ISOTOPE-GEOCHRONOLOGICAL DATA ON GRANULITES OF THE ILYINSKAYA BLOCK AS AN EVIDENCE OF THE ANABAR SHIELD HETEROGENEITY
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The Lišov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized ∼360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lišov granulites peak metamorphic conditions probably did not exceed 800–900°C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670–770°C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lišov granulites are thought to have originated during Viséan metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.
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High-pressure (HP) granulites widely occur as enclaves within tonalite-trondhjemite-granodiorite (TTG) gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton (NCC). Based on cathodoluminescence (CL), laser Raman spectroscopy and in-situ U-Pb dating, we characterize the zircons from the HP granulites and group them into three main types: inherited (magmatic) zircon, HP metamorphic zircon and retrograde zircon. The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites, and 207Pb/206Pb ages of 2915–2890 Ma and 2763–2510 Ma, correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet, clinopyroxene, plagioclase, quartz, rutile and apatite, and yield 207Pb/206Pb ages between 1900 and 1850 Ma, marking the timing of peak HP granulite facies metamorphism. The retrograde zircons contain inclusions of orthopyroxene, plagioclase, quartz, apatite and amphibole, and yield the youngest 207Pb/206Pb ages of 1840–1820 Ma among the three groups, which we correlate to the medium to low-pressure granulite facies retrograde metamorphism. The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic (1900–1850 Ma). Subsequently, the HP granulites were exhumated to upper crust levels, and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca. 1840–820 Ma.
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s 41 MAFIC GRANULITES OF THE LISOV MASSIF, SOUTHERN BOHEMIA: RELICS OF A LATE DEVONIAN MAGMATIC ARC? VOJTĚCH JANOUSEK, AXEL GERDES, STANISLAV VRANA, FRITZ FINGER & VOJTĚCH ERBAN 1 Division of Mineralogy and Material Science, Universitat Salzburg, Hellbrunnerstrase 34, Austria; vojtech.janousek@sbg.ac.at; friedrich.finger@sbg.ac.at 2 Institute of Mineralogy, J. W. Goethe–University, Senckenberganlage 28, 60054 Frankfurt, Germany; gerdes@em.uni-frankfurt.de 3 Czech Geological Survey, Klarov 3, 118 21 Prague 1, Czech Republic; vrana@cgu.cz The Lisov granulite Massif (LGM) differs from other granulite bodies in southern Bohemia by a higher proportion of intermediate/mafic and Opx-bearing types (gabbroic granulites to charnockites: Vrana & Jakes, 1982), association with Spl peridotites and pyroxenites as well as absence of eclogites. The evidence for the HP metamorphism is limited (Kotkova, 1998), being largely wiped out by a prolonged MP–HT equilibration (Vrana, 1990). Available is only a single U–Pb Zrc age from a tonalitic granulite (345 ± 5 Ma: van Breemen et al., 1982). The nature and age of protolith to mafic granulites have remained enigmatic. For this reason we have undertaken a combined study of internal zoning (CL) and LA ICP-MS dating of zircon in a quartz diorite Li–4 (Zvikov) and a tonalite Li–3 (Vlkovice). In both cases the zircon populations reflect two periods of growth. Many prismatic to needle-like crystals preserve igneous zoning; only some are perhaps recrystallized (blurred primary zones, convoluted zoning: Hoskin & Black, 2000) and/or resorbed and overgrown by irregular, featureless metamorphic rims. Other grains are oval-shaped/anhedral, lacking the internal structure. While the weighted average of Pb/U ages for metamorphic grains/overgrowths in Li-4 is 341 ± 5 Ma (2σ, n = 8), the igneous-looking crystals give 360 ± 4 Ma (MSWD = 0.6, n = 8). A few of the grains contain even older inherited cores (c. 430–600 Ma). An average of Pb/U ages for zircons with metamorphic appearance from Li-3 is 335 ± 6 Ma (MSWD = 1.3, n = 7); igneous grains yield 362 ± 4 Ma (MSWD = 1.2, n = 14). The both data sets thus point consistently to a protolith age of c. 360 Ma and HP metamorphism at c. 340 Ma, the latter age corresponding to the established timing of the metamorphism in the region (Kroner et al., 2000). The two-pyroxene, essentially Grt-free quartz dioritic–gabbroic granulites (qtzDG) constitute two larger (~4 km) bodies in the E part of the LGM. Their mineral assemblage is Opx + Cpx + Bt + Pl + Qtz (± Kfs, Hbl). Accessories include Ilm, Ap and Zrc. The protolith to the qtzDG was intruded by thin fine-grained picritic dykes. Even though the dykes show the lowest SiO2 (~43.5 %) and e Nd = +2.7, they cannot be little fractionated mantle-derived magmas due to their low Cr (~70 ppm), Ni (~26 ppm), mg# (~62) and high FeO*/MgO (~1.1) (Tatsumi & Eggins, 1995). The qtzDG preserve locally well-developed modal layering; elsewhere they enclose small pyroxenite enclaves (e Nd = +0.3) that seem to represent disrupted mafic cumulates crystallized from a magma parental to the qtzDG. The metaluminous granulites qtzDG are rather Na-rich (K2O/Na2O = 0.2–0.8), with moderately LREE-enriched REE patterns (CeN/YbN = 5.8–6.4) and slight Eu anomalies (Eu/Eu* = 0.80–0.86). The spiderplots are enriched in LILE, starting at >40 × NMORB (Cs, Rb) and falling to ~0.8 × NMORB (HREE). Most of the patterns feature troughs in Nb as well as bumps in Ba and Pb; P and Zr are depleted only in some samples. Similar LILE/HFSE enrichments are typical of basic igneous rocks from continental arc settings (Tatsumi & Eggins, 1995). The qtzDG show Sr–Nd isotopic signature more primitive (Sr/Sr340 = 0.706–0.707; e Nd = -1.7 to -2.4) than the tonalitic, charnockitic and granitic granulites from the LGM (Sr/Sr340 = 0.709–0.730; e Nd = -4.2 to -5.4) (Valbracht et al., 1994; Janousek et al., 2003).
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