Stoichiometric consequences of metamorphic mineral-forming reactions in pelitic rocks     

Kodjopa Attoh 《Lithos》1976,9(2):75-84

Metamorphic mineral-forming reactions are used to predict the volume proportions (percentages) of minerals in metapelitic rocks. Only minerals whose composition can be expressed in an idealised pelitic system (SiO₂Al₂O₂FeOK₂OH₂O) are considered in postulating the chemical reactions. The volume proportions predicted from the products of the chemical reactions are compared with the volume proportions of the minerals actually present in selected samples of metapelitic rocks from the Michigamme Formation (Michigan, U.S.A.).Evidence is presented to show that the reaction: 4 $\text{muscovite}\text{+3(}\text{Fe, Mg}\text{)}{}^{\mathrm{2+}}\text{? 3}\text{staurolite}\text{+6}\text{quartz}\text{+4}\text{K}{}^{\mathrm{+}}\text{+2}\text{H}{}^{\mathrm{+}}$ produces staurolite poikiloblasts in which the estimated quartz to staurolite ratio is 1:2.28.  相似文献   

The role of Pan-African structures in intraplate seismicity near the termination of the Romanche fracture zone, West Africa     

Kodjopa Attoh  Larry Brown  Joel Haenlein 《Journal of African Earth Sciences》2005,43(5):111

The distribution of epicenters of both historic earthquakes and recent seismic events in southeastern Ghana, compiled from local and teleseismic networks, show strong correlation with the Pan-African structures onshore and indicate an alignment with disruptions on seismic sections offshore. The seismic reflection sections reveal basement structures of the external zone of the Pan-African Dahomeyide orogen and these structures can be traced to offsets of shelf strata and seabottom reflectors, providing direct evidence, for the first time, for neotectonic activity that may be responsible for seismicity in the area. The deep structure of the external zone consists of moderately-dipping reflectors inferred to represent high-strain zones in the variably deformed margin of the West African craton. Taken together, the available data suggest that active tectonics in this intraplate environment may involve inversion of the Pan-African thrust structures but that this activity is apparently not related to reactivation of the nearby Romanche Fracture Zone.  相似文献   

Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan     

Attoh K 《The Journal of geology》2000,108(3):353-361

It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleoproterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamorphism. Heat production was anomalously high ( approximately 10.6 μWm-3) where andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels ( approximately 550 degrees -600 degrees C at approximately 3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600 degrees -650 degrees C at 6-7 kbar, heat production in the Archean basement was lower ( approximately 3.7 μWm-3). The effect of heat production rates on the metamorphic pressure-temperature paths was tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite-bearing assemblages formed, was significantly <6.0 μWm-3, the estimated pressure at peak temperatures (PTmax) would be much higher and lie in the sillimanite or kyanite stability fields; and (2) differences between PTmax estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involved significant crustal thickening as well as moderate unroofing rates.  相似文献   

Structure,gravity models and stratigraphy of an early Proterozoic volcanic—sedimentary belt in northeastern Ghana     

Kodjopa Attoh 《Precambrian Research》1982,18(3):275-290

The structure of an early Proterozoic volcanic—sedimentary belt in northeastern Ghana is inferred from the distribution of lithologic units and interpretation of Bouguer gravity anomaly associated with the belt. It is shown from gravity modelling that the vertical thickness (depth) of the volcanic—sedimentary succession is ca. 3 km and that the structure of the western part of the belt is an overturned anticline, an interpretation consistent with facing data. This structure provides the basis from which the stratigraphic order of the mapped lithic units is deduced: (1) fine-grained epiclastic sediments interbedded with minor felsic tuffs, followed by (2) tholeiitic basaltic lavas, which are overlain by (3) calc-alkaline andesitic and dacitic lavas and tuffs; the youngest volcanic unit belonging to the sequence is a calc-alkaline mafic tuff (4). A manganese-rich chemical sediment is preserved at the boundary between the tholeiitic mafic lava and calc-alkaline intermediate volcanic rock units. The early Proterozoic sequence, which is unconformably overlain by coarse fluviatile sediments, is estimated to be ca. 8500 m thick. The stratigraphic sequence in the study area contrasts strongly with the conceptual stratigraphic schemes which are currently held to be valid for similar lithologic associations of early Proterozoic age (Birimian) in the West African shield.  相似文献