Lead isotopes are used to distinguish between Precambrian mineralization and Mesozoic or Cenozoic mineralization in an area of Belt Supergroup rocks in northwestern Montana and northern Idaho. The Precambrian lead is characterized by a comparatively uniform isotopic composition (Pb 206 /Pb 204 = 16.15-16.73; Pb 207 /Pb 204 = 15.37-15.45; Pb 208 /Pb 204 = 35.88-36.46) and model ages of 1.5-1.2 b.y. We interpret it to be the product of approximate single-stage development, although the model ages need not be exact geochronologic ages. The possibility that some vein-type deposits formed at a later time by remobilization of this lead already in Belt rocks is also discussed. The Mesozoic or Cenozoic lead, which has undergone at least a two-stage development, is generally more radiogenic and ranges widely in isotopic composition.Deposits associated with the Osburn fault system, including those of the Coeur d'Alene mining district, and with a second belt of mineralization which extends southward from near Bonners Ferry, Idaho, and joins the Osburn fault system near Thompson Falls, Montana, contain the Precambrian type of lead. Deposits adjacent to the Kootenay Arc mobile belt in Idaho and occurrences throughout southern Lincoln and Sanders Counties, Montana, appear to be largely Mesozoic or Cenozoic. Small, isolated deposits of Precambrian and Mesozoic or Cenozoic ages have been identified in the weakly mineralized northeastern part of the study area. The ability to distinguish, by their lead isotopes, two generations of ore widely spaced in time but overlapping geographically promises to be a valuable aid in unraveling the structural history and in carrying out future prospecting within this area.
Abstract Over the Earth’s evolutionary history, the style of plate subduction has evolved through time due to the secular cooling of the mantle. While continuous subduction is a typical feature of modern plate tectonics, a stagnant-lid tectonic regime with localized episodic subduction likely characterized the early Earth. The timing of the transition between these two subduction styles bears important insights into Earth’s cooling history. Here we apply a statistical analysis to a large geochemical dataset of mafic rocks spanning the last 3.5 Ga, which shows an increasing magnitude of alkali basaltic magmatism beginning at ca. 2.1 Ga. We propose that the rapid rise of continental alkali basalts correlates with an abruptly decreasing degree of mantle melting resulting from the enhanced cooling of the mantle at ca. 2.1 Ga. This might be a consequence of the initiation of continuous subduction, which recycled increasing volumes of cold oceanic crust into the mantle.
Recent efforts to understand the ongm of igneous rocks have emphasized the association between petrographic suites and geologic environment. Plate tectonic theory has been particularly useful in relating certain types of calc-alkalic rocks to their position near active plate boundaries. In contrast, alkalic rocks are often emplaced after cessation of orogenic forces or in completely anorogenic settings of the craton. The role of plate tectonics in dictating such magmatism is not readily understandable. One obviou s prerequisite to any effort at iden tification of under lying causes is the accurate dating of the igneou s rocks. My objective in this paper is to review our prescnt state of knowledge concerning a chronology for some alkalic rocks of eastern and central United States. Although'representing less than five percent of the volume of all igneous rocks, the alkalic clan includes a variety of interesting petrographic types, such as syenite, alkalic granite, alkalic gabbro, lamprophyre, and mica peridotite. These rocks typically occupy discordant stocks, plugs, dikes, sills, and diatremes at relatively shallow levels in the crust; and they may also include eruptive counterparts. The literature abounds with contributions regarding their structure, petrography, mineralogy, and geochemistry; and I do not stray too far from my intended objective into these areas. My working definition of an alkalic rock corresponds essentially to that of Sorensen (1974), but I also include certain u ltram afic rock s (Wyllie 1967) that are often found in petrologic and spatial association. Generally, rock n ames are accepted as the original authors used them without defending the specific genetic implication sometimes att�ched to the name. This paper focuses upon several petrographically cohcrent groups of alkalic rocks, or provinces, that occur within the eastern and central U nited States. Although each province contains some felsic and some mafic members, two rather different
The sub-vertical carbonate-silica veins filling the Bow Ridge Fault, where exposed in Trench 14 on the east side of Yucca Mountain, carry a lead isotopic signature that can be explained in terms of local sources. Two isotopically distinguishable--silicate and carbonate--fractions of lead are recognized within the vein system as well as in overlying surficial calcrete deposits. The acid-insoluble silicate fraction is contributed largely from the decomposing Miocene volcanic tuff, which forms the wall rock of the fault zone and is a ubiquitous component of the overlying soil. Lead contained in the silicate fraction approaches in isotopic composition that of the Miocene volcanic rocks of Yucca Mountain, but diverges from it in some samples by being more enriched in uranogenic isotopes. The carbonate fraction of lead in both vein and calcrete samples resides dominantly in the HCl- and CH{sub 3}COOH-soluble calcite. HCl evidently also attacks and removes lead from silicate phases, but the milder CH{sub 3}COOH dissolution procedure oftentimes identifies a significantly more radiogenic lead in the calcite. Wind-blown particulate matter brought to the area from Paleozoic and Late Proterozoic limestones in surrounding mountains may be the ultimate source of the calcite. Isotopically more uniform samples suggest that locally the basaltic ash and other volcanic rock have contributed most of the lead to both fractions of the vein system. An important finding of this study is that the data does not require the more exotic mechanisms or origins that have been proposed for the veins. Instead, the remarkably similar lead isotopic properties of the veins to those of the soil calcretes support their interpretation as a surficial, pedogenic phenomenon.
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