L'arc intraocéanique allochtone jurassique supérieur-crétacé inférieur du domaine cordillerain mexicain ('Guerrero Terrane') : pétrographie, géochimie et minéralisations associées des segments de Guanajuato et Palmar Chico-Arcelia ; conséquences paléogéographiques pour le Mésozoïque au Mexique

摘要

The Late Jurassic-Early Cretaceous allocthonous arc sequences of the Mexican Cordillera belong to the Guerrero terrane. They accreted to the North American craton during the late Early Cretaceous. The Guanajuato, Palmar Chico-Arcelia and San Miguel de Allende igneous arc sequences represent the different stages of the arc growth. The Guanajuato plutono-volcanic sequence (central Mexico), forms the base of the arc and represents the nascent stage of this arc. It consists of a complete tholeiitic suite, composed of plutonic ultrmatic-mafic cumulates grading upwards into diorites. and K-poor siliceous rocks, and mafIc dykes feeding a thick pillow basaltic pile. The ultramafic cumulates are intruded by ankaramitic dykes and Apuan-Albian ca!c-.alkaline mafic plutons associated with magmatic brecclas. The ankaramatic dykes suggest rifting in the arc while the Aptian-Albian bodies represent likely the plutonic roots of the widespread calc-alkaline volcanism developed at the end of the arc growth. The Guanajuato sequence is thrusted over the Lower Cretaceous Arperos FormatIon. The latter Iis formed of intra-oceanic alkaline (OIB) pillow basalts and dolerites assoclated wIth pelaglc sediments. The Arperos Formation represents the remnants of an oceanlc basin, fringing the western margin of the North American craton. The Tlthonian to pre-Cenomanian Palmar Chico-Arcelia sequence (southern Mexico) is composed of arc tholelltIc orthopyroxene-phyric pillow basalts and their feeding dykes. Locally, the uppermost lavas show calc-alkaline affinities. Small bodies of serpentinized ultramatic cumulates assoclated wlth diorites are thrusted on the lavas and associated sediments. These rocks likely represent the plutonic counterparts ot the tholeiitic arc lavas and dykes. Thus, the Palmar Chico-Arcelia sequence, showing both tholeiitic and calc-alkaline affinities, represents an intermediate. stage in the arc development. The Alblan San Miguel de Allende volcano-sedimentary sequence is formed of calcalkaline basalts and andesltes erupted in a pelagtic environment and emitted at the end of the arc growth. . The Guanajuato and Arcelia matic tholeuiitic and calc-alkaline rocks show high eNd values common in oceanic magmatic arcs. The Guanajuato siliceous rocks show slighly lower eNd va!ues which may be explained by an assimilation-fractional-crystallization process (AFC). The siliceous melts differentiated by crustal fractionation from the tholeiitic mantle magma and assimilated material such as metamorphosed oceanic crust and sediments which formed the wall rocks of the magma chamber. The diversity of the magmas during the arc growth may be explained. by various processes such as different degrees of partial melting of the mantle, magma mixing between mafic and evolved melts, the precipitation order of the Fe-Ti oxides in the sequence of cristallisation. The presence of stratiform polymetallic ore deposits associated with siliceous calc-alkaline domes could he linked to the rifting event affecting the arc during its development. Thus, the Mesozolc arc sequences exposed in central-southern Mexico represent the remnants of an intra-oceanic arc which development was likely linked to the subduction of the Arperos basin under the Pacific plate. This subduction was dipping west-south-west. Late Early Cretaceous, at the end of its development, the arc was problably located near the margin of cratonal America, collided with this margin and was accreted to North America.