Alpine subduction zone metamorphism in the Paleozoic successions of the Monti Romani (Northern Apennines, Italy) (original) (raw)
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Italian Journal of Geosciences, 2018
In the Sesia-Lanzo Zone (SLZ), the subunit Rocca Canavese Thrust Sheet (RCT) is characterised by a mixture of mantle- and crust-derived lithologies and can be a good candidate to be a former subduction-related mélange of the Austroalpine domain. The unit consists of metapelites, metagranitoids, metabasics, metagabbro and serpentinised lherzolite lenses from metre to hundred-metre size. According to the literature, PT peak conditions for all lithologies are 0.8-1 GPa at 300-400° C, in lawsonite-blueschist facies conditions recorded during the Alpine subduction. However recent work describes different mineral assemblages for the metamorphic peak, separating rocks with lawsonite from those with jadeite. Therefore, we refined the meso and microstructural analysis of the tectonic slices of RCT and we performed a detailed thermo-barometry of different metamorphic stages in order to quantify Alpine peak conditions and P-T-d-t paths, and to test whether the RCT represents a subduction-related mélange. We focus on metagabbros, Jd-bearing and Lws-bearing glaucophanites since they have the most suitable chemistry allowing to reconstruct the complex evolution of the mixing. Metagabbros and Jd-bearing glaucophanites experienced a D1a metamorphic stage characterised by a pressure of 1.3-1.8 GPa and temperature of 450-550° C, in eclogite facies condition. Lws- bearing glaucophanites experienced a D1b metamorphic stage at a temperature <470° and pressure of ca. 1.2-1.5 GPa, in Lws-blueschist facies condition. The two tectono- metamorphic units (TMUs) were coupled together during the exhumation at D2 stage, under Ep- blueschist facies conditions. Successive evolution occurs at lower pressure, under greenschist facies conditions. D1a peak conditions are compatible with a thermal gradient between a cold and a warm subduction zone while D1b peak is recorded in a thermal gradient compatible with a cold subduction. The coupling between the two TMUs occurred under a cold thermal gradient, suggesting a still active subduction. The different origin and P-T-d-t paths of the blocks, the intense shearing experienced by all lithologies during their coupling and the abundance of serpentinites in the tectonic mixture agree with the interpretation of a subduction-related mélange for RCT, in analogy with other zones of the Alps.
Journal of Metamorphic Geology, 1992
In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low-grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium-grade zone, a later low-pressure, high-temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure-retrograde paths in each sector: (1) in the Val Vedello basement ( W B ) rocks, a first metamorphic imprint characterized by P = 7-9 kbar and T = 570-610" C was followed by a greenschist retrogression ( P < 4 kbar and T <500°C); ( 2 ) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P = 7-9 kbar and T = 550-630" C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.
The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Syn-orogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13–10 Ma rocks were already at shallow crustal levels (b 9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at ~ 6–4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6–4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional " out-of-sequence " or " re-activation " event occurring in the internal sector of Tuscany already affected by a regional extensional regime.
Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps
Lithos, 2018
In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing am- phibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and mi- nor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P=0.7- 1.1 GPa, T=580-660◦C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P=0.6-1.0 GPa, T>780◦C). The two tectono- metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P=0.4-0.6 GPa, T=620-750◦C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distin- guished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.
P–T evolution of Alpine metamorphism in the southern Aspromonte Massif (Calabria – Italy)
Swiss Journal of Geosciences
The crystalline basement of the southern Calabrian-Peloritani nappe pile edifice (Aspromonte Massif and Peloritani Mountains) consists of several tectonic slices, which were stacked during the Miocene Apennine thrust sheet em-placement (Fig. 1). The two lowermost tectonic slices of this nappe-edifice, here called respectively Aspromonte-Peloritani Unit and Samo-Africo Complex, tectonically overlap along a thick Late Oligocene mylonitic shear zone, which involves both remnants of the Hercynian chain and Alpine metamorphic rocks (Figs. 1–2). A structural and petrological study was employed to reconstruct both the entire P–T evolution of the two units and the kinematics of orogenic transport, which characterise the Late Oligocene mylonitic stage. P–T estimates were made using an integrated approach derived from conventional thermobarometry, computation of metamorphic equilibria in the NaCaKFMASH system, and an analysis of the deformation behaviour of quartz, feldspar and garnet. The sh...
Geological Society, …, 2008
Major discoveries in metamorphic petrology, as well as other geological disciplines, have been made in the Alps. The regional distribution of Late Cretaceous-Tertiary metamorphic conditions, documented in post-Hercynian metasediments across the entire Alpine belt from Corsica-Tuscany in the west to Vienna in the east, is presented in this paper. In view of the uneven distribution of information, we concentrate on type and grade of metamorphism; and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously, or where the maximum temperature was reached at lower pressures, after a significant temperature increase on the decompression path.