Hercynian basement rocks were recovered at the oceanward edge of the thinned continental crust at Sites 1067 and 1068 on the same structural high as Leg 149 Site 900 (Fig. F2). The rock types included metagabbros, amphibolites, microamphibolites, meta-tonalites, and meta-anorthosites. The Leg 173 samples have been pervasively metamorphosed during the development of foliation under greenschist facies conditions and the growth of metamorphic phases (Whitmarsh, Beslier, Wallace, et al., 1998). In this report, only the metagabbros, amphibolites, and microamphibolites will be discussed. The Site 1067 amphibolites consist of blue-green amphibole and plagioclase with minor amounts of epidote, chlorite, Fe-Ti oxides, apatite, zircon, sphene, and quartz. The amphibolites exhibit strong foliation in the upper and middle portions of the core and weak foliation in the lower portions of the core. The Site 1068 samples occur as clasts within a sedimentary breccia unit overlying serpentinized peridotite and include amphibolite, microamphibolite, and metagabbro rock types. The Site 1068 amphibolites are similar in appearance to the Site 1067 amphibolites. Site 1068 microamphibolites exhibit fine-grained textures of green amphibole and plagioclase compared to the other amphibolites. Site 1068 metagabbros display clinopyroxene preserved with plagioclase, green hornblende, Fe-Ti oxides, and chlorite. Based on the geochemical results presented below, the protoliths of the Leg 173 amphibolites and metagabbros are believed to have been basalts, diabases, and/or fine-grained gabbros representing liquid compositions.
In the Site 1067 amphibolites, Mg numbers = 49.1-59.4, TiO2 contents = 1.47-2.74 wt%, Zr contents = 75-338 ppm, and Y contents = 23.0-62.9 ppm (Tables T1, T2; Figs. F3, F4). The Site 1068 whole-rock compositions exhibit larger variations from primitive to more evolved compared to the continuous core of amphibolite from Site 1067. In the Site 1068 amphibolites, Mg numbers = 48.6-71.3, TiO2 contents = 0.22-1.58 wt%, Zr contents = 13-143 ppm, and Y contents = 6.3-20.1 ppm. The Site 1068 microamphibolite and metagabbro compositions generally lie within the range of the Site 1068 amphibolites. Sites 1067 and 1068 samples display a continuum from primitive to more evolved compositions for both the Leg 173 shipboard analyses and the results presented here (Figs. F3, F4). When compared with Leg 149 results for the Site 900 metagabbros and Site 899 basalts and diabases, the Sites 1067 and 1068 samples exhibit a larger range with more trace element- enriched compositions (Figs. F3, F4). However, the Site 899 basalt and diabase major element compositions of MgO, CaO, and K2O have been affected by seafloor weathering and low-grade metamorphism (Seifert and Brunotte, 1996), which produced the high Mg numbers shown in Figure F3. When compared with Gorringe Bank gabbros and dolerites and with oceanic gabbros recovered from the Mid-Atlantic Ridge (Leg 153 and Hayes Fracture Zone) and from the Southwest Indian Ridge (Leg 118), the Sites 1067 and 1068 samples are comparable but more enriched in Zr (Fig. F4). Similar enrichment trends are apparent for the ophiolitic gabbros, amphibolites, and diabases from the Bay of Islands Complex (BOIC), Newfoundland, Canada, and Leg 173 samples (Figs. F3, F4) that are distinct from the Zr and Y trends of the Gorringe Bank samples and the oceanic gabbros.
The REE compositions combined with other trace elements, however, provide the most diagnostic geochemical signature for comparison of the Leg 173 amphibolites and metagabbros with other rock suites and identification of the protoliths of these metamorphic rocks. The (La)cn contents and (La/Yb)cn ratios exhibit light rare earth element (LREE) enrichments. The Site 1067 amphibolites have (La)cn contents and (La/Yb)cn ratios, respectively, of 27-89 and 1.2-4.7; the Site 1068 amphibolites have 11-73 and 1.7-9.2, microamphibolites have ~26 and ~1.7, and metagabbros have ~29 and ~3.5 (Table T2; Fig. F5). The oceanic gabbros (Leg 153 and Hayes Fracture Zone), Gorringe Bank gabbros and dolerites, and Site 900 metagabbros contain much lower REE contents with flatter REE patterns (lower La/Yb ratios) typical of cumulate gabbros, except when highly enriched in modal plagioclase, compared to most of the enriched Leg 173 amphibolites and metagabbros. The Site 1067 amphibolites exhibit enriched REE contents but with low La/Yb ratios similar to the BOIC gabbros and diabases (Fig. F5). The Leg 149 basalts and diabases, however, exhibit similarly enriched REE contents with high La/Yb ratios compared to the Site 1068 amphibolites and metagabbros.
