Abstract

Major and trace element, U-Pb zircon geochronology and Pb-isotopic data are presented on plutons from the San Nicolás batholith of central and southern Peru.

The major oxides of 9 samples indicate a high-K calc-alkaline trend, and trace element discriminant diagrams reveal affinities to volcanic arc or late- to post-collision calc-alkaline intrusions. The apparent contradiction implied by the Rb-Hf-Ta triangular diagram (within-plate) strongly suggests loss of Rb, an interpretation supported by the extensive deuteric alteration and sericitization of feldspar. Ta/Zr, Ta/Hf and Ta/Nb ratios are considerably higher than those of volcanic arc intrusions, and favour the late or post-collision calc-alkaline classification.

U-Pb zircon geochronology shows that the San Nicolás batholith was emplaced in two short-lived magmatic events at approximately 425 Ma and 394-388 Ma ago. The presence of an inherited component of radiogenic Pb of Precambrian age in the zircons suggests xenocrystic cores, which were most likely scavenged from Precambrian basement rocks of the Arequipa massif.

Time-adjusted Pb isotopic compositions of feldspars have the following ranges: 206Pb/204Pb = 17.798-18.272, 207Pb/204Pb = 15.571-15.626, and 208Pb/204Pb = 38.182-38.291, indicating that Precambrian basement was incorporated into the magmas of the batholith. Other components are believed to have originated in an ‘enriched’ upper mantle or lower crustal wedge pervasively metasomatized by fluids extracted largely from subducted sediments.

U-Pb zircon age distribution, trace element characteristics and the linear nature of the plutons firmly establish the San Nicolás batholith as the plutonic substructure of an early Palaeozoic continental arc, which may extend northwards in Peru and southwards

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