Rare-earth and selected trace elements (Ti, Cr, V, Sr and Zr) have been analyzed by secondary ion mass spectrometry (SIMS) in pyroxenes and amphiboles from both the lower and upper zones of the post-orogenic Proterozoic Ikasaulak intrusive complex (Nagssugtoqidian mobile belt of southeast Greenland). The orthopyroxenes from the harzburgites and olivine-orthopyroxenites (lower zone) are LREE-depleted. In contrast, orthopyroxenes from gabbros (the most mafic members of the upper zone) display U-shaped chondrite-normalized patterns and REE contents which increase from core to rim. In the lower zone, clinopyroxene coexists with pargasitic amphibole, both exhibiting unusually high LREE, Sr and Zr contents. In the upper zone, clinopyroxene is less LREE-enriched and is replaced by late- or post-magmatic hornblende. The variations observed in both the major- and trace-element composition of the ferromagnesian phases indicate that the Ikasaulak intrusive complex did not result from a single magma injection. An early injection of magma was responsible for the lower zone and strongly interacted with the country rocks. Fractional crystallization of this magma was probably accompanied by metasomatism effected by highly LREE-enriched hydrous fluids (Ce abundances up to 1300 X chondrite) The most mafic members from the upper zone were derived from a late injection of magma. This magma probably mixed with the residual liquid of the earlier injection, thus retaining the geochemical signature of the contamination processes which had occurred in the lower zone. Mixing and assimilation processes led the pyroxenes to concentrate REE contents higher than those theoretically predicted on the basis of the usual distribution coefficients for basaltic systems. The late growth of hornblende is unrelated to the LREE-enrichment processes. The ion microprobe analyses of the main mafic rock types of the Ikasaulak intrusive complex indicate that: (i) the parental magmas (tholeiites probably marked by a slight LREE-enrichment) were contaminated during the early cumulus processes; (ii) the degree of LREE-enrichment and HFSE (high field strength elements) depletion in parental magmas, as deduced from the whole-rock chemistry, may be overestimated.
SIMS ANALYSIS OF REE IN PYROXENES AND AMPHIBOLES FROM THE PROTEROZOIC IKASAULAK INTRUSIVE COMPLEX (SE GREENLAND) - IMPLICATIONS FOR LREE ENRICHMENT PROCESSES DURING POSTOROGENIC PLUTONISM
1991
Abstract
Rare-earth and selected trace elements (Ti, Cr, V, Sr and Zr) have been analyzed by secondary ion mass spectrometry (SIMS) in pyroxenes and amphiboles from both the lower and upper zones of the post-orogenic Proterozoic Ikasaulak intrusive complex (Nagssugtoqidian mobile belt of southeast Greenland). The orthopyroxenes from the harzburgites and olivine-orthopyroxenites (lower zone) are LREE-depleted. In contrast, orthopyroxenes from gabbros (the most mafic members of the upper zone) display U-shaped chondrite-normalized patterns and REE contents which increase from core to rim. In the lower zone, clinopyroxene coexists with pargasitic amphibole, both exhibiting unusually high LREE, Sr and Zr contents. In the upper zone, clinopyroxene is less LREE-enriched and is replaced by late- or post-magmatic hornblende. The variations observed in both the major- and trace-element composition of the ferromagnesian phases indicate that the Ikasaulak intrusive complex did not result from a single magma injection. An early injection of magma was responsible for the lower zone and strongly interacted with the country rocks. Fractional crystallization of this magma was probably accompanied by metasomatism effected by highly LREE-enriched hydrous fluids (Ce abundances up to 1300 X chondrite) The most mafic members from the upper zone were derived from a late injection of magma. This magma probably mixed with the residual liquid of the earlier injection, thus retaining the geochemical signature of the contamination processes which had occurred in the lower zone. Mixing and assimilation processes led the pyroxenes to concentrate REE contents higher than those theoretically predicted on the basis of the usual distribution coefficients for basaltic systems. The late growth of hornblende is unrelated to the LREE-enrichment processes. The ion microprobe analyses of the main mafic rock types of the Ikasaulak intrusive complex indicate that: (i) the parental magmas (tholeiites probably marked by a slight LREE-enrichment) were contaminated during the early cumulus processes; (ii) the degree of LREE-enrichment and HFSE (high field strength elements) depletion in parental magmas, as deduced from the whole-rock chemistry, may be overestimated.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
