Simultaneous measurements of chlorite and illite crystallinity: a more reliable tool for monitoring low- to very low grade metamorphisms in metapelites. A case study from the Southern Alps (NE Italy)
Arkai, Peter; Sassi, Francesco P.; Sassi, Raffaele
European Journal of Mineralogy Volume 7 Number 5 (1995), p. 1115 - 1128
published: Oct 5, 1995
manuscript accepted: Apr 13, 1995
manuscript received: Jan 10, 1995
ArtNo. ESP147050705007, Price: 29.00 €
Abstract Illite and chlorite crystallinity indices (XRD peak widths, respectively IC and ChC for short) were determined and their variation compared in the Palaeozoic, pelitic-silty complex of the South-Alpine basement of the Eastern Alps. A suite of 238 samples derived from eight localities between S. Stefano di Cadore and Pontebba (Northern Italy) was investigated, each locality represented by a sample group consisting of 28 to 32 rock specimens. Metamorphic grade in this region (related to the high-thermal-gradient Variscan metamorphism) ranges from the diagenetic zone in the east through the anchizone up to the epizone in the west. Significant positive linear correlations (r = 0.62-0.85) were found between IC and ChC values, the latter having been measured on the 14 Å and 7 Å diffraction peaks of chlorite [ChC (001) and ChC (002) respectively]. The ChC boundaries of the anchizone were therefore established by linear correlation between IC and ChC data. These ChC boundaries are very close to those obtained by Arkai (1991) from a smaller shale-slate-phyllite sequence (n = 82) in the Biikkium, innermost Western Carpathians, N-Hungary. Within the 8 sample groups, perfect agreement between the IC- and ChC-based classification of the specific low grade zones was found in 5 groups. The perfect agreement extends to cover all eight sample populations where only the ChC(001) data at low goniometer speed are considered, depending on the better correlation between ChC (001) and IC (002). The applicability of ChC for monitoring metamorphic grade in the very low to low T range was also checked by significance tests of the differences between the crystallinity average values of the sample groups. Out of the investigated 28 sample group-pairs, these differences prove to be significant at the probability level of P = 1 % in 25 cases for IC, 21 cases for ChC(001) and 22 cases for ChC(002). Consequently, the ChC method is less sensitive for identifying differences in metamorphic grade than the IC method. This fact can be explained by the smaller Δ°2θ range of the ChC scale compared to the IC scale. Nevertheless, the comparison of the average ChC values among different sample groups with samples regularly spaced in a prograde sequence ranging from the diagenetic zone through the anchizone up to the epizone, proves to be a reliable indicator of the increase in metamorphic grade. Furthermore, chlorite crystallinity can be considered as a useful complementary tool, especially where the interpretation of illite crystallinity is hindered by disturbing factors (e.g. the presence of discrete or mixed-layer paragonitic phases, margarite, etc.).