Timing of diagenesis and very low-grade metamorphism in the eastern sector of the Sierra de Cameros (Iberian Range, Spain): a U–Pb SHRIMP study on monazite

The Sierra de Cameros is an intracontinental orogen and represents the north-western part of the Iberian Range in Northern Spain. It comprises a thick sequence of syn-rift continental sediments (mainly sandstones and carbonates) deposited during lower Cretaceous times. A unique characteristic of the Sierra de Cameros in relation to the rest of the Iberian Range is the presence of low-grade metamorphism in certain parts of the basin, an event that predates basin inversion. This paper describes the presence, textural relationships and geochemical aspects of authigenic and ⁄ or metamorphic monazite within different lithologies from the deepest parts of the basin. Sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of monazite records the presence of two age populations: the first with 206Pb ⁄ 238U ages ranging from 122 to 116 Ma which is considered as diagenetic in origin, whilst the second is dated at 99 ± 2 Ma and postdates the metamorphic climax

Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt

The Gran Paradiso massif, situated in the internal part of the Western Italian Alps, records a complex tectonometamorphic history involving high-pressure metamorphism and subsequent exhumation during retrograde metamorphism. The exact timing of deformation and, consequently, the geodynamic evolution of this part of the Western Alps is still debated and is addressed here by the application of Rb/Sr geochronology, 40Ar/39Ar step heating and 40Ar/39Ar total fusion dating techniques. Geochronological results are presented from shear zone samples in the core of the Gran Paradiso massif (Piantonetto Valley), and in the area closer to the contact with the overlying Piedmont ophiolitic domain (south and southwest of Pont Valsavarenche). The shear zones operated during crustal thinning and exhumation of the Gran Paradiso massif. 40Ar/39Ar step heating results from shear zones in the Piantonetto Valley show acceptable plateau ages that are interpreted to represent two events of mica growth. Similar ages, and an additional younger age cluster, are recognised in the 40Ar/39Ar total fusion analyses, indicating that specific cleavage domains operated at 39.2±0.2, 36.5±0.6 and 33.3±0.4 Ma. P–T pseudosections show a progressive decrease in metamorphic conditions during deformation, suggesting that the age of incipient exhumation and the related deformation in the Piantonetto Valley is equal to or older than 39.2±0.2 Ma. In the Pont area, the last increments of deformation in a top-to-W shear zone postdate 36.6±0.6 Ma (Rb/Sr mineral data), whereas the present-day top-to-Wcontact of the Gran Paradiso massif with the overlying Piedmont domain is dated at 41.2±1.1 Ma (Rb/Sr multi-mineral isochron age). We propose a model that considers exhumation of the Gran Paradiso nappe at 41–34 Ma. During this period, the nappe was coupled with the Zermatt-Saas zone, forming an extruding wedge. The kinematics associated with this wedge involved top-to-W shearing within the Gran Paradiso nappe (e.g. Pont area shear zones) and top-to-E shearing at the top of the extruding wedge (e.g. Orco shear zone). Subsequent deformation (after ~34 Ma) was characterised by coaxial strain involving orogenic-scale backfolding and backthrusting

The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: A petrological and geochronological study of silvery micaschists

We present a petrological and geochronological study of "silvery micaschists" that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P-T conditions and age of the Alpine high-pressure metamorphism. The studied "silvery micaschists", which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U-Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite + talc + chloritoid + allanite peak assemblage was stable at P = 1.9-2.7 GPa and T = 515-600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7 ± 1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4 ± 0.9 Ma, implying a minimum duration of ∼ 2-4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼ 1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges