UHT metamorphism of HP rocks? A case study from the Adula nappe complex (Central Alps, N Italy)

The Adula-Cima Lunga nappe complex represents the highest of the Lower Penninic units of the Central Alps. Garnet lherzolite bodies crop out at three localities, from west to east: Cima di Gagnone, Alpe Arami and Mt. Duria. This study concerns an outcrop close to Monte Duria, where retrogressed garnet lherzolites occur as m-to hm-sized boudins hosted within amphibole-bearing gneisses that contain also some dm-to-m-sized boudins of more or less preserved eclogites. This rock association is in turn embedded within the migmatitic gneisses that form most of the southern sector of the Adula nappe. Petrographic and chemical analyses indicate that garnet peridotite is composed of olivine (XMg=0.88), orthopyroxene, clinopyroxene and garnet (Py 68; Cr2O3 up to 1.45 wt%) with inclusions of Cr-rich spinel (up to Cr/(Al+Cr)=0.55) surrounded by kelyphitic symplectites of opx + cpx/amph + spl. These reaction produced double coronas, one composed of opx (former ol) and one composed of cpx + opx+ spl. In one kelyphite, we observed the uncommon occurrence of ZrO2 (baddeleyite) and ZrTi2O6 (srilankite). Tiny crystals of these two Zr- bearing phases (~1 μm) are invariably located in the opx corona after ol. The cpx + opx + spl corona (after grt) contains, instead, zircon. Baddeleyite should have formed through a reaction of the type Mg2SiO4 + ZrSiO4 = MgSiO3 + ZrO2. ZrO2 and ZrTi2O6 display a low amount of solid solution. These compositions are consistent with T below 1200°C, but an improvement of the thermodynamic model is needed in order to better constrain the T of the granulitic overprint on the basis of these Zr-bearing phases. In eclogites, the HP association consists of garnet (Py40Alm37Sp20), omphacite (preserved as inclusion, containing Jd30 and XMg 0.87), kyanite, K-feldspar, zoisite and minor quartz. Omphacite is almost always replaced by cpx (Jd5) + plag (An55) symplectites. Garnet is surrounded by plag (An33) + opx (En70) symplectites, more often by amphibole+plagioclase. Kyanite is replaced by plag (An84) + spinel ± sapphirine ± corundum. The spinel-sapphirine Fe-Mg thermometer suggests T of about 850°C for this granulite-facies overprint. Corundum-bearing symplectites after kyanite are common at the contact between eclogites and the host amphibole-bearing gneiss. In this contact, cm-sized emerald green zoisite has been found, replaced by anorthitic plagioclase ± clinopyroxene ± spinel ± calcite. The observed assemblages point to a diffuse static granulitization of both peridotites and eclogite of Mt. Duria, suggesting a nearly isothermal decompression from peak-pressure conditions. The surrounding migmatitic gneiss do not display evidence of such granulitic event, recording T<700°C. The mechanism and timing of emplacement of the garnet peridotite and associated HP-HT rocks in the country migmatites, and whether or not the subduction event is related to the Alpine or to an older orogenic cycle are still a matter of debate.