Geochemistry of the Gabbro--Diorite--Tonalite--Trondhjemite Suite of southwest Finland and its Implications for the Origin of Tonalitic and Trondhjemitic Magmas

Rocks of the Proterozoic gabbro—diorite—tonalite—trondhjemitesuite of southwest Finland were analyzed for major elements,REE, Rb, Sr, Ba, U, Th, and isotopic composition of Sr and O.Petrographic continuity from hornblendite through hornblendegabbro, hornblende—biotite diorite, hornblende—biotitetonalite and trondhjemite is reflected in regular variationof major and trace elements in samples ranging from 42 to 74per cent of SiO₂. The suite is calc alkaline—trondhjemiticand is distinguished from ‘normal’ calc-alkalinesuites by the increase in Na₂O and decrease in K₂O and REE concentrationsfrom intermediate to silicic rocks. Prior to solidification, the magmas may have undergone additionof water from metamorphic country rock producing a variationof whole-rock O₁₈ values of from 6.0 to 11.3 per mil. This processmay have been accompanied by introduction of Rb and Sr and removalof Ba from intermediate to trondhjemitic magmas. A linear whole-rockisochron age of 1.9 x 10⁹ years may only be approximate owingto the possibility of Rb and Sr exchange. REE concentrationsshow regular variation through the suite and were not noticeablyaffected by exchange processes. Samples ranging from 50 to 60per cent SiO₂ show chondrite-normalized light REE contents rangingfrom 40 to 160 whereas heavy REE contents are constant at 10,and there are no Eu anomalies. Samples ranging from 60 to 74per cent SiO₂ have REE patterns which decrease in both lightand heavy REE with increasing SiO₂ to values of 10 and 1 timeschondritic values respectively, and show increasingly positiveEu anomalies. Three possible models for magma genesis are consistent withthe petrographic, major and trace element variations, isotopicdata, and experimental petrologic studies: (1) fractional crystallizationof a gabbroic liquid involving hornblende, plagioclase, andbiotite as the major precipitating phases; (2) partial meltingof amphibolite leaving a hornblende-rich residue; (3) partialmelting of amphibolite or eclogite leaving an eclogitic residue.Model (1) is preferred because of the presence of hornblende-cumulaterocks, and because there is a continuum of compositions fromgabbro to trondhjemite.