Petrological and Geochemical Features

Petrological and geochemical features of the Siljan impact structure were investigated by Aldahan (1990, 1991) and Ramseyer et al. (1992). The igneous rocks used for these investigations include samples from outcrops, several shallow (down to 600 m depth) and the two deep bore holes Gravberg-1 and

Fig. 13. Maps of observed alterations (left) of mineral cathodoluminescence. Drill hole locations are marked with diamonds. Stars denote outcrop samples. Profile across the Siljan central uplift dome (right). Triangles mark observations projected into the profile. Dotted areas: Alteration of quartz, K-feldspar and plagioclase, respectively. Dashed lines show the approximate boundary between Siljan and Jarna granite types. After Ramseyer et al. (1992).

Fig. 13. Maps of observed alterations (left) of mineral cathodoluminescence. Drill hole locations are marked with diamonds. Stars denote outcrop samples. Profile across the Siljan central uplift dome (right). Triangles mark observations projected into the profile. Dotted areas: Alteration of quartz, K-feldspar and plagioclase, respectively. Dashed lines show the approximate boundary between Siljan and Jarna granite types. After Ramseyer et al. (1992).

Stenberg-1 (to depth of about 6.5 km). The majority of the data from the deep bore holes concern the Siljan and Jarna granite types that dominate the central uplift, and some minor dolerite.

Three major types of deformation and alteration are observed in the granites that have been related to the meteorite impact, intrusion of dolerite, and regional tectonic movements, respectively. The impact related alterations are estimated to extend down to 3 km depth in the Gravberg-1 borehole and to about 4.5 km depth in the more centrally located Stenberg-1 borehole. The impact-related alterations include 1) pseudotachylitic breccia veins, 2) planar deformation features (PDF) in quartz grains, 3) abundant fractures, 4) kinking and fragmentation of feldspars, 5) extensive saussuritisation of feldspars and chloritisation of micas.

Cathodoluminescence and microprobe analysis have been performed on quartz, plagioclase, and K-feldspar. Anomalous secondary brownish luminescence was found in albite and reddish luminescence in quartz. Primary magmatic quartz, plagioclase, and K-feldspar have blue, yellowish, and bluish luminescence colors, respectively. The change in luminescence is independent of the granite types. The depths to which anomalous cathodoluminescence is observed in the Gravberg-1 drill hole are for quartz 2.2 km, for K-feldspar 4.6 km and for plagioclase 4 km (Fig. 13). The change in luminescence of quartz is interpreted to be caused by the shock induced heating and subsequent high temperature in the granites. Most of the alteration of the feldspar occurred during the subsequent cooling period, by the introduction of meteoric water into the fractured rocks (Ramseyer et al. 1992).

Was this article helpful?

0 0

Post a comment