Crustal Evolution along the Early Ordovician Proto-Andean Margin of Gondwana: Trace Element and Isotope Evidence from the Complejo Igneo Pocitos(Northwest Argentina)

摘要

The Paleozoic geodynamic evolution of the proto-Andean margin of Gondwana was characterized by extensive sub-duction processes associated with the docking of several terranes.The major and trace element as well as Sr-Nd-Pb isotope composition of plutonic rocks from the Complejo Igneo Pocitos(CIP)are used here to constrain the processes of magma genesis in the Early Ordovician continental arc along the proto-Andean margin of Gondwana in northern Chile and northwestern Argentina.U-Pb sphene ages of 476 (+-) 1 Ma for the CIP date the time of intrusion.The CIP comprises two magmatic suites,each with a distinct and homogeneous isotope composition that was acquired by magmatic processes in the deep crust or mantle.Both suites are strongly enriched in incompatible trace elements and display a Sr-Nd isotope composition intermediate between mid-ocean ridge basalt(MORE)and Andean continental crust.The petrological,geochemical,and isotopic characteristics are best modeled by generation of the CIP melts in a subduction setting.Multicomponent modeling shows that the trace element and isotope inventory of group 1 samples can be generated by admixture of~0.5%melts from subducted sediments and l%-2%of MORB-derived fluids to the mantle source.Constant initial Sr and Nd isotope compositions at varying SiO_2 contents for both suites exclude significant modification of the melts by assimilation and fractional crystallization processes in upper-crustal levels.Assimilation of group 1 parental magmas in the deep crustal MASH(melting,assimilation,storage,and homogenization)zone with 20%-25%crustal material,however,produced the group 2 parental magmas.The CIP can be considered to be the northern continuation of the Early Ordovician Famatinian arc known from central Argentina.As indicated by the Nd-Pb isotope composition of the CIP,this arc was built on continental crust of Middle Proterozoic age.Similar protolith ages have been reported for the Famatinian arc in the southern Andes and for the Late Precambrian-Early Paleozoic metamorphic basement of the central Andes.The identification of MASH processes requires a thick continental crust in order to enable melting and assimilation at the base of the crust.Thickening of the crust can be related to the Pampean orogeny at~500 Ma.Magmatic rocks from the Famatinian arc in the southern Andes display a more crustlike Sr-Nd isotope signature compared with the CIP,suggesting that crustal thicknesses in the Early Ordovician continental arc decreased from south to north.