Department of Geology

Permanent URI for this collection

https://repository.udsm.ac.tz/handle/123456789/48

Browse

Browsing Department of Geology by Author "Appel, Peter"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    The Age of Au–Cu–Pb-bearing Veins in the Poly-orogenic Ubendian Belt (Tanzania): U–Th–total Pb Dating of Hydrothermally Altered Monazite
    (Springer, 2015) Kazimoto, Emmanuel O.; Schenk, Volker; Appel, Peter
    The age of gold–copper–lead mineralization in the Katuma Block of the Ubendian Belt remains controversial because of the lack of radiometric ages that correlate with the age of tectonothermal events of this polyorogenic belt. Previous studies reported whole rock and mineral Pb–Pb ages ranging between 1,660 and 720 Ma. In this study, we report U–Th–total Pb ages of monazite from hydrothermally altered metapelites that host the Au–Cu– Pb-bearing veins. Three types of chemically and texturally distinct types of monazite grains or zones of grains were identified: monazite cores, which yielded a metamorphic age of 1,938 ± 11 Ma (n = 40), corresponding to known ages of a regional metamorphic event, deformation and granitic plutonism in the belt; metamorphic overgrowths that date a subsequent metamorphic event at 1,827 ± 10 Ma (n = 44) that postdates known eclogite metamorphism (at ca. 1,880 Ma) in the belt; hydrothermally altered poikilitic monazite, formed by dissolution–precipitation processes, representing the third type of monazite, constrain the age of a hydrothermal alteration event at 1,171 ± 17 Ma (n = 19). This Mesoproterozoic age of the hydrothermal alteration coincides with the first amphibolite grade metamorphism of metasediments in the Wakole Block, which adjoins with a tectonic contact the vein-bearing Katuma Block to the southwest. The obtained distinct monazite ages not only constrain the ages of metamorphic events in the Ubendian Belt, but also provide a link between the metamorphism of the Wakole metasediments and the generation of the hydrothermal fluids responsible for the formation of the gold– copper–lead veins in the Katuma Block.
  • Thumbnail Image
    Item
    The Age of Au–Cu–Pb-bearing Veins in the Poly-orogenic Ubendian Belt (Tanzania): U–Th–total Pb Dating of Hydrothermally Altered Monazite
    (Springer, 2015) Kazimoto, Emmanuel O.; Schenk, Volker; Appel, Peter
    The age of gold–copper–lead mineralization in the Katuma Block of the Ubendian Belt remains controversial because of the lack of radiometric ages that correlate with the age of tectonothermal events of this polyorogenic belt. Previous studies reported whole rock and mineral Pb–Pb ages ranging between 1,660 and 720 Ma. In this study, we report U–Th–total Pb ages of monazite from hydrothermally altered metapelites that host the Au–Cu– Pb-bearing veins. Three types of chemically and texturally distinct types of monazite grains or zones of grains were identified: monazite cores, which yielded a metamorphic age of 1,938 ± 11 Ma (n = 40), corresponding to known ages of a regional metamorphic event, deformation and granitic plutonism in the belt; metamorphic overgrowths that date a subsequent metamorphic event at 1,827 ± 10 Ma (n = 44) that postdates known eclogite metamorphism (at ca. 1,880 Ma) in the belt; hydrothermally altered poikilitic monazite, formed by dissolution–precipitation processes, representing the third type of monazite, constrain the age of a hydrothermal alteration event at 1,171 ± 17 Ma (n = 19). This Mesoproterozoic age of the hydrothermal alteration coincides with the first amphibolite grade metamorphismof metasediments in the Wakole Block, which adjoins with a tectonic contact the vein-bearing Katuma Block to the southwest. The obtained distinct monazite ages not only constrain the ages of metamorphic events in the Ubendian Belt, but also provide a link between the metamorphism of the Wakole metasediments and the generation of the hydrothermal fluids responsible for the formation of the gold– copper–lead veins in the Katuma Block.
