Abstract This paper explores the unusual sulphide–graphite association of a selection of Beni Bousera garnet clinopyroxenites that initially equilibrated within the diamond stability field. Compared with common graphite-free garnet pyroxenites analysed so far, these rocks display tenfold S enrichment with concentrations up to 5550 μg g–1. Fe–Ni–Cu sulphides (up to 1·5 wt%) consist of large (up to 3 mm across), low-Ni pyrrrhotite (<0·1 wt% Ni) of troilite composition, along with volumetrically minor chalcopyrite and pentlandite. Such assemblages are interpreted as low-temperature (<100 °C) subsolidus exsolution products from homogeneous monosulphide solid solution. Troilite compositions of the pyrrhotite indicate strongly reducing conditions that are estimated to be slightly above the iron–wüstite (IW) buffer. Bulk-sulphide compositions are closer to the FeS end-member (i.e. Cu- and Ni-depleted) than other sulphide occurrences in mantle-derived pyroxenites described so far. Moreover, troilite contains trace metal microphases (Pb and Ag tellurides, molybdenite) that have never been reported before from mantle-derived garnet pyroxenites but occur in diamond-hosted eclogitic sulphide inclusions. Beni Bousera sulphides also show strong similarities to diamond-hosted sulphide inclusions of eclogitic affinity for a wide range of chalcophile–siderophile trace element contents. In view of the widespread molybdenite exsolution, coupled with Mo and S/Se/Te systematics of sulphide compositions (7872 < S/Se < 19 776; 15 < Se/Te < 31), black-shale pyrite is a potential sedimentary component to contribute to the petrogenesis of Beni Bousera garnet clinopyroxenites. Black shales would have recycled along with cumulates from the oceanic crust in the mantle source of Beni Bousera pyroxenites. Pyrite underwent desulfidation and replacement by troilite during subduction and prograde metamorphism, releasing its fluid-mobile elements (As, Sb, Pb) while suffering minimum S loss because of the strongly reduced conditions. Taken as a whole, our body of data supports a common origin for carbon (−27 ‰ < δ13C < −17 ‰) and sulphur and concomitant formation of diamond and sulphides. Both elements were delivered by an extraneous sedimentary component mixed with the altered oceanic crust rocks that was involved in the genesis of Beni Bousera garnet pyroxenites, prior to a Proterozoic partial melting event.
The Altiplano, a high plateau 200 km-wide and 1500 km-long is undoubtely the major feature of the Central Andes. One of its most characteristic feature is the presence of varied magmatic events, referred to as the Inner Arc Magmatic Domain by Clark et al. (1990). This magmatism started about 48 Ma ago and shows a greater diversity compared to the Main Arc Domain of the Western Andean Cordillera. Several occurrences of phlogopite lamproites, phlogopite-diopside-sanidine lamproites and ultrapotassic minettes have recently been discovered (Carlier et al., 1996) in addition to the peraluminous magmatism of dominantly crustal origin and high-K calc-alkaline to shoshonitic volcanism previously identified (Pichavant et al.. 1988; Clark et al., 1990). This led to a reappraisal of the Cenozoic Inner Arc magmatism in Southern Peru, which is presented here along with new age determinations and geochemical data on the ultrapotassic lavas, the high-K calc-alkaline to shoshonitic volcanism and the calc-alkaline plutons. These data provide important informations on the evolution of Central Andes for the past 50 Ma.
Separated and leached minerals from >40 samples from Eastern Pyrenean ultramafic massifs have been analysed for rare earth elements (REEs), and Sr and Nd isotope ratios. The sub-continental lithospheric mantle in this region has interacted with several different episodes of niagmatic activity since at least the Palaeozoic. The range of isotopic composition in the massifs is extreme for mantle material (87Sr/86Sr =0·7020−0·7050; 143Nd/144Nd = 0·5139−0·5121), although the dominant lithologies, spinel lherzolite and spinel websterite, are isotopically much more homogeneous. Light rare earth element (LREE)-depleted patterns occur in clinopyroxenes from protogranular and porphyroclastic spinel lherzolites. These rocks have high ɛNd and lowɛSr values (mid-ocean ridge basalt—MORB-type) and represent lithospheric mantle unaffected or only slightly affected by subsequent magmatic events. With increasing refractory nature of the peridotites, heavy rare earth element (HREE) content and ɛNd of clinopyroxene decrease, and ɛSr and (Ce/Yb)N increase. Harzburgite bands may be formed by the passage of tholeitic melts unrelated to the host peridotites. Layered pyroxenites originated as crystal segregates from tholeiitic magmas. Clinopyroxenes from garnet clinopyroxenites have bell-shaped REE patterns, but websterites have reacted with host peridotites, acquiring similar REE patterns. Layered pyroxenites generally have higher ɛSr values than spinel peridotites, and show a wide range of ɛNd, resulting in part from time-integrated Sm/Nd fractionation. Some layered pyroxenites have extremely low ɛNdvalues (−5·6 to −10·2) resembling enriched mantle 1 (EMI); others have high ɛNd values (+ 25 to +28), although many have ɛNd values similar to those of their host peridotites. Cross-cutting amphibole pyroxenite and hornblendite veins were formed by 100-Ma-old alkali basaltic magmas which have reacted with pre-existing peridotite. The metasomatism of the Caussou massif is also related to this magmatism. REE patterns in clinopyroxene and amphibole from hydrous veins and metasomatized peridotites resemble those of megacrysts from alkali basalts. Magmatic minerals in hydrous veins have homogeneous #x025B;Nd values (+ 5·7 to +7·2), whereas metasomatic minerals have slightly higher values (+ 6·5 to +9·4). The highɛSr(+ 6·4 to +7·8) of some amphiboles from Caussou may be related to crustal fluids.
