Eclogitic metatrondhjemites from metaophiolites of the Western Alps

In the Urtier valley (southern Aosta Valley, Italy), the Piemonte metaophiolites mainly consist of serpentinized peridotites including pods and boudinaged layers of Fe-metagabbro and trondhjemite transposed in the main eclogitic foliation. The contact between serpentinized peridotites and Fe-metagabbro/trondhjemite is locally lined by chloriteschist and rodingite. The high pressure parageneses in the Fe-metagabbro are omphacite-garnet-rutile-glaucophane-phengite, and in the metatrondhjemite plagioclase-quartz-phengite-clinozoisite-epidote-garnet, respectively. Bulk-rock major and trace elements in addition to O isotope analyses were performed in both rock types. Fe-metagabbros are characterized by MgO wt% ranging between 6.11 and 9.63%, EREE= 20-101 ppm, (La/Yb)N = 0.22-0.91; trondhjemites have SiO2 43%, Al2O3 ranging between 21 and 24%, CaO ranging between 17 and 20%, EREE = 172 - 272 ppm, (La/Yb)N ranging between 7.78 and 13.70. The 18O is 5.9 per-mil in a Fe-metagabbro sample and 7.4 per-mil in a trondhjemite sample, suggesting that these rocks have been affected by a weak oceanic low temperature alteration. The high CaO content may indicate a metasomatic process which could have occurred during the oceanic stage or at high pressure conditions

A neoproterozoic age for the chromitite and gabbro of the Tapo Ultramafic Massif, Eastern Cordillera, Central Peru, and its tectonic implications

The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites showa tholeiitic affiliation and a flat REE spider diagram, with a slight LREE depletion and a positive Eu anomaly suggesting magmatic accumulation of plagioclase, in an ocean ridge or ocean island environment. Sm-Nd isotopic analyses were performed on chromite as well as on whole rock from the gabbro. All samples yielded an Sm-Nd isochrone age of718 ± 47 Ma with an initial 143Nd/l44Nd of0.51213 ± 0.00005. The Nd (718 Ma) values calculated for both chromite and gabbro are in close agreement, around 8.0, implying that they were formed at the same time from the same mantelic magma source. Furthermore a K-Ar age on amphibole of 448 ± 26 Ma was obtained, interpreted as the cooling age of a younger orogenic event. These rocks represent slices of oceanic crust (from a dismembered ophiolitic complex), metamorphosed and later overthrust on upper Palaeozoic continental formations

An ERTS multispectral scanner experiment for mapping iron compounds

There are no author-identified significant results in this report

Mineral magnetic properties of granodiorite, metagabbro and microgabbro of Petermann Island, West Antarctica

The research focuses on studying the magnetic properties and mineralogy of iron-bearing minerals of granodiorite, metagabbro, and microgabbro of Petermann Island, West Antarctica. The predominant iron-bearing minerals of the rocks are ilmenite, magnetite, and iron sulphides. Magnetite in metagabbro and microgabbro is pointed out to be present as two morphological types with different grain size and morphology. The rocks owe their magnetic properties to the presence of different amounts of magnetite with the Curie temperatures of 570–575°C for granodiorite, 555–560°C for metagabbro and 560–565°C for microgabbro. Magnetite in the rocks is stable under heating to 650°C. A slight decrease in magnetisation at 350–400°C is attributed to the conversion of maghemite or maghemite-like phase into hematite. Variation of the magnetite content within each sample has a strong expression in the saturation magnetisation. The latter increases in sequence: granodiorite (0.8–1.3 Am2/kg), microgabbro (1.8–3 Am2/kg) and metagabbro (3.1–3.5 Am2/kg). The saturation magnetisation of rocks increases with the increasing content of iron. However, the inverse relation is observed for metagabbro and microgabbro due to the replacement of titanite for magnetite in the latter. The magnetic fraction of microgabbro reveals the wasp-waisted hysteresis loop suggesting bimodal size distribution. According to X-Ray Diffraction, the characteristic peaks (d-spacing) of pure magnetite are identified for magnetic fraction of granodiorite and metagabbro, while magnetite of microgabbro form stable intergrowth with titanite and chlorite

Strain Development and Partitioning Across a Transpressional Shear Zone Along a Quartzite- Metagabbro Contact in the Black Hills Uplift, South Dakota

The Nemo region of South Dakota’s Black Hills offers an ideal location to study transpressional shear zones because it hosts an exposed Archean lithological boundary between two contrasting rheological units, the Boxelder Creek Quartzite (BCQ), a rift-depositional quartzite, and the Blue Draw Metagabbro (BDM), a metagabbro sill, deformed within a ductile shear zone that represents the beginning of main-phase formation of the North American continent as we know it today. The tectonic setting of the Black Hills is at the eastern edge of the Archean Wyoming province, located near the Trans-Hudson Orogeny suture zone that formed between the Wyoming and Superior Provinces. The methods used in this study to achieve an updated analysis of the Nemo Group Shear Zone (NGSZ) are: (a) measure deformation fabrics (foliation and lineation) of discrete shear zones at the meter to kilometer scale to determine local and regional differences in strain and vorticity; (b) measure the aspect ratios and acute angles of “diamond-shaped” lozenges formed by shear bands observed in the NGSZ metagabbros and quartzites to quantify strain gradients across the NGSZ; (c) compare field data to kinematic models; and (d) supplement mesoscale analysis with microstructural analysis to identify shear sense indicators and record general mineralogical orientations within the Blue Draw Metagabbro and the Boxelder Creek Quartzite. In order to analyze the Nemo Group Shear Zone (NGSZ) kinematic models with varying inputs for shear obliquity (ɸ), extrusion obliquity (ʋ), and kinematic vorticity (Wk), and shear zone boundary strike and dip were compared to field measurement of lineation and foliation orientations and used to constrain transpression models. Modeling results suggest that the NGSZ zone strikes ~340 with steeply inclined boundaries. It accommodated both left-lateral simple shear and shortening accommodated by pure shear with steep to subvertical extrusion. Modeling results determined the fabrics most closely matched simple shear dominated models with high kinematic vorticity numbers (0.8-0.9). Lineation orientations vary significantly throughout the NGSZ, which is likely due to variations in strain magnitude and small changes in the pure shear related extrusion direction. Modeling results determined that extrusion likely deviated up to 20° from vertical in either NW or SE directions. Field measurements indicate strain partitioning within the high-strain, heterogeneous metagabbro is dominated by bands of strong foliation that encircle meter to kilometer scale blocks with less deformation. Deformation character ranges from mylonitic to ultra-mylonitic within zones of high strain, and little to no deformation moving to the east and west into zones of lower strain, away from central shear zone. Strain is more homogenous in the BCQ unit, with closely spaced foliations throughout. The bulk structural data indicates consistency with a transpression model, but the deformation was manifested differently in each lithology. Within the quartzite, the strain was more homogeneous indicated by consistent tightly spaced foliations. Strain geometries indicate plane strain to moderate flattening. Indications of previous deformation fabrics and folds were not generally observed. Within the metagabbro, strain was extremely heterogeneous on the meter scale, and the deformation was at least partially controlled by the influence of prior folds. Fold hinges nucleated blocks of low strain while high strain zones wrapped around these blocks. Strain shapes ranged from mild constriction to plane strain to flattening to extreme flattening, with most locations having either plane strain or flattening. The kinematic models and strain recorded by the lozenges, primarily suggests simultaneous left-lateral shearing and shortening at the NGSZ. This heterogeneity of deformation may be related to the influence of D1, D2, and D3 fabrics that predated or formed syn-deformational in the NGSZ or may be due to heterogeneity in kinematics or degree of strain within the NGSZ

Fluid Disturbed K-Ar Mineral Ages from the Dalradian Rocks of Connemara, Western Ireland