  • Thumbnail Image
    Item
    Granulite-facies Metamorphic Events in the Northwestern Ubendian Belt of Tanzania: Implications for the Neoarchean to Paleoproterozoic Crustal Evolution
    (Elsevier, 2015) Kazimoto, Emmanuel O.; Schenk, Volker; Appel, Peter
    We present a geological evolution model for the Paleoproterozoic Ubendian Belt. This model is deduced from the metamorphic histories of metasediments and metamafites combined with previously obtained crust formation and metamorphic ages obtained from different rock types of the Katuma Block in the NW Ubendian Belt. Geothermobarometry and pseudosection modelling of metabasites indicate that the granulite-facies coronas containing garnet–clinopyroxene–quartz–hornblende formed at about 8.9–6.6 kbar and 790–700 °C. The formation of the corona textures is attributed to the post magmatic cooling history in the deep crust following their intrusion at about 2.65 Ga. This period correlates with the age of deposition of sediments in the Katuma Block, as deduced from the age of detrital zircon grains. The metamorphic P–T path of these sediments contrasts with that of the Archean mafic rocks. The common occurrence of sillimanite pseudomorphs after cm-sized kyanite crystals in migmatitic metapelites provides evidence that an early stage of metamorphism took place in the kyanite stability field whereas the subsequent peak metamorphism was characterised by the stability of the mineral assemblage sillimanite-garnet/cordierite-K-feldspar. Modelling of the XMg ratios of compositionally homogenous cores of garnet porphyroblasts together with GASP barometry suggest peak P-T conditions of about 7 kbar and 770 °C. The formation of plagioclase coronas around garnet in metapelites, the decrease in XMg and an increase of the spessartine fraction in rims of garnet porphyroblasts point to a near isothermal uplift after peak metamorphism. Texturally controlled in situ U–Th–total Pb microprobe dating of monazite in metapelites resulted in two ages for metamorphic events. The monazite of the two dated samples is mostly complex and patchy zoned. The cores record ages of 1957 ± 10 Ma and 1967 ± 16 Ma, whereas the rims give ages of 1837 ± 6 Ma and 1848 ± 16 Ma. As the two ages of monazite growth zones (core and rims) are found in monazite of the rock matrix and in monazite inclusions of garnet porphyroblasts, we conclude that garnet growth occurred during or after the second metamorphic event at 1840 Ma. This interpretation is in agreement with the depletion of HREE and Y in the monazite rims. We correlate the second, high-grade event with the collisional stage between the Tanzania Craton and the Bangweulu Block. The first event that preceded the collision for about 120 Ma is attributed to the kyanite grade metamorphism during accretionary processes and associated calc-alkaline magmatism along the continental margin of the Tanzania Craton. Combining our new data with those of previous studies on the geochemistry and zircon geochronology we develop a new evolutionary model for the Paleoproterozoic orogenic cycle. The geologic history in the Ubendian Belt began in the Neoarchean (2.7–2.6 Ga) with a magmatic crust formation phase in an active continental margin setting. In the following Neoarchean–Paleoproterozoic (2.65–2.05 Ga) stage of a tectonically inactive, passive continental margin the protoliths of metabasites cooled under near-isobaric conditions and sediments were deposited on the Neoarchean crust. Subsequently, there was a protracted period of subduction (2.05–1.84 Ga) at an active continental margin, which was associated with magmatic additions and metamorphic events during tectonic accretions, that led to kyanite-grade metamorphism in wedge sediments. The final collision at 1.84 Ga leading to garnet–sillimanite–cordierite grade metamorphism in metapelites most likely was responsible for the exhumation of the 1880–1860 Ma MORB-type eclogites in the Ubendian Belt.
