STRONTIUM AND ARGON ISOTOPIC HOMOGENIZATION OF PELITIC SEDIMENTS DURING LOW-GRADE REGIONAL METAMORPHISM - PAN-AFRICAN UPPER DAMARA SEQUENCE OF NORTHERN NAMIBIA (SOUTHWEST AFRICA)

被引:49
作者
CLAUER, N [1 ]
KRONER, A [1 ]
机构
[1] UNIV MAINZ,INST GEOWISSENSCH,D-6500 MAINZ,FED REP GER
关键词
D O I
10.1016/0012-821X(79)90161-4
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
The fine mineral fractions < 2 μm extracted from pelitic sediments of the Eocambrian upper Damaran Mulden Group were subjected to a detailed isotopic study by the Rb-Sr and K-Ar methods in combination with a mineralogical analysis by X-ray diffractometry. The sediments contain two major parageneses of metamorphic origin which can be related to two separate low-grade regional metamorphic events of anchizonal intensity with temperatures between 250 and 300°C and pressures up to 2 kbar. These events are dated at about 535 m.y. and 455 m.y. respectively (with λ(87Rb) = 1.42 × 10-11 yr-1). Anomalously high K-Ar ages on samples from specific stratigraphic horizons can be related to open system behaviour and K migration during the formation of stilpnomelane from ferromagnesian illites. This study shows that age dating of sedimentary rocks subjected to low-grade regional metamorphism can only yield meaningful results if the interpretation of isotopic analyses is based on a clear understanding of the mineralogical processes that led to the formation of those parageneses which characterize the rock unit dated. © 1979.
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页码:117 / 131
页数:15
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