Trace element redistribution in high temperature deformed gabbros from East Ligurian ophiolites (northern Apennines, Italy): Constraints on the origin of syndeformation fluids

Abstract Mg‐gabbros from East Ligurian ophiolites (Northern Apennines, Italy) display a high‐temperature/low‐pressure recrystallization localized along ductile shear zones. In deformed gabbros, the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red‐brown amphibole. The latter displays a pargasitic composition, with high amounts of AlIV Na(A) and Ti (± 1.8, 0.7 and 0.4 atoms per formula unit, respectively). Major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800–950° C. Red‐brown Ti‐pargasite also occurs as a minor interstitial constituent, presumably growing from a residual trapped liquid, in the differentiated lithologies (Fe‐Ti‐diorites) of the plutonic ophiolitic complex. By means of ion microprobe (SIMS technique), rare earth (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) and selected trace elements (Sr, Y, Cr, V, Sc, Zr, Ti) have been analysed in igneous and neoblastic diopside, as well as in Ti‐pargasites. Ti‐pargasites have also been analysed for F and Cl, and compared with the halogen composition of the amphiboles, mainly hornblendes to actinolites, which are related to the subsequent low‐temperature brittle evolution. Neoblastic Ti‐pargasite from deformed Mg‐gabbros bears close compositional similarities with igneous Ti‐pargasite from undeformed Fe‐Ti‐diorites, whereas it is geochemically distinct from the amphiboles post‐dating the ductile event. In particular, Ti‐pargasites have relatively high contents of F, REE, Y, Zr and Sr, which are not consistent with crystallization in the presence of seawater‐derived hydrothermal fluids. High‐grade recrystallization probably developed in the presence of volatile‐rich igneous fluids, either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system. An alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in Ti‐pargasite‐rich layers. The petrological features of deformed gabbros recovered from present‐day slow‐spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros. In East Ligurian ophiolites, high‐grade ductile shear zones have been related to the initial stages of the uplift of the gabbro‐peridotite complex to the sea‐floor. Copyright © 1995, Wiley Blackwell. All rights reserved