Origin of mixed-layered (R1) muscovite-chlorite in an anchizonal slate from Puncoviscana Formation (Salta Province, Argentina)

Mica-chlorite mixed-layering was identified by X-ray diffraction (XRD) as a major or subordinate constituent in several slates of the Puncoviscana Formation from Sierra de Mojotoro (Eastern Cordillera, NW Argentina). In order to determine the crystallochemical characteristics of these mixed-layered sequences and interpret their petrological meaning, anchizonal slate P90 was chosen for TEM observations. In this slate, dioctahedral mica and chlorite form interleaved phyllosilicate grains (IPG) or stacks, up to 110 mu m long, preferentially oriented with (001) planes at a high angle to the slaty cleavage but also oblique to S (sub 0) . In agreement with XRD results, the main phyllosilicates identified by transmission electron microscopy (TEM) were dioctahedral mica and random mixed-layer muscovite-chlorite, with chlorite in subordinate amounts and scarce smectite. In the lattice-fringe images of mixed-layer packets, a sequence of irregular stacking that produced apparent 24 Aa (10 + 14) layers was observed, but it was frequently possible to distinguish the 10 Aa layers from adjacent 14 Aa layers. In nearly all packets, 14 Aa layers prevail, exhibiting 14 Aa :10 Aa ratios between 1:1 and 3:1. Some elongated lenticular fissures which are probably a consequence of layer collapse caused by the TEM vacuum were identified in these packets. The straight, continuous appearance of lattice fringes plus the scarce evidence of collapsed layers identified suggest that these packets correspond principally to mixed-layer muscovite-chlorite, which is confirmed by analytical electron microscopy analyses. However, smectite-like layers are probably the third component of some of these mixed-layer sequences, which may account for their high Si and low (Fe + Mg) contents, their low interlayer charge in relation to theoretical interlayer muscovite-chlorite, and for the presence of Ca in the interlayer site. Textural relationships between chlorite and muscovite packets in IPG along with the observed transformations from 14 Aa to 10 Aa along the layer, is compatible with a prograde metamorphic replacement of chlorite in stacks by dioctahedral mica layers, probably in the presence of an aqueous fluid.