Geochemistry of fine-grained clastic sedimentary rocks of the Neoproterozoic Ikorongo Group, NE Tanzania: Implications for provenance and source rock weathering
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Felsic
Protolith
Detritus
Andesites
Arenite
The isotope geochemical study for Iceland is reviewed. Iceland yields not only tholeiitic basalts but also tholeiitic andesites or rhyolites. Many geochemical studies were done to decipher the origin of the felsic volcanic rock which constitutes about 10% of the Icelandic crust. However, the origin was not clear. To solve this problem, the combination of Sr, Nd, Pb, He and O isotope tracers with U-Th disequilibrium has been attempted, and the involvement of the Icelandic hydrothermally altered crust into the source of the felsic rock is infered. In near future, application of new Li and B tracers will constrain the origin of the felsic rock in more detail.
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The mineral assemblages of ductile shear zones, acting as open systems, may be strongly influenced by the occurrence of mass transfer processes induced by channeling H2O-rich fluids and mobilizing major elements. In this respect, the mineralogy of ductilely sheared felsic rocks under metamorphic conditions is believed to be controlled mainly by the following variables: the infiltrated fluid fluxes, the thermobaric deformation conditions of felsic rocks, the tectonic context and the likely fluid source. To investigate the presence of common features regarding the gain and loss of mobilized major elements with respect to these variables, a review of literature case studies dealing with felsic sheared protoliths has been undertaken. Qualitative results suggest that: (i) the mobilization of preferred major elements seems not related to fixed values of time-integrated fluid flux; (ii) no general relations exist between the mobility of major elements and the tectonic context for shearing, with some exception; (iii) H2O, K, Ca, Na, Mg, Si and Fe exhibit the greatest mobility whatever P-T metamorphic conditions; (iv) fluids coming from far-field and near-field sources induced a some preferred mobility of major elements within ductile shear zones; (v) the chemical selection of major elements seems greatly related to the fluid chemistry, while the amounts of fluids seems only to drive at completion the metamorphic fluid-rock reactions within ductile shear zones.
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Abstract U-Pb zircon and Rb-Sr geochronological, and Sm-Nd isotopic studies have been carried out on the so-called leptyno-amphibolitic complex of the central part of the Maures Massif. The emplacement of the protolith of the felsic end-member (« leptynites ») has been dated at 548 Ma, an age much older than those (lower Ordovician) previously obtained on other leptyno-amphibolitic complexes. Rb-Sr data obtained on whole rocks and on mineral separates give an age of 348 Ma for the amphibolite-facies metamorphism. Nd isotopes indicate that the amphibolites display clear-cut mantle-derived signatures, whereas a significant crustal contribution is recorded in the three analyzed felsic facies. One of these acidic terms can be interpreted in terms of a simple mixing between two components, respectively similar to the amphibolites and to the two other felsic samples. These latter involve another mantle source, distinct from that of the amphibolites, and comparable to that of continental alkali basalts. These data indicate that the central part of the Maures Massif and the southern Massif Central were possibly part of the same pre-Variscan structural unit. The lack of evidence for a clear genetic relationship between the respective protoliths of the two end-members of the leptyno-amphibolitic complex raises once again the problem of the geodynamic significance of these formations.
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Abstract The use of Cs to discriminate between igneous rocks derived from an igneous' protolith (I‐type) and those derived from a metasedimentary protolith (5‐type) is evaluated for selected eastern Australian granitic and felsic volcanic rocks. For this investigation a 'flame emission' technique has been developed and tested for the rapid determination of Cs in rock samples. In the Berridale Batholith, where the I‐type, 5‐type subdivision was first recognized, the Cs content of the granites clearly distinguished the two granite types. In the New England Batholith the leucocratic 5‐types have higher Cs abundances, in most cases, than the similar I‐type granites, but the Cs values of the more mafic 5‐type granites overlap those of the I‐type. Problems in assigning origins for volcanics are complicated by alteration, but Cs may be useful in distinguishing reworked tuffs from volcanic rocks in volcanic sequences. Caesium may also find application in distinguishing volcanics and sediments in Precambrian terrains.
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