The Seiland Igneous Province, of which 5400 km2 is exposed, is hosted within a
discrete terrane within the northernmost part of the Caledonian orogenic belt. The
Province consists of numerous mafic and ultramafic plutons emplaced into a
sedimentary succession indicative of a continental setting. Accompanying this mafic
magmatism is a significant volume of intermediate monzonitic and dioritic rock
(10% of the total exposed igneous rock), as well as numerous nepheline syenite and
carbonatitic intrusions.
This study reports ID-TIMS U-Pb analyses on magmatic zircons from a range of
intrusions, which indicate that the bulk of the Seiland magmatism took place between
560 Ma and 570 Ma, whereas previous studies had produced a range of ages between
420 Ma and 830 Ma. The data indicate that only one magmatic episode is represented
in the rocks of the Seiland Igneous Province, invalidating previous models involving
multiple rifting events over a period of 300 m.y.
Detailed geochemical investigation of several plutons from an evolved high alkali
suite of gabbroic intrusions in the Seiland Igneous Province has shown that these
plutons are generally enriched in trace elements compared to layered intrusions from
other areas across the globe, but that geochemically the gabbros are relatively
homogenous. The rocks yield εHf and εNd values for the gabbroic rocks ranging from
+8 to -6 and from +4 to -4, respectively, indicative of the contamination of mantlederived
material with crustal material. The most primitive isotopic values are similar
to those obtained from the carbonatites and nepheline syenites, indicating the same
mantle source gave rise to the magmas that were subsequently emplaced as the
Seiland Igneous Province. The homogeneous trace element content of the different
mafic rocks most likely indicates a relatively homogeneous mantle source for the
original magmas of the province, which has subsequently been affected by processes
of assimilation and crustal contamination. The monzonitic and dioritic bodies in the
Seiland Igneous Province are not derived from melted silicic crustal material and
may have been formed by the melting of pre-existing mafic material.
The new geochronology invalidates the metamorphic framework previously
proposed for the Seiland Igneous Province, which postulated several orogenic events
between the emplacement of the magmas and the Caledonian Orogeny. There is no
evidence for metamorphic activity in the period between 570 Ma and 420 Ma, and
there are monazites in gneissic rocks hosted within mafic rocks of Seiland age that
preserve an age of 640 Ma. This leads to the conclusion that only one metamorphic
event, the 420 Ma Caledonian Orogeny caused by the collision of Baltica and
Laurentia, affected the Seiland terrane after the emplacement of the Seiland magmas.
The new data obtained lead to a model for the evolution of the Seiland Province in
which a number of heavily modified and contaminated mantle-derived mafic
magmas derived from the mantle were emplaced into the continental crust of the
Seiland nappe between 560 and 570 Ma. This magmatism was accompanied by the
injection of alkaline magmas into the same area of the crust, and the melting of mafic
rock emplaced earlier. This magmatic event is considered to have occurred in an
extensional stress regime, possibly during intracontinental rifting or back-arc
spreading. This event took place well before the 420 Ma Caledonian Orogeny, and
thus the Seiland Igneous Province can be considered a remnant of an older geological
terrane that was emplaced onto the margin of Baltica during the Caledonian
Orogeny.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/4993 |
| Date | 27 June 2008 |
| Creators | Roberts, Richard James |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 61815062 bytes, application/pdf, application/pdf |
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