LAWSONITE-BEARING ASSEMBLAGES IN CONTINENTAL CRUST: WHICH TECTONIC MECHANISM MAY PRODUCE SUCH A COLD CONTINENTAL SUBDUCTION?

Structural and metamorphic settings of the lithosphere along active margins can be reorganized by subduction to generate a crust-mantle mixing that may be preserved in the axial part of orogenic belts after continental collision. This mega-mélange of nappes and tectono-metamorphic units (TMUs), proper of suture zones, is allowed by tectonic erosion and ablative subduction, which prior to collision may drag continental crustal materials to great depths, where HP-LT or UHP-LT metamorphic transformations are accomplished. Where the tectonic system implies ocean subduction underneath a continental plate, the wedge dynamics drives the exhumation of buried continental units, successively involved in continental collision (e.g.: Cloos, 1982; 1984; 1985; Platt, 1986; Polino et al., 1990; Ernst, 2001; Gerya & Stoeckhert, 2006; Roda et al., 2010). In the Western Alps, HP-LT metamorphism widely affects not only units derived from the Tethyan ocean, but also large volumes of pre-Alpine continental crust, as is the case of the Sesia-Lanzo Zone (SLZ). This portion of Adriatic crust is the largest continental slice with the early-Alpine eclogite-facies imprint and is also one of the rare examples of continental lawsonite-assemblages. Structural and metamorphic evolution of Lws-rocks of SLZ nappe is investigated to unravel coupling and decoupling mechanisms active during continental crust subduction in such a depressed thermal regime by defining number, size and shape of tectono-metamorphic units (TMUs). Three Lws-occurrences are studied: two are located in the southern part of the SLZ (Mt Soglio and Mt Croass), in different formerly defined complexes (the Eclogitic Micaschists C. (EMC) and the Rocca Canavese Thrust Sheet (RCTS)); the third is central to SLZ (Ivozio) and belongs to the EMC. The Ivozio complex consists of metabasics where, during the last stages of the prograde tectono-metamorphic evolution, Lws overgrew the prograde foliation (S1) marked by SPO of Amp, Zo, Grt and Omp. It was in turn, replaced by Ky and Zo and successively by Pg-aggregates. The first replacement occurred at the Lws HP break-down, at the end of the prograde path; the second developed at lower P, during the retrograde path, accomplished at slightly higher T. The inferred P-T-d-t path shows that the prograde Lws assemblage developed at P ≥ 1.8 GPa and T ≤ 600°C (Zucali et alii, 2004; Zucali & Spalla, subm.). In Mt Soglio EMC eclogitic boudins preserve a pervasive foliation marked by eclogite facies minerals SPO (Omp, Ep, Rt, Qz ± Grt). A blueschist facies foliation (S2) wraps the boudins and is marked by SPO of Gl, Czo, Qz, ± Grt, Ttn, ± Wm, ± Lws in metabasics and by SPO of Wm, Gl, ± Cld, Qz, Grt, Ttn ± Lws in micaschists. Lws is replaced by a fine-grained aggregate of Ep, ± Ab, Chl, ± Pg. The Lws-assemblages developed during the exhumation path at P ≤ 1.6 GPa and T ≤ 450°C (Pognante 1989; Spalla & Zulbati, 2003). At Mt Croass a complex of metasediments, characterized by a pre-Alpine dominant metamorphic imprint under granulite facies conditions, comprises silicate marbles that are characterized by the occurrence of Lws + Pmp marking the earlier Alpine fabric. The early Alpine foliation is marked by Cld, Gl, ± Ky and Wm in the metasediments, indicating blueschist facies peak conditions. In metabasic layers the Alpine foliation is marked by colorless Amp, Czo, Wm and Ttn. This TMU is bounded by ten meter-thick discontinuous mylonitic horizons, in which the mylonitic foliation is marked by colorless Amp, Chl, Ep, Qz and Ttn. These mylonites separate Mt Croass TMU from the surrounding gneisses and micaschists of the SLZ that recorded the earlier Alpine eclogitic imprint. The parageneses associated with this structural evolution are similar to those described in the RCTS (Pognante 1989; Spalla and Zulbati, 2003), where the metamorphic climax occurred at P ≥ 1.0 GPa and T = 300-400°C and followed by a P-decrease at constant T (Pognante 1989); therefore the same PT evolution is proposed for Mt Croass.

In summary Lws may be prograde or retrograde, or mark peak-conditions in SLZ, indicating that contrasted tectonic trajectories can characterize not only different volumes of a single Alpine nappe (SLZ) but also portions of the same metamorphic complex (EMC). These results indicate that the multiscale structural analysis supported by petrologic investigations is the tool to individuate corresponding TMUs within different formerly defined metamorphic complexes (Spalla et alii, 2010).

The inferred PT burial and exhumation trajectories of these different TMUs in the SLZ, even if contrasted, are all accomplished under a depressed thermal regime. To verify if such high P/T ratios, recorded in the continental lithosphere, result from tectonic erosion, ablative subduction and exhumation supported by the recycling in the mantle wedge, the oceanic subduction beneath a continent has been simulated by a 2D numerical model, considering hydration in the mantle wedge. Comparison between model predictions and natural data show that the simulated geodynamic scenario generates a thermal regime coherent with that affecting the SLZ, which may have been long time stable during Alpine subduction, allowing rocks to accomplish their burial and exhumation path under an active subduction regime.