Dike intrusion and deformation during growth of the Half Dome pluton, Yosemite National Park, California

Meter-scale mapping of the Late Cretaceous Half Dome Granodiorite of the Tuolumne Intrusive Suite (TIS) near Tenaya Lake, Yosemite National Park, defines an intricate internal structure that reflects a combination of incremental pluton growth by diking and internal deformation as the pluton grew. At least four ages of dikes of layered granodiorite are defined by crosscutting relations. Because dikes thicker than 1 m invariably contain multiple cycles of layering that field relations indicate record multiple intrusive increments, dozens of discrete intrusive events are likely. The kinematic pattern of dilation across dikes, offset lithologic markers across dikes, shearing of mafic enclaves and magmatic layering, and folding of dikes defines a synintrusive bulk strain field characterized by E-W extension and N-S contraction, with net volume increase in the extension direction. The geometric and kinematic pattern of the deformation are consistent with current understanding of Late Cretaceous Cordilleran tectonics and suggest that regional tectonic dilation played a significant role in making upper-crustal space for the growing pluton. Narrow shear zones offset lithologic markers and produced extreme strains, yet no rock fabric is preserved in the zones. This indicates that late-magmatic to subsolidus recrystallization, previously inferred in the TIS based on textural and mineralogical observations, greatly modified rock textures and obscured both the intricate internal structure of the pluton and the importance of synemplacement deformation

A more informative way to name plutonic rocks

The International Union of Geological Sciences (IUGS) system for rock classification, introduced more than 40 years ago, has served geologists well but suffers from the problem of dividing a continuum of rock compositions into arbitrary bins. As a result, closely related rocks can be given unrelated names (e.g., granodiorite and tonalite), and the names themselves, which were generally derived from the names of places or people, rarely contribute to understanding the processes that generate the diversity of igneous rocks. Here we propose a quantitative modification to the IUGS system that reduces the number of distinct names but more effectively communicates the inherent variability of plutonic rocks. The system recognizes that mapped plutonic rock units are characterized by recognizable textures and mineral assemblages, but that mineral proportions within those units can be highly variable. Adding quantitative data to rock names is an important step toward moving geologic field observations into quantitative digital form and preparing them for advanced data mining and analysis

Aplite diking and infiltration: a differentiation mechanism restricted to plutonic rocks

The Half Dome Granodiorite in Yosemite National Park, California, contains multiple crosscutting generations of high-silica dikes that vary in texture but share the distinctive trace element characteristics of fine-grained aplite dikes of the region (e.g., low Y and middle REE). Dikes of the youngest generation are typical fine-grained, sugary aplites with sharp planar contacts, whereas progressively older dikes are increasingly coarse-grained. The oldest ghostly forms recognizable as dikes are similar in grain size to the host granodiorite and their contacts are irregular at the grain scale, making them easy to overlook in the field. Mineral compositions and microstructures of felsic dikes indicate that all dikes, including fine-grained aplite, have undergone significant recrystallization. We propose that older dikes were originally fine-grained aplites that, after emplacement, recrystallized together with the host to a typical granitic texture. Extraction, transport, and redistribution of aplitic melt by various mechanisms explains major and trace element variation in the pluton and likely was the dominant differentiation mechanism at the level of emplacement. This late-stage differentiation process can only occur in a largely crystallized host capable of sustaining tensile cracks, and therefore cannot play a role in the differentiation of volcanic rocks. Predicted geochemical effects of aplite redistribution are evident in trace-element geochemical patterns of Circum-Pacific plutonic rocks but are absent from corresponding volcanic rocks. This indicates that aplite infiltration may be an important and widespread late-stage process of pluton differentiation

Is chemical zonation in plutonic rocks driven by changes in source magma composition or shallow-crustal differentiation?

Lithologic and magnetic-susceptibility mapping of the western Half Dome Granodiorite of the Tuolumne Intrusive Suite of California reveals seven km-scale lithologic cycles, each of which is bounded by a sharp west-dipping contact that is subparallel to the external contact of the pluton. Crosscutting relations indicate that the cycles become younger to the east. Each cycle is inferred to have been a zone of partial melt in which an eastern melt-depleted base grades westward to a melt-rich top now preserved as a leucocratic facies of the Half Dome Granodiorite. Sharp contacts between cycles may record freezing episodes when the rate of heat input into the growing pluton dropped below that required to maintain interstitial melt. Thus, the interstitial melt zone migrated with time and its size at any given time need not have differed greatly from the ~1km thickness of the cycles. Cycles occur on the outer, older margins of the suite, and disappear toward the interior, younger intrusions. Inward disappearance of cycles likely reflects thermal maturation of the system such that melt was continuously present until the final migration of the solidus through the intrusive suite. Although the cycles span the compositional range from granodiorite to leucogranite, trace-element trends preserved in the cycles differ dramatically from those of both the Tuolumne Intrusive Suite and other Cretaceous zoned plutons of the eastern Sierra Nevada batholith. We suggest that (1) the compositional variations of the intrusive suite and the batholith reflect a signal from the source of the magmas, and (2) the geochemistry within the km-scale cycles reflects in situ crystal/liquid separation

Are plutons assembled over millions of years by amalgamation from small magma chambers?

