Grant M. Young

Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations

Abstract The exposed crust consists mainly of plagioclase (35%), quartz (20%), K-feldspar (11%), volcanic glass (12%), biotite (8%), and muscovite (5%). Quartz is a resistate, thus feldspars and glass represent approximately 75 percent of the labile minerals. The weathering characteristics of these constituents are summarized in the context of thermodynamic, mass balance and kinetic considerations. Experimentally determined release rate constants were used to predict the proportions of Ca, Na and K released by feldspars of plutonic rocks (granites to gabbros) to weathering solutions. The chemical weathering trends of the weathered residues, calculated from the kinetic data, conform closely to the initial trends observed in some recent weathering profiles, demonstrating the accuracy of the predictions. Since the weathering of feldspars is controlled by processes that should not change through geological time, the relative release rates of Ca, Na, and K from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering. Experimentally determined release rate constants are not available for a wide range of volcanic glass compositions, but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks. The kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile.

Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance

Lutites are commonly metasomatized during diagenesis, but the analysis presented here accounts for most postdepositional change. Potassium metasomatism is particularly common, and typically involves the conversion of kaolin (residual weathering product) to illite by reaction with K + -bearing pore waters. Sandstones also undergo K metasomatism, which involves the replacement of plagioclase by potassium feldspar. These changes can be identified petrographically and are quantitatively accounted for by techniques discussed herein. Bulk chemical analyses and ternary diagrams are used to determine the amount of K addition, premetasomatized sediment composition, and composition of provenance areas. The premetasomatized mineralogy of paleosols can be compared with the mineralogy of recent soil profiles and thus, climate and topographic conditions determined for past weathering events. Some weathering indices lead to erroneous conclusions because, by excluding K 2 O from consideration, correction cannot be made for metasomatic effects.

Quartz and Feldspar Stability, Steady and Non‐Steady‐State Weathering, and Petrogenesis of Siliciclastic Sands and Muds

The kinetic and thermodynamic properties of quartz, plagioclase, and K‐feldspars, which constitute 70 to 80% of the upper crust, provide a framework for prediction of mineralogical and chemical changes involved in the production of siliciclastic sediments. Chemical weathering of bedrock may produce weathering profiles with distinct mineralogical zones, compositionally much different from the parent rock. Mass wasting of such profiles produces sediments that reflect the mineralogy of the zones exposed to mechanical erosion, rather than the composition of fresh bedrock. The relative rates of chemical weathering and mechanical erosion determine which mineralogical zones are exposed to mass wasting, and therefore control the compositions of siliciclastic sediments. Stable rates of chemical weathering and erosion result in steady‐state weathering, so that thickness and the mineralogical composition of eroded soil zones, and therefore the mineralogy of derived sediments, remain unchanged while steady‐state weathering prevails. Non‐steady‐state weathering occurs where climate and tectonism vary. Changing conditions alter rates of chemical weathering and physical erosion, resulting in exposure of different, and sometimes all, weathering zones of profiles, or exposure of bedrock. Such conditions result in production of sediments with diverse mineralogy, reflecting incipiently‐to‐highly weathered zones of profiles. Steady‐ and non‐steady‐state weathering of granitic rocks therefore can be assessed by study of variations in quartz and feldspar contents of sands and variations in bulk compositions of muds, as shown for sediments derived from the Sierra Nevada and Bega batholiths, and the Appalachian Piedmont. Compositional variation of sediments, or its absence, are controlled by the relative rates of chemical weathering and erosion and provides insight into climatic and tectonic conditions in source lands, as well as information about provenance composition.

Are Neoproterozoic glacial deposits preserved on the margins of Laurentia related to the fragmentation of two supercontinents

Remarkably similar deposits representing two Neoproterozoic glaciations are present on the west and east sides of Laurentia. Although now located near the margins of Laurentia, these glaciogenic successions were formed within supercontinents. The older glaciogenic succession (Rapitan-Sturtian, ∼700 Ma) is preserved in a series of pull-apart basins formed when the supercontinent Kanatia fragmented to produce the proto–Pacific ocean. The younger Varangerian glaciogenic rocks (∼600 Ma) are now scattered throughout the North Atlantic region, but formed in basins that reflect the demise of a second Neoproterozoic supercontinent (Rodinia) and heralded the formation of the Iapetus ocean.

