Origin of titanian pargasite in gabbroic rocks from the Northern Apennine ophiolites (Italy): insights into the late-magmatic evolution of a MOR-type intrusive sequence

Gabbroic rocks from Northern Apennine ophiolites contain accessory titanian pargasite, in interstices between plagioclase and clinopyroxene, and as rims around interstitial Fe-Ti-oxide phases. The origin of titanian pargasite has been evaluated by combining major, volatile and trace element microanalyses. Titanian pargasites show variable amounts of F and low Cl (0.03-0.23 and ≤0.03 wt%, respectively), and mg value ranging from 0.78 to 0.70 and from 0.61 to 0.53 in Mg- and Fe-rich rocks, respectively. Geothermometric calculations based on amphibole-plagioclase equilibrium yield temperatures of 900 ± 50°C and 840 ± 50°C for Mg- and Fe-rich rocks, respectively. Titanian pargasites are characterized by LREE depletion, nearly flat HREE and variable negative Eu anomalies. Total REE contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites, respectively. In the chondrite-normalized patterns of titanian pargasites, Ba, K and Sr are markedly depleted relative to LREE, whereas Nb, Zr and Ti are slightly enriched to slightly depleted relative to neighboring REE. A separate of titanian pargasite from a Fe-rich gabbroic rock was analyzed for Sr isotopic composition; its initial 87Sr/86Sr falls within the range of modern N-MORB and is consistent with those of fresh Mg-rich gabbroic rocks. Titanian pargasite most likely formed by an igneous liquid with relatively high H2O content (4.4-6.1 wt%) and a slight LREE enrichment. The origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a H2O-rich agent of igneous origin, WHICH could be a trondhjemite liquid or an exsolved fluid. Probably, such interaction triggered a post-cumulus crystallization process that finally yielded the precipitation of titanian pargasite.