MAGMATIC RECYCLING DURING CONSTRUCTION OF A BATHOLITH

Once melt is generated, recycling processes (contamination by or assimilation of older parts of the system) can result in dramatic effects on evolving magma petrology, estimates of magma addition rates and crustal growth, and thermal and emplacement histories. To establish types and magnitudes of recycling processes in the Peruvian Coastal Batholith (PCB) near Ica we used evidence from 100 days of field work, 130 thin sections, 117 whole rock geochemical, and 46 isotope analyses. The Ica area PCB consists of Linga, Pampahuasi, and Tiabaya complexes. Precambrian gneisses are the basement rocks and volcanic and sedimentary rocks of the Jurassic Yura and Cretaceous Quilmana Groups form the cover. Mafic plutons both precede and are contemporaneous with the PCB.

Field observations reveal widespread recycling of older volcanic and plutonic units into younger plutons at km-scale cauldron subsidence, widespread meter-scale stoping, cm-scale disaggregation of larger blocks, and magma mingling resulting in enclaves. Field observations indicate that 15-20% of pre-PCB envelop is recycled into the Ica PCB and 10-15% of early PCB units may be recycled into later PCB units. Petrographic examination reveals some chemical contamination and plagioclase and zircon crystals record evidence of recycling processes such as corroded or resorbed rims, complexly zoned crystals, and inherited cores.

Whole rock geochemistry provides recycling estimates. Qualitatively, the Tiabaya shows more evidence for recycling than the other units: alumina plots suggest more envelop sediments and Gd/Yb and Sr/Y plots indicate a deeper source and thicker crustal traverse for assimilation. Quantitatively, Pb, Sr, and δ¹⁸O isotopes indicate that perhaps 20% comes from lower crust Precambrian gneisses. AFC calculations indicate 20-30% assimilation from envelop volcanics. The MgO Harker diagram geochemistry of the Linga plutons requires magma mixing in addition to partial melting and fractional crystallization. In summary, the Ica PCB composition appears to be 50% mantle, 20% recycled Precambrian lower crust, and 20-30% recycled upper crust volcanics and older plutonic units.