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An upper-crustal intrusive network in the 201.5 Ma, rift-related Central Atlantic Magmatic Province is exposed in the western Newark basin (PA, USA). Alpha-MELTS modeling was used to track magma ev...

The central Appalachian Piedmont lies in the critical juncture between the northern and southern Appalachians, portions of the orogen with distinct middle to late Paleozoic accretionary histories. Orogen-scale compilation maps link the central and southern Appalachians, but until recently, limited geochronological data prevented robust tectonic comparisons between high-grade metamorphic rocks in different parts of the orogen. We report the results of in situ U-Th-total Pb monazite geochronology that date significant deformation and metamorphism as middle Silurian (~425 Ma) through middle Devonian (~385 Ma) and demonstrate the diachronous nature of orogen development. The Rosemont Shear Zone is identified as a major tectonic boundary in southeastern Pennsylvania and northern Delaware separating the rifted Laurentian margin from younger rock units that formed in a magmatic arc setting. The Laurentian margin rocks occur in a series of nappes in which the metamorphic grade decreases from the structurally highest nappe to the lowest. The in situ monazite ages show that maximum temperature in the lowest nappe may have been attained some 15 million years after maximum temperature in the highest nappe. We interpret this to be the result of successive nappe emplacement, with the warmer overriding sheets contributing heat to lower levels. Combining geochronologic and thermobarometric results with the geometry of deformation results in a new picture of the tectonic development of the central Appalachian Piedmont that further links the evolution of the southern and northern Appalachians. For the Laurentian margin rocks, tectonism resulted from the approach and collision of peri-Gondwanan terranes during the Silurian to early Devonian in a dominantly sinistral, transpressive tectonic regime. This portion of the Pennsylvania-Delaware Piedmont inboard of the Rosemont Shear Zone is contiguous with comparable rocks in the southern Appalachians. In contrast, arc-related rock units outboard of the Rosemont Shear Zone experienced primarily thermal metamorphism in the Silurian, while crustal thickening and associated regional metamorphism is middle Devonian in age and likely the result of the accretion of Avalonia during the Acadian orogeny. These arc-related and younger rocks probably originated to the north of their present location as part of the northern Appalachians. They were ultimately emplaced in a right-lateral transcurrent regime sometime after the middle Devonian. Thus, it is in this portion of the central Appalachian Piedmont that the northern and southern Appalachians are joined.

To motivate student learning, the affective domain—emotion, attitude, and motivation—must be engaged. We propose a model that is specific to the geosciences with theoretical components of motivation and emotion from the field of educational psychology, and a term we are proposing, “connections with Earth” based on research in the fields of environmental education and art education. When all three of these components (motivation, emotion, and connections with Earth) are combined in the classroom, students may experience greater interest in and connection to the content. This interest and connection may lead to greater motivation to learn and value the content. We use our model to evaluate three practices in geoscience education and show that their demonstrated success in achieving student learning lies in the attention to students’ affective needs as well as to delivery of content. We propose a future research agenda using currently developed, validated instruments that can measure these motivational and attitudinal shifts to determine what practices work best for our students from both cognitive and affective perspectives. Although this was conducted in both Europe and the United States, the implications of this research may extend across cultures and nationalities. Additional research needs to be conducted to understand these implications. V C 2011 National Association of Geoscience Teachers. [DOI: 10.5408/1.3543934]

Geoscience teaching has primarily been oriented toward the value of science to explain natural systems. However, many kinds of values guide people’s responses to environmental problems, which originate when human expectations fail to match the behavior of natural systems. Examples from the literature show that practical environmental decision-making recognizes, and is formed on the basis of, diverse values. We propose a ‘values of nature’ framework based on Stephen Kellert’s (1996) values of life to provide a set of concepts and a terminology that engages students to recognize the values they bring to environmental issues. We show from our experiences in two different introductory courses that students using the values framework can develop thoughtful attitudes about the environment and can appreciate the views of those with different values.

High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.

