Mafic forearc cumulates and associated rocks in the central high-pressure belt of the Acatlan Complex of southern Mexico: geochemical constraints

摘要

The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO_2 (38-52 wt.%) and high Mg# (～48-70), Ni (100-470 ppm), and Cr (180-1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus min erals. Their Nb/Y and Zr/Tio_2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ε_(Nd(t)) = 5.3-8.5 and ~(147)Sm/~(144)Nd = 0.18-0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian-early Carboniferous arc developed along the west margin of Gondwana.