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Geochemistry and origin of chloritoid schist from the Alpi Apuane, Italy: evidence of a prevailing lateritic signature

¹ Dipartimento di Scienze della Terra, via Trentino 51, Università di Cagliari, I–09127 Cagliari, Italy
² Istituto di Geoscienze e Georisorse C.N.R., via Moruzzi 1, I–56124 Pisa, Italy
³ Dipartimento di Scienze della Terra, Via Valperga Caluso 37, Università di Torino, I–10125 Torino, Italy

^* Corresponding author, e-mail: francmar{at}unica.it

Dedicated to thememory of Luciano Ungaretti

Chloritoid schist layers frequently occur as the uppermost part of the Rhaetian Brecce di Seravezza Formation, interpreted as rockfall talus metabreccias intercalated between Norian and Liassic marbles, in the Alpi Apuane Tuscany, Italy. The chloritoid schist is characterised by mm-sized chloritoid porphyroblasts (20–50 % modal range) and a matrix made up of chlorite, muscovite, pyrophyllite, chloritoid, hematite, calcite, quartz and epidote. Most chloritoid porphyroblasts show cores with abundant hematite inclusions. Many pieces of duricrust, pisoid and composite pebbles come from older lateritic bauxite deposits of supposed Kasimovian to Sakmarian age. Their uneven to well rounded shapes indicate variable duration of transport and multistage reworking.

In comparison with many bauxite and metabauxites, Apuan chloritoid schist shows: great enrichment in SiO₂, K₂O, Na₂O, a slight increase in CaO, MgO, remarkable depletion in Al₂O₃ and Fe₂O₃, a slight decrease in TiO₂ and P₂O₅, strong depletion in Cu, Pb, Zn, Bi, and As, unexpected Ni, Co and Ba enrichments; slight to strong depletion for Ga, Cr, Nb, Zr, and V reflecting variable dilution of primary bauxite detritus. The REE spider diagram shows enrichment factors of 2–3 for LREE, of 1–2 for HREE. Strong enrichment to strong depletion in Sm, Eu, Gd, and Tb is correlated with maximum and minimum contents of duricrust fragments respectively.

Four detrital components were distinguished. Two of them represent about 90–95 % of the whole protolith: 1. Fe₂O_3tot-Al₂O₃-SiO₂-rich lateritic bauxite detritus; 2. illite-rich desert soil particles with high K₂O, SiO₂, Al₂O₃ contents. The calculated relative proportions are: component 1): 61 %, component 2): 39 %. Minor detrital components are given by: 1) Ba-rich mineralised rock fragments, a witness to Late Triassic hydrothermal circuits; 2) detritus supplied by Na, Ca, Mg-rich salts abundant in hot desert soils. The very low TiO₂ and very high SiO₂ contents of the calculated lateritic bauxite component strongly suggest a granite to rhyolite protolith for the primary bauxite deposit, the ancestor of the Apuan lateritic bauxite component.

Key-words: chloritoid schist, geochemistry, lateritic signature, reworking, Rhaetian, Alpi Apuane, Northern Apennines.