Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Cambrian-Ordovician evolution of Eastern Alps: New evidences constrain from magmatic rocks in the Schladming Complex (Austroalpine unit)

<p>The pre-Mesozoic basements in the Eastern Alps overprinted by the Variscan and alpine metamorphism (Neubauer and Frisch, 1993), which still remained the pre-Variscan tectonic evolution evidences. Many of these basements left away from their lithospheric roots due to large-scale tectonic activities (von Raumer et al., 2001), whereas their origin and tectonic history can be recorded by detailed geochemistry and geochronology. Here we present a study on the Schladming Complex, one part of Silvretta-Seckau nappe system in Austroalpine Unit, that located in the northern part of Alps to discuss their ages, origin, and tectonic relationship with the Proto-Tethys Ocean.</p><p>The Schladming Complex basement mainly comprises biotite-plagioclase gneiss, hornblende-gneiss, mica-schists, together with some amphibolites, orthogneisses, paragneisses, metagabbro and migmatites, which covered by sequence of metasedimentary (Slapansky and Frank, 1987). It underwent the medium- to high-grade metamorphism during the Variscan event and is overprinted by the greenschist facies metamorphism during the Alpine orogeny (Slapansky and Frank, 1987).</p><p>Granodioritic gneisses (539~538 Ma) and fine-grained amphibolite (531±2 Ma) in the basement represent a bimodal magmatism. Geochemical data show that the granodioritic gneisses belong to A<sub>2</sub>-type granite and originated from the lower crust, while the fine-grained amphibolites have an E-MORB affinity and the magma origined from the lithospheric mantle and contaminated by the arc-related materials. The data implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian.</p><p>A medium-grained amphibolite gives an age of 495±5 Ma, exhibits ocean island basalt-like geochemical features and zircons positive εHf(t) values (+5.3~+10.9) indicating that the medium-grained amphibolite derived from a depleted mantle source. The monzonite granitic gneiss and plagioclase gneiss yields ages of 464±4 Ma for and 487±3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive εHf(t) values of +5.9~+7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite.</p><p>In summary, our data presents record of the Cambrian to Ordovician magmatism in the Schladming Complex, which provided new evidence of tectonic evolution history between Proto-Tethys and Gondwana. According to the data, we proposed that a series of rift developed in the northern margin of Gondwana during 540-530 Ma, the rifts continually expanded into a back-arc ocean in ~490 Ma and was closed around 460 Ma with S-type granitic magma intruded.</p><p><strong>References</strong></p><p>Neubauer, F., Frisch, W. 1993. The Austroalpine metamorphic basement east of the Tauern window.  In: Raumer, J. von & Neubauer, F. (eds.): Pre-Mesozoic Geology in the Alps. Berlin (Springer), pp. 515–536.</p><p>von Raumer, J., Stampfli, G., Borel, G., Bussy, F., 2001. Organization of pre-Variscan basement areas at the north-Gondwanan margin. International Journal of Earth Sciences 91, 35-52.</p><p>Slapansky, P., Frank, W. 1987. Structural evolution and geochronology of the northern margin of the Austroalpine in the northwestern Schladming crystalline (NE Tadstädter Tauern). In: Flügel, H. W. & Faupl, P. (eds.), Geodynamics of the Eastern Alps, pp. 244-262.</p>

Biotitická plagioklasová rula z Koutů nad Desnou a Rejhotic (vrbenská skupina v zóně Červenohorského sedla): nové mineralogické údaje

The paper deals with metamorphic rocks of the Vrbno Group in the Červenohorské sedlo Zone near the Kouty nad Desnou village (north-eastern part of the Bohemian Massif). The studied rocks are named simply as biotite plagioclase gneiss in geological maps. Research has shown that most of the rocks in the area correspond to the graphite-bearing staurolite-biotite plagioclase gneiss. This gneiss merges into mica schist with the same mineral association but with a higher content of biotite. Plagioclase is a major and sometimes even dominant constituent of the rocks. Its composition varies from An39 to An84. Accessory minerals include ilmenite, apatite-(CaF), less often zircon, tourmaline (dravite), garnet (almandine), pyrite, and chalcopyrite. In the mentioned rocks, thin graphite-bearing chlorite plagioclase gneiss intercalations were found. Plagioclase has an extremely high basicity (An88-91). It is obvious from the chemical and mineral composition of the studied rocks that their protolith had to be rich in iron, aluminum and calcium. Clay rocks can be assumed as protolith. The origin of clay sediments or clay residue was possibly linked to the weathering of basic volcanism products.

SHRIMP U-PB DATA OF THE PALEOPROTEROZOIC, REGIÃO DOS LAGOS COMPLEX, RIO DE JANEIRO, BRAZIL: IMPLICATIONS TO RIBEIRA BELT EVOLUTION

The studied region is located in the Rio de Janeiro State (SE, Brazil) and is part of the Ribeira Belt, in the central portion of the Mantiqueira Province. The rocks of this region were included in the Brasiliano/Pan-African orogeny, from Neoproterozoic to the early Paleozoic. The basement is comprised of Paleoproterozoic rocks, sedimentary and volcano-sedimentary rocks from Mesoproterozoic and Neoproterozoic and intrusive granitic rocks from the Neoproterozoic. These rocks are interpreted as associations of magmatic arcs, sedimentary sequences of passive margins and tardi-kinematic granites, respectively. The study area is located in Cabo Frio Terrane, where the basement is represented by the Região dos Lagos Complex.The Região dos Lagos Complex can be divided into: granitic rocks (biotite-orthoclase-plagioclase gneiss), tonalitic rocks (biotite-microcline-plagioclase gneiss). The main aims of the SHRIMP U-Pb zircon studies of the Região dos Lagos Complex is to identify the crystallization ages of this complex and to define the magmatic events in the Ribeira Belt. The obtained U-Pb ages are: sample CRL-04, 1960 ± 19 Ma; sample CRL-05, 1986 ± 24 Ma; sample CRL-06, 1985 ± 34 Ma; sample CRL-07, 1942 ± 16 Ma; sample CRL-08, 1970 ± 17 Ma and; sample CRL-09, 1989 ± 22 Ma.These results suggest that the Região dos Lagos complex has crystallization from 1989 Ma to 1942 Ma, and was formed during an important magmatic event related to the Ribeira Belt formed previously to the Gondwana amalgamation probably related to Congo craton basement.