LATE CRETACEOUS UNDERPLATING OF AN OCEANICPLATEAU AND THE GENESIS
OF PALEOCENE PORPHYRY COPPER DEPOSITS ALONG THE SOUTH PERUVIAN MARGIN
PIERRE SOLER
Institut Francais de Recherche Scientifique pour le Development en Cooperation - Orstom, 213 rue
Lafayette, 75010 Paris, France.
Along the Andean margin of South America, Jurassic to Pliocene porphyry copper deposits are distributed along a
series of belts parallel to the Andean range and the Peru-Chile trench. Each belt is restricted to a segment of
the Andes and, in most cases, comprises Cu-mineralizations of a specific age range. One of the most important of
these belts is the southernmost Peruvian belt, which includes the Cerro Verde, Cuajone, Quelleveco and Toquepala
deposits. Along this 200 km-long belt, Cu-mineralizations are associated with a series of calc-alkaline plutons
emplaced over a very short time span (60 to 57 Ma), which are the youngest plutons of the southern "Toquepala
segment" of the Peruvian coastal batholith.
Previously, during the late Cretaceous, the southernmost Peruvian margin showed a series of overthrusting faults
involving the Precambrian basement, a gap in magmatic activity between 84 and 70 Ma, syntectonic filling of foreland
continental basins which were specific of this area when compared with northern and southern parts of the Andes
at that time. These features have been interpreted as consequences of the subduction and underplating of an oceanic
plateau (the Mollendo Ridge), which is supposed to have occurred in this area between 85 and 70 Ma, with a nearly
S-N direction of convergence between Farallon and South American plates. The Mollendo Ridge was an eastern equivalent
of the present-day Manihiki plateau of the western Pacific Ocean.
Underplating of the Mollendo Ridge resulted in a local thickening of the South American continental crust, the
base of which was then mainly made of MORB basalts and gabbros of the oceanic plateau, and in the creation of some
heterogeneity within the subcontinental lithospheric mantle. The Paleocene calc-alkaline magmas, which are demonstrated
to be the result of the partial melting of the asthenospheric mantle wedge above the newly subducted slab, suffered
a Cu-enrichment by interaction with the Mollendo Ridge MORB component during their ascent through the heterogeneous
lithospheric mantle (e.g. by zone refining) and their evolution in deep magma chambers at the base of the continental
crust (e.g. by assimilation).
Thus, some participation of MORB in the genesis of the Paleocene porphyry copper deposits of southernmost Peru
is evidenced, but it does not correspond to the partial melting of the descending slab, as it has long been suggested.
The existence of a late Cretaceous local heterogeneity of the continental lithosphere explains satisfactorily the
distribution of the Paleocene porphyry copper deposits of southernmost Peru in space and time. The applicability
of such a model to other Andean belts of Cu-mineralizations is discussed.
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