Robert Bolhar

Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?

Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation.

Timing, slip rate, displacement and cooling history of the Mykonos detachment footwall, Cyclades, Greece, and implications for the opening of the Aegean sea basin

We constrain the slip and cooling history of the Mykonos detachment footwall using thermochronometry. A U–Pb zircon age of 13.5 ± 0.3 Ma dates intrusion of the Mykonos monzogranite. 40Ar/39Ar hornblende and biotite ages from the monzogranite are 12.7 ± 0.6 Ma and 10.9 ± 0.6 Ma, whereas zircon and apatite fission-track ages range from 13 ± 0.8 Ma to 10.7 ± 0.8 Ma and 12.5 ± 2.2 Ma to 10.5 ± 1.8 Ma. (U–Th)/He ages range from 13.6 ± 0.6 Ma to 9.0 ± 0.7 Ma for zircon and 11.1 ± 0.5 Ma to 8.9 ± 0.4 Ma for apatite. The ages in part overlap within 2σ errors and together with the long apatite fission-track lengths (>14 μm) support rapid cooling at rates >100 °C Ma−1. The low-temperature thermochronometric ages decrease east-northeastwards in the direction of hanging-wall transport on the Mykonos detachment. Age–distance relationships show that the Mykonos detachment slipped at an average rate of 6.0 +9.2/−2.4 km Ma−1 causing c. 30 km of offset and c. 12 km of exhumation. This result indicates that Miocene low-angle normal faulting was not important for the exhumation of the Cycladic blueschist unit. The opening of the Aegean Sea basin in the Miocene was controlled by a few large-magnitude low-angle normal faults.

The geochemistry of Archaean shales derived from a mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: Provenance, source area unroofing and submarine versus subaerial weathering

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2–2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1–100 m thick units intercalated with shallow–water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84–4.45) and no significant Eu anomaly (Eu/Eu* = 0.93–0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69–2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.

Pb- and Nd-isotope systematics of stromatolitic limestones from the 2.7 Ga Ngezi Group of the Belingwe Greenstone Belt: constraints on timing of deposition and provenance

Abstract Pb–Pb isochrons have been obtained for stromatolitic limestones from the late Archaean Belingwe Greenstone Belt of Zimbabwe, providing direct age constraints on the deposition of these shallow water marine sediments. Samples from the Manjeri Formation and stratigraphically higher Cheshire Formation yield age estimates of 2706±49 Ma (MSWD=11.5) and 2601±49 Ma (MSWD=0.93), respectively. These data are in agreement with published U–Pb zircon and Pb–Pb whole rock ages of associated volcanics, and we, therefore, interpret our Pb–Pb ages as representing the timing of early diagenesis, thus providing a minimum age for carbonate precipitation. A 2543±70 Ma age (MSWD=5.1) for one sample from the Cheshire Formation is considered to reflect either late-stage diagenesis, a craton wide thermal/chemical disturbance or tectonic activity along the crustal-scale Mtshingwe fault. Calculated model μ1-values for Manjeri and Cheshire limestones are 8.40±0.02 and 9.02±0.01, similar to values for approximately 3.5 and 2.9 Ga Archaean basement units adjacent to the Belingwe Belt. Negative eNd (Tdeposition) and fSm/Nd suggest derivation of the REE from old, LREE enriched continental crust. Two-stage Nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3.5 Ga (Cheshire Formation) and 3.3 Ga (Manjeri Formation), in good agreement with mantle-extraction ages for local basement units (model TDM: 3.5–3.3 Ga).

Continental setting inferred for emplacement of the 2.9–2.7 Ga Belingwe Greenstone Belt, Zimbabwe

New major element, trace element, and Nd isotope data are reported for volcanic rocks from the 2.9 Ga Lower Greenstone and 2.7 Ga Upper Greenstone assemblages of the Belingwe Greenstone Belt. The majority of samples from five distinct stratigraphic units have generally unfractionated to slightly depleted trace element patterns similar to those observed in the small number of previous studies. However, previously unrecognized incompatible element–enriched samples with distinct high field strength anomalies invariably also occur in close association. These are reminiscent of crustally contaminated rocks from other well-documented greenstone settings. Numerical modeling of combined assimilation-fractionation processes using both trace and major elements suggests 10%–30% incorporation of model Archean continental crust in their genesis, in accord with Nd isotope compositions. These new observations are not consistent with a mid-ocean-ridge or oceanic plateau–type setting for Belingwe Greenstone Belt volcanism, but instead favor a model in which melts interacted with preexisting continental crust.

Carbon dioxide-rich coals of the Oaky Creek area, central Bowen Basin: a natural analogue for carbon sequestration in coal systems

High-CO2-containing coal seams in the Oaky Creek area of the Bowen Basin, eastern Australia provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. We conducted mineralogical, stable and radiogenic isotope and major element analyses of mudstones and sandstones adjacent to the coal seams and stable isotope and compositional studies of coal seam gas desorbed from the coals to establish the impact of the high CO2 levels and the mechanisms that keep the CO2 naturally sequestered. Siderite is the earliest carbonate phase present and occurs with kaolinite in mudstones and sandstones. It is interpreted to have formed under low-temperature, reducing conditions where methanogenesis has produced residual 13C-enriched CO2. Enhanced kaolinite concentrations adjacent to a low-CO2-containing coal seam reflect interaction with acidic fluids produced during the coalification of organic matter. Stable isotope data for carbonates and Rb–Sr isochron ages for illitic clays indicate that illitic clay–carbonate assemblages adjacent to both coal seams formed as a result of meteoric hydrothermal activity in the Upper Triassic with more intensive mineralogical reactions evident in the high-CO2 coals. The present-day CO2 in the high-CO2 coals at Oaky Creek was emplaced in the Upper Triassic based on dating of illitic clay minerals from the high-CO2 well and is magmatic or deep crustal in origin. Methane in the coals is of mixed origin, with secondary biogenic CH4 formed by microbial reduction of CO2 predominant in the high-CO2 coals. This suggests that methanogenesis may provide an additional sequestration mechanism for CO2 in coal seams.

