Abstract The isotopic composition of lead was investigated in and around Kuroko deposits of the Hokuroku district, Japan. Although the ore leads of these deposits were found to occupy a narrow isotopic range, each ore deposit has a characteristic isotopic composition. Within a given ore deposit, black ore has a uniform isotopic composition but is significantly higher in radiogenic lead than yellow ore. The differences between ore types are, however, smaller than those between ore deposits. The volcanic host rocks are in general lower in radiogenic lead than the ores, whereas the deeper, older formations, in particular the Sasahata Formation and the Paleozoic basement, have more radiogenic lead than the ores. On the basis of the isotopic distribution we conclude that a major part of the lead in the Kuroko deposits was derived from igneous, probably volcanic rocks with an uncertain but significant contribution coming from the underlying pre-Nishikurozawa formations. The ore fluids reached the Sasahata Formation and most likely also the Paleozoic basement. Each ore deposit within the district was formed by a local hydrothermal system. The difference in isotopic composition between the yellow and black ores reflects a shift in the proportions coming from the two major sources due to the temperature evolution of the hydrothermal system. The yellow ore seems to have a greater igneous rock lead component than does the black ore.

In the Minnesota River Valley, an epizonal, anorogenic granite that is often referred to as the granite of section 28 (lat 44°49.73′N, long 95°33.90′W) has been found to have a Pb-Pb age of 1.84 ± 0.05 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. The Th-Pb age of the feldspar-whole-rock pair is 1.9 b.y., which is in satisfactory agreement with the Pb-Pb age; but the U-Pb age is greater than the Pb-Pb age and probably indicates that uranium has been leached from the whole rock within the past several hundred million years, perhaps as a consequence of dilatancy resulting from uplift and erosion. Another granite, the mesozonal, late-tectonic Precambrian Sacred Heart Granite in the Minnesota River Valley (lat 44°41.3′N, long 95°21.5′W) is found to have a Pb-Pb age of 2.605 ± 0.006 b.y. on the basis of data obtained on HF leached and unleached feldspars and HCl leached and unleached whole rocks. Both the Th-Pb and U-Pb isochron ages are much older than the Pb-Pb age. An older age was expected for the U-Pb system as it had been previously found in the epizonal granite, but to also find it for the Th-Pb system was surprising. As is predicted for these kinds of granites in this type of tectonic environment on the basis of Mesozoic and Cenozoic analogues, the initial leads in the granites indicate that they were derived from source material having values of 238 U/ 204 Pb <9 normalized to the present day. This feature is common in Mesozoic and Cenozoic igneous rocks penetrating Precambrian terranes but is rarely observed in pre-Mesozoic igneous rocks. The Sacred Heart Granite is the oldest igneous rock known to show this effect and is the first representative of a mesozonal granite. The uranium depletion event appears to have been a granulite-facies metamorphism, but the age of that metamorphism cannot be determined from the available data. The model-lead-age information, however, suggests that it occurred before 2.78 b.y. ago. The source materials for both granites also underwent an earlier stage of extensive but unknown duration during which 238 U/ 204 Pb >9. In Phanerozoic rocks, such values are characteristic of ensialic tectonic environments. Similar development of ensialic environments was apparently occurring also in perancient times.

Compositions of initial, generally basaltic, rocks of geosynclines are virtually identical regardless of their time and place of formation within the past 3,500 m.y. This constancy of composition through time and place is matched by a similarity of the entire stratigraphic column in each of the studied geosynclines of different age and location. These observations lead to the conclusion that the major element content of the upper mantle has been essentially unchanged during the past 3,500 m.y. except by processes related to geosynclinal development and orogeny. The resulting compositional changes are caused largely by gradual segregation of new continental crust out of the mantle during geosynclinal, orogenic, and postorogenic stages. General enlargement of cratons may also explain the inception of major miogeosynclinal sedimentation in Proterozoic time and platform sedimentation in Paleozoic time.

Sr 87 /Sr 86 values of Columbia River bottom sediments vary consistently with the age and compositional nature of provenance as indicated by previous mineralogic studies. These values range from 0.717 at Grand Coulee Reservoir where sediments are derived in part from a terrane of older metamorphic, plutonic, and sedimentary rocks, to 0.704 near the mouth of the Columbia where young volcanic rocks of the Cascades are the dominant source. Isotopic variation in sediments along parts of the Columbia River—Grand Coulee to McNary—can be explained by simple mixing of end-member detrital components. These results suggest that strontium isotopes may provide a useful clue in reconstruction of source terranes for graywackes in which mineralogic evidence may be destroyed or masked by diagenetic or low-grade metamorphic processes.

