Metagabbros, metasyenites and related rocks in the Outokumpu assemblage, east Finland

Bulletin of the Geological Society of Finland

关键词: metagabbro;    hornblendite;    amphibolites;    pyroxenite;    syenites;    geochemistry;    genesis;    proterozoic;    Kaavi;    Juankoski;    Finland;   

DOI  :  10.17741/bgsf/64.1.007

来源: DOAJ

【 摘 要 】

A diverse group of minor intrusive rocks in the early Proterozoic Outokumpu assemblage of eastern Finland include metagabbro, hornblendite, amphibolite, pyroxenite, syenite and chloritized dykes. All are deformed and/or extensively altered and recrystallized. Intrusive relationships with serpentinite and metavolcanic rocks suggest they include the youngest pre-tectonic rocks in the Outokumpu assemblage. Geochemistry and rare relict mineral assemblages identify one, possibly two suites of minor intrusions that are alkaline in affinity, but not consistently peralkaline, with all but the amphibolites being derived from cumulates. They range from ultramafic diopside + olivine rocks, through quartz-dolerite, to diopside + plagioclase + microcline ± olivine syenites with normative nepheline and leucite. Elevated levels of Na, K and Ca are characteristic, giving the syenitic rocks an apparent kinship with shoshonites, and the most extreme compositions a resemblance to shonkinite. Pervasive carbonation and phlogopitization preceded deformation, and trace element patterns suggest this was a late stage magmatic process with the bulk addition of CO2, rather than bulk assimilation of the carbonate metasediments of the Outokumpu assemblage. The presence of alkaline rocks in the Outokumpu assemblage has profound implications for the original tectonic setting of these rocks, requiring, in particular, modification of the »ophiolite» model. These rocks show broad affinities with undersaturated calc-alkaline rocks of the shoshonite trend towards shonkinites (particularly Group III ultrapotassic igneous rocks), reinforcing an intraoceanic island arc interpretation for part, if not all, the Outokumpu assemblage.

【 授权许可】

Unknown