1. Most samples, especially in the more evolved rock types, have negative Eu anomalies. The more evolved samples (andesites through rhyolites) typically have concave patterns, reflecting depletion of the middle (M)REEs.
Major- and trace-element compositions for 24 whole-rock samples (eight samples from Site 977 and 16 from Site 978) are reported in Table 5. The samples from both sites range in composition from basalt to rhyolite (Fig. 6A) and belong to the tholeiitic, calc-alkaline, high-K calc-alkaline, and shoshonite series (Fig. 6B). The samples show a greater range in chemical composition than those from Cabo de Gata and extend to more mafic compositions (Fig. 6A).
As is illustrated in Figure 7, the major elements and compatible trace elements show systematic variations of SiO2 content. Relatively immobile elements, such as Al2O3, FeO* (total iron as FeO), TiO2, V, Co, and Cr, decrease with increasing SiO2. Dacite sample 7647-U (161-978A-47R-1, 0-6 cm) has elevated TiO2 and FeO*, which probably reflect the large amount of titanomagnetite in this sample (~10%). If the four samples with H2O > 4 are excluded, then CaO, CaO/Al2O3, MgO, P2O5, and Sr show inverse correlations with SiO2. In the four samples with H2O > 4, the higher MgO, but lower CaO, CaO/Al2O3, P2O5, and Sr than in the other samples at similar SiO2, most likely reflect the effects of seawater alteration, consistent with the greater degree of alteration observed in the thin sections of these samples. Sample 7647-H (161-978A-47R-1, 0-6 cm) has anomalously low Al2O3, TiO2, and Sr and anomalously high CaO/Al2O3 and MgO (Table 5). Most incompatible elements show crude positive correlations with SiO2, but correlate much better with each other (Fig. 8).
Compared to both mid-ocean ridge basalt (MORB) and ocean-island basalt (OIB), basaltic samples from the Alboran have high ratios of "fluid-mobile" (also often referred to as the large-ion-lithophile elements [LILE]) to "fluid-immobile" (Th, high-field-strength elements [HFSE], and REE) elements (e.g., Ba/Nb [21-65], Ba/Ta [248-870], Ba/La [7-34], K/Nb [617-10,917], K/Ta [7308-127,077]), and of light (L)REE to HFSE (e.g., La/Nb [1.0-6.2] and La/Ta [12-83]), but low ratios of immobile to mobile elements (e.g., Nb/U [1.2-5.6] and Ce/Pb [0.7-15]). The ratios in the Alboran samples, however, are characteristic of subduction-related volcanic rocks.
The REE patterns, normalized to chondrite using the values of Boynton (1984), are shown in Figure 9. The basalts and basaltic andesites can be divided into two groups based on their REE abundances: (1) the LREE-depleted group has patterns similar to those observed in normal (N) and enriched (E) MORB, and (2) the LREE-enriched group has patterns characteristic of OIB (e.g., basalts from Saint Helena and Gomera (Canary Islands; Fig. 9A). The LREE-depleted group contains Samples 7520 (161-977A-60X-1, 27-31 cm), 7647-H (978A-47R-1, 0-6 cm), 7647-B (978A-47R-1, 0-6 cm), and 7518 (978A-47R-1, 0-6 cm), and the LREE-enriched group contains samples 7647-Q (978A-47R-1, 0-6 cm), 7523 (977A-62X-1, 13-19 cm), and 7519 (978A-46R-1, 4-9 cm). The andesites, dacites, and rhyolites generally show LREE enrichment with (La/Yb)N 1. Most samples, especially in the more evolved rock types, have negative Eu anomalies. The more evolved samples (andesites through rhyolites) typically have concave patterns, reflecting depletion of the middle (M)REEs.
The incompatible-element diagrams (Fig. 9), normalized to primitive mantle after Hofmann (1988), are strongly spiked. Pronounced troughs occur at Nb and Ta in all samples. Peaks occur in most, but not all samples, at U and Pb. In general, mobile elements, such as Rb, Ba, U, K, and Sr, are enriched relative to less mobile elements such as high field strength elements (HFSE, for example Nb, Ta, Zr, Hf) and the REE. Th generally behaves similar to the LREEs (e.g., samples with high La have high Th and vice versa). The basalt groups also display distinct characteristics on multi-element diagrams (Fig. 9A). The LREE-enriched basalts are also characterized by relative enrichments in Th and U but depletions in Rb, Ba, K, Zr and Ti, as well as Nb and Ta. The LREE-depleted basalts show relative enrichments in Rb, U, K, Pb and Sr but relative depletion in Th, as well as in Nb and Ta.
As is illustrated in Figure 10, Figure 11, and Figure 12, the basalt groups also have distinct immobile, incompatible element ratios. For example, the LREE-enriched group has lower Ta/Nd, Sm/Nd, Y/Nb and Zr/Nb, but higher Ta/Yb, Th/Yb, Sm/Yb, Gd/Yb and La/Nb than the LREE-depleted group. These ratios are unlikely to have been significantly affected by low-temperature or hydrothermal alteration.