The anorthosite arc of the Bergen area contains a thick series of gabbroic to anorthositic rocks, with garnet-pyroxene-plagioclase mineralogy. These rocks are infolded with amphibolite-facies Cambro-Silurian supracrustal rocks, deformed during the Caledonian orogeny. Eclogites occur as thick layers and as crosscutting dikes in the anorthosites. Olivine pods, apparently formed from disrupted layers, have reacted with plagioclase during cooling, passing through the generalized reactions: 1. olivine + plagioclase → aluminous pyroxenes + spinel, and 2. aluminous pyroxenes + spinel + plagioclase → low-Al pyroxenes + garnet. The resultant corona structures have orthopyroxene cores, surrounded successively by shells of low-Al clinopyroxene and garnet (+ inclusions of aluminous clinopyroxene and spinel). Microprobe analyses show the aluminous clinopyroxene to be enriched in Tschermak’s silicate (Ts) and low in jadeite (Jd), whereas the low-Al clinopyroxene has Jd/Ts > ½. Slow uplift has forced simultaneous exsolution of jadeite from the low-Al clinopyroxene and of grossular from garnet, leading to a strong zoning of the garnet shell and eventual appearance of a plagioclase shell between the clinopyroxene and the garnet. The eclogites found as conformable layers are quartz free and the pyroxenes are low in Na, but with high Jd/Ts ratios; the eclogites originally may have been cumulate olivine-pyroxene-plagioclase layers. The crosscutting eclogites are quartz bearing and the pyroxenes are omphacites or chloromelanites, typical of low-T eclogites. The mineralogy of the coronas and eclogites suggests that the anorthositic rocks originally crystallized from a magma at P < 8 kb, and were buried to 10 to 12 kb during cooling. The great difference in metamorphic grade between the anorthosites and the Cambro-Silurian rocks rules out interpretation of the anorthositic rocks as either Caledonian intrusives or as a late Precambian sequence conformably underlying the Cambro-Silurian rocks. Decompression breakdown of garnet and Tschermakitic pyroxene is common in the anorthosites of the Caledonian Jotun nappes, but is absent in the Bergen anorthosites. This difference suggests there was no rapid vertical movement in the P-T history of Bergen anorthosites, and thus that they are tectonically distinct from the anorthosites of the Jotun nappes.

The Precambrian migmatites in the central Front Range were formed during high-grade regional metamorphism, prior to the emplacement of the major granitic bodies. The leucosomes of the migmatites consist essentially of varying proportions of microcline and plagioclase together with 30 to 50 percent quartz. The concentrations of biotite along the margins of the leucosomes suggest that the migmatite formed by local segregation. This interpretation is supported by data indicating equilibrium in the concentrations of alkali and alkaline earth elements between the leucosomes and the melanosomes. Strontium isotopic compositions are also in equilibrium between adjacent leucosomes and melanosomes. All the parameters considered in this study are consistent with the migmatites having formed by segregation of metamorphic rocks; the process apparently did not involve partial melting.

Calcic amphibole and coexisting albite + white mica + chlorite + epidote from 11 metabasaltic and 2 metasedimentary schists have been investigated from a portion of the relatively high pressure, low temperature (blueschist type) Sanba-gawa terrane. The host rocks are representative of two adjacent and intergradational parts of a progressive metamorphic series. Although Ca-amphibole, white mica, and, to some extent, chlorite show enrichments in A1 with progressive metamorphism, the plagioclases change only from an average of An 00 3 to An 01.7 .The Ca/Na ratios of calcic amphiboles do not appear to be related simply to metamorphic grade or to the An contents of coexisting plagioclases. The TiO 2 contents of the mafic phases and white micas increase with grade. The distributions of the large Mn 2+ and small Ti 4+ ions between coexisting phases are: for manganese, chlorite > Ca-amphibole > epidote > white mica; and for titanium, Ca-amphibole = white mica > epidote > chlorite. These fractionations reflect in part the different sizes of the structural sites in the competing minerals. Although the paragenesis is roughly similar to those of other, higher temperature type terranes, there is a slight tendency for Shirataki Ca-amphibole (SiO 2 +Al 2 O 3 ) sums and soda contents to be higher and (total iron + magnesia) sums, as well as titania and lime contents, to be lower than values for calcic amphiboles from non-blueschist terranes. In contrast to higher temperature terranes elsewhere, the continuous change in composition of Shirataki Ca-amphiboles from actinolite at lower metamorphic grade through barroisitic amphibole to hornblende at higher grade argues against the presence of a solvus in this series under relatively high pressure, low temperature (glaucophane schist type) conditions.

