Analysis of timing of deformation, porphyroblast growth and metamorphic conditions in the Lukmanier Pass area: implications for metamorphism in the Central Alps
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The Lukmanier Pass area is situated in the centre of the Penninic domain in the central Alps. This domain consists of crystalline basement and Mesozoic sedimentary units. These sediments have gone through complementary geodynamic processes, although there are regional heterogeneities observed. The complexity of the area can be visualized in a PT(t) path for the rock units in the area. The peak metamorphic pressure conditions are related to subduction of the margins of micro-continent Adria beneath the distal margins of the European crust. This is followed by continued exhumation of the Alpine orogen due to continental collision and nappe stacking. The blueschist-eclogite pressure dominated areas related to subduction (with phengite, glaucophane, carpholite and lawsonite) were later truncated by a Barrovian-type setting (with almandine, staurolite, kyanite and muscovite) related to orogenic build-up. In a final stage this setting was overprinted by a late stage heating event characterized by Buchan-type metamorphism (kyanite, ilmenite, muscovite and oligoclase porphyroblasts). The aim of this study is to analyse the mineralogical composition of P58b metapelitic samples in a Mesozoic sedimentary succession and link them to observed deformation phases in the central Alps. A PT(t) path was modelled based on EMP analyses and optical microscopy. The role of fluids in the rocks is also briefly mentioned, as fluids are an important medium in many geological processes. Since the metapelitic rocks contain many fluid-rich minerals, a water saturated system is assumed. Our samples P58b were collected during a 4-day fieldtrip in the Lukmanier Pass area and are studied in the lab at the Earth Sciences institute at Utrecht University. They were analysed using optical microscopy for microstructural relationships, X-ray fluorescence (XRF) for bulk rock composition and EMP to acquire compositional information. The data obtained was processed to create P-T pseudosections to indicate mineral stability fields over a range of 0 - 20 kbar and 500 – 1000 K. This was done using thermodynamic software like Perple_X (Connolly, 2005). These results are compared by using the TheriakDomino software developed by De Capitani (1994 a.o.). Thereafter, a practical approach is used to calculate stable mineral assemblages. Geothermobarometry was applied to several mineral clusters to obtain PT relationships. The metapelitic chemical bulk rock composition is relatively rich in Na, Fe and Al. The main porphyroblasts observed are almandine, ilmenite, kyanite, staurolite, muscovite and oligoclase. Inside the matrix, minerals like paragonite, biotite and chlorite indicated the enrichment in sodium and iron. Minor phases as magnetite, rutile and tourmaline are present in small volume-percentages. The garnet porphyroblasts showed a slightly chemical zonation. Microstructural relationships between the minerals and deformation phases showed a progressive development towards a higher metamorphic grade. The rocks underwent multiple deformation phases. According to the microstructural relationships, a late stage heating event would have influenced the region. The rocks show Barrovian-type metamorphism under greenschist facies conditions, indicated by a stable mineral assemblage of almandine, ilmenite, muscovite, chlorite and kyanite. Towards peak metamorphic pressures at 10-12 kbar, amphibolite facies minerals like almandine, staurolite, kyanite and muscovite form a stable mineral assemblage. Isothermal retrograde metamorphism was subsequently followed by a Buchan-type late stage heating event which heated the rocks ≈ 100 oC up to 650-680 oC. This heating caused porphyroblasts of oligoclase, ilmenite, kyanite and muscovite to grow which do not show any signs of deformation. In terms of geodynamics, this late stage heating event is most likely caused by a combination of geodynamical processes. Slab break-off at depth seems a viable mechanism to account for the relatively rapid rise in temperature, combined with accretion of large crustal masses with radiogenic heat production, like the Lepontine dome. However, regional influences increase the complexity of the area and more quantitative research is recommended to explain the evolution in the central Alps at Lukmanier Pass.