Field and geochronologic evidence indicate that large and broadly homogeneous plutons can accumulate incrementally over millions of years. This contradicts the common assumption that plutons form from large, mobile bodies of magma. Incremental assembly is consistent with seismic results from active volcanic areas which rarely locate masses that contain more than 10% melt. At such a low melt fraction, a material is incapable of bulk flow as a liquid and perhaps should not even be termed magma. Volumes with higher melt fractions may be present in these areas if they are small, and this is consistent with geologic evidence for plutons growing in small increments. The large melt volumes required for eruption of large ignimbrites are rare and ephemeral, and links between these and emplacement of most plutons are open to doubt. We suggest that plutons may commonly form incrementally without ever existing as a large magma body. If so, then many widely accepted magma ascent and emplacement processes (e.g., diapirism and stoping) may be uncommon in nature, and many aspects of the petrochemical evolution of magmatic systems (e.g., in situ crystal fractionation and magma mixing) need to be reconsidered

Effect of age, sex and gender on pain sensitivity: A narrative review

© 2017 Eltumi And Tashani. Introduction: An increasing body of literature on sex and gender differences in pain sensitivity has been accumulated in recent years. There is also evidence from epidemiological research that painful conditions are more prevalent in older people. The aim of this narrative review is to critically appraise the relevant literature investigating the presence of age and sex differences in clinical and experimental pain conditions. Methods: A scoping search of the literature identifying relevant peer reviewed articles was conducted on May 2016. Information and evidence from the key articles were narratively described and data was quantitatively synthesised to identify gaps of knowledge in the research literature concerning age and sex differences in pain responses. Results: This critical appraisal of the literature suggests that the results of the experimental and clinical studies regarding age and sex differences in pain contain some contradictions as far as age differences in pain are concerned. While data from the clinical studies are more consistent and seem to point towards the fact that chronic pain prevalence increases in the elderly findings from the experimental studies on the other hand were inconsistent, with pain threshold increasing with age in some studies and decreasing with age in others. Conclusion: There is a need for further research using the latest advanced quantitative sensory testing protocols to measure the function of small nerve fibres that are involved in nociception and pain sensitivity across the human life span. Implications: Findings from these studies should feed into and inform evidence emerging from other types of studies (e.g. brain imaging technique and psychometrics) suggesting that pain in the older humans may have unique characteristics that affect how old patients respond to intervention

THE GRANITE AQUEDUCT AND AUTOMETAMORPHISM OF PLUTONS

Ian Carmichael wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that subduction zone magmas contain 5 wt % or more H2O. Most of the water is released from crystallizing intrusions to play a central role in contact metamorphism and the genesis of ore deposits, but it also has important effects on the plutonic rocks themselves. Many plutons were constructed incrementally from the top down over million-year time scales. Early-formed increments are wall rocks to later increments; heat and water released as each increment crystallizes pass through older increments before exiting the pluton. The water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Pipe-like conduits in Yosemite National Park, California, are located in or near aplite-pegmatite dikes, which themselves are products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by fluid-mediated subsolidus modification of mineral compositions and textures. The flood of magmatic water carries a large fraction of the total thermal energy of the magma and transmits that energy much more rapidly than conduction, thus enhancing the fluctuating postemplacement thermal histories that result from incremental pluton growth. The effects of water released by subduction zone magmas are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and the thermal history of the plutons themselves

The Rocks Don't Lie, But They Can Be Misunderstood

Although the adage "the rocks don't lie" is true-rocks are literal ground truth-their message can be misinterpreted. More generally, it is misguided to favor one form of inquiry, such as field observation, over others, including laboratory analyses, physical experiments, and mathematical or computational simulations. This was recognized more than a century ago by T.C. Chamberlin, who warned against premature adherence to a "ruling theory," and by G.K. Gilbert, who emphasized the investigative nature of geological reasoning. Geologic research involves a search for fruitful, coherent, and causal hypotheses that are consistent with all the relevant evidence and tests provided by the natural world, and field observation is perhaps the most fertile source of new geologic hypotheses. Hypotheses that are consistent with other relevant evidence survive and are strengthened; those that conflict with relevant evidence must be either revised or discarded. © 2022 Geological Society of America. All rights reserved.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]