Timescales and Mechanisms of Batholith Construction, Coastal Cordillera, Northern Chile, From Precise U-Pb Zircon Ages and Regional Geochronological Data

HR: 10:35h
AN: V52B-02 [Abstracts]
TI: Timescales and Mechanisms of Batholith Construction, Coastal Cordillera, Northern Chile, From Precise U-Pb Zircon Ages and Regional Geochronological Data
AU: * Cruden, A R
EM: cruden@utm.utoronto.ca
AF: University of Toronto, Dept. of Geology, 22 Russell St., Toronto, ON M5S 3B1 Canada
AU: Carlos, A
EM: carevalo@sernageomin.cl
AF: Servicio Nacional de Geolog｛ y Miner｛, Avda. Santa Mar｛ 0104, Providencia, Santiago, Correo 21 Chile
AU: Davis, D D
EM: dond@geology.utoronto.ca
AF: University of Toronto, Dept. of Geology, 22 Russell St., Toronto, ON M5S 3B1 Canada
AU: Grocott, J
EM: j.grocott@kingston.ac.uk
AF: Kingston University, School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston-upon-Thames, KT1 2EE United Kingdom
AB: Analysis of $>$200 K-Ar and Ar-Ar ages on intrusive rocks combined with new single zircon U-Pb analyses on individual composition layers within three plutonic complexes of the Coastal Cordillera batholith of N Chile define distinctive patterns that provide insight into the upper crustal growth of a Mesozoic batholith in space and time. Triassic-Jurassic plutons, which dominate the western part of the arc display a diffuse age distribution pattern with no temporal zonation, indicating a period of widespread extension and magmatism. Cretaceous plutons young systematically west to east and define an average magmatic migration rate of about 500 m/Myr. In detail 20-30 km wide Cretaceous plutonic complexes remain fixed in space for ca. 3 Myr, then the locus of magmatism jumps 2-10 km east during a 1-3 Myr time gap. The 2-3 Myr emplacement times for individual plutons is confirmed by precise U-Pb zircon ages on subhorizontal, 100-500 m thick layers that make up the intrusions. The Carrizal Bajo pluton grew in ca. 2 Myr by initial injection of a diorite sheet dated at 207.7+/-1.3 and 207.5+/-0.5 Ma that was split by a 206.2+/-0.3 Ma granodiorite sheet and fed by a sheeted dyke complex with units dated at 208.5+/-0.5 Ma and 208.2+/-0.2. The Infiernillo pluton yielded ages from top to bottom of 131.4+/-0.2, 128.7+/-0.2, and 129.0+/-0.2 Ma and layers in La Borracha pluton were dated at 116.3+/-0.2, 114.5+/-0.2, and 113.0+/-0.1 Ma. Both plutons grew from the top down by floor depression over ca. 3 Myr. The two ages from the lower unit of Infiernillo indicate incremental filling from bottom to top during progressive, asymmetric fault-assisted floor subsidence over $<$600 Kyr. Both the vertical growth of plutonic complexes and their west-east localization within the batholith are attributed to reactivation of strands of arc-parallel faults during the Cretaceous. As magmatism migrated eastward across the arc, melt transport and pluton growth was controlled and stabilized for ca. 3 Myr intervals by the nearest crustal-scale fault. Further migration eventually led to the capture of the magma transport system by the adjacent fault strand, and the observed across-arc jumps in intrusive ages. West-to-east migration of magmatism and concurrent regional transtension are attributed to the interaction between westward slab roll-back and the dynamics of upper plate extension. The spatial-temporal distribution of upper crustal plutons in the batholith was controlled by the migration rate and direction of lower crustal melting and the horizontal spacing of crustal-scale faults.
DE: 8434 Magma migration
DE: 9360 South America
DE: 8035 Pluton emplacement
DE: 8105 Continental margins and sedimentary basins
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting