Abstract

A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith.

The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario.

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