Mineralogical heterogeneity of UHP garnet peridotite in the Moldanubian Zone of the Bohemian Massif (Nove Dvory, Czech Republic)

摘要

Previous reports on Nove Dvory garnet peridotites indicated that they lacked primary spinel, and prograde meta-morphism evidence was uncommon. This study revealed the presence of Al- and Cr-rich spinels, clinopyroxenes with a lower content of Na and Fe than what has been previously reported, and chemical heterogeneity in garnet in Nove Dvory garnet peridotites. Cr-poor (0.06-0.12 a.p.f.u.) and Cr-rich (0.10-0.27 a.p.f.u.) garnet populations were identified, and they had contrasting coronas around garnet. The garnet peridotite samples were classified into three types on the basis of the chemical composition of garnet and constituent minerals: type A that includes Cr-rich spinel and Cr-rich garnet; type B that includes Cr-poor garnet and no spinel; and type C that includes both Al- and Cr-rich spinel and both Cr-rich and Cr-poor garnet. The finding of spinel relics {Cr# = [100 Cr/ (Cr + Al)] ～ 60-70} in garnet from type A peridotite suggests that the Nove Dvory peridotite body may have been located at relatively shallow depths prior to the ultrahigh-pressure metamorphic stage of ＞4 Gpa. Suitable geo-thermobarometers were used to establish the P-T history of the rocks. The core compositions of clinopyroxene inclusions in garnet and host garnet yielded 978-1002 °C, 4.87-5.12 Gpa (type B) and 1034 °C, 4.93 Gpa (type C); stage Ⅰ. Garnet porphyroblasts in type A peridotite lack clinopyroxene inclusions. The core compositions of garnet and pyroxenes yielded 1005-1072 °C, 4.42-4.46 Gpa (type A); stage Ⅱ. The innermost garnet rims and cores of matrix pyroxenes surrounding garnet yielded 1222-1325 °C, 5.03-5.67 Gpa (type B) and 1189-1267 °C, 5.59-6.97 Gpa (type C); stage Ⅱ. Thus, Nove Dvory peridotite has experienced an increase in the P-Tconditions, and its chemical heterogeneity (especially in the Cr content of garnet) in type C peridotites may have been created by the mechanical mixing of different rock types (i.e., Cr-rich and Cr-poor types) during the compression and/or decompression stage(s).