<p>Cameroon Volcanic Line (CVL) is a ca. 1600 km long Cenozoic volcanic chain which&#160;crosses the boundary between ocean and continent in West Africa. Its origin, as well as &#160;the nature and age of the underlying continental lithospheric mantle (CLM), is still a matter of debate. Some of the CVL lavas contain peridotite xenoliths that can provide data elucidating the role of the CLM in the sustained magma generation along the line. In this abstract we describe xenolith suite from the Befang pyroclastic cone (< 1Ma) in the Oku Massif in the continental part of CVL, consisting of 14 spinel lherzolites, one spinel harzburgite and one websterite. The xenoliths are between 3 and 21 cm in diameter and have porphyroclastic to serial or equigranular texture, with porphyroclasts of olivine or orthopyroxene up to 9 mm in diameter. Some are weakly foliated. Olivine is Fo 88.6-90.4, contains 0.36 to 0.42 wt.% NiO and 180-750 ppm of Ca. Orthopyroxene (Mg# 0.89-0.91) contains 0.14 &#8211; 0.19 atoms of Al pfu, and clinopyroxene (Mg# 0.90-0.92) contains 0.24 &#8211; 0.31 atoms of Al pfu. The Cr# of lherzolite spinel is 0.09-0.15, in the harzburgitic one it is 0.18-0.19. Pyroxenes in all studied peridotites record a temperature range of 910 &#8211; 1010&#176;C (Brey and K&#246;hler 1990). Clinopyroxenes&#8217; REE patterns are flat at HREE-MREE and make a spectrum from slightly LREE-depleted to slightly LREE-enriched (La<sub>N</sub>/Lu<sub>N</sub> from 0.08 to 2.65). The trace-element patterns are flat except well-defined negative Nb-Ta and positive Th-U anomalies. Orthopyroxenes&#8217; REE patterns are variably depleted from HREE to LREE (LaN/LuN from 0.001 to 0.037). The REE pattern of clinopyroxene occurring in websterite exhibits enrichment from HREE towards LREE with hump in Sm/Nd, typical of silicate melt crystallization. The REE pattern of clinopyroxene The Befang lherzolites represent CLM metasomatised by melts produced by various, but generally low degrees of melting of DMM-like (Depleted MORB Mantle) source. Conversely, the harzburgite was formed by low degrees (few percent) of melting of DMM.</p><p>Acknowledgements. The study was funded by Polish National Centre for Science project UMO-2017/27/B/ST10/00365 to JP. EPMA analyses were done thanks to the Polish-Austrian project WTZ PL 08/2018.</p><p><strong>References:</strong></p><p>Brey, G.P. & K&#246;hler, T. (1990). Geothermobarometry in four-phase lherzolites II. New&#160;thermobarometers and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353-1378.</p>
DEFORMATION AND METAMORPHISM OF ROCK SERIES EAST OF THE SOWIE GORY BLOCK - NEW DATA AND INTERPRETATIONS
Summary
An important NNE-SSW - trending tectonic boundary, located between the Gory Sowie Gneiss Block to the west and the Strzelin Crystalline Unit to the east (Fig. 1), separates the structures of the West and East Sudetes in the area of the Fore-Sudetic Block. The West Sudetes have been traditionally included into the Saxothuringian and the East Sudetes into the Moravo-Silesian major facies-structural zones of the European Variscan belt. The boundary between the two zones continues further southwestward along the SE margin of the Bohemian Massif into the Moldanubian thrust [45] which separates Moldanubian and Moravian nappe piles. The ductile, NNE-directed, synmetamorphic displacements with a significant dextral, strike-slip component have been documented in this area throughout the entire length of the SE margin of the Bohemian Massif [26, 42, 21]. In contrast, the mylonites of the Niemcza Zone, located immediately to the west of the Western/ Eastern Sudetes boundary (Fig. 1), recorded effects of sinistral strike-slip displacements [28]. Metamorphic rocks cropping out to the east of the Gory Sowie Block are subdivided into three regional units. From west to east these are: the Niemcza Shear Zone, the Niemcza-Kamieniec Metamorphic Unit and the Doboszowice Metamorphic Unit (Fig. 1). The orientation of foliations and lineations in these units are shown in Figures 3, 4 and 7. On the basis of our field data, we established a tentative sequence of three tectonic events in the rock sequences of the study area (Fig. 8). The DJ event was related to E-directed tectonic transport under amphibolite facies conditions. The DJ fabric is preserved in paragneisses comprising the eastern part of the Doboszowice Metamorphic Unit (Fig. 6). The western part of this unit is composed of orthogneiss body (Fig. 6) representing a syntectonic granite intrusion emplaced during the D2 event. The D2 structures, well developed in this orthogneiss body and in coarse-grained mica schists exposed near to Kamieniec Ząbkowicki (Fig. 5) recorded a top-to-NE shearing under amphibolites fades conditions. The D3 event involved sinistral, strike-slip displacement in the Niemcza Shear Zone (Fig. 2) and a top-to-SW shearing in the Niemcza-Kamieniec Unit. The Niemcza Zone (Fig. 2), extending along the eastern edge of the Gory Sowie Block, consists of mylonites derived from the Gory Sowie gneisses during the D3. The mylonites occur as high- and low-temperature varieties produced under the amphibolite and greenschists facies conditions, respectively. The D2 event corresponds to synmetamorphic NNE-directed thrusting recognized along the entire SE margin of the Bohemian Massif. The displacements towards NNE were preceded by a separate stage of E-directed tectonic transport. The sinistral sense of shear in the Niemcza Zone is related to the subsequent D3. It seems to be comparable with late-orogenic, sinistral shearing localized in several NNE-SSW shear zones and ductile to brittle faults in the S and SE part of the Bohemian Massif [10].