  • Thumbnail Image
    Item
    Granulite-Facies Metamorphic Events in the Northwestern Ubendian Belt of Tanzania: Implications for the Neoarchean to Paleoproterozoic Crustal Evolution
    (Elsevier, 2015) Kazimoto, Emmanuel O.; Schenk, Volker; Appel, Peter
    We present a geological evolution model for the Paleoproterozoic Ubendian Belt. This model is deduced from the metamorphic histories of metasediments and metamafites combined with previously obtained crust formation and metamorphic ages obtained from different rock types of the Katuma Block in the NW Ubendian Belt. Geothermobarometry and pseudosection modelling of metabasites indicate that the granulite-facies coronas containing garnet–clinopyroxene–quartz–hornblende formed at about 8.9–6.6 kbar and 790–700 °C. The formation of the corona textures is attributed to the post magmatic cooling history in the deep crust following their intrusion at about 2.65 Ga. This period correlates with the age of deposition of sediments in the Katuma Block, as deduced from the age of detrital zircon grains. The metamorphic P–T path of these sediments contrasts with that of the Archean mafic rocks. The common occurrence of sillimanite pseudomorphs after cm-sized kyanite crystals in migmatitic metapelites provides evidence that an early stage of metamorphism took place in the kyanite stability field whereas the subsequent peak metamorphism was characterised by the stability of the mineral assemblage sillimanite-garnet/cordierite-K-feldspar. Modelling of the XMg ratios of compositionally homogenous cores of garnet porphyroblasts together with GASP barometry suggest peak P-T conditions of about 7 kbar and 770 °C. The formation of plagioclase coronas around garnet in metapelites, the decrease in XMg and an increase of the spessartine fraction in rims of garnet porphyroblasts point to a near isothermal uplift after peak metamorphism. Texturally controlled in situ U–Th–total Pb microprobe dating of monazite in metapelites resulted in two ages for metamorphic events. The monazite of the two dated samples is mostly complex and patchy zoned. The cores record ages of 1957 ± 10 Ma and 1967 ± 16 Ma, whereas the rims give ages of 1837 ± 6 Ma and 1848 ± 16 Ma. As the two ages of monazite growth zones (core and rims) are found in monazite of the rock matrix and in monazite inclusions of garnet porphyroblasts, we conclude that garnet growth occurred during or after the second metamorphic event at 1840 Ma. This interpretation is in agreement with the depletion of HREE and Y in the monazite rims. We correlate the second, high-grade event with the collisional stage between the Tanzania Craton and the Bangweulu Block. The first event that preceded the collision for about 120 Ma is attributed to the kyanite grade metamorphism during accretionary processes and associated calc-alkaline magmatism along the continental margin of the Tanzania Craton. Combining our new data with those of previous studies on the geochemistry and zircon geochronology we develop a new evolutionary model for the Paleoproterozoic orogenic cycle. The geologic history in the Ubendian Belt began in the Neoarchean (2.7–2.6 Ga) with a magmatic crust formation phase in an active continental margin setting. In the following Neoarchean–Paleoproterozoic (2.65–2.05 Ga) stage of a tectonically inactive, passive continental margin the protoliths of metabasites cooled under near-isobaric conditions and sediments were deposited on the Neoarchean crust. Subsequently, there was a protracted period of subduction (2.05–1.84 Ga) at an active continental margin, which was associated with magmatic additions and metamorphic events during tectonic accretions, that led to kyanite-grade metamorphism in wedge sediments. The final collision at 1.84 Ga leading to garnet–sillimanite–cordierite grade metamorphism in metapelites most likely was responsible for the exhumation of the 1880–1860 Ma MORB-type eclogites in the Ubendian Belt.
  • Thumbnail Image
    Item
    Mesoproterozoic High-Grade Metamorphism in Pelitic Rocks of the Northwestern Ubendian Belt: Implication for the Extension of the Kibaran Intra-Continental Basins to Tanzania
    (Elsevier, 2007) Boniface, Nelson; Schenk, Volker; Appel, Peter
    Paleoproterozoic basement rocks are thought to form the northwestern end of the Ubendian Belt in Tanzania that disappears towards the north below a Mesoproterozoic sedimentary cover. The northwestern end of the Ubendian Belt is known to constitute three litho-tectonic terranes of Katuma, Wakole and Ubende. Through dating of zircon (SHRIMP U–Pb) and monazite (U–Th–total Pb electron microprobe ages) of high-grade metasedimentary rocks of the Wakole Terrane we have detected solely Mesoproterozoic ages, showing no sign of reworking of older Paleoproterozoic basement. These findings signify that the Wakole Terrane hosts younger sediments of Mesoproterozoic times metamorphosed to high-grade P–T conditions (peaked at 670–680 °C/8.5–8.9 kbar). Two distinct phases of Mesoproterozoic metamorphic events separated by 160 Ma have been dated at 1166 ± 14 Ma and 1007 ± 6 Ma (SHRIMP U–Pb zircon). Zircon ages are supported by in-situ dating of monazite with ages at 1170 ± 10 Ma, and 1022 ± 5–1016 ± 10 Ma. The first age is closely related to the period of S-type granitoid emplacement at about 1200 Ma in the Karagwe-Ankolean and Kibaran Belts. The second age cluster overlaps with a period of global Mesoproterozoic orogenic cycle, also recorded in the neighboring Irumide and Kibaran Belts. This age group is associated with the assembly of the hypothetical Mesoproterozoic Rodinia Supercontinent. The recent model for the evolution of the Kibaran Belt suitably explains the spatial and temporal settings of Mesoproterozoic metasedimentary rocks of the Wakole Terrane overlying the Paleoproterozoic Ubendian Belt. Due to its proximity to the Kibaran Belt and by being bound by the Paleoproterozoic Terranes of Ubende and Katuma, it can be interpreted that the Wakole Terrane metasediments initially were deposited in an intra-continental basin that was later squeezed between these old terranes by the regional ca. 1000 Ma compressional event recorded in the Irumide, Kibaran and Karagwe-Ankolean Belts.