We offer a course that our majors enroll in concurrently with their calculus course that “shadows” the topics covered by the calculus professor. In the shadow course, the students work collaboratively to build mathematical skills and apply calculus concepts to solve geoscience problems provided by the instructor (two examples are given). Students earn a grade of “pass” by demonstrating their involvement in learning. It is not feasible to assess statistically the effect of the shadow course on students' grades in the calculus courses; however, the shadow course has led to other positive outcomes. Our majors have developed more positive attitudes toward mathematics, including calculus, and get the message that geoscience faculty value and actively support their learning of calculus. We have gained insight into our students' strengths and weaknesses in mathematics that may help us to incorporate mathematics more effectively into geoscience courses. The shadow course is helping to foster active collaboration ...

(2000). Building the Quantitative Skills of Students in Geoscience Courses. Journal of Geoscience Education: Vol. 48, No. 4, pp. 409-412.

We present a quantitative model for in situ crystallization within a solidification zone (or boundary layer) based on the trace and major element compositions of plutonic igneous rocks and related geochemical data. We developed the model to account for the characteristics of a suite of granitic rocks: nearly uniform mineral compositions in rocks that range widely in bulk composition (e.g., 58–76 wt.% SiO 2 ); linear variation and correlation of all major and trace elements analyzed except Ba; large and apparently random variations in Ba concentration. These characteristics cannot be explained by any standard petrogenetic model involving fractionation, mixing, or restite unmixing, but are successfully reproduced by our model of residual melt migration. We do not attempt to model the entire crystallization history of a pluton but, rather, only that interval during which melt migration processes have recognizable geochemical effects. For the granitic suite, the chemical signature of residual melt migration resulted from the change in Ba compatibility with the onset of orthoclase crystallization at a granite minimum. Our results demonstrate that plutonic rocks can develop large compositional variations, comparable to those expected to result from extreme differentiation within a large magma body, over short distances by melt migration under conditions of high permeability or slow crystallization rate within the solidification zone. Melt migration is probably a common process that could be overlooked in large plutons if sampling is sparse and if variations in some components that appear random are not considered. Our model equations yield estimates for parameters that describe the proportions of residual melt that crystallize within and migrate out of the solidification zone. Values of these parameters can be used to infer information about permeability and melt mobility within the solidification zone. The model parameters derived for the granitic suite have a roughly concentric spatial pattern in the pluton, suggesting that residual melt was trapped near the margins, accumulated in the interior, with a zone of enhanced melt mobility and possibly compositional convection in between. Our model may be of general usefulness because it requires no assumptions about magma chamber geometry or magma dynamics, it is applicable to magmas of any composition, and the equations could be formulated to include those variables best constrained by a particular suite of plutonic igneous rocks.

Cooperative-learning techniques are becoming increasingly popular strategies to improve learning and student performance. We have found that writing assignments complement and enhance these techniques at all levels of the geology curriculum. In large general-education sections of introductory geology, short writing assignments focus group work on specific problems or concepts and enhance student learning. In the geology curriculum, we use group activities called Class Research Modules (CRM), which are term-long research projects integrated into the content of the course. A series of writing assignments throughout the semester guides project development. The final product of a CRM consists of the written assignments assembled into a report or poster. In these projects, writing serves to organize and manage group effort, structure group activities, focus student attention, promote the exchange of ideas and constructive criticism, and enhance understanding. Writing assignments also help track individual cont...

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Two types of enclaves are found in the main syenite unit of the Cretaceous igneous complex at Ascutney Mountain, Vermont. Metasedimentary xenoliths consist of fine-grained hornfels with bands and lenses of coarser-grained quartz. Mafic enclaves of igneous origin contain phenocrysts in an aphanitic groundmass. We have reconstructed the three-dimensional shapes of a metasedimentary xenolith and 15 mafic enclaves from serial sections created by sawing blocks of syenite containing enclaves into parallel slabs. Enclave outlines on each side of the slabs were digitized and computer software was used to reconstruct three-dimensional images. Results are presented for three typical mafic enclaves and for both the hornfels and quartz portions of the metasedimentary enclave. Stereographic drawings show that the metasedimentary and the mafic enclaves have complex shapes in three dimensions, with branches, lobes, and concavities. Syenite from the host magma penetrated the xenolith along hornfels-quartz boundaries, producing complementary concave-convex surfaces. Adjacent mafic enclaves commonly have complementary surfaces and are not observed to impinge on each other. Syenite also penetrated both the metasedimentary and mafic enclaves along finger-like projections. Syenite infiltration into the metasedimentary xenolith apparently occurred by a combination of mechanical and chemical processes, controlled by the presence of compositionally favorable boundaries. A similar model is suggested for the mafic enclaves, in which syenite infiltration occurred preferentially along quartz-feldspar grain boundaries or involved larger-scale compositional heterogeneities within the mafic enclaves. Three-dimensional views of enclaves provide information complementary to chemical and petrologic studies of assimilation.

