Polyphase white mica growth in low-grade metapelites from La Cebila Metamorphic Complex (Famatinian Belt, Argentina): evidence from microstructural and XRD investigations.

Abstract

Two tectono-thermal metamorphic events, M1-D1 (S1, with associated white mica and chlorite: WM1-Chl1) and M2-D2 (S2, with development of WM2-Chl2), are established from polyphase white mica growth for low-grade units from the Ordovician metasedimentary successions of La Cebila Metamorphic Complex in the Famatinian belt (western-central Argentina). The thermobarometric characterization of the M1 main event was carried out by means of clay-mineral analysis and crystallo-chemical parameter measurements. Epizonal (temperatures between 300 and 400ºC) and low-pressure conditions are suggested for M1 event, based in Kübler index values ranging from 0.23 to 0.17 Δº2θ, white mica b parameter values between 9.004 and 9.022 Å (mean of 9.014 Å, n=16) and Si contents between 3.13-3.29 a.p.f.u. Temperatures of ~180-270ºC are estimated for the M2 event, with Kübler index values ranging from 0.31 to 0.46 Δº2θ. The M1-D1 event of La Cebila could be linked to highstrain heating tectono-metamorphic Ordovician regime recorded in others complexes from Famatinian foreland region of Sierras Pampeanas.