Trace element fractionation between biotite, allanite, and granitic melt

The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with Dbiotite/melt of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with Dbiotite/melt < 0.9. This is likely one of the reasons why Cu is enriched together with Mo (Dbiotite/melt = 0.29) in porphyry deposits associated with intermediate to felsic plutons, while the other transition metals are not. Both Nb and Ta are mildly compatible in biotite with Dbiotite/melt being larger for Nb (3.69) than for Ta (1.89). Moderate (15—30%) biotite fractionation would be sufficient to reduce the Nb/Ta ratio from the chondritic value to the range observed in the continental crust. Moreover, the strong partitioning of Ti into biotite implies that already modest biotite fractionation suppresses the saturation of Ti-oxide phases and thereby indirectly facilitates the enrichment of Ta over Nb in the residual melt. The heavy alkalis, alkaline earths, and Pb are only mildly fractionated between biotite and melt (Dbiotite/melt = 3.8 for Rb, 0.6 for Cs, 0.6 for Sr, 1.8 for Ba, 0.7 for Pb). The rare earth elements are generally incompatible in biotite, with a minimum for Dbiotite/melt of 0.03–0.06 at Gd, Tb, and Dy, while both the light and heavy rare earths are less incompatible (e.g. Dbiotite/melt = 0.6 for La and 0.3 for Yb). This behavior probably reflects a partitioning into two sites, the K site for the light rare earths and the octahedral Mg site for the heavy rare earths. There is no obvious dependence of the rare earth partition coefficients on tetrahedral Al in the biotite, presumably because charge balancing by cation vacancies is possible. Allanite was found as run product in some experiments. For the light rare earths, Dallanite/melt is very high (e.g. 385 to 963 for Ce and Nd) and appears to increase with decreasing temperatures. However, the rather high solubility of allanite in the melts implies that it likely only crystallizes during the last stages of cooling of most magmas, except if the source magma is unusually enriched in rare earths.

AGE SPAN FORMATION IN GRANITOIDS OF UKRAINIAN SHIELD

The paper discusses Berdychiv type granitoids that have always stirred up interest among researchers of the Ukrainian Shield. These rocks include minerals that are uncharacteristic of granites - cordierite, sillimanite, andalusite - and are closely related to rocks of the supposed substratum. At the same time, there still remain questions about the genetic nature of Berdychiv granites. Despite the fact that most researchers treat these granites as intrusive (anatectic) formations, there are other concepts according to which these granites are considered to be the products of metasomatic transformation (magmatic replacement) of primarily argillaceous and marly series of substratum. The Stryzhavka open pit, which is located in the Upper Bug region of the Ukrainian Shield displays differently cataclised porphyry-like plagiogranitoids (predominantly, garnet-bearing varieties in its southwestern area, locally rock-forming potassic feldspar varieties), and garnet-bearing leucocratic granites similar to those of Berdychiv type granites. The monazites of biotite-garnet granodiorite and biotite-garnet granite were dated by uranium-lead (U-Pb) isotopic dating method. The monazite of granodioritеs is dated 2049,3±3,5 million years based on the upper intersection of concordia with discordia. Significantly lower values of age (about 15 million years younger) for the monazite selected from granite, approximating 2035,1±1,9 million years were established. We assume it to be caused by prolonged crystallization of granitic melt and rather early crystallization of more basiс granodioritic magma in comparison with granitic one. Thus, the duration of the granitoid formation uncovered in the Stryzhavka open pit exceeds 15 million years, which correlates accurately with the estimates for occurrence duration of the granitic melts, which gave rise to various granites of the Ukrainian Shield.