TY - JOUR

T1 - Composition of Li-F granite melt and its evolution during the formation of the ore-bearing Orlovka massif in Eastern Transbaikalia

AU - Badanina, E. V.

AU - Syritso, L. F.

AU - Volkova, E. V.

AU - Thomas, R.

AU - Trumbull, R. B.

PY - 2010/5/14

Y1 - 2010/5/14

N2 - By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12-1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B2O3 contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO2 (by up to 6 wt %), Na2O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H2O. Two stages were distinguished on the basis of the behavior of trace elements and fluorine in the melts and rocks. The early stage, from the biotite granites of the parental massif to the microcline-albite granites with the pea-shaped quartz is characterized by a decrease in Si, Fe, Ca, Mg, Zr, Hf, and REE and accumulation of Al, Na, Li, Rb, F, Ta, and Nb, which correspond to the ongonite differentiation trend. During the second stage, amazonite-bearing rocks with Li micas and Ta mineralization (columbite-tantalite and microlite) were produced, and the melt was depleted in Al, Na, Li, F, Nb, and Ta. A considerable difference appeared between the compositions of rocks and aluminosilicate melts. There is a paradoxical discrepancy between the high contents of Li, Ta, and Nb in the rock (2289, 446, and 269 ppm, respectively) and the low contents of these elements in the melt from the amazonite granites (554 ppm Li, 23 ppm Ta, and 116 ppm Nb). This discrepancy could be related to quartz crystallization after the fractionation of Li-F micas, albite, topaz, columbite-tantalite, and microlite, which resulted in that the quartz trapped inclusions of already depleted melt. On the other hand, the dramatic depletion of the residual melt in Na, Al, Li, F, Ta, and Nb suggests the possible separation of a specific hydrosaline aluminofluoride melt, which accumulated Nb, Ta, and REE in some experimental systems. This suggestion was supported by the results of the investigation by melt and fluid inclusions in beryl from the pegmatoid bodies of Orlovka, which established the coexistence of two immiscible aluminosilicate melts and a CO2-rich supercritical aqueous fluid (Thomas et al., 2009). One of these melts closely corresponds to the volatile-rich hydrosaline and relatively subalkaline melt with high Li (2.09 wt %) and F contents (2.93 wt %). The Raman spectra of the coexisting fluid indicated the presence of columbite, which implies a Ta content in the fluid of approximately 6500 ppm.

AB - By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12-1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B2O3 contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO2 (by up to 6 wt %), Na2O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H2O. Two stages were distinguished on the basis of the behavior of trace elements and fluorine in the melts and rocks. The early stage, from the biotite granites of the parental massif to the microcline-albite granites with the pea-shaped quartz is characterized by a decrease in Si, Fe, Ca, Mg, Zr, Hf, and REE and accumulation of Al, Na, Li, Rb, F, Ta, and Nb, which correspond to the ongonite differentiation trend. During the second stage, amazonite-bearing rocks with Li micas and Ta mineralization (columbite-tantalite and microlite) were produced, and the melt was depleted in Al, Na, Li, F, Nb, and Ta. A considerable difference appeared between the compositions of rocks and aluminosilicate melts. There is a paradoxical discrepancy between the high contents of Li, Ta, and Nb in the rock (2289, 446, and 269 ppm, respectively) and the low contents of these elements in the melt from the amazonite granites (554 ppm Li, 23 ppm Ta, and 116 ppm Nb). This discrepancy could be related to quartz crystallization after the fractionation of Li-F micas, albite, topaz, columbite-tantalite, and microlite, which resulted in that the quartz trapped inclusions of already depleted melt. On the other hand, the dramatic depletion of the residual melt in Na, Al, Li, F, Ta, and Nb suggests the possible separation of a specific hydrosaline aluminofluoride melt, which accumulated Nb, Ta, and REE in some experimental systems. This suggestion was supported by the results of the investigation by melt and fluid inclusions in beryl from the pegmatoid bodies of Orlovka, which established the coexistence of two immiscible aluminosilicate melts and a CO2-rich supercritical aqueous fluid (Thomas et al., 2009). One of these melts closely corresponds to the volatile-rich hydrosaline and relatively subalkaline melt with high Li (2.09 wt %) and F contents (2.93 wt %). The Raman spectra of the coexisting fluid indicated the presence of columbite, which implies a Ta content in the fluid of approximately 6500 ppm.

UR - http://www.scopus.com/inward/record.url?scp=77952021967&partnerID=8YFLogxK

U2 - 10.1134/S0869591110020037

DO - 10.1134/S0869591110020037

M3 - Article

AN - SCOPUS:77952021967

VL - 18

SP - 131

EP - 157

JO - Petrology

JF - Petrology

SN - 0869-5911

IS - 2

ER -