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

摘要: Chloritoid schist layers frequently occur as the uppermost part of Rhaetian Brecce di Seravezza Formation, interpreted rockfall talus metabreccias intercalated between Norian and Liassic marbles, in Alpi Apuane Tuscany, Italy. The chloritoid is characterised by mm-sized porphyroblasts (20–50 % modal range) a matrix made up chlorite, muscovite, pyrophyllite, chloritoid, hematite, calcite, quartz epidote. Most show cores with abundant hematite inclusions. Many pieces duricrust, pisoid composite pebbles come from older lateritic bauxite deposits supposed Kasimovian to Sakmarian age. Their uneven well rounded shapes indicate variable duration transport multistage reworking. In comparison many metabauxites, Apuan shows: great enrichment SiO 2 , K O, Na slight increase CaO, MgO, remarkable depletion Al O 3 Fe decrease TiO P 5 strong Cu, Pb, Zn, Bi, As, unexpected Ni, Co Ba enrichments; for Ga, Cr, Nb, Zr, V reflecting dilution primary detritus. REE spider diagram shows factors 2–3 LREE, 1–2 HREE. Strong Sm, Eu, Gd, Tb correlated maximum minimum contents duricrust fragments respectively. Four detrital components were distinguished. Two them represent about 90–95 whole protolith: 1. 3tot -Al -SiO -rich detritus; 2. illite-rich desert soil particles high contents. calculated relative proportions are: component 1): 61 %, 2): 39 %. Minor are given by: 1) Ba-rich mineralised rock fragments, witness Late Triassic hydrothermal circuits; 2) detritus supplied Na, Ca, Mg-rich salts hot soils. very low strongly suggest granite rhyolite protolith deposit, ancestor component.