Cook Island artifact geochemistry demonstrates spatial and temporal extent of pre-European interarchipelago voyaging in East Polynesia

Significance Oceania, the last region settled on Earth, witnessed the greatest maritime migration in human history. Scholars have debated how and when islands were colonized and the role of postsettlement voyaging in maintaining founding colonies and in subsequent diversification of island societies. We geochemically “fingerprinted” exotic stone artifacts from a well-dated archaeological site in the Cook Islands, matching artifacts to their geological sources and demonstrating that the geographical voyaging network extended beyond the Cook Islands to include the Austral, Samoa, and Marquesas archipelagos—up to 2,400 km distant. We further demonstrate that Polynesian interarchipelago voyaging lasted from about AD 1300 to the 1600s, suggesting that long-distance interaction continued to influence the development of social structures in East Polynesia well after initial colonization. The Cook Islands are considered the “gateway” for human colonization of East Polynesia, the final chapter of Oceanic settlement and the last major region occupied on Earth. Indeed, East Polynesia witnessed the culmination of the greatest maritime migration in human history. Perennial debates have critiqued whether Oceanic settlement was purposeful or accidental, the timing and pathways of colonization, and the nature and extent of postcolonization voyaging—essential for small founding groups securing a lifeline between parent and daughter communities. Centering on the well-dated Tangatatau rockshelter, Mangaia, Southern Cook Islands, we charted the temporal duration and geographic spread of exotic stone adze materials—essential woodworking tools found throughout Polynesia— imported for more than 300 y beginning in the early AD 1300s. Using a technique requiring only 200 mg of sample for the geochemical analysis of trace elements and isotopes of fine-grained basalt adzes, we assigned all artifacts to an island or archipelago of origin. Adze material was identified from the chiefly complex on the Austral Islands, from the major adze quarry complex on Tutuila (Samoa), and from the Marquesas Islands more than 2,400 km distant. This interaction is the only dated example of down-the-line exchange in central East Polynesia where intermediate groups transferred commodities attesting to the interconnectedness and complexity of social relations fostered during postsettlement voyaging. For the Cook Islands, this exchange may have lasted into the 1600s, at least a century later than other East Polynesian archipelagos, suggesting that interarchipelago interaction contributed to the later development of social hierarchies.

In situ LA–ICP-MS and EPMA trace element characterization of Fe–Ti oxides from the phoscorite–carbonatite association at Phalaborwa, South Africa

In situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) and electron probe microanalysis (EPMA) are used to characterize magnetite and ilmenite of the phoscorite–carbonatite association at Phalaborwa. We trace the behavior of the compatible elements for two different generations of magnetite related to (1) a magmatic stage, with variable Ti–V content, which pre-dates the copper mineralization, and (2) a late hydrothermal, low-Ti, low-temperature event, mostly post-dating sulfide formation. Magnetite is shown to be a robust petrogenetic indicator; no influence on its chemical composition is detected from the intergrowth with the accompanying phases, including the interaction with coexisting sulfides. High spatial resolution EPMA characterize the tiny late-stage hydrothermal magnetite veins, as well as the ilmenite granular and lamellar exsolutions mostly developed in the magnetite from the phoscorite. By combining geochemical data with geothermo-oxybarometry calculations for magnetite–ilmenite pairs, we infer that the most primitive magnetite probably formed at oxygen fugacity above the nickel nickel oxide (NNO) buffer, revealing an evolutionary trend of decreasing temperature and oxygen fugacity. Geochemical similarity exists between magnetite from phoscorite and carbonatite, thus supporting a common mantle source for the phoscorite–carbonatite association.

Light rare earth element systematics as a tool for investigating the petrogenesis of phoscorite-carbonatite associations, as exemplified by the Phalaborwa Complex, South Africa

In-situ trace element analyses of fluorapatite, calcite, dolomite, olivine, and phlogopite have been undertaken on representative phoscorite and carbonatite rocks of the Palaeoproterozoic Phalaborwa Complex. Textural and compositional characterization reveals uniformity of fluorapatite and calcite among most of the intrusions, and seems to favor a common genetic origin for the phoscorite-carbonatite association. Representing major repositories for rare earth elements (REE), fluorapatite and calcite exhibit tightly correlated light REE (LREE) abundances, suggesting that partitioning of LREE into these rock forming minerals was principally controlled by simple igneous differentiation. However, light rare earth element distribution in apatite and calcite cannot be adequately explained by equilibrium and fractional crystallization and instead favors a complex crystallization history involving mixing of compositionally distinct magma batches, in agreement with previously reported mineral isotope variability that requires open-system behaviour.