Extended REE spidergrams for the Leg 173 amphibolites and metagabbros (Fig. F6) show the LREE-enriched nature of these samples with steep patterns (higher La/Yb). Zr, Ti, and Sr are also included in these spidergrams to provide a comparison between the incompatible trace elements and the rare earth elements. The rare earth elements Zr, Y, and Ti generally are considered to be immobile during metamorphism with respect to major rock-forming minerals (Ludden and Thompson, 1979; Grauch, 1989). Because Sr may be mobile during metamorphism and weathering, Sr is used here only as a comparison with Eu contents as a ratio (Sr/Sr*) in order to examine the sample patterns. Rocks with plagioclase accumulations (i.e., gabbros) may exhibit positive Eu/Eu* and Sr/Sr* anomalies (Fig. F7), whereas rocks that have had plagioclase removed (i.e., basalts) may show negative Eu/Eu* and Sr/Sr* anomalies (Smith, 1994). Also shown in Figure F6 are the Mg numbers for selected samples that show that the more evolved compositions (lower Mg numbers) have higher REE enrichments compared to the primitive compositions, indicating that the range in REE enrichments is probably the result of fractionating basaltic magmas within each group of samples rather than variable degrees of partial melting in the mantle source region.
The Site 1067 amphibolites (Fig. F6A, F6B) display generally zero to slightly negative Eu/Eu* anomalies of 0.8-1.1 and negative Sr/Sr* anomalies, unlike the Site 900 metagabbros (Fig. F7). The Site 900 metagabbros have strongly positive Eu/Eu* and Sr/Sr* anomalies that are more typical of plutonic cumulate rocks with plagioclase. The Site 1067 amphibolites also display positive to negative Zr/Zr* values of 0.7-1.6, which can be directly related to the presence or absence of zircon in these samples. Site 1068 amphibolites (Fig. F6C) have steeper patterns (higher La/Yb), unlike the Site 1067 samples and also approximately zero Eu/Eu* anomalies of 1.0-1.1. Site 1068 microamphibolites and metagabbros (Fig. F6D, F6E) exhibit flatter patterns (lower La/Yb), zero Eu/Eu* anomalies, and zero to positive Sr/Sr* anomalies of 1.1-1.5. The Site 899 basalts and diabases exhibit Eu/Eu* (0.9-1.3), Sr/Sr* (0.2-2.9), Zr/Zr* (0.6-1.1), and Ti/Ti* (0.7-1.0) anomalies that are more comparable to the ranges displayed by the Sites 1067 and 1068 samples. The lack of significant positive Eu/Eu* and Sr/Sr* anomalies in either Site 1067 or 1068 samples is distinct from the Site 900 metagabbros, indicating that the Leg 173 amphibolites and metagabbros probably did not form as cumulate gabbros, as has been suggested for the Site 900 metagabbros (Seifert et al., 1996; Cornen et al., 1996). In fact, the REE patterns of the Site 1067 amphibolite samples appear to be more similar to the Site 899 basalts and the Site 1068 samples are more similar to the Site 899 diabases (Fig. F7). These Leg 173 amphibolites and metagabbros may represent basaltic, diabasic, and/or fine-grained gabbroic protoliths from incompatible trace element-enriched liquid compositions rather than accumulations of fractionated minerals from a magma chamber or conduit system. These basaltic liquids could have been emplaced and trapped, allowing for the crystallization of the mineral phases such as plagioclase and clinopyroxene, with later metamorphism producing the observed mineral assemblages and grain coarsening.