Petrogenesis of Coarse-grained Intrusives from Tahiti Nui and Raiatea (Society Islands, French Polynesia)

This study is based on a set of coarse-grained igneous rockscollected from two zoned plutons located in the central partof Tahiti Nui and Raiatea. The Ahititera pluton (central depressionof Tahiti Nui) comprises a great diversity of rocks, rangingfrom ultrabasic to felsic in composition. It shows a concentriczonation with nepheline-free rocks in its periphery and nepheline-bearingrocks in its central part. The Faaroa pluton (central depressionof Raiatea) is entirely mafic and includes only gabbros andtheralites. The two plutons have variable Nd–Sr isotopicsignatures, especially the Ahititera rocks, which are subdividedinto three groups based on their mineralogy, geochemistry andisotope composition. The isotopic variability probably reflectslocal heterogeneities in the Society mantle plume. Petrographicand isotopic data have been used to define two magmatic suitesin Ahititera, identifiable from their degree of Si undersaturation.The evolution of the mildly Si-undersaturated suite is controlledby simple fractional crystallization, whereas the strongly Si-undersaturatedsuite requires additional H₂O influx. The third isotopic groupincludes only theralites. The rare earth element (REE) compositionsof the mafic rocks from both plutons do not correlate with theirisotopic signature. The REE patterns of the most Si-undersaturatedrocks are systematically characterized by steeper slopes. Suchfeatures are also observed in lavas from seamounts located withinthe present-day hotspot area. It appears that REE concentrationsin Society lavas and intrusives are probably mainly governedby variable degrees of partial melting of a garnet-free mantlesource and are independent of their isotopic signature. KEY WORDS: cumulates; fractional crystallization; partial melting; French Polynesia; plutonic rocks; Society Islands; Tahiti; Raiatea     

40Ar/39Ar ages of flood basalt provinces in Russia and China and their possible link to global faunal extinction events: A cautionary tale regarding alteration and loss of 40Ar1

The hypothesis that the Permo–Triassic boundary (PTB) mass extinctions were caused by flood basalt volcanism in Russia (Siberian Traps) and/or China (the Emeishan Traps) is investigated from the point of view of time of occurrence (⁴⁰Ar/³⁹Ar ages). Numerous published ages in the literature are rejected as good estimates of the time of crystallization. The filters applied in this respect are (a) statistical reliability of plateau/isochron sections of stepheating data and (b) the alteration state of the material that was dated. Alteration appears to be ubiquitous, unsurprising since most of the material dated was used without acid leaching – a procedure that is effective in yielding fresh(er) samples. Of ∼70 ages in the literature for the main pulse of Siberian Trap volcanism, less than ten prove to be reliable ages. Similar techniques applied to ⁴⁰Ar/³⁹Ar for the Emeishan Traps, leaves only a single reliable age for the magmatic episode. These ages are compared to both published and new ⁴⁰Ar/³⁹Ar ages for the PTB as based on analysis of minerals from critical ash beds in China. There is good overlap in the ages (PTB – 250.0 ± 0.1 Ma, Siberian Trap lavas – 250.1 ± 0.4 Ma), lending credence to a genetic link between the formation of the Siberian Traps and the faunal extinction event at the PTB. A similar link for the formation of the Viluy Traps (Russia) and the Late Devonian extinction event is investigated; only a single reliable ⁴⁰Ar/³⁹Ar age is available for the Viluy Traps, and falls close to the interpolated age for the Frasnian–Fammenian boundary. The use of the unspiked K–Ar technique to yield accurate ages for such (altered) samples is questioned.A review of U–Pb data pertinent to these problems suggests a close temporal link between the formation of the Siberian Traps and the PTB. Comparison of U–Pb and ⁴⁰Ar/³⁹Ar ages for the PTB, raises questions about the accuracy of high precision sanidine ages, possibly resulting from very slow leakage of ⁴⁰Ar¹ from this mineral.     

The baryte-bearing beryl-phosphate pegmatite Plössberg—A missing link between pegmatitic and vein-type barium mineralization in NE Bavaria, Germany

Pegmatites and aplites enriched in P, Be, Nb, Ta and Li occur in the high-temperature metamorphic lithological units of the NE Bavarian Basement, SE Germany. They are accompanied by Ba mineralization, in vein-type deposits in the basement as well as in its foreland. Locally, Ba minerals are encountered in the late Variscan pegmatites and aplites too. The shallow discordant stock-like pegmatites (Hagendorf-type) are barren as to Ba, but in the tabular, concordant aplites and pegmatites Ba was concentrated (Plössberg-type). These concordant pegmatites and aplites are supposed to be the root zone of the intrusive pegmatites. In the rare case of low sulfur fugacity, Ba forms Ba–Zr–K–Sc phosphates/silicates in the pegmatites (transition of magmatic into the hydrothermal stages I/II). Under high sulfur fugacity, Ba is accommodated within the same stages in the structure of baryte. Barium is not accommodated in the lattice of phosphates during or in the immediate aftermaths of the emplacement of these Be–P–Nb–Ta pegmatites (stage III). This element shows up again in APS minerals during supergene alteration under acidic conditions (stage IV). Considering the host rocks of baryte mineralization, the Sr contents of baryte increased from the early Paleozoic to the Late Triassic. The Sr contents of baryte are a function of the depth below ground in the vein-type deposits and in the shear-zones bounding the tabular concordant pegmatites. Beryl is not only a marker mineral for the shear-zone-hosted pegmatites but can also be used as a tool for the geodynamic positioning of these pegmatites using its oxygen isotopes. A subdivision of the pegmatites into intrusive and shear-zone hosted may be achieved by its REE and minor elements.     

Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault,Wollaston Forland,NE Greenland)

Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered.