Tectonic and magmatic evolution of the Andes of northern Argentina and Chile

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.

The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources

Two calc-alkaline plutonic complexes. Bazman and Natanz, intruded through the south-eastern active and south-western ancient continental margins of Central Iran, Have been dated by the Rb-Sr whole-rock isochron method at 74 ± 2 and 24 ± 4.5 Ma, respectively. Detailed trace element studies together with low 87 Sr/ 86 Sr initial ratios ( 87 Sr/ 86 Sr ratios and the gabbros, at least, were intruded some 10 Ma earlier.

Late Cretaceous and early Miocene Andean-type plutonic activity in northern Makran and Central Iran

A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge

The address is a wide-ranging commentary on the physical aspects of granitic magma and deals especially with the mode of emplacement. Recommendations on the nomenclature of plutons and batholiths involve both a description of the actual shape and their modeling by experimental and salt-diapir analogues. The nature of granitic crystal-mushes, wet or dry, is discussed in the light of the special properties of pseudoplastics, particularly the influence of the physical state on the sinking of crystals and xenoliths, on magma mobility in zoned plutons, and on the final rock structure. The longevity of plutonic events and their episodic character are commented on in relation to the subduction process. The generation of granitic magmas, their upwelling and the structural control of their intrusion, whether forceful or permissive, are discussed. The structural form of batholiths in depth is described in terms of a batholith denudation series particularly in the circum-Pacific context. Finally, two main orogenic environments of granites are contrasted in terms of the Hercynotype and Andinotype, and composition and structure are interrelated in a proposed general model for the emplacement of batholiths.

Simon J. Haynes

Papers

Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26° and 29° south

K-Ar evidence for the post-paleozoic migration of granitic intrusion foci in the Andes of northern Chile

A rapid method for the determination of chlorine in silicate rocks using ion-selective electrodes