The Eruptive History Of The Mascota Volcanic Field, Western Mexico: Age And Volume Constraints On The Origin Of Andesite Among A Diverse Suite Of Lamprophyric And calc-alkaline Lavas

摘要

The Mascota volcanic field is located in the Jalisco Block of western Mexico, where the Rivera Plate subducts beneath the North American Plate. It spans an area of ～ 2000 km~2 and contains ～ 87 small cones and lava flows of minette, absarokite, basic hornblende lamprophyre, basaltic andesite, and andesite. There are no contemporary dacite or rhyolite lavas. New ~(40)Ar/~(39)Ar ages are presented for 35 samples, which are combined with nine dates from the literature to document the eruptive history of this volcanic field. The oldest lavas (2.4 to 0.5 Ma) are found in the southern part of the field area, whereas the youngest lavas (predominantly <0.5 Ma) are found in the northern portion. On the basis of these ages, field mapping, and the use of ortho aerial photographs and digital elevation models, it is estimated that a combined volume of 6.8 ±3.1 km~3 erupted in the last 2.4 Myr, which leads to an average eruption rate of ～0.003 km~3/kyr, and an average volume per eruptive unit of <0.1 km~3. The dominant lava type is andesite (2.1 ±0.9 km3), followed by absarokite (1.6+0.8 km~3), basaltic andesite (1.2±0.5 km~3), basic hornblende lamprophyre (1.0+0.4 km~3), and minette (0.9±0.5 km~3). Thus, the medium-K andesite and basaltic andesite comprise approximately half (49%) of the erupted magma, with twice as much andesite as basaltic andesite, and they occur in close spatial and temporal association with the highly potassic, lamprophyric lavas. There is no time progression to the type of magma erupted. A wide variety of evidence indicate that the high-MgO (8-9 wt.% ) basaltic andesites (52-53% wt.% SiO_2) were formed by H_2O flux melting of the asthenopheric arc mantle wedge, whereas the mafic minettes and absarokites were formed by partial melting (induced by thermal erosion) of depleted lithospheric mantle containing phlogopite-bearing veins. There is only limited differentiation of the potassic magmas, with none more evolved than 55.4 wt.% SiO_2 and 4.4 wt.% MgO. This may be attributable to rapid crystallization of the mantle-derived melts in the deep crust, owing to their low volumes. Thus, the andesites (58-63 wt.% SiO_2) are notable for being both the most voluminous and the most evolved of all lava types in the Mascota volcanic field, which is not consistent with their extraction from extensively crystallized (60-70%), low-volume intrusions. Instead, the evidence supports the origin of the andesites by partial melting of amphibolitized, mafic lower crust, driven by the emplacement of the minettes, absarokites, and the high-Mg basaltic andesites.