Isotopic Studies of Northern Andean Crustal Evolution and Ore Metal Sources
Ore lead isotope provinces in the central Andes between 6° S and 32°S correspond in part to broad differences in the ages and types of rocks exposed in each area. If these provinces reflect scavenging oflead from upper crustal rocks and reconcentration into ore deposits, ore lead isotope ratios reflect the average upper crustal composition in each region. If the ore metals have a deeper source, the provinces instead reflect differences in magma sources or generation processes among the provinces. Ores from province II (the high Andes of Perú) show steep lead isotope arrays indicative of source mixing. The igneous rocks in the Hualgayoc district in northern Perú overlap with the radiogenic end of the province I array and are representative of the nonradiogenic end of the province II mixing trends; exposed supracrustal rocks are candidates for the radiogenic end member. The origins of both isotopic signatures are investigated to examine the relationship between hydrothermal metal budgets and magma sources.
The exposed crust in the northern Peruvian Andes consists of middle Cretaceous platform carbonates, sandstones, and shales that overlie a similar Jurassic sequence and probably a Precambrian to early Paleozoic metamorphic basement. The metamorphic basement and overlying sediments have broadly similar Pb-Sr-Nd isotope systematics. Whole-rock Pb isotope and U/P systematics of the sediments suggest U/Pb fractionation in the sediment source at approximately 1800 Ma, followed by evolution with elevated U/Pb ratios. ∊Nd values of the metamorphic basement and Cretaceous sedimentary rocks range from −11.6 to −16.5, with TDM equal to 1.43 to 2.06 Ga. Northern Perú basement rocks have much higher 206Pb/204Pb values than metamorphic basement terranes in eastern Colombia, southern Perú, and northern Chile, and their isotopes more closely resemble basement terranes to the east in Brazil.
The sedimentary rocks were intruded in the middle to late Miocene by numerous felsic igneous bodies associated with hydrothermal Ag-Zn-Cu-Pb mineralization. The intrusive rocks are intermediate to high K andesitic intrusions and rhyodacitic volcanic domes. Fresh igneous rocks have rare earth element (REE) abundances less than 100 times chondrites, lack significant Ce and Eu anomalies, and are relatively depleted in Ti and Nb. The isotopic compositions and homogeneity of the igneous rocks with respect to Pb, Sr, and Nd suggest that they assimilated little shallow crust and were derived largely from deeper sources in the upper sub-Andean mantle or the lower sub-Andean crust. Because no exposed Andean basement rocks resemble the compositions of province I ores, and because subducted sediment has recently been shown to be an important source of lead in arc magmas, the role of subducted sediment in producing a province I-like signature is evaluated. A simple numerical model for the enrichment of a possible mantlewedge source region by subducted sediments is presented. The model suggests that subducted sediment can account for the lead isotope signature of province I ores, and that the quantity of subducted material along the Perú-Chile trench could produce a mantle source with this signature within a few million years of the onset of subduction.
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Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner