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GEOCHEMISTRY OF PHANEROZOIC BASITES FROM THE EASTERN PART OF THE SIBERIAN PLATFORM
Abstract
In the eastern part of the Siberian platform, Phanerozoic magmatism manifested itself in various tectonic structures. It was related, in Middle Paleozoic and Lower Triassic times, to rifting processes, and in Late Paleozoic ? Early Mesozoic times ? to the initiation and development of traps. Varying geodynamic regime of magma generation resulted in a geochemical variety of rocks. The Permo-Triassic traps are characterized by a low content of incompatible elements, insignificant REE totals (60-70 ppm), a moderate degree of fractionation (La/Yb)n=2,0-2,8 and the presence of a Ta-Nb minimum. By the amount of incompatible elements and their position on correlation diagrams the traps are similar to E-MORB, which suggests that source rocks of this composition played a leading role in their formation. Middle Paleozoic rifting was accompanied by the formation of dike swarms and by eruptions of basalts. Characteristic of the rift-related Devonian basites are a high content of incompatible elements, significant REE differentiation (La/Yb)n=4,3-5,4 and a positive Ta-Nb anomaly. Geochemically, dolerites of the dike swarm are close to OIB, and effusive rocks on spider and correlation diagrams of elements are plotted between the values for E-MORB and OIB. A distinctive geochemical feature of trachydolerites is the highest content of incompatible elements, the character of REE distribution is distinguished by significant fractionation (La/Yb)n=23. On the correlation diagrams of incompatible elements, compositions of Phanerozoic basites exhibit a general trend from the E-MORB field through OIB to the field of higher contents in trachydolerites.
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