Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana Formation (Cordillera Oriental, NW Argentina). Clay Minerals vol. 38: 459-481.

MARGARITA DIANA DO CAMPO; FERNANDO NIETO,

CLAY MINERALS (PRINT)

The Mineralogical Society

Lugar: Londres; Año: 2003 vol. 38 p. 459 - 481

0009-8558

The Puncoviscana Formation, largely cropping out in NW, Argentina is mainly composed of a pelite-greywacke turbidite sequence affected by incipient regional metamorphism and polyphase deformation. Metapelites, mainly composed of quartz, albite, dioctahedral mica and chlorite, were sampled in the Lules-Puncoviscana and Choromoro belts. Lattice-fringe images, selected area electron diffraction and AEM analyses, coupled with previous data of white mica crystallinitiy index (IC), indicate a state of reaction progress for Puncoviscana slates consistent with medium anchizone to epizone grade metamorphism. The 2M polytype prevails in dioctahedral micas, coexisting in a few cases with the 1Md polytype as a consequence of lack of equilibrium. The 2M polytype coexists with 3T in two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3T and long-range stacking sequences present b values characteristic of intermediate-high pressure metamorphism and ordered chlorites (1L, 2L, 3L and 7L) prevail. Based on the Si contents of dioctahedral micas and considering peak temperatures of around 350-400 ºC, pressures from 5 kbar and 5 - 7 kbar were derived for metapelites from the Lules-Puncoviscana and Choromoro belts, respectively. These values agree with facies series derived from the b values. Micas with a wide range of phengitic substitution, as evidenced by Fe + Mg and Si contents, coexist. These variations could not arise from the disturbing effect of detrital white k-mica because TEM evidences indicate that they are absent or represent less than 10% of mica population. Thus, compositional variations suggest that dioctahedral micas of individual slates crystallised at different pressure conditions in response to the P-T path of the metamorphism. Moreover, in several biotite-free slates the IC values lead to an underestimation of the metamorphic grade attained in these rocks. The coexistence of IC predominately corresponding to anchizone and the occurrence of biotite in some slates and felsic metavolcanic rocks intercalated in the Puncoviscana metasediments are interpreted as the result of a metamorphic path including a relatively high-pressure/low-temperature (HP/LT) event, followed by a lower-pressure overprint possibly at higher temperatures than the HP/LT event. Small micas formed during the high-pressure stage would prevail in the < 2 µm fraction, producing anchizone IC.