High HFSE Research Articles

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge, Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ɛNd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (ɛNd=+7.5−+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ɛNd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.

Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks

Isotopic and geochemical data are presented from the northern and central Lesser Antilles arc. Island arc tholeiites from the northern islands are restricted in 143Nd 144Nd and 87Sr 86Sr and show large enrichments in LILE relative to other trace elements, as exemplified particularly in high Ba/La ratios. These volcanic rocks represent typical island arc compositions, similar to those described from the Aleutians, Marianas and South Sandwich Islands. In contrast, samples from the central islands (Martinique and St. Lucia) are highly variable in isotopic and trace element ratios, and reflect overprinting of primitive island arc magma chemistry by subsequent crustal contamination. Such magmas therefore do not represent island arc mantle source characteristics. Northern Lesser Antilles arc mantle sources are displaced from MORB Pb isotopic compositions towards the higher 207Pb 204Pb ratios characteristic of sediments. Also, Sr and Nd isotopic compositions are highly restricted and distinct from those of MORB. As a result, mixing models between MORB source and subducted sediment are tightly constrained in terms of the allowable proportion of sediment. The characteristically high LILE REE and LILE HFSE ratios of island arc volcanics cannot be easily reconciled with sediment + MORB source mixing. Buffering of isotopic compositions together with considerable relative LILE enrichment might be achieved by mixing between mantle and a hydrous fluid which has equilibrated with not only sediment but also a large proportion of variably altered oceanic crust. The fluid has isotopic compositions closer to those of MORB, thereby permitting a greater range of mixing proportions. At the same time, the LILE characteristics of the modified mantle source are dominated by the very high concentrations of these elements in the fluid. High degrees of partial melting of mantle modified by a hydrous fluid reproduce the trace element pattern of the source in the derivative volcanic rocks and account for the depletion of HFSE and HREE relative to MORB.

Early Proterozoic Volcanic Regimes in Southwestern North America

Summary Five early Proterozoic volcanic terranes of the following ages occur in the southwestern United States: 1760–1800 Ma, 1730–1740 Ma, 1720 Ma, 1680–1700 Ma, and 1650 Ma. Volcanics are typically bimodal in composition and, for the most part, represent submarine eruptions. In terms of the least mobile incompatible element distributions, basalts from the 1760–1800 Ma and 1720 Ma terranes are similar to basalts in evolved oceanic arcs and associated incipiently opened back-arc basins. Basalts from the other three terranes resemble basalts from continental-margin arcs and associated back-arc basins. Andesites are important volcanic components at only two localities and they are similar chemically to andesites from evolved oceanic and continental-margin arcs. Felsic volcanics define two major geochemical populations. Those from the 1760–1800 Ma and 1720 Ma terranes, which have low HFSE and heavy REE contents, are similar in composition to felsic volcanics from modern arc systems while those from the other three terranes, which have relatively high HFSE and heavy REE contents, resemble felsic volcanics from rifts or back-arc basins in or near continental crust. The 1760–1800 Ma terrane appears to reflect a succession of oceanic arcs accreted to the Archaean craton from the southeast and the 1730–1740 Ma terrane, an incipient back-arc basin formed within the 1760–1800 Ma terrane. The 1720 Ma terrane is interpreted as an evolved back-arc basin in which depleted mantle is tapped. The 1680–1700 Ma and 1650 Ma terranes are remnants of an extensive continental-margin arc system and associated back-arc basins. Bimodal volcanism may reflect differences in preservation of arc and back-arc successions.