Formation of metamorphic and metamorphosed garnets in the low-T/UHP metagranite during continental collision in the Dabie orogen

摘要

A combined study of major and trace elements in garnet was carried out for low-T/UHP metagranite in the Dabie orogen. The results show different fashions of element zoning in the garnet, suggesting occurrence of both metamorphic and metamorphosed garnets. Three generations of garnet growth are distinguished on the basis of the assumption that Ca contents, Mn contents and Fe/Mg ratios of garnet in metagranites are a function of pressure and temperature. The first generation of garnet (Grt-I) occurs in the core of a skeletal garnet, showing homogeneously low X_(Grs) values and Fe/Mg ratios but high X_(Sps) values. This implies that the core garnet grew at the highest temperature but the lowest pressure, representing a metamorphosed residue of magmatic garnet in protolith granite. The second generation of garnet (Grt-II) occurs in the mantles and cores of many garnet grains, exhibiting increased X_(Grs) values but decreased Fe/Mg ratios. This is ascribed to a continuous increase in temperature and pressure till the peak pressure, corresponding to metamorphic growth (or overgrowth) during the prograde subduction. The third generation of garnet (Grt-III) occurs in the rims of all garnet grains, displaying decreased X_(Grs) values and Fe/Mg ratios in response to a pressure decrease but a temperature increase till the peak temperature. These rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation. Grt-I shows steep MREE-HREE patterns and profoundly negative Eu anoalies, consistent with growth from granitic melt. This kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements (such as REE, Rb, Ba, Sr, Pb, Th, U, Nb and Ta) despite the low-T/UHP metamorphism. Grt-II and Grt-III in one sample exhibit steep MREE-HREE patterns, with a continuous decrease in REE contents. This suggests their growth from the almost same matrix of mineral assemblages (plagioclase + K-feldspar + muscovite) during metamorphism. However, Grt-II in the other sample displays flat to steep MREE-HREE patterns, with an increase of REE contents from core to mantle. This implies that the matrix of mineral assemblages for Grt-II changes from common rock-forming minerals (e.g., feldspar, muscovite and biotite) to REE-rich minerals (e.g., epidote,allanite, zircon, amphibole, apatite and titanite). Grt-III in the two samples all displays significantly lowered REE contents compared to those of Grt-I and Grt-II. This may be due to a decrease in pressure and an increase in temperature during the initial exhumation. Therefore, the two-stage growth of metamorphic garnet is evident in the low-T/UHP metagranite during the continental subduction-zone metamorphism. In addition, the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims. This suggests that the peak pressure (P_(max)) did not occur contemporarily at the peak temperature (T_(max)). corresponding to the "hot" exhumation.