Triaxial compression tests were run on seven orientations of three rocks with pervasive planar anisotropy—slate, phyllite, and schist—to evaluate the effects of cohesion and granularity on anisotropic strength variation and deformational mode. The rocks show a gradation in the percentage of platy minerals, and in the size, shape, and percentage of quartz grains present. Experiments were carried out at confining pressures of 500, 1,000, and 2,000 bars, and all tests were duplicated. The tests on slate were duplicated in two different apparatuses, with and without specimen end lubrication, to evaluate the effects of testing conditions on the experimental results. The strength data obtained were analyzed in terms of cohesive strength and internal friction. Cohesive strength increases with increasing granularity, but the coefficient of internal friction becomes the principal contributor to the strength variation. The coefficient of internal friction does not appear to change significantly with increased granularity; moreover, it appears to be less sensitive to material variability than does the cohesive strength. Cohesive strength becomes the more significant contributor to strength variation with increased percentage of platy minerals. Cohesion can have a pronounced effect on the mode of deformation. Curves of differential stress at failure versus anisotropy orientation are concave upward with pronounced strength minima for maximum compression at 30° to 45° to the anisotropy, depending upon the granularity and percentage of platy minerals. Maximum strength occurs in the 90° or 0° orientation, also as a function of material characteristics. The observed strength variation among different orientations is in excellent accord with the extended variable cohesive strength theory of Jaeger (1960). Strength for a given orientation increases linearly for phyllite at pressures up to 2,000 bars; the increase is nonlinear for slate above 1,000 bars. Significant shear stress developed on the ends of specimens under certain testing conditions that, for one orientation, resulted in the rotation of the maximum principal stress from the vertical loading direction by as much as 30°. As a consequence, for these test conditions measured values of strength for this orientation can be about 25 percent lower than the true strength. Lubrication of specimen ends and a rigid loading piston are believed to give the most accurate results.

The Archean of the Canadian Shield occurs as islandlike areas of supracrustal rocks in a sea of invasive granite—the latter having been emplaced no less than 2.5 b.y. ago. These “islands” consist of metavolcanic rocks that are mainly andesitic and basaltic greenstones with a much lesser volume of felsites, and also metasediments that in some places are altered graywacke and slate with locally interbedded conglomerates and iron-bearing formation and elsewhere are much more extensive quartz mica schists and paragneisses, at times heavily migmatized. A Shield-wide stratigraphy has not yet been established either by geological field studies or by geochronologic methods. Archean time is long enough to encompass several magmatic and orogenic cycles. Geologic evidence supports this view, but geochronology has generally failed to discriminate between Archean rocks thought to be significantly different from one another in age. Archean sediments belong to the eugeosynclinal suite. These are apparently deep-water, probably marine, deposits of the flysch turbidite facies. Shelf carbonates and orthoquartzites are missing. There is no basic difference in bulk chemical composition between the Archean graywackes and slates and those of later times. A sialic source for the sands is indicated by the quartz budget and the granitoid clasts in the associated conglomerates. The Archean strata have been massively invaded by granites. The greenstones are cut by domelike diapiric stocks and batholiths of granitoid rocks, mainly tonalites and granodiorites. The sediments outside of the greenstone belts are intimately interleaved with granite sheets and are in places much granitized and engulfed. Excluding the relatively small areas of infolded metasediments, and ignoring the extensive invasion and disruption by granites, the greenstones are organized into broad belts, traceable across the southern part of the Shield. These major belts are separated by equally broad and continuous belts of sediments that were apparently deposited in protogeosynclinal tracts. The conglomerates and iron formations are confined to the outlying infolds of sediments or to the marginal areas of the geosynclinal belts. Unlike younger geosynclines, these seem to contain only the flysch facies and seemingly were filled from both sides and hence, unlike younger geosynclines, have no polarity.