Cenozoic basanite forming the Ostrzyca Proboszczowicka Hill in SW Poland contains 1.5 — 3.0 cm long clinopyroxene “megacrysts.” Th ey contain intergrowths of euhedral apatite up to 7 mm long and older clinopyroxene grains. Th e megacrysts have the composition of aluminian-sodian diopside Mg# 0.61–0.70, their REE contents are slightly 1–10 x greater than those of primitive mantle. The REE patterns of megacrysts are enriched in light REE relative to heavy REE. Trace element patterns exhibit strong positive Zr, Hf and Ta anomalies and pronounced negative Sr and Ti anomalies. Apatite has composition of fluor-apatite. The REE concentrations in apatite reach up to 1000 x primitive mantle values for LREE, while trace element patterns are characterized by strong negative anomalies of HFSE and Pb. Megacrysts come from the not solidified coarse-grained cumulate, which was formed at mid-crustal level from the fractionated silicate alkaline magma batch. The new pulse of primitive basanitic magma entrained the megacrysts and brought them to the surface.W obrebie kenozoicznego bazanitu budującego wzgorze Ostrzyca Proboszczowicka SW Polska wystepują liczne „megakrysztaly” klinopiroksenu osiągające dlugośc od 1,5 do 3 cm. Megakrysztaly zawierają wrostki euhedralnego apatytu dlugości do 7 mm i starszego klinopiroksenu. Klinopiroksen tworzący megakrysztaly ma sklad glinowo-sodowego diopsydu mg# 0,61–0,70 a zawartośc REE jest nieznacznie 1–10 razy wyzsza od ich zawartości w prymitywnym plaszczu. Diagramy pajecze dla REE wykazują wzbogacenie w lekkie ziemie rzadkie. Klinopiroksen wykazuje pozytywne anomalie zawartości Zr, Hf i Ta oraz negatywne anomalie zawartości Sr i Ti. Apatyt ma sklad apatytu fluorowego i jest silnie wzbogacony w pierwiastki ziem rzadkich do 1000 wartości w prymitywnym plaszczu. Apatyt wykazuje silne negatywne anomalie w zawartości HFSE i Pb. Megakrysztaly klinopiroksenu pochodzą z niekonsolidowanego, gruboziarnistego kumulatu, ktory wykrystalizowal ze sfrakcjonowanej magmy alkalicznej na glebokościach opowiadających środkowej skorupie. Nowy puls prymitywnej magmy bazanitowej spowodowal dezintegracje kumulatu i wyniosl jego fragmenty na powierzchnie.
