Other
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.108...61l&link_type=abstract
Earth and Planetary Science Letters, Volume 108, Issue 1-3, p. 61-77.
Other
11
Scientific paper
The Late Jurassic-Early Cretaceous Alisitos-Teloloapan intra-oceanic arc, which extends along the Pacific coast of Mexico, is overlain by the extensive thick calc-alkaline Tertiary volcanics of the Sierra Madre Occidental. In central-southern Mexico this arc can be divided in to two major units. Its base, which is mainly exposed in the Guanajuato-Leon area, consists of a volcano-plutonic assemblage refered to as the Guanajuato magmatic sequence. The top consists of pillowed calc-alkaline basalts and andesites overlain and/or interbedded with a thick volcaniclastic sedimentary pile in which lenses of reworked Albian-Aptian reefal limestones are interlayered. The Guanajuato magmatic sequence represents the crustal section of an intra-oceanic nascent island arc and from top to base is made up of: (1) volcaniclastic sedimentary cover, (2) a predominantly pillowed basaltic sequence, (3) a doleritic and basaltic dyke complex, (4) silicic plutonic rocks and salite diorite, (5) Mg-hornblende diorite, (6) layered cumulate gabbros and, (7) ultramafic-mafic cumulates. Plutonic rocks of intermediate to silicic composition are volumetrically abundant. All the rocks have suffered low-grade metamorphism with magmatic textures preserved. Metamorphic textures do not occur. Mg-rich olivine is restricted to the ultramafic cumulates, whereas Ca-rich clinopyroxene is present in all the rock types, although with the exception of the most differentiated silica rich plutonic rocks. The textural relationships and mineral chemistry of the clinopyroxenes and hornblende in the ultramafic cumulates and in the clinopyroxene gabbros point to an origin by igneous accumulation from a tholeiitic parent magma at shallow levels above the mantle-crust boundary.
Al-rich basalts and dolerites belong to a low-K tholeiitic suite. The Nb contents, which are abnormaly high for arc tholeiites, are in the range of N-MORB. Such enrichments have been related to an immature stage of the arc development, the mantle wedge being still enriched. This is in agreement with the evidence that the Guanajuato tholeiitic magma represents the earliest products of the Alisitos-Teloloapan arc magmatic activity. The Guanajuato basic rocks and hornblende diorite show similar high ɛNd ratios which fall in the range of the values of oceanic arcs. The most siliceous plutonic rocks differ from the basic suite by virtue of their slightly lower ɛNd and higher ɛSr ratios, which may be explained by an assimilation-fractional crystallization (AFC) process. The siliceous melts differentiated by crystal fractionation from the tholeiitic mantle magma and assimilated material such as metamorphosed oceanic crust and sediments, which form the wall rocks of the magma chamber. The high ɛSr ratios of the basalts and their feeder dykes are probably related to the polyphase low-grade metamorphism (i.e. hydrothermal oceanic alteration and regional metamorphism) that has affected the uppermost levels of the arc.
Abouchami Wafa
Coulon Christian
Lapierre Henriette
Monod Olivier
Ortiz Luis Enrique
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