Contrasting Old and Young Volcanism from Aitutaki, Cook Islands: Implications for the Origins of the Cook–Austral Volcanic Chain

The Cook–Austral volcanic lineament is thought to be the product of three overlapping South Pacific hotspot tracks: Macdonald, Arago, and Rarotonga. However, the entire Rarotonga hotspot track consists of volcanism on just two islands, Rarotonga and Aitutaki, making it difficult to evaluate a hotspot origin for the young volcanism on these islands. We present new Sr–Nd–Hf–Pb–He isotopic and major and trace element data on Aitutaki—which has two periods of volcanism, older (9·39 Ma) and younger (1·382–1·941 Ma)—and Atiu in the Cook Islands to evaluate a hotspot origin. Like Atiu, the ‘older volcanic series’ at Aitutaki was tentatively linked with the Arago hotspot based on its age, but this link has never been tested with geochemistry. New geochemical data on the older Aitutaki volcanic series show clear affinities to the Arago hotspot volcano Atiu. In contrast, the younger Aitutaki volcanic series, erupted on a near-atoll, exhibits evidence for a rejuvenated, non-plume volcanic origin. If Rarotonga (1·157–1·697 Ma) and the young series at Aitutaki share a common heritage and mechanism for formation as previously proposed—they erupted only 260 km apart over an overlapping time period and exhibit overlapping radiogenic isotopic compositions—then Rarotonga lavas may also represent a subaerial rejuvenated volcanic stage capping an older volcano related to a different hotspot, consistent with no direct involvement from a mantle plume in the origin of subaerial lavas at Rarotonga. Rarotonga, Aitutaki, and Atiu exhibit some of the most extreme enriched mantle (EM) signatures in the Cook–Austral volcanic lineament. There are three geographically separated geochemical domains along the Cook–Austral volcanic lineament—a northern geochemical domain with EM geochemical signatures is separated from a southern EM domain by a 1000 km long central HIMU (‘high μ’, or high 238U/204Pb) domain—and Rarotonga, Aitutaki, and Atiu are part of the northernmost geochemical domain. There is a possible role for lithospheric thickness in controlling geochemical compositions of melt extracted from the mantle along the Cook–Austral volcanic lineament, particularly across the Austral Fracture Zone. However, this is complicated by the lineament’s juxtaposition with the South Pacific Superswell, which may modify lithospheric behavior and complicate a simple lithospheric thickness model describing variable hotspot volcano compositions in the region.

Early Jurassic magmatism of the Kobyuminsky system of grabens (Verkhoyansk frame of the Siberian platform) and its geodynamic nature

The Kobyuminsky system of grabens was formed in the Early Jurassic within the Verkhoyansk passive margin of the Siberian platform. The volcanic complex of grabens is represented by basalts with elevated TiO2 contents (> 1.8 wt%). According to geochemical characteristics, the rocks of the volcanic series occupy an intermediate position between the basalts of the oceanic islands (OIB) and the basalt-rich mid-oceanic ridges (E-MORB). The data obtained led to the conclusion that the Kobuminsky system of grabens within the passive margin of the Siberian continent arose in connection with the activity of a small mantle plume.

Palaeoecological analysis of Sentolo Formation based on foraminifera fossil assemblage from stratigraphic cross-sectional measurements in Pengasih, Kulon Progo, Yogyakarta

Sentolo formation that is included in the Kulon Progo stratigraphic series has a wide distribution. Although there have been a number of studies on it, more detailed research is needed, as there are still many unexplored outcrop locations. Moreover, there are still a lot of questions about the stratigraphic relationship between Sentolo Formation and Oligocene-Miocene volcanic series of Kulon Progo. Based on this background, this study is focused on the existence of foraminifera fossil in Sentolo Formation as an indicator of palaeoecological conditions at the time Sentolo Formation was deposited. This research is very important to get a detail understanding about the indicators that are influential at the time of deposition process of Sentolo Formation in research area. Based on palaeoecological analysis, the palaeoecology at the time Sentolo Formation deposited was normal marine continental slope waters. Meanwhile, the community structure of foraminifera indicates a stable water conditions in a marine environment that was not disrupted by volcanism, so it becomes a good factor for the growth of foraminifera. Whereas the presence of volcaniclastic material mixed with carbonate which is occurred in the lower part of Sentolo Formation were interpreted as reworked of older volcanic rock.

Potential hydrocarbon reservoirs of Albian–Paleocene age in the Nuussuaq Basin, West Greenland

The onshore Nuussuaq Basin in West Greenland is important for hydrocarbon exploration since many of the key petroleum systems components are well exposed and accessible for study. The basin has thus long served as an analogue for offshore exploration. The discovery of oil seeps on Disko, Nuussuaq, Ubekendt Ejland, and Svartenhuk Halvø (Fig. 1) in the early 1990s resulted in exploration onshore as well. In several wells, oil stains were observed in both the siliciclastic sandstone and in the volcanic series. An important aspect of any petroleum system is a high quality reservoir rock. The aim of this paper is to review petrophysical aspects of the reservoir potential of key stratigraphic intervals within the Nuussuaq and West Greenland Basalt groups. Reservoir parameters and porosity–permeability trends for potential siliciclastic and volcanic reservoirs within the relevant formations of the Nuussuaq Basin are discussed below.