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GeoRef Subject
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Lyra Basin
Alkalic magmatism in the Lyra Basin: A missing link in the late-stage evolution of the Ontong Java Plateau
The Lyra Basin is believed to be a contiguous part of the Ontong Java Plateau (OJP), based on geophysical studies. Volcaniclastic rocks dredged at two sites in the Lyra Basin document another post-plateau episode of magmatism on the OJP; they are olivine-titanaugite-phyric alkali basalts with as much as ~30% modal phenocrysts. Lyra Basin basalts have compositions that vary from picritic (MgO ~22 wt%) to more evolved (MgO ~5 wt%) and have low SiO 2 (41–46 wt%), high TiO 2 (2–4 wt%), and high Na 2 O + K 2 O (1–5 wt%) contents that are distinctly different from tholeiites that compose the main OJP. The 40 Ar- 39 Ar weighted mean age of Lyra Basin basalts is 65.3 ± 1.1 Ma, determined using a single-grain laser fusion method of the ground-mass from the least altered alkali basalt and of biotite separates from differentiated samples. This age is interesting because it is much younger than the main stage of OJP formation (122 Ma) and no ca. 65 Ma alkaline basalts have been found previously near or on the OJP. Incompatible trace element modeling suggests that the volcanic rocks of the Lyra Basin may have been formed by a low degree of partial melting (~3%), predominantly at the garnet-lherzolite stability field from the same OJP mantle source preserved in its thick lithospheric root. However, major and trace elements and isotopic compositions can be better explained by magma mixing of Rarotongan alkali magma and magma derived from OJP-source mantle melting (12% partial melting at garnet stability field) in the ratio of 1:2. Although the trace element compositions of Lyra basalts can be reproduced by OJP-source mantle melting with or without contribution from the Rarotongan hotspot, the lower potassium content of the calculated Rarotongan hotspot-influenced melt is more compatible with that of an average composition of Lyra basalt. These results are consistent with previous reconstruction of the OJP path from 120 Ma to its present position, indicating that it may have passed over the Rarotongan hotspot at 65 Ma. In either case, the petrogenesis of Lyra Basin basalts highlights the role of the thick lithospheric root of the OJP in the late-stage development of the plateau. Additional evidence for episodic late-stage magmatic activity on the OJP helps to elucidate the magmatic evolution of the plateau and may provide insights into the origins of other large igneous provinces.
The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with ( 87 Sr/ 86 Sr) t = 0.704513–0.705105, ( 143 Nd/ 144 Nd) t = 0.512709–0.512749, ε Nd (t) = +3.0 to +3.8, ( 206 Pb/ 204 Pb) t = 18.488–18.722, ( 207 Pb/ 204 Pb) t = 15.558–15.577, and ( 208 Pb/ 204 Pb) t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected ( 187 Os/ 188 Os) t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their ( 176 Hf/ 177 Hf) t = 0.28295–0.28299 and ε Hf (t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.
Tectonic interpretations of the East Caroline and Lyra Basins from reflection-profiling investigations
Location of Ontong Java, Manihiki, and Hikurangi Plateaus; Nauru, East Mari...
British Lower Jurassic Sequence Stratigraphy
Abstract Biostratigraphically well-calibrated exposures of Lower Jurassic rocks in the Wessex, Bristol Channel, Cleveland and Hebrides basins have been remeasured and interpreted in the context of sequence stratigraphy. The aim has been to see whether the stratigraphy and facies are compatible with the hypothesis that relative sea-level changes were synchronous across all these basins. The Lower Jurassic Series can be subdivided into four large-scale (so-called 2nd-order) lithologic cycles, with durations of approximately 3–10 my, that appear to be synchronously developed in all onshore British basins; the cyclic changes in facies become more extreme as the cycles young. Candidate maximum flooding surfaces in the large-scale cycles, identified on the basis of distal starvation, or facies successions indicative of maximal accommodation space in proximal areas, occur in the lower semicostatum zone (Lower Sinemurian), obtusum–oxynotum zones (upper Sinemurian), lower jamesoni zone (lower Pliensbachian) and falciferum–bifrons zones (lower Toarcian). Candidate sequence boundaries in the large-scale cycles, defined on the basis of major unconformities or facies successions indicative of minimal accommodation space in proximal areas, are recognized in the upper turneri zone (mid-Sinemurian), mid -raricostatum zone (upper Sinemurian), basal margaritatus zone (mid-Pliensbachian) and levesquei zone (upper Toarcian). In general, at the large scale, the Lower Jurassic Series of the Dorset area of the Wessex Basin shows the most distal pattern of sediment accumulation, in which condensed sections (limestone or mudrock) correspond to relative sea-level rise or highstand and expanded sections (mudrock or sandstone) correspond to relative sea-level fall or lowstand. In contrast, the Lower Jurassic Series of the Skye, Pabay and Raasay areas of the Hebrides Basin exemplify the proximal pattern of sedimentation in which expanded sections (sandstone and mudstone) correspond to relative sea-level rise or highstand, and condensed sections (sandstone) correspond to relative sea-level fall or lowstand. The Yorkshire coast successions of the Cleveland Basin exemplify an intermediate setting. Significant divergence from this pattern is evident in the Toarcian (and through the Middle Jurassic) deposits over which interval the style of accumulation in the Hebrides is intermediate between that of the Wessex Basin and that of the Cleveland Basin. This indicates a reduction of clastic supply or increase in creation of proximal accommodation space in the Hebrides area relative to Yorkshire that began in the early Toarcian. Lithologic cyclicity at the scale of ammonite zones and subzones (so-called 3rd-order) is recognized in a variety of facies; durations are inferred to be approximately 0.5 to 3 my. In a manner that contrasts with the large-scale cycles, the medium-scale cycles become more weakly expressed upwards through Lower Jurassic successions. The link between medium-scale sedimentary cycles and relative sea-level change is more interpretative than is the case for the large-scale cycles. There are few surfaces that have a definitive expression in all basins considered here; those that do are: candidate maximum flooding surfaces in the lyra and taylori subzones, and at the stokesi–subnodosus subzonal boundary (all major); and candidate sequence boundaries in the mid-jamesoni zone (moderate), and at the base of the stokesi subzone (major). Similarly, there are few surfaces that appear strongly localized, the best examples being candidate sequence boundaries in the subnodosus and gibbosus subzones, which are developed mainly in the south and north respectively. In hemipelagic-dominated, distal facies, there is evidence to suggest that stratigraphic condensation is a consequence of relative sea-level fall rather than rise; relative sea-level rises in these settings appear to have generated erosion surfaces. A new relative sea-level curve is presented with medium- and large-scale cycles shown that are compatible with all the successions considered in this study.