Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley, Sierra de Valle Fértil, Argentina: implications for the evolution of magmas in the lower crust of the Famatinian arc

OTAMENDI, J.E., CRISTOFOLINI, E., TIBALDI, A.M., QUEVEDO, F.I., BALIANI, I.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2010 vol. 29 p. 685 - 704

0895-9811

This work presents the field setting, petrography, mineralogy and geochemistry of agabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites,diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one amongseveral examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and LaHuerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc incentral-western Argentina. The layered sequence grew at deep crustal levels (20-25 km)within a mafic lower crust. The base of the layered body was detached off during the tectonicuplift of the Famatinian lower crust, whereas the roof of the layered body is thorough exposedon the present-day eastern zone. At the inferred roof, olivine-bearing rocks vanish, cumulatetextures are less frequent, and the igneous sequence becomes dominated by massive or thinlybanded gabbronorites. Mainly based on the petrographic relationships, the inferred order ofcrystallization in the gabbroic and peridotitic layered sequence is: 1- Cr-Al-spinel + olivine, 2-Cr-Al-spinel + olivine + clinopyroxene + magnetite, 3- Cr-Al-spinel + olivine + plagioclase +magnetite ± orthopyroxene, and 4- Al-spinel + orthopyroxene + amphibole. A strong linearnegative correlation between olivine and plagioclase modal proportions combined with field,petrographic and geochemical observations are used to demonstrate that the physical separationof olivine and plagioclase renders rock diversity from few centimeters to tens of meters scales.However, the composition of olivine (Fo ~ 0.81) and plagioclase (An > 94%) remains similarthroughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, anddisplay chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic andperidotitic layered rocks have trace element concentrations reflecting they are cumulate of earlycrystallizing minerals. The trace element patterns still retain the typical features of subductionrelatedarc magmatism, reflecting that the process of cumulate formation did not obscure thetrace element signature of the parental magma. Using the composition of cumulus minerals andwhole-rocks chemical trends, we show that the parent magma was mafic (SiO2 ~ 48 wt.%) withMg-number around 0.6, and hydrous. The oxygen fugacity (fO2) of the parent magma estimatedbetween +0.8 and -0.6 log fO2 units around the fayalite-magnetite-quartz (FMQ) buffer is alsocharacteristic of primitive hydrous arc magmas. The initially high water content of the parentalmagma allowed amphibole to crystallize as an interstitial phase all over the crystallizationevolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblagegives rise to the retention of many trace elements which would otherwise be incompatible withthe mineral assemblage of mafic-ultramafic cumulates. This study shows that there existstrongly mafic and primitive magmas that are both generated and emplaced within the lowercrustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentinacannot be regarded as a typical Andean-type tectono-magmatic setting.