Early Cambrian oceanic plagiogranite in the Silvretta Nappe,eastern Alps: geochemical,zircon U-Pb and Rb-Sr data from garnet-hornblende-plagioclase gneisses |
| |
Authors: | Bernhard Müller Urs Schaltegger Urs Klötzli Markus Flisch |
| |
Institution: | (1) Department of Earth Sciences, Institute of Isotope Geology and Mineral Resources, ETH Zentrum, CH-8092 Zurich, Switzerland Fax: ++41 1 632 11 79 E-mail: Mueller@erdw.ethz.ch, CH;(2) Department of Geology, Laboratory for Geochronology and Isotope Geology, University of Vienna, Franz Grill Str. 9, A-1030 Vienna, Austria, AT;(3) Brückfeldstr. 23, CH-3012 Bern, Switzerland, CH |
| |
Abstract: | Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine
Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses
were formerly oceanic plagiogranites. An εNd(
530
) value of +5.6 for the gneisses as well as initial 87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this
rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors
were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene
norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield
an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same
area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels.
The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early
Palaeozoic off the northern continental margin of Gondwana.
Received: 26 April 1996 / Accepted: 2 August 1996 |
| |
Keywords: | Austro-Alpine Eastern Alps Oceanic rocks Rb Sr Zircon U Pb Magmatism |
本文献已被 SpringerLink 等数据库收录! |
|