Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 71

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to Eocene, lying between Danian and Eocene marine beds (48).
This might be around 55—65 My. But the presently exposed
basalts must have been buried under an unknown thickness of
rocks for at least some 40 My.
Already before 1939, the petrographic studies of these rocks
revealed their state of alteration, the effects of temperature
and water under burial. We shall give some quotations as to
the state of the traps, after Wadia’s description ((48), p. 215
—217): “Wide-spread meteoric and chemical changes are many:
calcite, quartz, chalcedony, glauconite, prenite, zeolites. A host
of other secondary minerales have been described from the
basalts of different localities — chlorophaeite, delessite, cela-
donite, serpentine, chlorites, iddingsite, and lussatite. By the
discoloration attending these changes, the original black colour
of the basalt is altered to a grey or greenish tint.” The inter-
calated tuffs have a “shaly aspect”. The following zeolites are
mentioned (p. 217): Stilbite, apophyllatite, heulandite, scole-
cite, ptilolite, laumontite, thomsonite, chabazite.
This assemblage of secondary minerals is considered to indi-
cate temperature of up to about 300°C at the maximum of
burial. Ade-Hall et al. (48 a) have demonstrated which changes
take place in the magnetic minerals by hydrothermal effects.
In the fresh cool lavas, there is a bi-modal distribution of the
Curie points. On one hand, there are minerals with Curie points
in the range about 100—200°C, on the other titanomagnetite
in the Curie point range about 500—600°C. With increasing
hydrothermal alteration the former change by oxidation to
minerals of higher and higher Curie points towards the range
of the titanomagnetite. Such changes have, accordingly, taken
place in the Deccan basalts (48 a, Fig. 16).
One might expect that during this series of processes the
direction of the total magnetization was greatly changed. This
was possibly the case for the Deccan traps; we have no evi-
dence to the contrary. But for some other cases of high de-
grees of regional hydrothermal alteration there are evidences
for the assumption that the direction of the original magne-
tization was retained. These evidences are close constancy of