IDENTIFYING SUBDUCTION COMPONENTS IN THE TRANS MEXICAN VOLCANIC BELT: A COMPLEX MAGMATIC ARC IN A COMPLEX SUBDUCTION ZONE REGIME

The Trans-Mexican Volcanic Belt (TMVB) consists of more than 8000 volcanic structures, including more than 20 stratocones, and extends in W-E direction from the Pacific to the Gulf of Mexico. Magmatism is related to the subduction of the Cocos and Rivera plates. Due to oblique subduction, the emplacement of the magmatic arc is not trench-parallel but, instead, oblique. Down-going dip and velocity of the subducted oceanic plates are changing considerably from NW to SE. The slab trajectory is heterogeneous from steep to nearly sub-horizontal and achieves a depth of ca. 100 km beneath the central TMVB, corresponding to pressures of ca. 35 kbar. Calc-alkaline rocks are predominant but minor OIB-type and alkaline volcanism also occurs. Direct evidence for the composition of the subducted oceanic crust is not very abundant. N-MORB and high-TiMORB derived material with different isotopic ratios is involved together with heterogeneous deep sea turbidites and sedimentary ooze. A chemical contribution from these sediments in andesitic and basaltic TMVB samples can be deduced from elevated Ba/Nb (40-250) or Ba/La ratios. Ba is usually concentrated in oceanic sediments containing hydrothermal minerals and clays. One of the most sensitive elements to detect contributions from subducted sediments is Pb. TMVB volcanics show Pb concentrations between 4 and 30 ppm. Sediments from Pacific DSDP sites have Pb concentrations around 100 ppm, whereas for the MORB source Pb contents of only 0.05 ppm are reported. The often cited negative Nb anomaly is not always a convincing parameter to detect the influence of the subducted plate in arc rocks. Amphibole, an important Nb-bearing mineral, can break down with increasing depth of the slab, releasing Nb. However, sub-horizontal slab trajectories like those observed underneath the TMVB can gradually decelerate amphibole breakdown and arc rocks then display only weak or even absent Nb anomalies. The presence of minor alkaline and OIB-type rocks in the overall TMVB can be explained by significant upper mantle heterogeneities. It has been shown that these heterogeneities can occur within a 10 km scale. Another model favors removal of mantle lithosphere and lower crustal material beneath high-K TMVB magmatic sites combined with the release of fluids and mantle upwelling around the sinking mass.