  • Thumbnail Image
    Item
    Paleoproterozoic Eclogites of MORB-Type Chemistry and Three Proterozoic Orogenic Cycles in the Ubendian Belt (Tanzania): Evidence from Monazite and Zircon Geochronology, And Geochemistry
    (Elsevier, 2012) Boniface, Nelson; Schenk, Volker; Appel, Peter
    Abstract Eclogites and metapelites from the Ubende Terrane of the Proterozoic Ubendian Belt were studied for the purpose of establishing their metamorphic history. Geochemical, petrological and geochronological data of these rocks indicate that oceanic lithosphere was subducted in the Paleoproterozoic and experienced repeated regional metamorphic cycles following their emplacement into continental lithosphere in the Proterozoic. Eclogite facies metamorphic conditions are recorded by well-preserved porphyroclasts of omphacite (Jd17), matrix plagioclase (Ab72) and garnet core, which give a minimum peak pressure of 15 kbar at 700 °C reflecting a geothermal gradient lower than 13 °C/km. These eclogites have chondrite normalized REE patterns that resemble those of N-MORB and E-MORB. U–Pb SHRIMP dating of zircon in the eclogites reveal metamorphic dates of 1886 ± 16 and 1866 ± 14 Ma. These data indicate that eclogites of the Ubende Terrane in the Ubendian Belt may represent former oceanic crust that was metamorphosed during Paleoproterozoic subduction. The subduction was followed by a regional metamorphic event dated at 1831 ± 11 Ma (monazite in metapelite) and 1817 ± 26 Ma (zircon in metapelite). Mylonitic textures are common in all lithologic units of the Ubende Terrane and reflect regional Mesoproterozoic and Neoproterozoic metamorphic overprints. The Mesoproterozoic event is dated at 1091 ± 9 Ma (U–Pb SHRIMP date on zircon rims in a metapelite), while Neoproterozoic dates of 596 ± 41 Ma (U–Pb SHRIMP zircon age of an eclogite) and 601 ± 7 Ma (U–Th total Pb age of a monazite rim from a metapelite) reflect later metamorphic events. The Neoproterozoic mylonitization of eclogites occurred under high-pressure amphibolite-facies conditions at 680–750 °C/10–11 kbar. Highlights ► The Ubendian Belt, southwest margin of the Tanzania Craton, has two suture zones marked by eclogites of the Paleoproterozoic and Neoproterozoic Eras. This paper focuses on the Paleoproterozoic suture zone located in the Terrane of Ubende within the Ubendian Belt. ► The Ubende Terrane eclogites have MORB-type geochemistry and experienced subduction-related metamorphism between 1890 Ma and 1870 Ma. We have inferred a geothermal gradient of less than 13 °C/km and a minimum pressure of 15 kbar. ► After the subduction-related metamorphism another metamorphic event of cryptic nature was recorded in metapelite to have occurred between 1830 Ma and 1820 Ma. This event is probably related to an accretion or a crustal thickening metamorphism that followed after the subduction of an oceanic lithosphere. ► During Mesoproterozoic (1180–1090 Ma) and Neoproterozoic (600–570 Ma) rocks of the Ubendian Belt experienced regional metamorphic events attributed to Kibaran and Pan-African orogenic cycles respectively.