Abstract: The Hardy Formation represents a latest Jurassic-Early Cretaceous volcanic arc that was located along the Pacific margin of southern South America. It was separated from the continent by a marginal basin floored by portions of an ophiolite sequence (the Rocas Verdes ophiolites). The transition between the arc and marginal basin occurs on Peninsula Hardy, southernmost Chile, where there is a lateral facies transition from arc deposits of the Hardy Formation into proximal marginal basin fill of the Yahgan Formation. Interfingering of arc and marginal basin sequences demonstrates that subduction-related arc magmatism was concurrent with marginal basin formation. The lateral facies transition is reflected in the geochemistry of volcanic rocks from the Hardy and Yahgan formations. Basalts, andesites and dacites of the arc sequence follow a calc-alkaline differentiation trend whereas basalts from the marginal basin follow a tholeiitic differentiation trend. Estimates of temperature and oxygen fugacity for crystallization of the arc andesites are similar to values reported for other calc-alkaline andesites. It is suggested that water activity influenced the early or late crystallization of Ti-magnetite and this controlled the style of differentiation of the magmas erupted on Peninsula Hardy. Magmas with high water contents evolved along the calc-alkaline differentiation trend whereas those with low water contents evolved along the tholeiitic differentiation trend. Some rhyolites are differentiated from the calc-alkaline andesites and dacites, but most appear to be the products of crustal anatexis on the basis of trace-element evidence.The arc basalts and some marginal basin basalts show relative enrichment in LILE, relative depletion in HFSE, and enrichment in LREE. Other marginal basin basalts are LREE depleted and show small relative depletions in HFSE. Basalts with both calc-alkaline and tholeiitic affinities can also be recognized in the Rocas Verdes ophiolites, where their eruption is correlated with temporal changes in the evolution of the marginal basin. Geochemical data are consistent with derivation of all basalts from a mantle source that is a mixture of MORB-like mantle and variable proportions of a subduction-related component. Basalts with calc-alkaline affinities were erupted initially in the marginal basin, when the locus of back-arc magmatism was near the active arc and the upper mantle source region was strongly influenced by components released from the subducted slab. MORB-like tholeiitic basalts were erupted at a later stage in the wider parts of the basin, when the angle of subduction had increased, the trench had rolled back, and the basin had widened. These MORB-like tholeiitic magmas reflect partial melting of mantle dominated by upwelling asthenosphere containing only a minor component, if any, derived from the subducted slab.

EXPLOR_STEP=$WICRI_ROOT/Wicri/Terre/explor/NissirosV1/Data/Main/Exploration

HfdSelect -h $EXPLOR_STEP/biblio.hfd -nk 000354 | SxmlIndent | more

HfdSelect -h $EXPLOR_AREA/Data/Main/Exploration/biblio.hfd -nk 000354 | SxmlIndent | more

{{Explor lien
   |wiki=    Wicri/Terre
   |area=    NissirosV1
   |flux=    Main
   |étape=   Exploration
   |type=    RBID
   |clé=     ISTEX:8CCA967513DBF99C3D314791CD165BCED9513AB3
   |texte=   An Early Cretaceous volcanic arc/marginal basin transition zone, Peninsula hardy, southernmost Chile
}}