MARGARET N. REES, EUGENE I. SMITH, and DEBORAH L. KEENAN, Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154
ERNEST M. DUEBENDORFER, Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011
A critical element in reconstructing the paleo-Pacific margin of Gondwanaland is the Ellsworth-Whitmore mountains terrane that lies between the Transantarctic Mountains and Antarctic Peninsula (Storey et al. 1988). Paleomagnetic data suggest that during the Cambrian, the terrane lay near the juncture of Africa and Antarctica (Grunow 1995). Nevertheless, much of the tectonic history of this terrane is equivocal and numerous conflicting models have been proposed regarding its tectonic setting and timing of magmatism (Vennum et al. 1992; Grunow 1995; Curtis and Storey 1996; Dalziel 1997). Thus, one aspect of our larger Ellsworth Mountains project focused on the geochemistry and geochronology of magmatic rocks in the northern Heritage Range of the Ellsworth Mountains (figure 1). We conducted fieldwork during the 1996-1997 austral summer and subsequently completed laboratory analyses.
The Cambrian Heritage Group is composed of volcanic and sedimentary rocks (figure 2) (Webers et al. 1992) that are unconformably overlain by the Ordovician(?)-Devonian siliciclastic Crashsite Group (Duebendorfer and Rees in press). The dominant structures in the range, which are attributed to the Triassic Ellsworth/Gondwanide Orogeny, are north-northwest-trending folds and a series of east-vergent stacked thrust sheets that have disrupted the stratigraphic succession. In addition, the Heritage Group preserves evidence of an earlier pre-Crashsite Group deformation that is attributed to deformation within the Ross orogen (Duebendorfer and Rees in press).
Volcanic rocks within the dominantly sedimentary succession of the Heritage Group are present in the Union Glacier and Springer Peak formations (figure 2) (Webers et al. 1992). In the Union Glacier Formation, basalt to andesite hyaloclastite deposits, and flows and interbedded sedimentary rocks locally are cut by dikes of basalt. The hyaloclastite deposits have yielded uranium/lead (U/Pb) zircon ages of 512±14 million years (Van Schmus personal communication). This date, together with other stratigraphic data (Duebendorfer and Rees in press) and the timescale of Shergold (1995), suggests deposition during the late Early Cambrian or early Middle Cambrian.
The Union Glacier volcanic rocks are subalkaline, tholeittic basalt and picritic basalt with 50 to 62 weight percent silica (SiO2). Alumina (Al2O3), titania (TiO2), ferric iron (FeO), lime (CaO), magnesia (MgO), and soda (Na2O) decrease with increasing SiO2. Their magnesium number (Mg#; magnesium divided by the sum of magnesium plus iron) varies from 42 to 65. These rocks are enriched in light rare earth elements (LREE) when compared to chondritic abundances (60-200x) and display negative niobium (Nb), tantalum (Ta), and titanium (Ti) anomalies. Epsilon neodymium (Nd) varies from +2 to -1, and initial strontium-87/strontium-86 (87Sr/86Sr) between 0.7043 and 0.7095. Their geochemistry is very similar to that of mid-oceanic ridge basalt from the Gulf of California (Saunders et al. 1982), and they have trace element abundances reflecting asthenospheric and lithospheric mantle and crustal components. Nd model ages of 0.9 to 1.0 billion years from the Union Glacier volcanic rocks (Walker personal communication) may suggest the age of the underlying lithosphere of the newly formed narrow ocean basin (figure 3A).
Pillow basalt and flows cut by diabase and gabbro dikes and sills are present in the Springer Peak Formation (figure 2). Locally, isolated basalt flow lobes are interbedded with latest Middle Cambrian fossiliferous shale and limestone that indicate mafic magmatism continued in the region until approximately 500 million years ago, using the timescale of Shergold (1995).
Springer Peak Formation volcanic rocks are subalkaline, calc-alkalic basalt, andesite, and trachyandesite with 37 to 50 percent SiO2. Al2O3, TiO2, and CaO decrease with increasing SiO2. Their Mg# varies from 42 to 65. These rocks have low rubidium (Rb), potassium (K), and Sr due to alteration and lack high field strength element (Nb, Ta) anomalies when normalized to primitive mantle. They are moderately enriched in LREE (40-80x chondrite), have an epsilon Nd of +5, and initial 87Sr/86Sr of 0.705. The basalt is similar to enriched mid-oceanic ridge basalt (MORB) although their higher barium (Ba) and Sr may suggest either source heterogeneity, alteration, or minor sediment input. We suggest that they erupted in an ocean basin wider than that represented by Union Glacier volcanics and in which the lithospheric mantle had been delaminated (figure 3 B ).
Dacite and rhyolite sills and dikes were observed in the Springer Peak Formation on Yochelson Ridge in the Heritage Range. These rocks, however, have yielded zircon U/Pb dates of 4986 million years (Van Schmus personal communication). Again using the timescale of Shergold (1995), these rocks could represent a Late Cambrian magmatic episode that postdated the Springer Peak Formation and predated deposition of the Crashsite Group.
These intrusions are calc-alkaline dacite and rhyolite with SiO2 content between 75 to 87 percent SiO2, and Mg# between 22 and 40. They are enriched in large-ion lithophile elements (LIL) and LREE (500x chondrite) but depleted in Ba and Sr. They have negative anomalies at Nb and zirconium (Zr), a very strong negative anomaly at Ti when compared to primitive mantle, and distinct negative europium (Eu) anomaly compared to chondrite. Epsilon Nd is +0.5 and intitial 87Sr/86Sr is 0.713. Tectonic discrimination diagrams suggest that the dacite and rhyolite intrusives formed in a continental arc setting. Furthermore, the geochemical and isotopic differences preclude these more felsic rocks of Yochelson Ridge from being produced by fractional crystallization of magmas that produced the mafic succession in the Springer Peak. Thus, the later felsic rocks represent closing of the narrow ocean basin and onset of subduction related magmatism (figure 3 C ).
Our geochemical and geochronological study of the Union Glacier and Springer Peak formations of the Heritage Group in the Ellsworth Mountains indicates opening of a narrow ocean basin during late Early through Middle Cambrian time. The subsequent Late Cambrian arc magmatism together with deformation and low-grade metamorphism of the Heritage Group and the angular unconformity at the base of the overlying Crashsite Group are compelling evidence that the Ellsworth-Whitmore mountains terrane lay within the Cambrian mobile belts of the paleo-Pacific-facing margin of Gondwanaland.
This research was supported by National Science Foundation grants OPP 92-20395 and OPP 93-12040.
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