IDENTIFICATION OF HIGH-PRESSURE GRANULITES IN ULTRA-HIGH-PRESSURE TERRANES THROUGH FIELD OBSERVATIONS AND MINERAL TEXTURES: DULAN, CHINA

The Dulan ultra-high-pressure (UHP) rocks of the North Qaidam UHP terrane in Western China represent continental crust that has been subducted 80-100 kilometers (above 2.5 GPa). Recently, high-pressure (HP) granulites have been found within the Dulan UHP rocks, posing a question of how rocks of such relatively low pressure became mixed with UHP rocks.

The HP-granulites are limited to a small area in the western edge of the Dulan UHP exposure. In the same area as these granulites is a small outcrop of ky - bt gneiss. This gneiss was found in contact with the granulites and HP melts of the area, suggesting it is the host rock for the granulites, though limited exposure made this hard to confirm.

Petrographic analysis of granulite samples shows a primary mineral assemblage of grt + cpx + pl, representing HP granulite facies, along with zo(czo) +qtz + rt ± ky. Inclusions in grt and cpx include pl, ky, zo, qtz, and rt, supporting HP-granulite conditions during formation. The host gneiss has a mineral assemblage of qtz + pl + kfs + bt + grt + ky + rt. The combination kfs + ky can define the P-T conditions for the area to >800 ^oC, past the Ms + Qtz = Ky + Kfs + H₂O reaction, and >10 kbar, above the Sill = Ky transition. The UHP host rock contains qtz + pl + ms ± bt ± kfs ± grt, and very rarely ky or st, which indicates lower temperatures compared to the granulite host-gneiss.

High-pressure and high-temperature conditions are further supported by the presence of kyanite and/or zoisite melt veins. These veins show no reaction rims with the surrounding rock, indicating melting occurred at peak conditions. This melting is in contrast to younger granitic melts, which show amphibole reaction boundaries with surrounding rock. These HP melt veins contain high amounts of ky and/or zo(czo) along with qtz + pl + grt ± cpx.

All granulites are coarse grained and textures represent peak metamorphic conditions. There are no textural indications of former UHP conditions such as symplectite textures after omphacite. Most samples have an amphibolite overprint consisting of hornblende halos around garnet, hornblende and plagioclase symplectites after cpx, and ilmenite halos around rutile. This amphibolite overprint is similar to that seen in many UHP samples from Dulan, and could indicate the HP-granulite and UHP units were juxtaposed during or prior to this amphibolite overprint.