Chemical petrology of polymetamorphic ultramafic rocks from Galicia, NW Spain

The investigated polymetamorphic peridotites occur associated with metabasic rocks in several complexes of probably Precambrian age in the northern part of the Hesperian massif (Iberian peninsula). Spinel-clinopyroxene-, spinel-pargasite-, spinel-hornblende- and chlorite-amphibole-peridotites, wehrlites, spinel-amphiboleand plagioclase wehrlites are found; most rocks are partly to completely serpentinized. Attention has been paid particularly to the two first-mentioned catazonal types in which bands, veins and lenses of garnet ± spinel pyroxenite and -pargasitite occur, while brown ceylonite orthopyroxenite and spinel-sensu-stricto clinopyroxenite are present in subordinate amounts. It is contended that these pyroxenites and pargasitites represent partial melting products of a parental ultramafic rock which contention is corroborated by experimental evidence. The partial melt had a picritic composition and crystallized as an aluminous pyroxene assemblage, in which garnet was formed under subsolidus conditions. Comparisons are drawn with peridotites from other occurrences in which garnet-bearing assemblages are also encountered. It is assumed that these peridotites were emplaced as spinel-clinopyroxene peridotites (Iherzolites) during a Precambrian orogenic cycle under high-pressure granulitefacies conditions and equilibrated at 1100°-1200°C under 15-20 kb pressure. Catazonal retrogradation (800°-900°C, 10-15 kb pressure) gave rise to large-scale development of pargasite. Garnet probably metastably coexisted with pargasite and the zonary character in the garnets was presumably obtained during this phase. Aluminous chlorite was formed during the Hercynian orogeny under mesozonal conditions; a second generation of pyroxenes and amphiboles possesses lower contents of Al2O3. The spinel-hornblende peridotite probably did not undergo high-pressure granulite-facies conditions during the Precambrian orogeny. The chlorite-amphibole peridotite is supposed to have a lower Paleozoic age. The wehrlites are considered to be partial melting products of a parental peridotite and crystallized under low pressures. Spinel-amphibole wehrlites are hydrated plagioclase wehrlites. New whole-rock analyses of 11 peridotites and 14 pyroxenites are given. Contents of some minor and some trace elements have been determined with neutron activation analysis. Electron microprobe analyses are presented of olivines, pyroxenes, amphiboles, garnets, spinels, chlorites, högbomites and ilmenites. The mineral compositions are compared to whole-rock chemistry and distribution coefficients are calculated from which temperature and pressure estimates are derived.