The melilite (Gh50) skarns of Oravita, Banat, Romania: transition to gehlenite (Gh85) and to vesuvianite.
Résumé
Almost monomineralic Mg-rich gehlenite (~Gh50Ak46Na-mel4) skarns occur in a very restricted area along the contact of a diorite intrusion at Oravita, Banat, in Romania, elsewhere characterized by more typical vesuvianite–garnet skarns. In the apparently vein-like body of unaltered gehlenite, the textural relations of the associated minerals (interstitial granditic garnet and, locally, monticellite, rare cases of exsolution of magnetite in the core zone of melilite grains) suggest that the original composition of the gehlenite may have been different, richer in Si, Mg, Fe (and perhaps Na), in accordance with the fact that skarns are the only terrestrial type of occurrence of gehlenite-dominant melilite. The same minerals, monticellite and a granditic garnet, appear in the retrograde evolution of the Mg-rich gehlenite toward compositions richer in Al, the successive stages of which are clearly displayed, along with the final transformation to vesuvianite. These changes include (1) the local development of small rounded patches with Al-rich compositions (Gh60 to Gh85), accompanied by monticellite, spurrite (or tilleyite, afwillite, kilchoanite) and, at a later stage, a granditic garnet, and (2) the complete transformation of gehlenite to vesuvianite, associated with minor clintonite (+ monticellite and probably ellestadite), usually along a sharp front typically rimmed by a 0.5-mm-wide zone of Al-rich gehlenite (Gh85). The gehlenite rim, as well as the gehlenite in the local modifications, show a remarkable correlation between the åkermanite and Na-melilite contents, probably of crystal-chemical origin. The local modifications (1) are interpreted as nearly closed-system (except for Na and Fe) retrograde reactions at moderate temperature (500–600°C), controlled by the localized presence of small amounts of fluid at low pressure, mainly involving the transformation of the åkermanite component to monticellite. The silica released by this transformation resulted in the formation of spurrite (among others) and of the Na-melilite component at first, and garnet later. The subsequent transformation (2) of gehlenite to vesuvianite and clintonite, which involved silica metasomatism, may result from the more pervasive infiltration of the same fluid, at higher pressure, probably related to the development of the garnet–vesuvianite skarns elsewhere along the intrusive contacts.