Physics
Scientific paper
Oct 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976gecoa..40.1261g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 40, Issue 10, pp.1261-1280
Physics
12
Scientific paper
Sixteen new major and trace element analyses of granitic and acid volcanic rocks from the Barberton Mountain Land, eastern Transvaal, are reported. The data indicate very low abundances of LIL elements in albite porphyries incorporated in the lower ultramafic to mafic succession of the volcanic Onverwacht Group, and in the `ancient tonalites' plutonic suite which intruded the volcanics at about 3.2-3.4 aeons. Some of the albite porphyries display highly fractionated REE patterns, which indicate equilibration with garnet and therefore high-pressure origin. The REE patterns of the `ancient tonalites' are characterized by pronounced positive Eu anomalies, and their derivation by partial melting of eclogite within the depth range of 30-50 km is suggested. Some of the albite porphyries may represent shallow-level hypabyssal equivalents of the `ancient tonalites'. The Dalmein and Bosmanskop plutons are regarded as late members of the `ancient tonalites' suite, derived by decreasing partial melting of basic source rocks. A contrasting model of origin is suggested for the ca . 3.0 aeons old Nelspruit migmatite and Hood granite, which are thought to have been derived by anatexis of the `ancient tonalites' and incorporated ultramafic to mafic xenoliths of the Onverwacht Group. The chemistry of greywackes of the Fig Tree Group, which overlie the Onverwacht Group, is consistent with the erosion of a terrain consisting of Onverwacht Group and `ancient tonalites'--including late K-rich members of the latter suite. The data lend support to models involving a secular transformation from an ensimatic to an ensialic tectonic environment, but important geochemical differences are indicated between the Archaean rocks and acid igneous suites of island arcs and oceanic domains.
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