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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.

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.