar-garnet paragneiss is considered in detail. This paragneiss is traced and sampled along a belt 35 miles long that extends across the Grenville Lowlands into the central massif of the Adirondack Mountains, New York. Geologic thermometers indicate temperatures of metamorphism of about 500° C. at the southwest end of the belt and about 600° C. near the perimeter of the massif. Minimum temperatures of metamorphism in the gneiss arc determined largely from solid solutions of magnesite in dolomite, FeS in sphalerite, paragonite in muscovitc, and TiO_2 in magnetite. Maximum temperatures of metamorphism are inferred principally from the absence of wollastonite in closely associated siliceous marbles. The gradient in T is checked by the δ O^(18) in quartz and coexisting magnetite in the gneiss. The composition of the paragneiss and its constituent minerals is determined from 75 new chemical analyses, 50 partial chemical analyses, 400 analyses of trace elements, and modal analyses of approximately 400 rocks. At the lower-temperature end of the belt the least altered gneiss is a quartz-biotile-oligoclase-muscovite gneiss averaging (weight per cent) 70.25 SiO_2, 0.67 TiO_2, 14.14 Al_2O_3, 0.55 Fe_2O_3, 3.83 FeO, 2.20 CaO, 1.76 MgO, 0.05 MnO, 3.43 Na_2O, 2.40 K_2O, and (in ppm) B, 10; Ba, 600; Co, 8; Cr, 35; Cu, 16; Ga, 11; Ni, 15; Pb, 12; Sc, 12; Sr, 300; V, 56; Y, 50; Yb, 3; Zr, 170. This is inferred to approximate the bulk composition of the parent sedimentary rock. With increasing temperature of metamorphism of the least altered gneiss, the mineral composition changes as follows: muscovite disappears, garnet appears, plagioclase increases in abundance, and average An content and quartz decrease. Complementary changes in chemical composition include an increase in Al, Fe^(++), total Fe, Mg, Ca, Cr, Ga, Ni, and V. Amounts of K, Si, Fe^(+++), H_2O, and Ba decrease. This “degranitization” or “basification” of the gneiss appears to be a metamorphic process that begins at about 550° C. and is well defined at 600° C. The mobilized Si, K, and H_2O appear to be partly liberated and partly trapped as a venitic migmatite. Granitization of parts of the gneiss is accompanied by an increase in K feldspar and Ab content of plagioclase and by a decrease in biotite, plagioclase, and quartz. Chemical changes in major elements include an increase in K and a decrease in Ti, Fe^(+++), Fe^(++), Mg, Ca, H_2O, and in Na–K ratio. Changes in the amounts of minor elements in granitized parts of the gneiss include increases in Ba and Pb and a decrease in Co, Cr, Ni, Sc, Sr, Ti, V, and Y. All granitizing substances in the gneiss in areas of lower-temperature metamorphism appear to be introduced either laterally or from below. Those in areas of highest-temperature metamorphism are partly introduced, partly derived locally from the gneiss. The implied basic front evolved during granitization of the gneiss may have been large, for the introduced granitizing substances replace one-third of the sedimentary rock throughout a zone over ½ mile thick, 75 miles wide, and more than 40 miles long. Calculations of the chemical composition of the more even-textured gneiss, from modal analyses and mineral analyses, show about the same deviation from the actual chemical analyses as exists in the 34 analyses of G-1 and W-1 as reported by Fairbairn et al, (1951).

