Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

摘要

Garnet peridotites from the southern Su-Lu ultra-high-pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low-P-T mineral assemblage (chlorite, hornblende, Na-gedrite, Na-phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO2-rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X-Mg values (Mg/(Mg+ Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca-Mg-Fe. The Mg-rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 degrees C, greater than or equal to 5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P-T (c. 4.2 GPa, c. 760 degrees C). Sm-Nd data give apparent isochron ages of c. 380 Ma and negative epsilon(Nd)(0) values (c.-9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb-Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of (. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low-P-T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su-Lu terrane have been subjected to peak metamorphism at P-T conditions much higher than presently estimated (greater than or equal to 2.7 GPa, 1800 degrees C). [References: 45]