The alteration state of basalt lavas is shown to be largely the result of the combined action of two alteration processes, deuteric oxidation and regional hydrothermal alteration. The former process is well described in the literature while the latter is shown here to be another widespread source of lava alteration. The environment of regional hydrothermal alteration is that of the zeolite metamorphic facies, with temperatures of up to 300°C, and abundant groundwater being the major elements. Independent and rapid spatial variation of the two alteration processes results in the wide range of observed lava properties. Microscopically undetectable titanomagnetite alteration, indicated by steady rise in strong field Curie point, is the first response of a non-deuterically oxidized basalt to regional hydrothermal alteration. With more extreme alteration, titanohematite etc. is seen to pseudomorph titanomagnetite grains. Ilmenite also shows very distinctive mineralogical changes. Initially, high deuteric oxidation specimens appear to be immune magnetically to moderate alteration but these too eventually succumb when conditions are extreme enough for the formation of prehnite in the rock.The geophysical implications of this widespread post eruption lava alteration process is discussed. We conclude that it is probably possible to obtain information on the original TRM of basalts at least in many cases but that the implications for potassium-argon dating need serious investigation.
This study is the first of a series in which a main concern will be to establish the Neogene absolute motion of the African Plate. A combined palaeomagnetic and K–Ar whole rock age dating study has been made of the Haruj Assuad basaltic volcanic area of central Libya. Seventy volcanic units, largely pahoehoe basaltic flows, were sampled in the north-central Haruj Assuad in an area centred on 27°45′ N, 017°30′E. Precisely defined paleomagnetic directions were obtained for 68 of the volcanic units and a weighted mean pole at 83°N, 171°E (δp = 5°, δm = 9°) obtained. This pole is just significantly different at the 95% level from the geographic pole. The difference between the paleomagnetic and geographic poles is thought to be largely the result of a degree of nonrepresentative sampling of the geomagnetic field. K–Ar radiometric age data indicate that volcanic activity in the Haruj Assuad area continued at least over the interval 6.0 to 0.4 my. The polarities of dated units are in agreement with the predictions of the time polarity scale. The ages of dated flows indicate that most of the flows presently exposed are younger than 2.2 my (i.e.) Upper Pliocene and younger. A scheme for assigning relative ages to groups of flows by degree of surface weathering is now calibrated in part with absolute ages.
Paleomagnetic and K/Ar whole rock absolute age data are described for material from the Garian area of Libya, centered at 13°E, 32°N. Within-unit cleaned paleomagnetic directions from the essentially unaltered lavas are very well defined and can almost certainly be taken as reliable measurements of the geomagnetic field direction during the initial cooling of each flow. However, the distributions of mean direction, from which the effect of repeated sampling of the field at one time has been removed, does not suggest that a reversing axial dipole field has been recorded in a representative manner. Both N and R groups of directions are azimuthally elongated, and the average poles for the N and R groups differ by 21°, or four times the 95% level uncertainty for each average pole. A number of possible physical explanations for the paleomagnetic results are discussed. The conventional overall average pole at 88°N, 123°E, δp: 3°, δm: 7 °does not differ significantly from the geographic pole, a result which agrees closely with that of Schult and Soffel (1973). However, the value of these overall average poles in estimating absolute plate motion must await an understanding of the sources of the asymmetries in the paleomagnetic data.
This study is the second of a series in which a main concern will be to try to establish the Neogene absolute motion of the African Plate. A combined paleomagnetic and K–Ar whole rock age dating study has been made of the Jebel Soda basaltic volcanic area of central Libya. Sixty basaltic flows were sampled in an area centered on 28°45′N, 15°30′E. Precisely defined paleomagnetic directions were obtained for 57 of the flows. Considerable attention has been given to the problem of data selection to obtain a best measure of the pole representing the dipole field during the period of volcanic activity. The equatorial nature of the directions, with equivalent poles extending continuously to 30°N latitude, provide a problem in data selection. Similar characteristics are apparent in the results of Schult and Soffel (1973) for the same area. A provisional average pole for the combined data sets, located at 73°N, 195°E(K: 26, A95: 5.6°) is significantly different from the geographic pole. Reasons for the 17 °divergence of paleomagnetic and geographic poles are discussed briefly.
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