Crustal Evolution and Global Tectonics: A Petrogenic View

Studies of the relative abundances and characteristics of rock types, series, and complexes through time reflect both general and subtle features of crustal and upper-mantle environments. These studies also suggest salient aspects of the dynamothermal and global tectonic history of the last 3,500 m.y. Among the many petrochemical indices, the ratio K 2 O/Na 2 O reflects both the degree of differentiation (fractionation) of igneous rocks and the more or less mature (residuate) nature of clastic sediments. Accordingly, this ratio is a guide to the thickness, composition, and stability of source and crusial site of rock emplacement. Average K 2 O/Na 2 O of rocks, rock complexes, and terranes decreasing from 1 are typical of rock series formed in primitive borderland and arc-to–oceanic-crustal sites. Average values of K 2 O/Na 2 O increasing from 1 characterize rock series evolving in and on more mature arc-to-continental sites. Relative abundances of the most common and characteristic rock assemblages of various ages and their weighted average K 2 O/Na 2 O suggest profound episodicity in crustal evolution and global tectonics. At least three macro-episodes of major significance are defined: the Archean, >2,500 m.y. B.P.; the Proterozoic-Paleozoic, 250 m.y. B.P.; and the Mesozoic-Cenozoic, ∼250 m.y. B.P. to present. Within these macro-episodes, there are innumerable subordinate episodes and variously developed rock cycles. Most major Archean rocks, rock complexes, and terranes have a K 2 O/Na 2 O of Archean thermal gradients varied abruptly both vertically and laterally from very steep to moderate and created highly unstable, thin to thick, labile lithosphere and protocrusts commonly inhospitable to the evolution of widespread, highly fractionated calc-alkaline series or mature sediments. Heat transfer was largely via convection and advection. The advective loss was undoubtedly large, associated especially with the extrusion of floods of ultramafic to felsic magmas and related upward streaming of volatiles. Culmination of major Archean orogenies and subsequent thermal decay about 2,500 m.y. B.P. induced relative crustal quiet and resetting of most Archean Rb/Sr and K/Ar radiomentric clocks between 2,600 and 2,400 m.y. B.P. In the early Proterozoic (∼1,700 to 2,300 m.y. B.P.), many segments of the megacontinent(s) were sufficiently cool, thickened, and fractionated to remain quasi-coherent, deforming and cracking internally as well as marginally above convecting mantle forces. Widespread, relatively ensialic orogenic regions evolved above many thinner sialic zones. These were repeatedly refractionated and redated as they became populated by igneous series with K 2 O/Na 2 O commonly 2 O/Na 2 O as high as 2 or more. Sedimentary K 2 O/Na 2 O increased much faster than did the igneous ratio, as blankets, basins, and prisms of more mature, frequently recycled sediments formed on all the continents. Two of the several major Proterozoic orogenic episodes culminated at about 1,700 m.y. B.P. and 1,000 m.y. B.P. The latter, often called the “Greenville Event,” between 1,000 and 1,200 m.y. B.P., is characterized by the evolution of both K- and Na-rich terranes and distinctive granulite and anorthositic-charnokitic complexes. Most of these Grenville rock complexes evolved between and on relatively thicker, more fractionated continental crust than did the Archean granulites, as indicated by the relative abundance of less telescoped prograde terranes. The Grenville thermal pulses and decay involving much advective and convective heat loss again reset numerous Rb/Sr and K/Ar ages of rocks emplaced 100 to 2,000 m.y. earlier. A post-Grenville relatively amagmatic period of 400 to 500 m.y. after 1,000 m.y. B.P., preceded the incipient rifting of large continental segments as the great Pan–African-Appalachian-Her-cyian-Caledonian orogenies waxed. But large-scale drift of most of these continental segments seems precluded by the continuing alignment of the older, including Archean, fold belts and by the predominantly continental rock component emplaced in most Proterozoic and Paleozoic orogens. Weighted average K 2 O/Na 2 O of Proterozoic igneous complexes are commonly > 1.2 and, in sedimentary sequences, often > 2. Nevertheless, the occurrences of lower Paleozoic ophiolite, blueschist, and less fractionated rock complexes in many Phanerozoic orogens suggest the onset of the large-scale continental rifting and global drift characteristic of post-Permian time. These events are reflected in variously depressed lower Paleozoic K 2 O/Na 2 O and, subsequently, in the plunge of average K 2 O/Na 2 O of major Mesozoic rock complexes to near Archean lows of ∼0.5 to 0.9, especially in the circum-Pacific. The petrochemical and structural data indicate the significant differences as well as similarities between the Archean and post-Permian. The Archean was dominated by the evolution and aggregation of protocontinents, arcs, and intervening oceanic crusts. The post-Permian was dominated by the unique fragmentation and widespread drift (>1,500 km) of large, thick, cool, fractionated, continental fragments and by the birth of new large ocean basins and island-arc chains.

Late Precambrian evolution of Afro-Arabian crust from ocean arc to craton

Large parts of northeast Africa and Saudi Arabia consist of a telescoped, Proterozoic island-arc ocean-basin complex less than 1 b.y. old. In the central Eastern Desert of Egypt, the oldest units in the complex are a mafic and ultramafic sequence representing an oceanic substrate. Concentrations of elements in constituent rocks least altered by metamorphism, including Cr, Ni, Ti, and REE in the ultramafic rocks and pillow basalts, are essentially those found in similar rocks of contemporary oceanic crust. Thick sequences of calc-alkaline volcanic rocks and related volcanogenic metasediments, including wackes, breccias, and banded iron formations, overlie the oceanic substrate. The andesitic volcanic rocks are similar to those in modern circum-Pacific island arcs, although amounts of Cr and Ni tend to be higher. Rare cobble beds in the metasediments in the Eastern Desert of Eygpt contain granitic and quartzitic clasts derived from old Proterozoic and Archean forelands, presumably those exposed west of the Nile River. Stratiform ultramafic sills as much as 1 km thick, as well as thinner gabbroic sheets, are intruded as magmas within the metasediments. The composition of the ultramafic sills approximates that of basaltic komatiites. In eastern Egypt, granitic plutons, ranging from syntectonic quartz diorites to a post-tectonic flood of LIL-enriched granite emplaced 550 to 570 m.y. B.P. partly engulf and surround much of the ocean-arc complex. Age studies suggest that the Egyptian segment of the ocean-arc complex could have evolved, been telescoped, and intruded by progressively more voluminous and fractionated granitic rocks between about 550 and 850 m.y. B.P. In southwest Saudi Arabia, limited radiometric data obtained by the U.S. Geological Survey suggest that these processes may have begun slightly earlier. Both the volcanic rocks within the complex and the granitic rocks intrusive into it have low initial ratios of Sr87/Sr86 (< 0.704), suggestive of a mantle origin. There is no evidence of older sialic roots beneath the complex.

Age and Composition of a Bounty Islands Granite and Age of a Seychelles Islands Granite

Concordant

GRENVILLE SERIES IN THE NORTHWEST ADIRONDACK MOUNTAINS, NEW YORK: PART II: ORIGIN AND METAMORPHISM OF THE MAJOR PARAGNEISS

Ar^{40}-K^{40} + Sr^{87}-Rb^{87}

PROGRESSIVE METAMORPHISM AND GRANITIZATION OF THE MAJOR PARAGNEISS, NORTHWEST ADIRONDACK MOUNTAINS, NEW YORK

ages of 189 m.y. were obtained on biotite from a Bounty Islands granite.

Grenville series in the northwest Adirondack Mountains, New York. Part I: General features of the Grenville series

Sr^{87}-Rb^{87}