LeeAnn Srogi

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

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.

The role of residual melt migration in producing compositional diversity in a suite of granitic rocks

Abstract 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.

A Values Framework for Students to Develop Thoughtful Attitudes about Citizenship and Stewardship.

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.

Monazite age constraints on the tectono-thermal evolution of the central Appalachian Piedmont

Abstract 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.