Evolution of arc crust and relations between contrasting sources: U-Pb (age), Nd and Sr isotope systematics of the ophiolitic terrain of SW Norway |
| |
| Authors: | Rolf B Pedersen Greg R Dunning |
| |
| Institution: | (1) Department of Geology, University of Bergen, N-5007 Bergen, Norway, NO;(2) Department of Earth Sciences, Memorial University of Newfoundland St. John's, Newfoundland, Canada A1B 3X5, CA |
| |
| Abstract: | The U/Pb dating of ophiolite and arc complexes in the Caledonides of SW Norway has demonstrated that these spatially associated
rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence
of calc-alkaline volcanics have been dated as 494 ± 2 Ma (2σ) and 473 ± 2 Ma respectively. Ophiolitic crust formed both prior
to, and during the first 10 Ma after the tholeiitic island arc volcanism. Boninitic and island arc tholeiitic dyke swarms
intruded the ophiolites soon after they formed and represent a second phase of spreading-related magmatism in the ca 20 Ma
period that separated the tholeiitic and the calc-alkaline island arc volcanism. The magmatism ended with the formation of
alkaline, ocean island basalt (OIB)-like magmas. Quartz dioritic and S-type granitic plutons, dated to 479 ± 5 Ma and 474 +3/−2 Ma
respectively, intruded into the base of the arc crust during and subsequent to the boninitic magmatism, and at the time when
calc-alkaline volcanic centres developed. The quartz dioritic and the granitic rocks contain inherited zircons of Precambrian
age which prove the involvement of a continental source. This together with the geology of the terrain and the geochemistry
of these plutons suggests that the granitic magmas were partly derived from subducted clastic sediments. The Sr and Nd isotope
systematics indicate that the same continental source was a component in the boninitic and the calc-alkaline magmas. While
the calc-alkaline magmas may have gained this continental component at a crustal level by assimilation, both geology and isotope
systematics suggest that the continental component in the boninitic rocks was introduced by source contamination – possibly
by a direct interaction between the mantle source and the S-type granitic magmas. A modified mid ocean ridge basalt-like mantle
source was the principal source during the earliest and the main crust forming stage. This source became replaced by an OIB-like
source during the later stages in the evolution of this ancient arc.
Received: 27 June 1994 / Accepted: 16 September 1996 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
