Strontium isotope and abundance systematics of âpureâ and altered/metasomatized garnet and diopside separates from the Bultfontein kimberlite (after Richardson et al., 1985). Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subductionzones. Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron. The oceanic affinity of mafic units was confirmed in further work by Sassi et al. Figure 29. Serpentinization of peridotites and lower crustal gabbros has also been suggested as the cause of higher anomalous skewness of anomalies at slow-spreading ridges (Dyment and Arkani-Hamed, 1995; Dyment et al., 1997). In cratonic xenoliths, both low-T secondary alteration and kimberlite contamination have been identified as major concerns to be addressed during the analysis of mineral separates (Richardson et al., 1985). As with sampling of the gabbroic portion of the oceanic crust, drilling at tectonic exposures (Site 895 at Hess Deep (Gillis et al., 1993), Sites 670 and 920 in the MARK area (Cannat et al., 1995; Detrick et al., 1988), and near the 15Â°20â fracture zone (Kelemen et al., 2004)) has significantly enhanced our understanding of the magnetic properties of oceanic ultramafic rocks. A. on top of the surface. In comparison, the ReâOs system is much more robust to the effects of alteration than the highly incompatible-element-based isotope systems and less affected by invasion of the host rock. Acid washed mineral separates give considerably different NdâSr isotope compositions than their whole-rocks for both peridotites (Richardson et al., 1985; Walker et al., 1989a) and eclogites (Neal et al., 1990; Figure 31), the isotopic signature of the whole-rocks is dominated by small amounts of infiltrated host kimberlite. batholith. This will be outlined below. Tick marks on mixing lines illustrate % of component added. Partial melting. At the same time (Middle Jurassic) rifting in the north in response to the opening of the Atlantic ocean resulted in opening of the Piedmont-Liguria ocean (Frisch, 1979). Jagersfontein cpx plots well off scale on Nd axis. The mineral compositions of igneous rocks are usually described as being felsic, intermediate, mafic, or ultramafic (as examples, see Figure 4.7 and Figure 4.8). In the past, dredging was used to keep the Swift Creek channel free of sediments and to reduce debris flow hazards. This component usually is characterized by much more radiogenic strontium isotopes and appears to be due to the effects of addition of phlogopite mica, which may not be related to the host kimberlite (Figure 31). B. close to the surface but also just below it. An earlier phase of long-lived crustal extension or rifting accompanied by HTâLP metamorphism and mafic magmatism is thought to have occurred in PermianâTriassic times (ThÃ¶ni, 2002; Schuster & StÃ¼we, 2008). Such data led to numerous erroneous conclusions about the relationship between mantle samples and basalts due to the pervasive alteration suffered by many ultramafic rocks, enhancing the amount of radiogenic strontium present. The dredged materials were used for road fill and home construction projects, which led to further distribution of the chrysotile-containing materials. Well-documented exposures of serpentinized peridotite at slow-spreading ridges (Cannat, 1993; Cannat et al., 1997) indicate that such upper mantle material undoubtedly contributes, at least locally, to magnetic anomalies. Ultramafic rocks, due to the abundance of materials with low shear strength such as serpentinite and asbestos, can commonly serve as source rocks for landslides and debris flows. The term âultrabasicâ is applied to igneous rocks which contain <Â 45Â wt% silica (SiO2) in their bulk chemical composition. (2004), who suggested that continued convergence within Apulia during the Cretaceous was accommodated northwestward of the Meliata suture by intracontinental subduction, probably along the location of a failed Permian rift, resulting in thrusting and nappe stacking onto the northern part of the Apulian continental block The subducted crust underwent high- to UHP metamorphism, which is still preserved in Eo-Alpine eclogites. Mafic flows erupting beneath water form bulbous masses called pillow lavas. mantle. Ultramafic-associated ore deposits: Lateritic nickel ore deposits, kambalda type komatiitic nickel ore deposits, Igneous differentiation, fractional crystallisation. When magma flows onto the surface of the Earth it is called lava. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks. Figure 30. Soluble hexavalent chromium released from the landslide rocks might also be a concern in some areas, resulting from the oxidation of trivalent chromium (which is relatively insoluble) by manganese in the surface environment (Wood et al., 2009). parallels the rock layers. Ultramafic rocks occur either as minor (some meters or tens of meters large) lenses associated with basic lithologies (i.e., in Central Rhodope) or as kilometer-large bodies in West and East Rhodope (Bonev & Dilek, 2010 and references therein). The majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. They have a color index greater than 90, where the term âcolor indexâ refers to the percentage of mafic minerals such as olivine and pyroxene present in the rock. Shaded region indicates the area defined by phlogopite addition. Most intrusions are differentiated into a lower ultramafic zone where Ni-Cu sulfide and chromitite deposits form, and an overlying mafic zone where Fe-Ti-oxides crystallise, with PGE deposits located close to the mafic-ultramafic boundary. As a result, a distinctive type of vegetation develops on these soils. Jan C.M. Stein, in Treatise on Geochemistry (Second Edition), 2014. Based on these relatively high temperatures and the simple demagnetization behavior of MARK area peridotites, Lawrence et al. Cumulate rocks and rock types: chromitite. Ultramafic rocks are scattered as lenses and pods throughout the orthogneisses and supracrustal rocks (Figs.Â 17.1 and 17.2). De Hoog, ... Keiko H. Hattori, in Ultrahigh-Pressure Metamorphism, 2011. The region has a complicated tectonic history, which was reconstructed by Schmid et al. Volcanoes made up of mafic flows are typically shield volcanoes and/or cinder cones. These rocks form a strongly foliated matrix around elongated lenses and boudins of eclogite and ultramafic rocks, including the SBUC (Kirst et al., 2010). cuts across the rock layers vertically. (1991). For <Â 75% serpentinization, early FeO-rich lizardite is formed, and with continued alteration, this lizardite is replaced by more FeO-poor chrysotile, with a concomitant release of iron and production of magnetite (Oufi et al., 2002). This article will cover ultramafic rocks that form part of the mantle and, to a lesser extent, those that occur as layered ultramafic rocks formed by cumulus processes from magmas in the oceanic and continental crust. less than 100 square kilometers. Although the Washington State Department of Health found no evidence of elevated rates of asbestos-related disease in nearby residents, the EPA (2011a) did find that exposures to asbestos and some metals in the Swift Creek area were high enough to warrant implementation of measures to help reduce such exposures. Distribution of values compared to lines of constant Q (KÃ¶nigsberger ratio of remanent to induced magnetization; calculated for field of 40Â AÂ mâÂ 1) shows that remanence is greater than induced magnetization in most samples. Most dredged peridotites and some drillcore samples (particularly from breccias or fault zones) have experienced postserpentinization low-temperature oxidation, as evidenced by the presence of maghemite (e.g., Nazarova et al., 2000). Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones. (2010) tested the 190Ptâ186Os chronometer using the well-known Bushveld PGE-deposits, replicating the age within 2%. Most kimberlite and lamproite occurrences occur as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown. Thin, dark retrograde bands, commonly developed at the contacts between eclogite and the host rock. Subduction zones occur whenever a dense oceanic plate is subducted underneath another oceanic or continental plate. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process to typical ultramafic rocks. Dabie-Sulu UHP eclogites from China were mainly of silica-rich basaltic composition of continental affinity, comparable to the mafic rocks of many Precambrian terranes. Garnet peridotites within the SBUC from the same localities as the ones from this study record peak pressures of up to 4Â GPa based on garnetâolivine MgâFe exchange thermometry and Al-in-opx barometry (JanÃ¡k et al., 2006). Agrinier and Cannat, 1997; FrÃ¼h-Green et al., 1996, Encyclopedia of Physical Science and Technology (Third Edition). Magma Composition Magnetizations of serpentinized peridotites, whether obtained by dredging or drilling, vary from <Â 0.1Â AÂ mâÂ 1 to values higher than 30Â AÂ mâÂ 1 that are comparable to typical values for older oceanic basalts (Figure 29; Table 2). Some layered peridotites have interlayers of melagabbro and are spatially associated with layered gabbros; as such they are interpreted as the (fragmented) mafic cumulates (e.g.,Â Dymek etÂ al., 1988; Friend etÂ al., 2002; Szilas etÂ al., 2015) from layered gabbroic intrusions. Over ultramafic rocks also contain calcium-rich varieties of plagioclase feldspar ( SiO2 ) in terms of an origin from continental. Bennett, in Encyclopedia of Physical Science and Technology ( Third Edition,. Binds the rock together serpentine soil is a mixture of liquid rock, crystals, and there are of... 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