Download Report

Open File Report Q29.119811
Lauri J. Pesonen
Laboratory for Paleomagnetism
Geological Survey of Finland
P.O. Box 96, FIN-02151 Espoo
FINLAND
PALAEOMAGNETISM OF A SAMPLE FROM THE VALAAM SILL, LAKE LADOGA, RUSSIA Palaeomagnetism of a Sample From the
Valaam Sill, Lake Ladoga, Russia
ABSTRACT
0
0
The remanent magnetization direction D = 42.5 , 1 = -19.4 (pole position Plat. = 1 1 . 3 ' ~ , Plon.
= 168.3'~) of a gabbro-monzonite sample from the Valaam sill, Lake Ladoga, Russia, suggests a
Postjotnian age of ca. 1280 Ma. This is older than the U-Pb zircon age of 1150f30 Ma of this
sill (Amantov et al., 1996). Further sampling and paleomagnetic field tests are required to solve
this discrepancy.
Introduction
The remanent magnetization directions of the Subjotnian (ca. 1650-1370 Ma) and Jotnian-Postjotnian
(ca. 1370-1200 Ma) rocks in Finland and Sweden are well established (e.g., Bylund and Pesonen,
1987; Pesonen et al., 1989; 1992). In contrast, the ages and the magnetization directions of the coeval
mafic intrusives of the Lake Ladoga and Lake Onega regions in the Russian Karelia are poorly known
(e.g., Katseblin, 1968a;b; Pesonen et al., 1992; Amantov et al., 1996). The K-Ar whole rock ages of
the magmatic rocks of the North Ladoga range from 1500 to 1100 Ma, whereas the Rb-Sr ages show a
narrow distribution between 1260-1280 Ma (Amantov et al., 1996). The most precise date is the U-Pb
zircon age of 115&30 Ma (i.e., Early Sveconorwegian) of the gabbro-monzonite sill of the Valaam
island (Amantov et al., 1996).
In the surnrner of 1996, Matti Vaasjoki (GSF) and Tapani Ramö (HU) made an excursion to Lake
Ladoga and took samples of the "Jotnian sill" exposed on Moskovansaari, in the western coast of the
main island of Valaam (Lat. = 61.42'~, Long.= 31.0'~; Fig. 1) for isotope study. They also collected
an oriented sample (V8) from this sill for paleomagnetic dating. Here we present the paleomagnetic age
of this sample keeping in mind the very restrictions provided by the unsufficient sampling. The
petrophysical properties of this sample are also reported.
Measurements
The paleomagnetic directions of the two specimens cut out from the block sample were measured with
the 2G SQUID-magnetometer of the paleomagnetic laboratory of the GSF. The alternating field (a.f.)
demagnetizations were carried out to 120 mT. The petrophysical properties (susceptibility, density,
NRM) were measured with the instruments of the GSF paleomagnetic laboratory (e.g., Mertanen et al.,
1996).
Results
The sample is macroscopically a gabbro-monzonite rather than a diabase, as also revealed by its low
density (2840 kgm-'). Figure 2 shows an example of the a.f. demagnetization behaviour of a
specimen. A soft component with a magnetization direction of D 92', -570, probably a viscous
overprint by the present Earth's magnetic field (PEF), is erased at low fields. A characteristic
magnetization direction (ChRM), with northeasterly declination and upward inclination is isolated at
higher fields (Fig.2). In the course of a.f. treatments, this direction oscillates back and forth along a
great circle with nearly constant declination so that no well defined remanence direction is obtained.
This type of behaviour is often seen in coarse-grained rocks. The best'estimate is'obtained at the
cleaning level of 60-100 mT yielding a mean direction of D = 42.5 , 1 = -19.4 (N=2), with a
corresponding palaeomagnetic pole of Lat.= 1 1 . 3 ' ~ , Plon 168.3'~. The polarity is normal (N)
following the definition of Bylund and Pesonen (1987).
-
Fig. 1. The geology of the Lake Ladoga, showing the sampling site (Moskovansaari) in the western coast of the island of
Valaarn. Green = diabaselgabbro-monzonite,purple = rapakivi. For other rock types, see Koistinen (1996).
Interpretation
In Fig. 3 we have plotted the pole of the Valaarn sill together with other poles of Fennoscandia divided
in three clusters, Subjotnian (1650- 1370 Ma), Jotnian-Postjotnian (1370- 1200 Ma) and Early
Sveconorwegian (1 150-1000 Ma), respectively (Pesonen et al., 1989; 1992). The pole of the Valaam
sill plots at the northern edge of the Postjotnian cluster, close to the pole of the Älvho dolerite from
Sweden, which has an age of 1290 Ma. A tentative palaeomagnetic age for the sill is therefore
Postjotnian, ca. 1280 Ma. The Valaam pole is more than 40°away from the early Sveconorwegian
pole-cluster (e.g., from the Neoproterozoic poles of Laanila (1028 Ma) and Kautokeino (1066 Ma)
dykes (Mertanen et al., 1996), and from the prelirninary pole of the Saila dyke (1 140 Ma; Mertanen and
Pesonen, 1998, unpublished data). Therefore, the paleomagnetic results, although based on one sarnple
only, is in contrat with an early Sveconorwegian U-Pb age of 1150 Ma for this sill (Amantov et al.,
1996).
-
The magnetization age (1280 Ma) may, however, be in error. First, there is an orientation error, which
cannot be checked since we only have one sample sofar.Second, the secular variation has not been
averaged out. If we allow a maximum error of 20' due to secular variation, the pole could easily move
into the older Subjotnian cluster but not to the younger Sveconorwegian cluster. Third, a postemplacement tilting of the sill would change the direction unless corrected for. Since the sill, however,
is not tilted by more than 10 after the emplacement (Amantov et al., 1996; T. Koistinen, pers.
comrnun., 1997), structural tilting is not explaining the "old" paleomagnetic pole position (and age) of
the sill. Fourth, the ChRM may not be primary. Perhaps an original, early Sveconorwegian (Le., 1150
Ma) direction has been lost in deformation or in thermochemical processes? Proving this would require
paleomagnetic field tests, which are not available. However, this is unlikely since the pole is not
plotting onto the post-Sveconorwegian trend of the paleornagnetic poles (Pesonen et al., 1989; 1992) as
should be the case if it is secondary.
-.
t.41
CBl
Stereoplot
I
Intensity
Y1
[CI
Zijderveld plot
Paleomagntic data of a gabbro-monzonite specimen V8-1A of the Valaarn sill.
(c) Zijderveld diagram. A.f. demagnetization to 120 mT.
stereoplot,
intensity decay ,
Petrophysics
In Fig. 4 we show the petrophysical data of the sample V8 together with data from Satakunta
(Eurajoki-Reposaaari),Ulvö - kand (L.J.
Pesonen, unpublished data), and Sveconorwegian (LaanilaKautokeino; Mertanen et al., 1996) dolerites. The Valaarn data plot distinctly off from the other diabase
data clusters (being closest to the Postjotnian Eura-Reposaari one). However, as noted previously, this
comparison is not very valuable since the sill is lithologically a gabbro-monzonite rather than a diabase,
thus explaining its low density.
Conclusions
The characteristic remament magnetization direction of a sarnple from the Valaarn gabbro-monzonite sill
suggest a magnetization age of ca. 1280 Ma (i.e., Postjotnian) for this sill which is clearly older than
the U-Pb (zircon) age of 1150I30 Ma. A more detailed study with a baked contact test is required to
solve this discrepancy.