A synthesis and comparison of the geochemistry of volcanic rocks of the Sierra Madre Occidental and Mexican Volcanic Belt

Abstract

The Cenozoic-Recent volcanic rocks of mainland Mexico are largely confined to three provinces: the Sierra Madre Occidental (SMO), the Trans-Mexican Volcanic Belt (MVB), and the Eastern Alkalic province. The first two provinces are dominated by calc-alkalic rock series, but minor amounts of contemporaneous alkaline rocks are also present. The third province is comprised largely of alkaline volcanics. Comparison of the two calc-alkalic provinces reveals that each has distinct characteristics. The mid-Tertiary SMO volcanics are comprised largely of rhyolitic ignimbrites whereas the MVB is characterized by prominent neovolcanic centers that are primarily andesitic in composition. Calc-alkalic andesites from the western MVB (e.g., Ceboruco) have chemical characteristics that are distinct from andesites of the central MVB (e.g., Colima, Paricutfn, Orizaba, Jorullo, Popocatepetl, Toluca). The andesites of Ceboruco have higher Na20+ K20, K20, Na20, Ti02, P20s, Ba, La, and (Ce/Yb)N and lower CaO, MgO, Cr, and Ba/La than andesites of the central MVB. Calc-alkalic andesites from the northern SMO (Batopilas area) and from the central MVB have similar total alkalies, K20, Na20, CaO, Sr, Zr and (Ce/Yb)N. The SMO andesites also have lower MgO and K/Rb and higher FeO*, Ti02, Rb, Th, La, andY than calc-alkalic rocks of the central MVB. Initial Sr isotopic ratios of compositionallyintermediate MVB volcanics are restricted to the range 0.7035-0.7047 whereas the intermediate-felsic volcanics of the SMO have Sri ranging between 0. 7033 and 0. 7614. Such ratios indicate significant interaction of parental magmas with old radiogenic crust for the SMO volcanics, but relatively minor, if any, interaction with magmas parental to the MVB volcanics. Limited Nd isotopic data for MVB calc-alkalic volcanics (Colima, Los Humeros, Paricutfn, and La Primavera) generally have ENd greater than Bulk Earth (up to +5 to +6). Volcanics of the northern SMO (Batopilas area) and southeastern Chihuahua have ENd within ±2.5 units of Bulk Earth. Evolution of the rock suites at individual eruptive centers appears to have been controlled largely by cristal fractionation. Other processes operative at the same time may include magma mixing and crustal assimilation (the latter especially in the SMO). The nature of the source regions for parental magmas of volcanics in both the MVB and SMO-SEC areas has yet to be addressed in a comprehensive manner.