Article
Petrogenesis of the Fiambalá Gabbroic Intrusion, Northwestern Argentina, a Deep Crustal Syntectonic Pluton in a Continental Magmatic Arc
Journal of Petrology
(1994)
Abstract
The Fiambaláa gabbroic intrusion is an Ordovician pluton, ∼30 km2 in outcrop area, situated in the Sierras Pampeanas of NW Argentina. It was emplaced at a depth of 21–24 km in an actively deforming continental magmatic arc, and it represents some of the earliest are magmatism in the southern Cordillera. Its formation was accompanied by the emplacement of voluminous gabbroic sills and dikes, and was synchronous with regional amphibolite facies metamorphism that is locally upgraded to granulite facies along the edges of the intrusion. The intrusion has been tilted to the east and now stands almost vertically on edge, striking NNW-SSE. It has a 200–500-m basal cumulate zone of dunite and websterite along its western margin, overlain stratigraphically to the east by 2.5 km of poorly layered two-pyroxene gabbro and homblende gabbro. Although olivine is present in the basal ultramafic cumulates, nowhere does it coexist with plagioclase. The common differentiation indices, such as An content of plagioclase, 100 Mg/(Mg+Fe) in the pyroxenes and the whole rocks, and the concentrations of various incompatible trace elements, all vary systematically through the intrusion. The parental magma was evidently a hydrous, high-Mg tholeiite with >11 wt.% MgO, 100 Mg/(Mg+Fe)=73–74, and REE abundances approximately 10 × chondrites. Its estimated trace element characteristics were strikingly similar to those of primary island are magmas from occanic regions. Most of the gabbroic rocks represent or approach liquid compositions; they are hypersthene-normative, almost silica-saturated tholeiites. Evolution from the most mafic compositions comparable to primitive island are basalts to derivative compositions with the typical characteristics of continental are basalts occurred through the combined effects of fractional crystallization, multiple magma batch injection, and crustal assimilation. There appears to be no need to invoke a special subcontinental mantle source to produce the trace element signatures that are so characteristic of continental are magmas. These signatures can be developed by limited crystallization and assimilation in the lower crust.
Keywords
- Magmatism,
- Gabbro
Disciplines
- Geochemistry and
- Geology
Publication Date
June, 1994
DOI
10.1093/petrology/35.3.679
Publisher Statement
Published by Oxford Academic
Citation Information
DeBari, S. M. (1994) Petrogenesis and physical evolution of the Fiambalá gabbronorite, northwestern Argentina: Syntectonic magmatism in the deep crust of a continental margin arc, Journal of Petrology, v. 35, no. 3, p. 679-713.