This thesis is concerned with explaining the cause of a wide range (about 100 Ma) of K-Ar mineral ages in the Connemara Schists, western Ireland. This range was first reported by Elias (1985) and Elias et al. (1988) who determined a total of over sixty biotite, muscovite and hornblende K-Ar ages. These authors concluded that the ages were the result of differential uplift and cooling of three, independent 'blocks' within Connemara. Examination of the age data of Elias (1985) and Elias et al. (1988) reveals, however, that at best this can only be a partial explanation because some regions within Connemara record mica K-Ar ages significantly older than amphibole K-Ar ages. This is the opposite to the order predicted from commonly accepted closure temperature estimates (Harrison 1981, Harrison et al. 1985) and the opposite to the order recorded in many other metamorphic terranes. It was apparent, therefore, that some process other than simple cooling of the rocks was responsible for, at least, some of the K-Ar mineral ages. Fourteen biotite, twenty muscovite and thirty hornblende K-Ar mineral ages determined during the course of this study exhibited an age range indistinguishable from that of Elias (1985) and Elias et al. (1988). Hydrogen isotopic ratios measured on seven biotites and six muscovites showed slight variation and averaged around -66 and -41‰, respectively. Hydrogen isotopic ratios measured on hornblendes showed a larger variation that ranged from -51.5 to -75.1‰. The variation in the hydrogen isotopic ratios from the micas and the hornblendes is attributed to variable, incomplete, exchange between the minerals and a fluid at temperatures about 300 to 350°C. Oxygen isotopic ratios were measured on thirteen of the hornblendes. These showed some variation, from +5.5 to +11‰, which reflects primary compositional variations in the hornblendes and not partial exchange with the fluid that caused the variation in hydrogen isotopic ratios. The spread in the K-Ar mineral ages is shown to be neither geographically correlated nor the result of diachronous, slow cooling of the rocks. There is a weak correlation between the hornblende K-Ar age and the Fe/(Fe+Mg) ratio that may indicate some compositional control over the Ar closure temperature. The range of K-Ar mineral ages is shown to be the result of variable, partial resetting of individual K-Ar 'clocks' that had all previously been totally rejuvenated by the intrusion, at 490+/-1 Ma (Jagger et al. 1988), of a large (>80 km x >20 km) metagabbro-gneiss complex. The subsequent, variable and partial resetting occurred at c. 400 Ma and was induced by the intrusion of a suite of Lower Devonian Granites into the Connemara rocks. The resetting was not simply a thermal event as witnessed by the lack of any spatial correlation between sample location and K-Ar mineral age. Instead, the variable and partial resetting was brought about by interaction of the minerals with the fluid identified from the hydrogen isotopic ratio measurements. This fluid was circulating in a hydrothermal system around the granites. Different mineral species interacted with this fluid in different ways. Muscovite lost Ar where it was in contact with the fluid with a temperature in excess of the muscovite Ar closure temperature, about 350°C, long enough for Ar diffusion to occur. Biotite was reset in a similar way, except that Ar loss would have been enhanced by chloritisation of the biotite. For both micas, a correlation exists between the K-Ar ages and the extent of alteration of the rock indicated by the degree of sericitisation of the plagioclase. Oldest K-Ar mica ages were yielded by samples with little or no visible alteration of the feldspar. Argon loss from hornblendes was more complicated and was primarily governed by the extent of heterogeneity in the crystal microstructure. TEM investigation revealed that sub-micron scale exsolution and phyllosilicate growth along cleavages existed in some samples. Similiar features have been shown to cause a reduction in the Ar closure temperature of hornblende to about that of biotite (Harrison & Fitz Gerald 1986). Direct evidence of a linkage between Ar loss from hornblendes and interaction with the fluid comes from a correlation between the hornblende K-Ar ages and hydrogen stable isotopic ratios. It is suggested that sometimes Ar loss in hornblende is coupled directly with stable isotopic exchange. In such situations a hornblende could have two Ar closure temperatures; one 'wet', in the presence of a fluid, and one 'dry', in the absence of a fluid. It is possible to identify samples which yield ages which were not reset and which are consequently, geologically significant as closure ages following emplacement of the metagabbro-gneiss complex

The effect of lithology and microstructure on the deformation of unstable rock slopes in northern Norway

Rock avalanches are a type of natural catastrophe in Norway with very high consequences, and the alpine landscape makes Troms and Finnmark County the region with the highest frequency of unstable rock slopes. The bedrock geology is highly relevant to assess and understand the failure mechanisms, and mechanical properties that influence deformation in rock slopes. Pre-existing structures and lithological changes may have the potential to affect the mechanical properties in unstable rock slopes. The meta-igneous bedrock at the unstable rock slope Dusnjarga in Kvænangen municipality and the meta-sedimentary bedrock at the high-risk object Gámanjunni 3 in Kåfjord municipality, are both located in Troms and Finnmark county, northern Norway. Failure of Dusnjarga and Gámanjunni 3 can cause catastrophic consequences with possible harm to life and infrastructure. Both Dusnjarga and Gámanjunni 3 have a high frequency of pre-existing large-scale discontinuities in the back scarp region and consistent brittle structures throughout the instabilities. Many of these discontinuities are recognized in microscopy, suggesting that small-scale structures affect the formation of large-scale discontinuities, and eventually, the presence of these structures can cause a point of rupture initiation on large instabilities. All investigated thin sections present consistent small-scale joint sets parallel and normal to foliation. Each rock type displays different structures and mineralogy, and the fracture behavior is not random. In addition, observed microfractures appear to propagate easier in a metamorphosed fabric and favor changes in texture. All small-scale observations can be linked to large-scale discontinuities in an unstable rock slope. Lithology has a strong control on the development and evolution of URS-related structures