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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

John N. Aleinikoff^,1, William S. Schenck², Margaret O. Plank², LeeAnn Srogi³, C. Mark Fanning⁴, Sandra L. Kamo⁵ and Howell Bosbyshell⁶

¹ U.S. Geological Survey, MS 963, Denver Federal Center, Denver, Colorado 80225, USA
² Delaware Geological Survey, Newark, Delaware 19716, USA
³ Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA
⁴ Research School of Earth Sciences, Australian National University, Mills Road, Canberra ACT 0200, Australia
⁵ Jack Satterly Geochronology Laboratory, Geology Department, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada
⁶ Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA

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.

Key Words: Wilmington Complex • U-Pb geochronology • SHRIMP • zircon • monazite

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