Yv. Amelin et al., CHRONOLOGY OF MULTIPHASE EMPLACEMENT OF THE SALMI-RAPAKIVI GRANITE-ANORTHOSITE COMPLEX, BALTIC SHIELD - IMPLICATIONS FOR MAGMATIC EVOLUTION, Contributions to Mineralogy and Petrology, 127(4), 1997, pp. 353-368
The U-Pb dating of 18 samples, representing the principal rock types o
f the 4000 km(2) Salmi anorthosite-rapakivi granite complex and its sa
tellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield, r
eveals that six temporally distinct episodes of igneous activity occur
red in a timespan of 17 million years. From oldest to youngest they ar
e: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 15
43.4 Ma; (3) early wiborgite and pyterlite at 1540.6-1537.9 Ma; (4) bi
otite granite and more evolved granite at 1538.4-1535 Ma; (5) late pyt
erlite at 1535.2 Ma; (6) olivine gabbro and biotite-amphibole granite
at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5
-5.0 million years. Early wiborgite and pyterlite (3, above) and bioti
te granite (4, above) probably crystallized from multiple magma intrus
ions. Age differences of 3.4 +/- 1.5 million years between zircon and
baddeleyite in olivine gabbro (6, above) are probably a result of xeno
crystic origin of baddeleyite extracted from an earlier mafic phase of
the Salmi complex. The ages and chemical features of early and late z
ircon populations, together with our modeling of magma crystallization
and zircon growth, show that the duration of magma crystallization an
d Pb-diffusion in zircon was short lived and insignificant compared to
the precision of dating of about +/- 1-2 million years. Hence, the ra
nge of U-Pb ages for each of the major rock types may approximate the
emplacement intervals of their respective magmas. Average rate of magm
a emplacement was about 0.01 km(3)/year for the most voluminous phase
of early biotite-amphibole rapakivi granite, and about 0.0024 km(3)/ye
ar for the Salmi complex as a whole. Compositional changes of the Salm
i magmas over time are in agreement with the model of magmatism relate
d to lithospheric extension.