Fluid-Modified Gehlenite Skarns of Oravita, Banat, Romania
Abstract
Almost monomineralic Mg-rich gehlenite (Gh50Ak46Na-mel4) skarns locally occur along the contacts of a diorite intrusion at Oravita, Banat, Romania (Upper Cretaceous calcalcaline magmatic belt), elsewhere characterized by typical vesuvianite-garnet skarns. In the vein-like body of apparently unaltered gehlenite, the textural relations of the associated minerals (interstitial granditic garnet, and, locally, monticellite, rare cases of exsolutions 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 Na, in accordance with the fact that skarns are the only terrestrial type of occurrence of gehlenite-dominant melilite. A possible protolith for the gehlenite may be a melilite of typical magmatic composition, a few crystals of which have been observed close to the intrusive contacts in another Romanian occurrence of gehlenite skarns (Magureaua Vatei). The same minerals, monticellite and 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 85), accompanied by monticellite, spurrite (or tilleyite, afwillite, kilchonite) 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 gehlenite Gh85. The gehlenite in the rim, as well as in the local modifications, show a remarkable correlation between the akermanite and Na-mel 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° ), controlled by the localized presence of small amounts of fluid at low pressure, mainly involving the transformation of the akermanite component to monticellite. The silica released by this transformation resulted in the formation of spurrite (among others) and of the Na-mel 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.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V51C..10K
- Keywords:
-
- 3620 Crystal chemistry;
- 3665 Mineral occurrences and deposits;
- 8424 Hydrothermal systems (8135)