Hydrothermal convection initiated by emplacement of the gabbro-syenite complex of Abu Khruq into the Egyptian basement 89 Ma ago systematically altered the trace element and isotopic compositions of the syenites. The scale of Sr transport in migrating solutions was far larger than the scale of Sr isotopic equilibration within rocks. As a result, Sr exchange was heterogeneous in the syenites, an effect which can be observed on three different scales. Within grains of a single mineral species, heterogeneities are related to grain boundaries and microfractures through which fluids migrated. Among minerals within rock samples, heterogeneities are related to differences in susceptibility to Sr alteration. Among samples within a single unit, heterogeneous alteration is apparently related to differences in permeability close to fracture zones. During the early stages of alteration radiogenic Sr derived from the country rocks was added to the syenites, causing small net changes in concentration (5 ppm ave.). Some Rb-Sr mineral isochrons from single rock samples yield the emplacement age because isotopic equilibration of this added Sr sometimes occurred within rock specimens. However, regressions of the whole-rock Rb-Sr data yield apparent ages that are about 10 Ma too old. Later stage alteration involved larger changes in whole-rock Sr concentration (45 ppm ave.) but had little further effect on the isotopic relationships because the Sr was derived from cogenetic gabbros rather than the country rocks. Alterations of Rb, Sr, and Sr isotopic compositions are not well correlated with changes in 18O/16O ratio because mineralogy played an important role in decoupling trace element and oxygen isotopic alteration. In general, the absence of such correlations for whole-rock data is not diagnostic of rocks with unaltered trace element and isotopic compositions. Mineral-scale Sr isotopic heterogeneities associated with grain boundaries and microfractures may be the most unambiguous evidence of trace element mobility.

Two high-grade metamorphic terranes of early Paleozoic age are exposed in southeastern Pennsylvania and northern Delaware. The Wilmington Complex in Delaware was metamorphosed at high temperatures to intermediate-pressure granulite facies during the early Paleozoic. In the West Chester prong, Pennsylvania, a granulite-facies metamorphism of Grenville age was overprinted by a high-pressure, moderate-temperature metamorphism during the Taconic orogeny. We propose a model to account for the different metamorphic conditions in these two terranes. In this model, the Piedmont of northern Delaware and southeastern Pennsylvania was the site of collision between a magmatic arc and the North American continent along an east-dipping subduction zone. The Wilmington Complex was the infrastructure of the arc, whereas the Grenville-age gneisses of the West Chester prong were basement of the North American continent. Between these two relatively rigid plates, there are nappes of remobilized basement and highly deformed forearc-basin and accretion-ary-prism sediments. The North American continent underthrust the hot infrastructure of the volcanic arc, resulting in the high-pressure (9-11 kb), moderate-temperature (650–700 °C), metamorphic overprint on Grenville assemblages of the West Chester prong. The Wilmington Complex was metamorphosed at moderate pressure (6–7 kb) and high temperature (∼800 °C) during the early Paleozoic, under the conditions of the high geothermal gradient that existed at the base of the volcanic arc. The intervening sediments and remobilized basement are all at amphibolite grade, but the grade is highest (above second sillimanite isograd) everywhere adjacent to the structurally overlying Wilmington Complex, suggesting that the isograds in the sediments are inverted beneath the Wilmington Complex. Uplift of this doubled thickness of crust was rapid enough that the gneisses of the West Chester prong were never heated above 700 °C.