The Precambrian rocks, older than 2,600 m.y., in the Saganaga Lake-Northern Light Lake area on the Minnesota-Ontario boundary include a thick succession of metavolcanic rocks, chiefly greenstone but with some intermediate to silicic types, and minor metasediments; the Northern Light Gneiss; the Saganaga Tonalite; alkalic syenodiorite and granodiorite; and the Knife Lake Group. A variety of dikes and sills in the older units are of different ages; the youngest are related to the Keweenawan igneous activity, approximately 1,100 m.y. ago. The Northern Light Gneiss consists principally of fine-grained biotite leucotonalite, or trondhjemite, that is interlayed with amphibolite and lesser amounts of metarhyodacite and metarhyolite. The gneiss is part of the volcanic pile of greenstone and related rocks formerly referred to the Keewatin. Both the greenstone and the Northern Light Gneiss were intruded by the Saganaga Tonalite. This rock was formerly called the Saganaga Granite, but the “granite” is a sheared and altered phase in which microcline was introduced, and small amounts of epidote, chlorite, sericite, and carbonate were formed at the expense of the primary hornblende, biotite, and oligoclase. The Saganaga Tonalite and the Northern Light Gneiss were intruded by a small pluton near Icarus Lake in the eastern part of the area. The pluton consists of a western phase of syenodiorite and an eastern phase of somewhat younger granodiorite. Both rocks are alkalic and are characterized by soda-rich pyroxene and amphibole. Two periods of folding in the region were previously recognized as the Lauren-tian and Algoman orogenies. The Saganaga Tonalite is a late-kinematic intrusion in the folded greenstone and related rocks. Uplift and vigorous erosion uncovered the tonalite which supplied large boulders and finer detritus to the sediments of the Knife Lake Group deposited on the western margin of the batholith. At Cache Bay the conglomerate resting on the tonalite is composed almost wholly of clasts of tonalite, but north of Cache Bay the conglomerate on the greenstone is composed largely of pieces derived from the greenstone and related rocks. The tonalite also supplied a large component of the graywacke-argillite sequence that is intercalated with the conglomerate at Cache Bay. The Knife Lake conglomerate and graywacke-argillite sequences were folded during the Algoman orogeny, and at Cache Bay the beds dip approximately 70° W. Although these events were deciphered from the geologic record, efforts to resolve the two periods of folding by radiometric age determinations have not been completely successful. The Saganaga Tonalite is dated by a whole-rock Rb-Sr isochron and by U-Pb ages on sphene and zircon at 2,730 m.y. The older Northern Light Gneiss probably was formed not more than 2,750 m.y. ago, and the younger Icarus pluton was emplaced approximately 2,700 m.y. ago; hence a complicated geologic history is encompassed in a span of 50 m.y. or less. The Saganaga Lake-Northern Light Lake area is part of a mobile belt in which folding and igneous activity were essentially continuous from the time of eruption of the greenstone and related rocks to the time of folding of the Knife Lake Group.

The trondhjemitic Northern Light Gneiss, Saganaga Tonalite, and syenodioritic to granodioritic Icarus pluton are part of a typical Archean igneous sequence that began with the extrusion of large volumes of basic volcanics, which make up large parts of present-day greenstone belts, and ended with a small post-kine-matic alkalic stock. Radiometric dating indicates that these events encompassed approximately 50 m.y., from 2,700 to 2,750 m.y. ago. K/Rb, Rb/Sr, Sr/Ba, and initial Sr 87 /Sr 86 ratios for the Saganaga Tonalite and Northern Light Gneiss are similar to those for Archean basalts. A rare earth analysis for the tonalite is strongly depleted in the heavy rare earths with an abundance similar to chondrites and a light rare earth abundance similar to ocean ridge basalts. The depletion in the heavy rare earths is strongly suggestive of separation of a melt from a garnet-rich residue. Quartz “eyes” in the tonalite are probably partially resorbed quartz phenocrysts. It is proposed that both the tonalite and the trondhjemitic gneiss are derived by partial melting of either quartz eclogite or garnet-rich amphibolite at mantle depths. For both rocks the parent is assumed to be Archean mafic volcanics.

The Rainy Lake region on the boundary between Ontario and Minnesota is a classical Precambrian area in which two periods of folding and igneous activity, Laurentian and Algoman, were recognized by Lawson. Early efforts to resolve the two orogenic periods on the basis of K-Ar and Rb-Sr age determinations on micas were not successful. More sophisticated whole-rock Rb-Sr and zircon and sphene U-Pb studies likewise have not been wholly successful, but the U-Pb data suggest that all the early Precambrian events, including the Laurentian and Algoman igneous activity, probably occurred within the interval from 2,700 to 2,750 m.y. ago. Whole-rock Rb-Sr isochron studies, however, give younger ages as follows: Coutchiching Series 2,615 ± 50 m.y. Keewatin Series 2,595 ± 45 m.y. Algoman granites Ontario 2,540 ± 90 m.y. Minnesota 2,680 ± 95 m.y. Mineral ages and a rock-mineral isochron clearly indicate that both the Rb-Sr and the K-Ar systems were affected by subsequent events, but it is not certain that the isochron ages date the time of specific events. It is more likely that the determined ages mark the end of periods of retrograde metamorphism, faulting, and shearing. The different apparent ages of stabilization of the Rb-Sr system probably resulted from differences in rock composition, in water content, and in local heat flow.