Original paper

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

Franceschelli, Marcello; Puxeddu, Mariano; Gattiglio, Marco

Abstract

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. Fe2O3tot-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.

Keywords

chloritoid schistgeochemistrylateritic signaturereworkingrhaetianalpi apuanenorthern apennines