HR: 0800h
AN: V41C-0719    [Abstracts]
TI: Peculiar Carbonate-Rich HP/UHP Rocks from the Tromso Nappe – Metacarbonatite or Remobilized Marble?
AU: Ravna, E
EM: Erling.Ravna@ig.uit.no
AF: University of Tromsø, Department of Geology, Tromsø, N-9037, Norway
AU: * Kullerud, K
EM: Kare.Kullerud@ig.uit.no
AF: University of Tromsø, Department of Geology, Tromsø, N-9037, Norway
AU: Davidsen, B
EM: Borre.Davidsen@ngu.no
AF: Norwegian Geological Survey, Geolab, Trondheim, N-7491, Norway
AU: Selbekk, R S
EM: r.s.selbekk@nhm.uio.no
AF: University of Oslo, Natural History Museum P.O.Box 1172 Blindern, Oslo, N-0318, Norway
AB: The common occurrence of marble and calc silicate gneiss interlayered with garnet mica schist and quartz- feldspathic hosting UHP mafic and ultramafic bodies within the uppermost allochthonous unit of the Scandinavian Caledonides of northern Norway is well known (Krogh et al. 1990; Ravna et al., 2006; Ravna & Roux, 2006), and the UHP (730 oC/3.36 GPa) event is dated to 452 +/- 1.7 Ma (U-Pb zircon; Corfu et al., 2003). Recently, a massive medium to coarse grained carbonate-rich body intimately associated with carbonate-bearing phlogopite-rich garnet pyroxenite and relatively pure banded marble has been discovered. The rock has an almost isotropic fabric with aggregates of silicate minerals evenly distributed in a matrix of calcite, giving it an igneous appearance. Garnet, clinopyroxene and biotite are easily recognized in hand specimens. Locally megacrysts (up to 4x6 cm) of green clinopyroxene occur. The contact toward the adjacent pyroxenite exposed in situ and in large blasted blocks shows an intricate and chaotic relationship. The massive carbonate rock and the marble show distinctly different primary mineralogy. The primary minerals of the massive carbonate rock is Sr-bearing Fe-Mg-calcite (Ca90Fe3Mg7), with lamellae and recrystallized grains of Fe-dolomite (Ca52Fe9Mg39), garnet (Grs26Sp1Alm44Py24And5), sodic clinopyroxene (Jd + Ac)20-25, Ti- (3.6 wt % TiO2) and Ba- (0.85 wt % BaO) rich biotite, F-rich apatite and rutile. Secondary minerals are hornblende after clinopyroxene, LREE-rich (up to 15 wt % LREE-ox) allanite growing on biotite and titanite (XAl = 0.09) after rutile. Garnet contains inclusions of apatite and calcite surrounded by radial cracks. The associated marbles contain calcite (Ca96Fe1Mg3), Ca-rich garnet (Grs56Sp1Alm30Py13), clinopyroxene (Jd + Ac)20-25, zoisite, K-feldspar and titanite (XAl = 0.20), with secondary diopside and plagioclase after Na-rich clinopyroxene. Major and trace element concentrations also display a marked difference between the massive carbonatite-like rock and marble. The former have high contents of TiO2 (1.7-4.0 wt %) and P2O5 (0.1-2.1 wt %) as compared to 0.2-0.3 wt% and <0.1 wt %, respectively, in the marble. REE-patterns of the carbonatite-like rocks show strong LREE enrichment (up to 1000 times chondrite vs. ca. 100 for marble) and (La/Yb)N of 31-77 vs. 12-17 for marble. The absence of a distinct Eu-anomaly in the massive carbonate rock also distinguishes it from the marble. Based on the distinct differences in bulk chemical composition between the massive carbonate rock and marble, they certainly are of different origin. The massive carbonate rock has REE-patterns similar to true carbonatites, whereas the patterns for the marble have typical limestone patterns. Also the high content of Ti, P and REE and the virtual absence of a europium anomaly in the massive carbonate rock bear similarities with carbonatites. We thus suggest that the massive carbonate rock is a meta-carbonatite, and the marble is of metasedimentary origin. This indicates that the eclogite-bearing Tromsø Nappe may represent a rifted continental margin subducted to UHP conditions during Ordovician time.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting