Strontium Isotopic Constraints Research Articles

Trace Element and Strontium Isotope Constraints on the Origin and Evolution of Paran Continental Flood Basalts of Santa Catarina State (Southern Brazil)

Forty-five selected samples from two different sequences, and from different sites spread over the Parana Basin have been analysed for major and trace elements and Sr isotope ratios. Results demonstrate the existence of two geochemically and isotopically distinct groups, namely a high phosphorus and titanium (HPT) and a low phosphorus and titanium (LPT) group. HPT rocks are relatively rich in LIL elements and have more homogeneous and markedly lower isotopic Sr ratios than LPT rocks. Since no relationship between the two groups was found, they may derive from different mantle sources, the HPT source being more enriched in most incompatible trace elements except Th, U and Rb than the LPT one. The LPT group, comprising basalts to rhyodacites, displays a large range in Sr isotopic ratios. This is interpreted as a result of a fractional crystallization process occurring at high crustal levels associated with assimilation of crustal material.

Basement and Sedimentary Recycling and Continental Evolution

The present day cumulative distribution of areas of continental basement age provinces (as defined by radiometric estimates) follows an exponential function. A similar distribution is displayed by the reserves of their associated economic deposits and by the thickness and areal distribution data for the overlying sediments. This exponential pattern, similar to isotope decay systematics, is a result of recycling (s.l.). Each theoretically possible growth model must be complemented by an appropriate average recycling constant in order to generate the observed present day distribution. Computer simulation of the system suggests that: (a) the theoretical recycling (radiometric rejuvenation) constant b for the continental basement is $$\leq 6.5 \times 10^{-4}/10^{6}a$$. Strontium isotopic constraints favor $$2 - 4 \times 10^{-4}/10^{6}a$$ as the most likely b. Correcting for the effect of recycling, the emerging pattern of continental accretion (based on the available age data) is of a slow growth during the e...