A small (280 x 140 m) outcrop of peridotite occurs on Popiel Hill (Sudetes, SW Poland) within the low-grade metabasic rocks of the Rudawy Janowickie Complex, which form the eastern and north-eastern cover of the Variscan Karkonosze granite. The peridotite is situated on the Intra-Sudetic Fault, one of the major Variscan dislocations in the region. The rock consists of strongly tectonised olivine (Fo 84-88 ) and orthopyroxene (Mg# 0.84–0.88) aggregates, overgrown by tremolite-magnesiohornblende, locally forming large crystals, embedded in serpentine. Spinel and magnetite are subordinate; ilmenite, Fe-sulfide, and apatite are accessories. The bulk-rock chemical composition suggests a lherzolitic composition and the occurrence of primary clinopyroxene, now completely replaced by tremolite and magnesiohornblende. Rare Earth Element patterns are flat, slightly enriched relatively to primitive mantle, as is typical of island arc tholeiites. Olivine, orthopyroxene, and spinel were the first to crystallize, and they represent relics of a primary igneous assemblage. They were followed by tremolite and serpentine, formed during uplift and related metamorphism. The last mineral to crystallize was magnesiohornblende, which originated due to contact metamorphism by the Karkonosze granite magma. The Popiel peridotite probably represents a small slice of lherzolitic cumulate, which originated in a tholeiitic magma chamber at the roots of a supra-subduction volcanic arc.
<p>Sulfides hosted by peridotites from Befang (Oku Volcanic Group, Cameroon) xenolith suite can play an important role in tracking migration of strategic metals such as Au, Ag, or Cu through the subcontinental lithospheric mantle (SCLM) beneath the Central African Orogenic Belt. Most peridotites are lherzolites, which are subdivided into two main groups differing by crystallographic preferred orientation (CPO) and rare-earth element (REE) composition of clinopyroxene. Group I is characterized by light REE (LREE)-depleted clinopyroxene (re-)crystallized during percolation of metasomatic melt. Group II contains LREE-enriched clinopyroxene with the CPO representing deformation before percolation of the melt (Tedonkenfack et al., 2021). Lherzolites of group I &#160;are interpreted to be metasomatized by MORB-like melts coming from &#160;Depleted MORB Mantle (DMM). Peridotites of &#160;group II are interpreted to be a protolith for the group I ones.</p><p>The sulfides form oval to slightly elongated grains enclosed usually in orthopyroxene, or rarely in clinopyroxene and olivine. They are composed of pyrrhotite (Po), pentlandite (Pn), and chalcopyrite (Ccp). Pyrrhotite is mostly predominant, whereas Pn forms exsolution lamellae in Po or massive crystals separating Po from Ccp. Chalcopyrite is present on the rims of grain or penetrates through the entire grain, occasionally containing cubanite exsolutions. The Group I lherzolites contain more sulfides (up to 0.031 vol.&#8240;), with larger grains (range: 14&#8722;250 &#181;m, 57 &#181;m on average) compared to the Group II sulfides (up to 0.002 vol.&#8240;, range: 12&#8722;45 &#181;m, 27 &#181;m on average respectively). Sulfides from Group I are richer in Po, and especially Ccp (Po<sub>77</sub>Pn<sub>12</sub>Ccp<sub>11</sub> on average) compared to Group II (Po<sub>72</sub>Pn<sub>23</sub>Ccp<sub>4</sub> on average). Ni/(Ni+Fe) in pyrrhotite from Group I (0.14&#8211;0.43) is more heterogeneous compared to group II (0.20&#8211;0.37).</p><p>Enrichment in Po and Ccp in the Befang Group I xenoliths suggests a significant role of melts in transporting sulfur and metals. Observed refertilization by DMM-derived melts may affect the chalcophile and highly siderophile metal budget of the SCLM. The degree of refertilizaton seems to depend on temperature and therefore is moderate in Befang (up to 0.031 vol.&#8240;) with moderate temperatures of orthopyroxene-clinopyroxene equilibration (938&#8211;997&#176;C; Tedonkenfack et al, 2021). In lower temperatures of Opx-Cpx equilibration (810&#8211;970&#176;C), we observe higher sulfide abundances (up to 0.062 vol.&#8240;), whereas in higher temperatures (1010&#8211;1120&#176;C) lower sulfide abundances (up to 0.00048 vol.&#8240;; Mazurek et al., 2021).</p><p>&#160;</p><p>This study was supported by the Diamond Grant project 093/DIA/2020/49.</p><p>&#160;</p><p>References</p><p>Mazurek, H., Ciazela, J., Matusiak-Ma&#322;ek, M., Pieterek, B., Puziewicz, J., Lazarov, M., Horn, I., Ntaflos, T.: Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland., EGU General Assembly 2021, online, 19&#8211;30 Apr 2021, EGU21-15992, https://doi.org/10.5194/egusphere-egu21-15992, 2021</p><p>Tedonkenfack SST., Puziewicz J., Aulbach S., Ntaflos T., Kaczmarek M-A., Matusiak-Ma&#322;ek M., Kuku&#322;a A., Ziobro M.: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line &#8211; a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Contributions to Mineralogy and Petrology, 176: 37.</p>