Sampling Laurentia: Detrital zircon geochronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen

Neoproterozoic fluvial quartzarenites from the Shaler Group on Victoria Island in the western Canadian Arctic yield a diverse suite of detrital zircons. U-Pb ages from single zircons fall into three modes: Archean (3.01-2.62 Ga), Paleoproterozoic (1.97-1.84 Ga), and Meso proterozoic (1.64-1.03 Ga). Particularly intriguing is the unusually high proportion (50%) of the Mesoproterozoic mode, as the nearest exposed source is the Grenville structural province, ∼3000 km southeast of Victoria Island. Unimodal northwest paleocurrents, obtained from cross-bedding, are compatible with a southeasterly provenance. These data, supported by sedimentological evidence from the Shaler Group and from correlative strata in the northwestern Canadian Cordillera, imply the existence of a large, perennial river system flowing north-westward from the Grenville orogen. An analogy with the Llanos drainage basin in Venezuela and Colombia is proposed on the basis of similar geography, climate, and sedimentation. These data also provide a maximum age for the Shaler Group (basal Reynolds Point Formation) of about 1.11 Ga.

The early Neoproterozoic sedimentary Succession B of northwestern Laurentia: Correlations and paleogeographic significance

The Mesoproterozoic–Neoproterozoic stratigraphic record of ancestral North America (Laurentia) comprises three, unconformity-bounded sedimentary successions that are termed, from oldest to youngest, A, B, and C. Recent and ongoing detailed stratigraphic studies of Succession B, along with improved geochronology, allow extension and refinement of existing correlation schemes for northwestern Canada and Alaska. Succession B strata include the Shaler Supergroup of the Amundsen Basin, Mackenzie Mountains supergroup of the Mackenzie Mountains fold belt, Pinguicula group of the Wernecke Mountains inlier, Fifteenmile group of the Ogilvie Mountains inliers, and the lower Tindir Group of Tatonduk inlier. The Katakturuk Dolomite, in the northeast Brooks Range of Alaska, is included with Succession B on the basis of platformal character, geochronology, and inferred paleogeographic affinity. The framework for regional lithostratigraphic correlation of Succession B is built on recognition of four distinctive lithostratigraphic assemblages: two thick stromatolitic platformal carbonate assemblages separated by two largely subaerial siliciclastic assemblages. The correlation is supported by geochronology of detrital zircons from the upper quartzarenite assemblage, which indicates a maximum age of ca. 1000 Ma for the lower part of Succession B. These rocks are interpreted to be remnants of a northwesterly trending (present coordinates) early Neoproterozoic basin-margin promontory (Amundsen-Ogilvie-Mackenzie platform) that developed within an intracratonic basin on the northwest margin of Laurentia. The Neoproterozoic stratigraphic record of northwestern North America bears striking similarity to contemporaneous stratigraphy on other continents, particularly in the Amadeus Basin and Adelaide fold belt of central and southern Australia. Reconstructions of the Neoproterozoic supercontinent juxtapose the eastern margin of ancestral Australia against the western margin of Laurentia during the time these strata were being deposited. The Amundsen-Ogilvie-Mackenzie platform consequently may represent a segment (of the margin) of a large intracratonic basin that rifted apart with the breakup of the supercontinent during the latest Proterozoic. This hypothesis provides a template for future sequence stratigraphic, chemostratigraphic, biostratigraphic, paleomagnetic, and geochronologic comparisons and has implications for predictive economic geology in both areas. Capitalization of group and supergroup indicates formalization according to the International Stratigraphic Code. Noncapitalization indicates that the names have not been formalized.

Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of the Stoer Group and two specimens from the unconformably overlying Torridon Group were analysed on the GSC SHRIMP II ion probe. 207Pb/206Pb ages of zircons from the Stoer Group range between 3.00 and 1.74 Ga with 95% concentrated between 2.93 and 2.48 Ga. The Bay of Stoer Formation has a small mode c. 2.55 Ga, matching overgrowth ages on some older grains. A few grains between 1.92 and 1.74 Ga occur in each of the Stoer Group specimens. These data are consistent with conventional provenance information and sedimentology, which indicate that the bulk of the Stoer Group probably was derived from local basement of the Lewisian Gneiss Complex and that deposition occurred adjacent to basin-margin growth faults. The two Torridon Group samples have similar detrital zircon age profiles with distinctive modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, interpreted to represent sources of Ketilidian, Labradorian and Grenvillian affinity, respectively. A less well defined cluster c. 2.85–2.55 Ga reflects reworking of the underlying Stoer Group and possibly direct contribution from the Lewisian Gneiss Complex. The youngest concordant detrital zircon yields a maximum age of 1060±18 Ma for the Torridon Group (Applecross Formation). Our data, together with palaeocurrents from the Applecross Formation, suggest that the Torridon Group could have been deposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.

Earth'S Oldest Reported Glaciation: Physical and Chemical Evidence From the Archean Mozaan Group (∼2.9 Ga) of South Africa

The Pongola Supergroup is a thick succession of Archean (∼2.9 Ga) supracrustal rocks in the eastern part of the Kaapvaal craton of southern Africa. Its upper part, the Mozaan Group (∼5000 m‐thick) includes diamictites with a highly varied suite of clasts, some of which are striated and faceted. Clasts in associated stratified siltstones are interpreted as ice‐rafted debris. A glacial origin is also supported by geochemical investigation of diamictite matrix materials and associated mudstones. Plots of SiO2:Al2O3 and TiO2:Al2O3 suggest that these rocks underwent little chemical weathering, an interpretation confirmed by calculation of a Chemical Index of Alteration with low values, typical of other glacial deposits. The analyzed rocks are rich in Fe (average Fe2O3(T) is ∼24% wt %), like rocks of many other glaciomarine successions. The diamictites and associated mudstones of the Mozaan Group have low Th/Sc ratios, suggesting preferential incorporation of mafic materials into the diamictite matrix material. Previous studies suggest that this part of the Kaapvaal craton underwent early cratonization, involving tectonic and plutonic events that may have contributed to the drawdown of atmospheric CO2 and eventually provided a suitable substrate for development of continental glaciers.

Middle and late Proterozoic evolution of the northern Canadian Cordillera and Shield

The thick Proterozoic succession of northwestern North America can be subdivided into three sequences: A (∼1.7 to ∼1.2 b.y.), B (∼1.2 to ∼0.8 b.y.), and C (∼0.8 to 0.57 b.y.). Deposition of the rocks of sequence A was followed by widespread mafic igneous activity about 1.2 b.y. ago. In the Cordilleran region these rocks were then folded during the Racklan (= Grenville?) orogeny. A second, mainly tensional, orogenic event, here called the “Hayhook orogeny, ” occurred near the time boundary between sequences B and C and may have culminated in continental fragmentation in Phanerozoic time. The older Proterozoic rocks of the Cordilleran region (sequences A and B) are considered to have been derived in part from rising orogens in the eastern part of the Canadian Shield. There is no definitive evidence of extensive oceanic areas in the Cordilleran region during the Proterozoic, but local small ocean basins may have existed at the time of deposition of sequence C.

Behavior of major and trace elements (including REE) during Paleoproterozoic pedogenesis and diagenetic alteration of an Archean granite near Ville Marie, Québec, Canada

Abstract This paper presents a complete set of chemical data from a pre-2.0-Ga weathering profile that formed on the Archean granite near Ville Marie, Quebec. Examination of data from two sections of the paleosol indicates that Na, Ca, P, and Sr were lost from the paleosol, whereas Ti Nb, Th, Zr, Hf, and Ta remained immobile during chemical weathering and all subsequent alteration. Rare earth mobility during genesis of the Ville Marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles. Average concentration of REE in both profiles is 20–40% lower than what is found in the fresh granite, suggesting mobilization during pedogenesis. HREEs seem to be less mobilized than the LREEs. (La/Sm)N ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith. (Gd/Yb)N values remain unchanged in the weathering profile. (La/Yb)N values in the weathering profile are quite variable and generally lower than that of the protolith. Eu/Eu∗ and Ce/Ce∗ values are essentially the same as those of the protolith, but two samples from the uppermost part of the profile have developed positive cerium anomaly of ≈0.3 relative to the protolith.