<p>In this study, two types of natural asbestos-like actinolite occurrences were sampled in order to understand their tectonic and metamorphic signification. Studied rocks were collected within two Variscan ophiolitic formations (Tr&#233;ogat and Pont de Barel Formations, South Armorican Massif, Western France), mainly composed of amphibolites, and which recorded amphibolite to greenschist facies metamorphism. In these localities, the natural asbestos-like actinolite occurrences are closely related with the development of tectonic structures such as extension veins, tension gashes, &#963; and &#948;-type boudins. Field and petrostructural studies together with optical microscope, SEM and electron-microprobe analyses (EPMA) allowed to link early steps of the retrograde deformation event, during which acicular hornblende crystallizes in extension veins showing fuzzy boundaries or in hosting rock, with the late step of the same deformation event, during which hornblende is downgraded into asbestos-like actinolite synchronous with felsic melt circulation and tectonic structures opening. Field and microtectonic observations point to a sinistral strike-slip shearing for Pont de Barel formation and to a sinistral transtensive shearing for the Tr&#233;ogat formation, which is consistent with the late regional variscan exhumation of the South Armorican Terrane. &#160;SEM observations show that asbestos-like actinolite originate from hornblende crystallographic plan fragmentation, starting first along the (110) plans and continue both along the (100) and (110) plans. EPMA analyses show that Na-Al-Si metasomatism is associated with this fragmentation. Temperature estimates of chlorite crystallization after hornblende are around 300&#176;C for the Tr&#233;ogat Formation and 200&#176;C for the Pont de Barel Formation, suggesting that amphibole fragmentation can occur over a wide temperature range. Additionally, Principal Component Analysis was performed using crystallographic sites distribution. Results show a clear correlation between actinolite Si(T) and hornblende Al(T), Al(C) and Na(A) crystallographic sites, suggesting that asbestos-like actinolite after hornblende fragmentation is rather due to a decrease of pressure within the tectonic structures, as Al in amphibole is pressure-dependent. This decrease could be due to the fluid pressure, which is supra-lithostatic during tectonic structures opening.</p>
Sulphur and Cu-Fe-Ni sulphides can record partial melting events and aspects of the reaction between depleted mantle and mafic melts. The incompatible nature of sulphur in mantle melting processes is demonstrated by a positive correlation between A12O3and S in massive peridotites sampled at some distance from pyroxenite layers. Extrapolation of this correlation yields 220 ppm S in the mantle source of the Lanzo massif, which agrees well with the composition of the convecting asthenospheric mantle. In contrast, Pyrenean lherzolites have been contaminated at the base of the sub-continental lithosphere and have slightly higher S contents (up to 390 ppm S). Refractory rocks (harzburgites and dunites) are characterized by a heterogeneous distribution of sulphur and sulphide grains. Some samples are distinctly enriched in S (up to 210 ppm) with respect to the Al vs. S partial melting trend. This enrichment suggests additional precipitation of sulphides from the tholeiitic melt which has circulated by porous flow within the refractory rocks. Pyroxenite layers, which are interpreted as crystalline segregates deposited by tholeiitic melts, are moderately enriched in 5 (< 1300 ppm) relative to the surrounding peridotites. Higher sulphur contents (up to 2000 ppm) characterize a late generation of alkali basalt-related amphibole-rich veins from the Eastern Pyrenean massifs. A process of in situ sulphide liquid immiscibility can account for the precipitation of sulphur in melt-filled cracks. Cu/S ratios are of use for discriminating between tholeiite- and alkali basalt-related melt percolation. All the rocks (lherzolites, harzburgites, dunites, or pyroxenites) which equilibrated with tholeiitic melts have high Cu/S ratios (0·1–0·4). Moreover, the variations in modal compositions of sulphides testify to strong Ni-Fe exchanges between tholeiite-filled cracks and wall rocks. In contrast, those rocks which have equilibrated with alkali basalts have low Cu/S ratios (0·05). Persistence and/or precipitation of sulphur at each stage in the evolution of orogenic peridotite massifs show that basaltic melts may become saturated with respect to sulphur before any fractional crystallization in the crust, in strong agreement with conclusions based on the study of MORB.
