Aspects and significations of plagioclase disequilibria in the rocks of the Ditrau Alkaline Intrusive Complex (Romania)

The Ditrau Alkaline Intrusive Complex (DAIC) is exposed in the southern part of the Crystalline-Mesozoic Zone of the East Carpathians. It is a complex Mesozoic intrusive body, which was incorporated in the Alpine Bucovinian Nappe during the Mid-Cretaceous shortening (the Bucovinian shear plan cut the DAIC at a depth of about 1800 m).
   In terms of petrography, DAIC is characterized by diverse rock types, lacking compositional constancy and gradual transitions from one petrographic type to another.
   By universal stage (US) plagioclase feldspars from hornblendite, diorite and monzonite cropping out in the left side of the Jolotca valley and along the way Ditrau-Hagota, plagioclase feldspars of nephelinsyenite from the Ditrău valley and plagioclase feldspars from ultramafics cropping out in the right side of the Teasc valley were analyzed. As a result of these analyses we found that a great part of the studied crystals emphasis, zoning, corroding and varying degrees of structural disorder. These plagioclases are generally twin and the twins frequently are complex twin (57.6% of cases) supplemented by parallel and normal twins, equal among themselves in share (21.2% of cases). Contents in anorthite determined by US were verified and detailed by microprobe. They are in the range An 0.09% - An 55% and show numerous frequency peaks. This, in conjunction with succession relations observed microscopically highlighted the existence of several feldspar populations: First plagioclase population that appears in gabbro is represented by an up to An 50% plagioclase. In diorites only exceptionally is present: in zoning structures, or in armoured structures (just in plagioclase core). A second population of plagioclase has a maximum frequency around An 27% - An 30% and appears to be the centre of crystals or around the cores containing over An 40%. It is found in diorite mainly, but in monzodiorite, monzonite or syenite as well. The third plagioclase population has the content around An 20%. It is found mainly in monzonite, syenite, and granite. The fourth population of plagioclase forms a peak at around 14% anorthite. It is not represented in gabbros but since diorite and ending with syenite this is omnipresent. The fifth plagioclase population (albite/oligoclase) appears mainly in the nepheline syenite. Like in all other types of rocks, the nepheline syenite presents a progressive decrease in calcium of the plagioclase feldspars due to the presence of several plagioclase phases. Here, however, the difference in composition between the phases is much smaller. The nepheline syenite oligoclase, rarely exceeds the An 10% - An 12% content but remains at approx. six, seven percent away to the albite which came later. The sixth plagioclases population (we consider the second plagioclase generation) has an albitic composition typically range between An 3% and An 8%. This albite is found in all the rock types from monzodiorite to syenite or nepheline syenite. The albite surrounds all other plagioclase crystals and edits the contact contour of early crystals.
   The albite (the second generation of plagioclase) is in a microcline, perthite and sometimes nepheline association while the calcium plagioclase is in a hornblende, sphene and sometimes pyroxene association.
   The process of reorganization of first plagioclase feldspars generation is developed to a high temperature level (between the “liquidus” and “solidus” lines of crystallizations diagram) so here an open system evolution process as magma mixing is. The existence of the high plagioclases (plotted on, or near the high temperature curves of the standard stereogram used in US determination) also the existence of the zoning structures, show that the process can not be a metasomatic one.