Further detrital zircon evidence for peri-Gondwanan blocks in the central Appalachian Piedmont Province, USA

Abstract

Evidence for exotic terranes in the central Appalachian Piedmont Province is fragmented between central Virginia, northern Maryland, and southeastern Pennsylvania. Here we present laser ablation-inductively coupled plasma mass spectrometry data from detrital zircon that support the presence of an exotic terrane in this region. U–Pb dating of detrital zircon from new samples of the Storck quartzite (central Virginia) and the Hoods Mill rocks (northern Maryland) confirms the presence of a major peak at ca. 630–610 Ma in these units. These ages are consistent with derivation from Gondwana, but not Ediacaran Laurentia. Further, modern εHf values of five of the ca. 670–580 Ma grains in these samples are inconsistent with derivation from the few plutons of this age in Ediacaran Laurentia. The Loch Raven Schist and a metasedimentary xenolith in the Wilmington Complex contain a smaller proportion of ca. 670–580 Ma grains than the Storck quartzite and the Hoods Mill rocks, but more such grains than in sediment derived from Ediacaran Laurentia, so we tentatively conclude that these two units also received sediment from Gondwana. Detrital zircon ages from the Piney Run Formation, Pleasant Grove Schist, Prettyboy Schist, and Wissahickon Formation allow sediment provenance solely in Ediacaran Laurentia. We also present new zircon spot U–Pb and Lu–Hf isotopic data from western Newfoundland plutons for comparison with these types of data from the detrital zircon. Intrusion ages of the Steel Mountain Anorthosite, Disappointment Hill Tonalite, and Round Pond Granite are 608 ± 12, 600 ± 8, and 590 ± 9 Ma, respectively. None of these units was derived entirely from the depleted mantle.

Mineralogy of the Steel Mountain Anorthosite Complex, Western Newfoundland Appalachians, Canada: Petrogenesis and Tectonic Affinity

“…It is tectonically juxtaposed with the Corner Brook Lake Block of Proterozoic and Silurian crystalline rocks [1]. U-Pb zircon dating of the Steel Mountain complex has yielded both 1.14 Ga and 0.6 Ga ages [2]. In the adjacent Corner Brook Lake Block, basement gneisses give U-Pb zircon ages of 1.5 Ga [1], and there are well-dated Ediacaran granitoid plutons [2].…”

“…U-Pb zircon dating of the Steel Mountain complex has yielded both 1.14 Ga and 0.6 Ga ages [2]. In the adjacent Corner Brook Lake Block, basement gneisses give U-Pb zircon ages of 1.5 Ga [1], and there are well-dated Ediacaran granitoid plutons [2]. The Steel Mountain complex is some 50 km long and consists principally of anorthosite and gabbronorite, previously mapped by van Berkel [11] and investigated mineralogically by Collins [12].…”

“…Two bulk samples of anorthosite and two of gabbronorite were analyzed for Nd/Sm isotopes (Table 2). The samples define an apparent isochron of 1.2 ± 0.2 Ga (Figure 9a) and have an average depleted-mantle-model age of 1.38 Ga. ε Nd was calculated for three possible emplacement ages of the rocks: 0.6 Ga (based on [2]), 1.2 Ga (our isochron), and 1.5 Ga (based on [1]) (Table 2; Figure 9b). The strongly negative values of ε Nd at 0.6 Ga are quite unlike any Proterozoic anorthosites in the literature (Figure 14 of [4]) and the values at 1.5 Ga are similar only to unusually high values found in the literature.…”

“…The age of the Steel Mountain complex is unresolved. U-Pb analysis of zircons has yielded ages of ~1.14 and ~0.6 Ga ages [2], as well as one poor quality 1.27 Ga age [3].…”

“…(1) To describe and interpret the unusual evidence for hydrous conditions in the evolution of the Steel Mountain anorthosite. (2) To use Nd-Sm isotopes, 40 Ar-39 Ar geochronology, and the mode of occurrence of zircon to further constrain the age and tectonic history of the anorthosite massif.…”